scholarly journals Wake behind circular cylinder excited by spanwise non-uniform disturbances

2021 ◽  
Vol 1 (1) ◽  
Author(s):  
Takumi Kamiyama ◽  
Mizuki Ino ◽  
Yudai Yokota ◽  
Jun Sakakibara
Keyword(s):  
Circular Cylinder ◽  
Shear Layer ◽  
Strouhal Number ◽  
Large Scale ◽  
Plasma Actuator ◽  
Spanwise Direction ◽  
Free Shear Layer ◽  
Constant Vorticity ◽  
Vortex Lines ◽  
Two Component

We studied a modification of wake behind a circular cylinder using a plasma actuator. The plasma actuators were arranged in the spanwise direction of the cylinder to give temporal periodic disturbances having Strouhal number St = 0.18-2.3 with a burst ratio BR = 20 and 40%. The Reynolds number was set in a rage of Re = 4200 to 8400. Two types of plasma actuator were prepared; one is a single strip of the actuator placed at each side of the cylinder to give a spanwise uniform disturbance, and another is an array of small piece of actuators placed at the same location to create a spanwise non-uniform disturbance with temporal phase difference, φ = 0 or π, between adjacent electrodes. A conventional two-component PIV and stereo PIV was used to measure the flow field. Figure 1 shows the instantaneous spanwise component of vorticity at Re = 4200 evaluated by two-component PIV. Under no disturbance condition, the laminar shear layer extends straight to around x / d = 1.5 and then forms a wake vortex, as shown in Fig.1(a). In the case of spanwise non-uniform forcing with St = 1.09 and φ =π, rapid roll up of the initial shear layer leads to arrangement of wake vortices closer to the cylinder., as shown in Fig.1(b). With higher Strouhal number case with St = 1.09 and φ = 0, shown in Fig.1(c), a series of fine scale vortices are generated behind both side of the cylinder without forming regular Karman vortices. The spanwise non-uniform forcing was effective to suppress the formation of large scale vortices just behind the cylinder. Figure 2 shows surface of constant vorticity magnitude and vortex lines under St =1.09 and φ = π case. These were computed from a phase-averaged threecomponents velocity field evaluated by stereo PIV. The value of the surface was selected to display the boundary layer formed on the cylinder, and the vortex lines are selected to visualize the vortex structure formed in the following shear layer. A bundle of vortex lines are shaped in a wavy pattern along spanwise direction with 180 degrees out of phase to the adjacent bundle upstream of downstream. This structure, so called ‘chain-line fence structure’ was already found in planar free shear layer [Nygaard, K.J. and Glezer, A., 1990, Phys. Fluids A, 2, 461] and planar jet [Sakakibara, J., Anzai, T., 2001, Phys. Fluids, 13, 1541], but it became evident to create it in the wake of circular cylinder in this study.

Effects of wavelength and amplitude of a wavy cylinder in cross-flow at low Reynolds numbers

2009 ◽  
Vol 620 ◽  
pp. 195-220 ◽  
Author(s):  
K. LAM ◽  
Y. F. LIN
Keyword(s):  
Reynolds Number ◽  
Laminar Flow ◽  
Circular Cylinder ◽  
Drag Coefficient ◽  
Shear Layer ◽  
Lift Coefficient ◽  
Three Dimensional ◽  
Spanwise Direction ◽  
Free Shear Layer ◽  
The Mean

Three-dimensional numerical simulations of laminar flow around a circular cylinder with sinusoidal variation of cross-section along the spanwise direction, named ‘wavy cylinder’, are performed. A series of wavy cylinders with different combinations of dimensionless wavelength (λ/Dm) and wave amplitude (a/Dm) are studied in detail at a Reynolds number of Re = U∞Dm/ν = 100, where U∞ is the free-stream velocity and Dm is the mean diameter of a wavy cylinder. The results of variation of mean drag coefficient and root mean square (r.m.s.) lift coefficient with dimensionless wavelength show that significant reduction of mean and fluctuating force coefficients occurs at optimal dimensionless wavelengths λ/Dm of around 2.5 and 6 respectively for the different amplitudes studied. Based on the variation of flow structures and force characteristics, the dimensionless wavelength from λ/Dm = 1 to λ/Dm = 10 is classified into three wavelength regimes corresponding to three types of wake structures. The wake structures at the near wake of different wavy cylinders are captured. For all wavy cylinders, the flow separation line varies along the spanwise direction. This leads to the development of a three-dimensional free shear layer with periodic repetition along the spanwise direction. The three-dimensional free shear layer of the wavy cylinder is larger and more stable than that of the circular cylinder, and in some cases the free shear layer even does not roll up into a mature vortex street behind the cylinder. As a result, the mean drag coefficients of some of the typical wavy cylinders are less than that of a corresponding circular cylinder with a maximum drag coefficient reduction up to 18%. The r.m.s. lift coefficients are greatly reduced to practically zero at optimal wavelengths. In the laminar flow regime (60 ≤ Re ≤ 150), the values of optimal wavelength are Reynolds number dependent.

Multiple-contour-dynamic simulation of eddy scales in the plane shear layer

1989 ◽  
Vol 199 ◽  
pp. 89-124 ◽  
Author(s):  
P. A. Jacobs ◽  
D. I. Pullin
Keyword(s):  
Reynolds Number ◽  
Shear Layer ◽  
Large Scale ◽  
Free Shear Layer ◽  
Plane Shear ◽  
Fine Detail ◽  
Vortex Lines ◽  
Spiral Vortex ◽  
Moderate Reynolds Number ◽  
Weakly Dependent

The method of contour dynamics (CD) is applied to several inviscid prototype flows typical of the motions found in the transition region of the free shear layer. Examples of the interaction between the fundamental streamwise-layer perturbation and its first subharmonic are presented that illustrate the events of pairing and tearing of two rolled-up cores and also the coalescence of three rolled-up cores. The present simulations of the temporally unstable two-dimensional layer, at effectively infinite Reynolds number, support the hypothesis that the dynamics of the large-scale roll-up is only weakly dependent on Reynolds number. However, we find fine-scale structure that is not apparent in previous simulations at moderate Reynolds number. Spiral filaments of rotational fluid wrap around the rolled-up vortex cores producing ‘spiky’ vorticity distributions together with the entanglement of large quantities of irrotational fluid into the layer. Simulations proceeded only until the first such event because we were unable to resolve the fine detail generated subsequently. The inclusion of prescribed vortex stretching parallel to the vortex lines is found to accelerate the initial roll-up and to enhance the production of spiral vortex filaments. In the fundamental-subharmonic interaction, vortex stretching slows but does not prevent pairing.

Self-Sustained Oscillations of High Speed Impinging Planar Jets

2010 ◽  
Author(s):  
David Arthurs ◽  
Samir Ziada
Keyword(s):  
Shear Layer ◽  
High Speed ◽  
Large Scale ◽  
Impinging Jets ◽  
Free Shear Layer ◽  
Planar Case ◽  
Axisymmetric Case ◽  
Choked Flow ◽  
Tone Generation ◽  
Flow Velocities

High speed impinging jets are frequently used in a variety of industrial applications including thermal and coating control processes. These flows are liable to the production of very intense narrow band acoustic tones, which are produced by a feedback mechanism between instabilities in the jet free shear layer which roll up to form large scale coherent structures, and pressure fluctuations produced by the impingement of these structures at the impingement surface. This paper examines tone generation of a high speed planar gas jet impinging normally on a flat, rigid surface. Experiments are performed over the complete range of subsonic and transonic jet flow velocities for which tones are generated, from U0 = 150m/s (M≈0.4) to choked flow (U0 = 343m/s, M = 1), and over the complete range of impingement distance for which tones occur. The effect of varying the jet thickness is also examined. The behavior of the planar impinging jet case is compared to that of the axisymmetric case, and found to be significantly different, with tones being excited at larger impingement distances, and at lower flow velocities. The Strouhal numbers associated with tone generation in the planar case are on average an order of magnitude lower than that of the axisymmetric case when using similar velocity and length scales. The frequency behavior of the resulting tones is predicted using a simple feedback model, which allows the identification of the various shear layer modes of the instabilities driving tone generation. Finally, a thorough dimensionless analysis is performed in order to quantify the system behavior in terms of the appropriate scales.

Far-field noise of a subsonic jet under controlled excitation

1985 ◽  
Vol 152 ◽  
pp. 83-111 ◽  
Author(s):  
K. B. M. Q. Zaman
Keyword(s):  
Flow Field ◽  
Shear Layer ◽  
Strouhal Number ◽  
Large Scale ◽  
Jet Noise ◽  
Large Scale Structure ◽  
Broadband Noise ◽  
Scale Structure ◽  
Initial Condition ◽  
High Speeds

The phenomena of excitation-induced suppression and amplification of broadband jet noise have been experimentally investigated in an effort to understand the mechanisms, especially in relation to the near flow-field large-scale structure dynamics. Suppression is found to occur only in jets at low speeds with laminar exit boundary layers, the optimum occurring for excitation at Stθ ≈ 0.017, where Stθ is the Strouhal number based on the initial shear-layer momentum thickness. The suppression mechanism is linked to an initial-condition effect on the large-scale structure dynamics. The interaction and evolution of laminar-like structures at low jet speeds produce more (normalized) noise and turbulence, compared to asymptotically lower levels at high speeds when the initial shear layer is no longer laminar. The effect of initial condition has been demonstrated by tripped versus untripped jet data. The excitation at Stθ ≈ 0.017 results in a quick roll-up and transition of the laminar shear-layer vortices, yielding coherent structures which are similar to those at high speeds. Thus, the broadband noise and turbulence are suppressed, but at the most to the asymptotically lower levels. When at the asymptotic level, the broadband jet noise can only be amplified by the excitation; the amplification is found to be maximum for excitation in the StD range of 0.65–0.85, StD being the Strouhal number based on the jet diameter. Excitation in this StD range also produces strongest vortexpairing activity. From spectral analysis of the flow-field and the near sound-pressure field, it is inferred that the pairing process induced by the excitation is at the origin of the broadband noise amplification.

The structure of a turbulent free shear layer in a rotating fluid

1992 ◽  
Vol 241 ◽  
pp. 469-502 ◽  
Author(s):  
A. A. Bidokhti ◽  
D. J. Tritton
Keyword(s):  
Shear Stress ◽  
Shear Layer ◽  
Reynolds Shear Stress ◽  
Three Dimensional ◽  
Longitudinal Direction ◽  
Turbulence Energy ◽  
Spanwise Direction ◽  
Rotating Fluid ◽  
Free Shear Layer ◽  
Stress Changes

An experimental investigation has been carried out on the effects of rotation on the development and structure of turbulence in a free shear layer, oriented so that its mean vorticity is parallel or antiparallel to the system vorticity. The effective local Rossby number extended down to about 1/3. The experimental methods were hydrogen-bubble flow visualization and hot-film anemometry.In summarizing the results we refer to stabilized flow when the system vorticity has the same sign as the shear vorticity and destabilized and subsequently restabilized when it has the opposite sign (Tritton 1992). The roller eddy pattern, familiar in non-rotating flow, was observed in all stabilized flows, but was almost completely disrupted by even weak destabilization. Notable features of the quantitative results were: reorientation by Coriolis effects of the Reynolds stress tensor (inferred from the ratio of the cross-stream to longitudinal turbulence intensity and the normalized shear stress); changes in the ratio of spanwise to longitudinal intensity similar to but weaker than changes in the ratio of cross-stream to longitudinal; a gradual decrease, with increasing stabilization, of the Reynolds shear stress leading ultimately to its changing sign; an increase of the Reynolds shear stress in the destabilized range followed by rapid collapse to almost zero with restabilization. Absolute intensities did not change in line with the turbulence energy production, implying enhancement of dissipation in destabilized flow and inhibition in stabilized and restabilized. Correlation measurements indicated changes of lengthscale in the spanwise direction, and spectra indicated changes in the longitudinal direction that suggest that this enhancement and inhibition are associated with variations between fully three-dimensional and partially two-dimensional turbulence. Data for a wake in a rotating fluid (Witt & Joubert 1985) show similarities to some of the above observations and can be incorporated into the interpretation.

On the interactions between large-scale structure and finegrained turbulence in a free shear flow II. The development of spatial interactions in the mean

1978 ◽  
Vol 359 (1699) ◽  
pp. 497-523 ◽  
Keyword(s):  
Shear Flow ◽  
Shear Layer ◽  
Large Scale ◽  
Large Scale Structure ◽  
Scale Structure ◽  
Turbulent Shear ◽  
Turbulent Shear Flow ◽  
Free Shear Layer ◽  
Mean Flow ◽  
The Mean

Organized structures in turbulent shear flow have been observed both in the laboratory and in the atmosphere and ocean. Recent work on modelling such structures in a temporally developing, horizontally homogeneous turbulent free shear layer (Liu & Merkine 19766) has been extended to the spatially developing mixing layer, there being no available rational transformation between the two nonlinear problems. We consider the kinetic energy development of the mean flow, large-scale structure and finegrained turbulence with a conditional average, supplementing the usual time average, to separate the non-random from the random part of the fluctuations. The integrated form of the energy equations and the accompanying shape assumptions are used to derive ‘ amplitude ’ equations for the mean flow, characterized by the shear layer thickness, the non-random and the random components of flow (which are characterized by their respective energy densities). The closure problem was overcome by the shape assumptions which entered into the interaction integrals: the instability-wavelike large-scale structure was taken to be two-dimensional and the local vertical distribution function was obtained by solving the Rayleigh equation for various local frequencies; the vertical shape of the mean stresses of the fine-grained turbulence was estimated by making use of experimental results; the vertical shapes of the wave-induced stresses were calculated locally from their corresponding equations.

Large-scale motions in a plane wall jet

2019 ◽  
Vol 877 ◽  
pp. 239-281 ◽  
Author(s):  
Ebenezer P. Gnanamanickam ◽  
Shibani Bhatt ◽  
Sravan Artham ◽  
Zheng Zhang
Keyword(s):  
Reynolds Number ◽  
Shear Layer ◽  
Large Scale ◽  
Plane Wall ◽  
Shear Stresses ◽  
Wall Jet ◽  
Wall Region ◽  
Free Shear Layer ◽  
Large Scale Structures ◽  
Scale Structures

The plane wall jet (PWJ) is a wall-bounded flow in which a wall shear layer develops in the presence of extremely energetic flow structures of the outer free-shear layer. The structure of a PWJ, developing in still air, was studied with the focus on the large scales in the flow. Wall-normal hot-wire anemometry (HWA) measurements along with double-frame particle image velocimetry (PIV) measurements (wall-normal–streamwise plane) were carried out at streamwise distances up to $162b$, where $b$ is the slot width of the PWJ exit. The nominal PWJ Reynolds number based on exit parameters was $Re_{j}\approx 5940$. Comparisons with a zero-pressure-gradient boundary layer (ZPGBL) at nominally matched friction Reynolds number $Re_{\unicode[STIX]{x1D70F}}$ were also carried out as appropriate, to highlight key features of the PWJ structure. Consistent with previous work, the PWJ showed a dependence of the peak turbulent stresses on the jet exit Reynolds number. The turbulent production showed a peak corresponding to the near-wall cycle similar to the peak seen in the ZPGBL. However, another turbulent production peak was observed in the outer free-shear layer that was an order of magnitude larger than the inner one. Along with the change in sign of the viscous and Reynolds shear stresses, the PWJ was shown to have a region of very low turbulent production between these two peaks. The dissipation rate increased over the PWJ layer with a peak also in the outer region. Visualizations of the flow and two-point correlations reveal that the most energetic large-scale structures within a PWJ are vortical motions in the wall-normal–streamwise plane similar to those structures seen in free-shear layers. These structures are referred to as J (for jet) type structures. In addition two-point correlations reveal the existence of large-scale structures in the wall region which have a signature similar to those structures seen in canonical boundary layers. These structures are referred to as W (for wall) type structures. Instantaneous PIV realizations and flow visualizations reveal that these W type large-scale features are consistent with the paradigm of hairpin vortex packets in the wall region. The J type structures were seen to intrude well into the wall region while the W type structures were also seen to extend into the outer shear layer. Further, these large-scale structures were shown to modulate the amplitude of the finer scales of the flow.

Effects of a geometrical surface disturbance on flow past a circular cylinder: a large-scale spanwise wire

2010 ◽  
Vol 665 ◽  
pp. 120-157 ◽  
Author(s):  
A. EKMEKCI ◽  
D. ROCKWELL
Keyword(s):  
Circular Cylinder ◽  
Shear Layer ◽  
Large Scale ◽  
Angular Position ◽  
Near Wake ◽  
Surface Disturbance ◽  
Significant Extension ◽  
Image Density ◽  
Karman Vortex ◽  
The Wire

Flow control induced by a single wire that is attached on the outer surface and parallel to the span of a stationary circular cylinder is investigated experimentally. The Reynolds number has a value of 10 000 and the wire diameter is nearly two orders of magnitude smaller than the cylinder diameter, while being larger than the thickness of the unperturbed boundary layer forming around the cylinder. A technique of high-image-density particle image velocimetry is used to characterize mean and unsteady structures of the separating shear layer and the near wake. Only one of the shear layers is directly perturbed by the surface wire. This disturbance, however, has important global consequences over the entire near wake, provided that the wire is located within a certain range of angular positions with respect to the approach flow. Over this range, there are two angles that can be defined as critical on the basis of the streamwise extent of the near-wake structure. In a simplified sense, these critical angles are associated with significant extension and contraction of the near wake, relative to the wake in the absence of the effect of a surface disturbance. The critical angle of the wire that yields the most significant extension of the near wake is also found to lead to bistable oscillations of the separating shear layer at irregular time intervals, much longer than the time scale associated with the classical Kármán vortex shedding. The foregoing two critical states of extension and contraction of the near wake are, respectively, linked to attenuation or enhancement of the Kármán instability. Moreover, the onset of the shear-layer instability, Reynolds stress, Strouhal number and the transverse extent of shear-layer flapping are all shown to depend on the angular position of the wire within the defined range of angles.

Numerical Study of the Particle Dispersion on a Supersonic Shear Layer

2015 ◽  
Vol 798 ◽  
pp. 536-540
Author(s):  
Altyn Makasheva ◽  
Altynshash Naimanova ◽  
Yerzhan Belyayev
Keyword(s):  
Boundary Condition ◽  
Shear Layer ◽  
Large Scale ◽  
Stokes Equations ◽  
Numerical Study ◽  
Turbulence Models ◽  
Euler Method ◽  
Particle Dispersion ◽  
Layer Growth ◽  
Free Shear Layer

The numerical study of the two-dimensional supersonic hydrogen-air mixing in the free shear layer is performed. The system of the Favre-Averaged Navier-Stokes equations for multispecies flow is solved using the ENO scheme of the third order accuracy. The k-ε two-equation turbulence models with compressibility correction are applied to calculate the eddy viscosity coefficient. The dispersion of the particles is studied by following their trajectories in the shear layer by Euler method. In order to produce the roll-up and pairing vortex rings, an unsteady boundary condition is applied at the inlet plane. At the outflow, the non-reflecting boundary condition is taken. The influence of different Mach numbers on the formation of vorticity structures and shear layer growth rate are studied. The obtained results are compared with the available experimental data and the numerical results of other authors. The numerical simulation of the particle dispersion in the shear layer with large scale vortical structure is conducted.

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