Characterisation of a high Reynolds number boundary layer subject to pressure gradient and separation

Abstract

The flow over a ramp model is characterised in detail in the present study. In the selected configuration, a turbulent boundary layer developed on a flat plate is first accelerated in a curved contraction. It is then submitted to a mild adverse pressure gradient on a flat plate followed by a separation above a flap. Inlet boundary conditions and pressure distribution are provided to allow numerical simulations. The flow in the mild adverse pressure gradient region is characterised with hot-wire anemometry. In this region, the boundary layer thickness is of the order of 20 cm, the momentum Reynolds number is about 11,000 and the Clauser pressure gradient parameter β in the stabilised region is about 0.4. Particular emphasis is laid on the separation to provide quantitative information to evaluate turbulence models. This is achieved through a large streamwise two-dimensional two-component particle image velocimetry (2D2C PIV) plane which contains all the separation bubble and part of the flow upstream and downstream of it. The separation border is detected using the backflow coefficient, resulting in a separation length of about 3.49 H_s (with H_s the flap step height) and a maximum height of about 0.17 H_s. The Reynolds stresses and their main production terms are also determined. A region of high turbulence intensity develops above the separation border for all the measured components. The production of dominates the production of turbulent kinetic energy which implies a redistribution from to to explain the increase observed. The production term drives the production of in the first part of the flap which is not the case for zero pressure gradient boundary layers. Finally, a high similarity is observed between and as the production of the latter is dominated by . This flow appears as a challenging test case for Reynolds averaged Navier–Stokes (RANS) and large eddy simulation (LES) validation.

Keywords:

• turbulent boundary layer
• flow separation
• adverse pressure gradient
• particle image velocimetry
• hot-wire anemometry

Acknowledgements

The authors would like to thank Dr Sebastien Coudert for help in setting the PIV experiment.

Additional information

Funding

The authors would like to acknowledge the financial support provided by the European Community under the contract AVERT/RTD REG/H.3(2006)A/142121; the International Campus on Safety and Intermodality in Transportation (CISIT).