| Hypersonic vehicle design has been focused decades in the aerospace engineering.Boundary layer transition prediction is one of the key factor and also much difficult in the design.Previous study on the boundary layer transition most focused on the flat plate or axis-symmetric geometries.However,the next generation hypersonic aircraft designing usually adopts the wave-rider configurations,which will lead to a complex three dimensional boundary layer flow.HIFiRE-5 is the fifth flight plan of the Hypersonic International Flight Research and Experimentation(HIFiRE)program conduct by America and Austrilia unitedly.The main mission for the HIFiRE-5 is to study the three-dimensional boundary layer transtion.The forebody of the HIFiRE-5 vehicle is a 2:1 ratio elliptic cone.This model was chosen for its similarity to engineering configurations.This paper will study the instability characteristics for HIFiRE-5 elliptic cone,simulated the disturbences propagation in the HIFiRE-5 boundary layer,and predict the boundary layer transition of HIFiRE-5.In this study,the basic flow was calculated under the same condition of wind tunnel experiment.Three dimensional characteristics of the basic flow was analyzed by comparison between the elliptic cone and the circular cone.Then,the instability characteristics of the elliptic cone was analyzed by LST method.The transition position was predicted by the eN method,and the results were compared to experimental data.Then the instability of the streamwise vortex near the minor axis of the elliptic cone was analyzed by Bi-global method.In the last,the disturbances propagation in the HIFi RE-5 elliptic cone boundary layer were simulated by the numerical method,including linear and nonlinear characteristics.The main conclusions of these works are shown as follows.1.The elliptic cone boundary layer is typically three dimensional for the presence of crossflow in the boundary layer.And there is a streamwise vortex structure near the elliptic cone minor axis.The width of this streamwise vortex is nearly unchanged in the downstream.However,the boundary layer thickness vary violently along the spanwise direction near the minor axis.Thus,the streamwise vortex region should be analyzed by the Bi-global method.Most of the boundary layer except the streamwise vortex region,possess a moderate varying basic flow and similarity characteristics in the spanwise direction.In this major region,the LST method can be adopted.2.In the major region of the elliptic cone: 1)The distribution of the growth rate becomes asymmetric to the spanwise wavenumber due to the existence of crossflow in the boundary layer.And the asymmetry level is related to the product ?_w.The asymmetry makes the zero frequency disturbance exist,which will affect the mechanism of transition.2)The Inhomogeneity of the boundary layer thickness along the spanwise direction affects the instability a lot.It makes the first mode disturbance frequencies become “concentration” which will result in the unstable region for the first mode instabilities covers nearly the whole model.On the contrary,the boundary layer thickness inhomogeneity makes the second mode disturbance frequencies become “scattering” which will result in the unstable region for the second mode instabilities covers only a narrow zone of the model.So,the first mode enveloped N factor is much larger than the second mode one.3)The contour lines of the analyzed first mode enveloped N factors show good agreement with experiment transition position under noise condition.The analyzed dominant frequency is 50 kHz which is also very close to the experiment result.The contour lines of analyzed stationary enveloped N factors show good agreement with quiet wind tunnel experimental result.3.In the streamwise vortex region,Bi-global analysis results show that there are two unstable modes,Y-mode and Z-mode.The growth rate of Y-mode is larger than that of Z-mode,so the Y-mode may dominate the transition process in this region.The frequency corresponding to the maximum Y-mode N factor is close to the experiment result.4.The simulation results of the small amplitude disturbance show that the linear stability theory can reasonably predict the evolution of each mode disturbances in the elliptic cone boundary layer.The ray tracing method predicts the variation of spanwise wavenumber very well,and predicts a little lower of the disturbance amplitude.However,the saddle point method predicts the variation of spanwise wavenumber with some error,but predicts very well of the disturbance amplitude.The non-parallelism effect maybe the reason for this inconformity which should take much effort in the future study.5.The simulation results of the limited amplitude disturbance show that the disturbance amplitude will become saturated due to the nonlinear effect.There are some differences for the saturation process between traveling and stationary crossflow waves: for the traveling crossflow waves,the nonlinear effect shows out until the disturbance amplitude reaches 0.2,and after the disturbance amplitude passes about 0.24,the disturbance amplitude becomes damping,finally,the disturbance amplitude sustains a saturated amplitude about 0.1;for the stationary crossflow waves,the nonlinear effect shows out until the disturbance amplitude reaches 0.3,and saturate at a relative larger amplitude about 0.4,then after a short distance of balance stage,the disturbance amplitude continue amplifying.It can be infer that the propagation process of stationary crossflow waves contains an insufficient saturation process.The traveling crossflow waves experience a much quicker saturation process with a much shorter distance between the position where nonlinear effect shows out obviously and that where the disturbance amplitude reaches the peak.So,the linear stability theory can be adopted to predict the position where the disturbance amplitude reaches the first peak which will provide a theoretical principle for the prediction of transition caused by traveling crossflow waves. |
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