The interaction of round synthetic jets with a turbulent boundary layer separating from a rounded ramp

Abstract

A computational large eddy simulation (LES) study is presented of the interaction between a turbulent boundary layer separating from a rounded ramp in a duct and a pair of spanwise-periodic, round synthetic jets, actuated upstream of the nominal separation line. Several scenarios are considered, for different injection angles and velocity ratios. In all cases, the actuation frequency corresponds to the sheddinginstability mode of the separated shear layer. Experimental data, available for both the baseline flow and one actuated configuration, are used to verify the validity of the computational solutions. The analysis includes a separation of coherent and stochastic contributions to the time-averaged statistics of the auto-and cross-correlations of the fluctuations. The control authority is examined by reference to the effects of the actuation on the size of the separated zone, the momentum thickness of the boundary layer, the velocity field, various turbulence quantities and phase-averaged properties. The study demonstrates that the principal aspect of the interaction, at mean-flow level, is an increase in mixing provoked by the formation of strong streamwise vortices away from the wall, the induction of much weaker streamwise vortices close to the wall, and the extra production of stochastic turbulence caused by unsteady straining. The coherent stresses arising from the periodic perturbations are high -typically 5 times the levels of the unperturbed flow -but only within about 5-7 diameters of the jet orifice, and 2 orifice diameters on each side of the jet, and these are dominant primarily in the outer parts of the boundary layer. Stochastic turbulence is also elevated, but more modestly. The global effect of the actuation is a reduction of 10-20 % in the length of the separated region and 20-40 % in the thickness of the reverse-flow layer, depending on the actuation scheme, counter-flow actuation being the most effective. This reduction is mainly associated with a delay in separation. These results highlight the need for synthetic jets to be placed close to the separation zone and for the inter-jet distance to be of order 5 or lower to achieve a high level of separation-control authority.

LEM Characterization of Synthetic Jet Actuators Driven by Piezoelectric Element: A Review

“…Exit jet velocities were measured in the external ambient at the saddle point, therefore in this study velocity experimental measurements U e and LEM predictions of peak jet velocity U max were correlated to each other by means of the relationship of Equation (21). The measurements were carried out with a standard Kanomax mini Pitot tube, connected with a Mouser pressure transducer whose output signal was phase-locked to the device excitation signal.…”

“…Hot-wire Anemometry, Laser Doppler Velocimetry (LDV) and Particle Image Velocimetry (PIV) are only some of the several measurement techniques applied to determine the flow field generated by these devices, see Mohseni and Mittal [18]. On the other hand, many CFD simulations have been conducted, ranging from the first RANS to more recent LES and DNS computations, with the aim to achieve further details of the flow fields inside the cavity as well as near the orifice, both in quiescent and cross-flow conditions (e.g., Rumsey et al [19], Dandois et al [20], Lardeau and Leschziner [21]). Another very useful tool, employed for design and manufacturing purposes, consists in the definition of a low-order model, able to predict the dynamic response of the actuator in a relatively quick way and with reasonable fidelity and accuracy.…”

Comparison of synthetic jet actuators based on sharp-edged and round-edged nozzles

“…The potential applications have been found especially in boundary-layer separation control [3][4][5][6][7][8][9][10], jet vectoring [11][12][13][14][15][16], heat transfer enhancement [17][18][19][20][21][22], and mixing [23][24][25]. A variant of the SJ, namely a non-zero-net-mass-flux jet (or hybrid SJ), has been investigated more recently [26-28, 21, 29-31].…”

Energetic efficiencies of synthetic and hybrid synthetic jet actuators driven by electrodynamic transducers

“…Under certain conditions (see [23]- [27]), the outlet fluid flow forms a flow field behind the nozzle that is called a synthetic jet and is similar to the continuous jet in the far field [28]. In the past two decades, SJs have been intensively investigated, and many potential applications of them have been found (boundary layer control [5], [29]- [33], heat mass transfer [34]- [36], jet mixing [37]- [40] and shaking [41]). An improved variant of the SJ named the hybrid synthetic jet (HSJ) was studied in [42]- [50].…”