Turbulent drag reduction through rotating discs

An active technique for friction drag reduction in a turbulent channel flow is studied by direct numerical simulations. The flow modification is induced by the steady rotation of rigid flush-mounted discs, located next to one another on the walls. The effect of the disc motion on the turbulent drag is investigated at a Reynolds number of Rτ = 180 , based on the friction velocity of the stationary-wall case and the half channel height. For a fixed maximum disc tip velocity, drag reduction can be achieved when the disc diameter is larger than a threshold, while below this threshold the drag increases. A maximum drag reduction of 23% is computed. The net power saved, obtained by taking into account the power spent to enforce the rotational motion against the fluid viscous resistance, is found to be positive and reach 10%. The disc-flow parameters required for commercial aircraft flight conditions and flows over high-speed trains and ship hulls are estimated and future implementations based on existing micro-electromagnetic motor and micro-air turbine technologies are discussed.