Circular Cylinder In Cross Flow Research Articles

Numerical Prediction of Flow Fields Around Circular Cylinders: Forced Motion and Dynamic Response Cases

At Re=2000, the predicted flow field around a circular cylinder in forced transverse oscillation is verified with experimental results. For coupled torsional and transverse oscillation cases, the numerical results indicate that lock-in depends on the relative phase between torsional and translational oscillations. The dynamic response of an elastically mounted circular cylinder in cross flow, obtained by solving the structural equations simultaneously with the Navier-Stokes equations, is in reasonable agreement with experimental data. The dynamic response results indicate that the change of wake pattern from 2S to 2P with increased frequency ratio, is not always simultaneous with the change in the relative phase between lift force and cylinder displacement. [S0098-2202(00)02003-4]

Vortex-induced Vibrations of a Circular Cylinder in Cross Flow at Supercritical Reynolds Numbers.

Vortex-induced vibrations were measured for a circular cylinder subjected to a water cross flow at supercritical Reynolds numbers for a wide range of reduced velocities. Turbulence intensities were changed from 1% to 13% in order to investigate the effect of the Strouhal number on the region of synchronization by symmetrical and Karman vortex shedding. The reduced damping of the test cylinder was about 0.1 in water. The surface roughness of the cylinder was a mirror-polished surface. Strouhal number decreased from about 0.48 to 0.29 with increasing turbulence intensity. Synchronized vibrations were observed even at supercritical Reynolds numbers where fluctuating fluid force was small. Reduced velocities at which drag and lift direction lock-in by Karman vortex shedding were initiated decreased with increasing Strouhal number. When Strouhal number was about 0.29, the self-excited vibration in drag direction by symmetrical vortex shedding began at which the frequency ratio of Karman vortex shedding frequency to the natural frequency of cylinder was 0.32.

Vortex Shedding Characteristics from a Stationary Circular Cylinder in Cross Flow at Supercritical Reynolds Numbers.

Fluctuating lift and drag forces by Karman vortex shedding and pressure distributions around a stationary circular cylinder were measured in a water cross flow at supercritical Reynolds numbers. The surface roughness of the cylinder was a mirror-polished surface. Turbulence intensities of the incoming filow were changed from 1% to 13% in order to investigate the effect on vortex shedding characteristics. As the turbulence intensity increased in supercritical region, flow separation location of the boundary layer on the cylinder surface moved forward, Strouhal number decreased from about 0.48 to 0.29 and the fluctuating lift force by Karman vortex shedding increased.

VORTEX-INDUCED OSCILLATION AND LIFT OF YAWED CIRCULAR CYLINDERS IN CROSS-FLOW

This paper presents the wind-tunnel test data for vortex-induced oscillations of yawed circular cylinders in cross-flow. The experiments were conducted on free oscillation models for yaw angles ofθ=0,15,30 and 45°, and for different values of reduced damping or Scruton number. The structural damping was measured on each of the models. The test Reynolds number, in the subcritical region, ranged from 8×103to 2×104. The amplitude of oscillation and the amplitude and phase angle of fluctuating lift were measured with respect to the reduced velocity. The flow velocity and force characteristics were computed by applying the “Cosine Law” for yawed cylinders. The data correlations for yawed cylinders based on the cosine law were in fair agreement with data previously presented for unyawed cylinders. The amplitude and phase angle of fluctuating lift in terms of the peak cylinder amplitude are shown. The lock-in region boundary of the reduced velocity and the peak cylinder amplitude in terms of the reduced damping are also given.

Topological structures near a heated rotating cylinder in cross flow

The heated rotating circular cylinder in cross flow is apt for a standard case in basic research of complex thermofluiddynamics. Respective investigations done at the Institute of Thermodynamics over the last decade were focused on the theoretical modeling and the experimental determination of various field properties, e.g. boundary layer separation points or the global Nusselt numbers, over a wide range of parameter combinations.

Experiments on cylinder wake stabilization in an electrolyte solution by means of electromagnetic forces localized on the cylinder surface

Electromagnetic body forces, i.e. Lorentz forces, have been used to modify the boundary layer around a circular cylinder in cross flow. Depending on the polarity of the applied electric field different effects on the flow can be obtained. Lorentz forces directed with the mean flow are able to prevent the boundary layer from separation. Therefore flow separation as well as the von Kármán vortex trail can be suppressed. When the momentum gain produced by the Lorentz forces in the boundary layer is high enough, thrust is produced, which results in a jet flow originated from the cylinder's backside. If the Lorentz forces are directed opposite to the mean flow, the separation points are shifted towards the front stagnation point, the recirculation region broadens and the von Kármán vortex trail is modified. The described technique gives a variety of opportunities to control the flow around the cylinder and the flow structure of the cylinder wake. Results from flow visualizations and numerical calculations are presented.

Heat transfer and flow pattern at a cylinder impinged by a slot jet during incipient and nucleate boiling

The heat transfer due to a submerged slot jet of water impinging on a circular cylinder in cross flow was experimentally studied in the region of incipient and fully developed nucleate boiling. The diameter of the cylinder and the width of the jet are the same—namely, 3 mm. The heat flux ranged from 4 × 10 4 to 1.5 × 106 W/m 2. The effects of the jet velocity and of the fluid subcooling were investigated in the range 0.37 < ν < 1.11 m/s and 0 < (ΔT) sub <_ 15 K both in upward and downward flow. The results showed that the influence of these parameters is limited to the region of incipient boiling, whereas, in fully developed boiling, all the boiling curves merge into a single one. A photographic study of the bubble flow pattern evidenced a different behavior with respect to cylinders in uniform cross flow.

OPEN BOUNDARY CONDITIONS FOR THE STREAMFUNCTION -VORTICITY FORMULATION OF UNSTEADY LAMINAR CONVECTION

The streamfunction-vorticity formulation is used to analyze unsteady laminar-convection problems in two-dimensional incompressible flows. The Bubnov-Galerkin finite-element method and a sequential procedure are employed to discretize and solve the governing differential equations. Very accurate results are obtained by employing “advective derivative conditions” at the outflow for all the variables involved. The boundary conditions for the streamfUnction at internal walls are imposed during the assembly process, and the vorticity at inflow and wall boundaries is evaluated in the framework of the stream/unction equation. The accuracy of the approach is demonstrated by the solution of two well-known benchmark problems concerning forced convection over a circular cylinder in cross flow and mixed convection in a plane channel heated from below.

Forced-Convection Heat Transfer from a Single Row of Circular Cylinders in Cross Flow. Correlation Equation.

Experimental investigations were conducted of heat transfer from a single row of circular cylinders, placed in the transverse direction of flows of air and water. Local and average Nusselt numbers for a cylinder at cylinder spacings L/d from 1.3 to 3.3 were measured precisely in the Reynolds number range from 102 to 5.0×104. Average Nusselt numbers are well correlated with the Reynolds numbers based on mean velocity in the flow channel between cylinders. The effect of the interaction between wake flows behind cylinders on average Nusselt number is relatively small.

Dynamic response of circular cylinders in cross-flow

This paper presents results of an experimental study conducted to evaluate the effects of an interfering stationary cylinder, located downstream, on the flow induced vibration of an upstream elastically mounted circular cylinder. The upstream cylinder was used with two different surface conditions; one relatively smooth and the other being rough. The interference effects were studied extensively by changing the diameter ratios ( d D ) of the two cylinders and the gap spacing ( g D ) between them. One additional free stream turbulence intensity level was used over chosen points to evaluate its influence over the interference effects. The single cylinder response was used as a reference to highlight the interference effects. Results indicate that the interference effect ceases for gap spacings g / D ⩾ 3–4 and of ⩾ 0.5–1.0 in the vortex-shedding and fluid elastic excitation regions, respectively; depending on the relative ratio d D . A delay in the initiation of motion was noted over that of a single cylinder which has not been previously reported. A limiting turbulence was observed beyond which excessive vibration is induced.

Analysis of Radiative-Convective Heat Transfer about a Circular Cylinder in Crossflow Using Finite Volume Radiation Solution Method

A finite volume radiation solution method was applied to a non- orthogonal coordinate system for the analysis of radiative-convective heat transfer about a circular cylinder in crossflow. The crossflow Reynolds number based on the cylinder radius was 20, and the fluid Prandtl number was 0.7. The radiative heat transfer coupled with convection was reasonably predicted by the finite volume radiation solution method. The investigation includes the effects of conduction- to-radiation parameter, optical thickness, scattering albedo and cylinder wall-emissivity on heat transfer about the cylinder. As the conduction- to-radiation parameter decreases, the radiative heat transfer rate increases and conduction rate as well due to the increase in temperature gradient on the cylinder wall which is caused by radiation enhancement. With an increase in the optical thickness, the Nusselt number increases significantly and the temperature gradient shows similar behavior. Though the radiative heat transfer increases with the scattering albedo, the total heat transfer decreases. This is because the decrease in the conduction heat transfer exceeds the increase in the radiation heat transfer. As the wall- emissivity increases, the radiation absorbed in the vicinity of the cylinder wall increases and thereby the total heat transfer increases, even though the conduction heat transfer decreases.

Mechanism of interference galloping of two identical circular cylinders in cross flow

Cylinders placed close together may be excited by a self-sustained oscillation, the so-called “interference galloping”. This phenomenon occurs at critical wind directions β crit when the wind speed exceeds a critical wind speed (onset velocity u 0) Large vibration amplitudes may be established and structural safety may be affected. This paper presents results of current experimental investigations of two identical cylinders placed in cross flow with small spacing ratios and low Scruton numbers. Ideas for a theoretical solution of the problem will be explained and discussed.

Impingement heat transfer at a circular cylinder due to a submerged slot jet of water

An experimental investigation was carried out on the heat transfer due to a submerged slot jet of water impinging on a circular cylinder in crossflow. The cylinder diameter and the slot width are of the same order of magnitude, specifically D s = 2.0 and 3.0 mm and D c = 2.5 and 3.0 mm . The experimental apparatus allowed variation of the slot width, the cylinder diameter, and the distance from nozxle exit to heater. Conditions of impingement from the bottom (ascending flow) were taken into consideration as well as impingement from above (descending flow). The Nusselt number was determined as a function of Reynolds and Prandtl numbers in the range 1.5 × 10 3 < Re < 2.0 × 10 4, 2.7 < Pr < 7.0, and 1.5 ≤ z/D s ≤ 10 . The experimental data were correlated with a simple equation that fits 90% of the data with a precision of 20%.

Effects of the length scale of free-stream turbulence and cylinder size on heat transfer in laminar separated flows

An experimental study is conducted to investigate the combined influence of length scale of free-stream turbulence, Λ, and cylinder diameter, D, on heat transfer from slightly heated circular cylinders in cross flow. By using a hot-wire anemometer, spectrum analysis of its signal with a Fourier analyzer is employed to investigate the separated shear layer formed behind the circular cylinder. Furthermore, a resistance thermometer is used to measure the time-averaged temperature behind the cylinder and the power-spectral density of the temperature fluctuation in the thermal boundary layer. The Nusselt number at the rear stagnation point varies with the relative scale Λ/D, although the Reynolds number is the same. Correspondingly, the laminar-to-turbulence transition region in the separated shear layer moves in the streamwise direction due to the change in the length scale ratio, and at the same time its variation gives a substantial increase in the power spectrum of the temperature fluctuation. It is disclosed that flow and heat transfer behind cylinders are affected by Λ/D, and are closely related with the streamwise movement of the transition region in the separated shear layer. A correlation equation including Λ/D as the parameter is derived to predict the heat transfer coefficient at the rear stagnation point.

Determination of transfer coefficients by psychrometry

This paper presents a method based on psychrometry for measuring simultaneously heat and mass transfer coefficients in the case of forced convective exchanges between air and a body surface. This method is specially well Actapted to bodies of complex shapes. The theoretical aspects are described. Errors linked to data treatments and to measurements are discussed. The method is tested for a circular cylinder in cross flow in the following conditions: 3500 < Re < 26000, Tu = 12%, T air, = 25°C. Discussion shows that errors can be reduced to less than 2% for mean transfer coefficients and less than 5% for local coefficients.

EFFECTS OF THE LENGTH SCALE OF FREE-STREAM TURBULENCE AND CYLINDER SIZE ON LOCAL HEAT TRANSFER IN LAMINAR SEPARATED FLOWS

An experimental study has been conducted to investigate the combined influence of length scale of free-stream turbulence, A, and cylinder diameter, D, on heal transfer from slightly heated circular cylinders in cross flow. By using a hot-wire anemometer, spectrum analysis of its signal with a Fourier analyzer is employed to investigate the separated shear layer formed behind the circular cylinder. A resistance thermometer is used to measure the time-averaged temperature behind the cylinder and the power-spectral density of the temperature fluctuation in the thermal boundary layer. It is found that (I) the Nusselt number at the rear stagnation point varies with the relative scale A / D, at the same Reynolds number, (2) the laminar-to-turbulence transition region in the separated shear layer moves in the streamwise direction with the change in the length-scale ratio, and (3) a streamwise shift of the transition region in the separated shear layer causes a substantial increase in the power spectrum of the temperature fluctuation. A correlation equation with A/ Das a parameter is derived to predict the heat transfer rate at the rear stagnation point.

Frost formation and heat transfer on circular cylinders in cross-flow

When humid air comes into contact with a surface whose temperature is below the dew point of water vapour in air and also below the freezing point, frost deposition takes place over the surface. Previous studies indicate that the heat transfer rate increases at the initial stages of deposition since the rough frost surface acts as a finned one. As the frost thickens, however, the insulating effect of the frost layer predominates resulting in a reduction in the heat transfer rate. This paper presents a transient model to predict the frosting process over a circular cylinder in a cross-flow of humid air. the transfer parameters are computed employing a numerical solution of the momentum, energy and diffusion boundary-layer equations along with the continuity equation, using a finite difference scheme. Empirical correlations for thermal conductivity and density are utilized for closure purposes. Model results are compared with existing experimental data and with numerical data of previous investigators and are found to agree well in the applicable temperature and humidity ranges of the frost density and conductivity correlations.

The Effects of a Wall Boundary Layer on Local Heat Transfer from a Circular Cylinder in Cross Flow.

The Effects of a Wall Boundary Layer on Local Heat Transfer from a Circular Cylinder in Cross Flow.

Convection Heat Transfer from Three Circular Cylinders to Water at Relatively Low Reynolds Numbers.

Experimental investigations were made of the measurement of heat transfer coefficients for a single row of three circular cylinders in cross flow, using a water tunnel. Average and local Nusselt numbers for the cylinders, placed at a longitudinal cylinder spacing from 2d to 5d, were measured precisely in the Reynolds number range from 70 to 8 × 103. Flow visualization was also conducted to examine the flow patterns around a cylinder. From the obtained results, it was found that heat transfer characteristics of cylinders, located downstream of the 1st cylinder, varied moderately with the change in flow pattern upstream.

Flow-Induced Vibrations in Arrays of Cylinders

There is extreme variety in flow-induced vibrations; they are observed in many diverse situations ranging from reeds in musical instruments to fluttering panels in supersonic airplanes. In order to keep the discussion within bounds, we will focus on one versatile set of simple geometries: arrays of circular cylinders in cross flow. By changing cylinder arrangement and the parameters of spacing, damping, and mass ratio, several different basic types of self-sustaining oscillations can be produced. Because of the industrial importance of tube bundles in heat exchangers, a wealth of observations and analyses has accumulated in the literature (Naudascher & Rockwell 1980). We limit ourselves to discussing cross flow because of the fundamental differences between parallel-flow instabilities and the separated-flow mech­ anisms occurring in cross flow. In parallel flow in or outside of tubes, the cylindrical walls form continuous flow boundaries which curve when there is a vibrational deflection. The dynamic for a single tube is a beam with gyroscopic equation terms added: