Effect of velocity ratio on the interaction between plasma synthetic jets and turbulent cross-flow

2019 ◽  
Vol 865 ◽  
pp. 928-962 ◽  
Author(s):  
Haohua Zong ◽  
Marios Kotsonis
Keyword(s):  
Boundary Layer ◽  
High Speed ◽  
Velocity Ratio ◽  
Vortex Pair ◽  
Synthetic Jets ◽  
Low Energy ◽  
Vortex Rings ◽  
Wall Region ◽  
Mean Velocity ◽  
Counter Rotating Vortex Pair

Plasma synthetic jet actuators (PSJAs) are particularly suited for high-Reynolds-number, high-speed flow control due to their unique capability of generating supersonic pulsed jets at high frequency (${>}5$  kHz). Different from conventional synthetic jets driven by oscillating piezoelectric diaphragms, the exit-velocity variation of plasma synthetic jets (PSJs) within one period is significantly asymmetric, with ingestion being relatively weaker (less than $20~\text{m}~\text{s}^{-1}$) and longer than ejection. In this study, high-speed phase-locked particle image velocimetry is employed to investigate the interaction between PSJAs (round exit orifice, diameter 2 mm) and a turbulent boundary layer at constant Strouhal number (0.02) and increasing mean velocity ratio ($r$, defined as the ratio of the time-mean velocity over the ejection phase to the free-stream velocity). Two distinct operational regimes are identified for all the tested cases, separated by a transition velocity ratio, lying between $r=0.7$ and $r=1.0$. At large velocity and stroke ratios (first regime, representative case $r=1.6$), vortex rings are followed by a trailing jet column and tilt downstream initially. This downstream tilting is transformed into upstream tilting after the pinch-off of the trailing jet column. The moment of this transformation relative to the discharge advances with decreasing velocity ratio. Shear-layer vortices (SVs) and a hanging vortex pair (HVP) are identified in the windward and leeward sides of the jet body, respectively. The HVP is initially erect and evolves into an inclined primary counter-rotating vortex pair ($p$-CVP) which branches from the middle of the front vortex ring and extends to the near-wall region. The two legs of the $p$-CVP are bridged by SVs, and a secondary counter-rotating vortex pair ($s$-CVP) is induced underneath these two legs. At low velocity and stroke ratios (second regime, representative case $r=0.7$), the trailing jet column and $p$-CVP are absent. Vortex rings always tilt upstream, and the pitching angle increases monotonically with time. An $s$-CVP in the near-wall region is induced directly by the two longitudinal edges of the ring. Inspection of spanwise planes ($yz$-plane) reveals that boundary-layer energization is realized by the downwash effect of either vortex rings or $p$-CVP. In addition, in the streamwise symmetry plane, the increasing wall shear stress is attributed to the removal of low-energy flow by ingestion. The downwash effect of the $s$-CVP does not benefit boundary-layer energization, as the flow swept to the wall is of low energy.

The behaviour of circular synthetic jets in a laminar boundary layer

2005 ◽  
Vol 109 (1100) ◽  
pp. 461-470 ◽  
Author(s):  
S. Zhong ◽  
F. Millet ◽  
N. J. Wood
Keyword(s):  
Boundary Layer ◽  
Velocity Ratio ◽  
Synthetic Jets ◽  
Operating Conditions ◽  
Vortex Rings ◽  
Hairpin Vortices ◽  
Vortical Structures ◽  
Flow Separation Control ◽  
Wall Flow ◽  
Near Wall

Abstract Dye flow visualisation of circular synthetic jets was carried out in laminar boundary layers developing over a flat plate at a range of actuator operating conditions and freestream velocities of 0·05 and 0·1ms–1. The purpose of this work was to study the interaction of synthetic jets with the boundary layer and the nature of vortical structures produced as a result of this interaction. The effects of Reynolds number (Re), velocity ratio (VR ) and Strouhal number (St) on the behaviour of synthetic jets were studied. At low Re and VR , the vortical structures produced by synthetic jets appear as highly stretched hairpin vortices attached to the wall. At intermediate Re and VR , these structures roll up into vortex rings which experience a considerable amount of tilting and stretching as they enter the boundary layer. These vortex rings will eventually propagate outside the boundary layer hence the influence of the synthetic jets on the near wall flow will be confined in the near field of the jet exit. At high Re and VR , the vortex rings appear to experience a certain amount of tilting but no obvious stretching. They penetrate the edge of the boundary layer quickly, producing very limited impact on the near wall flow. Hence it is believed that the hairpin vortices produced at low Re and VR are likely to be the desirable structures for effective flow separation control. In this paper, a vortex model was also described to explain the mechanism of vortex tilting.

Organized motions in a jet in crossflow

2001 ◽  
Vol 444 ◽  
pp. 117-149 ◽  
Author(s):  
A. RIVERO ◽  
J. A. FERRÉ ◽  
FRANCESC GIRALT
Keyword(s):  
Velocity Field ◽  
Velocity Ratio ◽  
Mean Field ◽  
Vortex Pair ◽  
Leeward Side ◽  
Mean Velocity ◽  
Jet In Crossflow ◽  
Planar Laser ◽  
The Mean ◽  
Counter Rotating Vortex Pair

An experimental study to identify the structures present in a jet in crossflow has been carried out at a jet-to-crossflow velocity ratio U/Ucf = 3.8 and Reynolds number Re = UcfD/v = 6600. The hot-wire velocity data measured with a rake of eight X-wires at x/D = 5 and 15 and flow visualizations using planar laser-induced fluorescence (PLIF) confirm that the well-established pair of counter-rotating vortices is a feature of the mean field and that the upright, tornado-like or Fric's vortices that are shed to the leeward side of the jet are connected to the jet flow at the core. The counter-rotating vortex pair is strongly modulated by a coherent velocity field that, in fact, is as important as the mean velocity field. Three different structures – folded vortex rings, horseshoe vortices and handle-type structures – contribute to this coherent field. The new handle-like structures identified in the current study link the boundary layer vorticity with the counter-rotating vortex pair through the upright tornado-like vortices. They are responsible for the modulation and meandering of the counter-rotating vortex pair observed both in video recordings of visualizations and in the instantaneous velocity field. These results corroborate that the genesis of the dominant counter-rotating vortex pair strongly depends on the high pressure gradients that develop in the region near the jet exit, both inside and outside the nozzle.

Wall region of a relaxing three-dimensional incompressible turbulent boundary layer

1978 ◽  
Vol 85 (1) ◽  
pp. 33-56 ◽  
Author(s):  
K. S. Hebbar ◽  
W. L. Melnik
Keyword(s):  
Boundary Layer ◽  
Flow Field ◽  
Velocity Ratio ◽  
Three Dimensional ◽  
Cross Flow ◽  
Wall Region ◽  
Mean Flow ◽  
Mean Velocity ◽  
Flow Angle ◽  
The Mean

An experimental investigation was conducted at selected locations in the wall region of a three-dimensional turbulent boundary layer relaxing in a nominally zero external pressure gradient behind a transverse hump (in the form of a 30° swept, 5 ft chord, wing-type model) faired into the side wall of a low-speed wind tunnel. The boundary layer (approximately 3·5 in. thick near the first survey station, where the length Reynolds number was 5·5 × 106) had a maximum cross-flow velocity ratio of 0·145 and a maximum cross-flow angle of 21·9° close to the wall. The hot-wire data indicated that the apparent dimensionless velocity profiles in the viscous sublayer are universal and that the wall influence on the hot wire is negligible beyond y+= 5. The existence of wall similarity in the relaxing flow field was confirmed in the form of a log law based on the resultant mean velocity and resultant friction velocity (obtained from the measured skin friction).The smallest collateral region extended from the point nearest to the wall (y+≈ 1) up to y+= 9·7, corresponding to a resultant mean velocity ratio (local to free-stream) of 0·187. The unusual feature of these profiles was the presence of a narrow region of slightly decreasing cross-flow angle (1° or less) that extended from the point of maximum cross-flow angle down to the outer limit of the collateral region. A sublayer analysis of the flow field using the measured local transverse pressure gradient slightly overestimated the decrease in cross-flow angle. It is concluded that, in the absence of these gradients, the skewing of the flow could have been much more pronounced practically down to the wall (limited only by the resolution of the sensor), implying a near-wallnon-collateralflow field consistent with the equations of motion in the neighbourhood of the wall.The streamwise relaxation of the mean flow field based on the decay of the cross-flow angle was much faster in the inner layer than in the outer layer. Over the stream-wise distance covered, the mean flow in the inner layer and the wall shear-stress vector relaxed to a two-dimensional state in approximately 10 boundary-layer thicknesses whereas the relaxation of the turbulence was slower and was not complete over the same distance.

A Parametric Investigation of Wake Loss Reduction by using Serrated Trailing Edge in a High-speed Compressor Cascade

2017 ◽  
Vol 0 (0) ◽  
Author(s):  
Huaping Liu ◽  
Yongchuan Yu ◽  
Feipeng Yu ◽  
Dongfei Zhang ◽  
Huanlong Chen
Keyword(s):  
High Speed ◽  
Pressure Rise ◽  
Vortex Pair ◽  
Low Energy ◽  
Loss Reduction ◽  
Compressor Cascade ◽  
Fluid Exchange ◽  
Trailing Edge ◽  
Blade Height ◽  
Counter Rotating Vortex Pair

AbstractThe influence of the serration with different widths, angles and spanwise locations on the wake of a high-speed compressor cascade is parametrically investigated by numerical simulation. The effect mechanisms of the serration on the cascade performance are discussed in detail and the potential at the off-design points is also validated. The results show that the momentum inputted by the serration jet and the fluid exchange induced by the dominant jet vortex could reenergize the low energy corner region and weaken the trailing edge shedding vortex. With the upstream shift of the serration valley, the loss reduction and pressure rise are enhanced due to the strengthened jet and counter-rotating vortex pair, whereas the flow turning is reduced by the decreased camber angle and the enhanced jet deflection. The serration at the blade height corresponding to the core of the low energy fluid could perform better for the loss reduction, whereas the serration close to the endwall could obtain higher pressure rise. For the optimal serration located at 12.5 % blade height with a width of 5 mm and an angle of 45 °, the loss is reduced by 8.9 % and the pressure rise is increased by 2.2 %, with the flow turning slightly reduced by 0.93 °

scholarly journals Active control of multiscale features in wall-bounded turbulence

2019 ◽  
Vol 36 (1) ◽  
pp. 12-21 ◽  
Author(s):  
Xiaotong Cui ◽  
Nan Jiang ◽  
Xiaobo Zheng ◽  
Zhanqi Tang
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Turbulent Energy ◽  
Streamwise Velocity ◽  
Discrete Wavelet ◽  
Wall Region ◽  
Mean Velocity ◽  
Pzt Actuator ◽  
The Impact ◽  
Near Wall

Abstract This study experimentally investigates the impact of a single piezoelectric (PZT) actuator on a turbulent boundary layer from a statistical viewpoint. The working conditions of the actuator include a range of frequencies and amplitudes. The streamwise velocity signals in the turbulent boundary layer flow are measured downstream of the actuator using a hot-wire anemometer. The mean velocity profiles and other basic parameters are reported. Spectra results obtained by discrete wavelet decomposition indicate that the PZT vibration primarily influences the near-wall region. The turbulent intensities at different scales suggest that the actuator redistributes the near-wall turbulent energy. The skewness and flatness distributions show that the actuator effectively alters the sweep events and reduces intermittency at smaller scales. Moreover, under the impact of the PZT actuator, the symmetry of vibration scales’ velocity signals is promoted and the structural composition appears in an orderly manner. Probability distribution function results indicate that perturbation causes the fluctuations in vibration scales and smaller scales with high intensity and low intermittency. Based on the flatness factor, the bursting process is also detected. The vibrations reduce the relative intensities of the burst events, indicating that the streamwise vortices in the buffer layer experience direct interference due to the PZT control.

Turbulent Wake of a Stack and the Influence of Velocity Ratio

2006 ◽  
Author(s):  
M. S. Adaramola ◽  
D. Sumner ◽  
D. J. Bergstrom
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Reynolds Shear Stress ◽  
Velocity Ratio ◽  
Cross Flow ◽  
Turbulent Wake ◽  
Mean Velocity ◽  
Distinct Structure ◽  
Flow Reynolds Number ◽  
Cross Flow Velocity

The effect of the jet-to-cross-flow velocity ratio, R, on the turbulent wake of a cylindrical stack of AR = 9 was investigated with two-component thermal anemometry. The cross-flow Reynolds number was ReD = 2.3×104, the jet Reynolds number ranged from Red = 7×103 to 4.6×104, and R was varied from 0 to 3. The stack was partially immersed in a flat-plate turbulent boundary layer, with a boundary layer thickness-to-height ratio of δ/H = 0.5 at the location of the stack. The flow around the stack was broadly classified into three flow regimes depending on the value of R, which were the downwash (R < 0.5), cross-wind dominated (0.5 < R < 1.5), and jet-dominated (R > 1.5) regimes. Each flow regime had a distinct structure to the mean velocity (streamwise and wall-normal directions), turbulence intensity (streamwise and wall-normal directions), and Reynolds shear stress fields.

The infrared measurement cascade: Connecting large scale meteorologically induced surface temperature perturbations to local spatial velocity structures

2020 ◽  
Author(s):  
Benjamin Schumacher ◽  
Marwan Katurji ◽  
Jiawei Zhang
Keyword(s):  
Boundary Layer ◽  
Velocity Field ◽  
Brightness Temperature ◽  
High Speed ◽  
Surface Brightness ◽  
Large Scale ◽  
Mean Velocity ◽  
Near Surface ◽  
Multi Scale ◽  
Infrared Cameras

<p>The evolution of micrometeorological measurements has been recently manifested by developments in methodological and analytical techniques using spatial surface brightness temperature captured by infrared cameras (Schumacher et al. 2019, Katurji and Zawar-Reza 2016). The Thermal Image Velocimetry (TIV) method can now produce accurate 2D advection-velocities using high speed (>20Hz) infrared imagery (Inagaki 2013, Schumacher 2019). However, to further develop TIV methods and achieve a novel micrometeorological measurement technique, all scales of motion within the boundary layer need to be captured.</p><p>Spatial observations of multi-frequency and multi-scale temperature perturbations are a result from the turbulent interaction of the overlying atmosphere and the surface. However, these surface signatures are connected to the larger scales of the atmospheric boundary layer (McNaughton 2002, Träumner 2015). When longer periods (a few hours to a few days) of spatial surface brightness temperatures are observed, the larger scale information needs to be accounted for to build a comprehensive understanding of surface-atmospheric spatial turbulent interactions. Additionally, the time-frequency decomposition of brightness temperature perturbations shows longer periods of 4-15 minutes superimposed over shorter periods of ~ 4–30 seconds. This suggests that that boundary layer dynamic scales (of longer periods) can influence brightness temperature perturbations on the local turbulent scale. An accurate TIV algorithm needs to account for all scales of motion when analysing the time-space variability of locally observed spatial brightness temperature patterns.</p><p>To analyse these propositions temporally high resolved geostationary satellite infrared data from the Himawari 8 satellite was compared to near-surface and high speed (20 Hz) measured air and brightness temperature using thermocouple measurements and infrared cameras. The satellite provides a temporal resolution of 10-minutes and a horizontal resolution of 2 by 2 km per pixel and therefore captures the atmospheric meso γ and micro α scale which signals are usually active for ~10 minutes to < 12 hours. Moreover, the Himawari 8 brightness temperature was used to create the near-surface mean velocity field using TIV. Afterwards, the velocity field was compared to the in-situ measured wind velocity over several days during January 2019.</p><p>The results show that the atmospheric forcing from the micro α scale to lower atmospheric scales has a major impact on the near-surface temperature over several minutes. A significant (p-value: 0.02) positive covariance between the Himawari 8 measurement and the local measured temperature 1.5 cm above the ground on a 10 minute average, specifically concerning cooling and heating patterns, has been found.</p><p>Further analysis demonstrates that the retrieved near-surface 2-D velocity field calculated from the Himawari 8 brightness temperature perturbations is correctly representing the mean velocity. This finding allows the classification of meso-scale atmospheric forcing and its direct connection to local scale turbulent 2-D velocity measurements. This extends the TIV algorithm by a multi-scale component which allows to address inter-scale boundary layer analysis from a new point of view. In respect to the current findings a new experiment will focus on the repeated induced local velocity patterns from large scale forcing which will be measured through the surface brightness temperature.</p>

Measurements in an Axisymmetric Turbulent Boundary Layer Along a Circular Cylinder

1976 ◽  
Vol 27 (3) ◽  
pp. 217-228 ◽  
Author(s):  
Noor Afzal ◽  
K P Singh
Keyword(s):  
Boundary Layer ◽  
Circular Cylinder ◽  
Turbulent Boundary Layer ◽  
Constant Pressure ◽  
Reynolds Shear Stress ◽  
Longitudinal Velocity ◽  
Outer Region ◽  
Wall Region ◽  
Mean Velocity ◽  
Dimensional Boundary

SummaryIn an axisymmetric turbulent boundary layer along a circular cylinder at constant pressure, measurements have been made of mean velocity profile and turbulence characteristics: longitudinal velocity fluctuations, Reynolds shear stress, transverse correlation and spectrum. It has been found that the qualitative behaviour of an axisymmetric turbulent boundary layer is similar to that of a two-dimensional boundary layer in the wall region, where as in the outer region the effects of transverse curvature are observed.

scholarly journals Contributions of the wall boundary layer to the formation of the counter-rotating vortex pair in transverse jets

2011 ◽  
Vol 676 ◽  
pp. 461-490 ◽  
Author(s):  
FABRICE SCHLEGEL ◽  
DAEHYUN WEE ◽  
YOUSSEF M. MARZOUK ◽  
AHMED F. GHONIEM
Keyword(s):  
Boundary Layer ◽  
Boundary Condition ◽  
Near Field ◽  
Vortex Pair ◽  
Transverse Jets ◽  
Vortical Structures ◽  
Wall Boundary Layer ◽  
Wall Boundary ◽  
Jet Trajectory ◽  
Counter Rotating Vortex Pair

Using high-resolution 3-D vortex simulations, this study seeks a mechanistic understanding of vorticity dynamics in transverse jets at a finite Reynolds number. A full no-slip boundary condition, rigorously formulated in terms of vorticity generation along the channel wall, captures unsteady interactions between the wall boundary layer and the jet – in particular, the separation of the wall boundary layer and its transport into the interior. For comparison, we also implement a reduced boundary condition that suppresses the separation of the wall boundary layer away from the jet nozzle. By contrasting results obtained with these two boundary conditions, we characterize near-field vortical structures formed as the wall boundary layer separates on the backside of the jet. Using various Eulerian and Lagrangian diagnostics, it is demonstrated that several near-wall vortical structures are formed as the wall boundary layer separates. The counter-rotating vortex pair, manifested by the presence of vortices aligned with the jet trajectory, is initiated closer to the jet exit. Moreover tornado-like wall-normal vortices originate from the separation of spanwise vorticity in the wall boundary layer at the side of the jet and from the entrainment of streamwise wall vortices in the recirculation zone on the lee side. These tornado-like vortices are absent in the case where separation is suppressed. Tornado-like vortices merge with counter-rotating vorticity originating in the jet shear layer, significantly increasing wall-normal circulation and causing deeper jet penetration into the crossflow stream.

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