Vortex Formation Region Research Articles

Influence of vortex street structure on the efficiency of energy separation

Regions with reduced and increased values of total enthalpy are observed in a time-averaged flow behind a bluff body. This energy redistribution takes place both in the vortex formation region and in the developed vortex wake. The present paper focuses on studying the effect of the structure of a vortex street on the intensity of energy redistribution. Two approaches are used. The first one is direct numerical simulation of the flow behind a transversely oscillating cylinder, which is known for a variety of vortex patterns in the wake. The simulations are based on a finite element solution of the Navier-Stokes equations for a compressible perfect viscous gas. The second approach is based on simplified point vortex models for infinite periodic vortex streets, which contain a finite number of vortex chains in equilibrium. It turns out that these simple models make it possible to obtain satisfactory qualitative results, particularly if a more precise approximation of velocity fields in the vortex cores (Rankine vortices) is implemented. It is shown that the effect of energy redistribution significantly depends on the vortex structure, namely the mutual arrangement of the vortices and their intensities. The estimates of the energy separation efficiency in the time-averaged flow are obtained for the general case of an arbitrary number of chains. A more detailed analysis is performed for vortex streets with 2, 3, and 4 vortex chains.

Near-Wake Characteristics and Acoustic Resonance Excitation of Crimped Spirally Finned Cylinders in Cross-Flow

This paper presents an experimental investigation of the near-wake flow characteristics for isolated crimped spirally finned cylinders in cross-flow and its influence on the generated sound pressure during flow-excited acoustic resonance. Four crimped spirally finned cylinders are investigated, which have pitch-to-root diameter ratio (p/Dr) ranging between 0.384 ≤ p/Dr ≤ 1. A new equivalent diameter equation (Dc) has been developed to better capture the vortex shedding frequency emanating from the crimped spirally finned cylinders. The addition of crimped spiral fins reduces the coherence of the vortex shedding process as compared to that of a bare cylinder. Moreover, the addition of crimped spiral fins causes an elongation in the vortex formation region, as well as induces a larger velocity deficit in the near-wake. Reduction in the pitch-to-diameter ratio (p/Dr) leads to a progressive increase in the strength and coherence of the vortex shedding process. It also results in a gradual reduction in the vortex formation length and velocity deficit. The near-wake flow characteristics of the crimped spirally finned cylinders inherently affect the sound pressure during flow-excited acoustic resonance. Furthermore, the helical fins impose an asymmetrical inclination of the acoustic particle velocity. This hinders the flow-acoustic coupling, leading to a weakened energy transfer between the flow and sound fields. The findings of this investigation provide better understanding of the complex flow-sound interaction mechanism from crimped spirally finned cylinders in heat exchanger tube bundle.

3D vortex structure investigation using Large Eddy Simulation of flow around a rotary oscillating circular cylinder

Three-dimensional turbulent flow over a circular cylinder performing sinusoidal rotational motions around its axis is investigated using Large Eddy Simulation (LES). The simulation is carried out for Reynolds number equal to 3900 with rotation rate (Ω=θosc*r∕U0, where θosc is oscillation amplitude, r cylinder radius, and U0 flow velocity) varying from 0.5 to 2; and with non-dimensional forcing frequencies (ratio of the frequency of the cylinder oscillation and that of the vortex-shedding from a stationary cylinder) from 0 to 8. Time-averaged flow parameters as well as mean drag and lift coefficients, mean base pressure coefficient, and separation angle and mean length of the vortex formation region are obtained and thoroughly analyzed. It was found that under forcing control, 50% of drag reduction was achieved and the flow three-dimensionality was reduced in the lock-on range.

Direct numerical simulation of energy separation effect in the near wake behind a circular cylinder

Based on direct numerical simulation of two-dimensional Navier-Stokes equations, the effect of energy separation in unsteady vortex flows is investigated with the reference to the problem of a compressible viscous flow past a thermally insulated circular cylinder. The range of Reynolds (Re⩽103), Prandtl (0.1⩽Pr⩽10) and Mach (M⩽0.6) numbers considered corresponds mainly to the periodic vortex shedding regime. The energy separation, associated with the vortex shedding process, manifests itself in the appearance of cold and hot (in terms of total temperature) spots in the near wake. The main attention is focused on the comparative analysis of different mechanisms of total-enthalpy variation in a fluid particle moving around the cylinder, such as the action of viscosity, thermal conductivity, and unsteadiness of the flow. It is shown that the time-averaged total-enthalpy stratification in the boundary layer is caused by dissipative mechanisms. In the vortex formation region and in the vortex street, a decrease in the time-averaged total enthalpy is attributable mainly to the streamline oscillations. The known Eckert-Weise effect of low equilibrium temperature at the rearmost stagnation point of the cylinder is associated with the non-uniformities in the temperature and density fields, created by the evolution of recirculation zones near the body surface. For both instantaneous and time-averaged flow patterns, the regions of local increase and decrease in the total enthalpy are distinguished. It turned out that, in the time-averaged flow, the region responsible for the total-enthalpy decrease in the vortex formation zone does not affect the decrease in the total enthalpy in the developed vortex wake, and vice versa.

Bluff body boundary-layer modification and its effect on the near-wake topology

Boundary-layer modification over a bullet-shaped body has been accomplished using wall suction and its effect on the near-wake topology is reported. Two-dimensional velocity fields in the Reynolds number range 6.7×103−5.5×104 are acquired by means of particle image velocimetry and the mean, periodic, and random velocity components are analyzed. Wall suction reduces the boundary-layer thickness and increases the wall-normal velocity gradient. When the boundary layer is laminar up to the point of separation, the modification of the boundary layer leads to a vortex formation region that is up to 19% shorter compared with the case without wall suction. The modification of the boundary layer is accompanied by an increase of the Reynolds normal and shear stresses, which corresponds to larger forces acting on a fictive control area enclosing the mean recirculation region. The increase in Reynolds normal and shear stresses is due to an increase of the random fluctuation component, while the contribution of the periodic fluctuation component remains constant. The altered topology of the vortex formation region is accompanied by a significant change of the vortex shedding frequency and the base pressure. Empirical relations between the wake inlet conditions and the wake characteristics are reported, which hold for both laminar and turbulent boundary layers. The curvature of the streamlines enclosing the rear part of the body was also quantified and found to be proportional to the base pressure.

The Formation of Vortex Structures in a Screen Cylinder Wake

The formation of vortex structures in a screen cylinder wake was investigated in a wind tunnel at a Reynolds number of 7000. The screen cylinder was made of a stainless steel wire mesh with an open area ratio of 67%. The results showed that the screen cylinder wake could be classified into two distinct regions. The first region was characterised by the development of the shear layer vortices which resulted from Kelvin-Helmholtz instability. At about x/d = 2 0 (where d is the diameter of the cylinder) the shear layer vortices started to interact with each other across the centreline, and evolved downstream to form the alternately arranged ‘large-scale’ coherent structures. These structures were most pronounced at x/d = 40. The vortex formation region was therefore extended significantly downstream compared with that of the solid cylinder wake. The second region involved a gradual decay of the fully-formed large-scale structures, evidenced by the weak vorticity exchange across the wake centreline.

Optimization of planar PIV-based pressure estimates in laminar and turbulent wakes

The performance of four pressure estimation techniques using Eulerian material acceleration estimates from planar, two-component Particle Image Velocimetry (PIV) data were evaluated in a bluff body wake. To allow for the ground truth comparison of the pressure estimates, direct numerical simulations of flow over a circular cylinder were used to obtain synthetic velocity fields. Direct numerical simulations were performed for $$Re_\mathrm{D} = 100$$ , 300, and 1575, spanning laminar, transitional, and turbulent wake regimes, respectively. A parametric study encompassing a range of temporal and spatial resolutions was performed for each $$Re_\mathrm{D}$$ . The effect of random noise typical of experimental velocity measurements was also evaluated. The results identified optimal temporal and spatial resolutions that minimize the propagation of random and truncation errors to the pressure field estimates. A model derived from linear error propagation through the material acceleration central difference estimators was developed to predict these optima, and showed good agreement with the results from common pressure estimation techniques. The results of the model are also shown to provide acceptable first-order approximations for sampling parameters that reduce error propagation when Lagrangian estimations of material acceleration are employed. For pressure integration based on planar PIV, the effect of flow three-dimensionality was also quantified, and shown to be most pronounced at higher Reynolds numbers downstream of the vortex formation region, where dominant vortices undergo substantial three-dimensional deformations. The results of the present study provide a priori recommendations for the use of pressure estimation techniques from experimental PIV measurements in vortex dominated laminar and turbulent wake flows.

Passive flow control in the near wake of a circular cylinder using attached permeable and inclined short plates

The main objective of this study is to assess the effectiveness of attached permeable plates in suppressing nominally two-dimensional vortex shedding from a fixed cylinder. The permeable plate along the entire span of the cylinder was made of a chrome-nickel screen and was attached normal to the cylinder surface. The main parameters of the study are porosity of the permeable plate, β, and the plate angle of the cylinder-plate arrangement, θ, with respect to the freestream. Experiments were performed using Particle Image Velocimetry (PIV) and Flow Visualization Techniques. Additionally, drag force measurements were carried out in a wind tunnel to analyze the effects of permeable plates on drag coefficient of the circular cylinder. The results revealed that the use of a permeable plate successfully suppresses the vortex shedding downstream of the circular cylinder by reducing the velocity fluctuations in the wake, elongating the vortex formation region further downstream and attenuating the vortex shedding frequency. The porosity values of β=0.4 and 0.5 with plate angles in the range of 35°≤θ≤90° were found to be effective on suppressing the vortex shedding. Furthermore, both the drag coefficients of the cylinder-plate arrangement and the <TKE>max were reduced almost for all porosity ratios for θ<30°.

Animating Wall-Bounded Turbulent Smoke via Filament-Mesh Particle-Particle Method.

Turbulent vortices in smoke flows are crucial for a visually interesting appearance. Unfortunately, it is challenging to efficiently simulate these appealing effects in the framework of vortex filament methods. The vortex filaments in grids scheme allows to efficiently generate turbulent smoke with macroscopic vortical structures, but suffers from the projection-related dissipation, and thus the small-scale vortical structures under grid resolution are hard to capture. In addition, this scheme cannot be applied in wall-bounded turbulent smoke simulation, which requires efficiently handling smoke-obstacle interaction and creating vorticity at the obstacle boundary. To tackle above issues, we propose an effective filament-mesh particle-particle (FMPP) method for fast wall-bounded turbulent smoke simulation with ample details. The Filament-Mesh component approximates the smooth long-range interactions by splatting vortex filaments on grid, solving the Poisson problem with a fast solver, and then interpolating back to smoke particles. The Particle-Particle component introduces smoothed particle hydrodynamics (SPH) turbulence model for particles in the same grid, where interactions between particles cannot be properly captured under grid resolution. Then, we sample the surface of obstacles with boundary particles, allowing the interaction between smoke and obstacle being treated as pressure forces in SPH. Besides, the vortex formation region is defined at the back of obstacles, providing smoke particles flowing by the separation particles with a vorticity force to simulate the subsequent vortex shedding phenomenon. The proposed approach can synthesize the lost small-scale vortical structures and also achieve the smoke-obstacle interaction with vortex shedding at obstacle boundaries in a lightweight manner. The experimental results demonstrate that our FMPP method can achieve more appealing visual effects than vortex filaments in grids scheme by efficiently simulating more vivid thin turbulent features.

Three dimensionality in the wake of the flow around a circular cylinder at Reynolds number 5000

Abstract The turbulent flow around a circular cylinder has been investigated at R e = 5000 using direct numerical simulations. Low frequency behavior, vortex undulation, vortex splitting, vortex dislocations and three dimensional flow within the wake were found to happen at this flow regime. In order to successfully capture the wake three dimensionality, different span-wise lengths were considered. It was found that a length L Z = 2 π D was enough to capture this behavior, correctly predicting different aspects of the flow such as drag coefficient, Strouhal number and pressure and velocity distributions when compared to experimental values. Two instability mechanisms were found to coexist in the present case study: a global type instability originating in the shear layer, which shows a characteristic frequency, and a convective type instability that seems to be constantly present in the near wake. Characteristics of both types of instabilities are identified and discussed in detail. As suggested by Norberg, a resonance-type effect takes place in the vortex formation region, as the coexistence of both instability mechanisms result in distorted vortex tubes. However, vortex coherence is never lost within the wake.

BİR KANAT PROFİLİ VE BİR DAİRESEL SİLİNDİR ETRAFINDAKİ AKIŞIN AERODİNAMİK KARAKTERİSKTİKLERİ

The flow characteristics around a symmetrical airfoil NACA 0012 at incidence and a circular cylinder placed in tandem have been studied experimentally at a Reynolds number of 1.5×10 5 based on the chord length of the airfoil C. The downstream circular cylinder of diameter D = 25 mm used as a bluff body was placed in the same axis in the flow direction in the wake of the airfoil. The dimensionless gap S/D, where S is the longitudinal spacing between the airfoil and the cylinder, and the attack angle of the airfoil, a, were varied from 0 to 4.3 and from 0° to 15°, respectively. The effects of the attack angle of the airfoil and the longitudinal spacing between the airfoil and the cylinder on the pressure distributions and vortex shedding were examined. Characteristics of the vortex formation region and locations of flow attachments, reattachments, and separations were observed by means of the flow visualizations. It has been seen that the airfoil and the cylinder have considerably affected by each other. The variation in the flow structures according to the attack angle of the airfoil and the longitudinal spacing between the airfoil and the cylinder are revealed utilizing the flow visualization photographs. Keywords: Airfoil, circular cylinder, aerodynamic characteristic, pressure distribution, Strouhal number

Vortex shedding noise from a beveled trailing edge

Coherent vortex shedding from blunt and beveled trailing edges generates tonal noise, which is usually undesired. To obtain a better understanding of the noise generation under such conditions, the flow field around a beveled trailing edge was characterized for Reynolds numbers based on the bluntness ranging from 2.5 × 104 to 5.1 × 104. Flow field statistics were obtained by means of planar high-speed two-component and stereoscopic particle image velocimetry measurements. The development of the shear layers and vortex roll-up is described in the present study. Related length scales, the vortex formation length, and wake thickness parameter were derived from the measurements. Noise emission due to vortex shedding was predicted from an analytic solution, derived from diffraction theory and the reversed Sears’ problem, and compared to acoustic phased array measurements. This approach has previously been shown to provide accurate results for sharply truncated edges, but questions with regard to the applicability with different trailing edge geometries remained open. The prediction required the auto-spectral density, correlation length, and convective velocity of the upwash velocity component in the vortex formation region. Direct application with data obtained from particle image velocimetry measurements showed an overestimation of about 20 dB when compared to the acoustic measurements. The results thus showed that the prediction of vortex shedding noise based on the simplified wake model and diffraction theory is not generally applicable.

Control of flow around a square cylinder at incidence by using a splitter plate

Passive control of vortex shedding behind a square cylinder at incidence has been conducted experimentally by using a stationary splitter plate for the Reynolds numbers of 3.0×104. The splitter plate was located at the center of the rear face of the square cylinder in tandem. The width of the cylinder and the plate were both chosen to be 30mm and the incidence angle of the square cylinder was rotated between 0° to 45°. In this study, the combined effects of the splitter plate and angle of incidence on the pressure distributions and vortex-shedding phenomenon were investigated. Vortex shedding frequency was obtained from velocity measurements and aerodynamic force coefficients acted on the cylinder were calculated from pressure distributions. Characteristics of the vortex formation region and location of the flow attachments, reattachments and separation were observed by using the smoke–wire flow visualization technique. For the case with the plate, there is a sudden jump in the Strouhal number in the vicinity of 13° which corresponds to a minimum value of the drag coefficient. At zero angle of incidence, Strouhal number and a drag coefficient of the square cylinder decreased about 20% by means of the splitter plate. Drag reduction was minimum at about 13° and reached its maximum value at about 20°.

Vortex shedding and structural loading characteristics of finned cylinders

The flow development and structural loading characteristics of cylinders with equispaced circular fins were studied experimentally for a range of fin pitches with constant fin thickness and diameter. The experiments were performed for a range of Reynolds numbers, corresponding to the shear layer transition turbulent shedding regime. Time-resolved planar Particle Image Velocimetry and direct mean drag and fluctuating lift measurements are employed to relate spatio-temporal flow development to structural loading. The results show that wake development is dominated by vortex shedding for all the cases examined. However, the fin pitch ratio has a significant effect on vortex shedding characteristics. The addition of fins increases the characteristic spatial and temporal scales of the main spanwise vortices forming in the near wake. As the fin pitch is decreased to a critical value, the coalescence of boundary layers between the adjacent fins leads to a significant enlargement of the vortex formation region. A modified vortex shedding frequency scaling is proposed, based on the effective diameter, that incorporates a Reynolds number dependence associated with the lateral boundary layers developing on the fin surfaces. A detailed analysis is conducted to characterize the strength of the vortical structures forming in the near wake. The addition of the fins is shown to produce a stabilizing effect on the roll-up process, associated with a reduction in the generation of smaller scale, three-dimensional structures. The results demonstrate that the addition of fins leads to an increase in the mean drag, which is driven primarily by the associated increase in skin friction. The significant effect of the fin pitch ratio on the characteristics of the shed vortices as well as the size of the vortex formation region is shown to lead to substantial variations in the fluctuating loads.

Wake Topology of a Cylinder Undergoing Vortex-Induced Vibrations With Elliptic Trajectories

Wake vortex shedding topology of a cylinder undergoing vortex-induced vibrations (VIV) is investigated experimentally. Vibration measurements and flow visualization are utilized to study the connection between the cylinder response and the wake topology. The experiments were performed for two different orientations of the elliptic trajectories relative to the incoming flow at a fixed Reynolds number, moment of inertia ratio, mass ratio, and reduced velocity. Similar to the classical 2P regime, two counter-rotating vortex pairs are produced per oscillating cycle for both cases of elliptic trajectories examined here. However, significant changes in wake vortex dynamics are observed along the cylinder span. These changes include merging of vortices, which leads to shedding patterns similar to 2S and P + S modes downstream of the vortex formation region. The observed changes in vortex dynamics are accompanied by splitting of spanwise vortex filament and are attributed primarily to the changes in the local amplitude of vibrations along the span of the pivoted cylinder. It is shown that, being dependent on both the local amplitude of vibrations and vortex dynamics, the observed wake topology cannot be captured by the classical map of shedding regimes developed for VIV of one degree-of-freedom (DOF) cylinders.

Influence of rotation on the flow over a cylinder at Re = 5000

The influence of the rotation ratio on the forces acting on a circular cylinder at Re=5000 has been investigated by means of direct numerical simulations (DNS). Spin ratios in the range 0⩽α=UT/Uref⩽5 have been considered. The results showed that rotation causes vortex shedding to cease for spin ratios α⩾2, in very good agreement with previous numerical and experimental research. Furthermore, as a consequence of the increased angular velocity of the cylinder the onset of Taylor–Görtler structures is also observed. Symmetry in the flow is broken as rotation ratio increases. This is specially evident in the shear layers as they start to curve towards the side with the lower pressure gradient causing the shrinkage of the vortex formation region. As these changes occur, both the stagnation point and the saddle point, formed in the closure of the recirculation region, shift in location coming closer to each other as the rotation ratio increases. For larger rotational speeds, the recirculation area behind the cylinder disappears and shear layers roll over the cylinder creating a “circumvolving” layer that greatly changes the wake topology. For α⩾4, the circumvolving layer forces the stagnation point off the cylinder surface, whereas for α=5, the accumulation of vorticity close to the stagnation point makes vortices to be shed on one side of the cylinder. Moreover, the changes that rotation cause on the aerodynamic forces on the cylinder are analyzed and discussed in detail.

Distributed forcing flow control in the wake of a blunt trailing edge profiled body using plasma actuators

A modern flow control technique for reducing the drag associated with the periodic shedding of von Kármán vortices in the wake of a blunt trailing edge profiled body is presented. The technique involves distributed forcing of the wake flow using an array of dielectric barrier discharge plasma actuators, with a spanwise spacing matched to the spanwise wavelength of the dominant secondary wake instability. The experiments include measurement of the velocity field in multiple vertical and horizontal planes in the wake using particle image velocimetry, as well as base pressure, at Reynolds numbers of 2000, 3000, and 5000 based on trailing edge thickness. The flow control technique causes elongation of the vortex formation region across the span, and significant reduction of the fluctuating and total drag forces, up to a maximum of 94% and 18%, respectively. The effectiveness of the flow control technique is shown to be dependent on the induced momentum coefficient. Proper orthogonal decomposition analysis is used to investigate the mechanism of interaction of the flow control technique with the wake flow. Two distinct flow regimes are observed depending on the induced momentum coefficient. The effect of the control on the wake flow structure in the first regime is similar to those observed in previous studies involving mild spanwise-periodic geometric perturbations at the trailing edge, where control leads to streamwise displacement of the vortices and a shift in shedding frequency. However, an incremental increase in the momentum coefficient leads to a second flow regime similar to those previously observed in the case of large-amplitude geometric perturbations, with an almost complete attenuation of vortex shedding in the near-wake region.

Turbulent wake behind a curved circular cylinder

AbstractThis paper reports results from a direct numerical simulation of the flow past a circular cylinder with axial curvature. The main objective is to explore the effects of spanwise curvature on the stability of the shear layers and the turbulent wake at the subcritical Reynolds number of 3900. The bluff-body geometry is adapted from a previous study conducted at lower Reynolds numbers, in which a quarter segment of a ring represented the deformed cylinder. A convex configuration in which the free-stream direction is towards the outer face of the ring is adopted here. The present results show a striking distinction between the upper and lower wake regions. Despite the turbulent character of the wake, the upper wake region is more coherent due to the periodic vortex shedding of primary vortical structures, which are in close alignment with the axial curvature. A mild axial flow develops upwards along the lee face of the curved cylinder, displacing the vortex formation region further downstream from the location expected for a straight cylinder at the same Reynolds number. In the lower wake region the vortex shedding strength is drastically reduced due to larger local inclination, resulting in higher three-dimensionality and loss of coherence. A strong downdraft with a swirling pattern is the dominating feature in the lower base region. This is associated with a substantial decrease of the base suction, and the suppression of the characteristic recirculating backflow.

On vortex shedding from low aspect ratio dual step cylinders

A dual-step cylinder is comprised of two cylinders of different diameters. A large diameter cylinder (D) with low aspect ratio (L/D) is attached to the mid-span of a small diameter cylinder (d). The present study investigates the effect of Reynolds number (ReD) and L/D on dual step cylinder wake development for D/d=2, 0.2≤L/D≤3, and two Reynolds numbers, ReD=1050 and 2100. Experiments have been performed in a water flume facility utilizing flow visualization, Laser Doppler Velocimetry (LDV), and Particle Image Velocimetry (PIV). The results show that vortex shedding occurs from both the large and small diameter cylinders for 1≤L/D≤3 at ReD=2100 and 2≤L/D≤3 at ReD=1050. At these conditions, large cylinder vortices predominantly form vortex loops in the wake and small cylinder vortices form half-loop vortex connections. At lower aspect ratios, vortex shedding from the large cylinder ceases, with the dominant frequency in the large cylinder wake attributed to the passage of vortex filaments connecting small cylinder vortices. At these lower aspect ratios, the presence of the large cylinder induces periodic vortex dislocations. Increasing L/D increases the frequency of occurrence of vortex dislocations and decreases the dominant frequency in the large cylinder wake. The identified changes in wake topology are related to substantial variations in the location of boundary layer separation on the large cylinder, and, consequently, changes in the size of the vortex formation region. The results also show that the Reynolds number has a substantial effect on wake vortex shedding frequency, which is more profound than that expected for a uniform cylinder.

Low-frequency unsteadiness in the vortex formation region of a circular cylinder

The presence of low-frequency fluctuations in the wake of bluff bodies have been observed in several investigations. Even though the flow past a circular cylinder at Re = 3900 (Re = UrefD/ν) has been the object of several experimental and numerical investigations, there is a large scattering in the average statistics in the near wake. In the present work, the flow dynamics of the near wake region behind a circular cylinder has been investigated by means of direct numerical simulations and statistics have been computed for more than 858 shedding cycles. The analysis of instantaneous velocity signals of several probes located in the vortex formation region, point out the existence of a low-frequency fluctuation at the non-dimensional frequency of fm = 0.0064. This large-scale almost periodic motion seems to be related with the modulation of the recirculation bubble which causes its shrinking and enlargement over the time. Two different configurations have been identified: (i) a high-energy mode with larger fluctuations in the shear-layer and in the vortex formation region (Mode H) and (ii) a low-energy mode with weaker fluctuations in the shear layer (Mode L). The influence of such a low-frequency in the wake topology has been studied not only by means of the phase-average flow field for each mode, but also by the analysis of the time-average first- and second-order statistics of each wake mode. The results are compared with the long-term averaged solution and with results in the existing literature.