Behavior of Free and Impinging Supersonic Microjets

The fluid dynamics of microflows has recently commanded considerable attention because of their potential applications. To date, most of this work has been limited to low velocity flows. The present study examines supersonic microjets in the range of 100 400 microns with exit velocities in the range of 400500 m/s. Such microjets are presently being used to actively control larger supersonic impinging jets, which occur in STOVL (Short Takeoff and Vertical Landing) aircraft. The flow field is visualized using a Micro-schlieren system with effective magnifications greater than 100x. Schlieren images, which to the best of our knowledge have never before been obtained at this scale, clearly show the characteristic shock cell structure observed in large-scale jets. Based on these images, the jet is clearly supersonic as far as 10-12 diameters downstream. Quantitative measurements providing jet decay and spreading rates as well as shock cell spacing are also obtained via pressure surveys using micro-pitot probes. Overall, the microjet properties are similar to larger supersonic jets, especially those operating at similar Reynolds number. However, pronounced viscous effects in the present microjets do lead to some differences. The impingement of these supersonic microjets on flat surfaces is also examined in this study. A comparison reveals that the flow structure of impinging microjets strongly resembles that of larger, macro supersonic impinging jets.