We have developed a series of MEMS-based optical wall-shear stress sensors. Capable of taking instantaneous measurements in both one and two dimensions.
Why measure wall-shear stress?
Whenever air flows over a commercial aircraft a thin layer of turbulence is generated close to the surface of the vehicle. The same turbulence occurs with a high speed train. It also happens when water flows around the hull of a ship.
This region of turbulence generates a resistive force - drag. In turn, this creates a wall-shear stress across the entire surface of the aircraft, train or ship. This wall-shear stress is typically responsible for more than half of the vehicle’s energy consumption.
Knowledge of wall-shear stress is of both fundamental and practical importance when designing machines to operate in flows of air or liquid.
Mean wall-shear stress is indicative of the overall state of the flow. Fluctuating wall-shear stresses provide a “footprint” of individual flow processes that transfer momentum to the wall. This is seen as the “holy-grail” of flow measurements.
Unfortunately, accurately measuring fluctuating wall-shear stresses is notoriously difficult.
Limitations of Existing Sensor Technology:
Many of the existing sensors measure wall-shear stress indirectly
Sensors are difficult to use and calibrate, and are expensive
Some systems are not resilient and have a high failure rate.
None of the current solutions can measure instantaneous wall-shear stress in two dimensions over a surface area
Accurately measure instantaneous wall-shear stress using a direct method
Greater than 2% accuracy
Measure flow in multiple directions (both 1D and 2D sensors)
Can be supplied as arrays – greatly increasing the number of sensors available in given space.
No need to calibrate – sensors can be supplied pre-calibrated
Sensors can be flush mounted so not to interfere with turbulent flows