Unsteady Unstart

Unstart in air-breathing inlets in an ongoing issue that results in large acoustic and thermal loads and vehicle failure. This work seeks to better understand the dynamic behavior of the strong starting shocks in the Mach 2 High-Speed Wind Tunnel.

The first case evaluated looked at when the flow is started upstream of the cylinder, as shown in the Schlieren animation above. With the flow from left to right, interface shock structures are seen upstream of the vertical cylinder, indicating an established Mach 2 flow; however, downstream of the cylinder, the blockage from the cylinder and the generated inviscid shock prevents the freestream flow from recovering, and the test section is in an "unstarted" state. Close to the back end of the cylinder, the inviscid shock impinging on the test section sidewalls can be seen. These images were collected at 50 Hz and are played back at 10 Hz and the cylinder moves upwards approximately 1/4 inch.

In the second case, the cylinder begins high enough so that the wind tunnel cannot start upstream. The "starting shock" is observed on the left-side of the above animation for the first few frames. As the cylinder height reduces, the flow will eventually start and the starting shock will move downstream. Once that happens, the flowfield looks the same as that in Case 1 above. The Schlieren animation was acquired at 50 Hz and is played back at 7 Hz with the flow from left to right.

This false-color oil flow animation shows the separation shock interacting with corner flow. As the wind tunnel is gradually turned on, the inviscid shock structure on the test section floor develops. Due to the large cylinder diameter, the inviscid shock interacts with the test section sidewall. This results in the oil following the shock shape and travel up the sidewall, seen on the left-hand side of the animation. The flow is from top to bottom and animation was acquired at 50 Hz and playback is at 5 Hz.

These false-color oil flow images show more specifically the sidewall interaction with the separation shock. In part (a) the shift in where the shock interacts with the sidewall as the cylinder diameter increases is shown. In (b) the oil moving with the separation shock on the wind tunnel sidewall is evident. Finally in (c) the resulting oil flow pattern from the outside of the test section is shown.