• local recirculations
• drag reduction
• flat plate

The object of this work is to analyse the effects of local recirculations on the total drag of a flat plate. Single and multiple appropriately shaped cavities are used.
First, simulations are carried out in laminar and turbulent regime for the flat plate and the results are validated against theoretical and experimental solutions. Then single and multiple cavities are introduced in the flat plate in
laminar regime and their effect is investigated by comparing the flow features with the ones of the reference configuration. First, a shape optimization is carried out for a single cavity to identify the geometry that minimizes the
total drag and a comparison with the reference flat plate is presented. The introduction of the optimal cavity geometry leads to a reduction of the total drag on the flat plate of 5%. Then, multiple cavities are introduced in the flat plate. The effects of different numbers of cavity and different widths of the cavities are analysed. The use of multiple cavities leads to maximum reduction of overall drag of the order of 4%. The most important geometrical parameters are also identified both for single and multiple cavities. This favourable effect both in single and multiple cavity geometries is mainly due
to the presence of small recirculation regions inside the cavities, characterized by very small velocities, which leads to a reduction of the tangential stresses.
Even if the pressure drag increases, the use of cavities leads to reductions of overall drag in laminar regime.