Effects of free-stream turbulence on the global pressure fluctuation of compressible transitional flows in a low-pressure turbine cascade

Abstract

PurposeThis paper aims to investigate global pressure fluctuations in compressible transitional flows in a low-pressure turbine cascade because of variations in the free-stream turbulence and its interaction with the boundary layers.Design/methodology/approachTransition process resolving numerical simulations are performed with different types of inflow turbulence. The unsteady three-dimensional fully compressible Navier–Stokes equations are solved using a sixth-order compact difference and a tenth-order filtering method. First, simulations of both K-regime and bypass transitions are conducted for a flat plate boundary layer to validate the use of the filter in computing different transition routes. Second, computations of the cascade flows are conducted. Cases of no free-stream turbulence, isotropic free-stream turbulence of 5 per cent and wakes from an upstream cylinder are compared. For wakes, variations in wake trajectory depending on the cylinder blade relative position are also taken into account.FindingsThe different transition routes are successfully reproduced by the present method even with strong filtering. When feedback phenomena occur near the trailing edge, high-frequency oscillations dominate in the flow field. Low-frequency oscillations become dominant when the blade boundary layer becomes turbulent. Thus, the effects of the free-stream turbulence and its interaction with the boundary layer appear as changes in the global pressure fluctuation.Originality/valueThe free-stream turbulence qualitatively affects global pressure fluctuations, which become a medium to convey boundary-layer information away from the cascade.