Blunt Trailing Edge Profiled Body Wake Control Using Synthetic Jets

Abstract

INTRODUCTION The flow over bluff bodies is an important area of research in fluid dynamics due to its many scientific and engineering applications. Bluff body wakes involve the interaction of separated shear layers, forming a system of antisymmetric vortices (i.e., a vortex street). The distance downstream of a body that a vortex street forms is determined by the energy of the separated shear layers and the entrainment demands of the von Karman vortices. A strong vortex street leads to highly bent shear layers and a short region of recirculating flow, resulting in low pressure, high drag, and undesirable periodic aerodynamic forces. Control of the wake by affecting the entrainment balance of the shear layers can attenuate vortex shedding and reduce the pressure drag. Different control methodologies to accomplish this goal have been studied with varying degrees of success. Examples of such techniques include inhibiting shear layer interaction with a splitter plate (Bearman, 1965), opposition control of the vortex street (Siegel et al., 2003), and synchronizing the roll up of the upper and lower separated shear layers to prevent asymmetry and decouple the wake and shear layers (Pastoor et al., 2008). In recent years, the three-dimensional spanwise features of wakes have received increased attention due to the contribution they also make to drag and the transition of the wake to turbulence. This has motivated control techniques that involve introducing spanwise variable disturbances into the wake to induce vortex dislocations, a strategy often referred to as distributed forcing. The presence of dislocations is associated with higher base pressure and lower fluctuating aerodynamic forces, and is therefore desirable for drag reduction (Williamson, 1989). Distributed forcing was pioneered as a passive flow control technique by Tombazis and Bearman (1997), who observed cellular shedding patterns in the wake of a bluff body outfitted with a spanwise wavy trailing edge, leading to a 34% increase in base pressure at Red = 40,000. As an active flow control technique, distributed forcing was first investigated by Kim and Choi (2005) with spanwise sinusoidal blowing and suction on a cylinder, and resulted in a 25% drag reduction at Reynolds numbers (Red) up to 3,900. Naghib-Lahouti et al. (2015) performed distributed forcing on a BTE body with a discrete number of plasma actuators spaced at 2.4d on the upper and lower surfaces and observed an increase of 40% in the base pressure at Red = 3,000. The spanwise spacing of 2.4d was selected to match a dominant cellular shedding wavelength observed in BTE body wakes at this Red (Naghib-Lahouti et al., 2014). The present study investigates a new distributed forcing system for a BTE body using an array of synthetic jet actuators. A large number of studies have investigated unsteady forcing to control vortex shedding, with the effects strongly dependent on the excitation frequency and the symmetry/arrangement of actuation (Colonius and Williams, 2011). Low-frequency forcing (near the shedding frequency) has the potential to directly interact with the large-scale wake structures, but can amplify fluctuations in the shear layer and wake, leading to increased drag (Barros et al., 2016). In contrast, high-frequency forcing (typically an order of magnitude greater than the dominant unstable frequencies of the base flow) has been shown to increase the turbulent dissipation rate in shear layers, and consequently, decrease the turbulent kinetic energy (TKE) (e.g., Wiltse and Glezer, 1998, Cain et al., 2001). In the context of bluff body wake control, high-frequency forcing has been applied to lower the entrainment, modify the shape of the mean recirculation region of the wake, and recover base pressure by 35% (Oxlade et al., 2015). The mechanism of control was attributed to increased dissipation in the shear layer, which amplified the energy of the small-scales of the flow and attenuated the large-scales by enhancing the energy cascade from large to small scales. This is consistent with the results of Vukasinovic et al. (2010), who introduced a high-frequency jet in a flow upstream of a step, and demonstrated significant modifications to the small and large scale structures of the flow downstream of actuation. In particular, they observed an initial increase in the turbulent kinetic energy (TKE) production and in the energy contained by small-scale structures, which was then followed by a general reduction of TKE farther downstream. Therefore, unsteady high-frequency forcing is applied in the present study to attenuate vortex shedding by controlling the shear layers and separating boundary layer fluid feeding the vortex street without amplifying the low-frequency shedding instability of the flow. The experimental design for this study is provided in the next section, followed by a presentation and discussion of the results of the study, and finally a summary of the principal findings.