Cylinder In Cross Flow Research Articles

Passive noise control technique for suppressing acoustic resonance excitation of spirally finned cylinders in cross-flow

This paper presents an innovative passive noise control technique for suppressing flow-excited acoustic resonance of spirally finned cylinders in cross-flow. This was achieved by maintaining the number of fins, but varying the fin density along the finned cylinder’s span to create a non-uniform finned cylinder. Two non-uniform finned cylinders were investigated with pitch ratios of p(mid)/p(side) = 0.5 and 1.7. To better understand the influence of the non-uniform finned cylinders on the wake structure, bare cylinders with the same effective diameter were investigated. In the non-uniform finned cylinder case, due to the variation in the fin density, the effective diameter along its span changes. Thus, the effective diameter of a non-uniform finned cylinder happens to be a dual diameter cylinder. Three dual diameter cylinders were investigated with diameter ratios ranging between 0.5 ≤ D(mid)/D(side) ≤ 2. These diameter ratios closely correspond to the pitch ratio (p(mid)/p(side)) and effective diameter ratio (Dc(mid)/Dc(side)) of the non-uniform finned cylinders investigated. The results show that the non-uniform finned cylinderscause spatial variations in the vortex shedding process due to dissimilarities in the flow behavior at different fin density regions. Temporal variations were also revealed due to changes in the vortex shedding frequency at different spanwise regions. This resulted in a significant decrease in the spanwise flow coherence and vortex shedding strength. The spatio-temporal variations in the shed vortices at different spanwise regions observed by the non-uniform finned cylinders were found to be analogous to dual diameter cylinders with diameter ratios equivalent to the pitch ratios of the non-uniform finned cylinder. As a result, substantial attenuation in the acoustic pressure during flow-excited acoustic resonance was achieved by the non-uniform finned cylinders. The results presented in this paper show that non-uniform finned cylinders are a viable noise control technique for suppressing flow-excited acoustic resonance in tube bundles of heat exchangers.

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.

Hydrodynamic force investigation of a rigid cylinder under the coupling CF and IL motion

Extensive researches have been conducted to simulate the two-degree-of-freedom (2DOF) vortex induced vibration (VIV) of a cylinder during the past few decades. However, there are still very few publications in terms of the hydrodynamic force of a cylinder with 2DOF motion in cross-flow (CF) and in-line (IL) directions. This study employs the Reynolds-Average-Navier–Stokes (RANS) equations and shear stress transport (SST) k-ω turbulence model to investigate the hydrodynamic force characteristics. The numerical model is firstly validated based on the 2DOF VIV experiment of an elastically supported cylinder in the literature. The results indicate that the predicted displacement and hydrodynamic force are approximately harmonic, and are in reasonable agreement with the experimental data. By imposing harmonic motion on the cylinder in CF and IL directions with period ratio of 2, parametric analyses are carried out to simply broaden the understanding of the sensitivity of the hydrodynamic coefficients including force coefficient, excitation coefficient and added mass coefficient, to the motion-related parameters, such as the motion phase angle, non-dimensional amplitude and frequency. There would be a sudden change for the excitation coefficient and added mass coefficient at a certain motion phase angle approximately corresponding to the peak of the force coefficient. The varying trend of the hydrodynamic coefficient with non-dimensional amplitude is different from that obtained from one-degree-of-freedom (1DOF) forced vibration test in literatures, and is significantly related with the motion phase angle and non-dimensional frequency. The non-dimensional frequency seems to mainly affect the excitation coefficient, added mass coefficient and mean drag coefficient in the lock-in range.

Modeling Heat Transfer and Skin Friction Frequency Responses of a Cylinder in Cross-Flow—a Unifying Perspective

ABSTRACTThe dynamic behavior of skin friction and heat transfer of a cylinder in pulsating cross-flow is investigated. Existing analytical solutions are presented as transfer functions versus frequency, known from control theory. New results for Reynolds number ranges where no appropriate model exists until now are derived. These results are obtained by the combination of computational fluid dynamics and system identification (CFD/SI). In the CFD/SI approach, time series for fluctuations of skin friction and the rate of heat transfer are generated by imposing broad-band inlet velocity fluctuations in a CFD simulation of laminar flow across a cylinder. Direct numerical simulations are conducted for mean flow Reynolds numbers between 0.1 and 40, solving the incompressible Navier–Stokes equations in a two-dimensional domain using a finite volume approach. The SI framework allows to estimate transfer functions for the response of skin friction and heat transfer to velocity fluctuations from time series data. Available analytical models for the dynamic behavior of skin friction and heat transfer usually match the simulated data up to a point, but do not give any dependence on Reynolds number. This shortcoming is addressed in this work. The identified models especially excel at Reynolds numbers of order 10.

Non linear model of rotational galloping of square, rectangular and bundle cylinder in cross-flow

The occurrence of rotational galloping for several geometries is assessed through two-dimensional flow simulations and fluid-structure interaction simulations. A finite element formulation specif­ically devised for fluid-structure interactions simulations has been used. Non-linear models aimed at predicting rotational galloping are determined based on cross-flow simulations around fixed cylinders for various angles of attack. The added torsional fluid damping coefficient is modelled based on the results of forced rotational oscillation simulations. The reliability of the models is assessed by confrontation with the results of free rotation simulations, where reduced velocity and maximum amplitude of galloping have been chosen as comparison criteria, when they can be measured. The quasi-steady model is shown to have strong limitations on stream­lined bodies. An extended quasi-steady model is introduced. This model does not consider history effects and considers damping up to second order. Contrarily to the previous model, this improved model can precisely predict unstable zones for a variety of section shapes. The torsional galloping of square, rectangular and bundle shape cross sections have been studied.

A comprehensive investigation of vortex induced vibration effects on the heat transfer from a circular cylinder

In this paper, the effect of vortex induced vibration (VIV) on convective heat transfer from an elastically mounted rigid circular cylinder in cross-flow is investigated numerically. The motion of cylinder is modeled by using a mass-spring-damping system. The effect of reduced velocity and damping ratio on the vortex formation, vortex shedding process, cylinder displacement amplitude, Nusselt number and the position of maximum local Nusselt number is studied. In this study the Reynolds Number and the mass ratio are 150 and 2, respectively. Furthermore, different values of reduced velocity and damping ratio are investigated (Ur= 3, 4, 5, 6, 7, 8 and ζ= 0. 0.01, 0.05, 0.1). The numerical results demonstrate that the reduced velocity and damping ratio can affect the heat transfer considerably. The beating phenomenon is occurs at Ur= 4 and ζ= 0.05 which leads to changes in the displacement amplitude and Nusselt number widely with respect to the time. In the beating phenomenon the total Nusselt number decreases in comparison with a stationary cylinder. The maximum heat transfer enhancement is obtained at Ur=4, ζ=0.

Flow-induced vibration of three unevenly spaced in-line cylinders in cross-flow

The flow over three cylinders arranged in-line with uneven spacing in cross-flow is numerically investigated at a Reynolds number of 200. The center-to-center distance between the upstream and the most downstream cylinder was kept constant at 4 diameters. The distance between the inline centers of the upstream and middle cylinders (x) was studied in the range of 1.05 to 2.95 diameters. The flow structure around the cylinders shows two distinctive patterns of vortex shedding, one of which occurs at x=1.60 and 1.80 diameters, and causes significantly higher oscillating lift and mean drag forces with a lower value of Strouhal number. At these middle cylinder locations, the shear layer reattaches on the downstream cylinder and results in vortex formation in the gap between the middle and downstream cylinders. On the other hand, at all other locations of the middle cylinder the shear layer does not reattach on the downstream cylinder. Moreover, when the middle cylinder is located at x=1.60 or 1.80 diameters, the vortex formation length is shorter and the vorticity in the downstream vortex street is higher than the case when the middle cylinder is located at all other locations. To understand the effect of these distinctive flow features on the flow-induced vibration response of the most downstream cylinder, the coupling between the flow field and the cylinder motion is numerically modeled. As the reduced velocity is increased, the oscillation amplitude of the downstream cylinder increases to its maximum at lock-in, then it starts to decrease similar to the case of a single cylinder. However, the oscillation amplitude reaches a higher value than that observed for the case of vortex-induced vibration of both a single cylinder and two tandem cylinders. Moreover, significant oscillation amplitude is pertained at high reduced velocities with dependency on the uneven spacing between the cylinders. The frequency of oscillations increases at different rates in three different ranges of reduced velocities, indicating different energy transfer mechanisms. Although the frequency of vibration is shifted away from the structure resonance frequency at high reduced velocities, the significant amplitude of oscillations is attributed to the interaction of two opposing mechanisms that occur when the cylinder is at different positions in the cycle. The wide range of reduced velocities at which the cylinder vibrates with a significant amplitude makes the configuration of three in-line cylinders a good candidate for flow energy harvesting.

Numerical Characterisation of Active Drag and Lift Control for a Circular Cylinder in Cross-Flow

Synthetic jet actuators have shown promise to control drag and lift for a bluff body in cross-flow. Using unsteady RANS CFD modelling, a significant modification of the drag coefficient for a circular cylinder in cross-flow at R e = 3900 is achieved by varying the actuation frequency. The variation in actuation frequency corresponds to a range in Stokes number of 2.4 < S t o < 6.4. The trends in drag coefficient modification largely agree with the findings of past publications, achieving a maximum drag reduction at S t o = 4.9 for a fixed jet Reynolds number of the synthetic jet of R e U ¯ o = 12. A decrease in the adverse pressure gradient near the jet orifice correlated with a momentum increase in the viscous sublayer and stronger vortical structures at the rear of the cylinder. In these same conditions, a decrease in turbulence intensity was observed in the far field wake, which is a relevant finding in the context of wind and tidal turbine arrays.

HF DBD plasma actuators for reduction of cylinder noise in flow

Surface high frequency dielectric barrier discharge (HF DBD) was used to reduce flow-induced noise, radiated by circular cylinder in cross flow. Effect of HF DBD actuators is studied for flow velocity up to 80 m s−1 (Reynolds numbers up to 2.18 · 105), corresponding to the typical aircraft landing approach speed. Noise measurements were performed by microphone array in anechoic chamber; averaged flow parameters were studied by particle image velocimetry (PIV). Actuator was powered by high-frequency voltage in hundreds kHz range in steady or modulated mode with the modulation frequency of 0.3–20 kHz (Strouhal number St of 0.4 to 20). It is demonstrated that upstream directed plasma actuators are able to reduce the vortex noise of a cylinder by 10 dB. Noise reduction is accompanied by significant reorganization of the wake behind a cylinder, decreasing both wake width and turbulence level. The physical mechanism related to broadband noise control by HF DBD actuator is also discussed.

Fluid flow and mixed convective heat transfer around a semi-circular cylinder at incidence with a tandem downstream square cylinder in cross flow

The present paper presents fluid flow and mixed convective heat transfer characteristics past a semi-circular cylinder at incidence with a tandem square cylinder in cross flow. Using air (Pr=0.71) as an operating medium, numerical simulations are performed for the range of Reynolds numbers, 10≤Re≤45 and incidence angles, 00≤α≤1800. The effect of thermal buoyancy is brought about by varying the Richardson number in the range 0≤Ri≤2. The mathematical model is firstly validated with the experimental and numerical results from the literature and found to be in good agreement. The steady separated flow is observed to become unsteady periodic under the superimposed thermal buoyancy. Furthermore, apart from thermal buoyancy effect, angle of incidence is found to play a pivotal role in bringing hydrodynamic instabilities and thereby vortex shedding for such steady mixed convective flows. Functional dependence of drag (CD), lift (CL), and moment (CM) coefficients on the combined influence of α and Ri, is explored and analysed in detail. Additionally, other global quantities, such as local and average Nusselt number distribution, Strouhal number (St) are determined with respect to the various ranges of parameters considered in the present investigation.

Turbulence intensity effects on heat transfer and fluid-flow for a circular cylinder in crossflow

The intrinsically unsteady heat transfer on the surface of a cylinder in crossflow has been investigated in detail by numerical simulation as a function of the freestream turbulence intensity and the Reynolds number. After a brief startup transient, a periodic steady state is established at all circumferential locations. The resulting timewise fluctuations were seen to be of different phase depending on where on the circumference they occur. On one side of the cylinder, maxima occurred at the same moment in time as minima occurred on the other side. This finding and comparisons of the magnitudes of the local heat transfer coefficients showed that side-to-side symmetry does not prevail in the presence of the unsteadiness. The fluctuation frequencies were found to be virtually uniform over the entire circumference of the cylinder and varied only slightly with the Reynolds number and the turbulence intensity. The full slate of results included: (a) timewise and circumferential variations of the local heat transfer coefficient, (b) timewise variations of the all-angle spatial-averaged heat transfer coefficient, (c) spatial variations of the timewise-averaged heat transfer coefficient, (d) spatial- and timewise-averaged heat transfer coefficients as a function of turbulence intensity and Reynolds number, (e) timewise fluctuation frequencies, (f) comparisons with the experimental literature, and (g) effect of the selected turbulence model. As expected, the magnitude of the heat transfer coefficient increases as the turbulence intensity increases.

Fines erosion: Turbophoresis can be harmful

Enabling turbulence dispersion in a computational fluid dynamics simulation of a developing liquid–solid pipe flow leads to a focus of low-Stokes number particles around the pipe axis. This phenomenon is found to concentrate their impacts on a centrally located target surface such that a local erosion spot develops. This result is counter-intuitive as low-Stokes particles are deemed to follow the carrier average streamlines going around the body, which diminishes their probability of impact. This is nevertheless a fact reported experimentally too. Analysis of the simulations reveals that turbulence tends to drive preferentially tiny particles from areas of high to low agitation. This phenomenon is sometimes named as turbophoresis. Long straight piping systems are typical candidates for turbulent pipe flows hosting an annular zone of turbulence that tends to disperse and concentrate fines towards the axis. At the approach of a body, like a cross-flow cylinder, particles may be somewhat re-scattered away by the carrier dragging them around. As such, this turbulence dispersion effect on fines concerns various industrial solid transport systems. Fine impacts concentration is likely to create unexpected, local wear zone.

Hybrid Reynolds-Averaged Navier–Stokes/Large-Eddy Simulation Closure for Separated Transitional Flows

The numerical prediction of transition from laminar to turbulent flow has proven to be an arduous challenge for computational fluid dynamics, with few approaches providing routine accurate results within the cost confines of engineering applications. The recently proposed transition model shows promise for predicting attached and mildly separated boundary layers in the transitional regime, but its accuracy diminishes for massively separated flows. In this effort, a new turbulence closure is proposed that combines the strengths of the local dynamic kinetic energy model and the widely adopted transition model using an additive hybrid filtering approach. This method has the potential for accurately capturing massively separated boundary layers in the transitional Reynolds number range at a reasonable computational cost. Comparisons are evaluated on several cases, including a transitional flat plate, NACA 63-415 wing, and circular cylinder in crossflow. The new closure captures the physics associated with a separated wake (circular cylinder) across a range of Reynolds numbers from 10 to 2 million () and performed significantly better in capturing performance and flowfield features of engineering interest than existing turbulence models. The transitional hybrid approach is numerically robust and requires less than 2% extra computational work per iteration as compared with the baseline Langtry–Menter transition model.

Water Evaporation and Condensation in Air With Radiation: The Self-Similar Spalding Model

Several simple ways of improving the accuracy of Spalding model predictions over common textbook conventions for air/water evaporation/condensation problems are illustrated using open-literature examples. First is the choice of thermodynamic reference state for enthalpy evaluation. The common practice of choosing the steam table reference point (0.01 °C) with water-vapor enthalpy of hfg (2501 kJ/kg) and air enthalpy of zero introduces an enthalpy mismatch between air and water vapor that unnecessarily compromises accuracy. Choosing the air/water interface temperature as the reference point and setting both air and water-vapor enthalpies at this point to the same numerical value gives the most accurate results of several methods tried. Second is judicious choice of the blowing factor in high-rate mass transfer situations. The laminar boundary layer blowing factor is more accurate than the common stagnant-film (Couette flow) blowing factor for flat-plate flow and may be more accurate for a cylinder in crossflow under laminar conditions, as illustrated by the example of air leak effect on steam condenser tube performance. Third is radiation modeling, often a problematic or ignored feature in this type of problem. Two common, but opposite, assumptions about radiation participation in water—transparent interface and opaque interface—are shown to be equivalent for most purposes. A methodology is introduced for modeling true interfacial absorption/emission associated with phase change if/when the amount of this effect becomes known well enough to justify its inclusion. The importance of including radiation is illustrated by several examples: cloud droplet evaporation–condensation, sweat cooling, and the wet-bulb psychrometer. Fourth is inaccuracy introduced by unnecessarily setting Lewis number to unity.

Flow pattern and heat transfer past two tandem arranged cylinders with oscillating inlet velocity

Unsteady laminar convective fluid flow and heat transfer past two cylinders in tandem arrangement is investigated. Besides forced mixing by the natural vortex streets, extra imposed oscillating inlet velocity are considered. The lift and drag coefficients and Strouhal number for a single cylinder and their dependence on Reynolds number are also provided to validate the calculations with previous experiments. Different frequencies and amplitudes of the oscillating flows are set as inlet boundary conditions. For the cycle effect, at some specified points of the cycle ratios, the heat transfer enhancement of the upstream cylinder and the downstream cylinder are relatively larger due to the combined-effect of the natural vortex streets and the imposed oscillating inlet flows. The interaction between the natural vortex streets and imposed oscillating inlet flow leads to non-regular shaped vortices with increased flow mixing. For the amplitude effect, Nusselt numbers of the two cylinders are both greatly increased with increased amplitude ratios. With increased amplitude ratios, the vortex street expands and vibrates in a larger region which leads to stronger flow mixing and higher heat transfer. Generally, oscillating inlet flows can efficiently increase the heat transfer of two cylinders in crossflow without increasing the flow rate of the coolants.

Laminar forced convection from a rotating horizontal cylinder in cross flow

The influence of non-dimensional rotational velocity, flow Reynolds number and Prandtl number of the fluid on laminar forced convection from a rotating horizontal cylinder subject to constant heat flux boundary condition is numerically investigated. The numerical simulations have been conducted using commercial Computational Fluid Dynamics package CFX available in ANSYS Workbench 14. Results are presented for the non-dimensional rotational velocity α ranging from 0 to 4, flow Reynolds number from 25 to 40 and Prandtl number of the fluid from 0.7 to 5.4. The rotational effects results in reduction in heat transfer compared to heat transfer from stationary heated cylinder due to thickening of boundary layer as consequence of the rotation of the cylinder. Heat transfer rate increases with increase in Prandtl number of the fluid.

Prediction of convection from a finned cylinder in cross flow using direct simulation, turbulence modeling, and correlation-based methods

ABSTRACTDirect numerical simulation (DNS), two shear-stress transport (SST) turbulence models, and three k-ε models are used to predict mixed convection associated with air in cross flow over an isothermal, finned cylinder. The DNS predictions reveal complex time-variation in the flow field. Convection heat transfer coefficients predicted by the SST models are in good agreement with those generated by DNS, whereas the k-ε models do not accurately predict heat fluxes. Correlation-based predictions of heat transfer coefficients are, in general, in poor agreement with the DNS and SST predictions. The impact of various geometrical modifications on convection coefficients is also presented.

Heat transfer in production and decay regions of grid-generated turbulence

Heat transfer measurements around the centreline circumference of a cylinder in crossflow are performed in a wind tunnel. The cylinder is placed at several stations downstream of three turbulence-generating grids with different geometries and different blockage ratios σg: a regular grid (RG60) with σg=32%, a fractal-square grid (FSG17) with σg=25% and a single-square grid (SSG) with σg=20%. Measurements are performed at 20 stations for 3 nominal Reynolds numbers (based on the diameter D of the cylinder) Re∞=11,100,24,500,37,900. Hot-wire measurements are performed along the centreline, without the cylinder in place, to characterise the flow downstream of the grids. The extent of the turbulence production region, where the turbulence intensity Tu increases with the streamwise distance x from the grid, is higher for SSG and more so for FSG17 than for RG60. The angular profiles of the Nusselt number Nu are measured in the production regions of these two grids and are compared to those obtained in the decay regions, where Tu decreases with x. This comparison is made at locations with approximately same Tu. It is found that, for SSG, Nu/Re0.5 on the front of the cylinder (boundary layer region) is lower in the production region than in the decay region. This is explained by the presence of clear and intense vortex shedding in the production region of SSG which reduces the turbulent fluctuations which are “effective” in enhancing the heat transfer across a laminar boundary layer. For higher Re∞, the values of Nu/Re0.5 on the front of the cylinder are higher in the production region of FSG17 than in that of SSG, despite Tu being higher for SSG. This is consistent with a lower intermittency of the flow for FSG17 caused by the presence of the fractal geometrical iterations. The recovery of Nu on the back of the cylinder (wake region) is appreciably higher in the production region than in the decay region for both FSG17 and for SSG. This can be due to the lower integral length scale ratio Lu/D in the production region and suggests, for the same Re∞, a reduction of the vortex formation length downstream of the cylinder, possibly promoted by the interaction between the wakes of the bars of the grid and the wake of the cylinder. At a large distance from the grids, the heat transfer enhancement is higher and it is more efficient for FSG17 and for SSG than for RG60. For high values of x in the turbulence decay region of the grids, the values of Nu‾ (circumferential average of Nu) are similar for FSG17 and for SSG and they are both appreciably higher than for RG60. This happens despite both FSG17 and SSG having a lower blockage ratio than RG60. The use of FSG17 has the practical advantage of combining high heat transfer rates on the cylinder with a weak vortex shedding from the grid.

Heat Transfer from a Cylinder in Cross-Flow of Single and Multiphase Flows

Heat Transfer from a Cylinder in Cross-Flow of Single and Multiphase Flows

Modulating flow and aerodynamic characteristics of a square cylinder in crossflow using a rear jet injection

The temporally evolved flow behaviors around a square cylinder subject to modulation of a planar jet issued from the cylinder’s downstream surface into the wake were studied using the laser-assisted smoke flow visualization method and synchronized hot-wire anemometers. The drag force asserted on the square cylinder was obtained by measuring the surface pressures. Four characteristic flow modes (wake-dominated, transitional, critical, and jet-dominated) were observed in different regimes of freestream Reynolds number and jet injection ratio. In the wake-dominated mode, the jet swung periodically back and forth on the downstream surface due to the wake vortex shedding. In the transitional mode, the vortex shedding in the wake vanished so that the flow around the cylinder presented no periodic oscillations. In the critical mode, the wake width became smaller and therefore made the vortex shedding frequency larger than that observed in the wake dominated mode. In the jet-dominated mode, the jet had a large momentum that entrained wake fluids and therefore stabilized the instabilities of the wake, separated boundary layers on lateral surfaces, and stagnation point on the upstream surface. Two standing vortices appeared in the near wake beside the high-momentum jet. The width of the wake was decreased substantially by jet entrainment. The drag coefficient decreased with an increase in the jet injection ratio. The downstream surface jet injection caused the pressure coefficients to decrease at the upstream surface and to increase at the downstream surface. Therefore, the drag coefficients were decreased significantly by 26%, 33%, and 38% at the injection ratios of 0.5, 1.5, and 2.5, respectively.