Objective When the aircraft works at high altitude, the surrounding air density is low, the unit Reynolds number of incoming flow is small, and it is difficult for the forebody compression surface to transition into turbulence, which makes the boundary layer easy to separate and reduces the performance of the aircraft. In order to solve this problem, a forced transition device can be installed on the compression surface to promote the transition. Forced transition methods can be divided into the following two types: one is passive transition control induced by rough element, and the other is active transition control based on jet flow. Because the jet penetration height can be changed by adjusting the jet pressure ratio, the transition position can be adjusted according to the demand, which has become an international research hotspot. Most of the related studies only use infrared cameras to obtain temperature distribution, lacking detailed flow field information. A few numerical studies explain the flow mechanism by obtaining the development of flow structures such as horseshoe vortex and sy mmetric vortex, but they have not been analyzed in combination with vorticity dynamics. The RM (Richtmyer Meshkov) instability is caused by baroclinic vorticity due to the existence of shock wave and vortex structure near the supersonic transverse jet, so the flow mechanism can be understood more clearly by combining the vorticity dynamics theory. The purpose of this study is to study the influence of jet pressure ratio on the boundary layer disturbance and transition position of wake vortex, and to analyze the flow mechanism combined with baroclinic vorticity theory in vorticity dynamics.

Methods The forebody configuration is taken as the research object (Fig.5), and the γ-Re_θ transition model is used for numerical calculation, and the high-pressure storage and low-pressure injection device with orifice diameter of 0.5 mm is used for experimental verification in a 1m high speed wind tunnel (Fig.2). The influence of jet pressure ratio on the transition is analyzed by the turbulent kinetic energy in the boundary layer, and the baroclinic vorticity theory of vorticity dynamics is introduced to analyze the intensity of RM instability and explain the flow mechanism. Considering the influence of jet on transition and flow velocity gradient in the near-wall region, the reason for the variation of infrared heat flux on the wall with jet pressure ratio is analyzed.

Results and Discussions Jet will change the flow velocity near the wall, and the flow velocity will decrease significantly before X=700 mm, and the flow velocity in most areas will decrease with the increase of jet pressure ratio; When X=800 mm and the normal distance h_w<0.5 mm, the effect of jet on velocity is small; When h_w>0.5 mm, the flow velocity decreases with the increase of jet pressure ratio. Increasing jet pressure ratio can effectively reduce the flow velocity in most near-wall areas, and then reduce the normal gradient of flow velocity (Fig.9). The jet induces a flow vortex pair, which gradually dissipates along the flow direction. Increasing the jet pressure ratio can increase the vortex intensity (Fig.11). Baroclinic vorticity is generated near the jet and increases with the increase of jet pressure ratio, which indicates that active jet control can generate baroclinic vorticity, and then cause RM (Richtmyer Meshkov) instability to promote boundary layer instability. With the increase of jet pressure ratio, RM instability and disturbance in boundary layer gradually increase, which will promote transition more effectively (Fig.13). At the section Z=21 mm, when Xϵ600 mm, 700 mm, the Stanton number St is laminar, jet pressure ratio 25, jet pressure ratio 40, no jet transition, jet pressure ratio 100, turbulence. At the section Z=28 mm, turbulent St is the largest. When X is 520 mm, 710 mm, the turbulence St without jet is greater than that with jet (Fig.14). Infrared heat flux on the wall varies irregularly with the jet pressure ratio, which may be affected by the normal gradient of flow velocity near the wall and the transition degree of boundary layer.

Conclusions When the jet is applied to the forced transition on the front compression surface, the flow velocity decreases along the normal gradient, resulting in the effect of reducing the surface friction resistance. At the same time, baroclinic vorticity will be generated near the jet, which will cause RM instability and promote the instability of the boundary layer. The intensity of instability increases with the increase of jet pressure ratio. Infrared heat flux on the compression surface is affected by velocity gradient and transition degree in the near-wall region at the same time, but the two factors have opposite effects, which leads to no consistent change of relative magnitude of infrared heat flux.