Effects of Criterion Functions on Intermittency in Heated Transitional Boundary Layers With and Without Streamwise Acceleration

Abstract

Attempting to understand the mechanisms of momentum and thermal transports in transitional boundary layers has resulted in the use of conditional sampling to separate the flow into turbulent and nonturbulent portions. The choice of a proper criterion function to discriminate between the two flow conditions is critical. A detailed experimental investigation was performed to determine the effects of different criterion functions on the determination of intermittency for application in heated transitional boundary layers with and without streamwise acceleration. Nine separate criterion functions were investigated for the baseline case without pressure gradient and three cases with streamwise pressure gradient. Inherent differences were found to exist between each criterion function’s turbulence recognition capabilities. The results indicate that using a criterion function based on Reynolds shear stress, (∂uv/∂τ)2 , for turbulent/nonturbulent discrimination in a heated transitional boundary layer is superior to a single velocity or temperature scheme. Peak values in intermittency for the early to midtransitional region were found to occur away from the wall at approximately y/δ = 0.3 for all cases. To match the universal intermittency distribution of Dhawan and Narasimha (1958), the minimum values of intermittency at y / δ ≈ 0.1 should be used as the representative “near-wall” values.