DNS of separating, low Reynolds number flow in a turbine cascade with incoming wakes

Abstract

This chapter reviews the threedimensional direct numerical simulations (DNS) of flow in a low-pressure turbine cascade at high angle of attack. The large angle of attack is known to cause separation at the leading edge and upstream of the trailing edge on the suction side of the blade. The separation bubble at the leading edge is small and any disturbances generated are generally damped downstream by the accelerating flow because of the favorable pressure gradient. It is noted that the boundary layer at the downstream, half of the suction side separates as soon as the impingement of free-stream disturbances, stemming from a passing wake ends. The separated boundary layer is however very unstable and after some transient time, small disturbances cause it to roll up. Near the trailing edge the flow remains turbulent at all times. Simulations performed exhibits longitudinal vortical structures along downstream and half of the pressure side of the blade is generally obtained by straining of passing wakes. Along the upstream, though half of the suction side similar longitudinal vortices are found to be formed by severe stretching of the wake by the strong flow. Opposed to the vortical structures found at the pressure side, the suction side vortical structures only exist for a short time.