The present study investigates turbulent boundary layer separation control by means of streamwise vortices with focus on the instantaneous vortex behavior. A turbulent boundary layer is exposed to a pressure gradient that generates a separation bubble with substantial backflow. The separation bubble is controlled by conventional passive vortex generators creating pairs of counterrotating vortices. Quantitative information is achieved by applying Particle Image Velocimetry (PIV) to the cross-stream plane of the vortices. The characteristics of a pair of counter-rotating vortices shed from a vortex generator is investigated in the near-field downstream of the vortex generator. The vortices were found to grow with the boundary layer in the downstream direction, and the maximum vorticity decreases as the circulation is conserved. The vortices are nonstationary, and the movements in the spanwise direction are larger than those in the wall-normal direction, due to the presence of the wall. The vortices fluctuate substantially and move over a spanwise distance, which is approximately equal to their size. The most probable instantaneous separation between the two vortices shed from one vortex generator equals the difference between their mean positions. The unsteadiness of the vortices contributes to the observed maxima in the Reynolds stresses around the mean vortex centers. The instantaneous vortex size and the instantaneous maximum vorticity are also fluctuating properties, and the instantaneous vortex is generally smaller and stronger than the mean vortex. A correlation was found between a large instantaneous vortex size and a low instantaneous maximum vorticity (and vice versa), suggesting that the vortices are subjected to vortex stretching.

A sophisticated sensor design of the AeroMEMS surface fence probe for wall shear stress measurements in turbulent flow is presented. The fence deflection (proportional to the wall shear stress) is directly measured by a piezoresistive Wheatstone bridge. The fence is 5 mm wide, max. 700 pm high and 7 pm and 13 pm thick, respectively. The cantilever height is 1700 pm or 2200 pm. The sensing element rests on a silicon body of 5200 x 7500 pm'. A sensitivity up to 6 mV/(V.N/m2), a resolution of up to IO4 N/m2 and a symmetrical behavior for bidirectional measurements were obtained in a measurement range of % 0.3 N/m2. Two pn-diodes were implemented for temperature monitoring (sensitivity: -2 m V K @ 100 PA).

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