Axisymmetric Bluff Body Research Articles

SPATIO-TEMPORAL RECONSTRUCTION OF VORTEX DYNAMICS IN AXISYMMETRIC WAKES

With time recording Digital-Particle-Image Velocimetry and spatio-temporal reconstruction technique, we obtained detailed quantitative results of the evolution of the velocity and vorticity field in the wake of axisymmetric bluff bodies—a sphere and an axially oriented cylinder with an elliptic nose and a blunt base. Experiments were carried out for Reynolds numbers of Re=500, 700 and 1000 in the transition range from “regular” to “irregular” shedding. DPIV-recordings in radial cross-sections at several distances downstream of the bodies allowed us to reconstruct the dynamics of the streamwise vorticity over a large number of shedding cycles. Our results prove that the wake in this regime consists of a double-sided chain of oppositely oriented hairpin vortices. In addition, the results show a well-defined low-frequency modulation of the vortex shedding, with a distinct peak in the frequency spectrum at a Strouhal number of about Sr≈0·05 (in case of the sphere). The wake pattern in this “irregular” shedding regime typically exhibits periodic packets of 3–4 “regular” shedding cycles which are interrupted by phases with less action. The results indicate the coexistence of a long-wave instability of axisymmetric wakes against helical waves in addition to the primary instability causing the vortex shedding process.

Numerical simulation of unsteady flow field behind bluff body

The transient incompressible flow behind the axisymmetric bluff body is numerically simulated using the random vortex method(RVM). Based on the vorticity formulation of the unsteady Navier-Stokes equations, the Lagrangian approach with a stochastic simulation of diffusion using random walk technique is employed to account for the transport processes of the vortex elements. The numerical solutions for 2-dimensional recirculating flow behind a backward-facing step in the laminar range of Reynolds number are compared with experimental data. The present simulation focuses on the transitional flow regime where the recirculation zone behind the bluff body becomes highly unsteady and large-scale vortex eddies are shed from the bluff body wake due to intrinsic shear layer instabilities. The unsteady vertical flow structures and the mixing characteristics behind the bluff body are discussed in detail.

Experimental and numerical study of a water spray in the wake of an axisymmetric bluff body

An experimental and numerical study was made of a water spray issuing into a recirculating flow behind a bluff body disk mounted in a nozzle. A two-component laser-Doppler/phase-Doppler anomometry system was used to characterize the mean and turbulent dispersed phase. The numerical prediction of the hollow-cone spray was based on an Eulerian-Lagrangian stochastic hybrid model. The continuous gas flow field was predicted using a differential Reynolds stress transport model whereas the particulate droplet flow field was predicted using an improved Lagrangian stochastic dispersion model. Comparison of numerical predictions and experimental measurements was carried out for droplet mean and fluctuating velocities, number mean diameters, and mass fluxes. Results indicate that the droplet dispersion characteristics are strongly influenced by the presence of flow recirculation due to different particle Stokes numbers, yielding recirculation or penetration of particles through the separated flow region. Predictions of droplet axial and radial rms velocities obtained with the Lagrangian stochastic model are less satisfactory than those of other simple gas spray flows.

Simulation of supersonic flows with detached shocks using a property convection method

AbstractAn algorithm based on the convection of fundamental flow quantities, i.e. mass, momentum and energy, has been described by the author previously.1 Computer codes to implement the algorithm have been developed for two‐dimensional and axisymmetric flows. This algorithm has been further developed to improve the accuracy and stability of the calculations. In this paper the algorithm is used to calculate the flows about two‐dimensional and axisymmetric bluff bodies. Bodies of practical interest in supersonic and hypersonic flows are often sufficiently bluff for a detached shock to develop, and correct prediction of the shock stand‐off distance and the position of the sonic line is a useful test of numerical methods.The programme is used to calculate flows with a detached shock around three standard shapes: a sphere, a body of revolution with a flat nose and a plate with a flat nose. The Mach number range for the flows described is from 1.177 to 6.0. Comparisons are given with standard experimental results for the steady state stand‐off.

Drag and wake modification of axisymmetric bluff bodies using Coanda blowing

This work investigates the ability of Coanda jet blowing to modify the base pressure of a cylindrical body aligned axially in a flow, and thereby, produce overall drag reduction. It is found that blowing through one or two slot jets concentric to the outer body circumference can significantly influence the entire base flow region. The recirculating wake is eliminated and is replaced by freestream fluid entrained by the Coanda blowing. Base pressure rises significantly and leads to drag reduction of up to 30% beyond the thrusting action of the Coanda jet. A comparison between the power savings through drag reduction and the power requirement of the Coanda jet demonstrates that net benefits are attainable at certain body geometries and flow conditions. By judiciously selecting the jet blowing velocity, it is possible to produce a nearly flat wake velocity profile requiring little net power. RAG reduction of immersed bodies is a subject with a long history, leading to the early concept of streamlining. The net drag force on a body may be considered the sum of viscous drag and pressure drag forces. For streamlined aero- dynamic and hydrodynamic bodies, the pressure drag is small, and current research is directed towards the application of laminar flow control (e.g., suction), turbulent viscous drag reduction (e.g., riblets and large eddy breakup devices (LEBUs)), and the use of nonlinear aerodynamics for induced drag reduction (e.g., winglets or crescent wing planforms). For bluff bodies, on the other hand, streamlining is usually not an option for reducing drag because the bluff shape is often dictated by other constraints. This situation is particu- larly true for trucks, buses, and most automobiles. For bluff bodies then, where pressure drag dominates, drag reduction is primarily through base flow modification, including flow separation control using airfoils, the use of plates, cavities, base bleed, and suction/blowing. Bluff bodies may be considered of two general classifica- tions—high and low aspect ratio. The former may be modeled as bodies of large span relative to a characteristic height, and are generally two-dimensional in nature. Examples would be the classic cylinder in crossflow, or a symmetric airfoil with a thick, blunt trailing edge. Drag reduction for such bodies has been performed13 with drag reductions of up to 64% reported in the literature. Low aspect ratio bodies have also been studied. Such bodies are characterized by three-dimensionality or axisymmetry, with a sphere being the classic example. Other examples would be cylinders of rectangular or circular cross section aligned axially in the flow direction. Work on drag reduction of such bodies has been reported in Refs. 4-12. A variation of these studies would include inclined base regions such as fast-back auto- mobiles and cargo transport aircraft. During the energy crisis of the seventies, renewed interest in vehicle drag led to many important new findings which are summarized in the books

212 軸対称鈍頭物体の後流場の渦構造

The vortical structure of the near wake of an axisymmetric bluff body in the region of low Reynolds number is experimentally investigated. Morel model which is the fundamental shape of a flying body is used as the bluff body. This model has interchangeable after-bodies with slanted bases having slant angles from 0° to 60°.The model is installed in a uniform flow in the test section of the water channel. And the velocity distribution in the near wake is measured by using laser doppler velocimeter. Also the structures of shedding vortices are examined by the flow visualization technique. Behind the model, it was contirmed that there were regular shedding vortices. As the slant angle of the after-body becomes larger, the frequency of the shedding vortices becomes shorter. The remarkable change of the vortex pattern owing to the variation in the slant angle of the after-body could not be confirmed. It was found out that the geometrical form of the vortex was an arch-type with a revolution.

Effects of velocity ratio on bluffbody flow dynamics

Introduction Axisymmetric bluffbodies are used to enhance mixing and stabilize combustion between two streams. They are frequently utilized in utility combustors and propulsion systems since they exhibit the desirable property of promoting mixing at the expense of small pressure loss. Nonreacting and reacting bluffbody flows have been a subject of extensive experimental investigation, include flows in liquids and gases, unconfined and confined flows, and largeand small-scale facilities. Experimental observations indicate that nonreacting and reacting bluffbody flows are unsteady and, in most cases, can be characterized by large-scale, periodic oscillations.

Measurements and modeling of a bluff body stabilized flame

An axisymmetric bluff body stabilized nonpremixed turbulent flame of 27.5% CO/32.3% H 2/40.2% N 2-in-air was investigated. The recirculation zone stabilized the flame and provided greater strain rates than possible in jet or even piloted-jet flames. Major species, density, and temperature were measured using a laser Raman scattering system, which was modified to operate in a chemiluminescent environment. The computational model was based on partial equilibrium in the radical pool, an assumed shape pdf over the two thermochemical variables required, and the k-ϵ turbulence model for closure of the density-weighted averaged Navier Stokes equations. The equations were solved in the elliptic form appropriate to recirculating flow. Enough grid was added to reduce the transverse cell Reynolds numbers to below two, ensuring second-order accurate and stable discretization of convection operators and so eliminating artificial diffusion. Mean properties such as density were obtained at each node by convolution with the joint pdf over the two thermochemical scalars. The k-ϵ turbulence model gave too rapid an initial decay. Agreement was encouraging on mixture fraction mean and variance, temperature, and species concentration fields. The bluff body provides an intensely turbulent flowfield for interactions with combustion chemistry, and is within the scope of numerical analysis. To improve the turbulence model, and to have a formalism that permits three or more scalars as required for hydrocarbon fuels, pdf transport methods should be merged with conventional solvers for the mean hydrodynamics.

Axisymmetric-type vortex shedders for vortex flowmeters

The fluid dynamics of flow over an axisymmetric bluff body in a circular pipe are surveyed for circular disks and rings. One important feature found is that shedding vortices convecting in the wake may induce flow near the wall in unsteady, periodic motion. This finding indicates that the vortex shedding frequency can be obtained with a sensor situated on the surface of the pipe wall. Guidelines for the optimal sizes of the vortex shedders and the optimal streamwise locations of the frequency sensors are suggested. A critical comparison between the present design and the current strut-type vortex flowmeter is also made.

Visual study on wakes behind solid and slotted axisymmetric bluff bodies

A flow visualization study was conducted for various configurations of solid and slotted axisymmetric bluff bodies. The geometries also included those of solid and ribbon parachute canopy models. The structures of the unsteady near wake patterns were surveyed and compared. In addition, the effect of rapid model acceleration and deceleration on the wake was visualized. This article presents, although somewhat qualitatively, analysis of results from photographs and videotapes and an overview of various pertinent phenomena observed. CLEAR understanding of the wake flow behind a parachute is critical for the safe operation of parachutes. Though detailed knowledge of the wake structure is essential to predict, such phenomena as the wake recontact problem,1 analysis of the entire wake region at once appears to be inaccessible. For example, the wakes of axisymmetric bluff bodies are found to be fully three dimensional even for the simplest form of axisymmetric geometry. The flowfield imme- diately downstream of a ribbon parachute is particularly complex because of irregular merging of high speed jets through multiple narrow gaps. Moreover, the detailed study of the flowfield during a full-scale drop test appears to be a nearly inaccessible task at present. Flow visualizations in carefully designed laboratory environments, on the other hand, can not only provide excellent clues to the complex flowfield but also guide quantitative measurements and com- putation and analysis of the flowfield. It is also to be noted that some modern flow visualization techniques have started to provide detailed quantitative information with advance- ment of applied optics and image processings, as seen in Ref. 2. In the past, low-speed flow visualizations have been con- ducted to simulate complex high Reynolds number airflows in aircraft flight conditions, and somewhat surprisingly to some, the results compared well with flight test data.3 This means that many vortex-dominated flows are not as dependent on the differences in viscous effects. Naturally, additional com- pressibility effects must not be overlooked, and interpretation of flow visualizations in unsteady flowfields requires extra caution, as is often pointed out (see, for example, Gad-el- Hak4). In the present experimental investigation, the wake regions of the axisymmetric bluff bodies were observed using various flow visualization techniques in both air and water. Configurations studied included a circular disk, solid parachute model, slotted disk, and a ribbon parachute model. Flow visualization was also conducted to study the effect of rapid model acceleration and deceleration. Minnesota. The test section was 1.5m long and the cross section measured 1.37 x 0.97m. Freestream velocity up to 23 m/s can be obtained, though flow visualization was con- ducted at a substantially lower velocity, as noted in the figures in terms of the Reynolds numbers based on the model diame- ter. The tunnel flow condition5 was found adequate for the present study. The models to be described in the Results section were either sting-mounted or supported by thin wire along the centerline of the test section. The streamlined particle injector was fabricated and placed upstream of the model, and fine oil fog generated by a Rosco Model 1500 smoke/fog generator was introduced into the test section. The visualization in the empty test section as a control revealed no unsteadiness in the freestream nor any adverse effect of the injector. The diameter of particles, which are nontoxic accord- ing to the manufacturer, measured 2 jum on the average. Illumination was provided by an argon ion laser passed through a set of cylindrical lenses and front surface mirrors to produce a thin sheet of light aligned with the centerline of the model. The experiment in water was carried out in a towing tank at the St. Anthony Falls Hydraulic Laboratory, measuring 39 x 61 cm in cross section and 15.2m in length. In some tests, the tank was partitioned into a shorter segment for ease of operation. Normally, the model was towed in a stationary fluid. The towing carriage was driven by a variable speed a.c. motor. A 35-mm camera or a video camera and the lighting

Vortex flowmeter designed with wall pressure measurement

An axisymmetric bluff body is proposed for a vortex flowmeter as the vortex shedder. It is found that vortex shedding frequencies measured can be nondimensionalized into a unified relation for area blockage ratios of circular disks up to 29.2% and Reynolds numbers in a range of 2.5×103–9.7×104. Vortex shedding frequency can be clearly sensed by a pressure transducer installed on the pipe wall in the neighborhood of the maximum pressure fluctuation being measured. This suggests a feasible design that the sensor of a flowmeter can be removed from the flow field.

Determination of Surface Pressure Distributions for Axisymmetric Bluff Bodies

The conditions governing liquid droplet breakup are particularly important to the process of fuel atomization for spark-ignition engines. A proper mathematical description of the problem requires knowledge of the pressure distributions about the droplet surface. This work presents the results of measurements of pressure distributions about the surfaces of certain bluff, axi-symmetric bodies in airflows at ReD = 3 × 104 and ReD = 105. The results of measurements from a sphere, disk, and two ellipsoids are used to develop a general method for estimating surface pressures. These estimates are compared with the results of pressure measurements about the surfaces of a concave and a nonellipsoidal convex body and with results obtained from a numerical study by other authors.

A note on vortex shedding from axisymmetric bluff bodies

The linear parallel and incompressible stability of a family of axisymmetric wake profiles is studied in the range of Reynolds numbers where helical vortex shedding from bluff bodies of revolution is observed. The family of mean flow profiles allows for the variation of the wake depth as well as for a variable ratio of wake width to mixing-layer thickness. It is found that, even without reverse flow, the first helical mode is absolutely unstable in the near wake for Reynolds numbers, based on wake diameter and free-stream velocity, in excess of 3.3 × 103. A survey of the region of local absolute instability as a function of profile parameters and Reynolds number suggests that the large-scale helical vortex shedding, which is observed between Reynolds numbers of 6000 and 3 × 105for spheres, may be ‘driven’ by a self-excited oscillation in the near wake.

Oscillations in confined disk-stabilized flames

The flammability and stability limits of confined premixed flames stabilized behind axisymmetric bluff bodies have been established as a function of the length of upstream duct, with and without sudden contractions. Adjustment of the upstream length between a sudden contraction of large area ratio and the flame stabilizer is shown to offer a possible way of suppressing instabilities. Similarly, the careful location of an orifice in an upstream length of straight pipe can suppress instabilities and criteria for the location and diameter of the orifice are given. The relationship between shedding and acoustic frequencies is also examined and appears to be important in the range of equivalence ratios over which transition from stable to unstable combustion takes place.

Velocity characteristics in the turbulent near wakes of confined axisymmetric bluff bodies

Measurements of the velocity characteristics and wall pressure are reported for the axisymmetric turbulent flow downstream of three bluff bodies (disks of 25% and 50% area blockage and a cone of 25% blockage) confined by a long pipe. The dimensions of the recirculation regions were found from the mean-velocity components, which were determined by a laser-Doppler velocimeter: the corresponding components of Reynolds stress were also recorded. The lengths and maximum widths of the recirculation bubbles (in bluff-body diameters), recirculating mass-flow rates (normalized by the average velocity in the plane of the baffle, U0, and the baffle diameter) and maximum turbulent kinetic energy (normalized by U02) were as follows: cone 1.55, 0.55, 0.19, 0.11; disk (25% blockage) 1.75, 0.62, 0.31, 0.19; disk (50% blockage) 2.20, 0.55, 0.26, 0.16. The increase in recirculation length with blockage is opposite to the trend in unconfined, annular jets. The distribution of Reynolds stresses is strongly dependent on blockage: for the smaller blockage both the disk and the cone have the maximum value of kinetic energy near the rear stagnation point. It is proposed that this is because the generation of turbulence by normal stresses is more important in the flow consequent on the smaller blockage.The measurements include profiles of the velocity characteristics at, as well as upstream of, the trailing edges of the baffles for use as boundary conditions in numerical solutions of the equations of motion.

Jet effects on near wake of an axisymmetric bluff body

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Longitudinal grooves for bluff body drag reduction

Wind tunnel test results are presented for four axisymmetric bluff body configurations in order to determine their effect on form and pressure drag. It was found that drag reductions on the order of 40% are obtainable with an afterbody incorporating four longitudinal 'V' grooves. Although this effect may be due to the functioning of the grooves as longitudinal, continuous vortex generators, it is concluded that further research is needed to elucidate the physical basis of the test results. Optimization of the effect will be useful in base drag reduction for such vehicles as automobiles and cargo aircraft with sharply upswept afterbodies.

An approximate theory for the streaming motion past axisymmetric bodies at Reynolds numbers of from 1 to approximately 100

In Weinbaum et al. (1976) a simple new pressure hypothesis is derived which enables one to take account of the displacement interaction, the geometrical change in streamline radius of curvature and centrifugal effects in the thick viscous layers surrounding two-dimensional bluff bodies in the intermediate Reynolds number range O(1) < Re < O(102) using conventional Prandtl boundary-layer equations. The new pressure hypothesis states that the streamwise pressure gradient as a function of distance from the forward stagnation point on the displacement body is equal to the wall pressure gradient as a function of distance along the original body. This hypothesis is shown to be equivalent to stretching the streamwise body co-ordinate in conventional first-order boundary-layer theory. The present investigation shows that the same pressure hypothesis applies for the intermediate Reynolds number flow past axisymmetric bluff bodies except that the viscous term in the conventional axisymmetric boundary-layer equation must also be modified for transverse curvature effects O(δ) in the divergence of the stress tensor. The approximate solutions presented for the location of separation and the detailed surface pressure and vorticity distribution for the flow past spheres, spheroids and paraboloids of revolution at various Reynolds numbers in the range O(1) < Re < O(102) are in good agreement with available numerical Navier–Stokes solutions.

Near wake properties of axisymmetric bluff body flows

Experiments have been made to measure some of the near wake properties of axisymmetric bluff body flows with fixed points of separation, including the detention or residence time of fluid borne scalar entities, base pressure coefficient, wake bubble length parameter and shape parameter. Measurements were made in smooth and turbulent air flow for Reynolds number in the range 2×103<Re<4×104. The results for a given bluff body were found to be uniquely controlled by a free-stream turbulence parameter. The data for all the shapes of bluff body in the class under consideration were found to collapse into unique inter-relationships by the introduction of the face pressure coefficient as a quantitative measure of “bluffness”.

Calculation of two-dimensional and axisymmetric bluff-body potential flow

A numerical method incorporating some of the ideas underlying the wake source model of Parkinson &amp; Jandali (1970) is presented for calculating the incompressible potential flow external to a bluff body and its wake. The effect of the wake is modelled by placing sources on the rear of the wetted surface of the body. Unlike Parkinson &amp; Jandali's method, however, the body shapes that can be treated are not limited by the restrictions imposed by the use of conformal transformation. In the present method the wetted surface of the body is represented by a distribution of discrete vortices. Good agreement has been found between the pressure distributions predicted by the numerical method and the analytic expressions of Parkinson &amp; Jandali for a ‘two-dimensional’ circular cylinder and flat plate. A flat plate at incidence and other asymmetric two-dimensional flows have also been treated. The method has been extended to axisymmetric bluff bodies and the results show good agreement with measured pressure distributions on a circular disk and a sphere.