Elsevier

Computers & Fluids

Volume 85, 1 October 2013, Pages 19-26
Computers & Fluids

Analysis of plasma-based flow control mechanisms through large-eddy simulations

https://doi.org/10.1016/j.compfluid.2012.09.004Get rights and content

Abstract

High-fidelity simulations will become ever more important in the future if their use is routinely extended beyond engineering prediction to scientific analysis of the underlying physics. In particular, carefully conducted simulations can provide insight into aspects that are difficult if not impossible to explore in experiments. In this paper, we highlight such use of CFD to understand plasma-based flow control techniques, with emphasis on how the disturbances couple to the flowfield for actuators that are heat (as opposed to force) based. We present results with a relatively new type of actuator, the Nano-Second Dielectric Barrier Discharge (NS-DBD), which is formed by a flush mounted and embedded electrode combination excited by sharp (O(50) ns) pulses. A numerical model is presented and then applied to explore control of a stalled NACA 0015 airfoil at 15° angle of attack. These new results are assimilated with prior efforts on Localized Arc Filament Plasma Actuators (LAFPAs), to examine the differences in response of free versus bounded shear layers. It is shown that the initial manifestation of the disturbance occurs in the form of weak streamwise vortices. At the low levels of excitation of interest in scalable application, pulsing is essential for amplification in both types of shear layers. Hairpin-like structures characteristic of entrainment are also a common feature. Differences are evident however due to the no-slip condition of the boundary layer, where multiple induced counter-rotating vortices are observed prior to breakdown.

Introduction

Plasma actuators for control of flow around aerodynamic bodies have numerous advantages over conventional actuators. Key among these are their relatively low inertia, high bandwidth and non-intrusiveness under off-design considerations. Numerous types of actuators have been developed, each operating with a different mechanism. Some of the most prevalent are the Alternating Current Dielectric Barrier Discharge [1], [2] (AC-DBD), the Localized Arc Filament Plasma Actuator [3], [4] (LAFPA) and most recently the nano-second pulsed NS-DBD device [5], [6]. The effectiveness and efficiency of such actuators have been strongly correlated with their operating characteristics and the ability of the flow to respond to the stimuli imposed by the actuator.

In this paper, we consider heating based actuators, specifically NS-DBDs [7] which have been very successful at eliminating wing stall at up to takeoff velocity [8] and even shock modification at Mach 5, and LAFPA, which have been employed to control jet noise [9]. The evidence for a primarily heating based effect for the NS-DBDs is clear: measurements in Ref. [5] show that whereas the AC-DBD generates a directional jet, the NS-DBD does not. LAFPAs generate distinct arcs, which dissipate Joule heating into the flow. However, their use in grooves to shield the arc, has led to the possibility, discussed in Ref. [10], that the main effect is a jet of air similar to a synthetic jet actuator. Experiments performed by Hahn et al. [11] on jet flow control showed however that control authority of the actuators was not dependent on the presence of the groove. Indeed, in our previous effort [12], a simple surface heating model reproduced all the key observations (coherent structures and near-field data) and trends of the experiments of Ref. [9], with different excitation modes (some of these results are summarized in Section 2.3.1 below). Furthermore, the mean flow results matched the theoretical predictions of Cohen and Wygnanski [13], who showed for example that the cross section of an initially axisymmetric jet becomes elliptical with the flapping mode (m=±1) while the second mixed mode (m=±2) yields a square cross-section jet.

Here we discuss the effects of a heating-based actuator in a more fundamental context, with the theme of using CFD to elucidate phenomena and effects that are difficult to elicit through experiments alone. That is, we seek to demonstrate the use of CFD to complement experiment. We choose phenomenological models for this effort – as described in Ref. [14], using first principles models for the plasma processes is impossible at this time not only because of substantial uncertainties in the plasma kinetics, but also because the computational requirements of coupling such a model to LES is far too large for existing resources.

After briefly outlining the numerical method, we examine the effect of a simple surface heating pulse in a quiescent medium to generate an acoustic wave through thermal viscous and acoustic effects (Section 2.1). In Section 2.2, we discuss the NS-DBD as a prototype for a thermal-based actuator and characterize its spatially varying perturbation. In Sections 2.3.1 Effect of discrete thermal disturbances on axisymmetric jet, 2.3.2 Effect of spatially discrete thermal disturbances on near wall flow, the manner in which such a disturbance interacts respectively with a free and bounded shear layer is characterized. In this context, new results with NS-DBDs and LAFPAs are assimilated with prior results with LAFPAs. For both types of shear layers, the disturbance initially yields streamwise vortices. However, their downstream evolution is strongly dependent on the type of shear layer. Finally, in Section 2.4, we discuss results with the NS-DBD actuator on control of flow past a NACA 0015 wing section.

Section snippets

Results

The governing equations are the compressible Navier–Stokes equations cast in strong conservative form. For the problems in Sections 2.1 Thermo-acoustic effects of heating pulse, 2.2 Thermal disturbance associated with the NS-DBD, 2.3 Effect of actuator on free and bounded shear layers, the inviscid terms are evaluated with the third-order upwind biased Roe scheme combined with the van Leer harmonic limiter and viscous terms are centered second order accurate [12]. For the airfoil stall control

Conclusion

Large-eddy simulations have been performed to understand the effect of thermal-based actuators on a variety of flows in the subsonic and supersonic regime. The results indicate that streamwise vortices and their evolution under local shear conditions provide key insights into the physics. Hair-pin vortices appear common to both free and bounded shear layers. The actuator-induced reorganization of coherent structures yields a rich variety of features. These results show that high-fidelity

Acknowledgements

The author is grateful for support from the Air Force Office of Scientific Research (Monitor: John Schmisseur), the Army Research Office (Monitor: Frederick Ferguson, Program W911NF-11-1-0537), the DoD HPC Modernization Program centers at AFRL, ERDC and NAVO and the Ohio Supercomputer Center. Discussions with Drs. Visbal, Nishihara, Adamovich, Samimy, Glaz and Dinavahi were very helpful in analyzing these results.

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