Ph.D. (Engg) : Development of an ultra-miniature wall-shear-stress sensor
January 16 @ 4:00 PM - 5:00 PM
Shear stress at the wall is a quantity of fundamental importance in wall-bounded flows. It determines skin-friction drag and the dynamics of flow separation. From an engineering standpoint, it is a key parameter which dictates the overall aerodynamic performance and structural loading of flight vehicles. Hence, there is a natural motivation for the development of new techniques and sensors that can offer well-resolved measurements of wall shear stress. Conventionally, the techniques of hot-film anemometry and oil-film interferometry are used for wall-shear-stress measurements. These techniques, however, are severely limited in the spatio-temporal resolution that they can offer. Advances in micro and nano-fabrication techniques over the past three decades have led to the advent of MEMS-based floating element sensors. While MEMS sensors offer better resolution than conventional methods, the inertia of the floating element limits their temporal response. Miniaturizing the sensing element of the thermal anemometry probe is a viable solution to obtain high-resolution measurements. This approach has been successfully demonstrated with velocity measurements in turbulent flows with ultra-miniature hot-wire probes, which are able to fully resolve the turbulence spectrum even at high Reynolds numbers.
Motivated by the success of ultra-miniature hot-wire probes in velocity measurements, the present effort is directed at the development, fabrication, and demonstration of an ultra-miniature sensor, based on the principles of thermal anemometry, for wall-shear-stress measurements. The sensor design essentially consists of platinum filaments deposited on a thermally oxidized silicon substrate with electrical contact pads. The fabrication is carried out by oxide growth on a clean silicon wafer, followed by two-layer electron beam lithography, metal deposition, and lift-off processes. Titanium is used for adhesion in the first layer, followed by platinum deposition for the sensing element in the second layer. Dry reactive ion etching is used, when needed, to suspend the sensing element. Basic voltage-current characterization of the sensor is carried out prior to packaging of the sensors for use.
A demonstration of the sensor is made in a turbulent boundary layer flow. The packaged sensor is integrated onto a flat plate in a low-speed wind tunnel facility, and wall-shear-stress measurements are made in the turbulent boundary layer flow over the flat plate in the momentum thickness Reynolds number range of 1500 to 2500. The sensor is calibrated in the boundary layer flow in an in-situ manner by estimating the mean wall-shear-stress through hot-wire measurements of the flow velocity profile at different freestream velocities. The sensor fully resolves the spectrum of turbulent fluctuations in wall shear stress. The probability density distributions of wall-shear-stress fluctuations are found to match well with data reported in the literature, thereby validating the sensor’s performance. Overall, this work demonstrates the viability of making high-fidelity wall-shear-stress measurements using ultra-miniature thermal anemometry sensors. It lays the foundation for the development of a practical sensing tool for application outside the laboratory, in a real-world environment.
Speaker : Keshanjali Gaur
Research Supervisor : Prof. Duvvuri Subrahmanyam