Femtosecond Laser Tagging Characterization of a Sweeping Jet Actuator Operating in the Compressible Regime

Abstract

A sweeping jet (SWJ) actuator operating over a range of nozzle pressure ratios (NPRs) was characterized with femtosecond laser electronic excitation tagging (FLEET), single hotwire anemometry (HWA) and high-speed/phase-averaged schlieren. FLEET velocimetry was successfully demonstrated in a highly unsteady, oscillatory flow containing subsonic through supersonic velocities. Qualitative comparisons between FLEET and HWA (which measured mass flux since the flow was compressible) showed relatively good agreement in the external flow profiles. The spreading rate was found to vary from 0.5 to 1.2 depending on the pressure ratio. The precision of FLEET velocity measurements in the external flow field was poorer (≈25 m/s) than reported in a previous study due to the use of relatively low laser fluences, impacting the velocity fluctuation measurements. FLEET enabled velocity measurements inside the device and showed that choking likely occurred for NPR ≥ 2.0, and no internal shockwaves were present. Qualitative oxygen concentration measurements using FLEET were explored in an effort to gauge the jet's mixing with the ambient. The jet was shown to mix well within roughly four throat diameters and mix fully within roughly eight throat diameters. Schlieren provided visualization of the internal and external flow fields and showed that the qualitative structure of the internal flow does not vary with pressure ratio and the sweeping mechanism observed for incompressible NPRs also probably holds for compressible NPRs.

FLEET Velocimetry Measurements on a Transonic Airfoil

“…Instead, the current hypothesis is that the loss in signal lifetime is a function not of the observable gradients, but the instantaneous out-of-plane velocity gradients, which could not be resolved but work to fragment the FLEET excitation volume. Measurements made inside a sweeping jet actuator 20 , which locally exhibit similar velocities and flow gradients to those seen within this wake, were not seen to suffer from the loss in signal lifetime, though the FLEET signal was broken up by the strong local gradients. This flow was nominally twodimensional, which suggests that the out-of-plane behavior is likely responsible for the observed trend in lifetime.…”

A Low-Cost System for Seismocardiography-Based Cardiac Triggering: A Practical Solution for Cardiovascular Magnetic Resonance Imaging at 3 Tesla

“…The Human Machine Interface (HMI) is not part of the selected cRIO model, so a PC, which is connected to the cRIO via an Ethernet interface, is also included in the control unit. The SCG signal is sensed using a GRAS 40PP microphone [65]- [67] with a frequency range from 10 Hz to 20 kHz and a sensitivity of 50 mW/Pa. The cRIO used has four slots for connecting input/output modules.…”

Effect of low temperatures and pressures on signal, lifetime, accuracy and precision of femtosecond laser tagging velocimetry

“…FLEET is an unseeded molecular tagging method capable of nonintrusive measurements of velocity [1][2][3], temperature [4], density [5] and species concentration [6] for gases containing molecular nitrogen. The method has been employed in many types of flows, including turbulent [7], cryogenic [8,9], highspeed [10][11][12][13], combusting [14,15], as well as highly unsteady flows with subsonic through supersonic components [16].…”