Experimental Investigation Of Supersonic Flow Separation And Its Micro-ramp Control

The flow separation is ubiquitous in inner and outer flow fields of supersonic or hypersonic vehicles, which involves many complex phenomena, such as shock wave boundary layer interactions(SWBLIs), recirculation region, bouandry layer transition, turbulence, etc. However, flow separation may be harmful to the stabilization and structure credibility. Thus, the study of flow separation and its control is very necessary. There is an important theoretical and engineering practical value in the investigation of fine structures and dynamic behaviors of supersonic flow separation and its control. Nano-tracer Planar Laser Scattering(NPLS) and Particle Image Velocimetry(PIV) are employed to experimentally investigate fine structures, dynamic behaviors of coherent structures and velocity fields of supersonic flow separation and its micro-ramp control. This paper contains four parts:Firstly, the NPLS technique is used to investigate spcial structures of supersonic flow over the forward facig step, elliptic surface model, double elliptic surface model and double wedge, as well as dynamic behaviors of coherent structures. When it is lanminar inflow, the process of boundary layer transition is observed; the transition is advanced owing to the adverse pressure gradient of the models; the transformation of coherent structures is large at the beginning of the transition, while the coherent structures at the neighborhood of the models transform very slowly; there is flow separation in the upstream area of the first three models. As it is turbulent inflow, there is obvious hierarchy and it indicates it is developed turbulence; the structures at the models are the obvious intermittent large-scale structures; there is also flow separation in the upstream area of the first three models, however, the domain is less than that of lanminar inflow; the coherent structures have the features of moving fast and transforming slow.Secondly, two-point spatial correlation functions are evaluated statistically on NPLS images to quantify average mean size, shape and structure angles. The mean structure is elliptic and it is consistent with flow imaging; the structure angle of both cases increases with its distance away from the wall. When it is lanminar inflow, structure angles are slightly small and no more than 40°; as it is turbulent inflow, structure angles are from 35°to 65°, in good agreement with results of other flow imaging.Thirdly, PIV is employed to investigate the velocity structure of supersonic flow over the forward facig step, elliptic surface model and double elliptic surface model. Proper orthogonal decomposition(POD) is appled to the velocity and vorticity fields of the forward facig step and elliptic surface model. It shows velocity fluctuation mainly lies in the boundary layer and around the separation region; the maximum vorticity exists at the beginning of the flow separation or above the models; there is obvious reverse region upstream of the models, moreover, the results of velocity fields shows that the area of the reverse region of laminar inflow is much more than that of turbulent inflow, which confirms the results of flow imaging. The POD velocity analysis of both inflows on the two models shows the energy contribution of the first mode is rather large, more than 97% and it indicates that the dominant on velocity fields is the structures of the first mode. The POD vorticity analysis of both inflows on the two models shows the energy contribution of the first mode is also very large, about 50% and it indicates that the dominant on vorticity fields is the structures of the first mode. However, the contributionof the second mode to the tenth mode is more than 1%, thus these mode can not also be neglected.Lastly, the NPLS and PIV are used to investigate the effect of Ashill and Anderson micro-ramps on the the forward facig step, elliptic surface model, double elliptic surface model and double wedge. The wake of Ashill micro-ramp can be divided into two regions; the first region is characteristic of a fairly darker stripe, the average elevation angle of which is about 6.7°, slightly larger than the lean angle of the micro-ramp’ up surface 5.7°; the second region is characteristic of intermittent large-scale structures, which are hairpin-like struutures. On the control of micro-ramps, the wake can also be divided into two regions; but the area of two regions, the elevation angle of the first region and structure of large-scale votices have some difference. The fine structures of these large-scale structures are imaged and the time evolution is given. These two micro-ramps can reduce the area of reverse region and weaken the flow separation.