MicroS^{3 ©} is the first commercially available micro optical shear stress sensor capable of measuring velocity gradients in the sub-layer of a submerged body. The schematic on the left below illustrates the principle of this sensor. Diverging interference fringes originate at the surface and extend into the flow.* The scattered light from particles is collected through a window at the surface of the sensor. The region defined by the intersection of the transmitter and receiver fields is centered at approximately 105 um above the surface (140um in water) and measures about 30 um high. The photographs at the bottom of this page show two surface mountable package designs.

 The diverging fringes originate at the surface and extend into the flow, as illustrated on the schematic on the left. The local fringe separation, d, was designed to increase linearly with the distance from the sensor, y, given by d = k·y, where k is the fringe divergence rate.
As particles in the fluid flow through the linearly diverging fringes, they scatter light with a frequency f_Doppler that is proportional to the instantaneous velocity and inversely proportional to the fringe separation at the location of particle trajectory. The velocity of the particle is calculated from

U = f_Doppler·d .

The Doppler frequency simply multiplied by the fringe divergence yields the velocity gradient, which is proportional to the wall shear in the quasi linear sub-layer region of the boundary layer.

t_wall = m·(^dU/_dy)_y=0 = m·k·f_Doppler

The light is carried to and from the sensor with optical fibers held in an umbilical cable. The opto-eletronics elements are contained in an enclosure shown on the left that can be located several meters from the sensor.The output of the sensor is a Doppler signal, similar to the laser Doppler anemometry.   The wall shear stress sensor performed successfully in a number of experiments. See our online publications for more information.

This sensor was designed using the latest advancements in diffractive and integrated optics technology. As a result, the sensor is extremely compact, as demonstrated in the figure below.

  The sensor provides results 99% accurate for flows up to Re_x=10⁶. The Micro S-3 has been successfully used at Re_x=10⁸.