An investigation of two-dimensional flow separation with reattachment

This thesis presents an experimental study and numerical predictions of the separated-reattaching flow around a bluff rectangular section. This laboratory configuration, chosen for its geometric simplicity, exhibits all main features of two-dimensional flow separation with reattachment.

Detailed turbulent flow measurements of the man and fluctuating flow field are reported. The measurement techniques used are: hot-wire anemometry, pulsed-wire anemometry and pulsed-wire surface shear stress probes. The separated shear layer appears to behave like a conventional mixing layer over the first half of the separation bubble, but exhibits a lower growth rate and higher turbulent intensities in the second half. In the reattachment region, the flow is found to be highly turbulent and unsteady.

A finite difference method is used, in conjunction with a modified version of the TEACH code, to predict the mean flow field. Two discretization schemes are used: the hybrid-upwind differencing (HD) scheme, and the bounded-skew-hybrid differencing (BSHD) scheme. Laminar flow computations are performed for Reynolds numbers in the range 100 to 325. The HD computations underpredict the separation-bubble length by up to 35% as a result of false diffusion. The BSHD predictions, on the other hand, are in excellent agreement with the experimental results reported in the literature.

Turbulent flow computations using the k-$\epsilon$ turbulence model and the BSHD scheme result in a reattachment length about 30% shorter than the present measured value. When a curvature correction is incorporated into the model, a reattachment length of 4.3D, compared to the experimental value of 4.7D, is predicted. The predicted mean flow, turbulent kinetic energy field and pressure distribution are in good agreement with experimental observations.

An alternative method of analysis, based on the momentum integral technique, is presented. The method is not applied to the blunt-rectangular plate problem, but its use is illustrated for the simpler case of the flow in a sudden expansion, and promising results are obtained.