Cylinder In Cross Flow Research Articles

Effect of acoustic resonance on the dynamic lift forces acting on two tandem cylinders in cross-flow

Abstract Direct measurements of the dynamic lift force acting on two tandem cylinders in cross-flow are performed in the presence and absence of acoustic resonance. The dynamic lift force is measured because it represents the integrated effect of the unsteady wake and therefore it is directly related to the dipole sound source generated by vortex shedding from the cylinder. Three spacing ratios inside the proximity interference region, L/D=1.75, 2.5 and 3 are considered. During the tests, the first transverse acoustic mode of the duct housing the cylinders is self-excited. In the absence of acoustic resonance, the measured dynamic lift coefficients agree with those reported in the literature. When the acoustic resonance is initiated, a drastic increase in the dynamic lift coefficient is observed, especially for the downstream cylinder. This can be associated with abrupt changes in the phase between the lift forces and the acoustic pressure. The dynamic lift forces on both cylinders are also decomposed into in-phase and out-of-phase components, with respect to the resonant sound pressure. The lift force components for the downstream cylinder are found to be dominant. Moreover, the out-of-phase component of the lift force on the downstream cylinder is found to become negative over two different ranges of flow velocity and to virtually vanish between these two ranges. Acoustic resonance of the first mode is therefore excited over two ranges of flow velocity separated by a non-resonant range near the velocity of frequency coincidence. It is therefore concluded that the occurrence of acoustic resonance is controlled by the out-of-phase lift component of the downstream cylinder, whereas the effect of the in-phase lift component is confined to causing small changes in the acoustic resonance frequency.

Forced‐convection heat transfer from tandem square cylinders in cross flow at low Reynolds numbers

AbstractThis paper presents the results of a numerical study on the flow characteristics and heat transfer over two equal square cylinders in a tandem arrangement. Spacing between the cylinders is five widths of the cylinder and the Reynolds number ranges from 1 to 200, Pr=0.71. Both steady and unsteady incompressible laminar flow in the 2D regime are performed with a finite volume code based on the SIMPLEC algorithm and non‐staggered grid. A study of the effects of spatial resolution and blockage on the results is provided. In this study, the instantaneous and mean streamlines, vorticity and isotherm patterns for different Reynolds numbers are presented and discussed. In addition, the global quantities such as pressure and viscous drag coefficients, RMS lift and drag coefficients, recirculation length, Strouhal number and Nusselt number are determined and discussed for various Reynolds numbers. Copyright © 2008 John Wiley & Sons, Ltd.

Flow-Induced Sound of Wall-Mounted Cylinders with Different Geometries

The objective of this work was to investigate the influence of changes in geometry of a wall-mounted square cylinder in crossflow on the generated aerodynamic noise. The flowfield and acoustic field of several geometries are obtained and their interaction is analyzed. The results of the subset of geometries investigated indicate that the reduction in turbulent kinetic energy does not necessarily also lead to a reduction in radiated noise. Reducing the fluctuations of the stagnation point in front of the cylinder, maintaining a fixed separation line, and reducing the symmetry of the vortex street and its stability seem to be effective measures for reducing aerodynamic noise, among other known approaches.

Plasma Actuators for Cylinder Flow Control and Noise Reduction

In this paper, the results of flow-control experiments using single dielectric barrier discharge plasma actuators to control flow separation and unsteady vortex shedding from a circular cylinder in crossflow are reported. This work is motivated by the need to reduce landing gear noise for commercial transport aircraft via an effective streamlining created by the actuators. The experiments are performed at Re D = 3.3 x 104. Using either steady or unsteady actuation, Karman shedding is totally eliminated, turbulence levels in the wake decrease significantly, and near-field sound pressure levels associated with shedding are reduced by 13.3 dB. In the case of unsteady plasma actuation, an actuation frequency of St D = 1 is found to be most effective. The unsteady actuation has the advantage that total suppression of shedding is achieved for a duty cycle of only 25%. However, because unsteady actuation is associated with an unsteady body force and produces a tone at the actuation frequency, steady actuation is more suitable for noise-control applications.

Convective Heat Transfer from a NACA Airfoil at Varying Angles of Attack

Airfoil icing in aircraft and wind turbine applications has an adverse impact on aerodynamic performance, controllability, and system efficiency. A detailed understanding of convective heat transfer from airfoils is critical in the development of effective de-icing methods. In this paper, experimental correlations of heat transfer at different angles of attack of a NACA 63-421 airfoil are developed. Various angles of attack between 0 to 25 deg are investigated at different Reynolds numbers. The experimental data are correlated with respect to the Nusselt and Reynolds numbers, through a modified Hilpert correlation with an angular dependence on angles of attack. Conduction within the airfoil is balanced against heat transfer by convection from the airfoil surface in steady-state conditions. Both average and spatial variations of the heat transfer coefficients are nondimensionalized, through modifications of a Hilpert correlation for cylinders in crossflow. It is shown that the functional form of the Hilpert equation can effectively correlate the measured data for the NACA airfoil over a range of Reynolds numbers and angles of attack.

Flow-excited acoustic resonance of two side-by-side cylinders in cross-flow

Abstract The aeroacoustic response of two side-by-side circular cylinders in cross-flow is investigated experimentally. In order to investigate the effect of the gap between the cylinders on the acoustic resonance mechanism, six spacing ratios between the cylinders, in the range of T / D =1.25–3, have been investigated, where D is the diameter of the cylinders and T the centre-to-centre distance between them. Special attention is given to the intermediate spacing ratio range, which exhibits bistable flow regimes in the absence of resonance. During the tests, the acoustic cross-modes of the duct housing the cylinders are self-excited. For the intermediate spacing ratios, T/D =1.25, 1.35, 1.46 and 1.75, two distinct vortex-shedding frequencies at the off-resonance conditions are observed. These are associated with the wide and narrow wakes of the cylinders, as described in the literature. In this case, acoustic resonances occur at a Strouhal number, which is between those observed before the onset of resonance. The acoustic resonance synchronizes vortex shedding in the two wakes and thereby eliminates the bistable flow phenomenon. For large spacing ratios, T/D =2.5 and 3, vortex shedding occurs at a single Strouhal number at which the acoustic resonance is excited.

T-RANS Simulations of Subcritical Flow with Heat Transfer Past a Circular Cylinder Surrounded by a Thin Porous Layer

We study flow and heat transfer to a cylinder in cross flow at Re = 3,900–80,000 by means of three-dimensional transient RANS (T-RANS) simulations, employing an RNG k???? turbulence model. Both the case of a bare solid cylinder and that of a solid cylinder surrounded at some fixed distance by a thin porous layer have been studied. The latter configuration is a standard test geometry for measuring the insulating and protective performance of garments. In this geometry, the flow in the space between the solid cylinder and the porous layer is laminar but periodic, whereas the outer flow is transitional and characterized by vortex shedding in the wake of the cylinder. The results from the T-RANS simulations are validated against data from Direct Numerical Simulations and experiments. It is found that T-RANS is very well suited for simulating this type of flow. The transient nature of the flow underneath the porous layer is well reproduced, as well as the influence of vortex shedding on the heat transfer in the downstream stagnation zone. T-RANS results are found to be in much better agreement with DNS and experimental data than results from steady-state RANS.

Numerical Investigation of Flow-pattern and Flow-induced Noise for Two Staggered Circular Cylinders in Cross-flow by LBM

The flowfield behind two cylinders and flow-induced noise generated from the cylinders in various arrangement are numerically investigated based on the finite difference lattice Boltzmann model with 21 velocity bits, which is introduced a flexible specific heat γ to simulate diatomic gases like air. In an isolated cylinder with two type of mesh, some flow parameters such as Strouhal number S t and acoustic pressure ? p simulated from the solution are given and quantitatively compared with those provided the previous works. The effects of the center-to-center pitch ratio L cc /d=2.0 in staggered circular cylinders as shown in Fig. 1 and angles of incidence α=30°(T cc /d=0.5), 45°(T cc /d=0.707) and 60° (T cc /d=0.866), respectively, are studied. Our analysis focuses on the small-scale instabilities of vortex shedding, which occurs in staggered arrangement. With the results of drag C d and lift C l coefficients and vorticity contours, the mechanisms of the interference phenomenon and its interaction with the two-dimensional vortical structures are present in the flowfields under Re ≤ 200. The results show that we successively capture very small pressure fluctuations, with the same frequency of vortex shedding, much smaller than the whole pressure fluctuation around pairs of circular cylinders. The upstream cylinder behaves like an isolated single cylinder, while the downstream one experiences wake-induced flutter. It is expected that, therefore, the relative position of the downstream cylinder has significant effects on the flow-induce noise, hydrodynamic force and vortex shedding characteristics of the cylinders.

Instabilities in the dynamics of neutrally buoyant particles

The asymptotic dynamics of finite-size particles is governed by a slow manifold that is globally attracting for sufficiently small Stokes numbers. For neutrally buoyant particles (suspensions), the slow dynamics coincide with that of infinitesimally small particles, therefore the suspension dynamics should synchronize with Lagrangian particle motions. Paradoxically, recent studies observe a scattering of suspension dynamics along Lagrangian particle motions. Here we resolve this paradox by proving that despite its global attractivity, the slow manifold has domains that repel nearby passing trajectories. We derive an explicit analytic expression for these unstable domains; we also obtain a necessary condition for the global attractivity of the slow manifold. We illustrate our results on neutrally buoyant particle motion in a two-dimensional model of vortex shedding behind a cylinder in crossflow and on the three-dimensional steady Arnold–Beltrami–Childress flow.

Where do inertial particles go in fluid flows?

We derive a general reduced-order equation for the asymptotic motion of finite-size particles in unsteady fluid flows. Our inertial equation is a small perturbation of passive fluid advection on a globally attracting slow manifold. Among other things, the inertial equation implies that particle clustering locations in two-dimensional steady flows can be described rigorously by the Q parameter, i.e., by one-half of the squared difference of the vorticity and the rate of strain. Use of the inertial equation also enables us to solve the numerically ill-posed problem of source inversion, i.e., locating initial positions from a current particle distribution. We illustrate these results on inertial particle motion in the Jung–Tél–Ziemniak model of vortex shedding behind a cylinder in crossflow.

Experiments on circular cylinders in crossflow at Reynolds numbers up to 7 million

This paper describes a series of experiments conducted on polished circular cylinders in the Reynolds number range from 100,000 to 7 million. The experiments were conducted in a pressurized wind tunnel wherein Mach number and Reynolds number could be independently varied. Three models were used with aspect ratios of 10, 5 and 2.5. Measurements were also made in two grid-generated turbulent flows with intensities of 5% and 13%. In addition to force balances at each end of the models, 32 pressure taps were embedded in a circumferential ring at mid-span and along the leeward generator. Both time-averaged and unsteady data are discussed. The pressure-tap data provide a detailed understanding of the unsteady flow, including vortex shedding, around the cylinder in different flow regimes. The presence of turbulence can change the flow state and hence the steady and unsteady loads. Compressibility effects are shown to exist above a Mach number of 0.3.

Effect of internal flux on the heat transfer coefficient in circular cylinders in cross flow

The present paper shows results of laboratory measurements carried out to study effect of perforations on the cooling of cylinders under forced convection. Measurements were carried out for Reynold’s numbers between 2000 and 15,000. Results indicate that the presence of holes and their orientation relative to air flux have a significant effect on the cooling process. Nusselt numbers have been found to increase as much as 20% as compared to a corresponding smooth cylinder.

Forced convection heat transfer from banks of helical coiled resistance wires

Forced convection heat transfer from banks of helical coiled resistance wires in a cross flow of air was investigated experimentally, with reference to various operative conditions and several coil geometries. The resistance banks were subdivided in two branches, independently powered by single-phase alternating current. Average convection heat transfer coefficients were determined for each branch. No significant interactions were detected between adjacent coils nor between upstream and downstream coils, possibly because of the high percentage of inter-spaces between wires. The experimentally measured Nusselt numbers were thus correlated by a modified version of the Churchill and Bernstein model, valid for a single cylinder in cross flow. To this purpose, an equivalent wire diameter was defined in order to take into account the influences of diameter and pitch of the coils. The average deviation between measured and calculated values of the Nusselt number is of the order of ± 7 % in the range of Reynolds numbers from 70 to 400.

An Experimental Investigation of the Separation Points on a Circular Rotating Cylinder in Cross Flow

The flow past a rotating cylinder placed within a uniform stream is investigated at Reynolds numbers ranging from 8500 to 17,000 to 34,000. The dimensionless rotation rate α (ratio of the cylinder peripheral speed to the free-stream velocity) varies from 0 to 7. The experimental investigation is based on laser-Doppler anemometry measurements and particle-image velocimetry (PIV) within a water channel. The analysis of the experimental results mainly concerns the location of the separation points as defined by various criteria. It is found that the criterion suggested by Moore, Rott and Sears (MRS) is met in the case of the downstream-moving walls. Moreover, this study shows that sufficient information was obtained to confirm that the MRS criterion is still valid even in the case of the upstream-moving walls. This is confirmed by the behavior of the vertical velocity component educed from the averaged two-dimensional flow field obtained by PIV measurements.

Two-dimensional simulation of unsteady heat transfer from a circular cylinder in crossflow

The heat transfer from the surface of a circular cylinder into a crossflow has been computed using a two-dimensional model, for a range of Reynolds numbers fromRe=200 to 15550. The boundary-layer separation, the local and overall heat-transfer rates, the eddy- and flare-detachment frequencies and the width of the flares were determined from the numerical simulations. In this range of Reynolds numbers, the heat-transfer process is unsteady and is characterized by a viscous length scale that is inversely proportional to the square root of the Reynolds number. To ensure uniform numerical accuracy for all Reynolds numbers, the dimensions of the computational mesh were selected in proportion to this viscous length scale. The small scales were resolved by at least three nodes within the boundary layers. The frequency of the heat flares increases, and the width of each flare decreases, with the Reynolds number, in proportion to the viscous time and length scales. Despite the presence of three-dimensional structures for the range of Reynolds numbers considered, the two-dimensional model captures the unsteady processes and produced results that were consistent with the available experimental data. It correctly simulated the overall, the front-stagnation and the back-to-total heat-transfer rates.

1112 直交流による円柱及び円柱群の流れ方向振動に関する研究(GS-10 流体関連振動)

1112 直交流による円柱及び円柱群の流れ方向振動に関する研究(GS-10 流体関連振動)

Experimental correlation of forced convection heat transfer from a NACA airfoil

The results of an experimental investigation of the heat transfer coefficients for forced convection from a NACA-63421 airfoil are presented. Wind tunnel measurements of convection coefficients are obtained for air flow temperatures from −30 to 20 °C. The experimental data is correlated with respect to the Nusselt and Reynolds numbers. Conduction within the airfoil balances heat transfer by convection from the airfoil surface in steady-state conditions. Both average and spatial variations of the heat transfer coefficients are non-dimensionalized through modifications of a classical Hilpert correlation for cylinders in crossflow. It is shown that the functional form of the Hilpert correlation can effectively accommodate measured data for the NACA airfoil over a range of Reynolds numbers. An uncertainty analysis is performed to yield a 7.34% measurement uncertainty for experimental data correlated with the Nusselt number.

Convective Heat Transfer in Open Cell Metal Foams

Convective heat transfer in aluminum metal foam sandwich panels is investigated with potential applications to actively cooled thermal protection systems in hypersonic and re-entry vehicles. The size effects of the metal foam core are experimentally investigated and the effects of foam thickness on convective transfer are established. Four metal foam specimens are utilized with a relative density of 0.08 and pore density of 20 pores per inch (ppi) in a range of thickness from 6.4mmto25.4mm, in increments of approximately 6mm. An exact-shape-function finite element model is developed that envisions the foam as randomly oriented cylinders in cross flow with an axially varying coolant temperature field. A fully developed velocity profile is obtained through a semi-empirical, volume-averaged form of the momentum equation for flow through porous media, and used in the numerical analysis. The experimental results show that larger foam thicknesses produce increased heat transfer levels, but that this effect diminishes for thicker foams. The finite element simulations capture the thickness dependence of the heat transfer process and good agreement between experimental and numerical results is obtained for larger foam thicknesses.

Mass Transfer and Concentration Contours around Surfaces Buried in Granular Beds and Exposed to Fluid Flow

This work describes the process of mass transfer which takes place when a fluid flows past a soluble surface buried in a packed bed of small inert spherical particles of uniform voidage. The fluid is assumed to have uniform velocity far from the buried surface and different surface geometries are considered; namely, cylinder in cross flow and in flow aligned with the axis, flat surface aligned with the flow and sphere. The differential equations describing fluid flow and mass transfer by advection and diffusion in the interstices of the bed are presented and the method for obtaining their numerical solution is indicated. From the near surface concentration fields, given by the numerical solution, rates of mass transfer from the surface are computed and expressed in the form of a Sherwood number (Sh). The dependence between Sh and the Peclet number for flow past the surface is then established for each of the flow geometries. Finally, equations are derived for the concentration contour surfaces at a large distance from the soluble solids, by substituting the information obtained on mass transfer rates in the equation describing solute spreading in uniform flow past a point (or line) source.

Normal forces on cylinders in near-axial flow

The transverse forces on submerged cylinders in nearly axial flow were studied experimentally in a towing tank. Two rigid cylinders with length to diameter ratios of 10 and 30 were towed with angles from 0∘ to 20∘ between their centerline and the incoming flow, and with a combination of forced harmonic oscillations in the normal direction and constant speeds along the cylinder axis. The most important parameters are found to be the angle of attack and a Reynolds number combining the longitudinal and normal components of the flow. The Reynolds number is important because the transition from laminar to turbulent boundary layer due to the axial component of the flow has a significant influence on the normal force. This seems to be true both for constant and oscillating angle of attack. The finite length of the cylinder and the Keulegan–Carpenter number associated with an oscillating cylinder in cross-flow are found to be less influential than expected. Time-domain simulation models for the normal force suitable for general motion of the body are discussed. For constant angles of attack their standard deviation is found to be comparable to the measurement accuracy, but in the case of forced transverse oscillations the estimated errors of the force models increase to a factor three or more of the measurement errors.