Boundary-Layer Transition Over Rough Surfaces With Elevated Free-Stream Turbulence

Abstract

This paper presents experimental results documenting the combined effects of surface roughness and free-stream turbulence level on boundary-layer transition. The experiments were conducted on a flat surface, upon which a pressure distribution similar to those prevailing on the suction side of turbine blades was imposed. The test matrix consists of four variations in the roughness conditions, at each of three free-stream turbulence levels and two flow Reynolds numbers. The ranges of these parameters considered in the study, which are typical of low-pressure turbines, resulted in both attached-flow and separation-bubble transition. The experimental results show that the transition inception location remains sensitive to surface roughness with increasing free-stream turbulence. Through spectral analysis of the velocity signals, this is shown to be due to earlier appearance and larger amplitude of Tollmien-Schlichting instability waves in both attached-flow and separation-bubble transition. In the test cases in which a separation-bubble is present, the rate of transition is seen to be insensitive to surface roughness, and only mildly sensitive to free-stream turbulence.