The Effect of Sensor Dimensions on the Performance of Flexible Hot Film Shear Stress Sensors

This paper presents a study to determine the effect of sensor dimensions (length, width, and thickness) on the performance of flexible hot film shear stress sensors. The sensing component of a hot film sensor is nickel thermistor, and the flexible substrate material is polyimide. Several groups of flexible hot film shear stress sensors with different lengths, widths, and thicknesses were studied. The temperature coefficient of resistance (TCR) was measured. The TCR increased slightly with increasing thickness. The frequency response (time constant) of the flexible hot film shear stress sensor was obtained by the square wave, while the sensitivity was tested in a wind tunnel. The study found that as the sensor length was shortened, the frequency response increased, and the sensitivity decreased.

A Flexible Hot-Film Sensor Array for Underwater Shear Stress and Transition Measurement

A flexible hot-film sensor array for wall shear stress, flow separation, and transition measurement has been fabricated and implemented in experiments. Parylene C waterproof layer is vapor phase deposited to encapsulate the sensor. Experimental studies of shear stress and flow transition on a flat plate have been undertaken in a water tunnel with the sensor array. Compared with the shear stress derived from velocity profile and empirical formulas, the measuring errors of the hot-film sensors are less than 5%. In addition, boundary layer transition of the flat plate has also been detected successfully. Ensemble-averaged mean, normalized root mean square, and power spectra of the sensor output voltage indicate that the Reynolds number when transition begins at where the sensor array located is 1.82 × 105, 50% intermittency transition is 2.52 × 105, and transition finishes is 3.96 × 105. These results have a good agreement with the transition Reynolds numbers, as measured by the Laser Doppler Velocimetry (LDV) system.

Localization of Flow Separation and Transition Over a Pitching NACA0012 Airfoil at Transitional Reynolds Numbers Using Hot-Films

Separation and transition of flow over a pitching airfoil at transitional Reynolds Number are analyzed experimentally. The method is based on a windowed correlation between hot-film sensor signals taken simultaneously or synchronized based on the airfoil pitch angle. A comparison with large eddy simulation (LES) simulation obtained in the same conditions is presented and discussed. Our results agree very well with the LES data for the separation location during the pitch-up. For the pitch-down, the results show differences with the LES. However, when the LES data are analyzed with the same correlation technique, the results are in a better agreement.