Periodic Excitation for Jet Vectoring and Enhanced Spreading (original) (raw)
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Influence of a synthetic jet excitation on the development of a turbulent mixing layer
International Journal of Heat and Fluid Flow, 2008
The paper presents an experimental study of a flow control of a turbulent plane mixing layer by normal synthetic jets deployed through an array of circular orifices. The experimental configuration represents a flow control strategy for deflecting a mixing layer which has application in the control of separated flows. The mean and turbulent fields have been investigated. The control jet interacts with the mixing layer and modifies its turbulent energy behaviour. It is observed that the more important effect is the enhancement of the mixing properties of the mixing layer when controlled. This effect creates a deflection of the mixing layer toward the wall.
Experiments in Fluids, 2008
An axisymmetric air jet exhausting from a 22degree-angle diffuser is investigated experimentally by particle image velocimetry (PIV) and stereo-PIV measurements. Two opposite dielectric barrier discharge (DBD) actuators are placed along the lips of the diffuser in order to force the mixing by a co-flow actuation. The electrohydrodynamic forces generated by both actuators modify and excite the turbulent shear layer at the diffuser jet exit. Primary air jet velocities from 10 to 40 m/s are studied (Reynolds numbers ranging from 3.2 to 12.8 9 10 4 ), and baseline and forced flows are compared by analysing streamwise and cross-stream PIV fields. The mixing enhancement in the near field region is characterized by the potential core length, the centreline turbulent kinetic energy (TKE), the integrated value of the TKE over various slices along the jet, the turbulent Reynolds stresses and the vorticity fields. The time-averaged fields demonstrate that an effective increase in mixing is achieved by a forced flow reattachment along the wall of the diffuser at 10 m/s, whereas mixing enhancement is realized by excitation of the coherent structures for a primary velocity of 20 and 30 m/s. The actuation introduces two pairs of contra-rotating vortices above each actuator. These structures entrain the higher speed core fluid toward the ambient air. Unsteady actuations over Strouhal numbers ranging from 0.08 to 1 are also studied. The results suggest that the excitation at a Strouhal number around 0.3 is more effective to enhance the turbulence kinetic energy in the near-field region for primary jet velocity up to 30 m/s.
Jet Mixing Enhancement by High-Amplitude Fluidic Actuation
AIAA Journal, 2000
Recent experiments have shown that properly designed high-amplitude, low mass ux pulsed slot jets blowing normal to a jet's shear layer near the nozzle can signi cantly alter the jet's development. In contrast to commonly used low-amplitude forcing, this strong excitation appears to overwhelm the turbulence, having nearly the same effect at high and low Reynolds numbers. It can, therefore, be studied in detail by direct numerical simulation. Direct numerical simulations of Mach 0.9, Reynolds number 3:6 £ £ 10 3 jets exhausting into quiescent uid are conducted. Physically realistic slot jet actuators are included in the simulation by adding localized body-force terms to the governing equations. Three cases are considered in detail: a baseline unforced case and two cases that are forced with apping modes at Strouhal numbers 0.2 and 0.4. (Sr = 0:4 was found to be the most ampli ed frequency in the unforced case.) Forcing at either frequency causes the jet to expand rapidly in the plane parallel with the actuators and to contract in the plane perpendicular to the actuators, as observed experimentally. It is found that the jet responds closer to the nozzle when forced at Sr = 0:4, but forcing at Sr = 0:2 is more effective at spreading the jet farther downstream. Several different measures of mixing (scalar dissipation, volume integrals of jet uid mixture fraction, and point measurements of mixture fraction) are considered, and it is shown that by most, but not all, measures forcing at Sr = 0:2 is the more effective of the two at mixing.
Flow, Turbulence and Combustion, 2009
Turbulent free jets issuing from five different nozzle geometries; smooth pipe, contracted circular, rectangular, triangular, and square, are experimentally investigated by using TSI 2-D laser Doppler velocimetry (LDV) to assess the effect of nozzle geometry and quarl (i.e. a cylindrical sudden expansion) on jet entrainment and spreading. The centerline mean velocity decay and the jet half-velocity width, which are indicators of jet entrainment and spreading rates, are determined for each nozzle's flow configuration, i.e. with and without sudden expansion. Furthermore, turbulence quantities, such as the flow mean velocities and their mean fluctuating components, as well as Reynolds shear stresses, are all measured along the centerline plane of the jet to facilitate understanding the extent of the effect of nozzle's geometry (i.e. nozzle's orifice shape and sudden expansion) on jet's entrainment and spreading. The main results show that the jet flow with the presence of sudden expansion exhibits higher rates of entrainment and spreading than without. In addition, these results reveal that sudden expansion exercises a greater effect on the asymmetric jet characteristics, especially for the triangular and rectangular nozzles compared to their axisymmetric counterparts (i.e. circular contracted nozzle).
Space-time modulation of turbulence in co-flow jets
International Journal of Heat and Fluid Flow, 2020
This paper focuses on the effects of a space-time dependent periodic stirring of a moderately turbulent planar coflow jet configuration. The baseline flow is agitated in time and in space by small-scale turbulent perturbations in combination with large-scale modulation imposed at the inflow plane of a rectangular domain of size L × L × 2L in the x, y and z directions respectively. The prescribed large-scale modulation is characterized by a single modulation frequency ω and modulation wave-number, K. A parametric study at different modulation frequencies and wave-numbers is performed. We evaluate the system response to the external agitation in terms of key dynamic properties of the flow, e.g., the total kinetic energy E T , the global averaged dissipation rate and additional flow mixing properties. For low modulation frequencies, e.g., = 0.5 , 0 where ω 0 is the large scaleturn over frequency, = U D / , 0 1 with U 1 and D being relevant velocity and length scales, and at given wavenumber K, we observe that E T follows the imposed oscillation with a periodic amplitude response that is sustained at locations further from the inflow plane, whereas for higher frequencies, the response amplitude rapidly decays. Results of the global dissipation rate show the development of a definite maximum value of the response amplitude at frequencies on the order of ω 0 for any modulation wave-number K. To investigate in more detail the effects of modulated turbulence on the jet mixing properties, a passive scalar was injected at the inflow plane. The spreading of the scalar surface in the agitated jet was monitored for a wide range of modulation frequencies. In general, results show enhanced mixing efficiency when the main jet is modulated at frequencies near ω 0 and low K values.
Effects of Axisymmetric Square-Wave Excitation on Transverse Jet Structure and Mixing
AIAA Journal, 2019
The influence of temporal square-wave excitation on structural and mixing characteristics of an equidensity, gaseous jet in crossflow (JICF) was explored in the present study. As in separate unforced and sinusoidally excited JICF experiments, acetone planar laser-induced fluorescence imaging enabled this detailed quantification for the JICF for mean jet-to-crossflow momentum flux ratios J ranging from J 41 (with a convective unstable upstream shear layer, or USL, in the absence of forcing) to J 5 (with a globally unstable USL). Such square-wave excitation of the jet fluid required adaptive feedforward control, not only to create more accurate temporal square waveforms but to enable more accurate comparison among alternative forcing conditions. Square-wave excitation of the JICF demonstrated a significant influence on the naturally globally unstable JICF, where specific nondimensional stroke ratios within J-dependent ranges could produce deeply penetrating, periodic vortices with improved jet penetration and spread. Enhanced jet penetration did not always correlate with better molecular mixing, however; there was a stronger correlation of improved mixing at higher J values with creation of a more symmetric jet cross section via square-wave excitation, especially one with a clear counter-rotating vortex pair structure.
Influence of amplitude and frequency modulation on flow created by a synthetic jet actuator
Sensors and Actuators A-physical, 2010
An experimental investigation of a cross-flow interaction between a synthetic jet and a flat plate laminar boundary layer is reported. The synthetic jet uses a piezo-actuator for displacing the diaphragm, thus enabling flow control in terms of the excitation amplitude and the modulation frequency. The role of these parameters on heat transfer enhancement from the flat plate is investigated. Measurements are carried out using hotwire anemometry for the flow field while the heat transfer coefficient and jet spreading are imaged respectively by liquid crystal thermography and the laser schlieren technique.
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.