Measuring Shear Stress with a Microfluidic Sensor to improve Aerodynamic Efficiency

Abstract

In aerodynamic structures, shear stress is the greatest contributor to a body's total parasitic skin friction drag. This drag is proportional to the local wall shear stress on a surface. Measurement difficulties, high errors and the cost of fabrication have motivated innumerable efforts to develop precise and inexpensive methods for measuring the local shear stress in fluid structures. This is especially important in the supersonic aerodynamic environment, where the interaction between the sensor and air flow induces even higher errors. In order to further improve the efficiency of aircraft and other aerodynamic bodies, sensitive measurements on small scales are required. The present study introduces a novel electrochemical microfluidic shear stress sensor enabling the measurement of the wall shear stress in wind tunnel models. Our company proposes a paradigm shift in shear stress measurements which will take advantage of the complete sensing package offered in micro electro-mechanical systems (MEMS) without the need for moving mechanical parts or expensive manufacturing. The sensor contains a cavity, capped by a thin membrane. The air flow above the membrane deflects the membrane and induces fluid motion within the cavity, which accordingly changes the conductance of the electrolyte solution inside the cavity. This allows the direct measurement of the shear stress by measuring the electrical current under a fixed voltage applied. The proposed sensor is tested inside a subsonic wind tunnel at different air flow rates, using optical experiments and image processing techniques. These measurements enable the comparison of the shear stress measured by the sensor to that obtained by boundary layer measurements and cavity convection measurements. These results imply that the electrochemical shear stress sensor offers a precise and robust measurement system capable of quantifying wall shear stress in air flows.