Corner effects for oblique shock wave/turbulent boundary layer interactions in rectangular channels
In a rectangular cross-section wind tunnel a separated oblique shock reflection is set to interact with the turbulent boundary layer (oblique SBLI) both on the bottom wall and in the corner formed by the intersection of the floor with the side-walls. In such a scenario, shock-induced separation is often seen in each of the streamwise corners, resulting in a highly three-dimensional flow field in the near-wall region. To examine how the corner separations can affect the ‘quasi-two-dimensional’ main interaction and by what mechanism this is achieved, an experimental investigation has been conducted. This examines how modifications to the corner separation influence an oblique shock reflection. The nature of the flow field is studied using flow visualisation, Pressure Sensitive Paint and Laser Doppler Anemometry. A nominal freestream Mach number of 2.5 is used for all experiments with a unit Reynolds number of 40×10$^{6}$m$^{−1}$, and the shock-generator angle is set to 8°. The flow conditions are chosen to result in substantial separations both in the corners and along the centreline for the baseline case, which is thought to be a good starting point for this study. The results show that the size and shape of central separation vary considerably when the onset and magnitude of corner separation change. The primary mechanism coupling these separated regions appears to be the generation of compression waves and expansion fans as a result of the displacement effect of the corner separation. The presence and strength of the expansion waves have been overlooked in previous studies. This is shown to modify the three-dimensional shock-structure and alter the adverse pressure gradient experienced by the tunnel floor boundary layer. It is suggested that a typical oblique SBLI in rectangular channels features several zones depending on the relative position of the corner waves and the main interaction domain. In particular, it has been shown that the position of the corner ‘shock’ crossing point, found by approximating the corner compression waves by a straight line, is a critical factor determining the main separation size and shape. Thus, corner effects can substantially modify the central separation. This can cause significant growth or contraction of the separation length measured along the symmetry line from the nominally two-dimensional baseline value, giving a fivefold increase from the smallest to the largest observed value. Moreover, the shape and flow topology of the centreline separation bubble is also considerably changed by varying corner effects.
boundary layer separation, high-speed flow, oblique shock waves
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