Conditionally sampled flow and thermal behavior of a transitional boundary layer at elevated free-stream turbulence

Abstract

Abstract In the elevated FSTI condition, the laminar flow is highly disturbed. Linear instability amplification, which would take place under low-disturbance conditions triggered by infinitesimal disturbances, is bypassed, hence the term “bypass transition”. Nonlinear instability, triggered by finite-amplitude disturbance, dominates in bypass transition. Due to the highly disturbed condition, the turbulent and non-turbulent parts of the transitional flow become difficult to separate. Because of this, the conventional conditional techniques for such sampling encounter difficulty and large uncertainty. A modified method was made. This modified method was convenient to apply and was theoretically verified. Use of the turbulence shear stress, uv , as the criterion function was found superior to the use of u ′ for separating the turbulent and non-turbulent signals in elevated FSTI conditions. The conditionally sampled results for FSTI, ranging from 0.5% to 6.4%, indicated that u ′, v ′, t ′, and ut are high in the non-turbulent part of the high FSTI cases contrary to the low FSTI cases, whereas ( uv and vt ) are low in the non-turbulent part, similar to the low FSTI cases. This implies that although the velocity trace of the non-turbulent part was hardly distinguishable from the turbulent part in high FSTI situations, the flow and thermal transport mechanisms of the non-turbulent part were distinctively different from the turbulent part. Evaluation of each part (turbulent and non-turbulent) is presented in detail in this paper.