The microS™ is the first commercially available micro-optical shear stress sensor capable of measuring velocity gradients in the sub-layer of a submerged body. The probe’s face is 0.375″ (9.5 mm) in diameter and the total length is 1.18″ (30 mm). The probe is meant to be mounted flush with the surface of the body. The probe volume is just 15 by 30 by 30 μm (x by y by z) and is centered either at 75 or 135 μm above the surface of the sensor*.
The optics of the sensor generate multiple linearly diverging fringes of light. As particles in the fluid pass through the fringes, they scatter light with a frequency that is proportional to the instantaneous velocity and inversely proportional to the fringe separation at the location of particle trajectory. From this the velocity gradient is calculated, and inside the sublayer the shear stress is proportional to this gradient. Because the fringes are not symmetrical with respect to the sensor face, the probe can measure direction as well as velocity gradient.
The sensor provides results 99% accurate for flows up to Rex=2×106. The microS has been successfully used at Rex=108. It can measure up to 6500 Pa of stress in water and 140 Pa in air.
The microS System consists of the microS probe, driver electronics, band-pass filter, and the BP-microS Burst Processor acquisition hardware and processing software package. Optionally available is a computer with the hardware and software installed and verified by MSE technicians.
The wall shear stress sensor performed successfully in a number of experiments. For more information, please contact us about implementing the microS System into your next project.
*These distances are measured in air. When aimed into an experimental vessel with a different fluid, the probe volume will be farther from the probe and it will be slightly larger. Please refer to the specifications at left for examples.
The sensor is based on a technique first developed by Naqwi and Reynolds using conventional optics. A.A. Naqwi and W.C. Reynolds, “Dual cylindrical wave laser Doppler method for measurement of skin friction in fluid flow,” Report No. TF-28, Stanford University (1987).