Thermal measurement of turbulent wall shear stress fluctuations: tackling the effects of substrate heat conduction.




Assadian, Elsa

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This thesis presents a computational analysis of multi-element guard-heated sensors designed to overcome the most severe limitation of conventional thermal sensors for wall shear stress (WSS) measurement in turbulent flows –that of indirect heat conduction through the substrate. The objectives of this thesis are the study of guard-heated sensors {i} to quantify the reduction, over conventional single-element sensors, of substrate heat conduction losses and resultant errors over a range of applied shear and {ii} to examine a range of values of guard heater geometric parameters, in two common fluids, air and water and identify the best designs. Wall-turbulence, the turbulent flow in the vicinity of solid boundaries, has proved difficult to model accurately, due to the lack of accurate WSS measurements. Examples of areas of impact are drag force reduction on transport vehicles in land, sea, air, which today largely translate to reduced fossil fuel use and dependence; aerodynamic noise and control for flight and for wind energy conversion; atmospheric and oceanic transport studies for weather, climate and for pollutant transport; riverbank erosion. Constant-temperature anemometry with MEMS devices, flush-mounted hot-film thermal sensors, is non-intrusive, affords the best temporal resolution and is well-established. However, these hot-film probes suffer from unwanted heat transport to the fluid through the substrate, with errors and nonlinearity large enough to overwhelm quantitative utility of the data. Microfabrication techniques have enabled multi-element guard-heated prototypes to be fabricated. Our results show that errors in sensing-element signals, contributing to spectral distortion, are sensitive to sensor location within the guard heater. These errors can be reduced to below 1% of the signal with proper location of the sensor. Guard heating also reduces the large variation in spatial averaging due to substrate conduction. This makes them suitable for turbulent flows with a large range of fluctuations.



wall shear stress, thermal sensors, turbulent flow, fluctuation