Influence of a synthetic jet excitation on the development of a turbulent mixing layer (original) (raw)

Synthetic Jet Design Criteria and Application for Boundary Layer Separation Control

… Transactions on Fluid …, 2010

With the purposes of better understanding the phenomena associated with the synthetic jet operation as well as identifying suitable design criteria to control boundary layer laminar separation, the aerodynamic performance of a synthetic jet system has been experimentally investigated. Both device design and its aerodynamic characterization are addressed in the paper, paying particular attention to the fundamental parameters to be set for the synthetic jet design. The synthetic jet behaviour has been surveyed adopting different measuring techniques. Power density spectra have been evaluated from jet velocity distributions measured by means of a hot-wire anemometer. In order to investigate the instantaneous flow field and to allow the identification of the jet organised flow structures, Particle Image Velocimetry has been adopted. Moreover, to distinguish the different jet dynamics during the peculiar phases of the device (blowing and suction), phaselocked ensemble averaging technique has been applied.

Influencing the Mixing Process in a Turbulent Boundary Layer by Pulsed Jet Actuators

Pulsed jet actuators are studied in a low-speed wind tunnel by means of phase-locked stereoscopic particle image velocimetry (PIV) to examine the interaction of the jet with a turbulent boundary layer flow along a flat plate. The aim is the transport of high-momentum fluid from the outer part of the boundary layer flow towards the wall to actively delay or avoid flow separation. It is shown that a properly arranged actuator jet produces a strong streamwise vortex, which is well suited to enhance the desired mixing process. The strength and position of this streamwise vortex is of primary importance for the efficiency of the actuator concept. Different jet-exit-hole geometries, their impact on the vortex-structure and their ability to suppress or delay separation are discussed.

Analysis of a jet–mixing layer interaction

International Journal of Heat and Fluid Flow, 2003

The improvement of mixing in free-shear flows via external jets has been proven efficient in subsonic and supersonic flows as well. However, the hyper-mixing process is not well known. The present study deals with an experimental and a numerical approach of the interaction of an external control jet with a turbulent mixing layer. The main conclusion is that an intermittent penetration of the control jet occurs both in supersonic and subsonic configurations. Moreover, all results tend to show that the control jet flapping frequency and the spacing between the structures involved downstream of the interaction are respectively very close to the frequency and wavelength of the Kelvin-Helmholtz structures at the impact location. Two hypotheses are provided in order to explain the mechanism of the interaction. The first one is based upon the interaction with the passage of Kelvin-Helmholtz structures in the mixing layer, the other deals with an intrinsic instability of such a flow configuration.

Influence of Issued Jet Conditions on Mixing of Confined Flows

The development of the velocity and scalar fields in a coaxial jet mixer has been experimentally investigated applying simultaneously Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF) methods. Mixing of a turbulent jet (water solution of Rhodamin 6G) issued from a long round nozzle (l/d = 60) with co-flow (water) was studied. Because of the nozzle length the boundary layers of the inner flow already merged upstream of the jet exit. The issued jet conditions were changed installing vortex generators (tabs) at the nozzle exit. The tabs of rectangular and triangular forms with heights of 13 15 % the inner nozzle diameter accelerated mixing significantly but the scalar field developed to the homogeneous state faster than the velocity one. Turbulent characteristics measured downstream of the jet exit gave evidence the mixing specific behind the rectangular and triangular tabs.

Transversally injected synthetic jets for wall turbulence control

2008

Fully developed turbulent channel flow has been forced by means of transversal forcing operated with arrays of adjacent synthetic jets. The main flow has been controlled injecting groups of synthetic jets tangentially to the horizontal upper channel wall and perpendicularly to the mean flow itself. The effects of the forcing at two different Reynolds numbers have been investigated varying the forcing frequency and the synthetic jets arrays configuration. Skin friction reduction and near wall turbulence attenuation have been observed up to long distance (X=153H) downstream from the forcing section. In the spanwise direction the effects of the forcing is persistent up to a distance ΔZ/B from the vertical wall of injection, that depend from the Reynolds number. Higher level of drag reduction and turbulence attenuation are evidenced at lower Reynolds number.

Direct Numerical Simulation of Jet Mixing Control Using Combined Jets

JSME International Journal Series B, 2006

In order to develop an efficient jet mixing method, direct numerical simulations of combined jets are carried out. The Reynolds number defined with a nozzle diameter is Re = 1 500. Spatial discretization is performed by adopting a hybrid scheme of a sixth order compact scheme in the streamwise direction and Fourier series in the cross section. The distance between two jets is fixed at six times the jet diameter, and the inclination angle of the jets is changed from 45 to 70 deg. The results reveal that the turbulence intensity increases with a decrease in the inclination angle and that the jet width increases via jet excitation. These findings suggest that the diverse requirements of jet mixing control can be satisfied by a flexible combination of jets.

Predictions of mixing enhancement for jets in Cross Flows

The efficiency of a set of mixing enhancement devices on an array of Jets In Cross-Flow (JICF) is studied using Large Eddy Simulation (LES). The baseline flow is a rectangular channel flow on which five JICF's are installed on each wall (upper and lower). Mixing devices are fixed tabs installed upstream of the jets. Instantaneous analyses of the LES fields reveal two large vortical structures developing downstream of the mixing device. These structures strongly enhance mixing. Comparisons of the statistically averaged LES predictions against experimental results validate the predictions. The mixing devices provide a better spatial and temporal homogeneity of the gas mixture at the exit of the main duct. Even though full temporal and spatial homogeneity of the gas mixture prior to combustion is not guaranteed with this design, the probability of finding strong inhomogeneous zones is reduced. More generally, this study confirms the power of LES to help design actuating devices for flow and mixing control.

A single circular synthetic jet issued into turbulent boundary layer

An experimental investigation has been undertaken to study the behaviour of a single circular synthetic jet issued into turbulent boundary layer produced on a flat plate in cross flow. At the given free-stream conditions, the jet is also issued into laminar boundary layer so that an effective evaluation on the interaction of the vortices with the changing boundary layer could be made. The flow visualization technique is used in conjunction with the stereoscopic imaging system to reveal a unique quasi three-dimensional recognition of the vortices formed in either type of boundary layer under varying synthetic jet actuator (SJA) operating conditions. Firstly, the laminar boundary layer is produced on the flat plate with zero pressure gradient and later on the same boundary layer is triggered to turbulence using a trigger device. The free-stream conditions are justified by PIV measurements, in that the velocity profiles are drawn at given streamwise locations for both laminar and turbu...

Passive control of mixing in a coaxial jet

2008

An experimental investigation regarding interacting shear layers in a coaxial jet geometry has been performed. The present paper confirms experimentally the theoretical result by Talamelli and Gavarini (2006), who proposed that the wake behind the separation wall between the two stream of a coaxial jet creates the condition for an absolute instability. This instability, by means of the induced vortex shedding, may provide a continuous forcing mechanism for the control of the flow field. The potential of this passive mechanism as an easy, effective and practical way to control the near-field of interacting shear layers has been demonstrated.