Some Behaviours of Coherent Structures Under Controlled Excitation in Coaxial Jet of Mean Velocity Ratio of 0.3

This paper describes an experimental investigation of the initial region of a subsonic cold coaxial jet at a mean-velocity ratio λ, outer to inner, of 1.25. Detailed measurements in the initial region have shown that similarity of the pressure intensity profiles exists in the three zones: the initial merging zone, the intermediate zone and the fully merged zone. Spectral measurements of the pressure fluctuations confirm the existence of coherent structures in the outer mixing region. Comparison of the coaxial jet results with those of the single jet has been attempted.

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Effect of Mean Velocity-to-Critical Velocity Ratios on Bed Topography and Incipient Motion in a Meandering Channel: Experimental Investigation

Water ◽

10.3390/w13070883 ◽

2021 ◽

Vol 13 (7) ◽

pp. 883

Author(s):

Nargess Moghaddassi ◽

Seyed Habib Musavi-Jahromi ◽

Mohammad Vaghefi ◽

Amir Khosrojerdi

Keyword(s):

Experimental Investigation ◽

Critical Velocity ◽

Velocity Ratio ◽

Scour Depth ◽

Incipient Motion ◽

Mean Velocity ◽

Straight Path ◽

Meandering Channel ◽

Bed Topography ◽

The Mean

As 180-degree meanders are observed in abundance in nature, a meandering channel with two consecutive 180-degree bends was designed and constructed to investigate bed topography variations. These two 180-degree mild bends are located between two upstream and downstream straight paths. In this study, different mean velocity-to-critical velocity ratios have been tested at the upstream straight path to determine the meander’s incipient motion. To this end, bed topography variations along the meander and the downstream straight path were addressed for different mean velocity-to-critical velocity ratios. In addition, the upstream bend’s effect on the downstream bend was investigated. Results indicated that the maximum scour depth at the downstream bend increased as a result of changing the mean velocity-to-critical velocity ratio from 0.8 to 0.84, 0.86, 0.89, 0.92, 0.95, and 0.98 by, respectively, 1.5, 2.5, 5, 10, 12, and 26 times. Moreover, increasing the ratio increased the maximum sedimentary height by 3, 10, 23, 48, 49, and 56 times. The upstream bend’s incipient motion was observed for the mean velocity-to-critical velocity ratio of 0.89, while the downstream bend’s incipient motion occurred for the ratio of 0.78.

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

Journal of Fluid Mechanics ◽

10.1017/jfm.2019.93 ◽

2019 ◽

Vol 865 ◽

pp. 928-962 ◽

Cited By ~ 7

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.

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Effect of wind turbine nacelle on turbine wake dynamics in large wind farms

Journal of Fluid Mechanics ◽

10.1017/jfm.2019.206 ◽

2019 ◽

Vol 869 ◽

pp. 1-26 ◽

Cited By ~ 7

Author(s):

Daniel Foti ◽

Xiaolei Yang ◽

Lian Shen ◽

Fotis Sotiropoulos

Keyword(s):

Wind Turbine ◽

Coherent Structures ◽

Large Scale ◽

Wind Farm ◽

Wind Farms ◽

Practical Significance ◽

Mean Velocity ◽

Velocity Deficit ◽

Farm Scale ◽

Turbine Wake

Wake meandering, a phenomenon of large-scale lateral oscillation of the wake, has significant effects on the velocity deficit and turbulence intensities in wind turbine wakes. Previous studies of a single turbine (Kang et al., J. Fluid. Mech., vol. 774, 2014, pp. 374–403; Foti et al., Phys. Rev. Fluids, vol. 1 (4), 2016, 044407) have shown that the turbine nacelle induces large-scale coherent structures in the near field that can have a significant effect on wake meandering. However, whether nacelle-induced coherent structures at the turbine scale impact the emergent turbine wake dynamics at the wind farm scale is still an open question of both fundamental and practical significance. We take on this question by carrying out large-eddy simulation of atmospheric turbulent flow over the Horns Rev wind farm using actuator surface parameterisations of the turbines without and with the turbine nacelle taken into account. While the computed mean turbine power output and the mean velocity field away from the nacelle wake are similar for both cases, considerable differences are found in the turbine power fluctuations and turbulence intensities. Furthermore, wake meandering amplitude and area defined by wake meanders, which indicates the turbine wake unsteadiness, are larger for the simulations with the turbine nacelle. The wake influenced area computed from the velocity deficit profiles, which describes the spanwise extent of the turbine wakes, and the spanwise growth rate, on the other hand, are smaller for some rows in the simulation with the nacelle model. Our work shows that incorporating the nacelle model in wind farm scale simulations is critical for accurate predictions of quantities that affect the wind farm levelised cost of energy, such as the dynamics of wake meandering and the dynamic loads on downwind turbines.

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Coherent structures in the linearized impulse response of turbulent channel flow

Journal of Fluid Mechanics ◽

10.1017/jfm.2019.15 ◽

2019 ◽

Vol 863 ◽

pp. 1190-1203 ◽

Cited By ~ 9

Author(s):

Sabarish B. Vadarevu ◽

Sean Symon ◽

Simon J. Illingworth ◽

Ivan Marusic

Keyword(s):

Channel Flow ◽

Coherent Structures ◽

Large Scale ◽

Body Force ◽

Stokes Equations ◽

Turbulent Channel Flow ◽

Kinetic Energy Density ◽

Reynolds Stresses ◽

Mean Velocity ◽

Velocity Fluctuations

We study the evolution of velocity fluctuations due to an isolated spatio-temporal impulse using the linearized Navier–Stokes equations. The impulse is introduced as an external body force in incompressible channel flow at $Re_{\unicode[STIX]{x1D70F}}=10\,000$. Velocity fluctuations are defined about the turbulent mean velocity profile. A turbulent eddy viscosity is added to the equations to fix the mean velocity as an exact solution, which also serves to model the dissipative effects of the background turbulence on large-scale fluctuations. An impulsive body force produces flow fields that evolve into coherent structures containing long streamwise velocity streaks that are flanked by quasi-streamwise vortices; some of these impulses produce hairpin vortices. As these vortex–streak structures evolve, they grow in size to be nominally self-similar geometrically with an aspect ratio (streamwise to wall-normal) of approximately 10, while their kinetic energy density decays monotonically. The topology of the vortex–streak structures is not sensitive to the location of the impulse, but is dependent on the direction of the impulsive body force. All of these vortex–streak structures are attached to the wall, and their Reynolds stresses collapse when scaled by distance from the wall, consistent with Townsend’s attached-eddy hypothesis.

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Turbulent Wake of a Stack and the Influence of Velocity Ratio

Volume 9: 6th FSI, AE and FIV and N Symposium ◽

10.1115/pvp2006-icpvt-11-93629 ◽

2006 ◽

Cited By ~ 1

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.

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Dynamics of Large-Scale Structures for Jets in a Crossflow

Journal of Turbomachinery ◽

10.1115/1.2841355 ◽

1999 ◽

Vol 121 (3) ◽

pp. 577-587 ◽

Cited By ~ 3

Author(s):

F. Muldoon ◽

S. Acharya

Keyword(s):

Shear Layer ◽

Large Scale ◽

Computational Study ◽

Near Field ◽

Velocity Ratio ◽

Leeward Side ◽

Dynamic Features ◽

Mean Velocity ◽

Large Scale Structures ◽

Scale Structures

Results of a three-dimensional unsteady computational study of a row of jets injected normal to a crossflow are presented with the aim of understanding the dynamics of the large-scale structures in the region near the jet. The jet to crossflow velocity ratio is 0.5. A modified version of the computer program (INS3D), which utilizes the method of artificial compressibility, is used for the computations. Results obtained clearly indicate that the near-field large-scale structures are extremely dynamic in nature, and undergo breakup and reconnection processes. The dynamic near-field structures identified include the counterrotating vortex pair (CVP), the horseshoe vortex, wake vortex, wall vortex, and shear layer vortex. The dynamic features of these vortices are presented in this paper. The CVP is observed to be a convoluted structure interacting with the wall and horseshoe vortices. The shear layer vortices are stripped by the crossflow, and undergo pairing and stretching events in the leeward side of the jet. The wall vortex is reoriented into the upright wake system. Comparison of the predictions with mean velocity measurements is made. Reasonable agreement is observed.

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The stability of countercurrent mixing layers in circular jets

Journal of Fluid Mechanics ◽

10.1017/s0022112091000137 ◽

1991 ◽

Vol 227 ◽

pp. 309-343 ◽

Cited By ~ 77

Author(s):

P. J. Strykowski ◽

D. L. Niccum

Keyword(s):

Shear Layer ◽

Critical Velocity ◽

Velocity Ratio ◽

Mixing Layer ◽

Mixing Layers ◽

Mean Velocity ◽

Axisymmetric Vortex ◽

Circular Jets ◽

Axisymmetric Shear ◽

The Stability

A spatially developing countercurrent mixing layer was established experimentally by applying suction to the periphery of an axisymmetric jet. A laminar mixing region was studied in detail for a velocity ratio R = ΔU/2U between 1 and 1.5, where ΔU describes the intensity of the shear across the layer and U is the average speed of the two streams. Above a critical velocity ratio Rr = 1.32 the shear layer displays energetic oscillations at a discrete frequency which are the result of very organized axisymmetric vortex structures in the mixing layer. The spatial order of the primary vortices inhibits the pairing process and dramatically alters the spatial development of the shear layer downstream. Consequently, the turbulence level in the jet core is significantly reduced, as is the decay rate of the mean velocity on the jet centreline. The response of the shear layer to controlled external forcing indicates that the shear layer oscillations at supercritical velocity ratios are self-excited. The experimentally determined critical velocity ratio of 1.32, established for very thin axisymmetric shear layers, compares favourably with the theoretically predicted value of 1.315 for the transition from convective to absolute instability in plane mixing layers (Huerre & Monkewitz 1985).

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Effect of Mean Velocity to Critical Velocity Ratios on Bed Topography and Incipient Motion in a Meandering Channel: Experimental Investigation

10.20944/preprints202102.0198.v1 ◽

2021 ◽

Author(s):

Nargess Moghaddassi ◽

Seyed Habib Musavi-Jahromi ◽

Mohammad Vaghefi ◽

Amir Khosrojerdi

Keyword(s):

Experimental Investigation ◽

Critical Velocity ◽

Velocity Ratio ◽

Scour Depth ◽

Incipient Motion ◽

Mean Velocity ◽

Straight Path ◽

Meandering Channel ◽

Bed Topography ◽

The Mean

As 180-degree meanders are observed in abundance in nature, a meandering channel with two consecutive 180-degree bends was designed and constructed to investigate bed topography variations. These two 180-degree mild bends are located between two upstream and downstream straight paths. In this study, different mean velocity to critical velocity ratios have been tested at the upstream straight path to determine the meander's incipient motion. To this end, bed topography variations along the meander and the downstream straight path were addressed for different mean velocity to critical velocity ratios. In addition, the upstream bend's effect on the downstream bend has been investigated. Results indicated that the maximum scour depth at the downstream bend has increased as a result of changing the mean velocity to critical velocity ratio from 0.8 to 0.84, 0.86, 0.89, 0.92, 0.95, and 0.98 by respectively 1.5, 2.5, 5, 10, 12, and 26 times. Moreover, increasing the ratio increased the maximum sedimentary height by 3, 10, 23, 48, 49, and 56 times. The upstream bend's incipient motion was observed for the mean velocity to critical velocity ratio of 0.89, while the downstream bend was equal to 0.78.

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NUMERICAL AND EXPERIMENTAL STUDY OF A FREE INCOMPRESSIBLE ISOTHERMAL TURBULENT COAXIAL JET

Revista de Engenharia Térmica ◽

10.5380/reterm.v9i1-2.61939 ◽

2010 ◽

Vol 9 (1-2) ◽

pp. 98 ◽

Cited By ~ 1

Author(s):

G. R. Ströher ◽

C. A. Martins ◽

C. R. De Andrade

Keyword(s):

Experimental Data ◽

Turbulence Model ◽

Velocity Ratio ◽

Radial Profile ◽

Mass Conservation ◽

Experimental Measurements ◽

Governing Equations ◽

Self Similarity ◽

Mean Velocity ◽

Volume Method

In the present study the free incompressible isothermal turbulent coaxial jet problem is numerically solved, and compared with experimental measurements for different velocity ratio between the inner and the outer streams of the jet. The radial profile of the axial mean velocity was obtained with hot anemometry at different axial positions. Governing equations (mass conservation, momentum, turbulence model) were discretized employing the finite volume method with a segregated solver. The analysis of the experimental results showed that coaxial jet flow fields did not present self-similarity up to z/D=25, and the numerical solution using the Shih’s k ε turbulence model did not match reasonably with the experimental data, with a difference of about ± 10%.

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