Influence of wake passing frequency and elevated turbulence intensity on transition

Experimental results from a study of the effects of passing wakes upon laminar-to-turbulent transition in a low-pressure turbine passage are presented. The test section simulates the effects of unsteady wakes resulting from rotor-stator interaction in turbine blade boundary layers and separated flow regions over suction surfaces. Single-wire thermal anemometry techniques are used to measure time-resolved and phase-averaged wall-normal profiles of streamwise velocity, turbulence intensity, and intermittency at multiple streamwise locations over the turbine airfoil suction surface. The Reynolds number based on suction surface length and stage exit velocity is 5 × 10⁴. This low Reynolds number would apply to small engines flying at high altitude. This study compares cases of either decreased wake passing frequency or elevated approach flow turbulence intensity to a base case. When compared to the base case, the decreased wake frequency case shows an earlier separation with a larger separation bubble, whereas the elevated freestream turbulence intensity case shows a further downstream separation, but a shorter, thinner separation bubble.