Abstract
There has been growing use of microscale flowfields in a diverse range of applications, including active flow control. While these actuators have proven effective, further investigation would increase the understanding of these flowfields from a fundamental fluid mechanics aspect and assist in their optimization. Current flow visualization techniques lack either temporal or spatial resolution necessary to resolve small-scale and high-speed flow structures. Therefore, a laser-based microschlieren system has been developed to image several microscale flowfields, including: a 1mm free jet, a small-scale Hartmann tube, several variations of a Resonance-Enhanced Microjet (REM) actuator, and a sparkjet actuator. Laser-induced breakdown in argon is used to generate a light source with a ~10 nanosecond pulse width, which is capable of freezing features present in each of these small-scale, high-speed flows. This light source is coupled with a high-magnification schlieren system with a resolution of 140 pixels/mm to acquire high spatial and temporal resolution schlieren images. Through the use of this technique, various measurements such as shock oscillation displacements, jet front velocities, phase correlations between the aeroacoustic structures, etc. were acquired and compared with acoustic, pressure, and temperature measurements. The results show that the REM actuator has the capability to produce microjets with velocities pulsing from near zero to supersonic, while operating at high frequencies (1-10kHz). Studies from the sparkjet actuator using this new technique found shocks in the exhaust indicating local high-speed flow, which has previously not been seen. Lastly, the instantaneous images captured with the laser-based microschlieren system of the sparkjet actuator and one variation of the REM actuator were compared with snapshots from CFD simulations. Favorable results were found for the simulation of the REM actuator, but the sparkjet actuator simulation requires refinement. This new flow visualization technique has generated exceptional results, and will be a useful tool for researchers in future studies.
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