In engineering applications (Like an ocean riser), fluid flow around bluff bodies generates substantial resistance, which can jeopardize structural integrity, lifespan, and escalate resource consumption. Therefore, em...In engineering applications (Like an ocean riser), fluid flow around bluff bodies generates substantial resistance, which can jeopardize structural integrity, lifespan, and escalate resource consumption. Therefore, employing drag reduction measures becomes particularly crucial. This paper employs the immersed boundary method to investigate the impact of transversely oriented appendage plate flexibility on the drag of cylinders under different Reynolds numbers and distances. The results indicate that flexible appendage plate exerts drag reduction effects on the downstream cylinder, with this effect gradually diminishing as Reynolds numbers increase. At identical Reynolds numbers, the drag reduction effect initially increases and then decreases with distance, with the optimal drag reduction distance observed at D = 2.5. Compared to cylinders without appendage plate, the maximum drag reduction achieved is 30.551%. Addressing the drag reduction issue in cylinders holds significant importance for ensuring engineering structural integrity, enhancing engineering efficiency, and developing novel underwater towing systems.展开更多
Drag reduction features in the transition regime of channel flow with fibre suspension were analyzed in terms of the linear stability theory. The modified stability equation was obtained based on the slender-body theo...Drag reduction features in the transition regime of channel flow with fibre suspension were analyzed in terms of the linear stability theory. The modified stability equation was obtained based on the slender-body theory and natural closure approximation. Results of the stability analysis show attenuating effects of fibre additives to the flow instability. For the cases leading to transition, drag reduction rate increases with the characteristic parameter H of fibres. The mechanism of drag reduction by fibres is revealed through the variation of velocity profile and the decrease of wall shear stress. The theoretical results are qualitatively consistent with some typical experiments.展开更多
This study investigates the heterogeneous structure and its influence on drag coefficient for concurrent up gas-solid flow.The energy-minimization multi-scale (EMMS) model is modified to simulate the variation of stru...This study investigates the heterogeneous structure and its influence on drag coefficient for concurrent up gas-solid flow.The energy-minimization multi-scale (EMMS) model is modified to simulate the variation of structure parameters with solids concentration,showing the tendency for particles to aggregated to form clusters and for fluid to pass around clusters.The global drag coefficient is resolved into that for the dense phase,for the dilutephase and for the so-called inter-phase,all of which can be obtained from their respective phase-specific structure parameters.The computational results show that the drag coefficients of the different phases are quite different,and the global drag coefficient calculated from the EMMS approach is much lower than that from the correlation of Wen and Yu.The simulation results demonstrate that the EMMS approach can well describe the heterogeneous flow structure,and is very promising for incorporation into the two-fluid model or the discrete particle model as the closure law for drag coefficient.展开更多
In this paper,an analytical model that represents the streamwise velocity distribution for open channel flow with submerged flexible vegetation is studied.In the present vegetated flow modelling,the whole flow field h...In this paper,an analytical model that represents the streamwise velocity distribution for open channel flow with submerged flexible vegetation is studied.In the present vegetated flow modelling,the whole flow field has been separated into two layers vertically: a vegetated layer and a non-vegetated free-water layer.Within the vegetated layer,an analysis of the mechanisms affecting water flow through flexible vegetation has been conducted.In the non-vegetated layer,a modified log-law equation that represents the velocity profile varying with vegetation height has been investigated.Based on the studied analytical model,a sensitivity analysis has been conducted to assess the influences of the drag (CD) and friction (Cf ) coefficients on the flow velocity.The investigated ranges of CD and Cf have also been compared to published values.The findings suggest that the CD and Cf values are non-constant at different depths and vegetation densities,unlike the constant values commonly suggested in literature.This phenomenon is particularly clear for flows with flexible vegetation,which is characterised by large deflection.展开更多
In this paper the effects of hydrophobic wall on skin-friction drag in the channel flow are investigated through large eddy simulation on the basis of weaklycompressible flow equations with the MacCormack’s scheme on...In this paper the effects of hydrophobic wall on skin-friction drag in the channel flow are investigated through large eddy simulation on the basis of weaklycompressible flow equations with the MacCormack’s scheme on collocated mesh in the FVM framework. The slip length model is adopted to describe the behavior of the slip velocities in the streamwise and spanwise directions at the interface between the hydrophobic wall and turbulent channel flow. Simulation results are presented by analyzing flow behaviors over hydrophobic wall with the Smagorinky subgrid-scale model and a dynamic model on computational meshes of different resolutions. Comparison and analysis are made on the distributions of timeaveraged velocity, velocity fluctuations, Reynolds stress as well as the skin-friction drag. Excellent agreement between the present study and previous results demonstrates the accuracy of the simple classical second-order scheme in representing turbulent vertox near hydrophobic wall. In addition, the relation of drag reduction efficiency versus time-averaged slip velocity is established. It is also foundthat the decrease of velocity gradient in the close wall region is responsible for the drag reduction. Considering its advantages of high calculation precision and efficiency, the present method has good prospect in its application to practical projects.展开更多
The direct numerical simulation (DNS) is carried out for the incompressible viscous turbulent flows over an anisotropic porous wall. Effects of the anisotropic porous wall on turbulence modifications as well as on the...The direct numerical simulation (DNS) is carried out for the incompressible viscous turbulent flows over an anisotropic porous wall. Effects of the anisotropic porous wall on turbulence modifications as well as on the turbulent drag reduction are investigated. The simulation is carried out at a friction Reynolds number of 180, which is based on the averaged friction velocity at the interface between the porous medium and the clear fluid domain. The depth of the porous layer ranges from 0.9 to 54 viscous units. The permeability in the spanwise direction is set to be lower than the other directions in the present simulation. The maximum drag reduction obtained is about 15.3% which occurs for a depth of 9 viscous units. The increasing of drag is addressed when the depth of the porous layer is more than 25 wall units. The thinner porous layer restricts the spanwise extension of the streamwise vortices which suppresses the bursting events near the wall. However, for the thicker porous layer, the wall-normal fluctuations are enhanced due to the weakening of the wall-blocking effect which can trigger strong turbulent structures near the wall.展开更多
The nanophotocatalysts were synthesized in four stages and evaluated by FTIR, FESEM and VSM analysis. The influence of nanofluids containing functionalized magnetic Ti O2 nanophotocatalyst and dipalmitoylphosphatidylc...The nanophotocatalysts were synthesized in four stages and evaluated by FTIR, FESEM and VSM analysis. The influence of nanofluids containing functionalized magnetic Ti O2 nanophotocatalyst and dipalmitoylphosphatidylcholine lecithin in drag reduction of turbulent flow in four horizontal pipelines was studied. The effective parameters on drag reduction(nanoparticle concentration, surfactant concentration, p H and Re number) were investigated and optimized in each pipeline using response surface method. The drag reduction in 1/2 " galvanized, 3/4" galvanized, 1/2 "five-layer and 1/2" cuprous pipelines was found 99.1%, 92.5%, 87.6% and 85.2%, respectively. The model adequacy was measured using ANOVA. Based on the high determination coefficient, more than 95% of variance of experimental data in all pipelines was described by quadratic model.展开更多
Fully developed turbulent flow fields with and without polymer solution at the same Reynolds number were measured by time-resolved particle image velocimetry (TRPIV) in a water channel to investigate the mechanism o...Fully developed turbulent flow fields with and without polymer solution at the same Reynolds number were measured by time-resolved particle image velocimetry (TRPIV) in a water channel to investigate the mechanism of drag-reducing solution from the view of coherent structures manipulation. The streamwise mean velocity and Reynolds stress profiles in the solution were compared with those in water. After adding the polymer solution, the Reynolds stress in the near-wall area decreases significantly. The result relates tightly to the decease of the coherent structures' bursting. The spatial topology of coherent structures during bursts has been extracted by the new mu-level criterion based on locally averaged velocity structure function. The effect of polymers on turbulent coherent structures mainly reflects in the intensity, not in the shape. In the solution, it is by suppressing the coherent structures that the wall friction is reduced.展开更多
Numerical simulations and experimental research are both carried out to investigate the controlled effect of spanwise oscillating Lorentz force on a turbulent channel flow. The variations of the streaks and the skin f...Numerical simulations and experimental research are both carried out to investigate the controlled effect of spanwise oscillating Lorentz force on a turbulent channel flow. The variations of the streaks and the skin friction drag are obtained through the PIV system and the drag measurement system, respectively. The flow field in the near-wall region is shown through direct numerical simulations utilizing spectral method. The experimental results are consistent with the numerical simulation results qualitatively, and both the results indicate that the streaks are tilted into the spanwise direction and the drag reduction utilizing spanwise oscillating Lorentz forces can be realized. The numerical simulation results reveal more detail of the drag reduction mechanism which can be explained, since the spanwise vorticity generated from the interaction between the induced Stokes layer and intrinsic turbulent flow in the near-wall region can make the longitudinal vortices tilt and oscillate, and leads to turbulence suppression and drag reduction.展开更多
In this paper, the characteristics of the three-dimensional flow field around the circular cylinder members forming a square biplane grid were experimentally investigated by using a wind tunnel and a water tunnel. In ...In this paper, the characteristics of the three-dimensional flow field around the circular cylinder members forming a square biplane grid were experimentally investigated by using a wind tunnel and a water tunnel. In the wind tunnel testing, the span wise and circumference pressure distributions of surface on the circular cylinder were measured on the center mesh members formed by biplane grid in detail. Local drag coefficient was calculated from the surface pressure distributions. In addition, the flow visualization was performed in the water tunnel. As a result, it was suggested that the flow penetrating the contact region produced secondary-flow behind the biplane grid. Accordingly, the drag reduction would be caused by the presence of the secondary-flow.展开更多
The drag reducing effect of polymer additive aqueous solution was investigated in flow boiling, and the polymer additives were two kinds of polyacrylamide (PAM) with relative molecular mass about 2.56×10 6 and 8....The drag reducing effect of polymer additive aqueous solution was investigated in flow boiling, and the polymer additives were two kinds of polyacrylamide (PAM) with relative molecular mass about 2.56×10 6 and 8.55×10 6. The frictional pressure drop was calculated according to the measured total pressure drop. The results show that the flow drag of flow boiling is reduced by adding a small amount of PAM to water when heat flux is in the range of 15.1 kW·m -2 to 47.0 kW·m -2 , when the mass fraction of PAM is higher than 2.0×10 -5 , the drag reducing effect is obvious. Drag reducing effect of PAM, whose relative molecular mass is 8.55×10 6, is slightly better than that of 2.56×10 6 at the same mass fraction, and the greater the flow rate of the additive solution, the better the effect of the drag reduction.展开更多
<Abstract>The influence of rheological parameters on vortex dynamics and the extent of drag reduction(DR) were deciphered via extensively analyzing the hi-fidelity direct numerical simulation results of the turb...<Abstract>The influence of rheological parameters on vortex dynamics and the extent of drag reduction(DR) were deciphered via extensively analyzing the hi-fidelity direct numerical simulation results of the turbulent channel flow with polymer solutions.It has been observed that in all drag reduction regimes from the onset of DR to maximum drag reduction(MDR) limit,the Deborah number is defined as the product of an effective Weissenberg number,and the root mean square streamwise vorticity fluctuation remains O(1) in the near wall region.The ratio of the average lifetime of axial vortices to the vortex rotating duration decreases with increasing DR,and MDR is achieved when these time scales become nearly equal.Based on these observations a simple framework is proposed adequately to describe the influence of polymer additives on the extent of DR from onset to MDR as well as the universality of the MDR in flow systems with polymer additives.展开更多
文摘In engineering applications (Like an ocean riser), fluid flow around bluff bodies generates substantial resistance, which can jeopardize structural integrity, lifespan, and escalate resource consumption. Therefore, employing drag reduction measures becomes particularly crucial. This paper employs the immersed boundary method to investigate the impact of transversely oriented appendage plate flexibility on the drag of cylinders under different Reynolds numbers and distances. The results indicate that flexible appendage plate exerts drag reduction effects on the downstream cylinder, with this effect gradually diminishing as Reynolds numbers increase. At identical Reynolds numbers, the drag reduction effect initially increases and then decreases with distance, with the optimal drag reduction distance observed at D = 2.5. Compared to cylinders without appendage plate, the maximum drag reduction achieved is 30.551%. Addressing the drag reduction issue in cylinders holds significant importance for ensuring engineering structural integrity, enhancing engineering efficiency, and developing novel underwater towing systems.
基金the National Natural Science Foundation of China (No. 10372090 and No. 10102017).
文摘Drag reduction features in the transition regime of channel flow with fibre suspension were analyzed in terms of the linear stability theory. The modified stability equation was obtained based on the slender-body theory and natural closure approximation. Results of the stability analysis show attenuating effects of fibre additives to the flow instability. For the cases leading to transition, drag reduction rate increases with the characteristic parameter H of fibres. The mechanism of drag reduction by fibres is revealed through the variation of velocity profile and the decrease of wall shear stress. The theoretical results are qualitatively consistent with some typical experiments.
基金Supported by the National Key Program for Developing Basic Sciences of China (No. G1999022103) and the National Natural Science Foundation of China (No. 20176059).
文摘This study investigates the heterogeneous structure and its influence on drag coefficient for concurrent up gas-solid flow.The energy-minimization multi-scale (EMMS) model is modified to simulate the variation of structure parameters with solids concentration,showing the tendency for particles to aggregated to form clusters and for fluid to pass around clusters.The global drag coefficient is resolved into that for the dense phase,for the dilutephase and for the so-called inter-phase,all of which can be obtained from their respective phase-specific structure parameters.The computational results show that the drag coefficients of the different phases are quite different,and the global drag coefficient calculated from the EMMS approach is much lower than that from the correlation of Wen and Yu.The simulation results demonstrate that the EMMS approach can well describe the heterogeneous flow structure,and is very promising for incorporation into the two-fluid model or the discrete particle model as the closure law for drag coefficient.
文摘In this paper,an analytical model that represents the streamwise velocity distribution for open channel flow with submerged flexible vegetation is studied.In the present vegetated flow modelling,the whole flow field has been separated into two layers vertically: a vegetated layer and a non-vegetated free-water layer.Within the vegetated layer,an analysis of the mechanisms affecting water flow through flexible vegetation has been conducted.In the non-vegetated layer,a modified log-law equation that represents the velocity profile varying with vegetation height has been investigated.Based on the studied analytical model,a sensitivity analysis has been conducted to assess the influences of the drag (CD) and friction (Cf ) coefficients on the flow velocity.The investigated ranges of CD and Cf have also been compared to published values.The findings suggest that the CD and Cf values are non-constant at different depths and vegetation densities,unlike the constant values commonly suggested in literature.This phenomenon is particularly clear for flows with flexible vegetation,which is characterised by large deflection.
基金supported by the National Basic Research and Development Program of China (2006CB403304)State Key Laboratory of Hydroscience and Engineering Program of China (2009-TC-2, 2009-ZY-8)the National Natural Science Foundation of China (50475012)
文摘In this paper the effects of hydrophobic wall on skin-friction drag in the channel flow are investigated through large eddy simulation on the basis of weaklycompressible flow equations with the MacCormack’s scheme on collocated mesh in the FVM framework. The slip length model is adopted to describe the behavior of the slip velocities in the streamwise and spanwise directions at the interface between the hydrophobic wall and turbulent channel flow. Simulation results are presented by analyzing flow behaviors over hydrophobic wall with the Smagorinky subgrid-scale model and a dynamic model on computational meshes of different resolutions. Comparison and analysis are made on the distributions of timeaveraged velocity, velocity fluctuations, Reynolds stress as well as the skin-friction drag. Excellent agreement between the present study and previous results demonstrates the accuracy of the simple classical second-order scheme in representing turbulent vertox near hydrophobic wall. In addition, the relation of drag reduction efficiency versus time-averaged slip velocity is established. It is also foundthat the decrease of velocity gradient in the close wall region is responsible for the drag reduction. Considering its advantages of high calculation precision and efficiency, the present method has good prospect in its application to practical projects.
基金Project supported by the National Natural Science Foundation of China(Nos.11572183,91852111,and 11825204)the Program of Shanghai Municipal Education Commission(No.2019-01-07-00-09-E00018)
文摘The direct numerical simulation (DNS) is carried out for the incompressible viscous turbulent flows over an anisotropic porous wall. Effects of the anisotropic porous wall on turbulence modifications as well as on the turbulent drag reduction are investigated. The simulation is carried out at a friction Reynolds number of 180, which is based on the averaged friction velocity at the interface between the porous medium and the clear fluid domain. The depth of the porous layer ranges from 0.9 to 54 viscous units. The permeability in the spanwise direction is set to be lower than the other directions in the present simulation. The maximum drag reduction obtained is about 15.3% which occurs for a depth of 9 viscous units. The increasing of drag is addressed when the depth of the porous layer is more than 25 wall units. The thinner porous layer restricts the spanwise extension of the streamwise vortices which suppresses the bursting events near the wall. However, for the thicker porous layer, the wall-normal fluctuations are enhanced due to the weakening of the wall-blocking effect which can trigger strong turbulent structures near the wall.
文摘The nanophotocatalysts were synthesized in four stages and evaluated by FTIR, FESEM and VSM analysis. The influence of nanofluids containing functionalized magnetic Ti O2 nanophotocatalyst and dipalmitoylphosphatidylcholine lecithin in drag reduction of turbulent flow in four horizontal pipelines was studied. The effective parameters on drag reduction(nanoparticle concentration, surfactant concentration, p H and Re number) were investigated and optimized in each pipeline using response surface method. The drag reduction in 1/2 " galvanized, 3/4" galvanized, 1/2 "five-layer and 1/2" cuprous pipelines was found 99.1%, 92.5%, 87.6% and 85.2%, respectively. The model adequacy was measured using ANOVA. Based on the high determination coefficient, more than 95% of variance of experimental data in all pipelines was described by quadratic model.
基金supported by the National Natural Science Foundation of China (11272233)National Basic Research Program (973 Program) (2012CB720101)2012 opening subjects of The State Key Laboratory of Nonlinear Mechanics (LNM), Institute of Mechanics, Chinese Academy of Sciences
文摘Fully developed turbulent flow fields with and without polymer solution at the same Reynolds number were measured by time-resolved particle image velocimetry (TRPIV) in a water channel to investigate the mechanism of drag-reducing solution from the view of coherent structures manipulation. The streamwise mean velocity and Reynolds stress profiles in the solution were compared with those in water. After adding the polymer solution, the Reynolds stress in the near-wall area decreases significantly. The result relates tightly to the decease of the coherent structures' bursting. The spatial topology of coherent structures during bursts has been extracted by the new mu-level criterion based on locally averaged velocity structure function. The effect of polymers on turbulent coherent structures mainly reflects in the intensity, not in the shape. In the solution, it is by suppressing the coherent structures that the wall friction is reduced.
文摘Numerical simulations and experimental research are both carried out to investigate the controlled effect of spanwise oscillating Lorentz force on a turbulent channel flow. The variations of the streaks and the skin friction drag are obtained through the PIV system and the drag measurement system, respectively. The flow field in the near-wall region is shown through direct numerical simulations utilizing spectral method. The experimental results are consistent with the numerical simulation results qualitatively, and both the results indicate that the streaks are tilted into the spanwise direction and the drag reduction utilizing spanwise oscillating Lorentz forces can be realized. The numerical simulation results reveal more detail of the drag reduction mechanism which can be explained, since the spanwise vorticity generated from the interaction between the induced Stokes layer and intrinsic turbulent flow in the near-wall region can make the longitudinal vortices tilt and oscillate, and leads to turbulence suppression and drag reduction.
文摘In this paper, the characteristics of the three-dimensional flow field around the circular cylinder members forming a square biplane grid were experimentally investigated by using a wind tunnel and a water tunnel. In the wind tunnel testing, the span wise and circumference pressure distributions of surface on the circular cylinder were measured on the center mesh members formed by biplane grid in detail. Local drag coefficient was calculated from the surface pressure distributions. In addition, the flow visualization was performed in the water tunnel. As a result, it was suggested that the flow penetrating the contact region produced secondary-flow behind the biplane grid. Accordingly, the drag reduction would be caused by the presence of the secondary-flow.
文摘The drag reducing effect of polymer additive aqueous solution was investigated in flow boiling, and the polymer additives were two kinds of polyacrylamide (PAM) with relative molecular mass about 2.56×10 6 and 8.55×10 6. The frictional pressure drop was calculated according to the measured total pressure drop. The results show that the flow drag of flow boiling is reduced by adding a small amount of PAM to water when heat flux is in the range of 15.1 kW·m -2 to 47.0 kW·m -2 , when the mass fraction of PAM is higher than 2.0×10 -5 , the drag reducing effect is obvious. Drag reducing effect of PAM, whose relative molecular mass is 8.55×10 6, is slightly better than that of 2.56×10 6 at the same mass fraction, and the greater the flow rate of the additive solution, the better the effect of the drag reduction.
基金Project (10672069) supported by the National Natural Science Foundation of China
文摘<Abstract>The influence of rheological parameters on vortex dynamics and the extent of drag reduction(DR) were deciphered via extensively analyzing the hi-fidelity direct numerical simulation results of the turbulent channel flow with polymer solutions.It has been observed that in all drag reduction regimes from the onset of DR to maximum drag reduction(MDR) limit,the Deborah number is defined as the product of an effective Weissenberg number,and the root mean square streamwise vorticity fluctuation remains O(1) in the near wall region.The ratio of the average lifetime of axial vortices to the vortex rotating duration decreases with increasing DR,and MDR is achieved when these time scales become nearly equal.Based on these observations a simple framework is proposed adequately to describe the influence of polymer additives on the extent of DR from onset to MDR as well as the universality of the MDR in flow systems with polymer additives.