The use of single flush-mounted thin-films for thermal sensing of wall shear stress fluctuations in turbulent flows has seen a decline, in spite of their non-intrusiveness, and the availability of microfabrication technologies to create very small films. The limitations of such single-element sensors are quite severe—their spatial resolution is not determined by their size alone, but modified by substrate heat conduction which creates variations in the effective sensor size (heat exchange area), dependent on strength and timescale of the fluctuations. Here a two-element design is investigated—with the hot-film sensor element surrounded by an electrically isolated guard heater film maintained at the same temperature as the sensor, but controlled by a separate anemometer circuit. Numerical studies are used to examine such guard heater designs over a range of shear stress values. The results show that if the sensor film center-location is biased towards the downstream end (75% and 65% of guard-heater length for water and air, respectively), with an appropriately-sized guard heater, 95% of the total heat generated in the sensing film can be transferred directly to the fluid, for strong turbulent fluctuations (Peclet number Pe > 8000) when the working fluid is water.
Shear Stress Sensing with Elastic Microfence Structures