Transitory Behavior of Synthetic Jets

2005 ◽  
Author(s):  
Michael Amitay ◽  
Florine Cannelle
Keyword(s):  
Reynolds Number ◽  
Steady State ◽  
Input Signal ◽  
Mass Flux ◽  
Reynolds Numbers ◽  
Synthetic Jet ◽  
Synthetic Jets ◽  
Hot Wire ◽  
Stroke Length ◽  
Zero Net Mass Flux

The transitory behavior of an isolated synthetic (zero net mass flux) jet was investigated experimentally using PIV and hot-wire anemometry. In the present work, the synthetic jet was produced over a broad range of length- and time-scales, where three formation frequencies, f = 300, 917, and 3100Hz, several stroke lengths (between 5 and 50 times the slit width) and Reynolds numbers (between 85 and 408) were tested. The transitory behavior, following the onset of the input signal, in planes along and across the slit was measured. It was found that the time it takes the synthetic jet to become fully developed depends on the stroke length, formation frequency and Reynolds number. In general, the transients consist of four stages associated with the merging of vortices in both cross-stream and spanwise planes that grow in size, which lead to the pinch off of the leading vortex before the jet reaches its steady-state.

An experimental study on the behaviours of circular synthetic jets at low Reynolds numbers

2012 ◽  
Vol 226 (11) ◽  
pp. 2686-2700 ◽  
Author(s):  
Qingfeng Xia ◽  
Shan Zhong
Keyword(s):  
Reynolds Number ◽  
Silicone Oil ◽  
Reynolds Numbers ◽  
Synthetic Jet ◽  
Synthetic Jets ◽  
Operating Conditions ◽  
Flow Visualisation ◽  
Stroke Length ◽  
Low Reynolds Numbers ◽  
Low Reynolds

In the work presented in this article, the behaviour of circular synthetic jets issuing into quiescent surrounding fluid at low Reynolds numbers is experimentally studied for potential mixing applications of synthetic jets at micro-scales or in highly viscous fluids. Sugar solutions and silicone oil are used as the flow media in order to achieve the required low Reynolds numbers. The conditions for jet instability, vortex rollup and synthetic jet formation are investigated using both flow visualisation techniques and particle image velocimetry, and the typical behaviour of synthetic jets at a Reynolds number around unity is also illustrated. The roles of Reynolds number, dimensionless stroke length and Stokes number in determining the characteristics of synthetic jets are examined and found to be largely consistent with the finding obtained at higher Reynolds numbers. Finally, a parameter map of synthetic jet flow patterns is produced based on the results from this study, which can be used to aid the choice of synthetic jet operating conditions for specific applications or anticipate if a desired vortex structure can be obtained at a given synthetic jet operating condition.

A Heat Transfer Comparison Between a Synthetic Jet and a Steady Jet at Low Reynolds Numbers

2008 ◽  
Author(s):  
Rayhaan Farrelly ◽  
Alan McGuinn ◽  
Tim Persoons ◽  
Darina B. Murray
Keyword(s):  
Heat Transfer ◽  
Reynolds Numbers ◽  
Synthetic Jet ◽  
Synthetic Jets ◽  
Stroke Length ◽  
Low Reynolds Numbers ◽  
Nusselt Numbers ◽  
Significant Difference ◽  
Low Reynolds ◽  
Surrounding Fluid

A study has been carried out to compare steady jet and synthetic jet heat transfer distributions at low Reynolds numbers. Both jets issued from a 5mm diameter orifice plate with air for the steady jet being supplied by a compressor via a plenum chamber. Tests were conducted for Reynolds numbers ranging from 1000 to 4000, and for non-dimensional surface to jet exit spacings (H/D) from 1 to 6. Dimensionless stroke length (Lo/D) for the synthetic jet was held constant at 8. A significant difference was observed between the steady and synthetic jet Nusselt numbers at low Reynolds numbers and low H/D. In comparison to steady jets, the stronger entrainment of surrounding fluid and the vigorous mixing near the impingement surface are characteristics of synthetic jets that are beneficial to heat transfer. Nonetheless, the steady jet yields higher Nusselt numbers for all test conditions.

Experimental Study of Flow Separation over NACA633018 Wing with Synthetic Jet Control at Low Reynolds Numbers

2012 ◽  
Vol 29 (1) ◽  
pp. 45-52 ◽  
Author(s):  
C.-Y. Lin ◽  
F.-B. Hsiao
Keyword(s):  
Reynolds Number ◽  
Flow Separation ◽  
Separation Bubble ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Synthetic Jet ◽  
Synthetic Jets ◽  
Laminar Separation ◽  
Low Reynolds Numbers ◽  
Jet Control

AbstractThis paper experimentally studies flow separation and aerodynamic performance of a NACA633018 wing using a series of piezoelectric-driven disks, which are located at 12% chord length from the leading edge to generate a spanwise-distributed synthetic jets to excite the passing flow. The experiment is conducted in an open-type wind tunnel with Reynolds numbers (Re) of 8 × 104 and 1.2 × 105, respectively, based on the wing chord. The oscillations of the synthetic jet actuators (SJAs) disturb the neighboring passage flow on the upper surface of the wing before the laminar separation takes place. The disturbances of energy influence the downstream development of boundary layers to eliminate or reduce the separation bubble on the upper surface of the wing. Significant lift increase and drag decrease are found at the tested Reynolds number of 8 × 104 due to the actuators excitation. Furthermore, the effect of drag also reduces dominant with increasing Reynolds number, but the increase on lift is reduced with the Reynolds number increased.

Evolution of elliptic synthetic jets at low Reynolds number

2019 ◽  
Vol 868 ◽  
pp. 66-96 ◽  
Author(s):  
Xu-Dong Shi ◽  
Li-Hao Feng ◽  
Jin-Jun Wang
Keyword(s):  
Reynolds Number ◽  
Aspect Ratio ◽  
Vortex Ring ◽  
Synthetic Jet ◽  
Synthetic Jets ◽  
Stereoscopic Particle Image Velocimetry ◽  
Entrainment Rate ◽  
Streamwise Vortices ◽  
Primary Vortex ◽  
Stroke Length

The influence of the nozzle aspect ratio ($AR=1$, 2 and 4), stroke length ($L_{0}=1.85$, 3.7 and 5.55) and Reynolds number ($Re=79$, 158, 316 and 632) on the behaviour of elliptic synthetic jets is studied experimentally. Laser-induced fluorescence and two-dimensional and stereoscopic particle image velocimetry are used to analyse the vortex dynamics and evolution mechanism. It is found that the fluid elements around the major axis of an elliptic vortex ring move downstream faster and tend to approach the centreline, while the fluid elements around the minor axis move downstream at a slower speed and away from the centreline, thereby resulting in the occurrence of the well-known axis-switching phenomenon for elliptic synthetic jets. During this process, a pair of arc-like vortices forms ahead of the primary vortex ring, and they are constituted by streamwise vortices in the leg part and spanwise vortices in the head part; two pairs of streamwise vortices form from the inside of the primary vortex ring and develop in the tails. The streamwise vortices are pushed away progressively from the centreline by the synthetic jet vortex rings that are formed during the subsequent periods. These additional vortical structures for non-circular synthetic jets show regular and periodic characteristics, which are quite different from the previous findings for non-circular jets. Their mutual interaction with the vortex ring causes significant changes in the topology of elliptic synthetic jets, which further results in the variation of the statistical characteristics. Increasing the aspect ratio, stroke length and Reynolds number will make the evolution of the synthetic jet become more unstable and complex. In addition, the entrainment rate of an elliptical synthetic jet is larger than that of a circular synthetic jet and it increases with the nozzle aspect ratio ($AR\leqslant 4$) and Reynolds number. It is indicated that the formation of streamwise vortices could enhance the entrainment rate. This finding provides substantial evidence for the potential application of elliptic synthetic jets for effective flow control.

scholarly journals Control of a Round Jet Intermittency and Transition to Turbulence by Means of an Annular Synthetic Jet

Actuators ◽  
2021 ◽  
Vol 10 (8) ◽  
pp. 185
Author(s):  
Zuzana Antošová ◽  
Zdeněk Trávníček
Keyword(s):  
Reynolds Number ◽  
Active Control ◽  
Flow Regimes ◽  
Velocity Profiles ◽  
Synthetic Jet ◽  
Transition To Turbulence ◽  
Jet Flows ◽  
Maximal Effect ◽  
Hot Wire ◽  
Round Jet

This paper deals with active control of a continuous jet issuing from a long pipe nozzle by means of a concentrically placed annular synthetic jet. The experiments in air cover regimes of laminar, transitional, and turbulent main jet flows (Reynolds number ranges 1082–5181). The velocity profiles (time-mean and fluctuation components) of unforced and forced jets were measured using hot-wire anemometry. Six flow regimes are distinguished, and their parameter map is proposed. The possibility of turbulence reduction by forcing in transitional jets is demonstrated, and the maximal effect is revealed at Re = 2555, where the ratio of the turbulence intensities of the forced and unforced jets is decreased up to 0.45.

Time-Dependent Laminar Backward-Facing Step Flow in a Two-Dimensional Duct

1988 ◽  
Vol 110 (3) ◽  
pp. 289-296 ◽  
Author(s):  
F. Durst ◽  
J. C. F. Pereira
Keyword(s):  
Reynolds Number ◽  
Steady State ◽  
Separated Flow ◽  
Flow Region ◽  
Reynolds Numbers ◽  
Steady State Flow ◽  
Backward Facing Step ◽  
Step Flow ◽  
Recirculating Flow ◽  
State Flow

This paper presents results of numerical studies of the impulsively starting backward-facing step flow with the step being mounted in a plane, two-dimensional duct. Results are presented for Reynolds numbers of Re = 10; 368 and 648 and for the last two Reynolds numbers comparisons are given between experimental and numerical results obtained for the final steady state flow conditions. In the computational scheme, the convective terms in the momentum equations are approximated by a 13-point quadratic upstream weighted finite-difference scheme and a fully implicit first order forward differencing scheme is used to discretize the temporal derivatives. The computations show that for the higher Reynolds numbers, the flow starts to separate on the lower and upper corners of the step yielding two disconnected recirculating flow regions for some time after the flow has been impulsively started. As time progresses, these two separated flow regions connect up and a single recirculating flow region emerges. This separated flow region stays attached to the step, grows in size and approaches, for the time t → ∞, the dimensions measured and predicted for the separation region for steady laminar backward-facing flow. For the Reynolds number Re = 10 the separation starts at the bottom of the backward-facing step and the separation region enlarges with time until the steady state flow pattern is reached. At the channel wall opposite to the step and for Reynolds number Re = 368, a separated flow region is observed and it is shown to occur for some finite time period of the developing, impulsively started backward-facing step flow. Its dimensions change with time and reduce to zero before the steady state flow pattern is reached. For the higher Reynolds number Re = 648, the secondary separated flow region opposite to the wall is also present and it is shown to remain present for t → ∞. Two kinds of the inlet conditions were considered; the inlet mean flow was assumed to be constant in a first study and was assumed to increase with time in a second one. The predicted flow field for t → ∞ turned out to be identical for both cases. They were also identical to the flow field predicted for steady, backward-facing step flow using the same numerical grid as for the time-dependent predictions.

Numerical Investigations of Active Flow Control Using Synthetic Jets on a Highly Loaded Compressor Stator Cascade

2010 ◽  
Author(s):  
Christoph Gmelin ◽  
Mathias Steger ◽  
Vincent Zander ◽  
Wolfgang Nitsche ◽  
Frank Thiele ◽  
...  
Keyword(s):  
Separation Bubble ◽  
Active Flow Control ◽  
Suction Side ◽  
Synthetic Jet ◽  
Synthetic Jets ◽  
Navier Stokes ◽  
Compressor Cascade ◽  
Time Resolved ◽  
Laminar Separation ◽  
Zero Net Mass Flux

Time-resolved Reynolds-Averaged Navier-Stokes simulations of a 3D stator compressor cascade are performed. At the design point of the airfoil under investigation, pronounced secondary flow effects are observed. Strong corner vortices emerge from the casing walls and the flow separates from the blade suction side towards the trailing edge. Transition from laminar to turbulent flow occurs within a laminar separation bubble. Using a commercial CFD software, the influence of the spatial resolution is investigated by means of a spanwise coarsening and refinement of the created mesh. Zero net mass flux synthetic jet actuation is used to control the separated regions. The work presents a variation of the temporal discretization and an analysis of the driving parameters of the actuation.

The performance of round synthetic jets in quiescent flow

2006 ◽  
Vol 110 (1108) ◽  
pp. 385-393 ◽  
Author(s):  
M. Jabbal ◽  
J. Wu ◽  
S. Zhong
Keyword(s):  
Near Field ◽  
Jet Flow ◽  
Synthetic Jet ◽  
Synthetic Jets ◽  
Operating Conditions ◽  
Stroke Length ◽  
Synthetic Jet Actuators ◽  
Macro Scale ◽  
Jet Actuators ◽  
The Impact

AbstractPIV measurements in the near-field region of a jet flow emanating from a round synthetic jet actuator into quiescent air were conducted over a range of operating conditions. The primary purpose of this work was to investigate the nature of synthetic jets at different operating conditions and to examine the jet flow parameters that dictate the behaviour of synthetic jet actuators. The effects of varying diaphragm displacement and oscillatory frequency for fixed actuator geometry were studied. It was observed that the characteristics of synthetic jets are largely determined by the Reynolds number and stroke length. An increase in the former is observed to increase the strength of consecutive vortex rings that compose a synthetic jet, whereas an increase in the latter results in an increase in relative vortex ring spacing and for further increases in stroke length, shedding of secondary vortices. Correlations were also made between the operating parameters and the performance parameters most effective for flow control and which therefore determine the impact of a synthetic jet on an external flow. Relations of time-averaged dimensionless mass flux, momentum flux and circulation with the jet flow conditions were established and found to widely support an analytical performance prediction model described in this paper. It is anticipated that the experimental data obtained in this study will also contribute towards providing a PIV database for macro-scale synthetic jet actuators.

Hot-wire spatial resolution issues in wall-bounded turbulence

2009 ◽  
Vol 635 ◽  
pp. 103-136 ◽  
Author(s):  
N. HUTCHINS ◽  
T. B. NICKELS ◽  
I. MARUSIC ◽  
M. S. CHONG
Keyword(s):  
Reynolds Number ◽  
Boundary Layers ◽  
Spatial Resolution ◽  
Reynolds Numbers ◽  
Energy Spectra ◽  
Small Scale ◽  
Wire Length ◽  
Hot Wire ◽  
Wide Range ◽  
Near Wall

Careful reassessment of new and pre-existing data shows that recorded scatter in the hot-wire-measured near-wall peak in viscous-scaled streamwise turbulence intensity is due in large part to the simultaneous competing effects of the Reynolds number and viscous-scaled wire length l+. An empirical expression is given to account for these effects. These competing factors can explain much of the disparity in existing literature, in particular explaining how previous studies have incorrectly concluded that the inner-scaled near-wall peak is independent of the Reynolds number. We also investigate the appearance of the so-called outer peak in the broadband streamwise intensity, found by some researchers to occur within the log region of high-Reynolds-number boundary layers. We show that the ‘outer peak’ is consistent with the attenuation of small scales due to large l+. For turbulent boundary layers, in the absence of spatial resolution problems, there is no outer peak up to the Reynolds numbers investigated here (Reτ = 18830). Beyond these Reynolds numbers – and for internal geometries – the existence of such peaks remains open to debate. Fully mapped energy spectra, obtained with a range of l+, are used to demonstrate this phenomenon. We also establish the basis for a ‘maximum flow frequency’, a minimum time scale that the full experimental system must be capable of resolving, in order to ensure that the energetic scales are not attenuated. It is shown that where this criterion is not met (in this instance due to insufficient anemometer/probe response), an outer peak can be reproduced in the streamwise intensity even in the absence of spatial resolution problems. It is also shown that attenuation due to wire length can erode the region of the streamwise energy spectra in which we would normally expect to see kx−1 scaling. In doing so, we are able to rationalize much of the disparity in pre-existing literature over the kx−1 region of self-similarity. Not surprisingly, the attenuated spectra also indicate that Kolmogorov-scaled spectra are subject to substantial errors due to wire spatial resolution issues. These errors persist to wavelengths far beyond those which we might otherwise assume from simple isotropic assumptions of small-scale motions. The effects of hot-wire length-to-diameter ratio (l/d) are also briefly investigated. For the moderate wire Reynolds numbers investigated here, reducing l/d from 200 to 100 has a detrimental effect on measured turbulent fluctuations at a wide range of energetic scales, affecting both the broadband intensity and the energy spectra.

Electronic Cooling Using Synthetic Jets

2005 ◽  
Author(s):  
Anna A. Pavlova ◽  
Michael Amitay
Keyword(s):  
Reynolds Number ◽  
Heated Surface ◽  
Low Frequency ◽  
Heat Removal ◽  
Jet Impingement ◽  
Constant Heat Flux ◽  
Synthetic Jet ◽  
Synthetic Jets ◽  
Impingement Cooling ◽  
Jet Cooling

Efficiency of synthetic jet impingement cooling and the mechanisms of heat removal from a constant heat flux surface were investigated experimentally. The effects of jet’s formation frequency and Reynolds number at different nozzle-to-surface distances were investigated and compared to steady jet cooling. It was found that synthetic jets are up to three times more effective than steady jets at the same Reynolds number. For smaller distances, high formation frequency (f = 1200 Hz) synthetic jets remove heat better than low frequency (f = 420 Hz) jets, whereas low frequency jets are more effective at larger distances, with an overlapping region. Using PIV, it was shown that at small distances between the synthetic jet and the heated surface, the higher formation frequency jet is associated with accumulation of vortices before they impinge on the surface. For the lower frequency jet, the wavelength between coherent structures is so large that vortex rings impinge on the surface separately.

Sign in / Sign up

Export Citation Format

Share Document