Experimental Investigation Of The Hypersonic Boundary Layer Transition On The Cones

The transition of hypersonic boundary layer has important effects on the aerodynamics and aerodynamic heat of hypersonic vehicles.The understanding of the hypersonic boundary layer transition mechanism and the accurate prediction of the boundary layer transition process for hypersonic speed aircraft are of great significance.Compared with the supersonic boundary layer,the natural transition process of the hypersonic boundary layer is different because of the appearance of the second mode waves and it play a key role in the transition of hypersonic boundary layer.In this paper,the second mode waves during the transition process of the hypersonic boundary layer has been experimentally studied through the Nanoparticle-tracer based Planar Laser Scattering?NPLS?technique and the measurements of the high frequency fluctuation pressure.In order to investigate the influence of Reynolds number on the transition of the boundary layer,the experiments about the boundary layer transition on a 7ohalf-angle straight cone are carried out.The position of the second mode wave and the parameters such as the characteristic frequency and wavelength of the disturbance waves are found to be greatly influenced by Reynolds number.The characteristic frequency of the second mode wave changes from 55 kHz to about 226 kHz when the Reynolds number increases from 2×10⁶ m^-1 to 8×10⁶ m^-1.The second mode wave appears at the position closer to the upstream with a higher disturbance growth speed under higher unit Reynolds number.As the second mode wave propagates downstream,its characteristic frequency gradually decreases.The cross-correlation analysis results show that the propagation velocity of the second mode wave in the boundary layer is about 0.8?0.9times of the local mainstream velocity.The clear"rope-like"second mode wave structures were observed in the boundary layer through the NPLS results.Then the influence of the angle of attack on the boundary layer is studied.Under the condition of1osmall angle of attack,the dominant mode in the boundary layer is still the second mode,but the development of the boundary layer on the windward and leeward is obviously different.The disturbance development in the boundary layer of the leeward is advanced,the second mode wave appears closer to the upstream,and the development of the disturbance in the windward is suppressed,and the characteristic frequency of the second mode wave is larger.In the study of disturbance wave development over the surface of the cone under the 1oangle of attack,it is found that the growth rate of the disturbance wave amplitude does not always decrease from the leeward to windward of the cone surface.On the leeward side of the cone,there is a region where the amplitude growth is relatively delayed.Thereby forming a nonmonotonic curve on the amplitude growth results of the leeward surface.When the angle of attack increases to 5o,the leeward flow is greatly affected by the cross-flow instability,and the transition position is greatly advanced.A disturbance wave with a characteristic frequency of 40 kHz is detected in the leeward boundary layer.In this paper,the impacts of the laser-generated perturbations on hypersonic boundary layer stability was investigated experimentally on the 7°half-angle straight cone under the condition of Mach 6 and unit Reynolds number of 3.1×10⁶ m^-1.Through the Short Time Fourier Transformation and Power Spectrum Density analysis of the pressure data,the results showed that when the laser-generated perturbation was added in the flow field,the position of the second mode wave is advanced and the amplitude of the disturbance wave is greatly increased.The laser-generated perturbation has a significant effect on encouraging the development of disturbance waves in the boundary layer.At the same time,laser-generated perturbations will have different impacts on the boundary layer when it was focused in different position.When the laser is focused on the location X=100mm,the amplitude of the disturbance wave with the frequency of 90 kHz in the boundary layer grows fastest,and the amplitude magnification at the position of X=500mm is 3.81 times.When the laser perturbation is added to the free flow in front of the cone,the frequency of the disturbance wave with the fastest amplitude growth rate is greatly reduced to 73 kHz.In the same range,the amplitude magnification is 4.51 times.It can be seen that when the laser focus position is located in the free stream upstream from the cone,its effect on the disturbance wave in the boundary layer is more significant.Under the condition of Mach 6,the experimental study on the second mode waves in the boundary layer of flared-cone was carried out.Through the NPLS results,the intertwined rope-like second mode wave structures in the hypersonic boundary layer are observed clearly and intuitively.The evolution process of the boundary layer disturbance waves under three different Reynolds numbers is analyzed.Correlation analysis is performed on large-scale flow structures such as the second mode waves through the temporal-correlated pictures to obtain the propagation velocity of unstable waves.The frequency domain information of the unstable wave is analyzed by converting the gray information of the spatial distribution in the NPLS images into the time series signal,and the characteristic frequency of the second mode wave obtained by NPLS results agree well with the results measured by PCB pressure sensor.It is found that the energy of the second mode wave firstly increases during the transition process.This process is accompanied by the appearance of higher harmonic components.The energy of the second mode wave begins to decay when it reaches to a certain degree and the component with frequency lower than the characteristic frequency of second mode wave increases rapidly.At the same time,through the bispectrum analysis,it is found that the nonlinear effect of the flow is more obvious at the stage where the waveform of the second mode wave is unstable and the second mode wave begins to disappear.