A body force resembling the streamwise Lorentz force which decays exponentially in the wall-normalwise direction is applied in the primary and secondary separation bubbles to modify the base flow and thereby adjust th...A body force resembling the streamwise Lorentz force which decays exponentially in the wall-normalwise direction is applied in the primary and secondary separation bubbles to modify the base flow and thereby adjust the amplification rate of the perturbation energy.The amplification mechanisms are investigated numerically via analyzing the characteristics of the terms in the Reynolds-Orr equation which describes the growth rate of the perturbation energy.The results demonstrate that the main convective term always promotes the increase in the growth rate while the viscous terms usually play the reverse role.The contours of the product of the wall-normalwise and streamwise perturbation velocities distribute on both sides of the isoline,which represents the zero wall-normalwise gradient of the streamwise velocity in the base flow,due to the Kelvin-Helmholtz(KH)instability.For the case without control,the isoline downstream the reattachment point of the primary separation bubble is closer to the lower wall,and thus the viscous term near the lower wall might suppress the amplification rate.For the case in which the body force only acts on the secondary separation bubble,the secondary separation bubble is removed,and the magnitude of the negative wall-normalwise gradient of the base flow streamwise velocity decreases along the streamwise direction,and thus the growth rate of the perturbation energy is smaller than that for the case without control.For the case where the body force acts on both the separation bubbles,the secondary separation bubble is removed,the isoline stays in the central part of the channel,and thereby the viscous term has less effects on the amplification rate of which the peak value could be the maximum one for some control number.展开更多
Flow over a backward-facing step was studied to investigate the effect of large-scale vortex structures on sediment incipience. The transient flow velocity field at the downstream of the backward-facing step was obtai...Flow over a backward-facing step was studied to investigate the effect of large-scale vortex structures on sediment incipience. The transient flow velocity field at the downstream of the backward-facing step was obtained using the technique of Particle Tracking Velocimetry （PTV）. The optical amplification technique was employed to measure the instantaneous flow velocities near the bed and the instantaneous bed shear stress was given. The experimental observations revealed a new insight into the oscillation of the large-scale structure and the three-dimensional characteristics of the flow. In particular, very high turbulence intensity, instantaneous horizontal velocity near the bed and the bed shear stress near the reattachment point were observed. The sediment incipient probability obtained from the sequent images of sediment particles near the bed indicates that the critical instantaneous shear stress of the sediment incipience is independent of flow conditions.展开更多
The tile-type electromagnetic actuator(TEA)and stripe-type electromagnetic actuator(SEA)are applied to the active control of the perturbation energy in the liquid metal flow over a backward-facing step(BFS).Three cont...The tile-type electromagnetic actuator(TEA)and stripe-type electromagnetic actuator(SEA)are applied to the active control of the perturbation energy in the liquid metal flow over a backward-facing step(BFS).Three control strategies consisting of base flow control(BFC),linear model control(LMC)and combined model control(CMC)are considered to change the amplification rate of the perturbation energy.CMC is the combination of BFC and LMC.SEA is utilized in BFC to produce the streamwise Lorentz force thus adjusting the amplification rate via modifying the flow structures,and the magnitude of the maximum amplification rate could reach to 6 orders.TEA is used in LMC to reduce the magnitude of the amplification rate via the wall-normalwise Lorentz force,and the magnitude could be decreased by 2 orders.Both TEA and SEA are employed in CMC where the magnitude of the amplification rate could be diminished by 3 orders.In other words,the control strategy of CMC could capably alter the flow instability of the liquid metal flow.展开更多
In this study,magneto-hydrodynamics (MHD) mixed convection effects of Al2O3-water nanofluid flow over a backward-facing step were investigated numerically for various electrical conductivity models of nanofluids.A uni...In this study,magneto-hydrodynamics (MHD) mixed convection effects of Al2O3-water nanofluid flow over a backward-facing step were investigated numerically for various electrical conductivity models of nanofluids.A uniform external magnetic field was applied to the flow and strength of magnetic field was varied with different values of dimensionless parameter Hartmann number (Ha=0,10,20,30,40).Three different electrical conductivity models were used to see the effects of MHD nanofluid flow.Besides,five different inclination angles between 0°-90° is used for the external magnetic field.The problem geometry is a backward-facing step which is used in many engineering applications where flow separation and reattachment phenomenon occurs.Mixed type convective heat transfer of backward-facing step was examined with various values of Richardson number (Ri=0.01,0.1,1,10) and four different nanoparticle volume fractions (Ф=0.01,0.015,0.020,0.025) considering different electrical conductivity models.Finite element method via commercial code COMSOL was used for computations.Results indicate that the addition of nanoparticles enhanced heat transfer significantly.Also increasing magnetic field strength and inclination angle increased heat transfer rate.Effects of different electrical conductivity models were also investigated and it was observed that they have significant effects on the fluid flow and heat transfer characteristics in the presence of magnetic field.展开更多
Large eddy simulation (LES) explicitly calculates the large-scale vortex field and parameterizes the small-scale vortices.In this study,LES and κ-ε models were developed for a specific geometrical configuration of b...Large eddy simulation (LES) explicitly calculates the large-scale vortex field and parameterizes the small-scale vortices.In this study,LES and κ-ε models were developed for a specific geometrical configuration of backward-facing step (BFS).The simulation results were validated with particle image velocimetry (PIV) measurements and direct numerical simulation (DNS).This LES simulation was carried out with a Reynolds number of 9000 in a pressurized water tunnel with an expansion ratio of 2.00.The results indicate that the LES model can reveal largescale vortex motion although with a larger grid-cell size.However,the LES model tends to overestimate the top wall separation and the Reynolds stress components for the BFS flow simulation without a sufficiently fine grid.Overall,LES is a potential tool for simulating separated flow controlled by large-scale vortices.展开更多
The backward-facing step is a critical problem existing in many engineering and industrial applications.In this study,a semi-porous baffle(the root of the baffle is a porous medium and the tip is solid) is placed behi...The backward-facing step is a critical problem existing in many engineering and industrial applications.In this study,a semi-porous baffle(the root of the baffle is a porous medium and the tip is solid) is placed behind the step.The effects of the length of the porous part,and the baffle location on the energy transfer and pressure drop are studied in different Reynolds numbers(Re=100,200,300,400,500).The effect of the Darcy number of the porous medium on the aforementioned parameters is also investigated.Both the local maximum and average relative Nusselt numbers(divided by the Nusselt of the base case with no baffle at the same Reynolds) and relative pressure drop(calculated as the relative Nusselt number) are reported.The results show that by adoption of the proper length of the porous medium,the average relative and maximum local Nusselt numbers could be enhanced by 20% and 90%,respectively.Low permeable porous media give better energy transfer.For example,porous media with Da=10^(-5) give 30% better maximum local Nusselt number and about 7% higher average Nusselt number with respect to the same case with Da=10^(-2).展开更多
Numerical simulations of a two-dimensional laminar forced convection flow adjacent to inclined backward-facing step in a rectangular duct are presented to examine effects of baffle on flow, heat transfer and entropy g...Numerical simulations of a two-dimensional laminar forced convection flow adjacent to inclined backward-facing step in a rectangular duct are presented to examine effects of baffle on flow, heat transfer and entropy generation distributions. The main aim of using baffles is to enhance the value of convection coefficient on the bottom wall. But the useful energy can be destroyed due to intrinsic irreversibilities in the flow by the baffle. In the present work, the amount of energy loss is estimated by the computation of entropy generation. The values of velocity and temperature which are the inputs of the entropy generation equation are obtained by the numerical solution of momentum and energy equations with blocked-off method using computational fluid dynamic technique. Discretized forms of the governing equations in the (x, y) plane are obtained by the control volume method and solved using the SIMPLE algorithm. Numerical expressions, in terms of Nusselt number, entropy generation number, Bejan number and coefficient of friction are derived in dimensionless form. Results show that although a baffle mounted onto the upper wall increases the magnitude of Nusselts number on the bottom wall, but a considerable increase in the amount of entropy generation number takes place because of this technique. For validation, the numerical results for the Nusselt number and entropy generation number are compared with theoretical findings by other investigators and reasonable agreement is found.展开更多
The mixing and combustion characteristics in a cavity flameholding combustor under inlet Mach number 2.92 are numerically investigated with ethylene injection.Dimensionless distance is defined as the ratio of the actu...The mixing and combustion characteristics in a cavity flameholding combustor under inlet Mach number 2.92 are numerically investigated with ethylene injection.Dimensionless distance is defined as the ratio of the actual distance to the height of the combustor entrance.The cavity shear-layer mode,the lifted cavity shear-layer mode,and jet wake mode with upstream separation are observed respectively with dimensionless distance equals to 1.5,4.5,and 7.5.In both non-reacting and reacting flow fields,the numerical results are essentially in agreement with the schlieren photography,flame chemiluminescence images,and wall pressure,which verify the reliability of the numerical method.The results of non-reacting flow fields show that the BackwardFacing Step(BFS)can promote the flow separation downstream at a fixed distance.The more forward the separation position is,the larger the separation zone is in the non-reacting flow field.Furthermore,the larger the separation zone is,the higher the intensity of combustion in the reacting flow field is.A reasonable distance can reduce the total pressure loss generated by the shock waves in the combustor.The flame presents remarkable three-dimensional characteristics in the reacting flow fields.When dimensionless distance equals to 4.5,there are flames near the side wall above the cavity and it is difficult for the flame stabilization in the center of the combustor,while the combustion intensity in the center of the combustor is higher than that near the side wall when dimensionless distance equals to 7.5.In the cavity flameholding combustors with a backward-facing step,the higher combustion intensity may bring much total pressure loss to the combustor.Thus,it is a good choice to achieve better thrust performance when dimensionless distance equals to 4.5 compared to the other two combustors.展开更多
The present study focuses on the flow of a yield-stress(Bingham)nanofluid,consisting of suspended Fe3O4 nanoparticles,subjected to a magnetic field in a backward-facing step duct(BFS)configuration.The duct is equipped...The present study focuses on the flow of a yield-stress(Bingham)nanofluid,consisting of suspended Fe3O4 nanoparticles,subjected to a magnetic field in a backward-facing step duct(BFS)configuration.The duct is equipped with a cylindrical obstacle,where the lower wall is kept at a constant temperature.The yield-stress nanofluid enters this duct at a cold temperature with fully developed velocity.The aim of the present investigation is to explore the influence of flow velocity(Re=10 to 200),nanoparticle concentration(ϕ=0 to 0.1),magnetic field intensity(Ha=0 to 100),and its inclination angle(γ=0 to 90)and nanofluid yield stress(Bn=0 to 20)on the thermal and hydrodynamic efficiency inside the backward-facing step.The numerical results have been obtained by resolving the momentum and energy balance equations using the Galerkin finite element method.The obtained results have indicated that an increase in Reynolds number and nanoparticle volume fraction enhances heat transfer.In contrast,a significant reduction is observed with an increase in Hartmann and Bingham numbers,resulting in quasi-immobilization of the fluid under the magnetic influence and radical solidification of this type of fluid,accompanied by the suppression of the vortex zone downstream of the cylindrical obstacle.This study sheds light on the complexity of this magnetically influenced fluid,with potential implications in various engineering and materials science fields.展开更多
During the last three decades, the introduction of new construction materials (e.g. RCC (Roller Compacted Concrete), strengthened gabions) has increased the interest for stepped channels and spillways. However stepped...During the last three decades, the introduction of new construction materials (e.g. RCC (Roller Compacted Concrete), strengthened gabions) has increased the interest for stepped channels and spillways. However stepped chute hydraulics is not simple, because of different flow regimes and importantly because of very-strong interactions between entrained air and turbu- lence. In this study, new air-water flow measurements were conducted in two large-size stepped chute facilities with two step heights in each facility to study experimental distortion caused by scale effects and the soundness of result extrapolation to pro- totypes. Experimental data included distributions of air concentration, air-water flow velocity, bubble frequency, bubble chord length and air-water flow turbulence intensity. For a Froude similitude, the results implied that scale effects were observed in both facilities, although the geometric scaling ratio was only Lr=2 in each case. The selection of the criterion for scale effects is a critical issue. For example, major differences (i.e. scale effects) were observed in terms of bubble chord sizes and turbulence levels al- though little scale effects were seen in terms of void fraction and velocity distributions. Overall the findings emphasize that physical modelling of stepped chutes based upon a Froude similitude is more sensitive to scale effects than classical smooth-invert chute studies, and this is consistent with basic dimensional analysis developed herein.展开更多
The flow structures in a separated shear layer actuated by a synthetic jet actuator were studied using experimental methods. When forced at a frequency much lower than the natural shedding frequency （fH/U = 0.042 orf...The flow structures in a separated shear layer actuated by a synthetic jet actuator were studied using experimental methods. When forced at a frequency much lower than the natural shedding frequency （fH/U = 0.042 orfXr/U = 0.24）, the vertical flapping motion of the shear layer downstream of the separation point became prominent. The size of the peak in the pressure spectra at the forcing frequency （Sta = fAH//U） measured near the separation point （x/H = 1） increased linearly with the forcing ampli- tude （u＇/U） suggesting a linear response of the pressure fluctuations to the forcing by the synthetic jet. The linear response did not hold for the pressure fluctuations away from the jet exit as the magnitude of the peak for StA measured at x/H = 3 soon saturated when the forcing amplitude became larger than 0.3.展开更多
This paper deals with some aspects of the air entrainment process along the chute of spillway and study of pressure fluctuations. The experimental study has been carried out using stepped spillway model located in the...This paper deals with some aspects of the air entrainment process along the chute of spillway and study of pressure fluctuations. The experimental study has been carried out using stepped spillway model located in the campus of Government College of Engineering, Amravati (India). It is observed that air concentration is increasing with discharge as well as with number of step. Air concentration is increasing along the length of spillway. It is also observed that the bottom mean air concentration increases with step height in the upstream reach of stepped spillway, which is prone to cavitation. The pressure profiles exhibit a wavy pattern down the stepped chute and pressure on each step increases with ratio of critical depth to step height (yc/h).展开更多
Small-scale roughness elements or imperfections are inevitable over the surface of a flight vehicle.The aerodynamics of these small-scale structures is difficult to predict but may play an important role in the design...Small-scale roughness elements or imperfections are inevitable over the surface of a flight vehicle.The aerodynamics of these small-scale structures is difficult to predict but may play an important role in the design of a flight vehicle at high speed.The forward-facing step is a typical type of roughness element.Many experiments have been conducted to study the aerodynamics of supersonic forward-facing step,especially with a step height larger than boundary layer thickness.However,few studies focus on small steps.To improve the understanding of small-scale forwardfacing step flow,we perform a series of simulations to analyze its aerodynamic influence on a Mach number 5 turbulent boundary layer.The general flow structures are analyzed and discussed.Several shock waves can be induced by the step even if the step height is much smaller than the boundary layer thickness.Two significant shocks are the separation shock and the reattachment shock.The influenced area by the step is limited.With the increase of the step height,the non-dimensional influence area decreases and gradually converges when the step height reaches the boundary layer thickness.There are two normalized distributions of the skin friction coefficient and pressure coefficient associated with step height.By using the normalized parameters,a power-law relationship between the step height and the drag increment coefficient is revealed and fits the simulation results well.It is further illustrated that this relationship still holds when changing the inlet angle of attack,but needs slight modification with the angle of attack.展开更多
As a widely used flood energy dissipator, the stepped spillway can significantly dissipate the kinetic or hydraulic energy due to the air-entrainment in skimming flow over the steps. The free-surface aeration involves...As a widely used flood energy dissipator, the stepped spillway can significantly dissipate the kinetic or hydraulic energy due to the air-entrainment in skimming flow over the steps. The free-surface aeration involves the sharp deformation of the free surface and the complex turbulent shear flows. In this study, the volume of fluid （VOF）, mixture, and Eulerian methods are utilized to simulate the air-entrainment by coupling with the Reynolds-averaged Navier-Stokes/large eddy simulation （RANS/LES） turbulence models. The free surface deformation, air volume fraction, pressure, and velocity are compared for the three different numerical methods. Only the Eulerian＋RANS method fails to capture the free-surface aeration. The air volume fraction predicted by the VOF＋LES method best matches the experimental measurement, while the mixture＋LES method predicts the inception point of the air entrainment more accurately.展开更多
Bunching of elementary steps by solution flow is still not yet clarified for protein crystals. Hence, in this study, we observed elementary steps on crystal surfaces of model protein hen egg-white lysozyme （HEWL） un...Bunching of elementary steps by solution flow is still not yet clarified for protein crystals. Hence, in this study, we observed elementary steps on crystal surfaces of model protein hen egg-white lysozyme （HEWL） under forced flow conditions, by our advanced optical microscopy. We found that in the case of a HEWL solution of 99.99% purity, forced flow changed bunched steps into elementary ones （debunching） on tetragonal HEWL crystals. In contrast, in the case of a HEWL solution of 98.5% purity, forced flow significantly induced bunching of elementary steps. These results indicate that in the case of HEWL crystals, the mass transfer of impurities is more significantly enhanced by forced solution flow than that of solute HEWL molecules. We also showed that forced flow induced the incorporation of microcrystals into a mother crystal and the subsequent formation of screw dislocations and spiral growth hillocks.展开更多
The experimental investigation is conducted with LDV and hydrogen bubble technique in water flow. The shear layer thickness. the vorticity thickness. the maximulll value of turbulence intensities. the turbulent coher...The experimental investigation is conducted with LDV and hydrogen bubble technique in water flow. The shear layer thickness. the vorticity thickness. the maximulll value of turbulence intensities. the turbulent coherent structure. the variations of wall shear stress and the boundary layer shape factor are obtained. In the redevelopment region. the detailed analysis is first made for the streak structures in the near wall region and the turbulent boundary layer is formed at (x-xr) / h = 20.展开更多
Supercritical flows are ubiquitous in natural environments;however,there is rare 3D anatomy of their deposits.This study uses high-quality 3D seismic datasets from the world’s largest submarine fan,Bengal Fan,to inte...Supercritical flows are ubiquitous in natural environments;however,there is rare 3D anatomy of their deposits.This study uses high-quality 3D seismic datasets from the world’s largest submarine fan,Bengal Fan,to interpret 3D architectures and flow processes of Pliocene undulating bedforms that were related to supercritical flows.Bengal undulating bedforms as documented in this study were developed in unconfined settings,and are seismically imaged as strike-elongated,crescentic bedforms in plan view and as rhythmically undulating,upstream migrating,erosive,discontinuous reflections in section view.Their lee sides are overall 3 to 4 times steeper(0.280 to 1.19°in slope)and 3 to 4 times shorter(117 to 419 m in length)than their stoss flanks and were ascribed to faster(high flow velocities of 2.70 to 3.98 m/s)supercritical flows(Froude numbers of 1.53 to 2.27).Their stoss sides,in contrast,are overall 3 to 4 times gentler(0.120 to 0.270 in slope)and 3 to 4 times longer(410 to 1139 m in length)than their lee flanks and were related to slower(low velocities of 2.35 to 3.05 m/s)subcritical flows(Froude numbers of 0.58 to 0.97).Bengal wave-like features were,thus,created by supercriticalto-subcritical flow transformations through internal hydraulic jumps(i.e.,cyclic steps).They have crests that are positive relative to the surrounding region of the seafloor,suggesting the predominant deposition of draping sediments associated with net-depositional cyclic steps.Turbidity currents forming Bengal wave-like features were,thus,dominated by deposition,resulting in net-depositional cyclic steps.Sandy deposits associated with Bengal net-depositional cyclic steps are imaged themselves as closely spaced,strike-elongated high RMS-attribute patches,thereby showing closely spaced,long and linear,strike-elongated distribution patterns.展开更多
In the current study, a numerical investigation of three-dimensional combined convection-radiation heat transfer over an inclined forward facing step (FFS) in a horizontal rectangular duct is presented. The fluid is t...In the current study, a numerical investigation of three-dimensional combined convection-radiation heat transfer over an inclined forward facing step (FFS) in a horizontal rectangular duct is presented. The fluid is treated as a gray, absorbing, emitting and scattering medium. To simulate the incline surface of FFS, the blocked-off method is employed in this study. The set of governing equations for gas flow are solved numerically using the CFD technique to obtain the temperature and velocity fields. Since the gas is considered as a radiating medium, all of the convection, conduction and radiation heat transfer mechanisms are presented in the energy equation. For computation of radiative term in energy equation, the radiative transfer equation (RTE) is solved numerically by the discrete ordinates method (DOM) to find the divergence of radiative heat flux distribution inside the radiating medium. The effects of optical thickness, radiation-conduction parameter and albedo coefficient on heat transfer behavior of the system are carried out.展开更多
Taking a C1x motor with a backward-facing step which can generate a typical corner vortex as a reference,a numerical methodology using large eddy simulation was established in this study.Based on this methodology,the ...Taking a C1x motor with a backward-facing step which can generate a typical corner vortex as a reference,a numerical methodology using large eddy simulation was established in this study.Based on this methodology,the position of the backward-facing step of the motor was computed and analyzed to determine a basic configuration.Two key geometrical parameters,the head cavity angle and submerged nozzle cavity height,were subsequently introduced.Their effects on the corner vortex motion and their interactions with the acoustic pressure downstream of the backward-facing step were analyzed.The phenomena of vortex acoustic coupling and characteristics of pressure oscillations were further explored.The results show that the maximum error between the simulations and experimental data on the dominant frequency of pressure oscillations is 5.23%,which indicates that the numerical methodology built in this study is highly accurate.When the step is located at less than 5/8 of the total length of the combustion chamber,vortex acoustic coupling occurs,which can increase the pressure oscillations in the motor.Both the vorticity and the scale of vortices in the downstream step increase when the head cavity angle is greater than 24°,which increases the amplitude of the pressure oscillation by maximum 63.0%.The submerged nozzle cavity mainly affects the vortices in the cavity itself rather than those in the downstream step.When the height of the cavity increases from 10 to 20 mm,the pressure oscillation amplitude under the main frequency increases by 39.1%.As this height continues to increase,the amplitude of pressure oscillations increases but the primary frequency decreases.展开更多
基金the National Natural Science Foundation of China (Nos. 51776087 and 51979125)。
文摘A body force resembling the streamwise Lorentz force which decays exponentially in the wall-normalwise direction is applied in the primary and secondary separation bubbles to modify the base flow and thereby adjust the amplification rate of the perturbation energy.The amplification mechanisms are investigated numerically via analyzing the characteristics of the terms in the Reynolds-Orr equation which describes the growth rate of the perturbation energy.The results demonstrate that the main convective term always promotes the increase in the growth rate while the viscous terms usually play the reverse role.The contours of the product of the wall-normalwise and streamwise perturbation velocities distribute on both sides of the isoline,which represents the zero wall-normalwise gradient of the streamwise velocity in the base flow,due to the Kelvin-Helmholtz(KH)instability.For the case without control,the isoline downstream the reattachment point of the primary separation bubble is closer to the lower wall,and thus the viscous term near the lower wall might suppress the amplification rate.For the case in which the body force only acts on the secondary separation bubble,the secondary separation bubble is removed,and the magnitude of the negative wall-normalwise gradient of the base flow streamwise velocity decreases along the streamwise direction,and thus the growth rate of the perturbation energy is smaller than that for the case without control.For the case where the body force acts on both the separation bubbles,the secondary separation bubble is removed,the isoline stays in the central part of the channel,and thereby the viscous term has less effects on the amplification rate of which the peak value could be the maximum one for some control number.
基金Project supported by the National Natural Science Foundation of China(Grant No.10602017).
文摘Flow over a backward-facing step was studied to investigate the effect of large-scale vortex structures on sediment incipience. The transient flow velocity field at the downstream of the backward-facing step was obtained using the technique of Particle Tracking Velocimetry （PTV）. The optical amplification technique was employed to measure the instantaneous flow velocities near the bed and the instantaneous bed shear stress was given. The experimental observations revealed a new insight into the oscillation of the large-scale structure and the three-dimensional characteristics of the flow. In particular, very high turbulence intensity, instantaneous horizontal velocity near the bed and the bed shear stress near the reattachment point were observed. The sediment incipient probability obtained from the sequent images of sediment particles near the bed indicates that the critical instantaneous shear stress of the sediment incipience is independent of flow conditions.
基金Project supported by the National Natural Science Foundation of China(Grant No.U2141246)
文摘The tile-type electromagnetic actuator(TEA)and stripe-type electromagnetic actuator(SEA)are applied to the active control of the perturbation energy in the liquid metal flow over a backward-facing step(BFS).Three control strategies consisting of base flow control(BFC),linear model control(LMC)and combined model control(CMC)are considered to change the amplification rate of the perturbation energy.CMC is the combination of BFC and LMC.SEA is utilized in BFC to produce the streamwise Lorentz force thus adjusting the amplification rate via modifying the flow structures,and the magnitude of the maximum amplification rate could reach to 6 orders.TEA is used in LMC to reduce the magnitude of the amplification rate via the wall-normalwise Lorentz force,and the magnitude could be decreased by 2 orders.Both TEA and SEA are employed in CMC where the magnitude of the amplification rate could be diminished by 3 orders.In other words,the control strategy of CMC could capably alter the flow instability of the liquid metal flow.
文摘In this study,magneto-hydrodynamics (MHD) mixed convection effects of Al2O3-water nanofluid flow over a backward-facing step were investigated numerically for various electrical conductivity models of nanofluids.A uniform external magnetic field was applied to the flow and strength of magnetic field was varied with different values of dimensionless parameter Hartmann number (Ha=0,10,20,30,40).Three different electrical conductivity models were used to see the effects of MHD nanofluid flow.Besides,five different inclination angles between 0°-90° is used for the external magnetic field.The problem geometry is a backward-facing step which is used in many engineering applications where flow separation and reattachment phenomenon occurs.Mixed type convective heat transfer of backward-facing step was examined with various values of Richardson number (Ri=0.01,0.1,1,10) and four different nanoparticle volume fractions (Ф=0.01,0.015,0.020,0.025) considering different electrical conductivity models.Finite element method via commercial code COMSOL was used for computations.Results indicate that the addition of nanoparticles enhanced heat transfer significantly.Also increasing magnetic field strength and inclination angle increased heat transfer rate.Effects of different electrical conductivity models were also investigated and it was observed that they have significant effects on the fluid flow and heat transfer characteristics in the presence of magnetic field.
基金supported by the National Natural Science Foundation of China(Grant No.51379128)
文摘Large eddy simulation (LES) explicitly calculates the large-scale vortex field and parameterizes the small-scale vortices.In this study,LES and κ-ε models were developed for a specific geometrical configuration of backward-facing step (BFS).The simulation results were validated with particle image velocimetry (PIV) measurements and direct numerical simulation (DNS).This LES simulation was carried out with a Reynolds number of 9000 in a pressurized water tunnel with an expansion ratio of 2.00.The results indicate that the LES model can reveal largescale vortex motion although with a larger grid-cell size.However,the LES model tends to overestimate the top wall separation and the Reynolds stress components for the BFS flow simulation without a sufficiently fine grid.Overall,LES is a potential tool for simulating separated flow controlled by large-scale vortices.
文摘The backward-facing step is a critical problem existing in many engineering and industrial applications.In this study,a semi-porous baffle(the root of the baffle is a porous medium and the tip is solid) is placed behind the step.The effects of the length of the porous part,and the baffle location on the energy transfer and pressure drop are studied in different Reynolds numbers(Re=100,200,300,400,500).The effect of the Darcy number of the porous medium on the aforementioned parameters is also investigated.Both the local maximum and average relative Nusselt numbers(divided by the Nusselt of the base case with no baffle at the same Reynolds) and relative pressure drop(calculated as the relative Nusselt number) are reported.The results show that by adoption of the proper length of the porous medium,the average relative and maximum local Nusselt numbers could be enhanced by 20% and 90%,respectively.Low permeable porous media give better energy transfer.For example,porous media with Da=10^(-5) give 30% better maximum local Nusselt number and about 7% higher average Nusselt number with respect to the same case with Da=10^(-2).
文摘Numerical simulations of a two-dimensional laminar forced convection flow adjacent to inclined backward-facing step in a rectangular duct are presented to examine effects of baffle on flow, heat transfer and entropy generation distributions. The main aim of using baffles is to enhance the value of convection coefficient on the bottom wall. But the useful energy can be destroyed due to intrinsic irreversibilities in the flow by the baffle. In the present work, the amount of energy loss is estimated by the computation of entropy generation. The values of velocity and temperature which are the inputs of the entropy generation equation are obtained by the numerical solution of momentum and energy equations with blocked-off method using computational fluid dynamic technique. Discretized forms of the governing equations in the (x, y) plane are obtained by the control volume method and solved using the SIMPLE algorithm. Numerical expressions, in terms of Nusselt number, entropy generation number, Bejan number and coefficient of friction are derived in dimensionless form. Results show that although a baffle mounted onto the upper wall increases the magnitude of Nusselts number on the bottom wall, but a considerable increase in the amount of entropy generation number takes place because of this technique. For validation, the numerical results for the Nusselt number and entropy generation number are compared with theoretical findings by other investigators and reasonable agreement is found.
基金supported by the National Natural Science Foundation of China(Nos.11925207 and 12002381)the Scientific Research Plan of the National University of Defense Technology in 2019,China(No.ZK19-02)the Science and Technology Foundation of State Key Laboratory,China(No.6142703200311).
文摘The mixing and combustion characteristics in a cavity flameholding combustor under inlet Mach number 2.92 are numerically investigated with ethylene injection.Dimensionless distance is defined as the ratio of the actual distance to the height of the combustor entrance.The cavity shear-layer mode,the lifted cavity shear-layer mode,and jet wake mode with upstream separation are observed respectively with dimensionless distance equals to 1.5,4.5,and 7.5.In both non-reacting and reacting flow fields,the numerical results are essentially in agreement with the schlieren photography,flame chemiluminescence images,and wall pressure,which verify the reliability of the numerical method.The results of non-reacting flow fields show that the BackwardFacing Step(BFS)can promote the flow separation downstream at a fixed distance.The more forward the separation position is,the larger the separation zone is in the non-reacting flow field.Furthermore,the larger the separation zone is,the higher the intensity of combustion in the reacting flow field is.A reasonable distance can reduce the total pressure loss generated by the shock waves in the combustor.The flame presents remarkable three-dimensional characteristics in the reacting flow fields.When dimensionless distance equals to 4.5,there are flames near the side wall above the cavity and it is difficult for the flame stabilization in the center of the combustor,while the combustion intensity in the center of the combustor is higher than that near the side wall when dimensionless distance equals to 7.5.In the cavity flameholding combustors with a backward-facing step,the higher combustion intensity may bring much total pressure loss to the combustor.Thus,it is a good choice to achieve better thrust performance when dimensionless distance equals to 4.5 compared to the other two combustors.
文摘The present study focuses on the flow of a yield-stress(Bingham)nanofluid,consisting of suspended Fe3O4 nanoparticles,subjected to a magnetic field in a backward-facing step duct(BFS)configuration.The duct is equipped with a cylindrical obstacle,where the lower wall is kept at a constant temperature.The yield-stress nanofluid enters this duct at a cold temperature with fully developed velocity.The aim of the present investigation is to explore the influence of flow velocity(Re=10 to 200),nanoparticle concentration(ϕ=0 to 0.1),magnetic field intensity(Ha=0 to 100),and its inclination angle(γ=0 to 90)and nanofluid yield stress(Bn=0 to 20)on the thermal and hydrodynamic efficiency inside the backward-facing step.The numerical results have been obtained by resolving the momentum and energy balance equations using the Galerkin finite element method.The obtained results have indicated that an increase in Reynolds number and nanoparticle volume fraction enhances heat transfer.In contrast,a significant reduction is observed with an increase in Hartmann and Bingham numbers,resulting in quasi-immobilization of the fluid under the magnetic influence and radical solidification of this type of fluid,accompanied by the suppression of the vortex zone downstream of the cylindrical obstacle.This study sheds light on the complexity of this magnetically influenced fluid,with potential implications in various engineering and materials science fields.
基金Project supported by the National Council for Science and Tech-nology of Mexico (CONACYT)
文摘During the last three decades, the introduction of new construction materials (e.g. RCC (Roller Compacted Concrete), strengthened gabions) has increased the interest for stepped channels and spillways. However stepped chute hydraulics is not simple, because of different flow regimes and importantly because of very-strong interactions between entrained air and turbu- lence. In this study, new air-water flow measurements were conducted in two large-size stepped chute facilities with two step heights in each facility to study experimental distortion caused by scale effects and the soundness of result extrapolation to pro- totypes. Experimental data included distributions of air concentration, air-water flow velocity, bubble frequency, bubble chord length and air-water flow turbulence intensity. For a Froude similitude, the results implied that scale effects were observed in both facilities, although the geometric scaling ratio was only Lr=2 in each case. The selection of the criterion for scale effects is a critical issue. For example, major differences (i.e. scale effects) were observed in terms of bubble chord sizes and turbulence levels al- though little scale effects were seen in terms of void fraction and velocity distributions. Overall the findings emphasize that physical modelling of stepped chutes based upon a Froude similitude is more sensitive to scale effects than classical smooth-invert chute studies, and this is consistent with basic dimensional analysis developed herein.
基金supported by 973 Plan (2014CB744100)Dalian University of Technology (DUT14LK07)
文摘The flow structures in a separated shear layer actuated by a synthetic jet actuator were studied using experimental methods. When forced at a frequency much lower than the natural shedding frequency （fH/U = 0.042 orfXr/U = 0.24）, the vertical flapping motion of the shear layer downstream of the separation point became prominent. The size of the peak in the pressure spectra at the forcing frequency （Sta = fAH//U） measured near the separation point （x/H = 1） increased linearly with the forcing ampli- tude （u＇/U） suggesting a linear response of the pressure fluctuations to the forcing by the synthetic jet. The linear response did not hold for the pressure fluctuations away from the jet exit as the magnitude of the peak for StA measured at x/H = 3 soon saturated when the forcing amplitude became larger than 0.3.
文摘This paper deals with some aspects of the air entrainment process along the chute of spillway and study of pressure fluctuations. The experimental study has been carried out using stepped spillway model located in the campus of Government College of Engineering, Amravati (India). It is observed that air concentration is increasing with discharge as well as with number of step. Air concentration is increasing along the length of spillway. It is also observed that the bottom mean air concentration increases with step height in the upstream reach of stepped spillway, which is prone to cavitation. The pressure profiles exhibit a wavy pattern down the stepped chute and pressure on each step increases with ratio of critical depth to step height (yc/h).
基金financial support from the National Natural Science Foundation of China(Nos.11602127 and 11572176)。
文摘Small-scale roughness elements or imperfections are inevitable over the surface of a flight vehicle.The aerodynamics of these small-scale structures is difficult to predict but may play an important role in the design of a flight vehicle at high speed.The forward-facing step is a typical type of roughness element.Many experiments have been conducted to study the aerodynamics of supersonic forward-facing step,especially with a step height larger than boundary layer thickness.However,few studies focus on small steps.To improve the understanding of small-scale forwardfacing step flow,we perform a series of simulations to analyze its aerodynamic influence on a Mach number 5 turbulent boundary layer.The general flow structures are analyzed and discussed.Several shock waves can be induced by the step even if the step height is much smaller than the boundary layer thickness.Two significant shocks are the separation shock and the reattachment shock.The influenced area by the step is limited.With the increase of the step height,the non-dimensional influence area decreases and gradually converges when the step height reaches the boundary layer thickness.There are two normalized distributions of the skin friction coefficient and pressure coefficient associated with step height.By using the normalized parameters,a power-law relationship between the step height and the drag increment coefficient is revealed and fits the simulation results well.It is further illustrated that this relationship still holds when changing the inlet angle of attack,but needs slight modification with the angle of attack.
基金supported by the Guangdong Special Research Fund of Public Welfare and Capacity Building(2015A020216008)the Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund(the second phase)
文摘As a widely used flood energy dissipator, the stepped spillway can significantly dissipate the kinetic or hydraulic energy due to the air-entrainment in skimming flow over the steps. The free-surface aeration involves the sharp deformation of the free surface and the complex turbulent shear flows. In this study, the volume of fluid （VOF）, mixture, and Eulerian methods are utilized to simulate the air-entrainment by coupling with the Reynolds-averaged Navier-Stokes/large eddy simulation （RANS/LES） turbulence models. The free surface deformation, air volume fraction, pressure, and velocity are compared for the three different numerical methods. Only the Eulerian＋RANS method fails to capture the free-surface aeration. The air volume fraction predicted by the VOF＋LES method best matches the experimental measurement, while the mixture＋LES method predicts the inception point of the air entrainment more accurately.
基金Chinese Academy of Sciences visiting professorship for senior international scientists(2013T2J0060)
文摘Bunching of elementary steps by solution flow is still not yet clarified for protein crystals. Hence, in this study, we observed elementary steps on crystal surfaces of model protein hen egg-white lysozyme （HEWL） under forced flow conditions, by our advanced optical microscopy. We found that in the case of a HEWL solution of 99.99% purity, forced flow changed bunched steps into elementary ones （debunching） on tetragonal HEWL crystals. In contrast, in the case of a HEWL solution of 98.5% purity, forced flow significantly induced bunching of elementary steps. These results indicate that in the case of HEWL crystals, the mass transfer of impurities is more significantly enhanced by forced solution flow than that of solute HEWL molecules. We also showed that forced flow induced the incorporation of microcrystals into a mother crystal and the subsequent formation of screw dislocations and spiral growth hillocks.
文摘The experimental investigation is conducted with LDV and hydrogen bubble technique in water flow. The shear layer thickness. the vorticity thickness. the maximulll value of turbulence intensities. the turbulent coherent structure. the variations of wall shear stress and the boundary layer shape factor are obtained. In the redevelopment region. the detailed analysis is first made for the streak structures in the near wall region and the turbulent boundary layer is formed at (x-xr) / h = 20.
基金jointly funded by PetroChina Hangzhou Research Institute of Geology(No.2019D-4309)。
文摘Supercritical flows are ubiquitous in natural environments;however,there is rare 3D anatomy of their deposits.This study uses high-quality 3D seismic datasets from the world’s largest submarine fan,Bengal Fan,to interpret 3D architectures and flow processes of Pliocene undulating bedforms that were related to supercritical flows.Bengal undulating bedforms as documented in this study were developed in unconfined settings,and are seismically imaged as strike-elongated,crescentic bedforms in plan view and as rhythmically undulating,upstream migrating,erosive,discontinuous reflections in section view.Their lee sides are overall 3 to 4 times steeper(0.280 to 1.19°in slope)and 3 to 4 times shorter(117 to 419 m in length)than their stoss flanks and were ascribed to faster(high flow velocities of 2.70 to 3.98 m/s)supercritical flows(Froude numbers of 1.53 to 2.27).Their stoss sides,in contrast,are overall 3 to 4 times gentler(0.120 to 0.270 in slope)and 3 to 4 times longer(410 to 1139 m in length)than their lee flanks and were related to slower(low velocities of 2.35 to 3.05 m/s)subcritical flows(Froude numbers of 0.58 to 0.97).Bengal wave-like features were,thus,created by supercriticalto-subcritical flow transformations through internal hydraulic jumps(i.e.,cyclic steps).They have crests that are positive relative to the surrounding region of the seafloor,suggesting the predominant deposition of draping sediments associated with net-depositional cyclic steps.Turbidity currents forming Bengal wave-like features were,thus,dominated by deposition,resulting in net-depositional cyclic steps.Sandy deposits associated with Bengal net-depositional cyclic steps are imaged themselves as closely spaced,strike-elongated high RMS-attribute patches,thereby showing closely spaced,long and linear,strike-elongated distribution patterns.
文摘In the current study, a numerical investigation of three-dimensional combined convection-radiation heat transfer over an inclined forward facing step (FFS) in a horizontal rectangular duct is presented. The fluid is treated as a gray, absorbing, emitting and scattering medium. To simulate the incline surface of FFS, the blocked-off method is employed in this study. The set of governing equations for gas flow are solved numerically using the CFD technique to obtain the temperature and velocity fields. Since the gas is considered as a radiating medium, all of the convection, conduction and radiation heat transfer mechanisms are presented in the energy equation. For computation of radiative term in energy equation, the radiative transfer equation (RTE) is solved numerically by the discrete ordinates method (DOM) to find the divergence of radiative heat flux distribution inside the radiating medium. The effects of optical thickness, radiation-conduction parameter and albedo coefficient on heat transfer behavior of the system are carried out.
基金Sponsored by the Natural Science Foundation of Shaanxi Province (Grant No. S2025-JC-YB-0532)the Practice and Innovation Funds for Graduate Students of Northwestern Polytechnical University (PF2024044)
文摘Taking a C1x motor with a backward-facing step which can generate a typical corner vortex as a reference,a numerical methodology using large eddy simulation was established in this study.Based on this methodology,the position of the backward-facing step of the motor was computed and analyzed to determine a basic configuration.Two key geometrical parameters,the head cavity angle and submerged nozzle cavity height,were subsequently introduced.Their effects on the corner vortex motion and their interactions with the acoustic pressure downstream of the backward-facing step were analyzed.The phenomena of vortex acoustic coupling and characteristics of pressure oscillations were further explored.The results show that the maximum error between the simulations and experimental data on the dominant frequency of pressure oscillations is 5.23%,which indicates that the numerical methodology built in this study is highly accurate.When the step is located at less than 5/8 of the total length of the combustion chamber,vortex acoustic coupling occurs,which can increase the pressure oscillations in the motor.Both the vorticity and the scale of vortices in the downstream step increase when the head cavity angle is greater than 24°,which increases the amplitude of the pressure oscillation by maximum 63.0%.The submerged nozzle cavity mainly affects the vortices in the cavity itself rather than those in the downstream step.When the height of the cavity increases from 10 to 20 mm,the pressure oscillation amplitude under the main frequency increases by 39.1%.As this height continues to increase,the amplitude of pressure oscillations increases but the primary frequency decreases.
