Experimental studies of zero pressure-gradient turbulent boundary layer flow

TLDR: In this paper, the authors investigated the high Reynolds number zero pressure gradient turbulent boundary layers in an incompressible flow without any effects of heat-transfer and found that the inner limit of overlap region was found to scale on the viscous length scale (ν/uτ) and was estimated to be y = 200.

Abstract: This thesis deals with the problem of high Reynolds number zero pressuregradient turbulent boundary layers in an incompressible flow without any effects of heat-transfer. The zero-pressure gradient turbulent boundary layer is one of the canonical shear flows important in many applications and of large theoretical interest. The investigation was carried out through an experimental study in the MTL wind-tunnel at KTH, where the fluctuating velocity components and the fluctuating wall-shear stress in a turbulent boundary layer were measured using hot-wire and hot-film anemometry. Attempts were made to answer some basic and “classical” questions concerning turbulent boundary boundary layers. The classical two layer theory was confirmed and constant values of the slope of the logarithmic overlap region (i.e. the von Karman constant) and the additive constants were found and estimated to κ = 0.38, B = 4.1 and B1 = 3.6 (δ = δ95). The inner limit of overlap region was found to scale on the viscous length scale (ν/uτ) and was estimated to be y = 200, i.e. considerably further out compared to previous knowledge. The outer limit of the overlap region was found to scale on the outer length scale and was estimated to be y/δ = 0.15. This also means that a universal overlap region only can exist for Reynolds numbers of at least Reθ ≈ 6000. The values of the newly determined limits explain the Reynolds number variation found in some earlier experiments. Measurements of the fluctuating wall-shear stress using the hot-wire-onthe-wall technique and a MEMS hot-film sensor show that the turbulence intensity τr.m.s./τw is close to 0.41 at Reθ ≈ 9800. A numerical and experimental investigation of the behavior of double wire probes were carried out and showed that the Peclet number based on wire separation should be larger than about 50 to ensure an acceptably low level of thermal interaction. Results are presented for two-point correlations between the wall-shear stress and the streamwise velocity component for separations in both the wallnormal-streamwise plane and the wall-normal-spanwise plane. Turbulence producing events are further investigated using conditional averaging of isolated shear-layer events. Comparisons are made with results from other experiments and numerical simulations. Descriptors: Fluid mechanics, turbulence, boundary layers, high Reynolds number, zero-pressure gradient, hot-wire, hot-film anemometry, oil-film interferometry, structures, streak spacing, micro-electro-mechanical-systems.