Cylinder Reynolds Number Research Articles

Effects of cylinder Reynolds number on the turbulent horseshoe vortex system and near wake of a surface-mounted circular cylinder

The turbulent horseshoe vortex (HV) system and the near-wake flow past a circular cylinder mounted on a flat bed in an open channel are investigated based on the results of eddy-resolving simulations and supporting flow visualizations. Of particular interest are the changes in the mean flow and turbulence statistics within the HV region as the necklace vortices wrap around the cylinder’s base and the variation of the mean flow and turbulence statistics in the near wake, in between the channel bed and the free surface. While it is well known that the drag crisis induces important changes in the flow past infinitely long circular cylinders, the changes are less understood and more complex for the case of flow past a surface-mounted cylinder. This is because even at very high cylinder Reynolds numbers, ReD, the flow regime remains subcritical in the vicinity of the bed surface due to the reduction of the incoming flow velocity within the bottom boundary layer. The paper provides a detailed discussion of the changes in the flow physics between cylinder Reynolds numbers at which the flow in the upstream part of the separated shear layers (SSLs) is laminar (ReD = 16 000, subcritical flow regime) and Reynolds numbers at which the transition occurs inside the attached boundary layers away from the bed and the flow within the SSLs is turbulent (ReD = 5 ∗ 105, supercritical flow regime). The changes between the two regimes in the dynamics and level of coherence of the large-scale coherent structures (necklace vortices, vortex tubes shed in the SSLs and roller vortices shed in the wake) and their capacity to induce high-magnitude bed friction velocities in the mean and instantaneous flow fields and to amplify the near-bed turbulence are analyzed. Being able to quantitatively and qualitatively describe these changes is critical to understand Reynolds-number-induced scale effects on sediment erosion mechanisms around cylinders mounted on a loose bed, which is a problem of great practical relevance (e.g., for pier scour studies).

Lock-exchange gravity currents propagating in a channel containing an array of obstacles

AbstractLarge eddy simulation (LES) is used to investigate the evolution of Boussinesq gravity currents propagating through a channel of height$H$containing a staggered array of identical cylinders of square cross-section and edge length$D$. The cylinders are positioned with their axes horizontal and perpendicular to the (streamwise) direction along which the lock-exchange flow develops. The effects of the volume fraction of solids,${\it\phi}$, the Reynolds number and geometrical parameters describing the array of obstacles on the structure of the lock-exchange flow, total drag force acting on the gravity current, front velocity and global energy budget are analysed. Simulation results show that the currents rapidly transition to a state in which the extra resistance provided by the cylinders strongly retards the motion and dominates the dissipative processes. A shallow layer model is also formulated and similarity solutions for the motion are found in the regime where the driving buoyancy forces are balanced by the drag arising from the interaction with the cylinders. The numerical simulations and this shallow layer model show that low-Reynolds-number currents transition to a drag-dominated regime in which the resistance is linearly proportional to the flow speed and, consequently, the front velocity,$U_{f}$, is proportional to$t^{-1/2}$, where$t$is the time measured starting at the gate release time. By contrast, high-Reynolds-number currents, for which the cylinder Reynolds number is sufficiently high that the drag coefficient for most of the cylinders can be considered constant, transition first to a quadratic drag-dominated regime in which the front speed determined from the simulations is given by$U_{f}\sim t^{-0.25}$, before undergoing a subsequent transition to the aforementioned linear drag regime in which$U_{f}\sim t^{-1/2}$. Meanwhile, away from the front, the depth-averaged gravity current velocity is proportional to$t^{-1/3}$, a result that is in agreement with the shallow water model. It is suggested that the difference between these two is due to mixing processes, which are shown to be significant in the numerical simulations, especially close to the front of the motion. Direct estimation of the drag coefficient$C_{D}$from the numerical simulations shows that the combined drag parameter for the porous medium,${\it\Gamma}_{D}=C_{D}{\it\phi}(H/D)/(1-{\it\phi})$, is the key dimensionless grouping of variables that determines the speed of propagation of the current within arrays with different$C_{D},{\it\phi}$and$D/H$.

Transient Growth of Axisymmetric Perturbations in Co-rotating Taylor-couette Flow of a Bingham Fluid

Linear transient growth of perturbation energy is studied in co-rotating Taylor-Couette flow of a Bingham fluid. The effects of yield stress on transient growth and structure of the optimal perturbation are mainly considered. For the wide gap case, the peak of the amplitude of optimal perturbation is shifted towards the inner cylinder with increasing yield stress. The anti-lift-up mechanism found in a range of inner cylinder Reynolds number Re1 is substituted by the classical lift-up view as strong yield stress is imposed. While for the narrow gap case, the peak value of the optimal perturbation is shifted towards the outer cylinder as the yield stress is increased, and the lift-up mechanism always exists.

The Influence of Different Reynolds Number on Flow across Three Tandem Cylinders with Different Diameter

Flow around a circular cylinder is a classical problem in fluid mechanics. The flowing problem about three tandem cylinders with different diameter is numerical simulated by a finite volume method. Through the simulation, the variations of flow field with different distance between the adjacent cylinder and different Reynolds number are investigated. The simulation result shows that three tandem cylinders with different diameter can evidently small columns minish width and aggrandize length of cylinder wakes compared with single cylinder.

Lattice Boltzmann simulation on liquid flow and mass transport in a bioreactor with cylinder bundle for hydrogen production

The lattice Boltzmann method is adopted to simulate hydrodynamics and mass transfer accompanying with biochemical reaction in a channel with cylinder bundle, which is the scenario of biohydrogen production by photosynthetic bacteria in the biofilm attached on the surface of cylinder bundle in photobioreactor. The effects of cylinder spacing, Reynolds number and cylinder arrangement are investigated. The numerical results reveal that highest glucose concentration and the lowest hydrogen concentration are obtained at the front of the first row cylinders for all cases. The staggered arrangement leads to an increment in average drag coefficient, Sherwood number and consumption efficiency of substrate under a given condition, and the increment in Sherwood number reaches up to 30 %, while that in drag coefficient is around 1 %, moreover, the increment in consumption efficiency reaches the maximum value of 12 %. The results indicate that the staggered arrangement is beneficial to the mass transfer and biochemical reaction.

Numerical Simulation of Flow Through Circular Array of Cylinders Using Multi-Body and Porous Models

Two two-dimensional (2D) models for flow through a circular patch of circular cylinders in a channel, the multi-body model and porous model, are established using the scale adaptive simulation (SAS) turbulent model. It is verified that the multi-body model could predict well the average velocity field, turbulent structures and vortex in all cases. The porous model could only predict well the flow pattern and the large-scale vortex motion and fails to predict the small-scale motion. For cases of high solid volume fraction or low cylinder Reynolds number, the differences in prediction between the two models are small even in the region inside the patch. For the case of low solid volume fraction, the performance of the porous model is acceptable only in the region away from the patch. If predictions are required only for large-scale motion away from the patch, the more computational efficient porous model is a good modeling tool.

Numerical investigation of fluid flow past circular cylinder with multiple control rods at low Reynolds number

Laminar flow past a circular cylinder with multiple small-diameter control rods is numerically investigated in this study. The effects of rod-to-cylinder spacing ratio, rod and cylinder diameter ratio, cylinder Reynolds number, number of control rods and angle of attack on the hydrodynamics of the main circular cylinder are investigated. Four different flow regimes are identified based on the mechanism of lift and drag reduction. The range of rod-to-cylinder spacing ratio where significant force suppression can be achieved is found to become narrower as the Reynolds number increases in the laminar regime, but is insensitive to the diameter ratio. The numerical results for the case with six identical small control rods at Re=200 show that the lift fluctuation on the main cylinder can be suppressed significantly for a large range of spacing ratio and various diameter ratios, while the drag reduction on the main cylinder is also achieved simultaneously. The six-control-rod arrangement has shown better performance in flow control than the arrangements with less control rods, especially in terms of force reduction at various angles of attack.

Scour and flow around submerged structures

The results of an experimental investigation on scour of non-cohesive uniform and non-uniform sediment beds and flow around fully submerged structures over a fixed plane bed are discussed. In the study, the submergence factor, Froude number and cylinder Reynolds number for all the experiments were kept in the ranges of 0·25–0·75, 0·23–0·70 and 13 500–58 650 respectively. The free surface profile over the submerged structures consists of distinct upper and lower layers; the lower layer is characterised by continuous mixing with the generated downstream wake whereas the characteristics of the upper layer depend on the submergence factor and the Froude number. Using observed scour depths at different times, the time variation of scour depth is scaled by an exponential law. The non-dimensional time scale decreases following a power-law relationship with an increase in densimetric Froude number. It is evident that there is a considerable reduction in the non-dimensional scour depth with an increase in the submergence ratio, mainly due to the decrease in the intensity of the horseshoe vortex. It is also evident that the normalised equilibrium scour depth decreases with an increase in the ratio of cylinder diameter to sediment size. A significant reduction of 40% of the scour depth in uniform sediment was observed in experiments with non-uniform sediments. The three-dimensional flow fields around the submerged structure over a fixed plane bed were measured with an acoustic Doppler velocimeter. The vertical velocity distributions indicate deceleration of the approaching flow, acceleration around the submerged body and the approach to the original state of open-channel flow downstream of the submerged structure.

Boundary layer dynamics at the transition between the classical and the ultimate regime of Taylor-Couette flow

Direct numerical simulations of turbulent Taylor-Couette flow are performed up to inner cylinder Reynolds numbers of Rei = 105 for a radius ratio of η = ri/ro = 0.714 between the inner and outer cylinders. With increasing Rei, the flow undergoes transitions between three different regimes: (i) a flow dominated by large coherent structures, (ii) an intermediate transitional regime, and (iii) a flow with developed turbulence. In the first regime the large-scale rolls completely drive the meridional flow, while in the second one the coherent structures recover only on average. The presence of a mean flow allows for the coexistence of laminar and turbulent boundary layer dynamics. In the third regime, the mean flow effects fade away and the flow becomes dominated by plumes. The effect of the local driving on the azimuthal and angular velocity profiles is quantified, in particular, we show when and where those profiles develop.

Compressive sensing based machine learning strategy for characterizing the flow around a cylinder with limited pressure measurements

Compressive sensing is used to determine the flow characteristics around a cylinder (Reynolds number and pressure/flow field) from a sparse number of pressure measurements on the cylinder. Using a supervised machine learning strategy, library elements encoding the dimensionally reduced dynamics are computed for various Reynolds numbers. Convex L1 optimization is then used with a limited number of pressure measurements on the cylinder to reconstruct, or decode, the full pressure field and the resulting flow field around the cylinder. Aside from the highly turbulent regime (large Reynolds number) where only the Reynolds number can be identified, accurate reconstruction of the pressure field and Reynolds number is achieved. The proposed data-driven strategy thus achieves encoding of the fluid dynamics using the L2 norm, and robust decoding (flow field reconstruction) using the sparsity promoting L1 norm.

Particle image velocimetry study of parameter influence for synthetic-jet application

Flow separation control of a circular cylinder using a synthetic jet positioned at the front stagnation point is experimentally investigated by the particle image velocimetry (PIV) technique. The control results for different excitation parameters, including the stroke length, the excitation frequency, and the momentum coefficient, are compared to distill the essential control parameters, and the influence of the cylinder Reynolds number on the control effect is discussed. The separation control mechanism for the present control configuration is also revealed. It is suggested that the effective control ability of the synthetic jet is attributed to the increment of the turbulent kinetic energy (TKE) and the dissipation rate surrounding the circular cylinder. High level of TKE enhances the dynamics of the fluids and thus flow around the leeward surface is endured a considerable vertical acceleration pointing to the centerline from both sides, which is more resistant to flow separation.

Numerical investigation of drag force on flow through circular array of cylinders

A 2-D model for flow through a circular patch with an array of vertical circular cylinders in a channel is established using the Navier-Stokes equations with a hybrid RANS/LES turbulence model–the Scale Adaptive Simulation (SAS) model. The applicability of the model is first validated by test cases where experimental data are available for comparison with the computed results. It is verified that the present model can predict well the average velocity and turbulence structure. The drag force and drag coefficient are then calculated using the present model for a number of cases with different solid volume fractions, cylinder Reynolds numbers and patch diameters. It is shown that the drag coefficient increases with increasing solid volume fraction, but decreases with increasing Reynolds number. However, the drag coefficient is independent of the diameter of circular batch when the solid volume fraction and Reynolds number are kept constant.

Immersed Boundary Method applied to single phase flow past crossing cylinders

Bubble column reactors suffer from bubble coalescence resulting in decreasing efficiency. In order to increase the mass transfer rate in bubble column reactors, a novel reactor is proposed, the micro-structured bubble column (MSBC). The new reactor makes use of a wire mesh serving the purpose of cutting bubbles into smaller bubbles. This increases bubble surface area and enhancing the mass transfer coefficient, due to the enhanced surface dynamics. The first step is understanding of fluid–structure interaction in the limiting case of single-phase flow past crossing cylinders not found in literature. To this end Direct Numerical Simulations (DNS) are performed, where the presence of the wire mesh consisting of intersecting cylinders is accounted for using an Immersed Boundary Method (IBM). The IBM is verified with existing literature data for flow past a single cylinder. Finally a new set of simulations is done for intersecting cylinders varying in cylinder diameter and Reynolds numbers. From these simulations a new drag relation is derived describing the drag as function of the Reynolds number and the pitch to cylinder diameter, p/D, ratio.

Characteristics of Turbulent Wakes Generated by Twin Parallel Cylinders

This research aims to study the characteristics of the wake generated by twin cylinders. The cylinders are arranged in parallel side-by-side and staggered arrangements. The mainstream velocity is varied between 18 m/s and 20 m/s, which are equivalent to cylinder Reynolds numbers between 4750 and 5300. The cross-stream spacing ratios are 2, 3, 4, and 6 times the cylinder's diameter for the side-by-side arrangements. For the staggered arrangement, the cross-stream and streamwise spacing were varied between 2 and 4 times the cylinder diameter. The results show that the spacing ratio s/d has a significant effect on the wake development and interactions. The wakes of the side-by-side cylinders tend to merge into a single wake for cross-stream spacing of 2d and 3d at early stations, equivalent to 15 and 30 times the cylinder diameter, respectively, and merge completely at 50 and 100 times the cylinder diameter for 4d and 6d, respectively. Velocity measurements are used to develop a correlation that relates the wake merging distance to the cylinder spacing. Turbulence measurements are used to develop a correlation between the turbulence intensity and the streamwise distance. The comprehensive survey of the results and the correlations developed are provided in order to facilitate numerical model development and evaluation.

Experimental investigation on control of vortex shedding mode of a circular cylinder using synthetic jets placed at stagnation points

Control of flow around a circular cylinder by synthetic jets has been experimentally investigated in a water tunnel with particle image velocimetry (PIV) technique. The synthetic jets are positioned at both the front and rear stagnation points. With power spectrum analysis, proper orthogonal decomposition (POD) method and other techniques for data processing, particular attention is paid to the control of vortical structures around the circular cylinder, in which the excitation frequency fe is one to three times of the natural frequency f0 and the cylinder Reynolds number and the excitation amplitude are fixed. The influenced-scope of the synthetic jet enlarges as the excitation frequency increases, and thus the synthetic jet dominates the global flow field gradually. For the natural case and the control case at fe/f0 = 1, the distributions of the first two POD modes and the power spectra for their POD coefficients all exhibit the characteristics of the natural shedding with antisymmetric mode. For fe/f0 = 2 and fe/f0 = 3, the variations in the third and fourth POD modes and the corresponding power spectra indicate that the wake vortex shedding mode changes and the dominant frequency becomes the excitation frequency. For fe/f0 = 2, the wake vortex sheds downstream with either the antisymmetric or the symmetric mode; for fe/f0 = 3, the synthetic jet vortex pair interacts with the near wake shear layers from both sides to induce a pair of the symmetric wake vortices, which is gradually converted into an antisymmetric mode when shedding downstream.

On energy transfer in flow around a row of transversely oscillating square cylinders at low Reynolds number

In this paper, the effects of cylinder spacing, cylinder oscillation frequency, amplitude of cylinder oscillations and Reynolds number on the ensuing flow regimes and energy transition for flow across a row of transversely oscillating cylinders have been studied numerically using the lattice Boltzmann method. The lift and drag coefficient signals are analyzed in detail for finding the extent of lock-on regime and wake interaction mechanism at different spacings. It is noticed that the magnitude of the mean drag coefficient is large at small spacings, which is consistent with a strong wake interaction at small spacings. The effect of wake interaction can also be noticed from the non-monotonic variation of rms lift. The average energy transfer per cylinder oscillation cycle is large when the cylinders oscillate with a frequency near to the natural vortex shedding frequency. The direction of energy transfer changes between positive and negative values with small changes in the cylinder oscillation frequency, suggesting that the direction of energy transfer is very sensitive to this parameter. It is shown that the instantaneous lift coefficient and the cylinder velocity govern the energy transfer from or to the fluid. While the different parameters affect the flow regimes, the cylinder oscillation frequency primarily governs the energy transfer.

Quantitative flow characteristics for side-by-side square cylinders via PIV

In this study, instantaneous and time-averaged flow structures downstream of the sharp-edged single and two and three side-by-side square cylinders (SCs) immersed in a uniform open channel water flow were studied by a technique of particle image velocimetry (PIV). Experimental results of wake flow structures were presented for gap ratios (G/D) in the range of 1.0�G/D�3.0 for Reynolds number values of 1050, 2450 and 3400. Flow structures depending on the square cylinder (SC) configurations and Reynolds number were discussed. It has been found that the development of the vortex shedding as well as the flow structure were substantially altered for side-by-side SCs comparing to the single SC. Asymmetrical and biased wake structures were observed because of the jet- like flow between the SCs for two SCs cases for the gap ratio less than 2.0. Depending on the gap spacing between the SCs, the interaction results of time- averaged vorticity, velocity vector field, Reynolds stress correlations 2 U v' u' and streamline patterns in the wake region form a distinguished flow structure. Strouhal numbers for the single square cylinder for 1050�Re�3400 are found in the range of 0.12-0.13. The present results have supported the previous works by providing detailed quantitative experimental information with PIV in the wake region of the SC and might be helpful for validation of numerical studies and designers.

Experimental investigation of torque scaling and coherent structures in turbulent Taylor–Couette flow

The effect of flow structures to the torque values of fully turbulent Taylor-Couette flow was experimentally studied using tomographic PIV. The measurements were performed for various relative cylinder rotation speeds and Reynolds numbers, based on a study of Ravelet et al. (2010). We confirmed that the flow structures are strongly influenced by the rotation number. Our analyses using time-averaged mean flow showed the presence of Taylor vortices for the two smallest rotation numbers that were studied. Increasing the rotation number initially resulted in the shape deformation of the Taylor vortices. Further increment towards only outer cylinder rotation, showed transition to the dominance of the small scale vortices and absence of Taylor vortex-like structures. We compared the transition of the flow structures with the curves of dimensionless torque. Sudden changes of the flow structures confirmed the presence of transition points on the torque curve, where the dominance of small and large scale vortical structures on the mean flow interchanges.

Turbulent flow field over fluvial obstacle marks generated in a laboratory flume

The study is aimed at investigating the mean flow and turbulence characteristics in scour geometry developed near a circular cylinder of length 10cm placed over the sand bed transverse to the flow. The obstacle placed on a sand bed, on the way of a unidirectional flow, develops a crescent-shaped scour mark on the bed. The scour is caused by generation of vortex developed on the upstream side of the obstacle. Sand grains eroded by this vortex, are deposited on the downstream side of the obstacle as wakes. The turbulent flow field within the scour mark was measured in a laboratory flume using an Acoustic Doppler Velocimeter (ADV). The scour marks named as current crescents preserved in geological record are traditionally used as indicators of palaeocurrent direction. The distribution of mean velocity components, turbulent intensities and Reynolds stresses at different positions of the mark are presented. The experimental evidence also shows that the geometric characteristics of the scour mark (width) depend primarily on the cylinder aspect ratio, cylinder Reynolds number and sediment Froude number.

Laminar Boundary Layer Development Around a Circular Cylinder: Fluid Flow and Heat-Mass Transfer Characteristics

This paper presents a comprehensive computational work on the hydrodynamic, thermal, and mass transfer characteristics of a circular cylinder, subjected to confined flow at the cylinder Reynolds number of Red=40. As the two-dimensional, steady and incompressible momentum and energy equations are solved using ANSYS-CFX (version 11.0), the moisture distributions are computed by a new alternating direction implicit method based software. The significant results, highlighting the influence of blockage (β=0.200–0.800) on the flow and heat transfer mechanism and clarifying the combined roles of β and moisture diffusivity (D=1×10−8–1×10−5 m2/s) on the mass transfer behavior, are obtained for practical applications. It is shown that the blockage augments the friction coefficients (Cf) and Nusselt numbers (Nu) on the complete cylinder surface, where the average Nu are evaluated as Nuave=3.66, 4.05, 4.97, and 6.51 for β=0.200, 0.333, 0.571, and 0.800. Moreover, the blockage shifts separation (θs) and maximum Cf locations (θCf−max) downstream to the positions of θs=54.10, 50.20, 41.98, and 37.30 deg and θCf−max=51.5, 53.4, 74.9, and 85.4 deg. The highest blockage of β=0.800 encourages the downstream backward velocity values, which as a consequence disturbs the boundary layer and weakens the fluid-solid contact. The center and average moisture contents differ significantly at the beginning of drying process, but in the last 5% of the drying period they vary only by 1.6%. Additionally, higher blockage augments mass transfer coefficients (hm) on the overall cylinder surface; however, the growing rate of back face mass transfer coefficients (hm−bf) is dominant to that of the front face values (hm−ff), with the interpreting ratios of h¯m−bf/h¯m=0.50 and 0.57 and h¯m−ff/h¯m=1.50 and 1.43 for β=0.200 and 0.800.