The characteristics of turbulent boundary layer over streamwise aligned drag reducing riblet surface under zero-pressure gradient are investigated using particle image velocimetry. The formation and distribution of la...The characteristics of turbulent boundary layer over streamwise aligned drag reducing riblet surface under zero-pressure gradient are investigated using particle image velocimetry. The formation and distribution of large-scale coherent structures and their effect on momentum partition are analyzed using two-point correlation and probability density function. Compared with smooth surface, the streamwise riblets reduce the friction velocity and Reynolds stress in the turbulent boundary layer, indicating the drag reduction effect. Strong correlation has been found between the occurrence of hairpin vortices and the momentum distribution. The number and streamwise length scale of hairpin vortices decrease over streamwise riblet surface. The correlation between number of uniform momentum zones and Reynolds number remains the same as smooth surface.展开更多
This paper presents a method based on riblet surfaces. Its advantage lies in that it is more with testing methods using instruments such as a the Spalding formula for testing drag-reduction on convenient and yields mo...This paper presents a method based on riblet surfaces. Its advantage lies in that it is more with testing methods using instruments such as a the Spalding formula for testing drag-reduction on convenient and yields more precise data compared scale. With this method, data is obtained from the velocity distribution within the inner layer, nearest the riblet surface. Precision of measurement of the velocity distribution is the key factor affecting the precision of the testing.展开更多
This work primarily focuses on the drag reduction characteristics and mechanism investigation of oblique riblets. First, a calculation model of the oblique riblets surface is established, and Reynolds stress model(RSM...This work primarily focuses on the drag reduction characteristics and mechanism investigation of oblique riblets. First, a calculation model of the oblique riblets surface is established, and Reynolds stress model(RSM) turbulence model is used for numerical simulation of the oblique riblets flow field. Subsequently, the influence of inclination angle between flow direction and arrangement direction of riblets on friction resistance and drag reduction rate is further analyzed. Through the investigation of the distribution of shear stress, pressure stress and velocity in oblique riblets boundary layer, the oblique riblets drag reduction mechanism is finally revealed. Results show that, with increase of velocity and inclination angle, the pressure resistance increases obviously, along with the decreasing of the viscous resistance distinctly. The maximum drag reduction rate of the oblique riblets is 7.33%. Moreover, when the inclination angle increases, the wall shear stress reduces on oblique riblets surface; while differential pressure increases at both sides of oblique riblets tips. In addition, when inclination angle is small, the secondary vortex at oblique riblets tips will disappear soon. New vortices will be formed inside the oblique riblets and cause the decrease of viscosity resistance. Thus, oblique riblets show a better drag reduction effect and have an effective control on boundary layer.展开更多
Direct numerical simulations (DNS) of turbulent flow over a drag-reducing and a drag-increasing riblet configuration are performed. Three-dimensional two-point statistics are presented for the first time to quantify t...Direct numerical simulations (DNS) of turbulent flow over a drag-reducing and a drag-increasing riblet configuration are performed. Three-dimensional two-point statistics are presented for the first time to quantify the interaction of the riblet surfaces with the coherent, energy-bearing eddy structures in the near-wall region. Results provide statistical evidence that the averaged organization of the streamwise vortices in the drag-reducing case is lifted above the riblet tip, while in the drag-increasing case the streamwise vortices are embedded further into the riblet cove. In the spanwise direction, the cores of the streamwise vortices over the riblet surfaces are shown to be closer to each other than those for flow over the smooth wall, and wider riblet spacing leads to more reduction on their spanwise distances. In the cases with riblets the streamwise vortices have longer streamwise lengths, but their inclination angles do not change much.展开更多
Time-resolved particle image velocimetry（TRPIV） experiments are performed to investigate the coherent structure＇s performance of riblets in a turbulent boundary layer（TBL） at a friction Reynolds number of 185. To...Time-resolved particle image velocimetry（TRPIV） experiments are performed to investigate the coherent structure＇s performance of riblets in a turbulent boundary layer（TBL） at a friction Reynolds number of 185. To visualize the energetic large-scale coherent structures（CSs） over a smooth surface and riblets, the proper orthogonal decomposition（POD） and finite-time Lyapunov exponent（FTLE） are used to identify the CSs in the TBL. Spatial-temporal correlation is implemented to obtain the characters and transport properties of typical CSs in the FTLE fields. The results demonstrate that the generic flow structures, such as hairpin-like vortices, are also observed in the boundary layer flow over the riblets, consistent with its smooth counterpart. Low-order POD modes are more sensitive to the riblets in comparison with the high-order ones,and the wall-normal movement of the most energy-containing structures are suppressed over riblets. The spatial correlation analysis of the FTLE fields indicates that the evolution process of the hairpin vortex over riblets are inhibited. An apparent decrease of the convection velocity over riblets is noted, which is believed to reduce the ejection/sweep motions associated with high shear stress from the viscous sublayer. These reductions exhibit inhibition of momentum transfer among the structures near the wall in the TBL flows.展开更多
The effect of riblets surface on flat plate boundary layer development hasbeen measured with laser Doppler velocimeter (LDV). In the present experiment, although the development of boundary layer can be divided into t...The effect of riblets surface on flat plate boundary layer development hasbeen measured with laser Doppler velocimeter (LDV). In the present experiment, although the development of boundary layer can be divided into three regions, the laminarboundary layer region is increased, and the transition occurs at a larger Reynolds number.The Reynolds number of transition to turbulence Rextr is 4. 3 tinies as great as that in thesmooth flat plate case. In the turbulent boundary layer region, greater value of integralconstant C is obtained in log-law for velocity distribution. This indicates that the ribletssurface has obviously drag reduction characteristics. The maximum value of turbulence intensity and its position are less than the smooth surface results in the turbulent boundarylayer region.展开更多
This research investigates the aerodynamic performance and flow characteristics of a delta wing with 65° sweep angle and with coarse axial riblets,and then compares with that of a smooth-surface delta wing.Partic...This research investigates the aerodynamic performance and flow characteristics of a delta wing with 65° sweep angle and with coarse axial riblets,and then compares with that of a smooth-surface delta wing.Particle Image Velocimetry(PIV)were utilized to visualize the flow over the wing at 6 cross-sections upright to the wing surface and parallel to the wing span,as well as 3 longitudinal sections on the leading edge,symmetry plane,and a plane between them at Angles of Attack(AOA)=20°and 30°and Re=1.2×10~5,2.4×10~5,and 3.6×10~5.The effects of the riblets were studied on the vortices diameter,vortex breakdown location,vortices distance from the wing surface,flow lines pattern nearby the wing,circulation distribution,and separation.The results show that the textured model has a positive effect on some of the parameters related to drag reduction and lift increase.The riblets increase the flow momentum near the wing’s upper surface except near the apex.They also increase the flow momentum behind the wing.展开更多
Nature has shown us that the microstructure of the skin of fast-swimming sharks in the ocean can reduce the skin friction drag due to the well-known shark-skin effect.In the present study,the effect of shark-skin-insp...Nature has shown us that the microstructure of the skin of fast-swimming sharks in the ocean can reduce the skin friction drag due to the well-known shark-skin effect.In the present study,the effect of shark-skin-inspired riblets on coherent vortex structures in a turbulent boundary layer（TBL） is investigated.This is done by means of tomographic particle image velocimetry（TPIV） measurements in channel fl ws over an acrylic plate of drag-reducing riblets at a friction Reynolds number of 190.The turbulent fl ws over drag-reducing riblets are verifie by a planar time-resolved particle image velocimetry（TRPIV） system initially,and then the TPIV measurements are performed.Two-dimensional（2D） experimental results with a dragreduction rate of around 4.81% are clearly visible over triangle riblets with a peak-to-peak spacing s＋of 14,indicating from the drag-reducing performance that the buffer layer within the TBL has thickened;the logarithmic law region has shifted upward and the Reynolds shear stress decreased.A comparison of the spatial topological distributions of the spanwise vorticity of coherent vortex structures extracted at different wall-normal heights through the improved quadrant splitting method shows that riblets weaken the amplitudesof the spanwise vorticity when ejection（Q2） and sweep（Q4） events occur at the near wall,having the greatest effect on Q4 events in particular.The so-called quadrupole statistical model for coherent structures in the whole TBL is verified Meanwhile,their spatial conditional-averaged topological shapes and the spatial scales of quadrupole coherent vortex structures as a whole in the overlying turbulent fl w over riblets are changed,suggesting that the riblets dampen the momentum and energy exchange between the regions of near-wall and outer portion of the TBL by depressing the bursting events（Q2 and Q4）,thereby reducing the skin friction drag.展开更多
It is commonly known that riblets with sharper tip generally have better turbulent drag reduction capacity,which,however,poses great challenges for manufacturing and makes the riblets vulnerable to tip erosion.In this...It is commonly known that riblets with sharper tip generally have better turbulent drag reduction capacity,which,however,poses great challenges for manufacturing and makes the riblets vulnerable to tip erosion.In this study,we show that a scalloped riblet which is not as sharp in the tip as corresponding triangular riblet with same height-width ratio,nevertheless has a larger protrusion height,a quantity solely depending on the riblet shape and calculated through a boundary element algorithm in this study,and thus a higher projected drag reduction rate.In addition,it is found that,when subjected to tip rounding,this scalloped riblet performs better in terms of protrusion height than corresponding parabolic riblet,which indicates stronger resilience to riblet tip erosion.With the class of scalloped riblets,designed by smoothly connecting two third-order polynomials and thus the tip sharpness and valley curvature can be well defined,it is revealed that two mechanisms,one for the valley curvature at the viscous limit and one for the tip sharpness at infinite deep limit,determine the protrusion height,and thus the projected drag reduction capacity.Direct numerical simulations are then carried out to investigate controlled boundary layer transition with the scalloped riblet of width s+=20 and 5+=60.A 7.8%drag reduction in the turbulent region is found for the smaller riblet with a preferable transition delay,while for the larger riblet transition is promoted and drag is increased in the turbulent region.It is also found that the area fraction of high drag region around the riblet tips is basically the same for the two cases.Surprisingly,even higher drag is found around the tip region for the smaller drag-reducing riblets.On the other hand,a much smaller drag coefficient is found in the valley of the smaller riblet,which results in the reduction of turbulent drag.It is thus inferred that the issue of sharp riblet tip,that hard to manufacture and deteriorate substantially when subjected to tip erosion,could be mitigated by optimization of the riblet geometry.展开更多
Drag reducing and increasing mechanism on riblet surface has been studied through computational fluid dynamics(CFD).Drag reduction is achieved through the optimization of riblet geometry which would affect flow struct...Drag reducing and increasing mechanism on riblet surface has been studied through computational fluid dynamics(CFD).Drag reduction is achieved through the optimization of riblet geometry which would affect flow structure inside riblet grooves.Force and flow structure on riblet surface are analyzed and compared with those of smooth surface based on the k-εturbulence model.Drag reducing and increasing mechanism is proved to be related to microvortexes induced inside riblets which lead to Reynolds shear stress reduction significantly and is considered to be the dominant factor resulting in wall friction reduction.Simulation results also show that the pressure drag generating from the deviation of static pressure on the front and rear ends of riblets occurs and grows exponentially with Mach number,which can cause drag increasing.Furthermore,near-wall vortical structures,Reynolds shear stress and static pressure on riblet surfaces are also analyzed in detail.展开更多
This paper studies the riblet drag reduction effect for an infinite swept wing under a low Reynolds number using a large-eddy simulation.The results show that the drag reduction ratio is not linear under different swe...This paper studies the riblet drag reduction effect for an infinite swept wing under a low Reynolds number using a large-eddy simulation.The results show that the drag reduction ratio is not linear under different sweep angles.The maximum drag reduction ratio in this study is 9.5%for a wing with a 45°sweep angle.The local surface streamline angle and turbulence quantities are calculated to analyze the drag reduction mechanism.The results demonstrate that the riblets considerably suppress the Reynolds stresses above the wing upper surface,while the turbulence kinetic energy in the near wake is increased.A possible relaminarization phenomenon is observed at the middle part of the wing.Quasi-two-dimensional flow structures are observed near the wall,and a peak frequency is considered as the dominant frequency of the region.展开更多
Reducing drag during take-off and nominal(cruise)conditions is a problem of fundamental importance in aeronautical engineering.Existing studies have demonstrated that v-shaped symmetrical riblets can effectively be us...Reducing drag during take-off and nominal(cruise)conditions is a problem of fundamental importance in aeronautical engineering.Existing studies have demonstrated that v-shaped symmetrical riblets can effectively be used for turbulence control,with those with dimensionless depth h+=15 and dimensionless width s+=15 having the best drag reduction effect.In the present study,experimental tests have been conducted considering two models of the same size,one with smooth surface,the other with v-shaped riblets of the h+=15 and s+=15 type.The results show that for an angle of attack in the 8°~20°range(take-off stage),the maximum lift coefficient can be increased by 22%.For angle of attack between 8°and 14°,a drag reduction effect can be produced using riblets,which increases with the Reynolds number,leading to a decrease in the drag coefficient maximum of 36%.Flow visualization experiments have been carried out by means of Laser Induced fluorescence.展开更多
The time series of velocity vector fields and their statistics in the turbulent boundary layer(TBL)over riblets and smooth plate were measured by utilizing a time-resolved particle image velocimetry(TR-PIV)system. The...The time series of velocity vector fields and their statistics in the turbulent boundary layer(TBL)over riblets and smooth plate were measured by utilizing a time-resolved particle image velocimetry(TR-PIV)system. The mean velocity profiles of the TBL were compared in the case of 0.13 m/s(the riblets with dimensionless peakto-peak spacing being approximately s?≈21)and 0.19 m/s( s?≈28)for these two kinds of plates, respectively. Two kinds of drag-reducing velocity profiles were illustrated and analyzed. Then the spatial topologies of the physical vorticity for the coherent spanwise structures were detected and extracted at the fourth scale by utilizing an improved quadrant splitting method(IQSM). Results revealed that nearly 6.17%, and 10.73%, of a drag reduction was separately achieved over the riblets surface. Besides, it was visualized that the drag-reduction was acquired by the riblets influencing the bursting ejection(Q2)and sweep(Q4)events of the coherent spanwise vortex structures, the Q4 events in particular. Based on such two drag-reducing cases of the riblets, lastly, a simplified KelvinHelmholtz-like linear instability model proposed initially by García-Mayoral and Jiménez(2011)has been discussed. It is still difficult to establish with certainty whether the observed phenomena, the appearance of coherent spanwise structures found at around or below y?≈20 in both cases of s?≈21 and s?≈28 and their topological changes, were consequences or causes of the breakdown of the viscous regime. We prefer to suggest that the interactions between those structures and the riblets, which contain the coherent spanwise structures extending toward the wall and penetrating into the riblet grooves, are the root causes.展开更多
The extensive investigations during the last decades have verified the potential of ideal shaped riblet geometries for viscous drag reduction in the turbulent boundary layer.Basic requirement is an adaption of the rib...The extensive investigations during the last decades have verified the potential of ideal shaped riblet geometries for viscous drag reduction in the turbulent boundary layer.Basic requirement is an adaption of the riblet dimensions in terms of the local flow conditions.However,due to the complex manufacturing process of micro-scale surface structures most experimental measurements were conducted using riblets with constant dimensions.Therefore,the drag reducing potential of riblets could not be exploited.Thanks to the rapid development in micro structuring technologies it is now possible to manufacture continuously adapted riblets in almost industrial processing scales.To determine an appropriated area for the riblet application and continuously adapted riblet dimensions for the National Renewable Energy Laboratory(NREL)airfoil S809 information on the location of the laminar-turbulent transition and the wall shear stress distribution are required.For this purpose,numerical simulations were conducted using the transitional kkL-ωandγ-ReΘturbulence models as well as the fully turbulent SST implementation.Both transitional models show a sufficient precise prediction of the transition onset location in comparison with experimental data.Depending on the applied turbulence model computed riblet dimensions exhibit a certain deviation whereby a significant effect on the drag reduction can be expected.展开更多
A new passive control approach,utilizing bionic slanting riblets,is employed to mitigate the flow close to the blade endwall in a linear cascade,and its effectiveness and mechanism in controlling corner separation are...A new passive control approach,utilizing bionic slanting riblets,is employed to mitigate the flow close to the blade endwall in a linear cascade,and its effectiveness and mechanism in controlling corner separation are investigated through numerical simulations.The slanting riblets are positioned at the endwall upstream of the cascade channel,and the influence of riblet height,yaw angle and relative position on the control of corner separation is investigated.The findings indicate that the application of slanting riblets can efficiently counteract corner separation across the stable operational range.Specifically,the introduction of riblets with a height of merely 0.1 times the boundary layer thickness results in a significant reduction in total pressure loss by up to 14.53%,while simultaneously enhancing the static pressure coefficient by 21.74%.Flow analysis reveals that minute vortices produced within the riblet channels tend to coalesce,forming a potent large-scale vortex near the boundary layer's base downstream.This phenomenon results in reduced additional losses compared to conventional vortex generators.Additionally,the induced vortex promotes enhanced mixing between the mainstream flow and boundary layer,inhibiting the lateral displacement of low-energy fluids within the endwall boundary layer.Consequently,this delays the onset of separation vortex formation and eliminates vortex rings in the corner region,ultimately enhancing the aerodynamic efficiency of the cascade.展开更多
基金supported by the National Natural Science Foundation of China (Nos. 11721202 and 11672020)
文摘The characteristics of turbulent boundary layer over streamwise aligned drag reducing riblet surface under zero-pressure gradient are investigated using particle image velocimetry. The formation and distribution of large-scale coherent structures and their effect on momentum partition are analyzed using two-point correlation and probability density function. Compared with smooth surface, the streamwise riblets reduce the friction velocity and Reynolds stress in the turbulent boundary layer, indicating the drag reduction effect. Strong correlation has been found between the occurrence of hairpin vortices and the momentum distribution. The number and streamwise length scale of hairpin vortices decrease over streamwise riblet surface. The correlation between number of uniform momentum zones and Reynolds number remains the same as smooth surface.
基金Supported by National Natural Science Foundation (50835009 and 10672136)Science and Technology Innovation Foundation of NWPU(2008KJ02012)
文摘This paper presents a method based on riblet surfaces. Its advantage lies in that it is more with testing methods using instruments such as a the Spalding formula for testing drag-reduction on convenient and yields more precise data compared scale. With this method, data is obtained from the velocity distribution within the inner layer, nearest the riblet surface. Precision of measurement of the velocity distribution is the key factor affecting the precision of the testing.
基金Project(51476144)supported by the National Natural Science Foundation of ChinaProject(LQ15E050005)supported by the Zhejiang Provincial Natural Science Foundation of China+2 种基金Project(2017C31025)supported by Zhejiang Province Department Public Welfare Industrial Projects,ChinaProject(2016M601736)supported by China Postdoctoral Science FoundationProject(1601028 C)supported by Postdoctoral Research Funding Plan in Jiangsu Province,China
文摘This work primarily focuses on the drag reduction characteristics and mechanism investigation of oblique riblets. First, a calculation model of the oblique riblets surface is established, and Reynolds stress model(RSM) turbulence model is used for numerical simulation of the oblique riblets flow field. Subsequently, the influence of inclination angle between flow direction and arrangement direction of riblets on friction resistance and drag reduction rate is further analyzed. Through the investigation of the distribution of shear stress, pressure stress and velocity in oblique riblets boundary layer, the oblique riblets drag reduction mechanism is finally revealed. Results show that, with increase of velocity and inclination angle, the pressure resistance increases obviously, along with the decreasing of the viscous resistance distinctly. The maximum drag reduction rate of the oblique riblets is 7.33%. Moreover, when the inclination angle increases, the wall shear stress reduces on oblique riblets surface; while differential pressure increases at both sides of oblique riblets tips. In addition, when inclination angle is small, the secondary vortex at oblique riblets tips will disappear soon. New vortices will be formed inside the oblique riblets and cause the decrease of viscosity resistance. Thus, oblique riblets show a better drag reduction effect and have an effective control on boundary layer.
基金the National Basic Research Program of China (973 program)(Grants 2014CB744802 and 2014CB744804)the National Natural Science Foundation of China (Grants 11772194 and 91441205).
文摘Direct numerical simulations (DNS) of turbulent flow over a drag-reducing and a drag-increasing riblet configuration are performed. Three-dimensional two-point statistics are presented for the first time to quantify the interaction of the riblet surfaces with the coherent, energy-bearing eddy structures in the near-wall region. Results provide statistical evidence that the averaged organization of the streamwise vortices in the drag-reducing case is lifted above the riblet tip, while in the drag-increasing case the streamwise vortices are embedded further into the riblet cove. In the spanwise direction, the cores of the streamwise vortices over the riblet surfaces are shown to be closer to each other than those for flow over the smooth wall, and wider riblet spacing leads to more reduction on their spanwise distances. In the cases with riblets the streamwise vortices have longer streamwise lengths, but their inclination angles do not change much.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11332006,11732010,11572221,and 11502066)the Natural Science Foundation of Tianjin City(Grant No.18JCQNJC5100)
文摘Time-resolved particle image velocimetry（TRPIV） experiments are performed to investigate the coherent structure＇s performance of riblets in a turbulent boundary layer（TBL） at a friction Reynolds number of 185. To visualize the energetic large-scale coherent structures（CSs） over a smooth surface and riblets, the proper orthogonal decomposition（POD） and finite-time Lyapunov exponent（FTLE） are used to identify the CSs in the TBL. Spatial-temporal correlation is implemented to obtain the characters and transport properties of typical CSs in the FTLE fields. The results demonstrate that the generic flow structures, such as hairpin-like vortices, are also observed in the boundary layer flow over the riblets, consistent with its smooth counterpart. Low-order POD modes are more sensitive to the riblets in comparison with the high-order ones,and the wall-normal movement of the most energy-containing structures are suppressed over riblets. The spatial correlation analysis of the FTLE fields indicates that the evolution process of the hairpin vortex over riblets are inhibited. An apparent decrease of the convection velocity over riblets is noted, which is believed to reduce the ejection/sweep motions associated with high shear stress from the viscous sublayer. These reductions exhibit inhibition of momentum transfer among the structures near the wall in the TBL flows.
文摘The effect of riblets surface on flat plate boundary layer development hasbeen measured with laser Doppler velocimeter (LDV). In the present experiment, although the development of boundary layer can be divided into three regions, the laminarboundary layer region is increased, and the transition occurs at a larger Reynolds number.The Reynolds number of transition to turbulence Rextr is 4. 3 tinies as great as that in thesmooth flat plate case. In the turbulent boundary layer region, greater value of integralconstant C is obtained in log-law for velocity distribution. This indicates that the ribletssurface has obviously drag reduction characteristics. The maximum value of turbulence intensity and its position are less than the smooth surface results in the turbulent boundarylayer region.
基金supported by the Brain Pool Program through the Korean Federation of Science and Technology Societies (KOFST), which is funded by the Ministry of Science, ICT and Future Planningprovided by the National Research Foundation of Korea (NRF) grant, which is funded by the Korean government (MSIT) (Nos. 2011-0030013, 2018R1A2B2007117 and NRF-2017K1A3A1A30084513)
文摘This research investigates the aerodynamic performance and flow characteristics of a delta wing with 65° sweep angle and with coarse axial riblets,and then compares with that of a smooth-surface delta wing.Particle Image Velocimetry(PIV)were utilized to visualize the flow over the wing at 6 cross-sections upright to the wing surface and parallel to the wing span,as well as 3 longitudinal sections on the leading edge,symmetry plane,and a plane between them at Angles of Attack(AOA)=20°and 30°and Re=1.2×10~5,2.4×10~5,and 3.6×10~5.The effects of the riblets were studied on the vortices diameter,vortex breakdown location,vortices distance from the wing surface,flow lines pattern nearby the wing,circulation distribution,and separation.The results show that the textured model has a positive effect on some of the parameters related to drag reduction and lift increase.The riblets increase the flow momentum near the wing’s upper surface except near the apex.They also increase the flow momentum behind the wing.
基金supported by the National Natural Science Foundation of China (Grants 11332006,11272233,and 11411130150)the foundation from the China Scholarship Council (CSC) (Grant 201306250092)the Foundation Project for Outstanding Doctoral Dissertations of Tianjin University
文摘Nature has shown us that the microstructure of the skin of fast-swimming sharks in the ocean can reduce the skin friction drag due to the well-known shark-skin effect.In the present study,the effect of shark-skin-inspired riblets on coherent vortex structures in a turbulent boundary layer（TBL） is investigated.This is done by means of tomographic particle image velocimetry（TPIV） measurements in channel fl ws over an acrylic plate of drag-reducing riblets at a friction Reynolds number of 190.The turbulent fl ws over drag-reducing riblets are verifie by a planar time-resolved particle image velocimetry（TRPIV） system initially,and then the TPIV measurements are performed.Two-dimensional（2D） experimental results with a dragreduction rate of around 4.81% are clearly visible over triangle riblets with a peak-to-peak spacing s＋of 14,indicating from the drag-reducing performance that the buffer layer within the TBL has thickened;the logarithmic law region has shifted upward and the Reynolds shear stress decreased.A comparison of the spatial topological distributions of the spanwise vorticity of coherent vortex structures extracted at different wall-normal heights through the improved quadrant splitting method shows that riblets weaken the amplitudesof the spanwise vorticity when ejection（Q2） and sweep（Q4） events occur at the near wall,having the greatest effect on Q4 events in particular.The so-called quadrupole statistical model for coherent structures in the whole TBL is verified Meanwhile,their spatial conditional-averaged topological shapes and the spatial scales of quadrupole coherent vortex structures as a whole in the overlying turbulent fl w over riblets are changed,suggesting that the riblets dampen the momentum and energy exchange between the regions of near-wall and outer portion of the TBL by depressing the bursting events（Q2 and Q4）,thereby reducing the skin friction drag.
基金the National Natural Science Foundation of China(Grant No.11702159)the EU-China Joint Project Drag Reduction via Turbulent Boundary Layer Flow Control(Grant No.690623).
文摘It is commonly known that riblets with sharper tip generally have better turbulent drag reduction capacity,which,however,poses great challenges for manufacturing and makes the riblets vulnerable to tip erosion.In this study,we show that a scalloped riblet which is not as sharp in the tip as corresponding triangular riblet with same height-width ratio,nevertheless has a larger protrusion height,a quantity solely depending on the riblet shape and calculated through a boundary element algorithm in this study,and thus a higher projected drag reduction rate.In addition,it is found that,when subjected to tip rounding,this scalloped riblet performs better in terms of protrusion height than corresponding parabolic riblet,which indicates stronger resilience to riblet tip erosion.With the class of scalloped riblets,designed by smoothly connecting two third-order polynomials and thus the tip sharpness and valley curvature can be well defined,it is revealed that two mechanisms,one for the valley curvature at the viscous limit and one for the tip sharpness at infinite deep limit,determine the protrusion height,and thus the projected drag reduction capacity.Direct numerical simulations are then carried out to investigate controlled boundary layer transition with the scalloped riblet of width s+=20 and 5+=60.A 7.8%drag reduction in the turbulent region is found for the smaller riblet with a preferable transition delay,while for the larger riblet transition is promoted and drag is increased in the turbulent region.It is also found that the area fraction of high drag region around the riblet tips is basically the same for the two cases.Surprisingly,even higher drag is found around the tip region for the smaller drag-reducing riblets.On the other hand,a much smaller drag coefficient is found in the valley of the smaller riblet,which results in the reduction of turbulent drag.It is thus inferred that the issue of sharp riblet tip,that hard to manufacture and deteriorate substantially when subjected to tip erosion,could be mitigated by optimization of the riblet geometry.
基金Supported by the Specialized Research Fund for the Doctoral Program of Higher Education(20130002120017)the Tribology Sciency Fund of State Key Laboratory of Tribology(SKLTKF12B16)
文摘Drag reducing and increasing mechanism on riblet surface has been studied through computational fluid dynamics(CFD).Drag reduction is achieved through the optimization of riblet geometry which would affect flow structure inside riblet grooves.Force and flow structure on riblet surface are analyzed and compared with those of smooth surface based on the k-εturbulence model.Drag reducing and increasing mechanism is proved to be related to microvortexes induced inside riblets which lead to Reynolds shear stress reduction significantly and is considered to be the dominant factor resulting in wall friction reduction.Simulation results also show that the pressure drag generating from the deviation of static pressure on the front and rear ends of riblets occurs and grows exponentially with Mach number,which can cause drag increasing.Furthermore,near-wall vortical structures,Reynolds shear stress and static pressure on riblet surfaces are also analyzed in detail.
基金supported by the National Natural Science Foundation of China(Nos.:91852108 and 11872230)Open Fund of Key Laboratory of Icing and Anti/De-icing of China(No.:IADL20190201)。
文摘This paper studies the riblet drag reduction effect for an infinite swept wing under a low Reynolds number using a large-eddy simulation.The results show that the drag reduction ratio is not linear under different sweep angles.The maximum drag reduction ratio in this study is 9.5%for a wing with a 45°sweep angle.The local surface streamline angle and turbulence quantities are calculated to analyze the drag reduction mechanism.The results demonstrate that the riblets considerably suppress the Reynolds stresses above the wing upper surface,while the turbulence kinetic energy in the near wake is increased.A possible relaminarization phenomenon is observed at the middle part of the wing.Quasi-two-dimensional flow structures are observed near the wall,and a peak frequency is considered as the dominant frequency of the region.
文摘Reducing drag during take-off and nominal(cruise)conditions is a problem of fundamental importance in aeronautical engineering.Existing studies have demonstrated that v-shaped symmetrical riblets can effectively be used for turbulence control,with those with dimensionless depth h+=15 and dimensionless width s+=15 having the best drag reduction effect.In the present study,experimental tests have been conducted considering two models of the same size,one with smooth surface,the other with v-shaped riblets of the h+=15 and s+=15 type.The results show that for an angle of attack in the 8°~20°range(take-off stage),the maximum lift coefficient can be increased by 22%.For angle of attack between 8°and 14°,a drag reduction effect can be produced using riblets,which increases with the Reynolds number,leading to a decrease in the drag coefficient maximum of 36%.Flow visualization experiments have been carried out by means of Laser Induced fluorescence.
基金Supported by the National Natural Science Foundation of China(No.11332006,No.11272233 and No.11411130150)the foundation from the China Scholarship Council(CSC)(No.201306250092)the Foundation Project for Outstanding Doctoral Dissertations of Tianjin University
文摘The time series of velocity vector fields and their statistics in the turbulent boundary layer(TBL)over riblets and smooth plate were measured by utilizing a time-resolved particle image velocimetry(TR-PIV)system. The mean velocity profiles of the TBL were compared in the case of 0.13 m/s(the riblets with dimensionless peakto-peak spacing being approximately s?≈21)and 0.19 m/s( s?≈28)for these two kinds of plates, respectively. Two kinds of drag-reducing velocity profiles were illustrated and analyzed. Then the spatial topologies of the physical vorticity for the coherent spanwise structures were detected and extracted at the fourth scale by utilizing an improved quadrant splitting method(IQSM). Results revealed that nearly 6.17%, and 10.73%, of a drag reduction was separately achieved over the riblets surface. Besides, it was visualized that the drag-reduction was acquired by the riblets influencing the bursting ejection(Q2)and sweep(Q4)events of the coherent spanwise vortex structures, the Q4 events in particular. Based on such two drag-reducing cases of the riblets, lastly, a simplified KelvinHelmholtz-like linear instability model proposed initially by García-Mayoral and Jiménez(2011)has been discussed. It is still difficult to establish with certainty whether the observed phenomena, the appearance of coherent spanwise structures found at around or below y?≈20 in both cases of s?≈21 and s?≈28 and their topological changes, were consequences or causes of the breakdown of the viscous regime. We prefer to suggest that the interactions between those structures and the riblets, which contain the coherent spanwise structures extending toward the wall and penetrating into the riblet grooves, are the root causes.
文摘The extensive investigations during the last decades have verified the potential of ideal shaped riblet geometries for viscous drag reduction in the turbulent boundary layer.Basic requirement is an adaption of the riblet dimensions in terms of the local flow conditions.However,due to the complex manufacturing process of micro-scale surface structures most experimental measurements were conducted using riblets with constant dimensions.Therefore,the drag reducing potential of riblets could not be exploited.Thanks to the rapid development in micro structuring technologies it is now possible to manufacture continuously adapted riblets in almost industrial processing scales.To determine an appropriated area for the riblet application and continuously adapted riblet dimensions for the National Renewable Energy Laboratory(NREL)airfoil S809 information on the location of the laminar-turbulent transition and the wall shear stress distribution are required.For this purpose,numerical simulations were conducted using the transitional kkL-ωandγ-ReΘturbulence models as well as the fully turbulent SST implementation.Both transitional models show a sufficient precise prediction of the transition onset location in comparison with experimental data.Depending on the applied turbulence model computed riblet dimensions exhibit a certain deviation whereby a significant effect on the drag reduction can be expected.
基金supported by the National Natural Science Foundation of China(Youth Program No.52306058)Natural Science Foundation of Tianjin Municipal Science and Technology Commission(Youth Program No.22JCQNJC00050)+1 种基金the Open Fund of Key Laboratory of Civil Aircraft Airworthiness Technology(No.SH2021111908)the Fundamental Research Funds for the Central Universities(No.3122024PT15)。
文摘A new passive control approach,utilizing bionic slanting riblets,is employed to mitigate the flow close to the blade endwall in a linear cascade,and its effectiveness and mechanism in controlling corner separation are investigated through numerical simulations.The slanting riblets are positioned at the endwall upstream of the cascade channel,and the influence of riblet height,yaw angle and relative position on the control of corner separation is investigated.The findings indicate that the application of slanting riblets can efficiently counteract corner separation across the stable operational range.Specifically,the introduction of riblets with a height of merely 0.1 times the boundary layer thickness results in a significant reduction in total pressure loss by up to 14.53%,while simultaneously enhancing the static pressure coefficient by 21.74%.Flow analysis reveals that minute vortices produced within the riblet channels tend to coalesce,forming a potent large-scale vortex near the boundary layer's base downstream.This phenomenon results in reduced additional losses compared to conventional vortex generators.Additionally,the induced vortex promotes enhanced mixing between the mainstream flow and boundary layer,inhibiting the lateral displacement of low-energy fluids within the endwall boundary layer.Consequently,this delays the onset of separation vortex formation and eliminates vortex rings in the corner region,ultimately enhancing the aerodynamic efficiency of the cascade.
