A computational investigation of the instability of the detached shear layers in the wake of a circular cylinder

2010 ◽  
Vol 659 ◽  
pp. 375-404 ◽  
Author(s):  
MAN MOHAN RAI
Keyword(s):  
Shear Layer ◽  
Shear Layers ◽  
Wake Flow ◽  
Recirculation Region ◽  
Computational Investigation ◽  
Near Wake ◽  
Layer Transition ◽  
Flow Phenomena ◽  
Considerable Impact ◽  
Velocity Spectra

Cylinder wakes have been studied extensively over several decades to better understand the basic flow phenomena encountered in such flows. The physics of the very near wake of the cylinder is perhaps the most challenging of them all. This region comprises the two detached shear layers, the recirculation region and wake flow. A study of the instability of the detached shear layers is important because these shear layers have a considerable impact on the dynamics of the very near wake. It has been observed experimentally that during certain periods of time that are randomly distributed, the measured fluctuating velocity component near the shear layers shows considerable amplification and it subsequently returns to its normal level (intermittency). Here, direct numerical simulations are used to accomplish a number of objectives such as confirming the presence of intermittency (computationally) and shedding light on processes that contribute significantly to intermittency and shear-layer transition/breakdown. Velocity time traces together with corresponding instantaneous vorticity contours are used in deciphering the fundamental processes underlying intermittency and shear-layer transition. The computed velocity spectra at three locations along the shear layer are provided. The computed shear-layer frequency agrees well with a power-law fit to experimental data.

Aspects of Shear Layer Unsteadiness in a Three-Dimensional Supersonic Wake

2005 ◽  
Vol 127 (6) ◽  
pp. 1085-1094 ◽  
Author(s):  
Alan L. Kastengren ◽  
J. Craig Dutton
Keyword(s):  
Shear Layer ◽  
Stream Flow ◽  
Free Stream ◽  
Angle Of Attack ◽  
Three Dimensional ◽  
Base Flow ◽  
Recirculation Region ◽  
Side View ◽  
Near Wake ◽  
Supersonic Wake

The near wake of a blunt-base cylinder at 10° angle-of-attack to a Mach 2.46 free-stream flow is visualized at several locations to study unsteady aspects of its structure. In both side-view and end-view images, the shear layer flapping grows monotonically as the shear layer develops, similar to the trends seen in a corresponding axisymmetric supersonic base flow. The interface convolution, a measure of the tortuousness of the shear layer, peaks for side-view and end-view images during recompression. The high convolution for a septum of fluid seen in the middle of the wake indicates that the septum actively entrains fluid from the recirculation region, which helps to explain the low base pressure for this wake compared to that for a corresponding axisymmetric wake.

On Integral Methods for Predicting Shear Layer Behavior

1969 ◽  
Vol 36 (4) ◽  
pp. 673-681 ◽  
Author(s):  
S. J. Shamroth
Keyword(s):  
Boundary Layer ◽  
Shear Layer ◽  
Integral Equations ◽  
Boundary Layers ◽  
Specific Problem ◽  
Shear Layers ◽  
Near Wake ◽  
Integral Methods ◽  
Layer Behavior ◽  
Momentum Integral

The origin and consequences of a nonphysical constraint which may arise when boundary-layer momentum integral equations are used to predict the behavior of shear layers are examined. It is pointed out that should the constraint occur within the domain of integration of the momentum integral equations, the effect may either be catastrophic or significantly constrain the solution. Several methods of solution having the usual advantages associated with boundary-layer momentum integral equations, but free from this constraint, are proposed for the specific problem of the plane turbulent near wake. One method developed to avoid this constraint in the case of a plane turbulent near wake appears to be perfectly general, and therefore, it may be possible to apply this method to both boundary layers and wakes.

scholarly journals Decomposition of wake dynamics in fluid–structure interaction via low-dimensional models

2019 ◽  
Vol 867 ◽  
pp. 723-764 ◽  
Author(s):  
T. P. Miyanawala ◽  
R. K. Jaiman
Keyword(s):  
Reynolds Number ◽  
Shear Layer ◽  
High Amplitude ◽  
Wake Flow ◽  
High Dimensional ◽  
Near Wake ◽  
Structure Interaction ◽  
Moderate Reynolds Number ◽  
Low Dimensional ◽  
Lock In

We present a dynamic decomposition analysis of the wake flow in fluid–structure interaction (FSI) systems under both laminar and turbulent flow conditions. Of particular interest is to provide the significance of low-dimensional wake flow features and their interaction dynamics to sustain the free vibration of a square cylinder at a relatively low mass ratio. To obtain the high-dimensional data, we employ a body-conforming variational FSI solver based on the recently developed partitioned iterative scheme and the dynamic subgrid-scale turbulence model for a moderate Reynolds number ($Re$). The snapshot data from high-dimensional FSI simulations are projected to a low-dimensional subspace using the proper orthogonal decomposition (POD). We utilize each corresponding POD mode to detect features of the organized motions, namely, the vortex street, the shear layer and the near-wake bubble. We find that the vortex shedding modes contribute solely to the lift force, while the near-wake and shear layer modes play a dominant role in the drag force. We further examine the fundamental mechanism of this dynamical behaviour and propose a force decomposition technique via low-dimensional approximation. To elucidate the frequency lock-in, we systematically analyse the decomposed modes and their dynamical contributions to the force fluctuations for a range of reduced velocity at low Reynolds number laminar flow. These quantitative mode energy contributions demonstrate that the shear layer feeds the vorticity flux to the wake vortices and the near-wake bubble during the wake–body synchronization. Based on the decomposition of wake dynamics, we suggest an interaction cycle for the frequency lock-in during the wake–body interaction, which provides the interrelationship between the high-amplitude motion and the dominating wake features. Through our investigation of wake–body synchronization below critical $Re$ range, we discover that the bluff body can undergo a synchronized high-amplitude vibration due to flexibility-induced unsteadiness. Owing to the wake turbulence at a moderate Reynolds number of $Re=22\,000$, a distorted set of POD modes and the broadband energy distribution are observed, while the interaction cycle for the wake synchronization is found to be valid for the turbulent wake flow.

scholarly journals Flow control over an airfoil using virtual Gurney flaps

2015 ◽  
Vol 767 ◽  
pp. 595-626 ◽  
Author(s):  
Li-Hao Feng ◽  
Kwing-So Choi ◽  
Jin-Jun Wang
Keyword(s):  
Flow Control ◽  
Shear Layer ◽  
Plasma Actuator ◽  
Recirculation Region ◽  
Dbd Plasma ◽  
Near Wake ◽  
Trailing Edge ◽  
Pressure Surface ◽  
Gurney Flaps ◽  
Gurney Flap

AbstractFlow control over a NACA 0012 airfoil is carried out using a dielectric barrier discharge (DBD) plasma actuator at the Reynolds number of 20 000. Here, the plasma actuator is placed over the pressure (lower) side of the airfoil near the trailing edge, which produces a wall jet against the free stream. This reverse flow creates a quasi-steady recirculation region, reducing the velocity over the pressure side of the airfoil. On the other hand, the air over the suction (upper) side of the airfoil is drawn by the recirculation, increasing its velocity. Measured phase-averaged vorticity and velocity fields also indicate that the recirculation region created by the plasma actuator over the pressure surface modifies the near-wake dynamics. These flow modifications around the airfoil lead to an increase in the lift coefficient, which is similar to the effect of a mechanical Gurney flap. This configuration of DBD plasma actuators, which is investigated for the first time in this study, is therefore called a virtual Gurney flap. The purpose of this investigation is to understand the mechanism of lift enhancement by virtual Gurney flaps by carefully studying the global flow behaviour over the airfoil. First, the recirculation region draws the air from the suction surface around the trailing edge. The upper shear layer then interacts with the opposite-signed shear layer from the pressure surface, creating a stronger vortex shedding from the airfoil. Secondly, the recirculation region created by a DBD plasma actuator over the pressure surface displaces the positive shear layer away from the airfoil, thereby shifting the near-wake region downwards. The virtual Gurney flap also changes the dynamics of laminar separation bubbles and associated vortical structures by accelerating laminar-to-turbulent transition through the Kelvin–Helmholtz instability mechanism. In particular, the separation point and the start of transition are advanced. The reattachment point also moves upstream with plasma control, although it is slightly delayed at a large angle of attack.

Analysis of the Near Wake of Bluff Bodies in Ground Proximity

2002 ◽  
Author(s):  
Szabolcs R. Balkanyi ◽  
Luis P. Bernal ◽  
Bahram Khalighi
Keyword(s):  
Vector Fields ◽  
Aerodynamic Drag ◽  
Base Pressure ◽  
Wake Flow ◽  
Vehicle Body ◽  
Recirculation Region ◽  
Bluff Bodies ◽  
Pressure Measurements ◽  
Near Wake ◽  
Mean Velocity

The effect of several drag reducing devices on the near wake of a generic ground vehicle body was investigated. Drag and base pressure measurements were conducted to identify the effects of the devices on the base drag. A Particle Image Velocimetry (PIV) study was conducted to determine changes of the near wake flow field. Averages of more than 200 PIV velocity vector fields were used to compute the mean velocity and turbulent stresses at several cross section planes. The results of the drag and base pressure measurements show that significant reductions of the total aerodynamic drag (as high as 48%) can be achieved with relatively simple devices. The results also indicated that models with base cavity have lower drag than their counter parts without it. The base pressure distributions showed a strong effect of the ground, resulting in decrease of pressure towards the lower half of the base. The PIV study showed that the extent of the recirculation region is not strongly affected by the drag reducing devices. The tested devices however, were found to have a strong effect on the underbody flow. A rapid upward deflection of the underbody flow in the near wake was observed. The devices were also found to reduce the turbulent stresses in the near wake. The turbulent stresses were found to decrease in magnitude with increasing drag reduction.

The dynamics of the transitional flow over a backward-facing step

2009 ◽  
Vol 623 ◽  
pp. 85-119 ◽  
Author(s):  
F. SCHÄFER ◽  
M. BREUER ◽  
F. DURST
Keyword(s):  
Shear Layer ◽  
Channel Wall ◽  
Shear Layers ◽  
Recirculation Region ◽  
Pressure Gradients ◽  
Reattachment Length ◽  
Backward Facing Step ◽  
Recirculation Bubble ◽  
Flowing Fluid ◽  
Rotational Movement

The internal flow over a backward-facing step in the transitional regime (ReD = 6000) was studied based on direct numerical simulations. The predictions were carried out with the help of a finite-volume Navier–Stokes solver equipped with a co-visualization facility which allows one to investigate the flow dynamics at high temporal resolution. First, grid-induced oscillations were precluded by a careful grid design. Second, the strong influence of the velocity profile approaching the step was studied and outlined. The main objective, however, was to provide a comprehensive insight into the dynamic flow behaviour, especially oscillations of the reattachment length of the primary recirculation region. The origin of this well-known flapping behaviour of the reattachment line is not yet completely understood. In the present work, the mechanisms leading to the oscillations of the reattachment length were extensively investigated by analysing the time-dependent flow. Besides the oscillations of the primary recirculation region, oscillations of the separation and the reattachment line of the secondary recirculation bubble at the upper channel wall were also observed. The results clearly show that in the present flow case the flapping of the primary reattachment and the secondary separation line is due to vortical structures in the unstable shear layers between the main flow and the recirculation bubbles. Vortices emerging in the shear layers and sweeping downstream convectively induce small zones of backward-flowing fluid at the channel walls while passing the recirculation regions. In the case of the primary recirculation region, the rotational movement of the shear-layer vortices impinging on the lower channel wall was found to cause zones of negative fluid velocity at the end of the recirculation bubble and thus flapping of the reattachment line. In contrast, in the case of the secondary recirculation region, the shear-layer vortices moved away from the upper channel wall so that their rotational movement did not reach the boundary. In this case, the pressure gradients originating from local pressure minima located in the shear-layer vortices were identified as being responsible for the oscillations of the separation line at the upper channel wall. While moving downstream with the shear-layer vortices, the pressure gradients were found to influence the top boundary of the channel and create alternating zones of forward- and backward-flowing fluid along the wall. All of these unsteady processes can best be seen from animations which are provided on the Journal of Fluid Mechanics website: journals.cambridge.org/FLM.

Optical Measurements of Interacting Lean Direct Injection Fuel Nozzles With Varying Spacing

2015 ◽  
Author(s):  
Brian Dolan ◽  
Rodrigo Villalva Gomez ◽  
Ephraim Gutmark
Keyword(s):  
Shear Layer ◽  
Flow Behavior ◽  
Direct Injection ◽  
Flame Structure ◽  
Shear Layers ◽  
The Other ◽  
Optical Measurements ◽  
Recirculation Region ◽  
Local Flow ◽  
Lower Velocity

Particle image velocimetry and OH planar laser induced fluorescence are used to examine the flow and flame structure resulting from two adjacent fuel/air nozzles. The distance between nozzles is varied from 1.1 to 2.72 nozzle diameters to change the degree of interaction between the nozzles. Non-reacting PIV shows a flowfield which is nearly symmetric between nozzles for all four spacings. For all but the widest spacing, there exist differences in the flow structures between the inner and outer sides of the nozzles. Less distance between the nozzles results in more rapid merging of the shear layers and higher axial velocities between the nozzles. At a spacing of 1.89 nozzle diameters, the shear layers toward the adjacent nozzle are intermittently pulled into the center of the combustor, resulting in a wider and lower velocity average flow between nozzles compared to the other cases. When a flame is added, the flowfields become much more asymmetric, both between nozzles and between the shear layers toward and away from the interacting nozzle. The outer shear layers, away from the other nozzle, are pushed to the domeplate by the expanding recirculation region. With a higher airflow, this behavior is negated. At the two further spacings, the shear layers toward the adjacent nozzle also become different between swirlers. The flow from the nozzle located on the right stays near the domeplate and joins the shear layer from the left nozzle near the nozzle lip and becomes a strong and penetrating jet at an angle to the axial coordinate. OH PLIF imaging shows that the flame fronts are generally located in the shear layer between the incoming reactants and the recirculating combustion products. This includes cases where the flow is highly asymmetric and stays close to the domeplate or penetrates deep into the combustor. So there is no evidence that the addition of an adjacent nozzle has an effect on the local flow/flame interaction. However, it is also clear that the presence of interacting swirling flows with combustion can lead to very dramatic changes to the global flow behavior relative to a single nozzle experiment.

Flow and Dispersion Characteristics of a Stack-Issued Backward Inclined Jet in Crossflow

2016 ◽  
Vol 33 (6) ◽  
pp. 841-852 ◽  
Author(s):  
M. G. Khouygani ◽  
R.-F. Huang ◽  
C.-M. Hsu
Keyword(s):  
Shear Layer ◽  
Inclination Angle ◽  
Flow Characteristics ◽  
Flux Ratio ◽  
Wake Flow ◽  
Flow Structures ◽  
Wake Vortex ◽  
Near Wake ◽  
Jet In Crossflow ◽  
Inclination Angles

AbstractThe effects of backward inclination angle on flow characteristics and jet dispersion properties of a stack-issued jet in crossflow were studied by means of instantaneous and long-exposure photography, particle image velocimetry (PIV), and tracer-gas concentration detections at a Reynolds number of 2,400, a jet-to-crossflow momentum flux ratio of 1.0, and the backward inclination angles θ = 0° - 60°. Three characteristic flow patterns featured by different near-wake flow structures were found within the surveyed span of the backward inclination angle: low (θ ≤ 25°), mediate (25° < θ < 50°), and high (θ ≥ 50°). In the range of low backward inclination angle, mushroom vortices appeared in the upwind shear layer. Jet fluids were entrained into the jet- and tube-wakes because the near wake region was characterized by a jet-wake vortex and a downwash flow. In the range of mediate backward inclination angle, forward-rolling vortices were formed in the upwind shear layer. Jet fluids were entrained into the jet wake but not appearing in the tube wake because the near wake was characterized by an isolated tube wake and up-going flows. In the range of high backward inclination angle, small-sized forward-rolling vortices were observed in the upwind shear layer. Jet fluids were not observed in both the jet- and tube-wakes because all flows went forward without reversal or vortex, which was similar to that in a jet in co-flow. Large turbulence intensities occurred around the jet-wake vortex and along sides of the tube wake bifurcation line, therefore the mixing at the low backward inclination angles presented better properties than those at mediate and high backward inclination angles owing to the featured flow structures and turbulence intensities.

A freely yawing axisymmetric bluff body controlled by near-wake flow coupling

2019 ◽  
Vol 863 ◽  
pp. 1123-1156 ◽  
Author(s):  
Thomas J. Lambert ◽  
Bojan Vukasinovic ◽  
Ari Glezer
Keyword(s):  
Shear Layer ◽  
Time Scales ◽  
Attitude Control ◽  
Bluff Body ◽  
Cross Flow ◽  
The Body ◽  
Wake Flow ◽  
Small Scale ◽  
Near Wake ◽  
Yaw Attitude

Flow-induced oscillations of a wire-mounted, freely yawing axisymmetric round bluff body and the induced loads are regulated in wind tunnel experiments (Reynolds number $60\,000<Re_{D}<200\,000$) by altering the reciprocal coupling between the body and its near wake. This coupling is controlled by exploiting the receptivity of the azimuthal separating shear layer at the body’s aft end to controlled pulsed perturbations effected by two diametrically opposed and independently controlled aft-facing rectangular synthetic jets. The model is supported by a thin vertical wire upstream of its centre of pressure, and prescribed modification of the time-dependent flow-induced loads enables active control of its yaw attitude. The dynamics of the interactions and coupling between the actuation and the cross-flow are investigated using simultaneous, time-resolved measurements of the body’s position and phase-locked particle image velocimetry measurements in the yawing plane. It is shown that the interactions between trains of small-scale actuation vortices and the local segment of the aft-separating azimuthal shear layer lead to partial attachment, and the ensuing asymmetric modifications of the near-wake vorticity field occur within 15 actuation cycles (approximately three convective time scales), which is in agreement with measurements of the flow loads in an earlier study. Open- and closed-loop actuation can be coupled to the natural, unstable motion of the body and thereby affect desired attitude control within 100 convective time scales, as is demonstrated by suppression or enhancement of the lateral motion.

Flow Around a Pipeline Near a Smooth Bed in Steady Current

2010 ◽  
Author(s):  
Xikun Wang ◽  
Zhiyong Hao ◽  
Soon Keat Tan
Keyword(s):  
Reynolds Number ◽  
Large Scale ◽  
Reynolds Shear Stress ◽  
Vortex Formation ◽  
Shear Layers ◽  
Wake Flow ◽  
Recirculation Region ◽  
Critical Gap ◽  
Gap Ratio ◽  
Gap Height

The flow around submarine pipelines is represented by a circular cylinder near a horizontal plane wall. The combined effects of Reynolds number (Re = 4000, 6400 and 9200) and gap ratio (S/D = 0.125–2, where S is the gap height between the cylinder bottom and the wall surface, and D is cylinder diameter) on the flow are investigated experimentally using Particle Image Velocimetry (PIV). Detailed velocity vector maps, contours of vorticity, root-mean-square (rms) velocities and Reynolds shear stress are provided. Results show that both the ensemble-averaged and the instantaneous flow fields are primarily dependent on S/D, yet the effects of Re are also obvious over the Re-range considered. Shear layers surrounding the recirculation region behind the cylinder are discussed in terms of the flow physics and vortex formation lengths of large-scale Karman vortices. It is found that the critical gap ratio is about 0.3, higher than which the wake flow is characterized by the alternate Karman-like vortex shedding from the two sides of the cylinder, whereas below which there is no mutual coupling between the two shear layers. Flow structures depending upon the gap ratio and Reynolds number are interpreted with quantitative representations.

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