Semiconductor Nanomembrane-Based Flight Sensors and Arrays

There are two broad classes of methods for measuring skin friction (or wall shear): indirect and direct. The direct methods generally employ a movable element of the actual surface surrounded by a very small gap and connected to some type of flexure. One then measures the displacement of the movable element or the strain in the flexure to obtain the skin friction force acting on the movable element directly. Some methods of detecting skin friction relied on optical interferometric measurements of the thickness of an oil film applied to a test surface. The motion of the oil due to airflow creates thickness variations that can be associated to the frictional forces acting on the surface. Mapping of the surface over a small area is possible, but practical use of this technique is limited due to contamination of the tunnel by the free-flowing oil.