Skin friction measurement in variable temperature flow

A new skin friction sensor with constant sensitivity in variable temperature flow has been developed. It is based on a specially designed temperature compensation bridge which contains two sub-bridges for flow and temperature sensors. In our case identical hot film elements (active surface 1.5 mm×2.5 mm), one operating as a hot film (skin friction) and one as a cold film (temperature) were mounted side by side on a small plate. Calibration shows the good performance of the system when the water temperature varies in a wide range and with strong temperature gradients. When the friction velocity u* meets condition L´ ²P_r>200 and L´<64P_r, good linearity between u*^1/2 and the output of the probe E² is obtained. In addition, for the case when u* is very high, a relationship based on calibration is presented