| Automotive aerodynamics affects the vehicle's power,fuel economy,handling stability,and driving safety directly.The 3/4 open return wind tunnel,as the main means for automotive aerodynamic testing,plays a vital role in the development and design of the product.However,its structure determines that the airflow from the nozzle forms a jet and produces a larger vortex structure which has a certain vortex frequency in the shear layer.This unstable free jet structure is regarded as an excitation which matches with the acoustic mode of the wind tunnel is the reason that the resonance phenomenon occurs.It destroys the temporal and spatial uniformity of the flow field in the test section and affects the acoustic information collection and aerodynamic measurement of the tested object seriously.This paper uses a combination of theoretical analysis,numerical simulation,and experimental verification to study the phenomenon of low frequency pressure pulsation in open return wind tunnels.The main work content is as follows:(1)Firstly,the theoretical model of wind tunnel acoustic modal is investigated.And combined with the geometric parameters of the tunnel body,the excitation frequencies and response frequencies of the wind tunnel are calculated.Secondly,the low frequency pressure pulsation phenomenon of the wind tunnel in the full wind speed range was measured experimentally.A preliminary judgment was made on the potential of various factors in forming the pulsation phenomenon.It can be seen from the comparative analysis of theoretical calculation results and experimental data that the effect of wind tunnel structure parameters on the pulsation involved in the nozzle natural vortex separation frequency,wedge feedback frequency,and loop response frequency are more prominent.(2)In view of the advantages of CFD simulation in studying the phenomenon of low frequency pressure pulsation,the transient numerical simulation method of this phenomenon was explored firstly,and the effectiveness of the simulation method was verified through the comparison of simulation and experimental data in the full wind speed range.Among them,the closed-loop is the important point to numerically solve the low frequency pressure pulsation phenomenon for ensuring that the pressure wave circulates in the pipeline;in the calculation and solution process,the air density parameter is set as a compressible ideal gas to ensure that the pressure propagation speed inside the medium is consistent with the actual process.Secondly,it focuses on the analysis of the transient pressure field in the whole circuit of the basic example and clarifies the basic principle of the pulsation phenomenon.When the pressure peak propagated in the loop is superimposed with the pressure accumulation caused by the vortex structure on the flow field of the jet core area in the test section,the pulsation will occur.(3)By changing the geometric parameters of the theoretical model,different wind tunnel models are obtained.The effects of excitation and response parameters on the low frequency pressure pulsation are compared and analyzed by the numerical simulation method of this paper.The results showed that the coupling of the length of the tunnel loop and the test section is the main cause of the low-frequency pressure pulsation phenomenon.The former affects the excitation in the flow field directly,and the latter directly affects the response in the flow field.In the wedge feedback effect,the natural vortex of the nozzle only exists in the form of initial excitation,and it is not the main reason for the formation of the final low-frequency pressure pulsation.The possibility that the normal frequency of the resident chamber and the resonance frequency of the cavity can be used as the response of the pulsating excitation in the test section is ruled out,but its suitable space size can inhibit the occurrence of resonance obviously.(4)It conducts an in-depth study on the transient information of the flow field in the test section based on the simplified model and clarifies the trajectory of vortex motion and its influence on the pressure propagation in the loop.The main reasons for the large errors in the existing theoretical models are analyzed:1)In the excitation model,when the pressure disturbance occurs in the flow field of the test section,the difference between the vortex position and the length of the test section cannot be ignored;2)In the response model,theoretical formula only expresses the sound velocity propagation part of the pressure in the loop and lacks the convection propagation process of the pressure in the test section.In the study of the variation of pulsation characteristics,it is found that:1)the pulsation peak wind speed and peak frequency are proportional to the length of the tunnel loop inversely.When the loop length is fixed,the peak frequency does not change.It means that the resonance frequency changes from the tunnel loop.The response is locked.2)when the overall scale of the test section is scaled,the pulsating peak wind speed changes in the same multiple relationships with the scaling ratio;3)only when the length of the test section is adjusted,the pulsating peak wind speed is linearly related to the length variable.On the base of the original theoretical model,according to the relationship between the geometric parameters and the pulsation characteristics of each wind tunnel,a semi empirical prediction model for the peak characteristics of pulsation is proposed.The model can predict the peak frequency and peak wind speed of the low-frequency pulsation phenomenon of the open return wind tunnel.Referring to other advanced wind tunnel test data in the world,the accuracy increases from 5%to 20%compared with the original theoretical model.(5)To meet the requirements of the flow field quality specification,when the excitation in the test section is unavoidable,numerical simulation and experimental verification are carried out on the measures to suppress the low-frequency pressure pulsation from the perspective of the loop.The results show that the proper position of the hole balance port can effectively release the pressure wave in the flow channel and destroy the loop response mechanism.Among them,under the condition of 22m/s wind speed,the pressure balance port at the power section can reduce the peak Cprms coefficient from 4.9%to 0.39%and the pulsation phenomenon basically disappears.At the same time,the sound field of the test section has also been significantly optimized.The sound pressure level at the peak frequency of 2.5 Hz is reduced by about 30 dB,and the high-order frequency at 5.0 Hz is reduced by about 17dB.Through the investigation and verification of numerical simulation methods of the pressure pulsation phenomenon,this paper promoted the development and application of CFD simulation technology effectively in the process of solving this problem.In terms of theory,the mechanism of the pulsation phenomenon is clarified,and the reason for the larger error in the original theoretical formula is clarified.In terms of engineering,a set of semi-empirical formulas have been summarized through the study of the change law between the geometric structure parameters of the wind tunnel and the peak pulsation characteristics,which can improve the accuracy of pulsation prediction effectively and it has important reference value for the design,flow field quality and performance evaluation of automotive wind tunnel. |
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