Linear cascade of Rolls-Royce T2 turbine blades tested above their design Mach number in a free jet tunnel

Distinctive fish tail shocks stem from the blade trailing edges. These reflect off neighbouring blades and affect the growth of the trailing edge shear layers further downstream. A lambda shock stems from the topmost blade. Secondary flows are visible: For instance, horse-shoe vortices rap around the blades and are marked by thin dark lines running parallel to the shear layers.

This picture was obtained with a double-pass schlieren method, using a mirrored side wall and a horizontal knife edge in place of traditional schlieren glass. A 200 microsecond flash was used to freeze the flow.

Reference:

A. Rona, J.P Gostelow, R. Paciorri, M. Geron: "Wall interference in the discharge flow in a linear cascade wind tunnel," AIAA 2003-0455, 41st Aerospace Sciences Meeting and Exhibit, 6-9 January 2003, Reno, NV, USA. Photograph by P. Wingrove.