It is observed that the feather surface exhibits anisotropic resistances for the streamwise and spanwise flows.To obtain a qualitative understanding about the effect of this anisotropic resistance feature of surface o...It is observed that the feather surface exhibits anisotropic resistances for the streamwise and spanwise flows.To obtain a qualitative understanding about the effect of this anisotropic resistance feature of surface on the boundary-layer transitional flow over a flat plate,a simple phenomenological model for the anisotropic resistance is established in this paper.By means of the large eddy simulation(LES)with high-order accurate finite difference method,the numerical investigations are conducted.The numerical results show that with the spanwise resistance hindering the formation of vortexes,the transition from laminar flow to turbulent flow can be delayed,and turbulence is weakened when the flow becomes fully turbulent,which leads to significant drag reduction for the plate.On the contrary,the streamwise resistance renders the flow less stable,which leads to the earlier transition and enhances turbulence in the turbulent region,causing a drag increase for the plate.Thus,it is indicated that a surface with large resistance for spanwise flow and small resistance for streamwise flow can achieve significant drag reduction.The present results highlight the anisotropic resistance characteristic near the feather surface for drag reduction,and shed a light on the study of bird’s efficient flight.展开更多
The design of high-lift Low-Pressure Turbines(LPTs)causes the separation of the boundary layer on the suction side of the blade and leads to a strong secondary flow.This present study aims to minimize secondary losses...The design of high-lift Low-Pressure Turbines(LPTs)causes the separation of the boundary layer on the suction side of the blade and leads to a strong secondary flow.This present study aims to minimize secondary losses through endwall slot suction and incoming wakes in a front-loaded high-lift LPT cascade with Zweifel of 1.58 under low Reynolds number of 25000.Two slotted schemes for the boundary layer of the endwall were designed(Plan A and Plan B),and the effects of suction mass flow on secondary flow were studied.The underlying physics of the endwall boundary layer of the suction and secondary flow under unsteady wakes was discussed.The results show that slot suction at the endwall boundary layer can significantly suppress the secondary flow by removing low-momentum fluids.Plans A and B significantly reduced the secondary kinetic energy by 44.2%and 36.9%,respectively,compared with the baseline cascade at the suction mass flow ratios of 1%.With an increase in the mass flow ratio of suction,the secondary flow was gradually reduced in both Plans A and B.It is more beneficial to control the secondary flow to destroy the intersection of the pressure side and suction side of the horseshoe vortex before it develops into a passage vortex.Under unsteady wakes,the combined effects of incoming wakes and endwall boundary layer suction can further suppress the secondary flow at the suction mass flow ratios of 2%for Plan A,because the positive and negative vorticity inside upstream wakes accelerated the mixing of the main flow and secondary flow and thus increased the energy of secondary vortices.展开更多
Hydrate phase transition may pose risks in pipeline blockage and severe challenges for offshore natural gas hydrate pro-duction.The present work involves the development of a multiphase gas-liquid-solid vertical slug ...Hydrate phase transition may pose risks in pipeline blockage and severe challenges for offshore natural gas hydrate pro-duction.The present work involves the development of a multiphase gas-liquid-solid vertical slug flow hydrodynamic model consi-dering hydrate phase transition kinetics with heat and mass transfer behaviors.The varying gas physical properties due to pressure and temperature variations are also introduced to evaluate vertical slug flow characteristics.The proposed model is used to carry out a series of numerical simulations to examine the interactions between hydrate phase transition and vertical slug flow hydrodynamics.Furthermore,the hydrate volumetric fractions under different pressure and temperature conditions are predicted.The results reveal that hydrate formation and gas expansion cause the mixture superficial velocity,and the gas and liquid fractions,void fraction in liq-uid slug,and unit length tend to decrease.The increase in outlet pressure leads to an increased hydrate formation rate,which not only increases the hydrate volumetric fraction along the pipe but also causes the upward shift of the hydrate phase transition critical point.展开更多
The increasing performance demands of modern aero engines necessitate the integrated design of compressor transition ducts with upstream components to reduce the axial length of the engine.However,this design approach...The increasing performance demands of modern aero engines necessitate the integrated design of compressor transition ducts with upstream components to reduce the axial length of the engine.However,this design approach narrows the spacing between the stator and the strut,making traditional research on transition ducts only with struts unsuitable.The numerical results and experimental oil flow visualization results were utilized to reconstruct the three-dimensional flow structures in the stator passages under various operating conditions.Additionally,numerical methods were employed to analyze the mechanisms of the strut's effect on the upstream stator in an aggressive transition duct.The results show that the strut potential field increases the load on the upstream stator,leading to severe blade surface separation and corner separation/stall,and redistributes the inflow angle of the upstream stators circumferentially,resulting in significant differences in the flow structures within the stator passages on both sides.The separation flows within the stator passages mainly manifest in five types:pressure surface separation vortex,suction surface concentrated shedding vortex,suction surface separation vortex,suction surface-corner stall separation vortex,and suction surface separation vortex pair.Under different operating conditions,the separation flows within the stator passages are always composed of a part of these five types or a transitional state between two of them.展开更多
This study presents a two-dimensional,transient model to simulate the flow and thermal behavior of CO_(2) within a fracturing wellbore.The model accounts for high-velocity flow within the tubing and radial heat exchan...This study presents a two-dimensional,transient model to simulate the flow and thermal behavior of CO_(2) within a fracturing wellbore.The model accounts for high-velocity flow within the tubing and radial heat exchange between the wellbore and surrounding formation.It captures the temporal evolution of temperature,pressure,flow velocity,and fluid density,enabling detailed analysis of phase transitions along different tubing sections.The influence of key operational and geological parameters,including wellhead pressure,injection velocity,inlet temperature,and formation temperature gradient,on the wellbore’s thermal and pressure fields is systematically investigated.Results indicate that due to intense convective transport by the high-speed CO_(2) flow,the temperature and velocity within the tubing are primarily governed by the inlet temperature and injection velocity,with relatively minor influence from radial heat transfer with the formation.The pressure,flow velocity,and density of CO_(2) within the tubing are strongly dependent on wellhead conditions.Frictional losses and well depth contribute to pressure variations,particularly in the horizontal section of the wellbore,where a noticeable pressurization effect increases the fluid density.During injection,liquid CO_(2) initially undergoes a rapid transition to a supercritical state,with the depth at which this phase change occurs stabilizing as injection progresses.展开更多
To meet the challenge of drag reduction for next-generation supersonic transport aircraft,increasing attention has been focused on Natural Laminar Flow(NLF)technology.However,the highly swept wings and high-Reynolds-n...To meet the challenge of drag reduction for next-generation supersonic transport aircraft,increasing attention has been focused on Natural Laminar Flow(NLF)technology.However,the highly swept wings and high-Reynolds-number conditions of such aircraft dramatically amplify Crossflow(CF)instabilities inside boundary layers,making it difficult to maintain a large laminar flow region.To explore novel NLF designs on supersonic wings,this article investigates the mechanisms underlying the attenuation of Tollmien-Schlichting(TS)and CF instabilities by modifying pressure distributions.The evolution of TS and CF instabilities are evaluated under typical pressure distributions with different leading-edge flow acceleration region lengths,pressure coefficient slopes and pressure coefficient deviations.The results show that shortening the leading-edge flow acceleration region and using a flat pressure distribution are favorable for suppressing CF instabilities,and keeping a balance of disturbance growth between positive and negative wave angles is favorable for attenuating TS instabilities.Based on the uncovered mechanisms,a strategy of supersonic NLF design is proposed.Examination of the proposed strategy at a 60°sweep angle and Ma=2 presents potential to exceed the conventional NLF limit and achieve a transition Reynolds number of 17.6million,which can provide guidance for NLF design on supersonic highly swept wings.展开更多
This article describes an experimental study on friction and heat transfer performances of a transitional airflow in a rectangular channel with stagger-arrayed short pin fins. Friction factors, average Nusselt numbers...This article describes an experimental study on friction and heat transfer performances of a transitional airflow in a rectangular channel with stagger-arrayed short pin fins. Friction factors, average Nusselt numbers and overall thermal performances of the transitional flow are obtained. The experimental study has showed that the pin fins enhance the heat transfer performance sig- nificantly, however increasing the flow frictional resistance considerably. After comparing the experimental results with the p...展开更多
This paper describes a simplified transition model based on the recently developed correlation-based γ - Reot transition model. The transport equation of transition momentum thick- ness Reynolds number is eliminated ...This paper describes a simplified transition model based on the recently developed correlation-based γ - Reot transition model. The transport equation of transition momentum thick- ness Reynolds number is eliminated for simplicity, and new transition length function and critical Reynolds number correlation are proposed. The new model is implemented into an in-house com- putational fluid dynamics （CFD） code and validated for low and high-speed flow cases, including the zero pressure flat plate, airfoils, hypersonic flat plate and double wedge. Comparisons between the simulation results and experimental data show that the boundary-layer transition phenomena can be reasonably illustrated by the new model, which gives rise to significant improvements over the fully laminar and fully turbulent results. Moreover, the new model has comparable features of accuracy and applicability when compared with the original 3＇ - Reot model. In the meantime, the newly proposed model takes only one transport equation of intermittency factor and requires fewer correlations, which simplifies the original model greatly. Further studies, especially on separation- induced transition flows, are required for the improvement of the new model.展开更多
Water-rich clay to sand suspensions show a shear rate dependent flow behavior and knowledge of the appropriate rheological model is relevant for sedimentological, industrial and hydraulic studies. We present experimen...Water-rich clay to sand suspensions show a shear rate dependent flow behavior and knowledge of the appropriate rheological model is relevant for sedimentological, industrial and hydraulic studies. We present experimental rheological measurements of water-rich(40 to 60 wt%) clay to silt(population A) and silt to sand(population B) suspensions mixed in different proportions. The data evidence a shear rate dependent shear thinning-shear thickening transition. At lower shear rates, the suspensions organize in chains of particles, whereas at higher shear rates, these chains disrupt so increasing the viscosity. The viscosity, consistency and yield stress decrease as the A+B fraction decreases as the content of B particles increases. This behavior reflects the competing effects of the lubrication and frictional processes as a function of particle size and water content. Transitional flows form by the incorporation of small amounts of the finer fraction while ‘oceanic floods’ form at the estuary of rivers and the submarine debris-flows increase their velocity by incorporating water. The critical Reynolds number of the studied suspensions is ～2000±100 suggesting that the grainsize plays a major role in the laminar to turbulent transition. Our results have implications for the modeling of sediment flows and the hazard related to floods.展开更多
The study presents an experimental exploration into the mode transition of an overunder TBCC(Turbine-Based Combined Cycle)inlet,with a specific emphasis on the flow characteristics at off-design transition Mach number...The study presents an experimental exploration into the mode transition of an overunder TBCC(Turbine-Based Combined Cycle)inlet,with a specific emphasis on the flow characteristics at off-design transition Mach number.A systematic investigation was undertaken into the mode transition characteristics in both unthrottled and throttled conditions within a highspeed duct,employing high speed Schlieren and dynamic pressure acquisition systems.The results show that the high-speed duct faced flow oscillations primarily dictated by the separation bubble near the duct entrance during the downward rotation of splitter,leading to the duct’s unstart under the unthrottled condition.During the splitter’s reverse rotation,a notable hysteresis of unstart/restart of the high-speed duct was observed.Conversely,hysteresis vanishes when the initial flowfield nears the critical state owing to downstream throttling.Moreover,the oscillatory diversity,a distinctive characteristic of the high-speed duct,was firstly observed during the mode transition induced by throttling.The flow evolution was divided into four stages:an initial instability stage characterized by low-frequency oscillations below 255 Hz induced by shock train self-excitation oscillation and high-frequency oscillations around 1367 Hz caused by the movement of separation bubble.This stage is succeeded by the“big buzz”phase,comprised of pressure accumulation/release within the overflow-free duct and shock motion outside the duct to retain dynamic flow balance.The dominant frequency escalated with the increase of the internal contraction ratio in the range of 280 Hz to 400 Hz.This was followed by a high-frequency oscillation stage around 453 Hz dominated by a large internal contraction ratio with low pulsating energy,accompanied by a continuous supersonic overflow.Lastly,as the splitter gradually intersected the boundary layer of the first-stage compression surface,the capture area and the turbulence intensity of the incoming flow underwent a sudden shift,leading to a more diverse flow oscillation within the duct,manifested as various forms of mixed buzz.展开更多
Wind-sand flow generates erosion and deposition around obstacles such as bridges and roadbeds, resulting in sand damage and endangering railway systems in sandy regions. Previous studies have mainly focused on the flo...Wind-sand flow generates erosion and deposition around obstacles such as bridges and roadbeds, resulting in sand damage and endangering railway systems in sandy regions. Previous studies have mainly focused on the flow field around roadbeds, overlooking detailed examinations of sand particle erosion and deposition patterns near bridges and roadbeds. This study employs numerical simulations to analyze the influence of varying heights and wind speeds on sand deposition and erosion characteristics at different locations: the bridge-road transition section(side piers), middle piers, and roadbeds. The results show that the side piers, experience greater accumulation than the middle piers. Similarly, the leeward side of the roadbed witnesses more deposition compared to the windward side. Another finding reveals a reduced sand deposition length as the vertical profile, in alignment with the wind direction, moves further from the bridge abutments at the same clearance height. As wind speeds rise, there’s a decline in sand deposition and a marked increase in erosion around the side piers, middle piers and roadbeds. In conclusion, a bridge clearance that’s too low can cause intense sand damage near the side piers, while an extremely high roadbed may lead to extensive surface sand deposition. Hence, railway bridges in areas prone to sandy winds should strike a balance in clearance height. This research provides valuable guidelines for determining the most suitable bridge and roadbed heights in regions affected by wind and sand.展开更多
Particle image velocimetry and numerical simulation results of juncture flows were analyzed to parametrically investigate topology transition. The vortex system evolutions from non-vortex to multi-vortex with variatio...Particle image velocimetry and numerical simulation results of juncture flows were analyzed to parametrically investigate topology transition. The vortex system evolutions from non-vortex to multi-vortex with variations in obstacle bluntness, obstacle width, flow velocity, and boundary layer thickness are discussed from the perspective of velocity characteristic lines. The velocity characteristic lines of u = 0, υ = 0, and ∇^(2) υ = 0 are adopted to describe the vortex system evolution. The motions of the characteristic lines with juncture flow parameters are described in detail, and the corresponding reflections of the vortex system patterns are illustrated. A panoramic picture of the development of velocity characteristic lines corresponding to the HSV topology transition from a non-vortex to a multi-vortex system with variations in the juncture flow parameter is established. Two methods for determining the attachment/separation pattern of the most upstream singularity are proposed. One method is based on the number of intersections of the u = 0 and υ = 0 velocity characteristic curve lines, and the other is based on the relative positions of the most upstream feet of the u = 0 and υ = 0 loop curves with both feet attached to the wall.展开更多
A new algebraic transition model is proposed based on a Structural Ensemble Dynamics(SED)theory of wall turbulence,for accurately predicting the hypersonic flow heat transfer on cone.The model defines the eddy viscosi...A new algebraic transition model is proposed based on a Structural Ensemble Dynamics(SED)theory of wall turbulence,for accurately predicting the hypersonic flow heat transfer on cone.The model defines the eddy viscosity in terms of a two-dimensional multi-regime distribution of a Stress Length(SL)function,and hence is named as SED-SL.This paper presents clear evidence of precise predictions of transition onset location and peak heat flux of a wide range of hypersonic Transitional Boundary Layers(TrBL)around straight cone at zero incidence,to an unprecedented accuracy as validated by over 70 measurements for varying five crucial influential factors(Mach number,temperature ratio,cone half angle,nose Reynolds number and noise level).The results demonstrate the universality of the postulated multi-regime similarity structure,in characterizing not only the spatial non-uniform distribution of the eddy viscosity in hypersonic TrBL on cone,but also the dependence of the transition onset location on the five influential factors.The latter yields a novel correlation formula for transition center Reynolds number which takes similar functional form as the SL function within the symmetry approach.It is concluded that the SED-SL model simulates TrBL around cone with uniformly high accuracy,and then points out to an optimistic alternative way to construct hypersonic transition model.展开更多
Mach reflection in steady supersonic flow is an important phenomenon having received extensive studies,among which simplified theoretical models to predict the size of Mach stem and other flow structure are of particu...Mach reflection in steady supersonic flow is an important phenomenon having received extensive studies,among which simplified theoretical models to predict the size of Mach stem and other flow structure are of particular interest.Past efforts for such models were based on inviscid assumption while in real cases the flow is viscous.Here in this paper we consider the influence of wedge boundary layer on the Mach stem height.This is done by including a simplified boundary layer model into a recently published inviscid model.In this viscous model,the wedge angle and the trailing edge height,which control the Mach stem height,are replaced by their equivalent ones accounting for the displacement effect of the wedge boundary layer,with the boundary layer assumed to be laminar or fully turbulent.This viscous model is shown to compare well with numerical results by computational fluid dynamics and gives a Mach stem height as function of the Reynolds number and Mach number.It is shown that due to the viscous effect,the Mach stem height is increased,through increasing the effective wedge angle.展开更多
This paper aims to present complete series solution of non-similarity boundary-layer flow of an incompressible viscous fluid over a porous wedge. The corresponding nonlinear partial differential equations are solved a...This paper aims to present complete series solution of non-similarity boundary-layer flow of an incompressible viscous fluid over a porous wedge. The corresponding nonlinear partial differential equations are solved analytically by means of the homotopy analysis method (HAM). An auxiliary parameter is introduced to ensure the convergence of solution series. As a result, series solutions valid for all physical parameters in the whole domain are given. Then, the effects of physical parameters γ and Prandtl number Pr on the local Nusselt number and momentum thickness are investigated. To the best of our knowledge, it is the first time that the series solutions of this kind of non-similarity boundary-layer flows are reported.展开更多
Hilly terrain pipeline is a common form of pipeline in oil and gas storage and transportation industry.Due to the hilly terrain influence, the liquid at the elbow of the gathering pipeline is easy to flow back and acc...Hilly terrain pipeline is a common form of pipeline in oil and gas storage and transportation industry.Due to the hilly terrain influence, the liquid at the elbow of the gathering pipeline is easy to flow back and accumulate to form slug flow, so it is necessary to remove the accumulated liquid by gas purging. In this paper, experiment is carried out in hilly terrain pipelines. Three flow patterns of stratified flow, slug flow and stratified entrained flow are observed. The process of gas purging accumulated liquid is divided into four stages, namely liquid accumulation, liquid rising, continuous outflow and tail outflow. At the same time, the flow pattern maps of each stage are drawn. The pressure drop signal is analyzed in time domain and frequency domain, and the contour map of pressure drop distribution is drawn. It is found that the ratio of range to average value can well distinguish the occurrence range of each flow pattern.Based on visualization, the transition process of slug flow to stratified flow and stratified entrained flow is studied, and the transition boundary prediction model is established. An image processing method is proposed to convert the image signal into a similarity curve, and PSD analysis is performed to calculate the slug frequency. The normal distribution is used to fit the slug frequency, and the predicted correlation is in good agreement with the experimental data.展开更多
The terminal velocity has been widely used in extensive fields, but the complexity of drag coefficient expression leads to the calculation of terminal velocity in transitional flow （1 〈 Re ≤ 1000） with much more d...The terminal velocity has been widely used in extensive fields, but the complexity of drag coefficient expression leads to the calculation of terminal velocity in transitional flow （1 〈 Re ≤ 1000） with much more difficulty than those in laminar flow （Re ≤ 1） and turbulent flow （Re ≥ 1000）. This paper summarized and compared 24 drag coefficient correlations, and developed an expression for calculating the terminal velocity in transitional flow, and also analyzed the effects of particle density and size, fluid density and viscosity on terminal velocity. The results show that 19 of 24 previously published correlations for drag coefficient have good prediction performance and can be used for calculating the terminal velocity in the entire transitional flow with higher accuracy. Adapting two dimensionless parameters （w＊, d＊）, a proposed explicit correlation, w＊=-25.68654 × exp （-d＊/77.02069）＋ 24.89826, is attained in transitional flow with good performance, which is helpful in calculating the terminal velocity.展开更多
The experiments of primary and secondary instabilities with controlled excitation are carried out on a swept flat plate to study the process leading to the final breakdown of laminar flow. Two types of high frequency ...The experiments of primary and secondary instabilities with controlled excitation are carried out on a swept flat plate to study the process leading to the final breakdown of laminar flow. Two types of high frequency secondary instabilities are identified. The most amplified mode is centered about the inflection point of the crosswise profile of the boundary layer and is interpreted as inflectional instability, the other occurs in the one third of the boundary layer from the wall. The high frequency disturbances are highly amplified but they also saturate similarly to the primary and nonlinearly generated disturbances. Their main effect in the final breakdown seems interact with the disturbances is developed and thus widens the frequency spectrum to turbulent state.展开更多
Local fluid flow（LFF） at the mesoscopic scale is the main dissipation mechanism of seismic waves in heterogeneous porous media within the seismic frequency band.LFF is easily influenced by the structure and boundary...Local fluid flow（LFF） at the mesoscopic scale is the main dissipation mechanism of seismic waves in heterogeneous porous media within the seismic frequency band.LFF is easily influenced by the structure and boundary conditions of the porous media,which leads to different behaviors of the peak frequency of attenuation.The associated transition frequency can provide detailed information about the trend of LFF;therefore,research on the transition frequency of LFF and its relationship with the peak frequency of the corresponding attenuation（i.e.,inverse of quality factor） facilitates the detailed understanding of the effect of inner structures and boundary conditions in porous media.In this study,we firstly obtain the transition frequency of fluid flux based on Biot＇s theory of poroelasticity and the fast Fourier transform algorithm in a sample containing one repeating unit cell（RUC）.We then analyze changes of these two frequencies in porous media with different porous properties.Finally,we extend our analysis to the influence of the undrained boundary condition on the transition frequency and peak frequency in porous media with multiple RUCs.This setup can facilitate the understanding of the effect from the undrained boundary condition.Results demonstrate that these two frequencies have the same trend at low water saturation,but amplitude variations differ between the frequencies as the amount of saturation increases.However,for cases of high water saturation,both the trend and the amplitude variation of these two frequencies fit well with each other.展开更多
It is very important to understand the annular multiphase flow behavior and the effect of hydrate phase transition during deep water drilling. The basic hydrodynamic models, including mass, momentum, and energy conser...It is very important to understand the annular multiphase flow behavior and the effect of hydrate phase transition during deep water drilling. The basic hydrodynamic models, including mass, momentum, and energy conservation equations, were established for annular flow with gas hydrate phase transition during gas kick. The behavior of annular multiphase flow with hydrate phase transition was investigated by analyzing the hydrate-forming region, the gas fraction in the fluid flowing in the annulus, pit gain, bottom hole pressure, and shut-in casing pressure. The simulation shows that it is possible to move the hydrate-forming region away from sea floor by increasing the circulation rate. The decrease in gas volume fraction in the annulus due to hydrate formation reduces pit gain, which can delay the detection of well kick and increase the risk of hydrate plugging in lines. Caution is needed when a well is monitored for gas kick at a relatively low gas production rate, because the possibility of hydrate presence is much greater than that at a relatively high production rate. The shut-in casing pressure cannot reflect the gas kick due to hydrate formation, which increases with time.展开更多
基金Project supported by the National Natural Science Foundation of China (No.12072281)the Foundation of National Key Laboratory of Science and Technology on Aerodynamic Design and Research (No.614220121030224)the Fundamental Research Funds for the Central Universities of China (No.D5000220178)。
文摘It is observed that the feather surface exhibits anisotropic resistances for the streamwise and spanwise flows.To obtain a qualitative understanding about the effect of this anisotropic resistance feature of surface on the boundary-layer transitional flow over a flat plate,a simple phenomenological model for the anisotropic resistance is established in this paper.By means of the large eddy simulation(LES)with high-order accurate finite difference method,the numerical investigations are conducted.The numerical results show that with the spanwise resistance hindering the formation of vortexes,the transition from laminar flow to turbulent flow can be delayed,and turbulence is weakened when the flow becomes fully turbulent,which leads to significant drag reduction for the plate.On the contrary,the streamwise resistance renders the flow less stable,which leads to the earlier transition and enhances turbulence in the turbulent region,causing a drag increase for the plate.Thus,it is indicated that a surface with large resistance for spanwise flow and small resistance for streamwise flow can achieve significant drag reduction.The present results highlight the anisotropic resistance characteristic near the feather surface for drag reduction,and shed a light on the study of bird’s efficient flight.
基金support for this work was provided by the National Natural Science Foundation of China(No.52206060)the National Science and Technology Major Project of China(Nos.J2019-Ⅱ-0021-0042 and J2019-Ⅱ-0002-0022).
文摘The design of high-lift Low-Pressure Turbines(LPTs)causes the separation of the boundary layer on the suction side of the blade and leads to a strong secondary flow.This present study aims to minimize secondary losses through endwall slot suction and incoming wakes in a front-loaded high-lift LPT cascade with Zweifel of 1.58 under low Reynolds number of 25000.Two slotted schemes for the boundary layer of the endwall were designed(Plan A and Plan B),and the effects of suction mass flow on secondary flow were studied.The underlying physics of the endwall boundary layer of the suction and secondary flow under unsteady wakes was discussed.The results show that slot suction at the endwall boundary layer can significantly suppress the secondary flow by removing low-momentum fluids.Plans A and B significantly reduced the secondary kinetic energy by 44.2%and 36.9%,respectively,compared with the baseline cascade at the suction mass flow ratios of 1%.With an increase in the mass flow ratio of suction,the secondary flow was gradually reduced in both Plans A and B.It is more beneficial to control the secondary flow to destroy the intersection of the pressure side and suction side of the horseshoe vortex before it develops into a passage vortex.Under unsteady wakes,the combined effects of incoming wakes and endwall boundary layer suction can further suppress the secondary flow at the suction mass flow ratios of 2%for Plan A,because the positive and negative vorticity inside upstream wakes accelerated the mixing of the main flow and secondary flow and thus increased the energy of secondary vortices.
基金supported by the National Natural Science Foundation of China(No.52301355)the Natu-ral Science Foundation of Qingdao Municipality(No.23-2-1-108-zyyd-jch)the China University of Petroleum(East China)Independent Innovation Research Project(Science and Engineering)-Leading Talent Cultivation Fund(No.24CX07001A).
文摘Hydrate phase transition may pose risks in pipeline blockage and severe challenges for offshore natural gas hydrate pro-duction.The present work involves the development of a multiphase gas-liquid-solid vertical slug flow hydrodynamic model consi-dering hydrate phase transition kinetics with heat and mass transfer behaviors.The varying gas physical properties due to pressure and temperature variations are also introduced to evaluate vertical slug flow characteristics.The proposed model is used to carry out a series of numerical simulations to examine the interactions between hydrate phase transition and vertical slug flow hydrodynamics.Furthermore,the hydrate volumetric fractions under different pressure and temperature conditions are predicted.The results reveal that hydrate formation and gas expansion cause the mixture superficial velocity,and the gas and liquid fractions,void fraction in liq-uid slug,and unit length tend to decrease.The increase in outlet pressure leads to an increased hydrate formation rate,which not only increases the hydrate volumetric fraction along the pipe but also causes the upward shift of the hydrate phase transition critical point.
基金supported by the National Natural Science Foundation of China(No.52276025)the Science Center for Gas Turbine Project of China(Nos.P2022-A-Ⅱ-001-001,P2022-A-Ⅱ-002-001 and P2022-B-Ⅱ-002-001)。
文摘The increasing performance demands of modern aero engines necessitate the integrated design of compressor transition ducts with upstream components to reduce the axial length of the engine.However,this design approach narrows the spacing between the stator and the strut,making traditional research on transition ducts only with struts unsuitable.The numerical results and experimental oil flow visualization results were utilized to reconstruct the three-dimensional flow structures in the stator passages under various operating conditions.Additionally,numerical methods were employed to analyze the mechanisms of the strut's effect on the upstream stator in an aggressive transition duct.The results show that the strut potential field increases the load on the upstream stator,leading to severe blade surface separation and corner separation/stall,and redistributes the inflow angle of the upstream stators circumferentially,resulting in significant differences in the flow structures within the stator passages on both sides.The separation flows within the stator passages mainly manifest in five types:pressure surface separation vortex,suction surface concentrated shedding vortex,suction surface separation vortex,suction surface-corner stall separation vortex,and suction surface separation vortex pair.Under different operating conditions,the separation flows within the stator passages are always composed of a part of these five types or a transitional state between two of them.
基金funded by National Natural Science Foundation of China(Mechanisms of proppant-carrying transport by magnetic cross-linked microparticle grids and their degradation patterns in CO_(2) fractured cracks).
文摘This study presents a two-dimensional,transient model to simulate the flow and thermal behavior of CO_(2) within a fracturing wellbore.The model accounts for high-velocity flow within the tubing and radial heat exchange between the wellbore and surrounding formation.It captures the temporal evolution of temperature,pressure,flow velocity,and fluid density,enabling detailed analysis of phase transitions along different tubing sections.The influence of key operational and geological parameters,including wellhead pressure,injection velocity,inlet temperature,and formation temperature gradient,on the wellbore’s thermal and pressure fields is systematically investigated.Results indicate that due to intense convective transport by the high-speed CO_(2) flow,the temperature and velocity within the tubing are primarily governed by the inlet temperature and injection velocity,with relatively minor influence from radial heat transfer with the formation.The pressure,flow velocity,and density of CO_(2) within the tubing are strongly dependent on wellhead conditions.Frictional losses and well depth contribute to pressure variations,particularly in the horizontal section of the wellbore,where a noticeable pressurization effect increases the fluid density.During injection,liquid CO_(2) initially undergoes a rapid transition to a supercritical state,with the depth at which this phase change occurs stabilizing as injection progresses.
基金supported by the National Natural Science Foundation of China(No.12072285)the National Key Research and Development Program of China(No.2023YFB3002800)the Youth Innovation Team of Shaanxi Universities,China。
文摘To meet the challenge of drag reduction for next-generation supersonic transport aircraft,increasing attention has been focused on Natural Laminar Flow(NLF)technology.However,the highly swept wings and high-Reynolds-number conditions of such aircraft dramatically amplify Crossflow(CF)instabilities inside boundary layers,making it difficult to maintain a large laminar flow region.To explore novel NLF designs on supersonic wings,this article investigates the mechanisms underlying the attenuation of Tollmien-Schlichting(TS)and CF instabilities by modifying pressure distributions.The evolution of TS and CF instabilities are evaluated under typical pressure distributions with different leading-edge flow acceleration region lengths,pressure coefficient slopes and pressure coefficient deviations.The results show that shortening the leading-edge flow acceleration region and using a flat pressure distribution are favorable for suppressing CF instabilities,and keeping a balance of disturbance growth between positive and negative wave angles is favorable for attenuating TS instabilities.Based on the uncovered mechanisms,a strategy of supersonic NLF design is proposed.Examination of the proposed strategy at a 60°sweep angle and Ma=2 presents potential to exceed the conventional NLF limit and achieve a transition Reynolds number of 17.6million,which can provide guidance for NLF design on supersonic highly swept wings.
基金National Natural Science Foundation of China (50806045)
文摘This article describes an experimental study on friction and heat transfer performances of a transitional airflow in a rectangular channel with stagger-arrayed short pin fins. Friction factors, average Nusselt numbers and overall thermal performances of the transitional flow are obtained. The experimental study has showed that the pin fins enhance the heat transfer performance sig- nificantly, however increasing the flow frictional resistance considerably. After comparing the experimental results with the p...
基金supported by the State Key Development Program for Basic Research of China(No.2014CB340201)
文摘This paper describes a simplified transition model based on the recently developed correlation-based γ - Reot transition model. The transport equation of transition momentum thick- ness Reynolds number is eliminated for simplicity, and new transition length function and critical Reynolds number correlation are proposed. The new model is implemented into an in-house com- putational fluid dynamics （CFD） code and validated for low and high-speed flow cases, including the zero pressure flat plate, airfoils, hypersonic flat plate and double wedge. Comparisons between the simulation results and experimental data show that the boundary-layer transition phenomena can be reasonably illustrated by the new model, which gives rise to significant improvements over the fully laminar and fully turbulent results. Moreover, the new model has comparable features of accuracy and applicability when compared with the original 3＇ - Reot model. In the meantime, the newly proposed model takes only one transport equation of intermittency factor and requires fewer correlations, which simplifies the original model greatly. Further studies, especially on separation- induced transition flows, are required for the improvement of the new model.
文摘Water-rich clay to sand suspensions show a shear rate dependent flow behavior and knowledge of the appropriate rheological model is relevant for sedimentological, industrial and hydraulic studies. We present experimental rheological measurements of water-rich(40 to 60 wt%) clay to silt(population A) and silt to sand(population B) suspensions mixed in different proportions. The data evidence a shear rate dependent shear thinning-shear thickening transition. At lower shear rates, the suspensions organize in chains of particles, whereas at higher shear rates, these chains disrupt so increasing the viscosity. The viscosity, consistency and yield stress decrease as the A+B fraction decreases as the content of B particles increases. This behavior reflects the competing effects of the lubrication and frictional processes as a function of particle size and water content. Transitional flows form by the incorporation of small amounts of the finer fraction while ‘oceanic floods’ form at the estuary of rivers and the submarine debris-flows increase their velocity by incorporating water. The critical Reynolds number of the studied suspensions is ～2000±100 suggesting that the grainsize plays a major role in the laminar to turbulent transition. Our results have implications for the modeling of sediment flows and the hazard related to floods.
基金funded by the National Natural Science Foundation of China(Nos.12025202,U20A2070 and 12172175)the National Science and Technology Major Project,China(No.J2019-Ⅱ-0014-0035)+2 种基金the Postdoctoral Fellowship Program of CPSF,China(No.GZB20230970)the Science Center for Gas Turbine Project,China(Nos.P2022-C-Ⅱ-002-001 and P2022-A-Ⅱ-002-001)the Young Scientific and Technological Talents Project of Jiangsu Association for Science and Technology,China(No.TJ-2021-052).
文摘The study presents an experimental exploration into the mode transition of an overunder TBCC(Turbine-Based Combined Cycle)inlet,with a specific emphasis on the flow characteristics at off-design transition Mach number.A systematic investigation was undertaken into the mode transition characteristics in both unthrottled and throttled conditions within a highspeed duct,employing high speed Schlieren and dynamic pressure acquisition systems.The results show that the high-speed duct faced flow oscillations primarily dictated by the separation bubble near the duct entrance during the downward rotation of splitter,leading to the duct’s unstart under the unthrottled condition.During the splitter’s reverse rotation,a notable hysteresis of unstart/restart of the high-speed duct was observed.Conversely,hysteresis vanishes when the initial flowfield nears the critical state owing to downstream throttling.Moreover,the oscillatory diversity,a distinctive characteristic of the high-speed duct,was firstly observed during the mode transition induced by throttling.The flow evolution was divided into four stages:an initial instability stage characterized by low-frequency oscillations below 255 Hz induced by shock train self-excitation oscillation and high-frequency oscillations around 1367 Hz caused by the movement of separation bubble.This stage is succeeded by the“big buzz”phase,comprised of pressure accumulation/release within the overflow-free duct and shock motion outside the duct to retain dynamic flow balance.The dominant frequency escalated with the increase of the internal contraction ratio in the range of 280 Hz to 400 Hz.This was followed by a high-frequency oscillation stage around 453 Hz dominated by a large internal contraction ratio with low pulsating energy,accompanied by a continuous supersonic overflow.Lastly,as the splitter gradually intersected the boundary layer of the first-stage compression surface,the capture area and the turbulence intensity of the incoming flow underwent a sudden shift,leading to a more diverse flow oscillation within the duct,manifested as various forms of mixed buzz.
基金financially supported by the fellowship of the China Postdoctoral Science Foundation (2021M703466)the Natural Science Foundation of Gansu Province, China (20JR10RA231)the Natural Science Foundation of Gansu Province, China (22JR5RA050)。
文摘Wind-sand flow generates erosion and deposition around obstacles such as bridges and roadbeds, resulting in sand damage and endangering railway systems in sandy regions. Previous studies have mainly focused on the flow field around roadbeds, overlooking detailed examinations of sand particle erosion and deposition patterns near bridges and roadbeds. This study employs numerical simulations to analyze the influence of varying heights and wind speeds on sand deposition and erosion characteristics at different locations: the bridge-road transition section(side piers), middle piers, and roadbeds. The results show that the side piers, experience greater accumulation than the middle piers. Similarly, the leeward side of the roadbed witnesses more deposition compared to the windward side. Another finding reveals a reduced sand deposition length as the vertical profile, in alignment with the wind direction, moves further from the bridge abutments at the same clearance height. As wind speeds rise, there’s a decline in sand deposition and a marked increase in erosion around the side piers, middle piers and roadbeds. In conclusion, a bridge clearance that’s too low can cause intense sand damage near the side piers, while an extremely high roadbed may lead to extensive surface sand deposition. Hence, railway bridges in areas prone to sandy winds should strike a balance in clearance height. This research provides valuable guidelines for determining the most suitable bridge and roadbed heights in regions affected by wind and sand.
基金supported by the State Key Laboratory of Mechanical Behavior and System Safety of Traffic Engineering Structures(Grant No.ZZ2021-04)the Science and Technology Project of the Hebei Education Department(Grant No.QN2021230)the Natural Science Foundation of Hebei Province(Grant No.E2019210026).
文摘Particle image velocimetry and numerical simulation results of juncture flows were analyzed to parametrically investigate topology transition. The vortex system evolutions from non-vortex to multi-vortex with variations in obstacle bluntness, obstacle width, flow velocity, and boundary layer thickness are discussed from the perspective of velocity characteristic lines. The velocity characteristic lines of u = 0, υ = 0, and ∇^(2) υ = 0 are adopted to describe the vortex system evolution. The motions of the characteristic lines with juncture flow parameters are described in detail, and the corresponding reflections of the vortex system patterns are illustrated. A panoramic picture of the development of velocity characteristic lines corresponding to the HSV topology transition from a non-vortex to a multi-vortex system with variations in the juncture flow parameter is established. Two methods for determining the attachment/separation pattern of the most upstream singularity are proposed. One method is based on the number of intersections of the u = 0 and υ = 0 velocity characteristic curve lines, and the other is based on the relative positions of the most upstream feet of the u = 0 and υ = 0 loop curves with both feet attached to the wall.
基金supported by the National Numerical WindTunnel Project,China(No.NNW2019ZT1-A03)the National Natural Science Foundation of China(Nos.91952201,11372008 and 11452002).
文摘A new algebraic transition model is proposed based on a Structural Ensemble Dynamics(SED)theory of wall turbulence,for accurately predicting the hypersonic flow heat transfer on cone.The model defines the eddy viscosity in terms of a two-dimensional multi-regime distribution of a Stress Length(SL)function,and hence is named as SED-SL.This paper presents clear evidence of precise predictions of transition onset location and peak heat flux of a wide range of hypersonic Transitional Boundary Layers(TrBL)around straight cone at zero incidence,to an unprecedented accuracy as validated by over 70 measurements for varying five crucial influential factors(Mach number,temperature ratio,cone half angle,nose Reynolds number and noise level).The results demonstrate the universality of the postulated multi-regime similarity structure,in characterizing not only the spatial non-uniform distribution of the eddy viscosity in hypersonic TrBL on cone,but also the dependence of the transition onset location on the five influential factors.The latter yields a novel correlation formula for transition center Reynolds number which takes similar functional form as the SL function within the symmetry approach.It is concluded that the SED-SL model simulates TrBL around cone with uniformly high accuracy,and then points out to an optimistic alternative way to construct hypersonic transition model.
基金supported by the Natural National Science Foundation of China(No.11802157).
文摘Mach reflection in steady supersonic flow is an important phenomenon having received extensive studies,among which simplified theoretical models to predict the size of Mach stem and other flow structure are of particular interest.Past efforts for such models were based on inviscid assumption while in real cases the flow is viscous.Here in this paper we consider the influence of wedge boundary layer on the Mach stem height.This is done by including a simplified boundary layer model into a recently published inviscid model.In this viscous model,the wedge angle and the trailing edge height,which control the Mach stem height,are replaced by their equivalent ones accounting for the displacement effect of the wedge boundary layer,with the boundary layer assumed to be laminar or fully turbulent.This viscous model is shown to compare well with numerical results by computational fluid dynamics and gives a Mach stem height as function of the Reynolds number and Mach number.It is shown that due to the viscous effect,the Mach stem height is increased,through increasing the effective wedge angle.
文摘This paper aims to present complete series solution of non-similarity boundary-layer flow of an incompressible viscous fluid over a porous wedge. The corresponding nonlinear partial differential equations are solved analytically by means of the homotopy analysis method (HAM). An auxiliary parameter is introduced to ensure the convergence of solution series. As a result, series solutions valid for all physical parameters in the whole domain are given. Then, the effects of physical parameters γ and Prandtl number Pr on the local Nusselt number and momentum thickness are investigated. To the best of our knowledge, it is the first time that the series solutions of this kind of non-similarity boundary-layer flows are reported.
基金supported by the Basic Science Center Program for Ordered Energy Conversion of the National Natural Science Foundation of China(No.52488201)the National Natural Science Foundation of China(No.52422606).
文摘Hilly terrain pipeline is a common form of pipeline in oil and gas storage and transportation industry.Due to the hilly terrain influence, the liquid at the elbow of the gathering pipeline is easy to flow back and accumulate to form slug flow, so it is necessary to remove the accumulated liquid by gas purging. In this paper, experiment is carried out in hilly terrain pipelines. Three flow patterns of stratified flow, slug flow and stratified entrained flow are observed. The process of gas purging accumulated liquid is divided into four stages, namely liquid accumulation, liquid rising, continuous outflow and tail outflow. At the same time, the flow pattern maps of each stage are drawn. The pressure drop signal is analyzed in time domain and frequency domain, and the contour map of pressure drop distribution is drawn. It is found that the ratio of range to average value can well distinguish the occurrence range of each flow pattern.Based on visualization, the transition process of slug flow to stratified flow and stratified entrained flow is studied, and the transition boundary prediction model is established. An image processing method is proposed to convert the image signal into a similarity curve, and PSD analysis is performed to calculate the slug frequency. The normal distribution is used to fit the slug frequency, and the predicted correlation is in good agreement with the experimental data.
文摘The terminal velocity has been widely used in extensive fields, but the complexity of drag coefficient expression leads to the calculation of terminal velocity in transitional flow （1 〈 Re ≤ 1000） with much more difficulty than those in laminar flow （Re ≤ 1） and turbulent flow （Re ≥ 1000）. This paper summarized and compared 24 drag coefficient correlations, and developed an expression for calculating the terminal velocity in transitional flow, and also analyzed the effects of particle density and size, fluid density and viscosity on terminal velocity. The results show that 19 of 24 previously published correlations for drag coefficient have good prediction performance and can be used for calculating the terminal velocity in the entire transitional flow with higher accuracy. Adapting two dimensionless parameters （w＊, d＊）, a proposed explicit correlation, w＊=-25.68654 × exp （-d＊/77.02069）＋ 24.89826, is attained in transitional flow with good performance, which is helpful in calculating the terminal velocity.
文摘The experiments of primary and secondary instabilities with controlled excitation are carried out on a swept flat plate to study the process leading to the final breakdown of laminar flow. Two types of high frequency secondary instabilities are identified. The most amplified mode is centered about the inflection point of the crosswise profile of the boundary layer and is interpreted as inflectional instability, the other occurs in the one third of the boundary layer from the wall. The high frequency disturbances are highly amplified but they also saturate similarly to the primary and nonlinearly generated disturbances. Their main effect in the final breakdown seems interact with the disturbances is developed and thus widens the frequency spectrum to turbulent state.
基金supported by National Natural Science Foundation of China(Grant No.41374116)the Fundamental Research Funds for Central Universities(Grant No.2014B39014)
文摘Local fluid flow（LFF） at the mesoscopic scale is the main dissipation mechanism of seismic waves in heterogeneous porous media within the seismic frequency band.LFF is easily influenced by the structure and boundary conditions of the porous media,which leads to different behaviors of the peak frequency of attenuation.The associated transition frequency can provide detailed information about the trend of LFF;therefore,research on the transition frequency of LFF and its relationship with the peak frequency of the corresponding attenuation（i.e.,inverse of quality factor） facilitates the detailed understanding of the effect of inner structures and boundary conditions in porous media.In this study,we firstly obtain the transition frequency of fluid flux based on Biot＇s theory of poroelasticity and the fast Fourier transform algorithm in a sample containing one repeating unit cell（RUC）.We then analyze changes of these two frequencies in porous media with different porous properties.Finally,we extend our analysis to the influence of the undrained boundary condition on the transition frequency and peak frequency in porous media with multiple RUCs.This setup can facilitate the understanding of the effect from the undrained boundary condition.Results demonstrate that these two frequencies have the same trend at low water saturation,but amplitude variations differ between the frequencies as the amount of saturation increases.However,for cases of high water saturation,both the trend and the amplitude variation of these two frequencies fit well with each other.
基金supported by the China National 863 Program (Grant No.2006AA09A106)the Doctoral Program of Higher Education of China (Grant No.20060425502)+1 种基金the National Natural Science Foundation of China (Grant No.50874116)Shandong Province Natural Science Foundation(Grant No.Z2007A01)
文摘It is very important to understand the annular multiphase flow behavior and the effect of hydrate phase transition during deep water drilling. The basic hydrodynamic models, including mass, momentum, and energy conservation equations, were established for annular flow with gas hydrate phase transition during gas kick. The behavior of annular multiphase flow with hydrate phase transition was investigated by analyzing the hydrate-forming region, the gas fraction in the fluid flowing in the annulus, pit gain, bottom hole pressure, and shut-in casing pressure. The simulation shows that it is possible to move the hydrate-forming region away from sea floor by increasing the circulation rate. The decrease in gas volume fraction in the annulus due to hydrate formation reduces pit gain, which can delay the detection of well kick and increase the risk of hydrate plugging in lines. Caution is needed when a well is monitored for gas kick at a relatively low gas production rate, because the possibility of hydrate presence is much greater than that at a relatively high production rate. The shut-in casing pressure cannot reflect the gas kick due to hydrate formation, which increases with time.
