This study includes an experimental and numerical analysis of the performances of a parabolic trough collector(PTC)with and without cylindrical turbulators.The PTC is designed with dimensions of 2.00 m in length and 1...This study includes an experimental and numerical analysis of the performances of a parabolic trough collector(PTC)with and without cylindrical turbulators.The PTC is designed with dimensions of 2.00 m in length and 1.00 m in width.The related reflector is made of lined sheets of aluminum,and the tubes are made of stainless steel used for the absorption of heat.They have an outer diameter of 0.051 m and a wall thickness of 0.002 m.Water,used as a heat transfer fluid(HTF),flows through the absorber tube at a mass flow rate of 0.7 kg/s.The dimensions of cylindrical turbulators are 0.04 m in length and 0.047 m in diameter.Simulations are performed using the ANSYS Fluent 2020 R2 software.The PTC performance is evaluated by comparing the experimental and numerical outcomes,namely,the outlet temperature,useful heat,and thermal efficiency for a modified tube(MT)(tube with novel cylindrical turbulators)and a plain tube(PT)(tube without novel cylindrical turbulators).According to the results,the experimental outlet temperatures recorded 63.2°C and 50.5°C for the MT and PT,respectively.The heat gain reaches 1137.5 Win the MT and 685.8 Win the PT.Compared to the PT collector,the PTC exhibited a(1.64 times)higher efficiency.展开更多
Mathematical modeling of heat exchange in air in pipes with turbulators with d/D=0.95÷0.90 and t/D=0.25÷1.00,as well as in rough pipes,with large Reynolds numbers(Re=106).The solution of the heat exchange pr...Mathematical modeling of heat exchange in air in pipes with turbulators with d/D=0.95÷0.90 and t/D=0.25÷1.00,as well as in rough pipes,with large Reynolds numbers(Re=106).The solution of the heat exchange problem for semicircular cross-section flow turbulizers based on multi-block computing technologies based on the factorized Reynolds equations(closed using the Menter shear stress transfer model)and the energy equation(on multi-scale intersecting structured grids)was considered.This method was previously successfully applied and verified by experiment in[1-4]for lower Reynolds numbers.The article continues the computational studies initiated in[1-4,25-27].展开更多
This paper details the results of a joint project between Rolls-Royce Deutschland (RRD) and the Northwestern Polytechnical University of China (NWPU). The objective of the project was the determination of the influenc...This paper details the results of a joint project between Rolls-Royce Deutschland (RRD) and the Northwestern Polytechnical University of China (NWPU). The objective of the project was the determination of the influence of tabulators in turbine blade cooling passages on film hole discharge coefficients (Cd coefficients). A large-scale plexiglas model was used by the NWPU to measure the turbulator influence on Cd coefficients for a wide range of different geometrical parameters, Reynolds numbers and cooling flow off take ratios. RRD specified the comprehensive test matrix and analysed the test data. The CFD code FLUENT was used by RRD for numerical simulation of the test cases with the main objective to support the interpretation of observed trends. Both, experimental and numerical results will be presented in this paper for a selection of test configurations.展开更多
Gas explosion in confined space often leads to significant pressure oscillation.It is widely recognized that structural damage can be severe when the oscillation frequency of the load resonates with the natural vibrat...Gas explosion in confined space often leads to significant pressure oscillation.It is widely recognized that structural damage can be severe when the oscillation frequency of the load resonates with the natural vibration frequency of the structure.To reveal the oscillation mechanism of gas explosion load,the experiment of gas explosion was conducted in a large-scale confined tube with the length of 30 m,and the explosion process was numerically analyzed using FLACS.The results show that the essential cause of oscillation effect is the reflection of the pressure wave.In addition,due to the difference in the propagation path of the pressure wave,the load oscillation frequency at the middle position of the tunnel is twice that at the end position.The average sound velocity can be used to calculate the oscillation frequency of overpressure accurately,and the error is less than 15%.The instability of the flame surface and the increase of flame turbulence caused by the interaction between the pressure wave and the flame surface are the main contributors to the increase in overpressure and amplitude.The overpressure peaks calculated by the existing flame instability model and turbulence disturbance model are 31.7%and 34.7%lower than the numerical results,respectively.The turbulence factor model established in this work can describe the turbulence enhancement effect caused by flame instability and oscillatory load,and the difference between the theoretical and numerical results is only 4.6%.In the theoretical derivation of the overpressure model,an improved model of dynamic turbulence factor is established,which can describe the enhancement effect of turbulence factor caused by flame instability and self-turbulence.Based on the one-dimensional propagation theory of pressure wave,the oscillatory effect of the load is derived to calculate the frequency and amplitude of pressure oscillation.The average error of amplitude and frequency is less than 20%.展开更多
Particle-fluid two-phase flows in rock fractures and fracture networks play a pivotal role in determining the efficiency and effectiveness of hydraulic fracturing operations,a vital component in unconventional oil and...Particle-fluid two-phase flows in rock fractures and fracture networks play a pivotal role in determining the efficiency and effectiveness of hydraulic fracturing operations,a vital component in unconventional oil and gas extraction.Central to this phenomenon is the transport of proppants,tiny solid particles injected into the fractures to prevent them from closing once the injection is stopped.However,effective transport and deposition of proppant is critical in keeping fracture pathways open,especially in lowpermeability reservoirs.This review explores,then quantifies,the important role of fluid inertia and turbulent flows in governing proppant transport.While traditional models predominantly assume and then characterise flow as laminar,this may not accurately capture the complexities inherent in realworld hydraulic fracturing and proppant emplacement.Recent investigations highlight the paramount importance of fluid inertia,especially at the high Reynolds numbers typically associated with fracturing operations.Fluid inertia,often overlooked,introduces crucial forces that influence particle settling velocities,particle-particle interactions,and the eventual deposition of proppants within fractures.With their inherent eddies and transient and chaotic nature,turbulent flows introduce additional complexities to proppant transport,crucially altering proppant settling velocities and dispersion patterns.The following comprehensive survey of experimental,numerical,and analytical studies elucidates controls on the intricate dynamics of proppant transport under fluid inertia and turbulence-towards providing a holistic understanding of the current state-of-the-art,guiding future research directions,and optimising hydraulic fracturing practices.展开更多
Large size titanium alloy parts are widely used in aerospace.However,they are difficult to manufacture using mechanical cutting technology because of severe tool wear.Electrochemical jet machining is a promising techn...Large size titanium alloy parts are widely used in aerospace.However,they are difficult to manufacture using mechanical cutting technology because of severe tool wear.Electrochemical jet machining is a promising technology to achieve high efficiency,because it has high machining flexibility and no machining tool wear.However,reports on the macro electrochemical jet machining of large size titanium alloy parts are very scarce,because it is difficult to achieve effective constraint of the flow field in macro electrochemical jet machining.In addition,titanium alloy is very sensitive to fluctuation of the flow field,and a turbulent flow field would lead to serious stray corrosion.This paper reports a series of investigations of the electrochemical jet machining of titanium alloy parts.Based on the flow analysis and experiments,the machining flow field was effectively constrained.TB6 titanium alloy part with a perimeter of one meter was machined.The machined surface was smooth with no obvious machining defects.The machining process was particularly stable with no obvious spark discharge.The research provides a reference for the application of electrochemical jet machining technology to achieve large allowance material removal in the machining of large titanium alloy parts.展开更多
During Donald Trump’s first term,the“Trump Shock”brought world politics into an era of uncertainties and pulled the transatlantic alliance down to its lowest point in history.The Trump 2.0 tsunami brewed by the 202...During Donald Trump’s first term,the“Trump Shock”brought world politics into an era of uncertainties and pulled the transatlantic alliance down to its lowest point in history.The Trump 2.0 tsunami brewed by the 2024 presidential election of the United States has plunged the U.S.-Europe relations into more gloomy waters,ushering in a more complex and turbulent period of adjustment.展开更多
Restoration of phase aberrations is crucial for addressing atmospheric turbulence in light propagation.Traditional restoration algorithms based on Zernike polynomials(ZPs)often encounter challenges related to high com...Restoration of phase aberrations is crucial for addressing atmospheric turbulence in light propagation.Traditional restoration algorithms based on Zernike polynomials(ZPs)often encounter challenges related to high computational complexity and insufficient capture of high-frequency phase aberration components,so we proposed a Principal-Component-Analysis-based method for representing phase aberrations.This paper discusses the factors influencing the accuracy of restoration,mainly including the sample space size and the sampling interval of D/r_(0),on the basis of characterizing phase aberrations by Principal Components(PCs).The experimental results show that a larger D/r_(0)sampling interval can ensure the generalization ability and robustness of the principal components in the case of a limited amount of original data,which can help to achieve high-precision deployment of the model in practical applications quickly.In the environment with relatively strong turbulence in the test set of D/r_(0)=24,the use of 34 terms of PCs can improve the corrected Strehl ratio(SR)from 0.007 to 0.1585,while the Strehl ratio of the light spot after restoration using 34 terms of ZPs is only 0.0215,demonstrating almost no correction effect.The results indicate that PCs can serve as a better alternative in representing and restoring the characteristics of atmospheric turbulence induced phase aberrations.These findings pave the way to use PCs of phase aberrations with fewer terms than traditional ZPs to achieve data dimensionality reduction,and offer a reference to accelerate and stabilize the model and deep learning based adaptive optics correction.展开更多
Temperature is a key meteorological factor that affects tropospheric ozone(O_(3)),with both humid-heat(sauna days)and dry-heat(roast days)conditions leading to O_(3) exceedances.However,the mechanisms influencing O_(3...Temperature is a key meteorological factor that affects tropospheric ozone(O_(3)),with both humid-heat(sauna days)and dry-heat(roast days)conditions leading to O_(3) exceedances.However,the mechanisms influencing O_(3) formation and degradation under these two weather conditions remain unclear.Therefore,experiments were conducted in Beijing from 2019 to 2021 to observe O_(3),its precursors,and related meteorological elements.A total of 18 days with O_(3) exceedances were selected,including 10 sauna days and 8 roast days.The results of this study revealed that on roast days,the sensible heat flux was 143.5 W/m^(2) greater and the wind speed gradient was 0.018 s^(-1) greater than those on sauna days,indicating more intense thermal and dynamic turbulence.The strong turbulence enhanced the vertical cycle of nitrogen dioxide(NO_(2))and O_(3),resulting in a 58.2μg/(m^(2)·h)increase in NO_(2) upward transport rate and a 1034.4μg/(m^(2)·h)increase in O_(3) downward transport rate on roast days than sauna days.Subsequently,a box model analysis was used to examine O_(3) formation under the two types of weather conditions,revealing that the NO_(2)-O_(3) vertical cycling speed dominated the O_(3) sensitivity.The O_(3) sensitivity was synergistically controlled by nitrogen oxides(NO_(x))and volatile organic compounds on sauna days,while it tended to be NO_(x)-limited on roast days.The aim of this study was to provide a scientific theoretical basis for the control of O_(3) under different types of high temperature weather conditions.展开更多
Atmospheric turbulence is an important parameter affecting laser atmospheric transmission.This paper reports on a self-developed atmospheric turbulence detection Li DAR system(scanning differential image motion Li DAR...Atmospheric turbulence is an important parameter affecting laser atmospheric transmission.This paper reports on a self-developed atmospheric turbulence detection Li DAR system(scanning differential image motion Li DAR(DIM-Li DAR)system).By designing and simulating the optical system of atmospheric turbulence detection Li DAR,the basic optical imaging accuracy has been determined.展开更多
Due to the arid and sandy surface of the Taklimakan Desert(TD)in China,the turbulence structure and vertical distribution of ozone exhibit unique and complex characteristics.However,few studies have focused on these i...Due to the arid and sandy surface of the Taklimakan Desert(TD)in China,the turbulence structure and vertical distribution of ozone exhibit unique and complex characteristics.However,few studies have focused on these issues.To reveal the variation characteristics of summertime atmospheric turbulence and ozone concentration over the TD,we conducted joint detection experiments in July 2016 and July 2021 at Tazhong in the hinterland of the TD using an eddy covariance detection system,a GPS(Global Positioning System)sounding system,and a meteorological gradient tower.Using methods such as statistical analysis,nonlinear fitting,and Fast Fourier Transform,this study analyzed and processed parameters including temperature,relative humidity,wind speed,turbulence parameters,turbulence spectra,and ozone concentration.The high average temperature is accompanied by low relative humidity over the TD,showing a negative correlation between the two.The temperature of the 10.0-cm-deep sand layer lags the near-surface air temperature by nearly 4 h.From 09:30 to 21:00(Beijing Time),under conditions where the sensible heat flux is positive but stability parameter(z/L,where z is the height and L is the Obukhov length)is negative,the atmosphere is heated by the land surface,with the occurrence of unstable stratification;however,the conditions are the opposite(sensible heat flux is negative and z/L is positive)after 22:00,which are accompanied with the cooling of the surface radiation,occurrence of temperature inversion in the lower atmosphere,and stable stratification.A positive correlation is identified between the diurnal variation of turbulent kinetic energy(TKE)and the atmospheric boundary layer(ABL)height,with significant contributions from both the buoyancy and shear terms during the daytime.Under unstable stratification,the normalized standard deviations of the three-dimensional wind speed,temperature,and humidity conform to the Monin-Obukhov Similarity Theory(MOST).As the stability parameter z/L transitions from strongly unstable to strongly stable,the energy of the dimensionless turbulent velocity spectra gradually decreases and conforms to the -2/3 power law within the inertial subrange.In the hinterland of the TD,the summertime tropospheric ozone concentration remains below approximately 0.70×10^(-6)(volume concentration).Above the troposphere,within the range of 16,500.0-30,000.0 m,a significant increasing trend is identified in the ozone concentration with altitude.At an altitude of 30,000.0 m,the maximum ozone concentration can reach up to 7.50×10^(-6).The research findings provide both theoretical and data foundations for future in-depth studies of turbulent motion and ozone concentration distribution in the TD,as well as in the similar areas around the world.展开更多
While Metaheuristic optimization techniques are known to work well for clustering and large-scale numerical optimization,algorithms in this category suffer from issues like reinforcement stagnation and poor late-stage...While Metaheuristic optimization techniques are known to work well for clustering and large-scale numerical optimization,algorithms in this category suffer from issues like reinforcement stagnation and poor late-stage refinement.In this paper,we propose the Improved Geyser-Inspired Optimization Algorithm(IGIOA),an enhancement of the Geyser-Inspired Optimization Algorithm(GIOA),which integrates two primary components:the Adaptive Turbulence Operator(ATO)and the Dynamic Pressure Equilibrium Operator(DPEO).ATO allows IGIOA to periodically disrupt stagnation and explore different regions by using turbulence,while DPEO ensures refinement in later iterations by adaptively modulating convergence pressure.We implemented IGIOA on 23 benchmark functions with both unimodal and multimodal contours,in addition to eight problems pertaining to cluster analysis at the UCI.IGIOA,out of all the tested methods,was able to converge most accurately while also achieving a stable convergence rate.The mitigation of premature convergence and low-level exploitation was made possible by the turbulence and pressure-based refinements.The findings from the tests confirm that the adaptation of baseline strategies by IGIOA helps deal with complex data distributions more effectively.However,additional hyperparameters which add complexity are introduced,along with increased computational cost.These include automatic tuning of parameters,ensemble or parallel variations,and hybridization with dedicated local search strategies to extend the reach of IGIOA for general optimization while also specializing it for clustering focused tasks and applications.展开更多
The primary objective in aircraft transportation is to minimize turbulent drag, thereby conserving energy and reducing emissions. We propose a sector-shaped counter-flow dielectric barrier discharge plasma actuator, w...The primary objective in aircraft transportation is to minimize turbulent drag, thereby conserving energy and reducing emissions. We propose a sector-shaped counter-flow dielectric barrier discharge plasma actuator, which leverages jet synthesis for drag reduction. A drag control experiment was conducted in a low-speed wind tunnel with a controlled flow velocity of 9.6 m/s(Re = 1.445 × 10^(4)). This study investigated the effects of varying pulse frequencies and actuation voltages on the turbulent boundary layer. Using a hot-wire measurement system, we analyzed the pulsating and time-averaged velocity distributions within the boundary layer to evaluate the streamwise turbulent drag reduction. The results show that the local TDR decreases as the pulse frequency increases, reaching a maximum reduction of approximately 20.97% at a pulse frequency of 50 Hz. In addition, as the actuation voltage increases, the friction coefficient decreases, increasing the drag reduction rate. The maximum drag reduction of approximately 33.34% is achieved at an actuation voltage of 10 kV.展开更多
Numerical simulations were conducted on a 10-blade Sevik rotor ingesting wake downstream of two turbulence-generating grids.These simulations were based on implicit large-eddy simulation(ILES)and the boundary data imm...Numerical simulations were conducted on a 10-blade Sevik rotor ingesting wake downstream of two turbulence-generating grids.These simulations were based on implicit large-eddy simulation(ILES)and the boundary data immersion method(BDIM)for compressible flows,which were solved using a fully self-programmed Fortran code.Results show that the predicted thrust spectrum aligns closely with the experimental measurements.In addition,it captures the thrust dipole directivity of the noise around the rotating propeller due to random pressure pulsations on the blades,as well as the flow structures simultaneously.Furthermore,the differences in the statistical characteristics,flow structures,and low-frequency broadband thrust spectra due to different turbulence levels were investigated.This analysis indicates that the interaction between the upstream,which is characterized by a lower turbulence level and a higher turbulent length of scale,and the rotating propeller results in a lower amplitude in force spectra and a slight increase in the scale of tip vortices.展开更多
Machine learning(ML)techniques have emerged as powerful tools for improving the predictive capabilities of Reynolds-averaged Navier-Stokes(RANS)turbulence models in separated flows.This improvement is achieved by leve...Machine learning(ML)techniques have emerged as powerful tools for improving the predictive capabilities of Reynolds-averaged Navier-Stokes(RANS)turbulence models in separated flows.This improvement is achieved by leveraging complex ML models,such as those developed using field inversion and machine learning(FIML),to dynamically adjust the constants within the baseline RANS model.However,the ML models often overlook the fundamental calibrations of the RANS turbulence model.Consequently,the basic calibration of the baseline RANS model is disrupted,leading to a degradation in the accuracy,particularly in basic wall-attached flows outside of the training set.To address this issue,a modified version of the Spalart-Allmaras(SA)turbulence model,known as Rubber-band SA(RBSA),has been proposed recently.This modification involves identifying and embedding constraints related to basic wall-attached flows directly into the model.It is shown that no matter how the parameters of the RBSA model are adjusted as constants throughout the flow field,its accuracy in wall-attached flows remains unaffected.In this paper,we propose a new constraint for the RBSA model,which better safeguards the law of wall in extreme conditions where the model parameter is adjusted dramatically.The resultant model is called the RBSA-poly model.We then show that when combined with FIML augmentation,the RBSA-poly model effectively preserves the accuracy of simple wall-attached flows,even when the adjusted parameters become functions of local flow variables rather than constants.A comparative analysis with the FIML-augmented original SA model reveals that the augmented RBSA-poly model reduces error in basic wall-attached flows by 50%while maintaining comparable accuracy in trained separated flows.These findings confirm the effectiveness of utilizing FIML in conjunction with the RBSA model,offering superior accuracy retention in cardinal flows.展开更多
The diffuse-interface immersed boundary method(IBM)possesses excellent capabilities for simulating flows around complex geometries and moving boundaries.In this method,the flow field is solved on a fixed Cartesian mes...The diffuse-interface immersed boundary method(IBM)possesses excellent capabilities for simulating flows around complex geometries and moving boundaries.In this method,the flow field is solved on a fixed Cartesian mesh,while the solid boundary is discretized into a series of Lagrangian points immersed in the flow field.The boundary condition is implemented by introducing a force term into the momentum equation,and the interaction between the immersed boundary and the fluid domain is achieved via an interpolation process.Over the past decades,the diffuse-interface IBM has gained popularity and spawned many variants,effectively handling a wide range of flow problems from isothermal to thermal flows,from laminar to turbulent flows,and from complex geometries to fluidstructure interaction scenarios.This paper first outlines the basic principles of the diffuse-interface IBM,then highlights recent advancements achieved by the authors’research group,and finally shows the method’s excellent numerical performance and wide applicability through several case studies involving complex moving boundary problems.展开更多
In the Arctic Ocean,turbulent mixing drives vertical heat flux,thereby affecting the sea ice variability.Internal wave is regarded as one of the important energy sources of mixing in this region.The high latitude and ...In the Arctic Ocean,turbulent mixing drives vertical heat flux,thereby affecting the sea ice variability.Internal wave is regarded as one of the important energy sources of mixing in this region.The high latitude and sea ice cover make internal wave in the Arctic Ocean apparently differs from that in mid-and low-latitude oceans.However,the internal wave and its underlying mechanism are less understood due to the lack of observations.This paper briefly reviews the recent studies and unresolved questions on the internal wave in the Arctic Ocean,including wind-driven near-inertial wave,internal tide,and high-frequency internal wave.The aim is to provide new insights for in-depth research in the future,with a focus on the mechanisms responsible for the evolution of internal wave under the rapidly changing Arctic climate.展开更多
Amid overlapping crises and uncertainties as the world enters an era of turbulence and transformation,the countries of the Global South are swiftly emerging from the periphery of world politics to the forefront of int...Amid overlapping crises and uncertainties as the world enters an era of turbulence and transformation,the countries of the Global South are swiftly emerging from the periphery of world politics to the forefront of international cooperation,playing an increasingly vital role in advancing humanity’s collective progress.展开更多
In this study,the flow characteristics around a group of three piers arranged in tandem were investigated both numerically and experimentally.The simulation utilised the volume of fluid(VOF)model in conjunction with t...In this study,the flow characteristics around a group of three piers arranged in tandem were investigated both numerically and experimentally.The simulation utilised the volume of fluid(VOF)model in conjunction with the k–ɛmethod(i.e.,for flow turbulence representations),implemented through the ANSYS FLUENT software,to model the free-surface flow.The simulation results were validated against laboratory measurements obtained using an acoustic Doppler velocimeter.The comparative analysis revealed discrepancies between the simulated and measured maximum velocities within the investigated flow field.However,the numerical results demonstrated a distinct vortex-induced flow pattern following the first pier and throughout the vicinity of the entire pier group,which aligned reasonably well with experimental data.In the heavily narrowed spaces between the piers,simulated velocity profiles were overestimated in the free-surface region and underestimated in the areas near the bed to the mid-stream when compared to measurements.These discrepancies diminished away from the regions with intense vortices,indicating that the employed model was capable of simulating relatively less disturbed flow turbulence.Furthermore,velocity results from both simulations and measurements were compared based on velocity distributions at three different depth ratios(0.15,0.40,and 0.62)to assess vortex characteristic around the piers.This comparison revealed consistent results between experimental and simulated data.This research contributes to a deeper understanding of flow dynamics around complex interactive pier systems,which is critical for designing stable and sustainable hydraulic structures.Furthermore,the insights gained from this study provide valuable information for engineers aiming to develop effective strategies for controlling scour and minimizing destructive vortex effects,thereby guiding the design and maintenance of sustainable infrastructure.展开更多
Research has shown considerable variability in whitecap coverage(W)under low to moderate wind conditions.During an expedition to the Northwestern Pacific,oceanographic variables and photographic measurements were coll...Research has shown considerable variability in whitecap coverage(W)under low to moderate wind conditions.During an expedition to the Northwestern Pacific,oceanographic variables and photographic measurements were collected to investigate the influence of wave-induced stress on W within these wind ranges.The friction velocity was recalculated based on turbulent stress,and wind profiles were modified to account for wave-induced stress and swell presence on the sea surface.The study examined W’s relationship with multiple parameters,including friction velocity(u*),breaking wave Reynolds numbers,wavesea Reynolds numbers,and wave age.The analysis utilized both conventional u*and turbulent stress-based friction velocity(u*turb).When utilizing u*turb rather than u*,the estimation model’s fitting results revealed an increase in correlation coefficient(R2)from 0.51 to 0.62,and a decrease in root mean square error(RMSE)from 0.0652 to 0.0574.Additionally,when parameterizing W using the windsea Reynolds number,with u_(*turb) replacing u*and wind wave height substituting mixed wave height,the R^(2) increased from 0.38 to 0.53,and the RMSE decreased from 0.0737 to 0.0668.The results demonstrate that calculating u*using the turbulent stress-based method,along with wind wave height and peak wave speed of mixed waves,yields stronger correlation with W.This correlation improvement stems from the inhibition of wave breaking by swell and wave-induced stress.The integration of turbulent stress and wind wave field measurements enhances the understanding of relationships between W and various parameters.However,swell effects on wind profiles do not substantially affect W estimation using wind speed-related parameters.展开更多
文摘This study includes an experimental and numerical analysis of the performances of a parabolic trough collector(PTC)with and without cylindrical turbulators.The PTC is designed with dimensions of 2.00 m in length and 1.00 m in width.The related reflector is made of lined sheets of aluminum,and the tubes are made of stainless steel used for the absorption of heat.They have an outer diameter of 0.051 m and a wall thickness of 0.002 m.Water,used as a heat transfer fluid(HTF),flows through the absorber tube at a mass flow rate of 0.7 kg/s.The dimensions of cylindrical turbulators are 0.04 m in length and 0.047 m in diameter.Simulations are performed using the ANSYS Fluent 2020 R2 software.The PTC performance is evaluated by comparing the experimental and numerical outcomes,namely,the outlet temperature,useful heat,and thermal efficiency for a modified tube(MT)(tube with novel cylindrical turbulators)and a plain tube(PT)(tube without novel cylindrical turbulators).According to the results,the experimental outlet temperatures recorded 63.2°C and 50.5°C for the MT and PT,respectively.The heat gain reaches 1137.5 Win the MT and 685.8 Win the PT.Compared to the PT collector,the PTC exhibited a(1.64 times)higher efficiency.
文摘Mathematical modeling of heat exchange in air in pipes with turbulators with d/D=0.95÷0.90 and t/D=0.25÷1.00,as well as in rough pipes,with large Reynolds numbers(Re=106).The solution of the heat exchange problem for semicircular cross-section flow turbulizers based on multi-block computing technologies based on the factorized Reynolds equations(closed using the Menter shear stress transfer model)and the energy equation(on multi-scale intersecting structured grids)was considered.This method was previously successfully applied and verified by experiment in[1-4]for lower Reynolds numbers.The article continues the computational studies initiated in[1-4,25-27].
文摘This paper details the results of a joint project between Rolls-Royce Deutschland (RRD) and the Northwestern Polytechnical University of China (NWPU). The objective of the project was the determination of the influence of tabulators in turbine blade cooling passages on film hole discharge coefficients (Cd coefficients). A large-scale plexiglas model was used by the NWPU to measure the turbulator influence on Cd coefficients for a wide range of different geometrical parameters, Reynolds numbers and cooling flow off take ratios. RRD specified the comprehensive test matrix and analysed the test data. The CFD code FLUENT was used by RRD for numerical simulation of the test cases with the main objective to support the interpretation of observed trends. Both, experimental and numerical results will be presented in this paper for a selection of test configurations.
基金financial support from National Natural Science Foundation of China(Grant No.52378488)Postgraduate Research&Practice Innovation Program of Jiangsu Province(Grant No.KYCX22_0222).
文摘Gas explosion in confined space often leads to significant pressure oscillation.It is widely recognized that structural damage can be severe when the oscillation frequency of the load resonates with the natural vibration frequency of the structure.To reveal the oscillation mechanism of gas explosion load,the experiment of gas explosion was conducted in a large-scale confined tube with the length of 30 m,and the explosion process was numerically analyzed using FLACS.The results show that the essential cause of oscillation effect is the reflection of the pressure wave.In addition,due to the difference in the propagation path of the pressure wave,the load oscillation frequency at the middle position of the tunnel is twice that at the end position.The average sound velocity can be used to calculate the oscillation frequency of overpressure accurately,and the error is less than 15%.The instability of the flame surface and the increase of flame turbulence caused by the interaction between the pressure wave and the flame surface are the main contributors to the increase in overpressure and amplitude.The overpressure peaks calculated by the existing flame instability model and turbulence disturbance model are 31.7%and 34.7%lower than the numerical results,respectively.The turbulence factor model established in this work can describe the turbulence enhancement effect caused by flame instability and oscillatory load,and the difference between the theoretical and numerical results is only 4.6%.In the theoretical derivation of the overpressure model,an improved model of dynamic turbulence factor is established,which can describe the enhancement effect of turbulence factor caused by flame instability and self-turbulence.Based on the one-dimensional propagation theory of pressure wave,the oscillatory effect of the load is derived to calculate the frequency and amplitude of pressure oscillation.The average error of amplitude and frequency is less than 20%.
基金the Australian Research Council Discovery Project(ARC DP 220100851)scheme and would acknowledge that.
文摘Particle-fluid two-phase flows in rock fractures and fracture networks play a pivotal role in determining the efficiency and effectiveness of hydraulic fracturing operations,a vital component in unconventional oil and gas extraction.Central to this phenomenon is the transport of proppants,tiny solid particles injected into the fractures to prevent them from closing once the injection is stopped.However,effective transport and deposition of proppant is critical in keeping fracture pathways open,especially in lowpermeability reservoirs.This review explores,then quantifies,the important role of fluid inertia and turbulent flows in governing proppant transport.While traditional models predominantly assume and then characterise flow as laminar,this may not accurately capture the complexities inherent in realworld hydraulic fracturing and proppant emplacement.Recent investigations highlight the paramount importance of fluid inertia,especially at the high Reynolds numbers typically associated with fracturing operations.Fluid inertia,often overlooked,introduces crucial forces that influence particle settling velocities,particle-particle interactions,and the eventual deposition of proppants within fractures.With their inherent eddies and transient and chaotic nature,turbulent flows introduce additional complexities to proppant transport,crucially altering proppant settling velocities and dispersion patterns.The following comprehensive survey of experimental,numerical,and analytical studies elucidates controls on the intricate dynamics of proppant transport under fluid inertia and turbulence-towards providing a holistic understanding of the current state-of-the-art,guiding future research directions,and optimising hydraulic fracturing practices.
基金the National Natural Science Foundation of China(No.52205468)China Postdoctoral Science Foundation(No.2022M710061 and No.2023T160277)Natural Science Foundation of Jiangsu Province(No.BK20210755)。
文摘Large size titanium alloy parts are widely used in aerospace.However,they are difficult to manufacture using mechanical cutting technology because of severe tool wear.Electrochemical jet machining is a promising technology to achieve high efficiency,because it has high machining flexibility and no machining tool wear.However,reports on the macro electrochemical jet machining of large size titanium alloy parts are very scarce,because it is difficult to achieve effective constraint of the flow field in macro electrochemical jet machining.In addition,titanium alloy is very sensitive to fluctuation of the flow field,and a turbulent flow field would lead to serious stray corrosion.This paper reports a series of investigations of the electrochemical jet machining of titanium alloy parts.Based on the flow analysis and experiments,the machining flow field was effectively constrained.TB6 titanium alloy part with a perimeter of one meter was machined.The machined surface was smooth with no obvious machining defects.The machining process was particularly stable with no obvious spark discharge.The research provides a reference for the application of electrochemical jet machining technology to achieve large allowance material removal in the machining of large titanium alloy parts.
文摘During Donald Trump’s first term,the“Trump Shock”brought world politics into an era of uncertainties and pulled the transatlantic alliance down to its lowest point in history.The Trump 2.0 tsunami brewed by the 2024 presidential election of the United States has plunged the U.S.-Europe relations into more gloomy waters,ushering in a more complex and turbulent period of adjustment.
文摘Restoration of phase aberrations is crucial for addressing atmospheric turbulence in light propagation.Traditional restoration algorithms based on Zernike polynomials(ZPs)often encounter challenges related to high computational complexity and insufficient capture of high-frequency phase aberration components,so we proposed a Principal-Component-Analysis-based method for representing phase aberrations.This paper discusses the factors influencing the accuracy of restoration,mainly including the sample space size and the sampling interval of D/r_(0),on the basis of characterizing phase aberrations by Principal Components(PCs).The experimental results show that a larger D/r_(0)sampling interval can ensure the generalization ability and robustness of the principal components in the case of a limited amount of original data,which can help to achieve high-precision deployment of the model in practical applications quickly.In the environment with relatively strong turbulence in the test set of D/r_(0)=24,the use of 34 terms of PCs can improve the corrected Strehl ratio(SR)from 0.007 to 0.1585,while the Strehl ratio of the light spot after restoration using 34 terms of ZPs is only 0.0215,demonstrating almost no correction effect.The results indicate that PCs can serve as a better alternative in representing and restoring the characteristics of atmospheric turbulence induced phase aberrations.These findings pave the way to use PCs of phase aberrations with fewer terms than traditional ZPs to achieve data dimensionality reduction,and offer a reference to accelerate and stabilize the model and deep learning based adaptive optics correction.
基金supported by the National Natural Science Foundation of China(No.42177081)the National Key R&D Program of China(No.2023YFC3706103)+1 种基金Beijing Municipal Natural Science Foundation(No.8222075)the Youth Cross Team Scientific Research Project of the Chinese Academy of Sciences(No.JCTD-2021–10).
文摘Temperature is a key meteorological factor that affects tropospheric ozone(O_(3)),with both humid-heat(sauna days)and dry-heat(roast days)conditions leading to O_(3) exceedances.However,the mechanisms influencing O_(3) formation and degradation under these two weather conditions remain unclear.Therefore,experiments were conducted in Beijing from 2019 to 2021 to observe O_(3),its precursors,and related meteorological elements.A total of 18 days with O_(3) exceedances were selected,including 10 sauna days and 8 roast days.The results of this study revealed that on roast days,the sensible heat flux was 143.5 W/m^(2) greater and the wind speed gradient was 0.018 s^(-1) greater than those on sauna days,indicating more intense thermal and dynamic turbulence.The strong turbulence enhanced the vertical cycle of nitrogen dioxide(NO_(2))and O_(3),resulting in a 58.2μg/(m^(2)·h)increase in NO_(2) upward transport rate and a 1034.4μg/(m^(2)·h)increase in O_(3) downward transport rate on roast days than sauna days.Subsequently,a box model analysis was used to examine O_(3) formation under the two types of weather conditions,revealing that the NO_(2)-O_(3) vertical cycling speed dominated the O_(3) sensitivity.The O_(3) sensitivity was synergistically controlled by nitrogen oxides(NO_(x))and volatile organic compounds on sauna days,while it tended to be NO_(x)-limited on roast days.The aim of this study was to provide a scientific theoretical basis for the control of O_(3) under different types of high temperature weather conditions.
基金jointly funded by the National Science Foundation of China(No.42405069)the University Natural Sciences Research Project of Anhui Province(Nos.2023AH052201 and 2023AH052184)+1 种基金the 2023 Talent Research Fund Project of Hefei University(No.23RC01)the Technical Development Project of Hefei University(Nos.902/22050124128,902/22050124148 and 902/22050124250)。
文摘Atmospheric turbulence is an important parameter affecting laser atmospheric transmission.This paper reports on a self-developed atmospheric turbulence detection Li DAR system(scanning differential image motion Li DAR(DIM-Li DAR)system).By designing and simulating the optical system of atmospheric turbulence detection Li DAR,the basic optical imaging accuracy has been determined.
基金financially supported by"Tianshan Talent"Training Program-Science and Technology Innovation Team(Tianshan Innovation Team)Project(2022TSYCTD0007)the Youth Innovation Team of China Meteorological Administration(CMA2024QN13)+2 种基金the Central Government Guidance Funds for Local Science and Technology Development Program(ZYYD2025ZY21)the S&T Development Fund of Chinese Academy of Meteorological Sciences(2021KJ034)the Xinjiang Science Fund for Distinguished Young Scholars(2022D01E07).
文摘Due to the arid and sandy surface of the Taklimakan Desert(TD)in China,the turbulence structure and vertical distribution of ozone exhibit unique and complex characteristics.However,few studies have focused on these issues.To reveal the variation characteristics of summertime atmospheric turbulence and ozone concentration over the TD,we conducted joint detection experiments in July 2016 and July 2021 at Tazhong in the hinterland of the TD using an eddy covariance detection system,a GPS(Global Positioning System)sounding system,and a meteorological gradient tower.Using methods such as statistical analysis,nonlinear fitting,and Fast Fourier Transform,this study analyzed and processed parameters including temperature,relative humidity,wind speed,turbulence parameters,turbulence spectra,and ozone concentration.The high average temperature is accompanied by low relative humidity over the TD,showing a negative correlation between the two.The temperature of the 10.0-cm-deep sand layer lags the near-surface air temperature by nearly 4 h.From 09:30 to 21:00(Beijing Time),under conditions where the sensible heat flux is positive but stability parameter(z/L,where z is the height and L is the Obukhov length)is negative,the atmosphere is heated by the land surface,with the occurrence of unstable stratification;however,the conditions are the opposite(sensible heat flux is negative and z/L is positive)after 22:00,which are accompanied with the cooling of the surface radiation,occurrence of temperature inversion in the lower atmosphere,and stable stratification.A positive correlation is identified between the diurnal variation of turbulent kinetic energy(TKE)and the atmospheric boundary layer(ABL)height,with significant contributions from both the buoyancy and shear terms during the daytime.Under unstable stratification,the normalized standard deviations of the three-dimensional wind speed,temperature,and humidity conform to the Monin-Obukhov Similarity Theory(MOST).As the stability parameter z/L transitions from strongly unstable to strongly stable,the energy of the dimensionless turbulent velocity spectra gradually decreases and conforms to the -2/3 power law within the inertial subrange.In the hinterland of the TD,the summertime tropospheric ozone concentration remains below approximately 0.70×10^(-6)(volume concentration).Above the troposphere,within the range of 16,500.0-30,000.0 m,a significant increasing trend is identified in the ozone concentration with altitude.At an altitude of 30,000.0 m,the maximum ozone concentration can reach up to 7.50×10^(-6).The research findings provide both theoretical and data foundations for future in-depth studies of turbulent motion and ozone concentration distribution in the TD,as well as in the similar areas around the world.
基金King Saud University for funding this work through Researchers Supporting Project number(RSPD2024R697),King Saud University,Riyadh,Saudi Arabiafinancial support European Union under the REFRESH-Research Excellence For REgion Sustainability and High-tech Industries project number CZ.10.03.01/00/22_/0000048 via the Operational Programme Just Transition.
文摘While Metaheuristic optimization techniques are known to work well for clustering and large-scale numerical optimization,algorithms in this category suffer from issues like reinforcement stagnation and poor late-stage refinement.In this paper,we propose the Improved Geyser-Inspired Optimization Algorithm(IGIOA),an enhancement of the Geyser-Inspired Optimization Algorithm(GIOA),which integrates two primary components:the Adaptive Turbulence Operator(ATO)and the Dynamic Pressure Equilibrium Operator(DPEO).ATO allows IGIOA to periodically disrupt stagnation and explore different regions by using turbulence,while DPEO ensures refinement in later iterations by adaptively modulating convergence pressure.We implemented IGIOA on 23 benchmark functions with both unimodal and multimodal contours,in addition to eight problems pertaining to cluster analysis at the UCI.IGIOA,out of all the tested methods,was able to converge most accurately while also achieving a stable convergence rate.The mitigation of premature convergence and low-level exploitation was made possible by the turbulence and pressure-based refinements.The findings from the tests confirm that the adaptation of baseline strategies by IGIOA helps deal with complex data distributions more effectively.However,additional hyperparameters which add complexity are introduced,along with increased computational cost.These include automatic tuning of parameters,ensemble or parallel variations,and hybridization with dedicated local search strategies to extend the reach of IGIOA for general optimization while also specializing it for clustering focused tasks and applications.
基金Project supported by the National Natural Science Foundation of China (Grant Nos. 61971345 and 52107174)。
文摘The primary objective in aircraft transportation is to minimize turbulent drag, thereby conserving energy and reducing emissions. We propose a sector-shaped counter-flow dielectric barrier discharge plasma actuator, which leverages jet synthesis for drag reduction. A drag control experiment was conducted in a low-speed wind tunnel with a controlled flow velocity of 9.6 m/s(Re = 1.445 × 10^(4)). This study investigated the effects of varying pulse frequencies and actuation voltages on the turbulent boundary layer. Using a hot-wire measurement system, we analyzed the pulsating and time-averaged velocity distributions within the boundary layer to evaluate the streamwise turbulent drag reduction. The results show that the local TDR decreases as the pulse frequency increases, reaching a maximum reduction of approximately 20.97% at a pulse frequency of 50 Hz. In addition, as the actuation voltage increases, the friction coefficient decreases, increasing the drag reduction rate. The maximum drag reduction of approximately 33.34% is achieved at an actuation voltage of 10 kV.
基金Supported by the National Key R&D Program of China(2022YFB3303500).
文摘Numerical simulations were conducted on a 10-blade Sevik rotor ingesting wake downstream of two turbulence-generating grids.These simulations were based on implicit large-eddy simulation(ILES)and the boundary data immersion method(BDIM)for compressible flows,which were solved using a fully self-programmed Fortran code.Results show that the predicted thrust spectrum aligns closely with the experimental measurements.In addition,it captures the thrust dipole directivity of the noise around the rotating propeller due to random pressure pulsations on the blades,as well as the flow structures simultaneously.Furthermore,the differences in the statistical characteristics,flow structures,and low-frequency broadband thrust spectra due to different turbulence levels were investigated.This analysis indicates that the interaction between the upstream,which is characterized by a lower turbulence level and a higher turbulent length of scale,and the rotating propeller results in a lower amplitude in force spectra and a slight increase in the scale of tip vortices.
基金supported by the National Natural Science Foundation of China(Grant Nos.12388101,12372288,U23A2069,and 92152301).
文摘Machine learning(ML)techniques have emerged as powerful tools for improving the predictive capabilities of Reynolds-averaged Navier-Stokes(RANS)turbulence models in separated flows.This improvement is achieved by leveraging complex ML models,such as those developed using field inversion and machine learning(FIML),to dynamically adjust the constants within the baseline RANS model.However,the ML models often overlook the fundamental calibrations of the RANS turbulence model.Consequently,the basic calibration of the baseline RANS model is disrupted,leading to a degradation in the accuracy,particularly in basic wall-attached flows outside of the training set.To address this issue,a modified version of the Spalart-Allmaras(SA)turbulence model,known as Rubber-band SA(RBSA),has been proposed recently.This modification involves identifying and embedding constraints related to basic wall-attached flows directly into the model.It is shown that no matter how the parameters of the RBSA model are adjusted as constants throughout the flow field,its accuracy in wall-attached flows remains unaffected.In this paper,we propose a new constraint for the RBSA model,which better safeguards the law of wall in extreme conditions where the model parameter is adjusted dramatically.The resultant model is called the RBSA-poly model.We then show that when combined with FIML augmentation,the RBSA-poly model effectively preserves the accuracy of simple wall-attached flows,even when the adjusted parameters become functions of local flow variables rather than constants.A comparative analysis with the FIML-augmented original SA model reveals that the augmented RBSA-poly model reduces error in basic wall-attached flows by 50%while maintaining comparable accuracy in trained separated flows.These findings confirm the effectiveness of utilizing FIML in conjunction with the RBSA model,offering superior accuracy retention in cardinal flows.
基金partially supported by the National Natural Science Foundation of China(Nos.92271103,12202191)。
文摘The diffuse-interface immersed boundary method(IBM)possesses excellent capabilities for simulating flows around complex geometries and moving boundaries.In this method,the flow field is solved on a fixed Cartesian mesh,while the solid boundary is discretized into a series of Lagrangian points immersed in the flow field.The boundary condition is implemented by introducing a force term into the momentum equation,and the interaction between the immersed boundary and the fluid domain is achieved via an interpolation process.Over the past decades,the diffuse-interface IBM has gained popularity and spawned many variants,effectively handling a wide range of flow problems from isothermal to thermal flows,from laminar to turbulent flows,and from complex geometries to fluidstructure interaction scenarios.This paper first outlines the basic principles of the diffuse-interface IBM,then highlights recent advancements achieved by the authors’research group,and finally shows the method’s excellent numerical performance and wide applicability through several case studies involving complex moving boundary problems.
基金supported by the National Natural Science Foundation of China(Grant no.42176244)CAS Key Deployment Project of Centre for Ocean Mega-Research of Science(Grant no.COMS2020Q07)。
文摘In the Arctic Ocean,turbulent mixing drives vertical heat flux,thereby affecting the sea ice variability.Internal wave is regarded as one of the important energy sources of mixing in this region.The high latitude and sea ice cover make internal wave in the Arctic Ocean apparently differs from that in mid-and low-latitude oceans.However,the internal wave and its underlying mechanism are less understood due to the lack of observations.This paper briefly reviews the recent studies and unresolved questions on the internal wave in the Arctic Ocean,including wind-driven near-inertial wave,internal tide,and high-frequency internal wave.The aim is to provide new insights for in-depth research in the future,with a focus on the mechanisms responsible for the evolution of internal wave under the rapidly changing Arctic climate.
文摘Amid overlapping crises and uncertainties as the world enters an era of turbulence and transformation,the countries of the Global South are swiftly emerging from the periphery of world politics to the forefront of international cooperation,playing an increasingly vital role in advancing humanity’s collective progress.
文摘In this study,the flow characteristics around a group of three piers arranged in tandem were investigated both numerically and experimentally.The simulation utilised the volume of fluid(VOF)model in conjunction with the k–ɛmethod(i.e.,for flow turbulence representations),implemented through the ANSYS FLUENT software,to model the free-surface flow.The simulation results were validated against laboratory measurements obtained using an acoustic Doppler velocimeter.The comparative analysis revealed discrepancies between the simulated and measured maximum velocities within the investigated flow field.However,the numerical results demonstrated a distinct vortex-induced flow pattern following the first pier and throughout the vicinity of the entire pier group,which aligned reasonably well with experimental data.In the heavily narrowed spaces between the piers,simulated velocity profiles were overestimated in the free-surface region and underestimated in the areas near the bed to the mid-stream when compared to measurements.These discrepancies diminished away from the regions with intense vortices,indicating that the employed model was capable of simulating relatively less disturbed flow turbulence.Furthermore,velocity results from both simulations and measurements were compared based on velocity distributions at three different depth ratios(0.15,0.40,and 0.62)to assess vortex characteristic around the piers.This comparison revealed consistent results between experimental and simulated data.This research contributes to a deeper understanding of flow dynamics around complex interactive pier systems,which is critical for designing stable and sustainable hydraulic structures.Furthermore,the insights gained from this study provide valuable information for engineers aiming to develop effective strategies for controlling scour and minimizing destructive vortex effects,thereby guiding the design and maintenance of sustainable infrastructure.
基金supported by the National Natural Science Foundation of China(Grant No.42276001)2024 Qinhuangdao Social Science Development Research Project(Grant No.2024LX206)Hebei Agricultural University Research Project for Talented scholars(Grant No.YJ201835).
文摘Research has shown considerable variability in whitecap coverage(W)under low to moderate wind conditions.During an expedition to the Northwestern Pacific,oceanographic variables and photographic measurements were collected to investigate the influence of wave-induced stress on W within these wind ranges.The friction velocity was recalculated based on turbulent stress,and wind profiles were modified to account for wave-induced stress and swell presence on the sea surface.The study examined W’s relationship with multiple parameters,including friction velocity(u*),breaking wave Reynolds numbers,wavesea Reynolds numbers,and wave age.The analysis utilized both conventional u*and turbulent stress-based friction velocity(u*turb).When utilizing u*turb rather than u*,the estimation model’s fitting results revealed an increase in correlation coefficient(R2)from 0.51 to 0.62,and a decrease in root mean square error(RMSE)from 0.0652 to 0.0574.Additionally,when parameterizing W using the windsea Reynolds number,with u_(*turb) replacing u*and wind wave height substituting mixed wave height,the R^(2) increased from 0.38 to 0.53,and the RMSE decreased from 0.0737 to 0.0668.The results demonstrate that calculating u*using the turbulent stress-based method,along with wind wave height and peak wave speed of mixed waves,yields stronger correlation with W.This correlation improvement stems from the inhibition of wave breaking by swell and wave-induced stress.The integration of turbulent stress and wind wave field measurements enhances the understanding of relationships between W and various parameters.However,swell effects on wind profiles do not substantially affect W estimation using wind speed-related parameters.
