实验流体力学

Zhang Wen, Liu Peiqing, Guo Hao, Qu Qiulin

2014, (6): 1-12,38. doi: 10.11729/syltlx20140030

Abstract(230) PDF(38)

Abstract:
Laminar-turbulent transition can greatly affect the characteristics of boundary layer flow like skin friction and heat transfer.Predicting transition accurately is of great importance in engineering design.The prediction of transition using transition models which develop quickly during past years is very appropriate in engineering applications.Transition models are divided into four types and each type is reviewed in detail respectively.Conclusions are made based on a-nalysis and discussion about the achievements as well as the drawbacks of transition models now-adays,and some suggestions are proposed for transition research and modeling in the future.

Experimental study on boundary-layer transition control by spanwise discrete suction

Guo Hui, Li Xiaobao, Wang Haiwen, Feng Yulong

2014, (6): 13-19. doi: 10.11729/syltlx20140027

Abstract(135) PDF(9)

Abstract:
Delaying boundary layer transition is an important way to reduce friction drag of ve-hicles in air and water.Introducing steady streaks upstream is a new idea proposed recently to de-lay transition.Available studies demonstrated that the steady streaks of higher amplitude have higher ability to depress T-S wave propagation,and thus are more effective in delaying transi-tion.However,only relatively low amplitude steady streaks (12%U,whereU is the freestream velocity)can be obtained by the roughness elements method commonly used previously.This is because the generation of higher amplitude steady streaks needs higher roughness elements, which will lead to the bypass transition due to the stronger perturbation of vortex shedding caused by the elements.Moreover,recent theoretical studies show that there are some kinds of streaks wich do not suppress but instead promote the transition because of their acceleration of secondary instability.But this has not been verified by experiments yet.Thus,at least two prob-lems are valuable to be studied for the flow control technique.One is to develop an effective method to generate higher amplitude steady streaks;the other is to test the control effect of dif-ferent streaks on transition.In our studies,a new method,called spanwise discrete suction,has been developed to generate high amplitude steady streaks.Based on this,the streak control on the artificially excited transition in flat plate boundary layer has been studied using hydrogen bub-ble timeline method,by detecting the variation of perturbation evolution before and after the steady streaks introduced.The influences of streak spacing (corresponding to the suction hole spacing)and streak amplitude (depending on the suction intensity)are tested.The results show that spanwise discrete suction is an effective method to generate high amplitude streaks.The maximum amplitude of steady streaks generated by this method reaches as high as 28.4%U,even
higher than the upper amplitude threshold value of 26%U predicted theoretically for steady streaks. Moreover,both of the two steady streaks with spacing of 14mm and 28 mm introduced in our study sup-press the flow breakdown,and thus delay the transition;The higher the streak amplitude or the narro-wer the streak spacing is,the more effective the streaks are in depressing the transition.

Experimental investigation on the influence of a Mini-TED on the flow separation of an airfoil at low Reynolds number using PIV

Cheng Jubo, Daichin

2014, (6): 20-26. doi: 10.11729/syltlx20140032

Abstract(198) PDF(15)

Abstract:
The wind tunnel experimental results of flow structures of a NACA23012 airfoil mounted with a rearward Mini-TED for low Reynolds number situations are introduced in this pa-per.The PIV technique was applied to capture the velocity fields around the airfoil for different angle of attack,and the detailed vortex structures near the trailing edge were measured as well. The velocity measurement results are compared with those of NACA23012 prototype airfoil,so as to investigate the influence of the Mini-TED on the flow separation characteristics above the airfoil surface.The Reynolds number is Re≈1.3×105 based upon the chord length of the airfoil. The results show that the flow separation on the suction surface of the airfoil,especially for the case of larger angle of attack,is obviously depressed due to the installation of Mini-TED,and the flow speed on the suction surface is much higher than that of NACA23012 prototype airfoil.A low speed region is locally formed in front of the Mini-TED,which leads to the increase of static pressure.Meanwhile,a double-vortex structure with low pressure is observed behind the Mini-TED.The resulting pressure difference causes the extra lift increment near the trailing edge of the airfoil,and the pitching moment is increased as well.The velocity differences between the suction surface and the pressure surface of airfoil with the Mini-TED are higher than those of the NACA23012 prototype airfoil,which can contribute to the increment of lift force.

Experimental study of effects of multi-winglets and tip blowing upon wingtip vortex

Yang Ke, Huang Hao, Xu Shengjin

2014, (6): 27-38. doi: 10.11729/syltlx20140024

Abstract(262) PDF(17)

Abstract:
The effects of multi-winglets and wingtip blowing on the wing tip vortex are exper-imentally studied.On the basis of those results,a wing tip vortex control strategy which com-bines multi-winglets and tip blowing is proposed and justified.The experiments have been con-ducted at a low speed wind tunnel.The basic wing model is a rectangular NACA 0015 airfoil. Reynolds number (Re)defined by the chord length of wing and the free flow speed is 5.3×104 . The blowing coefficient (Cμ)defined by the momentum ratio of blowing to free flow is 0.017. The results show that the multi-winglets divide the wing tip vortex into several co-rotating vorti-ces which decreases the effect of the downwash flow generated by wingtip vortex,and thus im-prove the quality of the local flow.The lift coefficient increases forα>4°.The maximum lift co-efficient increases up to 1 2 .3%.Blowing strengthens the “wandering”of vortex core to change the position of the tip vortex.Blowing suppresses the formation of the wingtip vortex to some ex-tent.The joint control strategies have achieved a well control effect,meanwhile,the blowing en-hances the interaction of the co-rotating vortices.The peak of mean instantaneous vorticity is de-creased by 37% compared with the individual “+0-”multi-winglets configuration,and by 79%compared with the basic wing.The control effect of the joint strategy is subjected to whether the blowing could enhance the interaction of the co-rotating vortices or not.The moving direction of wingtip vortex also has a substantial influence on the control effects.

Experimental study on the flow field and impulse of a two-electrode plasma high energy synthetic j et

Zhang Yu, Luo Zhenbing, Wang Che, Wang Lin, Xia Zhixun

2014, (6): 39-44. doi: 10.11729/syltlx20140054

Abstract(130) PDF(5)

Abstract:
The gas inside the cavity of the two-electrode plasma high energy synthetic j et actu-ator is electrothermally heated by the transient discharge of electric arc,that leads to a rapid in-crease in pressure within the cavity.This high-pressure gas ejects through the orifice and forms the high-speed jet.Considering the fast response character of the high energy synthetic jet,a sin-gle line torsion pendulum system for the impulse measurement is designed.Combined with the high-speed shadowgraphy,the development of the flow field and the impulse of the plasma jet are experimentally studied.The results show that the response time of the two-electrode plasma high energy synthetic jet is less than 10μs,and the duration time of the single-pulse jet is about 1ms. The maximum velocity of the jet front is about 190m/s,and there are shocks spreading at the speed of sound.The measurement accuracy of the single line torsion pendulum system isμN·s. The impulse of the two-electrode plasma high energy synthetic jet actuator is about 32μN·s. The total impulse increases linearly with the discharge frequency when the discharge frequency is low.

An experimental investigation on separation control of an airfoil by beveled-slit-synthetic-j et-actuator

Zuo Wei, Gu Yunsong, Cheng Keming, Liu Yuan

2014, (6): 45-50. doi: 10.11729/syltlx20140063

Abstract(152) PDF(15)

Abstract:
Active flow control on a three-dimensional NACA633-421 airfoil has been investiga-ted experimentally using beveled-slit-synthetic-j et-actuator (BSSJA).Balance force measurement results show that the synthetic jet can control the flow separation effectively,improve the maxi-mum lift coefficient by 10.4% and increase the stall angle of attack by 4°.Both boundary layer velocity measurement and particle image velocimetry (PIV)technology have been performed un-der selected experimental conditions to investigate the mechanism of synthetic jet flow control. Results reveal that the shape factor of boundary layer velocity profile decreases with BSSJA con-trol and the promotion of the power of the boundary layer is responsible for the control effect. The instantaneous flow maps and ensemble-averaged PIV results reveal that synthetic j et tends to enhance the flow dilution and transfer the mass flux so that the separated flow could attach to the wall of NACA633-421 airfoil again.In addition,the turbulent kinetic energy (TKE)and Reyn-olds stress are increased near the wall compared to the situation without control.

A historical review for the 50 th anniversary of laser Doppler velocimetry

Shen Xiong

2014, (6): 51-55. doi: 10.11729/syltlx20140011

Abstract(205) PDF(15)

Abstract:
A historical review has been presented for the 50th anniversary of laser Doppler ve-locimetry.Main successes developed in this period are pointed out in memory with the important contributions from a number of scientists and engineers.It is noticed that the new age has already been opened up for the complex turbulent flow research with the accurate,quantitative and dy-namical measurements.

Experimental study on thereshocked RM instability of two kinds of heavy gas cylinder

He Huiqin, Zhai Zhigang, Si Ting, Luo Xisheng

2014, (6): 56-60. doi: 10.11729/syltlx20130077

Abstract(170) PDF(5)

Abstract:
The evolutions of the Richtmyer-Meshkov instability of two kinds of membrane-less heavy gas cylinder (SF6 and argon),under reshock condition with a specific reflected dis-tance,are experimentally studied in a horizontal square shock tube.Based on the jet technique, SF6 and argon gas cylinders are generated respectively to study the effect of Atwood number on the development of the Richtmyer-Meshkov instability.For the visualization of the flow,the tested gases are mixed with glycol droplets which are generated by the fog generator.Illuminated by a continuous laser sheet with a width of 80mm and a thickness of 1 mm,the interface mor-phologies after incident shock and reshock impact are captured in a single test run with the help of the high speed camera.The results show that different evolving rates and interface morphologies before and after reshock are observed for SF6 and argon gas cylinders due to the different Atwood numbers.For the same reflected end wall,the visible two reversed vortex rings are generated for the SF6 gas cylinder after the incident shock passage,which are not apparently observed for the argon gas cylinder because of less vorticity deposition on the interface.Moreover,after the re-shock impact,secondary vortex rings which have reversed rotating directions to the original vor-tex rings are quickly generated in the SF6 gas cylinder and dominate the flow field at the later stage,becoming the primary feature of the flow.However,in the argon gas cylinder,the second-ary vortex rings,though generated with opposite directions of rotation to the original ones,are not fully developed all the time due to the smaller Atwood number that results in less production of reversed baroclinic vorticity on the argon interface.The present results illustrate the pronounced influence of the Atwood number on the development of the Richtmyer-Meshkov instability.

Investigation on piston-typed synthetic j et technology and its application

Wu Jifei, Luo Xinfu, Xu Laiwu, Fan Zhaolin

2014, (6): 61-65. doi: 10.11729/syltlx20130076

Abstract(211) PDF(8)

Abstract:
A piston-typed actuator system is designed to study the characteristics and influence factors of synthetic jet.An experiment is conducted in the high speed wind tunnel to investigate the effect of synthetic jet on aero-acoustic noise suppression of the cavity flow field.Results indi-cate that the synthetic jet actuator is reliable and reasonable,which can produce very high speed jet with the maximum of plus velocity reaching about 160m/s.The frequency of the generated synthetic jet is consistent with the actuator excitation frequency.Some factors,such as the fre-quency of the actuator,the distance of the piston stroke and the shape of the jet exit,may exert great influence on the synthetic jet velocity.Synthetic jet characteristics are insensitive to the thickness of the jet exit.The aero-acoustic noise suppression effect of this flow control method on the cavity flow field depends strongly on the Mach number.Aero-acoustic environment of the cavity can be improved by using synthetic jet as a flow control method at transonic speed.But it is basically ineffective at supersonic speed.

Analysis of the measurement error in the cross section perpendicular to flow using 2D-PIV

Cao Yongfei, Gu Yunsong, Cheng Keming

2014, (6): 66-72. doi: 10.11729/syltlx20140042

Abstract(171) PDF(18)

Abstract:
As an important means of field testing,conventional two dimensional Particle Im-age Velocimetry (2D-PIV)has been widely applied to various flow measurements.Measurement error arises when 2D-PIV is applied to measure the cross section perpendicular to the flow due to geometric perspective relations.The error factors are analyzed and a model of the error in the measurement plane is established.The correctness of the error model is verified by experimental tests .It is concluded that the normal speed and off-axis angle of view are the key influencing fac-tors.The calculation results show that the maximum perspective error can be up to 9 .3%of nor-mal speed.According to the analysis of error model,the effect of perspective error on measure-ment of streamwise vortex flow field mainly exhibits in three aspects:changing the velocity value of flow field,changing the shape of vortex,and changing the position of the vortex.Finally, some approaches to reduce the error are proposed,which can improve the measurement for the application of 2D-PIV in the cross section perpendicular to the flow.

An integrated sensor technology for measurements of specific heat ratio of combustion gases in supersonic flow

Wang Tiejin, Chen Jun

2014, (6): 73-79. doi: 10.11729/syltlx20140026

Abstract(183) PDF(3)

Abstract:
We present an integrated probe system for measuring the specific heat ratioγof combustion gases by combining Pitot tube and laser beam tracking system.The key benefit of the measurement system is the elimination of the complicated gas chromatography apparatus.The pressure ratio and the angle of refraction of a laser beam across the normal shock wave generated by Pitot tube are simultaneously measured with this system.The angle of refraction of the laser beam is expected to be accurately recorded by the laser tracking system consisting of two CCD cameras and a mobile spatial coordinate measuring system (MScMS).Moreover,the density rati-o across the shock wave is deduced with Gladstone-Dale relation and Snell’s law.The specific heat ratioγis accordingly quantified by its relation to the pressure ratio and the density ratio across the shock wave.This technique may effectively reduce the time taken to obtainγ.Uncer-tainty estimation shows that the uncertainty brought by the laser beam tracking system and Pitot tube measurement are limited within 5 .5%for any Mach number if the stagnation temperature is lower than 1000K.Feasibility analysis present that the technique is physically realizable but a few technical issues need to be addressed before it can be implemented.

Determination of the interfacial evanescent wave base intensity I0 in total internal reflection velocimetry (TIRV)

Shi Fei, Zheng Xu, Chen Rongqian, Li Zhanhua

2014, (6): 80-85. doi: 10.11729/syltlx20140048

Abstract(110) PDF(6)

Abstract:
The total internal reflection velocimetry (TIRV)based on evanescent wave is an ef-ficient method for velocity measurement within a few hundred nanometers to the wall.The eva-nescent wave intensity,I(z),decays exponentially with the distance z away from the wall.When a nanotracer located in this field is illuminated by evanescent wave,its radiation intensity will also obey the exponential decay low,and thus it is possible to determine the nanotracer’s z position by its illuminated intensity.Obviously,one of the key issues of this technology is to determine the base intensity I0 . In this paper,a numerical solution of the nanotracer intensity probability density function is given to predict the intensity distribution according to the exponential decay of the evanescent wave,the tracer concentration distribution and the tracer size variation.By com-paring the measured intensity distribution with the theoretical prediction,the base intensity I0 can be quantitatively determined.Fluorescent polystyrene spheres ofφ100nm andφ250nm are used in the measurements.The validity of this method is verified.The effect of the nanotracer size variation on the determination of I0 is also analyzed.Furthermore,φ100nm nanotracers are used to measure the velocity close to the wall,and the result verifies the validity of our method.

Design and validation of a vertical annular shock tube for RM instability study

Long Tong, Zhai Zhigang, Si Ting, Luo Xisheng

2014, (6): 86-91. doi: 10.11729/syltlx20130106

Abstract(136) PDF(7)

Abstract:
A vertical annular coaxial diaphragm-less shock tube is designed based on the prin-cipal proposed by Hosseini and Takayama and modified in order to conveniently install the initial interface in the test section and visualize the flow field for the investigation of the Richtmyer-Meshkov (RM)instability.Parametric study is carried out both experimentally and numerically to explore the characteristics of the annular coaxial cylindrical converging shock wave.The varia-tion of pressure behind the shock shows the feasibility and reliability of this shock tube to generate the annular coaxial cylindrical converging shock wave.The pressure variations with time at differ-ent positions in the test section are acquired from the experiment and numerical simulation,and the converging effect of the shock wave is emphasized.After the validation of the converging shock wave,the experiment of RM instability induced by this converging shock wave is con-cerned.For this purpose,a regular octagon air/SF6 interface (the distance from each vertex to the center is 20mm)is generated in the test section by using eight thin wires to restrict the soap films.In this way,the initial interface shape,which is crucial to RM instability study,can be precisely controlled.The influence of the thin wires on the interface evolution is also assessed by numerical simulation and the results indicate that the thin wires have limited effect on the inter-face development at the very early stage.Moreover,because the height of the interface generated is only 5mm,the influence of the gravity can be neglected and the regular octagon soap interface
can be treated as two dimensional.For visualizing the flow field,a continuous laser sheet combined with the high-speed camera is employed and the evolution of the regular octagon air/SF6 interface accelerated by the annular coaxial converging shock wave and its reflected shock wave is captured through the Mie scatting light from the droplets of the soap film.Corresponding numerical simulation is also performed for comparing with the experiment,and a good agreement is found between these two results.During the evolution,the inter-face is first compressed after the incident converging shock wave passage and then the “spike”and “bubble”configurations are generated due to the deposition of the baroclinic vorticity on the interface.Phase reversal occurs on the interface after the reflected shock wave impacts on it,which creates an opposite pressure gradi-ent compared with the initial incident shock.The secondary “spike”configuration is generated at the original“bubble”position while the secondary“bubble”configuration is generated at the original“spike”position and they grow gradually with time.From the observation,it can be found that the interface evolution is quite symmetric which once again verifies the reliability of the shock tube to generate the converging shock wave and the interface formation method.

Research on vectoring thrust test technology in low-speed wind tunnel

Jia Yi, Zheng Fang, Huang Hao, Yin Shibo, Lang Weidong

2014, (6): 92-97. doi: 10.11729/syltlx20130116

Abstract(140) PDF(3)

Abstract:
A vectoring thrust test system for YF-1 6 standard model has been developed by FD-09 low-speed wind tunnel laboratory of China Academy of Aerospace Aerodynamics (CAAA).The maximum 2 .0 MPa compressed air flow through vectoring thrust pipe and j etted from nozzle can be generated to simulate the influence of the vectoring thrust j et on the aerody-namic characteristics of the whole plane.The system takes the full advantage of the existing high angle of attack device to achieve convenient conversion between the ventral and the dorsal air-sup-ply.The range of angle of attack can be adjusted from-6 degree to 90 degree,and the maximum Nozzle Pressure Ratio (NPR)is more than 5 .In tests an ordinary balance,a vectoring thrust force measurement sensor and other instruments are applied to obtain the influence of the j et flow on the aerodynamic characteristics of the whole plane and the nozzle aerodynamic performance. This paper describes the vectoring thrust test system design,the instruments and devices employed,and the typical results obtained.The system can be used for proj ect tests after making further improvements in sting interference correcting;sensor calibration in jetting status and pipe optimization design.

Experimental study on pressure fluctuation sensor-based diagnostic for boundary layer transition with long-fine tubing system

Gao Yongwei, Huang Peng, Zhu Qiliang, Liang Chenggang

2014, (6): 98-103. doi: 10.11729/syltlx20140021

Abstract(125) PDF(5)

Abstract:
Low speed flow transition zone of boundary layer can be detected by means of pres-sure fluctuation measurement over the surface of the test model.It is convenient that fluctuation pressure transducers are arranged outside the model through long-fine tubing system.The distor-tion of pressure signal caused by the tubing system should be recognized.Under the static condi-tion(V= 0m/s),the authors measured a series of sound signals over an airfoil model surface through the tubing system.The sound signal source come from a speaker actuated by a signal generation.The model chord length is 0.8m and the model span is 1.6m in length.The tubes are about 1.2m long and their inner diameters are 0.8mm.The tubing system has proved to be able to transmit the characteristics of the surface pressure fluctuation for the transition detection.Un-der the condition of section flow velocity V= 30m/s,signals of the surface pressure fluctuation transmitted along the chord of the model were measured and the boundary layer transition zones were located.This experiment was conducted in NF-3 low-speed wind tunnel of Northwestern Polytechnical University.The authors suggest that the pressure fluctuation transition diagnostic method with long-fine tubing system is promising and further researches should be carried out.

2014 Vol. 28, No. 6