Reduction of Pressure Losses by the In-Plane Contraction/Relaxation Waves

Channel flow modifications caused by the in-plane contraction/relaxation waves applied at the bounding walls were investigated. The question was posed regarding the change in the pressure gradient required to drive the same flow rate with and without waves, or as the change in flow rate when the pressure gradient was kept constant. It was determined that the waves generate a pumping effect that acts in the opposite direction to wave propagation as well as a propulsive effect. The pumping effect may increase/decrease pressure losses in the flow. Negative waves, i.e., waves propagating against the flow, always reduce pressure losses. Positive waves, i.e., waves propagating with the flow, reduce losses only if the flow is sufficiently faster than the waves. A significant increase in pressure losses occurs when the properties of the waves match the natural frequencies of the flow.