Boundary layer development after a separated region

Abstract

Measurements obtained in boundary layers developing downstream of the highly turbulent, separated flow generated at the leading edge of a blunt flat plate are presented. Two cases are considered : first, when there is only very low (wind tunnel) turbulence present in the free-stream flow and, second, when roughly isotropic, homogeneous turbulence is introduced. With conditions adjusted to ensure that the separated region was of the same length in both cases, the flow around reattachment was significantly different and subsequent differences in the development rate of the two boundary layers are identified. The paper complements, but is much more extensive than, the earlier presentation of some of the basic data (Castro & Epik 1996), confirming not only that the development process is very slow, but also that it is nonmonotonic. Turbulence stress levels fall below those typical of zero-pressure-gradient boundary layers and, in many ways, the boundary layer has features similar to those found in standard boundary layers perturbed by free-stream turbulence. It is argued that, at least as far as the turbulence structure is concerned, the inner layer region develops no more quickly than does the outer flow and it is the latter which essentially determines the overall rate of development of the whole flow. Some numerical computations are used to assess the extent to which current turbulence models are adequate for such flows.

Tomographic-PIV measurement of the flow around a zigzag boundary layer trip

“…For example, hu 0 u 0 i and hu 0 v 0 i are amplified by a factor 7-10 at a distance of three boundary layer thicknesses downstream of average reattachment (x/d 0 & 8 in Fig. 5), which is in agreement with the enhancement of the Reynolds stresses downstream of a laminar separation, as reported by Castro and Epik (1998). Moreover, the order of magnitudes of the Reynolds stresses is consistent with results obtained just behind a turbulent separation (Alving and Fernholz 1996).…”

“…This occurred after the boundary layer reached a Reynolds number based on momentum thickness of 2,175, which, for the present conditions (U e = 0.21 m/s), is estimated to be 4.6 m (*1,000d 0 ) downstream of the trip. Similarly, the dominance of energetic and very slowly decaying largescale flow structures was also observed from the spectra taken downstream of the turbulent reattachment of a laminar separation bubble (Castro and Epik 1998).…”

On the Formation Mechanisms of Artificially Generated High Reynolds Number Turbulent Boundary Layers

“…On the contrary, shed vortices with vorticity in z, i.e. the recirculation-like structures present for the Saw case and also reported in Alving and Fernholz (1996) and Castro and Epik (1998), would enhance the wake influence on the wall generating a wake-driven mechanism.…”

“…These trips with non-uniform blockage create a distorted wake which, after an adaptation region, may influence the formation of the inner structures up to a larger extent than in the former case, where the inner structures may not be as disturbed due to the low blockage at y = 0. This influence of the wake in the near-wall region has also been reported as a characteristic of re-attached flows downstream of a separation (Alving and Fernholz 1996;Castro and Epik 1998). Different heights and degrees of immersion in the boundary layer are tested in order to assess the influence of these parameters on the recovery length for canonical properties of the generated boundary layer.…”

Spectral analysis of a transitional separated-reattached flow using Fourier and wavelet transforms

“…For laminar separation, as in the current case, the transition could be delayed leading to a longer bubble than the case for turbulent separation. A relevant type of flow to the FFS is the blunt plate experiment of Castro and Epik [13] (Re D = 6500) in which the reattachment is reported to be 7.7D, where D is the plate thickness. This is comparable to the mean reattachment length for the current simulation.…”