Characteristics of a hot wire microsensor for time-dependent wall shear stress measurements

Hot-wire microsensors for the purpose of measuring the instantaneous velocity gradient close to a wall have been designed and their characteristics are evaluated. The sensors were made using MEMS (microelectromechanical systems) technology, which permits the fabrication of various micro geometrical configurations with high precision and good repeatability. The design is based on estimates of the heat rates from the sensor wire to the air, through the supports, and to the wall. Several hot-wire configurations were fabricated with wires positioned in the range of 50-250 μm from the wall. Requirements for the design and details of the fabrication methodology are outlined. The hot-wire microsensors were calibrated and tested in a flat-plate boundary layer with and without pressure gradient and were found to have good steady-state characteristics. In addition, the developed sensors were used for preliminary studies of transitional phenomena and turbulence, and the sensors were found to have good time-dependent response as well