Measurements in separated and transitional boundary layers under low-pressure turbine airfoil conditions

Derailed velocity measurements were made along a flat plate subject to the same dimensionless pressure gradient as the suction side of a modern low-pr essure turbine airfoil. Reynolds numbers based on wetted plate length and n ominal exit velocity were varied SI om 50,000 to 300,000, covering cruise t o takeoff conditions. Low and high inlet free-stream turbulence intensities (0.2 and 7 percent) were set using passive grids. The location of boundary -layer separation does not depend strongly on the free-stream turbulence le vel oi Reynolds number, as long as the boundary layer remains nonturbulent prior to separation. Strong acceleration prevents transition on the upstrea m part of the plate in all cases. Both free-stream turbulence and Reynolds number have strong effects on transition irt the adverse pressure gi-adient region. Under low free-stream turbulence conditions, transition is induced by instability waves in the shear layer of the separation bubble. Reattach ment generally occurs at the transition start. At Re = 50,000 the separatio n bubble does not close before the trailing edge of the modeled airfoil. Ar higher Re, transition moves upstream, and the boundary, layer reattaches. With high free-stream turbulence levels, transition appears to occur in a b ypass mode, similar to that in attached boundary layers. Transition moves u pstream, resulting in shorter separation regions. At Be above 200,000 trans ition begins before separation. Mean velocity, turbulence, and intermittenc y profiles are presented.