The influence of periodic excitation on a turbulent separation bubble

Turbulent separation limits the performance in many engineering applications, for example creating pressure losses in diffuser like flows or stall on aircraft wings. In the present study the turbulent boundary layer flow over a flat plate separating due to an adverse pressure gradient is studied as a model problem and the effect of periodic excitation in both time and space is investigated through direct numerical simulations. Linear stability analysis is used to analyse the sensitivity of the flow with respect to time-periodic excitations. The dependence on position, amplitude and frequency of the forcing is investigated. For a certain frequency range at sufficiently high amplitudes, it is possible to eliminate the separated region. Furthermore, three-dimensional effects are studied by applying a steady spanwise forcing as well as a both time-dependent and spanwise varying forcing. A forcing varying in spanwise direction is shown to be the most effective in eliminating the separated region, whereas two-dimensional time-periodic excitation was not as efficient as it was expected.