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.

Three-dimensional mapping of small areas was accomplished by measuring thickness variations. Optical measurement of the displacement of thin polymer films for skin friction measurement is also under study. One drawback to this technique is that the polymer undergoes a significant shear modulus change during the first 60 days of use, and therefore needs to be aged until the polymer is stable if the shear modulus cannot be measured prior to wind tunnel testing. The second drawback is that it relies on cameras and line-ofsight measurements during tunnel testing. Moreover, temperature effects cannot be overlooked for polymer-based sensors.

Semiconductor nanomembrane-based flight sensors and arrays are being developed on flexible substrates. Such lowmodulus, conformal nanomembrane (NM) sensor skins with integrated interconnect elements and electronic devices can be applied to new or existing wind tunnel models for skin friction analysis, or to lightweight UAVs (unmanned aerial vehicles) such as the X-56A. The feasibility of NM transducer materials in such sensor skins for the measurement of dynamic shear stress and normal pressure has been demonstrated.

Semiconductor NM sensor skins are thin, mechanically and chemically robust materials that may be patterned in two dimensions to create multi-sensor element arrays that can be embedded into small probe tips or conformally attached onto vehicle and model surfaces. Sensors may be connected to external support instrumentation either through thin film and ribbon cable interconnects, or potentially wirelessly using RF communication directly from electronic networks incorporated into the sensor skin material.

Nanomembrane sheet material is formed by cutting the semiconductor membranes from the active semiconductor layer-on-insulator (SOI in case of Si) using conventional CMOS (complementary metal-oxide semiconductor) processing techniques. It permits crystalline membrane and advanced polymers to be co-processed in a way that controls the sensitivity and dynamic range of the sensor elements. The NM sensors combine the advantages of both crystalline/inorganic sensors and polymer/organicbased sensors.

Tailoring both the electrical conductivity and mechanical modulus allows a means of optimizing stress-induced strain transducer response. NM thin film flow sensors act as mechanoelectrical transducers to convert stress-induced tangential surface forces and normal stress into electrical output signals.

The sensor tips are thin and surfacemounted, causing minimal interaction with the flow, or potentially can be applied as an applique, and require no cavities or recesses in the test articles other than holes to connect the sensor array leads to data acquisition wiring. The NM material is also resistant to normal fluids and solvents, can potentially operate over a temperature range of –65 to +800 °C (all-silicon version), and is capable of withstanding rain and dust erosion.

This work was done by Hang Ruan and Echo Kang of Nanosonic Inc., and Wing Ng of Virginia Tech for Kennedy Space Center. KSC-13897

NASA Tech Briefs Magazine

This article first appeared in the December, 2015 issue of NASA Tech Briefs Magazine.

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