Wake Centerline Research Articles

Higher-order moments of velocity fluctuations in the wake of a short stack

The effect of jet velocity relative to the crossflow velocity on the higher-order moments of velocity fluctuations characteristics in the wake of stack is reported in this study. The cross-flow Reynolds number was ReD = 2.3×104 and the jet-to-cross-flow velocity ratio was varied from R = 0 to 3. The stack was partially immersed in a flat-plate turbulent boundary layer, with a boundary layer thickness-to-stack-height ratio of δ/H = 0.5 at the location of the stack. The skewness factor, flatness factor and triple correlations are found to be influenced by the flow regime. The deviation of skewness and flatness factors from the Gaussian fluctuation values of zero and 3 are more pronounced outside the wake centre region due to the presence of the separated shear layers and vortex structures. The high values of the skewness and flatness factors for all values of velocity ratio within the vicinity of the stack free end are related to the tip vortex structures near the stack free end which decrease in strength as the value of R increases. In addition, the separated shear layers from both sides of the stack contribute to the high values observed away from the wake centreline.

Counter-hairpin vortices in the turbulent wake of a sharp trailing edge

AbstractThe unsteady organization and evolution of coherent structures within the turbulent boundary layer and subsequent wake of the sharp symmetric trailing edge of a NACA0012 aerofoil are investigated. The experiments are conducted in an open test-section wind tunnel at ${\mathit{Re}}_{c} = \text{386\hspace{0.167em}000} $ based on the aerofoil chord and ${\mathit{Re}}_{\theta } = 1300$ based on the boundary layer momentum thickness. An initial characterization of the flow field using two-component particle image velocimetry (PIV) is followed by the investigation of the unsteady organization and evolution of coherent structures by time-resolved three-dimensional PIV based on a tomographic approach (Tomo-PIV). The inspection of the turbulent boundary layer prior to the trailing edge in the region between 0.15 and $0. 8\hspace{0.167em} {\delta }_{99} $ demonstrated streaks of low- and high-speed flow, while the low-speed streaks are observed to be more coherent along with strong interaction with hairpin-type vortical structures similar to a turbulent boundary layer at zero pressure gradient. The wake region demonstrated gradual deterioration of both the low- and the high-speed streaks with downstream progress. However, the low-speed streaks are observed to lose their coherence at a faster rate relative to the high-speed streaks as the turbulent flow develops towards the far wake. The weakening of the low-speed streaks is due to the disappearance of the viscous sublayer after the trailing edge and gradual mixing through the transport of the remaining low-speed flow towards the free stream. This transport of low-speed flow is performed by the ejection events induced by the hairpin vortices as they also persist into the developing wake. The higher persistence of the high-speed streaks is associated with counter-hairpin vortical activities as they oppose the deterioration of the high-speed streaks by frequently sweeping the high-speed flow towards the wake centreline. These vortical structures are regarded as counter-hairpin vortices as they exhibit opposite characteristics relative to the hairpin vortices of a turbulent boundary layer. They are topologically similar to the hairpins as they appear to be U-shaped but with inverted orientation, as the spanwise portion is in the vicinity of the wake centreline and the legs are inclined at an approximately $6{0}^{\ensuremath{\circ} } $ to the wake axis in the downstream direction demonstrating a strain-dominated topology. The counter-hairpin vortices are partially wrapped around the high-speed streaks and contribute to the wake development by transporting high-speed flow towards the wake centreline. Similar to the hairpin vortices of a turbulent boundary layer, the occurrence of a complete counter-hairpin vortex is occasional while its derivatives (portions of spanwise or quasi-streamwise vortices) are more frequently observed. Therefore, a pattern recognition algorithm is applied to establish characterization based on an ensemble-averaged counter-hairpin vortex. The formation of the counter-hairpin vortices is due to an additional degree of interaction between the low- and high-speed streaks after the trailing edge across the wake centreline. The shear layer produced along the wake centreline by neighbouring low- and high-speed streaks promotes the formation of spanwise vortices that form the counter-hairpin vortices by connection to quasi-streamwise vortices. Finally, a conceptual model is proposed to depict the three-dimensional unsteady organization and evolution of coherent structures in the wake region based on the hairpin and counter-hairpin vortex signatures.

A mathematical model of 2P and 2C vortex wakes

We present a mathematical model of the vortex wake modes that appear behind neighboring and/or oscillating, flapping, and swimming bodies in which there are four vortices generated in an anti-symmetric pattern during each shedding cycle. The two-dimensional potential flow model consists of four point vortices with strengths ± Γ in a spatially periodic domain. The relative vortex positions are restricted by a discrete symmetry that is motivated by the spatial symmetry observed in experimental wakes. The strength restriction and the imposed symmetry result in the model system being an integrable Hamiltonian dynamical system. We find that the point vortex motion can be one of four distinct types based on the values of linear impulse and Hamiltonian. Two of these types correspond to 2P wakes and consist of two oppositely signed, counter-rotating vortex pairs. One of these types corresponds to 2C wakes and consists of two like-signed, co-rotating vortex pairs. The fourth type is an exchanging mode in which the two vortices near the wake centerline translate faster than the outer two vortices. Scaled comparisons of the model with both a 2P and a 2C experimental wake show good representation of the experimentally observed vortex dynamics and lead to estimates of the experimental vortex strengths.

Effects of Splitter Plates on Turbulent Quantities on a Boundary Layer Developing on a Flat Plate near the Trailing Edge

An experimental investigation was made on a turbulent boundary layer near the trailing edge on a long flat plate with a blunt trailing edge. The flow was controlled by an additional splitter plate attached to the trailing edge along the wake center line. The length of the splitter plate, l, was varied from 0.3 to 3 times the trailing edge thickness, h. Measurement of fluctuating velocity was made under the freestream zero pressure gradient. Almost independent of the installation of the splitter plate, the turbulent intensity and the Reynolds shear stress in the inner layer decreased when approaching the trailing edge of the flat plate (x/h=0), compared with that in the fully-developed turbulent boundary layer under the zero pressure gradient. The only exception was the distribution at x/h=0 near the wall in the case without the splitter plate (l/h=0), where the degree of the decrease in urms/Ue was relatively small, and vrms/Ue and -uv / Ue2 were larger than those of the zero pressure gradient. The effect of l/h on turbulent quantities was shown just upstream of the trailing edge upon close comparison, and was classified into two groups: l/hl/h≧1. The tendency of the turbulent energy production terms approaching x/h=0 coincided with the decrease in the streamwise turbulent intensity and in the Reynolds shear stress.

Numerical Simulation of the Wind-Stress Effect on SAR Imagery of Far Wakes of Ships

Centerline wakes of ships in synthetic aperture radar (SAR) images were modeled in 2-D with the computational fluid dynamics (CFD) software Fluent and a radar-imaging algorithm. We initialized the model with a pair of vortices generated by a ship hull and applied wind stress perpendicular to the ship wake. Results of the CFD simulation using a nonhydrostatic model have demonstrated ship-wake asymmetry with respect to the wind-stress direction relative to the ship course. Due to the wind stress, flow convergence increased on the upwind side of the centerline wake and reduced on the downwind side of the wake. The radar-imaging algorithm processed with the surface velocity field produced by the CFD model revealed ship-wake asymmetry relative to the wind direction. These results are qualitatively consistent with SAR images from the TerraSAR-X satellite and representative statistics of photographic images of the ship wake collected from a volunteer observing ship.

An assessment of turbulence models for the prediction of flow past a circular cylinder with momentum injection

Reynolds averaged Navier–Stokes equations (RANS) are solved to simulate the flow past a circular cylinder. Momentum injection through Moving Surface Boundary-layer Control (MSBC) with zero net mass injection is implemented to achieve wake control. Popular eddy viscosity based closure models are assessed for their predictive capability of turbulent wake characteristics. For the present simulation, sub-critical Reynolds number ( Re) of 3900 is chosen, where extensive validations are available. A stabilization approach is proposed to model, predict and control vortex shedding behind a circular cylinder. Along with the mass, momentum conservation, turbulent kinetic energy (TKE) and its rate of dissipation equations are solved with the objective of achieving the annihilation of wake structures. To enable momentum injection, two simple rotary type control cylinders are located at 120°, behind the main cylinder. The ratio of the main cylinder, control cylinder and gap between them are fixed at D: 0.1D: 0.01D, respectively. These control cylinders, which are like externally controllable actuators, are assessed for their ability to influence momentum injection and hence wake patterns. The popular finite volume based Semi Implicit Pressure Linked Equations (SIMPLE) scheme is employed for the numerical calculations. Detailed assessment of different eddy viscosity based turbulence models viz., standard k – ε , Renormalization Group (RNG) k – ε , realizable k – ε and k – ε version of Kato–Launder (KL) is carried out. As a precursor, validation of turbulence statistics such as, mean streamwise velocity along the wake centerline, mean pressure coefficient on the cylinder surface and time averaged Reynolds stresses etc. is carried out against known experimental and numerical computations. The role of externally controllable actuators on the fluid flow patterns past a circular configuration is assessed with the help of streaklines, streamlines, vorticity, Reynolds stress contours etc. Complete suppression of vortex shedding is observed for an injection parameter (defined as the ratio of control cylinder velocity ( U c ) to that of the freestream ( U ∞ )) ξ = 6.0 . The results clearly demonstrate the effectiveness of a rather simple momentum injection strategy in suppressing turbulent vortex structures.

Effect of a splitter plate on flow over a semi-circular cylinder

In this study, two-dimensional unsteady laminar flow over a semi-circular cylinder (SCC) near a splitter plate was investigated numerically. Adding a splitter plate along the wake centreline downstream of the bluff bodies is an efficient passive means of controlling fully developed vortex shedding. Numerical simulations were done for different Reynolds numbers ranging from 100 to 500 in three different gaps ( g: g = 0.0 D to 4.5 D) and two different splitter lengths ( LSP = 1 and 2 D). Results indicate that by increasing the gap ratio, vortex shedding formed in the wake is suppressed and the oscillating amplitude of the lift coefficient decreases. On the other hand, drag, lift, and pressure coefficients representing the flow characteristics also vary as a function of the gap ratio at different Reynolds numbers. Extensive results on streamline patterns, wake characteristics (recirculation length), drag, lift, and pressure coefficients are presented to elucidate the interplay between the gap ratio and the Reynolds number and their influence on drag. Using the splitter plate at the downstream of the SCC causes the stretching of the vortex core and moves the centre of the vortex cores towards the leading edge of the plate. This phenomenon is caused by the reduction of the lift and drag coefficients.

Momentum and heat transport in a finite-length cylinder wake

This paper reports an experimental study of turbulent momentum and heat transport in the wake of a wall-mounted finite-length square cylinder, with its length-to-width ratio L/d = 3–7. The cylinder was slightly heated so that heat produced could be considered as a passive scalar. A moveable three-wire probe (a combination of an X-wire and a cold wire) was used to measure velocity and temperature fluctuations at a Reynolds number of 7,300 based on d and the free-stream velocity. Measurements were performed at 10 and 20d downstream of the cylinder at various spanwise locations. Results indicate that L/d has a pronounced effect on Reynolds stresses, temperature variance and heat fluxes. The downwash flow from the free end of the cylinder acts to suppress spanwise vortices and, along with the upwash flow from the cylinder base, makes the finite-length cylinder wake highly three-dimensional. Reynolds stresses, especially the lateral normal stress, are significantly reduced as a result of suppressed spanwise vortices at a small L/d. The downwash flow acts to separate the two rows of spanwise vortices further apart from the wake centerline, resulting in a twin-peak distribution in temperature variance. While the downwash flow entrains high-speed fluid into the wake, responsible for a small deficit in the time-averaged streamwise velocity near the free end, it does not alter appreciably the distribution of time-averaged temperature. It has been found that the downwash flow gives rise to a counter-gradient transport of momentum about the central region of the wake near the free end of the cylinder, though such a counter-gradient transport does not occur for heat transport.

Aerodynamic Characteristics of Asymmetric Bluff Bodies

The wake of asymmetric bluff bodies was experimentally measured using particle imaging velocimetry, laser Doppler anemometry, load cell, hotwire, and flow visualization techniques at Re=2600–8500 based on the freestream velocity and the characteristic height of the bluff bodies. Asymmetry is produced by rounding some corners of a square cylinder and leaving others unrounded. It is found that, with increasing corner radius, the flow reversal region is expanded, and the vortex formation length is prolonged. Accordingly, the vortex shedding frequency increases and the base pressure rises, resulting in a reduction in the mean drag as well as the fluctuating drag and lift. It is further found that, while the asymmetric cross section of the cylinder causes the wake centerline to shift toward the sharp corner side of the bluff body, the wake remains globally symmetric about the shifted centerline. The near wake of asymmetric bluff bodies is characterized in detail, including the Reynolds stresses, characteristic velocity, and length scale, and is further compared with that of the symmetric ones.

Turbulence Within a Turbomachine Rotor Wake Subject to Nonuniform Contraction

The flow and turbulence in a rotor near wake located within a nonuniform field, generated by a row of inlet guide vanes, are investigated experimentally in a refractive-index-matched facility that provides unobstructed view of the entire flowfield. Stereoscopic particle image velocimetry measurements, performed in closely spaced radial planes, enable measurements of all the components of the phase-averaged strain rate, Reynolds stress, and triple correlation tensors. The rotor wake is bent and compressed as a result of exposure to regions with high axial momentum (jets), which fill the gaps between inlet guide vanes wakes. As the rotor wake propagates away from the blade, this process of bending and compression by the jets leads to formation of distinct wake kinks containing regions of high turbulence (turbulent hot spots). We focus on an early stage of hot-spot formation. On the suction side of the wake, compression by a jet increases turbulence production, causing nonuniform and asymmetric distribution of Reynolds stresses. In a curvilinear coordinate system aligned with the wake centerline, all the Reynolds stresses are higher on the suction side of the wake where the decay rate of turbulence is much slower than that expected in wakes. The production rate of streamwise stress is dominated by interactions of the shear stress with the cross-stream gradients of the phase-averaged streamwise velocity. The latter also interact with the cross-stream stress to cause high production of shear stress, especially on the suction side. The locations of peak streamwise and shear stresses are consistent with those of the corresponding production rates. The similar locations of peak streamwise and cross-stream stresses, together with low production rate of the latter, suggest a significant contribution by intercomponent transfer from the streamwise to the cross-stream stress. The effects of streamwise curvature and rotation on the evolution of turbulent stresses are marginal. The paper also provides distributions of advection by phase-averaged flow and turbulent diffusion components. The diffusion terms oppose the peaks of production rates, but also have high values along the perimeter of the wake. We compare the distribution of diffusion of turbulent kinetic energy to a model based on the gradient-diffusion hypothesis. The model predicts the diffusion peak that opposes the production rate maximum quite well, but underpredicts the diffusion along the perimeter of the wake and overpredicts it near the wake center.

Onset of mixing layer instability in flow past a plate

AbstractA global stability analysis of the flow past a flat plate, normal to the flow, is carried out to determine the critical Re for the onset of convective instability of the mixing‐layer‐type modes. To suppress the primary wake mode, which leads to von Karman vortex shedding, flow past one half of the plate is studied and symmetry conditions are enforced along the wake centerline. The linearized perturbation equations for an incompressible flow are written in a moving frame of reference that travels with the perturbation and solved using a stabilized finite element method. The critical Re for the onset of convective instability is found to be 22, approximately, for H=5. Here, H is the distance of the lateral boundary from the plate centerline. Computations have also been carried out for H=10 and 20. The critical Re decreases with increase in H. Excellent agreement is observed between the results from global stability analysis and direct time integration of linearized disturbance equations. The results from the global analysis are compared with earlier reported results from local analysis of a Gaussian wake profile. Copyright © 2008 John Wiley & Sons, Ltd.

Flow structure behind two staggered circular cylinders. Part 2. Heat and momentum transport

This work aims to study flow structures, heat and momentum transport in the wake of two staggered circular cylinders. In order to characterize heat transport in the flow, both cylinders were slightly heated so that heat generated could be treated as a passive scalar. The velocity and temperature fluctuations were simultaneously measured by traversing a three-wire (one cross-wire plus one cold wire) probe across the wake, along with a fixed cross-wire, which acted to provide a reference signal. Four distinct flow structures, i.e. two single-street modes (S-I and S-II) and two twin-street modes (T-I and T-II), are identified based on the phase-averaged vorticity contours, sectional streamlines, and their entrainment characteristics. Mode S-I is characterized by a vortex street approximately antisymmetric about the centreline. This mode is further divided into S-Ia and S-Ib, which differ greatly in the strength of vortices. The vortex street of Mode S-II is significantly asymmetric about the centreline, the strenth of vortices near the downstream cylinder exceeding by 50% that on the other side. Mode T-I consists of two alternately arranged vortex streets; the downstream-cylinder-generated street is significantly stronger than that generated by the upstream cylinder. In contrast, Mode T-II displays two streets approximately antisymmetrical about the wake centreline. Free-stream fluid is almost equally entrained from either side into the wake in Modes S-Ia and T-II, but largely entrained from the downstream cylinder side in Modes S-II and T-I. The entrainment motion in Mode S-Ib is very weak owing to the very weak vortex strength. Vortices decay considerably more rapidly in the twin-street modes, under vigorous interactions between the streets, than in the single-street modes. This rapid decay is particularly evident for the inner vortices near the wake centreline in Modes T-II and T-I. Other than flow structures, heat and momentum transport characteristics are examined in detail. Their possible connection to the initial conditions is also discussed.

The Effects of Splitter Plates on Turbulent Boundary Layer on a Long Flat Plate Near the Trailing Edge

An experimental investigation has been made on a turbulent boundary layer near the trailing edge on a long flat plate. The flow was controlled by an additional splitter plate fitted to the trailing edge along the wake centerline. The length of the splitter plate, l, was varied from a half, to five times the trailing edge thickness, h. Measurements of base pressure behind the trailing edge and of mean velocity and pressure distribution in the turbulent boundary layer on the flat plate were made under the freestream zero-pressure gradient. The absolute value of the base pressure coefficient of the long flat plate was considerably smaller than that of the short flat plate without the splitter plate. A significant increase in the base pressure coefficient was achieved with the splitter plate (l∕h≧1), fitted to the long flat plate. Within an inner layer in the turbulent boundary layer near the trailing edge, the mean velocity increased more than that in the upstream position in the case without the splitter plate. With the splitter plate, however, the base pressure rise made the mean velocity distribution more closely approach that of a fully developed turbulent boundary layer.

Experimental investigation of a circular radially deforming cylinder near wake using an infrared technique

The present experimental study aims at developing a method to control the circular cylinder near wake by radial deformation and understand the underlying physics. Using an infra-red camera, we examine the temperature distribution of the near wake center line of a sinusoidal law radially deforming circular cylinder. From these measurements, the near wake is characterized by the length of the recirculation zone, the vortex formation zone length, the temperature fluctuation maximum intensity and the vortex street shedding frequency. For several deformations frequencies, we study the radial deformation influence on the near wake characteristics. It is noted that the wake structure is strongly affected by the deformation frequency. Among other things, we note the recirculation zone length reduction and the vortex formation zone length reduction when the radial vibrations are close to the “Lock-in” fundamental range. It is also noted that the variations of the vortex shedding frequency depend on the deformation frequency.

Wake states of a tethered cylinder

This paper describes an experimental investigation of a buoyant,m*<1, tethered cylinder which is free to move in an arc about its pivot points. The response of the cylinder, in particular its layover angle and flow-induced motion, is considered for a range of flow velocities and mass ratios. At pertinent parameters, the flow fields were also measured using particle image velocimetry (PIV). At lower mass ratios, 0.54≤m*≤0.72, two distinct states are observed, the low-amplitude and upper states. The transition from the low-amplitude state to the upper state is characterized by abrupt jumps in the amplitude of oscillation, the mean tether angle and the drag coefficient as well as distinct changes in the cylinder's wake. At higher mass ratios, the jump does not occur; however, asm* approaches unity at low flow velocities the cylinder's motion is more periodic than that observed at lowerm*. The flow fields indicate that the low-amplitude state exhibits a 2S Kármán wake. The wake of the upper state has long shear layers extending well across the wake centreline, is not fully symmetric and is often consistent with either the 2P or P+S shedding modes. There is a collapse of the response data, in particular an excellent collapse of the mean layover angle, when the response parameters are plotted against the buoyancy Froude number,Frbuoyancy=U/((1-m*)gD)0.5. When the data collapses, the two states described above are clearly delineated.

Absolute and convective instabilities of two-dimensional bluff body wakes in ground effect

Local and global instabilities are investigated of wakes of general two-dimensional bluff bodies placed near and parallel to a plane boundary or ground. A spatio-temporal linear stability analysis is first applied to a four-parameter family of local wake profiles to investigate the fundamental local stability characteristics of the wake in ground effect. The analysis shows significant dependencies of the stability characteristics of the wake on the distance from the wake centreline to the ground (normalised by the wake width), and also on the velocity ratio of the near- and far-ground sides of the wake. The analysis is then compared with earlier experiments on a circular cylinder to examine, according to the transition scenario of the steep global modes, the streamwise variation of the local stability characteristics of the wake in ground effect. The comparison indicates that the near wake region of the cylinder changes from being absolutely unstable to being convectively unstable when the cylinder comes down into the near-ground range in which the von Kármán-type vortex shedding from the cylinder is suppressed, being qualitatively consistent with the transition scenario for general wake-type flows. A possible explanation is also given for the counter-intuitive relation between the thickness of the boundary layer on the ground and the critical gap distance for the cessation of the von Kármán-type vortex shedding in ground effect.

Temporal evolution of wake behind a rotationally oscillating circular cylinder

The temporal evolution of the near-wake behind a circular cylinder undergoing rotational oscillatory motion with a relatively high forcing frequency has been investigated experimentally using a dynamic particle image velocimetry velocity field measurement technique. Experiments were carried out varying the frequency ratio FR (=ff∕fn), the ratio of the forcing frequency ff to the natural vortex shedding frequency fn, in the range of 0.0 (stationary) to 1.0 at an oscillation amplitude of θA=30° and Reynolds number of Re=4.14×103. Depending on the forcing condition (FR), the near-wake flow showed markedly different flow regimes: stationary (FR=0.0), non-lock-on (FR=0.4) and lock-on (FR=1.0) regimes. However, at all the frequency ratios tested, the power spectral density distributions showed a single dominant peak at the same vortex shedding frequency. As the forcing frequency was increased, the rotational oscillatory motion of the cylinder decreased the length of the vortex formation region and enhanced the mutual interaction between large-scale vortices across the wake centerline. In particular, the entrainment of ambient fluid seemed to play an important role in controlling the near-wake flow and shear-layer instability. The shear-layer instability showed different mechanisms compared with the case of small oscillation amplitude. A strong vortex motion appeared in the near-wake region. In addition, the rotational oscillatory motion of the cylinder changed the local mean flow structure significantly; thereby mean velocity profiles for each frequency ratio varied markedly in the vortex formation region. The configuration of vorticity contours shrunk toward the cylinder as the frequency ratio was increased. The temporally resolved flow information extracted in the present work is useful for understanding the evolution of vortex structure and analyzing the effects of open-loop active flow control technique on the near-wake flow structure.

PIV measurements of the wake behind a rotationally oscillating circular cylinder

The near-wake behind a circular cylinder undergoing rotational oscillatory motion with a relatively high forcing frequency has been investigated experimentally. Experiments were carried out varying the ratio of the forcing frequency f f to the natural vortex shedding frequency f n in the range of 0.0 (stationary) to 1.6 at an oscillation amplitude of θ A =30° and Reynolds number of Re=4.14×10 3 . Depending on the frequency ratio ( F R = f f / f n ), the near-wake flow could be divided into three regimes—non-lock-on ( F R =0.4), transition ( F R =0.8, 1.6) and lock-on ( F R =1.0) regimes—with markedly different flow structures. When the frequency ratio was less than 1.0 ( F R ⩽1.0), the rotational oscillatory motion of the cylinder decreased the length of the vortex formation region and enhanced the mutual interaction between large-scale vortices across the wake centerline . The entrainment of ambient fluid seemed to play an important role in controlling the near-wake flow and shear-layer instability. In addition, strong vortex motion was observed throughout the near-wake region. The flow characteristics changed markedly beyond the lock-on flow regime ( F R =1.0) due to the high frequency forcing. At F R =1.6, the high frequency forcing decreased the size of the large-scale vortices by suppressing the lateral extent of the wake. In addition, the interactions between the vortices shed from both sides of the cylinder were not so strong at this forcing frequency. As a consequence, the flow entrainment and momentum transfer into the wake center region were reduced. The turbulent kinetic energy was large in the region near the edge of the recirculation region, where the vortices shed from both sides of the cylinder cross the wake centerline for all frequency ratios except for the case of F R =1.6. The temporally resolved quantitative flow information extracted in the present work is useful for understanding the effects of open-loop active flow control on the near-wake flow structure.

Flow around an impulsively arrested circular cylinder

The vortex dynamics of the flow around a suddenly arrested translating circular cylinder is investigated by direct numerical simulation and water tank experiments. In the numerical study, a method of visualization of streaklines in simulated-particle tracking computations is proposed, which is based on a variable-variance two-dimensional Gaussian-weighted summation of particles in the vicinity of each interpolation point, and for which a close similarity with physical dye visualizations is found. This technique is used to identify the trajectory of both the wake vortices, as well as the secondary vortices induced as the original wake convects over the arrested cylinder. Observations show that, in a fashion similar to the flow past an arresting sphere, each wake vortex induces a counter-rotating vortex pair, which subsequently self-propels over a range of sometimes surprising trajectories as the Reynolds number and cylinder translation distance are varied. At low Reynolds numbers and short translation distances, the wake vortices propel past the cylinder, continuing in the direction of the original cylinder motion. At higher Reynolds numbers, the vortices deviate outwards in circular arcs of increasing curvature, even to the extent that the vortex pairs collide behind the cylinder. These trajectory curvatures are analyzed with respect to the circulation of the vortex pairs. At sufficiently long translation distances, a wake instability destroys the reflective symmetry about the wake centerline. This regime is investigated by both comparison with experiment and analysis of the discrepancy between the vorticity and particle fields at large post-arrest times.

Zonal-Detached-Eddy Simulation of Projectiles in the Subsonic and Transonic Regimes

This paper presents nonspinning-projectile computations with advanced turbulence modeling to demonstrate the relevance of using a hybrid method in projectile simulations. A zonal-detached-eddy simulation methodology is used to improve the base-flow prediction without deteriorating the incoming attached flow upstream of the base in the subsonic and transonic regimes. The numerical results are found to compare fairly well with the available experimental wall-pressure data. Both time-averaged and unsteady features are then discussed. Particular attention was paid to the near-wake flowfield and its dependency upon the freestream Mach number. In these calculations, some classical features of massively separated flows, such as the pressure evolution along the base or the wake centerline characteristics, are poorly predicted with the Spalart-Allmaras model. The use of a hybrid method leads to promising results and allows for a physical analysis of the separated flowfield. The flow appears to exhibit self-similar properties, even in the recirculation area, independently of the Mach number value in the range of freestream conditions investigated. Moreover, the instability process leading to the shear-layer growth is found to be similar in the range of parameters investigated and is in accordance with previous results concerning the compressibility effects in free shear flows. Finally, base-pressure spectra are reported and compared with axisymmetric base-flow data.