UCLA

This work describes the experimental exploration of the influence of external asymmetric excitation on the equidensity gaseous jet in crossflow (JICF). Asymmetric forcing was applied via an array of speakers flush mounted around the exterior of the jet exit, each embedded in the injection wall of the wind tunnel. The speakers were individually operated with sinusoidal temporal excitation at differing phases with respect to one another, allowing for controlled directional azimuthal forcing about the jet exit, for example, in counterclockwise or clockwise directions, and with variable forcing amplitudes and frequencies. Operation of one, two, or

all four speakers was explored here in terms of the influence of local or more circumferential asymmetric excitation on transverse jet response. The amplitudes of pressure perturbation were very low as compared to previous axisymmetric forcing studies by Shoji (2017) and Shoji et al. (2019c), in many cases by at least an order of magnitude. There was a special focus here on high jet-to-cross flow momentum flux ratios (e.g., J = 61 and 41), which are known to have a convectively unstable upstream shear layer (USL) and to create asymmetric cross-sections with typically poorer mixing characteristics (Gevorkyan et al., 2016) than for lower J values with an absolutely unstable USL.

The results of hotwire-based spectral measurements in the transverse jet's upstream shear layer revealed that asymmetric forcing at a frequency ff within the fundamental range of the jet USL instability created a strong lock-in of the USL to the forcing frequency, as expected, whereas excitation at frequencies ff further from fo required higher amplitude excitation for clear lock-in. Similar to sinusoidal excitation studies on the free jet (Li and Juniper, 2013c) and JICF subject to axisymmetric forcing (Shoji et al., 2019b), quasiperiodic behavior on the run-up to lock-in was observed for forcing cases outside the fundamental range. While forcing frequencies below the fundamental tended to enable lock-in for sufficiently high amplitudes, for forcing frequencies ff that were well above fo, as ff approached 2fo, the jet USL did not lock-in to the external forcing, even at relatively high pressure perturbation amplitudes. Additionally, differing shear layer responses were often observed when employing different directional forcing strategies for a fixed forcing frequency ff and amplitude P', suggesting a different susceptibility to clockwise and counter-clockwise orientations of flow perturbation and thus differing rates of growth of asymmetric instabilities under these high J conditions.

Acetone planar laser induced florescence (PLIF) imaging showed that asymmetric forcing at frequencies near the fundamental frequency associated with the USL can greatly influence jet cross-sectional structure. In many cases such forcing creates enhanced symmetrization of the counter-rotating vortex pair (CVP), more typical of transverse jets at lower J values

and a naturally absolutely unstable USL. Symmetrization of the jet cross-sectional structure at high J values with asymmetric forcing was associated with improvements in molecular mixing, as had been seen for the unforced JICF at low J values (Gevorkyan et al., 2016). For all forcing conditions in which 1:1 lock-in of the USL occurred, mixing was generally enhanced in both the centerplane and cross-sectional views. Moreover, results in this study show that in general, asymmetric forcing enhanced mixing to some degree, even in instances when the USL was known to exhibit quasiperiodic behavior or was not locked-in to the asymmetric forcing. Yet cases where the USL was locked-in to the forcing virtually always provided better mixing enhancement (lower Unmixedness) than cases where the USL exhibited quasiperiodic behavior in the USL in response spectra, or cases which were neither locked-in or quasiperiodic.

Simultaneous acetone PLIF and stereo particle image velocimetry (PIV) measurements quantified the interaction of the transverse jet's velocity field and scalar concentration field in response to asymmetric forcing for the J = 41 condition. Asymmetric forcing clearly demonstrated influence on the flow field velocity and a moderate influence on the local associated strain rate, primarily causing the spatial rate of increase in local strain rate to occur closer to the jet exit than in the absence of forcing. Cross-sectional PLIF/PIV results showed small natural asymmetries in both the mean vorticity field and mean scalar concentration field at the upstream edge of the jet orifice, and that asymmetric forcing influenced these natural structures in different ways, depending on the orientation and localization of the excitation.

Proper orthogonal decomposition (POD) analysis of the transverse jet's near-field scalar and velocity fields was performed, and the phase space of POD mode coefficients was mapped for dominant modes associated with in a given forcing condition. Hence the dynamics of a larger region than just the upstream shear layer could be quantified here. Sometimes, coherent shapes emerged from the POD coefficient phase space, and those shapes strongly resembled a variety of strange attractors, potentially representing non-periodic solutions. All forcing cases which produced strange attractor-like structures had a strongly locked-in upstream shear layer. Other groups (Bonetti and Boon, 1989; Williams-Stuber and Gharib, 1990; Aref et al., 1987; Guzman and Amon, 1994; Guan et al., 2018) have found evidence for strange attractors in other flowfields in the run-up to chaotic behavior, which may suggest that the asymmetric forcing cases for the transverse jet in which coherent phase space shapes appear may be associated with a transition of the flow, especially but not exclusively in the transverse jet's upstream shear layer.

Overall, then, asymmetric perturbations of the flow in the vicinity of the exit of the flush injected transverse jet can have a substantial impact on many key aspects of jet behavior: the dynamical character of the upstream shear layer and nearfield dynamics, the jet centerplane and especially cross-sectional structure, and molecular mixing characteristics of the jet. This study provides evidence of the rich potential that strategic asymmetric perturbations can provide in both understanding and controlling key features of the transverse jet, opening new questions worthy of future exploration.