System and method for using a flexible composite surface for pressure-drop free heat transfer enhancement and flow drag reduction

A method of reducing velocity and pressure fluctuations in a non-zero pressure gradient boundary layer flow using a flexible composite sheet, the flexible sheet comprising a membrane;

• a substrate supporting the said membrane;
• a plurality of equispaced ridges with two distinct heights coupled to the substrate between the said membrane and the said substrate;
• and a gas enclosed in the space between the substrate and the membrane such that modes of flexural oscillation of the said membrane which force the said gas over the shorter of the said ridges experience higher viscous damping compared to other modes
• the method comprises the steps of:
• exposing the membrane to a region in a flow of fluid where the streamwise flow pressure gradient changes from favorable to adverse thereby creating the condition of efficient transfer of vibratory motion from the flow to at least one segment of a membrane spanning a distance(s) over two of the said ridges, wherein the vibratory motion is reflected from the at least one segment of the membrane to the flow, and;
• wherein a reduction in fluctuations is caused in the flow pressure gradient and freestream velocity U at all frequencies except around f, where f≈U/s.