A relatively high aerodynamic drag is an important factor that hinders the further acceleration of high-speed trains.Using the shear stress transport(SST)k-ωturbulence model,the effect of various vortex generator typ...A relatively high aerodynamic drag is an important factor that hinders the further acceleration of high-speed trains.Using the shear stress transport(SST)k-ωturbulence model,the effect of various vortex generator types on the aerodynamic characteristics of an ICE2(Inter-city Electricity)train has been investigated.The results indi-cate that the vortex generators with wider triangle,trapezoid,and micro-ramp arranged on the surface of the tail car can significantly change the distribution of surface pressure and affect the vorticity intensity in the wake.This alteration effectively reduces the resistance of the tail car.Meanwhile,the micro-ramp vortex generator with its convergent structure at the rear exhibits enhancedflow-guiding capabilities,resulting in a 15.4%reduction in the drag of the tail car.展开更多
The performance of compact,aggressive ducts in advanced propulsion systems is limited by the internal flow separation coupled with the formation of secondary counter-rotating vortices that give rise to intensive flow ...The performance of compact,aggressive ducts in advanced propulsion systems is limited by the internal flow separation coupled with the formation of secondary counter-rotating vortices that give rise to intensive flow distortions at the duct exit.An experimental investigation was conducted to study the flow field and passive suppression of flow separation and Aerodynamic Interface Plane (AIP) distortion in a serpentine air inlet duct.Tests were performed by a turbofan engine at several Engine Operating Points (EOPs) from 56%(idle) to 100%(max).A large total pressure deficit region arose at the upper part of the AIP,which was associated with the upper surface flow separation.Using the new mechanical S-type vortex generators in two longitudinal positions (VGl and VG2),separation and loss were diminished at the upper part of the duct and AIP.The VG2 arrangement attained the maximum reductions in distortion coefficients which were 73.72%,60.7% and 37.8% in DC(90°),DC(60°) and△P_C/P metrics,respectively.In the next step of the study,some unsteady aspects of the flow field were analyzed inside the duct.The separation onset and reattachment points were determined by the standard deviation of static pressure on the upper surface.The AIP spectral distribution showed that the boundary of low pressure and high pressure recovery regions was dominated by the maximum fluctuations.Furthermore,the PSD diagram of several probes at AIP revealed the vortex shedding frequency and its higher harmonics at separation region.The energy content of distinct unsteady spectral features in the bare configuration was significantly reduced using VGs,which showed the improvement of flow at the duct exit.展开更多
The hydrothermal behavior of air inside a solar channel heat exchanger equipped with various shaped ribs is analyzed numerically.The bottom wall of the exchanger is kept adiabatic,while a constant value of the tempera...The hydrothermal behavior of air inside a solar channel heat exchanger equipped with various shaped ribs is analyzed numerically.The bottom wall of the exchanger is kept adiabatic,while a constant value of the temperature is set at the upper wall.The duct is equipped with a flat rectangular fin on the upper wall and an upstream V-shaped baffle on the lower wall.Furthermore,five hot wall-attached rib shapes are considered:trapezoidal,square,triangular pointing upstream(type Ⅰ),triangular pointing downstream(type Ⅱ),and equilateral-triangular(type Ⅲ)cross sections.Effects of the flow rates are also inspected for various Reynolds numbers in the turbulent regime(1.2×10^(4)-3.2×10^(4)).The highest performance(η)value is given for the Ⅱ-triangular rib case in all Re values,while the square-shaped ribs show a significant decrease in the η along the achieved Re range.The η value at Remax is 2.567 for the Ⅱ-triangular roughness case.Compared with the other simulated cases,this performance is decreased by about 3.768%in the case of Ⅰ-triangular ribs,15.249% in the case of Ⅲ-triangular ribs,20.802% in the case of trapezoidal ribs,while 27.541% in the case of square ribs,at the same Remax.Also,a comparison ismade with air-heat exchangers that have non-rough walls and contain cross-shaped VGs presented previously,in order to highlight the effectiveness of the rough surface presence in the baffled and finned channels.The obtained results indicated that the triangular-shaped rib(type Ⅱ)has the most significant hydrothermal behavior than the other cases.This indicates the necessity of roughness heat transfer surfaces for finned and baffled channels to improve significantly the performance of the air-heat exchangers they contain.展开更多
Re-engineering the channel heat exchangers(CHEs)is the goal of many recent studies,due to their great importance in the scope of energy transport in various industrial and environmental fields.Changing the internal ge...Re-engineering the channel heat exchangers(CHEs)is the goal of many recent studies,due to their great importance in the scope of energy transport in various industrial and environmental fields.Changing the internal geometry of the CHEs by using extended surfaces,i.e.,VGs(vortex generators),is the most common technique to enhance the efficiency of heat exchangers.This work aims to develop a newdesign of solar collectors to improve the overall energy efficiency.The study presents a new channel design by introducing VGs.The FVM(finite volume method)was adopted as a numerical technique to solve the problem,with the use of Oil/MWCNT(oil/multi-walled carbon nano-tubes)nanofluid to raise the thermal conductivity of the flow field.The study is achieved for a Re number ranging from12×10^(3) to 27×10^(3),while the concentration(φ)of solid particles in the fluid(Oil)is set to 4%.The computational results showed that the hydrothermal characteristics depend strongly on the flow patterns with the presence of VGs within the CHE.Increasing the Oil/MWCNT rates with the presence of VGs generates negative turbulent velocities with high amounts,which promotes the good agitation of nanofluid particles,resulting in enhanced great transfer rates.展开更多
The utilization of vortex generators to increase heat transfer from cylinders installed inside a duct is investigated.In particular,a channel containing eight cylinders with volumetric heat sources is considered for d...The utilization of vortex generators to increase heat transfer from cylinders installed inside a duct is investigated.In particular,a channel containing eight cylinders with volumetric heat sources is considered for different values of the Reynolds number.The effective possibility to use vortex generators with different sizes to increase heat transfer and,consequently,reduce the surface temperature of the cylinders is examined.Also,the amount of pressure drop inside the channel due to the presence of vortex generators is considered and compared with the cases without vortex generators.The results show that although the addition of generators increases the pressure drop,it strongly contributes to increase the heat transfer coefficient inside the duct(up to 80–90%).展开更多
The effect of streamwise vortices generated from passive vortex generators was investigated to manipulate the separation bubble behind the vertical fence. The experiments were carried out in a circulating water channe...The effect of streamwise vortices generated from passive vortex generators was investigated to manipulate the separation bubble behind the vertical fence. The experiments were carried out in a circulating water channel and the velocity fields were measured using 2D and stereoscopic PIV method.The distance between the vortex generator and fence and the effect of the Reynolds number were investigated. In addition,the effect of boundary layer thickness was also investigated. The averaged recirculation lengths were compared with that of uncontrolled fence flow. The results showed the oscillatory variation of recirculation region appeared under the existence of vortex generators. The reduction of the separation bubble became larger when the fence was submerged in the thick boundary layer with increasing the distance between the generator and fence. When the boundary layer is thin,vortex generator can only suppress the separation bubble under the specific condition.展开更多
A numerical model of delta-wing type vortex generator was developed in two steps.The first step was to obtain a parameterized model of the shedding vortex based on delta-wing theory,which relates the geometry paramete...A numerical model of delta-wing type vortex generator was developed in two steps.The first step was to obtain a parameterized model of the shedding vortex based on delta-wing theory,which relates the geometry parameters and flow field parameters to the strength of shedding vortex which directly decides the source term.In the second step,a method was proposed to add source terms into the flow control equations so that the shedding vortex could be simulated numerically.As soon as the numerical model was completed,two cases:One for a plate and another for an airfoil segment were investigated for test.Comparison showed that the flow field structure and aerodynamic performance agreed well with those obtained from cases with real vortex generators.展开更多
Unsteady cavitation causes noise,damage,and performance decline in the marine engineering and fluid machinery systems.Therefore,finding a method to control the cavitation and its destructive effects is important for t...Unsteady cavitation causes noise,damage,and performance decline in the marine engineering and fluid machinery systems.Therefore,finding a method to control the cavitation and its destructive effects is important for the industrial applications.In this work,we proposed a passive method to control the unsteady behavior of transient cavitation at the medium Reynolds number.For this aim,we performed an experimental study using a high-speed camera to analyze the effects of hemispherical vortex generators(VGs)on the cavitation dynamics around a hydrofoil surface.In addition,the pressure pulsations induced by the collapse of the cavity structures in the wake region of the hydrofoil were captured with a pressure transducer mounted on the wall downstream of the hydrofoil.The results showed that the instability behaviors of the cavity structures on the hydrofoil were mitigated using the proposed cavitation passive control method.In addition,the pressure pulsations in the wake region of the hydrofoil were reduced significantly.It can be concluded that the suppression of cavitation instabilities can improve the operating life and reliability of the marine and hydraulic systems.展开更多
For the problem that the flow separation on the flap lowers the aerodynamic performance of high lift system,an investigation was carried out on using micro vortex generators(VGs) to control the separation on flap of t...For the problem that the flow separation on the flap lowers the aerodynamic performance of high lift system,an investigation was carried out on using micro vortex generators(VGs) to control the separation on flap of the swept constant chord half-model(SCCH) high-lift configuration,at a small to medium angle of attack,by experimental and numerical methods.The basic flow characteristics of SCCH landing configuration were analyzed by using numerical method to provide required information for the design of micro VGs.Then,by keeping the cruise configuration intact,the preliminary design procedure and design methods of micro VGs were established.In addition,the micro VGs were designed.The effects of VG's arrangement and geometric parameters,such as the arrangement mode,chordwise position,arrangement angle,height and spanwise distance,on controlling efficiency were investigated by using numerical method.Then the parameters of preliminary VGs were adjusted as the basis configuration for wind tunnel test.The experiments were accomplished in NH-2 wind tunnel for validating the numerical method,as well as obtaining the design principles and methods of micro VGs.The parameters of VGs were also optimized based on the experiments.The experimental results showed that the numerical design method can serve as an efficient and accurate design tool.The lift and drag were increased by 10% and 14%,respectively in landing state,which satisfied the requirements for landing.Finally,it was concluded that the established design principles and methods for micro VGs in this investigation can be used in engineering application.展开更多
The interaction between a shock wave and a boundary layer on a suction side of gas turbine profile,namely Transition Location Effect on Shock Wave Boundary Layer Interaction,was one of main objectives of TFAST project...The interaction between a shock wave and a boundary layer on a suction side of gas turbine profile,namely Transition Location Effect on Shock Wave Boundary Layer Interaction,was one of main objectives of TFAST project.A generic test section in a transonic wind tunnel was designed to carry out such investigations.The design criteria were to reproduce flow conditions on the profile in wind tunnel as the one existing on the suction side of the turbine guide vane.In this paper,the effect of film cooling and jet vortex generators on the shock wave boundary layer interaction and shock induced separation is presented.Numerical results for Explicit Algebraic Reynolds Stress Model with transition modeling are compared with experimental data.展开更多
In this paper,a novel composite heat transfer enhancement technique comprised of louvered fins(LFs)and rectangular wing vortex generators(RWVGs)is proposed to improve the LF side thermal-hydraulic performance of louve...In this paper,a novel composite heat transfer enhancement technique comprised of louvered fins(LFs)and rectangular wing vortex generators(RWVGs)is proposed to improve the LF side thermal-hydraulic performance of louvered fin and flat tube heat exchangers(LFHEs).After validation of the LF side pressure dropΔP and heat transfer coefficient hLFof the baseline by experiments,the numerical method is applied to investigate the influential mechanisms of the RWVG parameters(the number N(7 to 15),attack angleβ(30°to 90°),height H_(VG)(0.8 mm to 2 mm)and width W_(VG)(0.8 mm to 1.2 mm))on the performance of the LFHE in the velocity range of 3 m/s to 10 m/s.Results show that thermal-hydraulic performance of the LFHE is significantly impacted by the RWVGs,and according to the performance evaluation criteria(PEC),the LFHE achieves its optimal thermal-hydraulic performance when N=7,β=45°,H_(VG)=1.8 mm and W_(VG)=1 mm.Compared to the baseline,the maximum,minimum and average increments of PEC for the optimal case are 13.85%,4.67%and 8.39%,respectively.展开更多
Taking the NACA0012 airfoil as the research object,the bio-inspired herringbone groove array,a new passive control method,is applied to relieve the flow separation under the large angle-of-attack conditions,and its ef...Taking the NACA0012 airfoil as the research object,the bio-inspired herringbone groove array,a new passive control method,is applied to relieve the flow separation under the large angle-of-attack conditions,and its effectiveness and mechanism in delaying airfoil stall are investigated by numerical simulations.The herringbone groove array is placed on the airfoil's upper surface near the trailing edge,and the effects of groove depth and yaw angle on the control effect are investigated.The results demonstrate that different designs of herringbone groove array can effectively broaden the stable operating range of the airfoil,and the application of herringbone groove array with a depth of only 0.00135 times the chord length and a yaw angle of 45°can result in a28.57%increase in the stable operating range.The flow details indicate that a pair of induced vortices with the same strength and opposite direction are formed near the converging line under the combined action of the accumulation effect of small-scale vortices in the grooves and the spanwise migration flow above the grooves.The induced vortices increase the mixing between the main flow and the boundary layer,allowing the boundary layer to gain sufficient energy to withstand the adverse pressure gradient at high angles of attack,effectively delaying airfoil stall.展开更多
In order to provide the line-of-sight blockage of the engine face for an advanced Uninhabited Combat Air Vehicle（UCAV）, a highly curved serpentine inlet is proposed and experimentally studied. Based on the static pr...In order to provide the line-of-sight blockage of the engine face for an advanced Uninhabited Combat Air Vehicle（UCAV）, a highly curved serpentine inlet is proposed and experimentally studied. Based on the static pressure distribution measurement along the wall, the flow separation is found at the top wall of the second S duct for the baseline inlet design, which yields a high flow distortion at the exit plane. To improve the flow uniformity, a single array of vortex generators （VGs） is employed within the inlet. In this experimental study, the effects of mass flow ratio, free stream Mach number, angle of attack and yaw on the performance of a serpentine inlet instrumented with VGs are obtained. Results indicate： （1） Compared with the baseline serpentine design without flow control, the application of the VGs promotes the mixing of core flow and the low momentum flow in the boundary layer and thus prevents the flow separation. Under the design condition, the exit flow distortion （-↑△σ0） decreases from 11.7% to 2.3% by using the VGs. （2） With the descent of the free stream Mach number the total pressure loss decreases. However, the circular total pressure distortion increases. When the angle of attack rises from - 4° to 8°, the total pressure recovery and the circular total pressure distortion both go down. In addition, with the increase of yaw the total pressure recovery is fairly constant, while the circular total pressure distortion ascends gradually. （3） When Mao = 0.6-0.8, a = -4°-8° and β = 0°-6°, the total pressure recovery varies between 0.936 and 0. 961, the circular total pressure distortion coefficient varies between 1.4 % and 5.4 % and the synthesis distortion coefficient has a ranges from 3.8 % to 7.0 %. The experimental results confirm the excellent performance of the newly designed serpentine inlet incorporating VGs.展开更多
It has been recently shown that Sub Boundary layer Vortex Generator(SBVG,abbreviated as VG hereafter)can suppress the Cross-Flow(CF),and therefore,can eliminate corner separation and increase aerodynamic loading when ...It has been recently shown that Sub Boundary layer Vortex Generator(SBVG,abbreviated as VG hereafter)can suppress the Cross-Flow(CF),and therefore,can eliminate corner separation and increase aerodynamic loading when installed on the end wall inside middle-load compressor passages.However,when VGs are applied in high-load compressors,it is difficult to achieve ideal results.This is because the definition of the VG attack angle in the presence of CF in existing research is confusing,and the stronger CF in high-load compressors worsens the problem and results in an improper design and optimization range of VG attack angle.Therefore,this paper clarifies the definition of the VG attack angle in the presence of CF and reveals the CF controlling mechanism of VG on a flat plate.The differences in the flow phenomena around a VG both with and without CF are also studied.The numerical results show that a larger height or attack angle of the VG generates a greater CF suppression effect.However,the cross velocity increases when surmounting the primary vortex induced by the VG,except that this enhanced CF is less conspicuous for larger VG heights.Compared to the cases without CF,the VG suffers an additional loss because of the stronger separation and primary vortex loss caused by the CF.展开更多
The fine space-time structure of a vortex generator （VG） in supersonic flow is studied with the nanoparticle-based planar laser scattering （NPLS） method in a quiet supersonic wind tunnel. The fine coherent structu...The fine space-time structure of a vortex generator （VG） in supersonic flow is studied with the nanoparticle-based planar laser scattering （NPLS） method in a quiet supersonic wind tunnel. The fine coherent structure at the symmetrical plane of the flow field around the VG is imaged with NPLS. The spatial structure and temporal evolution characteristics of the vortical structure are analyzed, which demonstrate periodic evolution and similar geometry, and the characteristics of rapid movement and slow change. Because the NPLS system yields the flow images at high temporal and spatial resolutions, from these images the position of a large scale structure can be extracted precisely. The position and velocity of the large scale structures can be evaluated with edge detection and correlation algorithms. The shocklet structures induced by vortices are imaged, from which the generation and development of shocklets are discussed in this paper.展开更多
To investigate the characteristics of vaporized RP-3 kerosene combustion in a scramjet combustor enhanced by the micro vortex generator(MVG),a series of experiments are carried out based on the advanced combustion dia...To investigate the characteristics of vaporized RP-3 kerosene combustion in a scramjet combustor enhanced by the micro vortex generator(MVG),a series of experiments are carried out based on the advanced combustion diagnosis technique.The high-enthalpy incoming flow is accelerated to supersonic through a Mach 2.52 nozzle,the total pressure and temperature of which are 1.6 MPa and 1486 K,respectively.The effect of MVG on the ignition process,flame distribution,and combustor pressure along the bottom wall is well revealed,and the effects of the position and number of MVGs on stable combustion performance are analyzed.The results indicate that the development processes of the initial flame kernel with and without an MVG during ignition process show a similar behavior.The installation of an MVG can lift the shear layer,promote the penetration of flame deeper into the mainstream,and expand the area of the reactive region.Reducing the distance between the MVG and the injection position and increasing the number of MVGs are regarded as effective ways of improving the mixing degree of fuel and air with a resultant intensification of chemical reactions and flame luminescence.The effect of mixing and subsequent combustion is enhanced by shortening the distance between the MVG and the injection position.As the layout schemes of the MVG vary,the pressure distribution between the injection position and the leading edge of the cavity changes considerably,while that in the cavity remains almost constant.Increasing the number of MVGs is also beneficial for improving the premixed degree of fuel and incoming flow and results in more violent chemical reactions downstream of the cavity.展开更多
This paper aims at probing the flow characteristics of a jet in supersonic crossflow(JISC)by installing a vortex generator(VG)upstream of the jet orifice.Nanoparticle planar laser scattering(NPLS)and stereo-particle i...This paper aims at probing the flow characteristics of a jet in supersonic crossflow(JISC)by installing a vortex generator(VG)upstream of the jet orifice.Nanoparticle planar laser scattering(NPLS)and stereo-particle image velocimetry(SPIV)technologies were employed to observe the flowfield,and three cases were designed for comparison.CASE0 stands for JISC without passive VG.In CASE1 and CASE2,VG is installed at 20 mm and 80 mm upstream away from the jet orifice,respectively.Transient flow structures show that two flow modes exist when the VG wake interacts with the JISC.In CASE1,vortices are induced from both sides of the jet plume because of the VG wake.This leads to a complex streamwise vortex system.Penetration and lateral diffusion are enhanced.In CASE2,intermittent large-scale eddies in the VG wake cause large streamwise vortices at the windward side of the jet.The penetration depth is also enhanced while the lateral diffusion is restrained.In addition,experimental results show that the penetration depth is approximately 8.5%higher in CASE1 than that in CASE0,and the lateral diffusion is larger by about 17.0%.In CASE2,the penetration is increased by about 26.2%,while the lateral diffusion is enhanced by just 0.5%.展开更多
To develop vortex generator jet （VGJ） method for flow control, the turbulence flow in a 14° conical diffuser with and without vortex generator jets are simulated by solving Navier-Stokes equations with k-ε tur...To develop vortex generator jet （VGJ） method for flow control, the turbulence flow in a 14° conical diffuser with and without vortex generator jets are simulated by solving Navier-Stokes equations with k-ε turbulence model. The diffuser performance, based on different velocity ratio （ratio of the jet speed to the mainstream velocity）, is investigated and compared with the experimental study. On the basis of the flow characteristics using computation fluid dynamics （CFD） method observed in the conical diffuser and the downstream development of the longitudinal vortices, attempt is made to correlate the pressure recovery coefficient with the behavior of vortices produced by vortex generator jets.展开更多
The effects of the vortex generator jet(VGJ)attached at the endwall on the corner separation/stall control are investigated by numerical simulation in a high-turning linear compressor cascade. The results show that th...The effects of the vortex generator jet(VGJ)attached at the endwall on the corner separation/stall control are investigated by numerical simulation in a high-turning linear compressor cascade. The results show that the corner separation could be reduced significantly, which results in a wider operation range as well as a more uniform exit flow angle and total pressure profile. At the near-stall operation point, the maximum relative reduction of the total pressure loss is up to 32.5%,, whereas the jet mass ratio is less than 0.4%,. Based on the analysis of the detailed flow structure, three principal effects of the VGJ on the endwall cross flow and corner separation are identified. One is to increase the tangential velocity component opposite to cross flow, thus inhibiting the endwall secondary flow near the jet exit. The second is to suppress the pitchwise extension of the passage vortex as an air fence. The third is to sweep the low energy fluids towards the mainstream on the up-washed side and to transport the mainstream fluids to the endwall to reenergize the boundary layer on the down-washed side.展开更多
One of the innovative ways to improve heat transfer properties of heat exchangers,is using nanofluids instead of traditional fluids.Due to presence of metal and oxides of metal particles in nanofluids structure,they h...One of the innovative ways to improve heat transfer properties of heat exchangers,is using nanofluids instead of traditional fluids.Due to presence of metal and oxides of metal particles in nanofluids structure,they have better potential in different environments and conditions than conventional fluids and having higher thermal conductivity causes improvements in heat transfer properties.In this research flow of two different nanofluids through a rectangular microchannel containing a different number of longitudinal vortex generators(Ivgs),has been investigated.Simulations conducted under laminar flow boundary condition and for varied Reynolds numbers of 100 to 250.Considered volumetric concentration in this paper is 1,1/6 and 2/3%Results showed,nanofluids and the LVGs notably improve the heat transtfer rates within the microchannel.havg improved with increasing the nanoparticles volume concentrations and Reynolds number,while the op posite trends recognized for pressure drop.havg improved for4 to 12 and 9 to 18%for TO2 and CuO nanofuids,respectively for different volume concentrations in simple microchannel.For lvg-enhanced microchannel the amount of improvements is about 9-14 and 5-10%for CuO and T0,respectively.Also using vortex generators alone improved havg for 15-25%for different number of Ivgs.展开更多
基金supported by the National Natural Science Foundation of China(12372049)Sichuan Science and Technology Program(2023JDRC0062)+1 种基金Science and Technology Program of China National Accreditation Service for Conformity Assessment(2022CNAS15)the Independent Project of State Key Laboratory of Rail Transit Vehicle System(2023TPL-T06).
文摘A relatively high aerodynamic drag is an important factor that hinders the further acceleration of high-speed trains.Using the shear stress transport(SST)k-ωturbulence model,the effect of various vortex generator types on the aerodynamic characteristics of an ICE2(Inter-city Electricity)train has been investigated.The results indi-cate that the vortex generators with wider triangle,trapezoid,and micro-ramp arranged on the surface of the tail car can significantly change the distribution of surface pressure and affect the vorticity intensity in the wake.This alteration effectively reduces the resistance of the tail car.Meanwhile,the micro-ramp vortex generator with its convergent structure at the rear exhibits enhancedflow-guiding capabilities,resulting in a 15.4%reduction in the drag of the tail car.
文摘The performance of compact,aggressive ducts in advanced propulsion systems is limited by the internal flow separation coupled with the formation of secondary counter-rotating vortices that give rise to intensive flow distortions at the duct exit.An experimental investigation was conducted to study the flow field and passive suppression of flow separation and Aerodynamic Interface Plane (AIP) distortion in a serpentine air inlet duct.Tests were performed by a turbofan engine at several Engine Operating Points (EOPs) from 56%(idle) to 100%(max).A large total pressure deficit region arose at the upper part of the AIP,which was associated with the upper surface flow separation.Using the new mechanical S-type vortex generators in two longitudinal positions (VGl and VG2),separation and loss were diminished at the upper part of the duct and AIP.The VG2 arrangement attained the maximum reductions in distortion coefficients which were 73.72%,60.7% and 37.8% in DC(90°),DC(60°) and△P_C/P metrics,respectively.In the next step of the study,some unsteady aspects of the flow field were analyzed inside the duct.The separation onset and reattachment points were determined by the standard deviation of static pressure on the upper surface.The AIP spectral distribution showed that the boundary of low pressure and high pressure recovery regions was dominated by the maximum fluctuations.Furthermore,the PSD diagram of several probes at AIP revealed the vortex shedding frequency and its higher harmonics at separation region.The energy content of distinct unsteady spectral features in the bare configuration was significantly reduced using VGs,which showed the improvement of flow at the duct exit.
文摘The hydrothermal behavior of air inside a solar channel heat exchanger equipped with various shaped ribs is analyzed numerically.The bottom wall of the exchanger is kept adiabatic,while a constant value of the temperature is set at the upper wall.The duct is equipped with a flat rectangular fin on the upper wall and an upstream V-shaped baffle on the lower wall.Furthermore,five hot wall-attached rib shapes are considered:trapezoidal,square,triangular pointing upstream(type Ⅰ),triangular pointing downstream(type Ⅱ),and equilateral-triangular(type Ⅲ)cross sections.Effects of the flow rates are also inspected for various Reynolds numbers in the turbulent regime(1.2×10^(4)-3.2×10^(4)).The highest performance(η)value is given for the Ⅱ-triangular rib case in all Re values,while the square-shaped ribs show a significant decrease in the η along the achieved Re range.The η value at Remax is 2.567 for the Ⅱ-triangular roughness case.Compared with the other simulated cases,this performance is decreased by about 3.768%in the case of Ⅰ-triangular ribs,15.249% in the case of Ⅲ-triangular ribs,20.802% in the case of trapezoidal ribs,while 27.541% in the case of square ribs,at the same Remax.Also,a comparison ismade with air-heat exchangers that have non-rough walls and contain cross-shaped VGs presented previously,in order to highlight the effectiveness of the rough surface presence in the baffled and finned channels.The obtained results indicated that the triangular-shaped rib(type Ⅱ)has the most significant hydrothermal behavior than the other cases.This indicates the necessity of roughness heat transfer surfaces for finned and baffled channels to improve significantly the performance of the air-heat exchangers they contain.
基金supported by the Natural Science Foundation of China(Grant Nos.61673169,11301127,11701176,11626101,11601485).
文摘Re-engineering the channel heat exchangers(CHEs)is the goal of many recent studies,due to their great importance in the scope of energy transport in various industrial and environmental fields.Changing the internal geometry of the CHEs by using extended surfaces,i.e.,VGs(vortex generators),is the most common technique to enhance the efficiency of heat exchangers.This work aims to develop a newdesign of solar collectors to improve the overall energy efficiency.The study presents a new channel design by introducing VGs.The FVM(finite volume method)was adopted as a numerical technique to solve the problem,with the use of Oil/MWCNT(oil/multi-walled carbon nano-tubes)nanofluid to raise the thermal conductivity of the flow field.The study is achieved for a Re number ranging from12×10^(3) to 27×10^(3),while the concentration(φ)of solid particles in the fluid(Oil)is set to 4%.The computational results showed that the hydrothermal characteristics depend strongly on the flow patterns with the presence of VGs within the CHE.Increasing the Oil/MWCNT rates with the presence of VGs generates negative turbulent velocities with high amounts,which promotes the good agitation of nanofluid particles,resulting in enhanced great transfer rates.
文摘The utilization of vortex generators to increase heat transfer from cylinders installed inside a duct is investigated.In particular,a channel containing eight cylinders with volumetric heat sources is considered for different values of the Reynolds number.The effective possibility to use vortex generators with different sizes to increase heat transfer and,consequently,reduce the surface temperature of the cylinders is examined.Also,the amount of pressure drop inside the channel due to the presence of vortex generators is considered and compared with the cases without vortex generators.The results show that although the addition of generators increases the pressure drop,it strongly contributes to increase the heat transfer coefficient inside the duct(up to 80–90%).
基金supported from the Priority Research Centers Program(2012-048078)Basic Science Research Program(2012-008918)+1 种基金through the National Research Foundation(NRF)a grant(08GASPLANTC04) from the program of KAIA funded by the Ministry of Land,Infrastructure and Transport of the Republic of Korea
文摘The effect of streamwise vortices generated from passive vortex generators was investigated to manipulate the separation bubble behind the vertical fence. The experiments were carried out in a circulating water channel and the velocity fields were measured using 2D and stereoscopic PIV method.The distance between the vortex generator and fence and the effect of the Reynolds number were investigated. In addition,the effect of boundary layer thickness was also investigated. The averaged recirculation lengths were compared with that of uncontrolled fence flow. The results showed the oscillatory variation of recirculation region appeared under the existence of vortex generators. The reduction of the separation bubble became larger when the fence was submerged in the thick boundary layer with increasing the distance between the generator and fence. When the boundary layer is thin,vortex generator can only suppress the separation bubble under the specific condition.
基金supported by the National Natural Science Foundation of China(Grant Nos.50836006,50976117)
文摘A numerical model of delta-wing type vortex generator was developed in two steps.The first step was to obtain a parameterized model of the shedding vortex based on delta-wing theory,which relates the geometry parameters and flow field parameters to the strength of shedding vortex which directly decides the source term.In the second step,a method was proposed to add source terms into the flow control equations so that the shedding vortex could be simulated numerically.As soon as the numerical model was completed,two cases:One for a plate and another for an airfoil segment were investigated for test.Comparison showed that the flow field structure and aerodynamic performance agreed well with those obtained from cases with real vortex generators.
文摘Unsteady cavitation causes noise,damage,and performance decline in the marine engineering and fluid machinery systems.Therefore,finding a method to control the cavitation and its destructive effects is important for the industrial applications.In this work,we proposed a passive method to control the unsteady behavior of transient cavitation at the medium Reynolds number.For this aim,we performed an experimental study using a high-speed camera to analyze the effects of hemispherical vortex generators(VGs)on the cavitation dynamics around a hydrofoil surface.In addition,the pressure pulsations induced by the collapse of the cavity structures in the wake region of the hydrofoil were captured with a pressure transducer mounted on the wall downstream of the hydrofoil.The results showed that the instability behaviors of the cavity structures on the hydrofoil were mitigated using the proposed cavitation passive control method.In addition,the pressure pulsations in the wake region of the hydrofoil were reduced significantly.It can be concluded that the suppression of cavitation instabilities can improve the operating life and reliability of the marine and hydraulic systems.
文摘For the problem that the flow separation on the flap lowers the aerodynamic performance of high lift system,an investigation was carried out on using micro vortex generators(VGs) to control the separation on flap of the swept constant chord half-model(SCCH) high-lift configuration,at a small to medium angle of attack,by experimental and numerical methods.The basic flow characteristics of SCCH landing configuration were analyzed by using numerical method to provide required information for the design of micro VGs.Then,by keeping the cruise configuration intact,the preliminary design procedure and design methods of micro VGs were established.In addition,the micro VGs were designed.The effects of VG's arrangement and geometric parameters,such as the arrangement mode,chordwise position,arrangement angle,height and spanwise distance,on controlling efficiency were investigated by using numerical method.Then the parameters of preliminary VGs were adjusted as the basis configuration for wind tunnel test.The experiments were accomplished in NH-2 wind tunnel for validating the numerical method,as well as obtaining the design principles and methods of micro VGs.The parameters of VGs were also optimized based on the experiments.The experimental results showed that the numerical design method can serve as an efficient and accurate design tool.The lift and drag were increased by 10% and 14%,respectively in landing state,which satisfied the requirements for landing.Finally,it was concluded that the established design principles and methods for micro VGs in this investigation can be used in engineering application.
基金supported by the 7 EU framework project and was carried out within the research project with the acronym TFAST(Transition Location Effect on Shock Wave Boundary Layer Interaction)supported by CI TASKPL-Grid Infrastructure。
文摘The interaction between a shock wave and a boundary layer on a suction side of gas turbine profile,namely Transition Location Effect on Shock Wave Boundary Layer Interaction,was one of main objectives of TFAST project.A generic test section in a transonic wind tunnel was designed to carry out such investigations.The design criteria were to reproduce flow conditions on the profile in wind tunnel as the one existing on the suction side of the turbine guide vane.In this paper,the effect of film cooling and jet vortex generators on the shock wave boundary layer interaction and shock induced separation is presented.Numerical results for Explicit Algebraic Reynolds Stress Model with transition modeling are compared with experimental data.
基金supported by the National Natural Science Foundation of China(51875238)。
文摘In this paper,a novel composite heat transfer enhancement technique comprised of louvered fins(LFs)and rectangular wing vortex generators(RWVGs)is proposed to improve the LF side thermal-hydraulic performance of louvered fin and flat tube heat exchangers(LFHEs).After validation of the LF side pressure dropΔP and heat transfer coefficient hLFof the baseline by experiments,the numerical method is applied to investigate the influential mechanisms of the RWVG parameters(the number N(7 to 15),attack angleβ(30°to 90°),height H_(VG)(0.8 mm to 2 mm)and width W_(VG)(0.8 mm to 1.2 mm))on the performance of the LFHE in the velocity range of 3 m/s to 10 m/s.Results show that thermal-hydraulic performance of the LFHE is significantly impacted by the RWVGs,and according to the performance evaluation criteria(PEC),the LFHE achieves its optimal thermal-hydraulic performance when N=7,β=45°,H_(VG)=1.8 mm and W_(VG)=1 mm.Compared to the baseline,the maximum,minimum and average increments of PEC for the optimal case are 13.85%,4.67%and 8.39%,respectively.
基金supported by the National Natural Science Foundation of China(No.52306058)the Natural Science Foundation of Tianjin Municipal Science and Technology Commission,China(No.22JCQNJC00050)。
文摘Taking the NACA0012 airfoil as the research object,the bio-inspired herringbone groove array,a new passive control method,is applied to relieve the flow separation under the large angle-of-attack conditions,and its effectiveness and mechanism in delaying airfoil stall are investigated by numerical simulations.The herringbone groove array is placed on the airfoil's upper surface near the trailing edge,and the effects of groove depth and yaw angle on the control effect are investigated.The results demonstrate that different designs of herringbone groove array can effectively broaden the stable operating range of the airfoil,and the application of herringbone groove array with a depth of only 0.00135 times the chord length and a yaw angle of 45°can result in a28.57%increase in the stable operating range.The flow details indicate that a pair of induced vortices with the same strength and opposite direction are formed near the converging line under the combined action of the accumulation effect of small-scale vortices in the grooves and the spanwise migration flow above the grooves.The induced vortices increase the mixing between the main flow and the boundary layer,allowing the boundary layer to gain sufficient energy to withstand the adverse pressure gradient at high angles of attack,effectively delaying airfoil stall.
文摘In order to provide the line-of-sight blockage of the engine face for an advanced Uninhabited Combat Air Vehicle（UCAV）, a highly curved serpentine inlet is proposed and experimentally studied. Based on the static pressure distribution measurement along the wall, the flow separation is found at the top wall of the second S duct for the baseline inlet design, which yields a high flow distortion at the exit plane. To improve the flow uniformity, a single array of vortex generators （VGs） is employed within the inlet. In this experimental study, the effects of mass flow ratio, free stream Mach number, angle of attack and yaw on the performance of a serpentine inlet instrumented with VGs are obtained. Results indicate： （1） Compared with the baseline serpentine design without flow control, the application of the VGs promotes the mixing of core flow and the low momentum flow in the boundary layer and thus prevents the flow separation. Under the design condition, the exit flow distortion （-↑△σ0） decreases from 11.7% to 2.3% by using the VGs. （2） With the descent of the free stream Mach number the total pressure loss decreases. However, the circular total pressure distortion increases. When the angle of attack rises from - 4° to 8°, the total pressure recovery and the circular total pressure distortion both go down. In addition, with the increase of yaw the total pressure recovery is fairly constant, while the circular total pressure distortion ascends gradually. （3） When Mao = 0.6-0.8, a = -4°-8° and β = 0°-6°, the total pressure recovery varies between 0.936 and 0. 961, the circular total pressure distortion coefficient varies between 1.4 % and 5.4 % and the synthesis distortion coefficient has a ranges from 3.8 % to 7.0 %. The experimental results confirm the excellent performance of the newly designed serpentine inlet incorporating VGs.
基金sponsored by the National Natural Science Foundation of China(Nos.51976010,52006011)National Major Science and Technology Projects of China(Nos.2017Ⅱ-0006-0020,2017-Ⅱ-0001-0013,J2019-Ⅱ-0003-0023)Beijing Institute of Technology Research Fund Program for Young Scholars,China。
文摘It has been recently shown that Sub Boundary layer Vortex Generator(SBVG,abbreviated as VG hereafter)can suppress the Cross-Flow(CF),and therefore,can eliminate corner separation and increase aerodynamic loading when installed on the end wall inside middle-load compressor passages.However,when VGs are applied in high-load compressors,it is difficult to achieve ideal results.This is because the definition of the VG attack angle in the presence of CF in existing research is confusing,and the stronger CF in high-load compressors worsens the problem and results in an improper design and optimization range of VG attack angle.Therefore,this paper clarifies the definition of the VG attack angle in the presence of CF and reveals the CF controlling mechanism of VG on a flat plate.The differences in the flow phenomena around a VG both with and without CF are also studied.The numerical results show that a larger height or attack angle of the VG generates a greater CF suppression effect.However,the cross velocity increases when surmounting the primary vortex induced by the VG,except that this enhanced CF is less conspicuous for larger VG heights.Compared to the cases without CF,the VG suffers an additional loss because of the stronger separation and primary vortex loss caused by the CF.
基金National Natural Science Foundation of China(11072264)
文摘The fine space-time structure of a vortex generator （VG） in supersonic flow is studied with the nanoparticle-based planar laser scattering （NPLS） method in a quiet supersonic wind tunnel. The fine coherent structure at the symmetrical plane of the flow field around the VG is imaged with NPLS. The spatial structure and temporal evolution characteristics of the vortical structure are analyzed, which demonstrate periodic evolution and similar geometry, and the characteristics of rapid movement and slow change. Because the NPLS system yields the flow images at high temporal and spatial resolutions, from these images the position of a large scale structure can be extracted precisely. The position and velocity of the large scale structures can be evaluated with edge detection and correlation algorithms. The shocklet structures induced by vortices are imaged, from which the generation and development of shocklets are discussed in this paper.
基金supported by the National Natural Science Foundation of China(No.12002373)。
文摘To investigate the characteristics of vaporized RP-3 kerosene combustion in a scramjet combustor enhanced by the micro vortex generator(MVG),a series of experiments are carried out based on the advanced combustion diagnosis technique.The high-enthalpy incoming flow is accelerated to supersonic through a Mach 2.52 nozzle,the total pressure and temperature of which are 1.6 MPa and 1486 K,respectively.The effect of MVG on the ignition process,flame distribution,and combustor pressure along the bottom wall is well revealed,and the effects of the position and number of MVGs on stable combustion performance are analyzed.The results indicate that the development processes of the initial flame kernel with and without an MVG during ignition process show a similar behavior.The installation of an MVG can lift the shear layer,promote the penetration of flame deeper into the mainstream,and expand the area of the reactive region.Reducing the distance between the MVG and the injection position and increasing the number of MVGs are regarded as effective ways of improving the mixing degree of fuel and air with a resultant intensification of chemical reactions and flame luminescence.The effect of mixing and subsequent combustion is enhanced by shortening the distance between the MVG and the injection position.As the layout schemes of the MVG vary,the pressure distribution between the injection position and the leading edge of the cavity changes considerably,while that in the cavity remains almost constant.Increasing the number of MVGs is also beneficial for improving the premixed degree of fuel and incoming flow and results in more violent chemical reactions downstream of the cavity.
基金supported by the National Natural Science Foundation of China(Nos.91541203 and 51676204)the Fenglei Youth Innovation Fund of China Aerodynamics Research and Development Center(No.PJD20170186)。
文摘This paper aims at probing the flow characteristics of a jet in supersonic crossflow(JISC)by installing a vortex generator(VG)upstream of the jet orifice.Nanoparticle planar laser scattering(NPLS)and stereo-particle image velocimetry(SPIV)technologies were employed to observe the flowfield,and three cases were designed for comparison.CASE0 stands for JISC without passive VG.In CASE1 and CASE2,VG is installed at 20 mm and 80 mm upstream away from the jet orifice,respectively.Transient flow structures show that two flow modes exist when the VG wake interacts with the JISC.In CASE1,vortices are induced from both sides of the jet plume because of the VG wake.This leads to a complex streamwise vortex system.Penetration and lateral diffusion are enhanced.In CASE2,intermittent large-scale eddies in the VG wake cause large streamwise vortices at the windward side of the jet.The penetration depth is also enhanced while the lateral diffusion is restrained.In addition,experimental results show that the penetration depth is approximately 8.5%higher in CASE1 than that in CASE0,and the lateral diffusion is larger by about 17.0%.In CASE2,the penetration is increased by about 26.2%,while the lateral diffusion is enhanced by just 0.5%.
基金This project is supported by Scientific Research Foundation of Ministry of Education of China for Returnee.
文摘To develop vortex generator jet （VGJ） method for flow control, the turbulence flow in a 14° conical diffuser with and without vortex generator jets are simulated by solving Navier-Stokes equations with k-ε turbulence model. The diffuser performance, based on different velocity ratio （ratio of the jet speed to the mainstream velocity）, is investigated and compared with the experimental study. On the basis of the flow characteristics using computation fluid dynamics （CFD） method observed in the conical diffuser and the downstream development of the longitudinal vortices, attempt is made to correlate the pressure recovery coefficient with the behavior of vortices produced by vortex generator jets.
基金Supported by the National Natural Science Foundation of China(No.51306042)
文摘The effects of the vortex generator jet(VGJ)attached at the endwall on the corner separation/stall control are investigated by numerical simulation in a high-turning linear compressor cascade. The results show that the corner separation could be reduced significantly, which results in a wider operation range as well as a more uniform exit flow angle and total pressure profile. At the near-stall operation point, the maximum relative reduction of the total pressure loss is up to 32.5%,, whereas the jet mass ratio is less than 0.4%,. Based on the analysis of the detailed flow structure, three principal effects of the VGJ on the endwall cross flow and corner separation are identified. One is to increase the tangential velocity component opposite to cross flow, thus inhibiting the endwall secondary flow near the jet exit. The second is to suppress the pitchwise extension of the passage vortex as an air fence. The third is to sweep the low energy fluids towards the mainstream on the up-washed side and to transport the mainstream fluids to the endwall to reenergize the boundary layer on the down-washed side.
文摘One of the innovative ways to improve heat transfer properties of heat exchangers,is using nanofluids instead of traditional fluids.Due to presence of metal and oxides of metal particles in nanofluids structure,they have better potential in different environments and conditions than conventional fluids and having higher thermal conductivity causes improvements in heat transfer properties.In this research flow of two different nanofluids through a rectangular microchannel containing a different number of longitudinal vortex generators(Ivgs),has been investigated.Simulations conducted under laminar flow boundary condition and for varied Reynolds numbers of 100 to 250.Considered volumetric concentration in this paper is 1,1/6 and 2/3%Results showed,nanofluids and the LVGs notably improve the heat transtfer rates within the microchannel.havg improved with increasing the nanoparticles volume concentrations and Reynolds number,while the op posite trends recognized for pressure drop.havg improved for4 to 12 and 9 to 18%for TO2 and CuO nanofuids,respectively for different volume concentrations in simple microchannel.For lvg-enhanced microchannel the amount of improvements is about 9-14 and 5-10%for CuO and T0,respectively.Also using vortex generators alone improved havg for 15-25%for different number of Ivgs.
