Cylinder Reynolds Number Research Articles

Influence of Reynolds number and rotational number on the features of a transitional flow in short Taylor-Couette cavity

The paper reports on the DNS results of the flow in co- and counter-rotating coaxial cylinders of aspect ratios Γ = H/(R2 − R1) between 3.8 and 4.05, and radius ratio η = R1/R2 = 0.5, with the end-walls rotating with the angular velocity of the inner cylinder Ω1. The computations are performed for a wide range of rotational number RΩ = (1 − η)(Re1 + Re2)/(ηRe2 − Re1), from − 1.069 to 0.0, which includes both the linearly unstable flows and the Rayleigh stable flows. The considered Reynolds numbers of the inner cylinder Re1 = Ω1R1(R2 − R1)/ν are up to 3000 (Re2 = Ω2R2(R2 − R1)/ν). The obtained flow structures appearing at various stages of the laminar-turbulent transition and the radial profiles of statistical parameters are discussed in the light of the data published by other authors. The critical bifurcation lines are determined as functions of the inner and outer cylinder Reynolds numbers. Many interesting phenomena have been found.

Simulation of laminar to transitional wakes past cylinders with a discontinuous Galerkin inviscid shallow water model

Laminar to transitional wakes occur in slow, quasi-steady flows past cylinders at low cylinder Reynolds numbers (R ed ≤ 250). Inviscid numerical solvers of the depth-averaged shallow water equations (SWE) introduce numerical dissipation that, depending on Red , may imitate the mechanisms of viscous turbulent models. However, the numerical dissipation rate in a second-order finite volume (FV2) SWE solver is so large at a practical resolution that this can instead hide these mechanisms. The extra numerical complexity of the second-order discontinuous Galerkin (DG2) SWE solver results in a lower dissipation rate, making it a potential alternative to the FV2 solver to reproduce cylinder wakes. This paper compares the DG2 and FV2 solvers, initially for wake formation behind one cylinder. The findings confirm that DG2 can reproduce the expected wake formations, which FV2 fails to capture, even at a 10-fold finer resolution. It is further demonstrated that DG2 is capable of reproducing key features of the flow fields observed in a laboratory random cylinder array.

Numerical investigation of mixed convection from rectangular cylinders subjected to upper cross flow

Unsteady combined natural and forced convection heat transfer from a rectangular cylinder subjected to upward cross flow is investigated numerically. The working fluids are air (Pr = 0.7) and water (Pr = 7), and the flow is laminar. The governing flow and energy equations are solved using a fractional step finite-difference scheme. The effects of the cylinder aspect ratio (a = L/D), Reynolds and Richardson numbers on the flow and heat transfer characteristics are investigated. Numerical simulations were carried out for the cylinders with aspect ratios of a = 0.5, 1, and 2, while the Richardson and Reynolds numbers varied between 0 to 10, and 100 to 200, respectively. The mean drag, mean rms lift coefficients, as well as the Strouhal number and the mean Nusselt number were computed for each simulation to determine the quantitative effects on flow and heat transfer. Steady flow is observed for Pr = 0.7, Ri = 0.5, and a = 0.5 or for Re ≥ 150 and Ri ≥ 1. For Ri = 10 and Re = 200, the flow becomes unsteady in the downstream region and trigger vortex street formation for a = 0.5, 1, and 2. Decreasing the aspect ratio, as well as increasing Reynolds number, increases the heat transfer from the cylinder. The rms lift coefficient decreases sharply at Ri < 1 for Pr = 0.7, Ri < 2 for Pr = 7 due to vortex breakdown. Further increase in the Richardson leads to a slight increase in rms lift coefficient except for Re = 200 where the sharp increase is observed due to triggered vortex formation again. For air, the increase in the mean Nusselt number in Ri = 10 case with respect to the forced convection case (Ri = 0) was 54–65% and 80–98%, respectively, for a = 0.5 and 2, while these values for water were realized as 50–68% and 53–63%, respectively. Using the computed data and nonlinear regression, the mean Nusselt number and the drag coefficients were developed.

Local Scour Patterns around a Bridge Pier with Cable-Wrapping

The performance of cable flow-altering bed scour countermeasures was experimentally evaluated based on the scour reduction, bed morphology, and the effects on the flow field. An unprotected 40 mm diameter pier was compared to piers protected with spiral cables (2, 4, 6, 8 and 10 mm diameters) wrapped at a 15-degree angle for two-bed sediment sizes with median grain sizes of 0.86 and 1.83 mm, for a cylinder Reynolds number of 7120. The scour depth was reduced by the cables by up to 52 percent compared to the unprotected pier case, a reduction that increased with increasing cable diameter for both sediment beds. Scour depth and sediment deposition varied by sediment size, where the scour hole was up to 45 percent deeper for the finer sediment bed than that of the coarser bed. Velocity and turbulence statistics showed that cables attenuated the flow within the scour hole by diminishing the downflow and horseshoe vortex, whereas in the case of finer sediment, spatially averaged turbulent kinetic energy and Reynolds shear stresses were respectively up to 1.4 and 1.8 times higher for the unprotected pier than the protected pier, resulting in scour depth reduction. The presence of the cable also reduced the vortex shedding frequency in the pier wake as indicated by a Strouhal number of around 0.175. The results demonstrate the potential of cable threading as a flow-altering scour countermeasure to reduce bridge pier scour.

Observations of pumping and vortex dynamics due to a cylinder oscillating normal to a plane wall

Understanding the fluid dynamics associated with a circular cylinder oscillating normal to a plane wall is important for safe design of offshore infrastructure, such as power cables and pipeline risers. This paper investigates the fluid dynamics of an oscillating cylinder with no imposed incident current experimentally using flow visualisation and force measurements where the ratio of the cylinder Reynolds number ( $Re$ ) to Keulegan–Carpenter number ( $KC$ ) is $\beta =500$ and $KC$ varies between 2 and 12. The minimum distance between the cylinder and wall was between 12.5 % and $50\,\%$ of the diameter. Across this parameter space three primary vortex flow regimes were observed: (i) for $KC\leq 5$ , the flow field is approximately symmetric about the cylinder centreline and the velocity field between the cylinder and the wall resembled a pumping flow in phase with cylinder motion, which is well predicted by potential theory for most of the cycle; (ii) for $5&lt; KC&lt;8$ , the flow field is increasingly asymmetric but with frequent switching of the side associated with vortex shedding; and (iii) for $KC\geq 8$ , the flow field is consistently asymmetric due to vortex shedding. The in-line force increases when the cylinder is near the wall due to dynamic pressures associated with pumping. This increase can be estimated using potential theory superimposed onto the force time history for an isolated cylinder at the same $KC$ and $Re$ . This study complements recent numerical modelling focused on low Reynolds number conditions and provides important insights into the fluid mechanics associated with trenching beneath cable and pipeline risers.

Effects of bed roughness on a horseshoe vortex in overland water flowing past a cylinder

A horseshoe vortex (HV) system is formed around a bed-mounted circular cylinder in overland water flowing past the cylinder; however, the effect of bed roughness on HV still lacks clear recognition. The present study experimentally investigated the characteristics of HV upstream of a bed-mounted circular cylinder in a flume, where sand of four grain sizes was glued to the flume bed to produce the bed roughness. A particle image velocimetry system was mounted on the flume to facilitate a high-resolution (63 pixel/mm) observation of a flow field. A total of 28 cases with flow depths varying from 0.39 to 1.09 cm and cylinder Reynolds numbers (ReD) varying from 2700 to 5800 were selected to evaluate the effects of the bed roughness on HV in overland flow conditions. Measurements were then conducted on the upstream flow of a cylinder in the symmetry plane. Bed roughness significantly affected HV, in turn destroying the three vortices. Subsequently, time-averaged primary HV features on the three beds with lower roughness levels were analyzed in terms of the separation point, location, radius, and swirling strength. With an increase in the bed roughness, the separation point moved toward the cylinder, and the primary HV moved toward a free surface; however, the effect on the streamwise location was nonsignificant. A positive relationship was observed between the radius of the primary HV and the bed roughness, whereas a negative relationship was observed between the swirling strength and bed roughness. For beds with the highest roughness levels, all vortices were destroyed, and there was no clear HV topology in the time average sense. Finally, the motion process of the HV was summarized as showing an increase in the bed roughness. The present study investigated HV features with different bed roughness levels and has the potential to enhance our theoretical understanding of the relationship between HV kinematic characteristics and the bed roughness.

On the behavior of nonlinear hydrodynamic coefficients of a submerged cylinder beneath the water surface

This study aims at numerically exploring the behavior of flow fields and nonlinear hydrodynamic coefficients of a horizontal cylinder beneath the free surface flow considering the effects of nonlinear surface waves and various cylinder shapes. The computational model is based on two-dimensional incompressible Navier-Stokes solvers along with the treatment of the free surface flow using the volume of fluid method. The effect of the turbulent flow is also considered by using the shear stress transport turbulence model. The simulation result of a benchmark case study of the submerged cylinder is first validated with available experiment data, where a mesh convergence analysis is also performed. Afterward, the flow fields and hydrodynamic force coefficients around the cylinder surface are analyzed, and the influences of various cylinder shapes and Reynolds numbers on the hydrodynamic coefficients are investigated. A state diagram representing the hydrodynamic behavior including stable and unstable stages is finally proposed; this is an important criterion for the practice design of submerged civil structures under the free surface flow.

Self-Preservation of Turbulence Statistics in the Wall-Wake Flow of a Bed-Mounted Horizontal Pipe

This research article analyzed the self-preserving behaviour of wall-wake region of a circular pipe mounted horizontally over a flat rigid sand bed in a shallow flow in terms of mean velocity, RSS, and turbulence intensities. The study aims to investigate self-preservation using appropriate length and velocity scales.in addition to that wall-normal distributions of the third-order correlations along the streamwise direction in the wake region are analyzed. An ADV probe was used to record the three-dimensional instantaneous velocities for four different hydraulic and physical conditions corresponding to four cylinder Reynolds numbers. The results revealed that the streamwise velocity deficits, RSS deficits, and turbulence intensities deficits distributions displayed good collapse on a narrow band when they were non-dimensionalized by their respective maximum deficits. The wall-normal distance was non-dimensionalized by the half velocity profile width for velocity distributions, while the half RSS profile width was used in the case of the RSS deficits and turbulence intensities deficits distributions. The results indicate the self-preserving nature of streamwise velocity, RSS, and turbulence intensities in the wall-wake region of the pipe. The third-order correlations distributions indicate that sweep is the dominant bursting event in the near-bed zone. At the same time, ejection is the dominant bursting event in the region above the cylinder height.

Polyelectrolyte solutions in Taylor-Couette flows

Taylor-Couette (TC) flow is ideal for studying the flow behavior of complex solutions due to the wide variety of available hydrodynamic flow states. For non-Newtonian solutions, the presence of polymers in solution changes the solution rheology, which in turn modifies accessible flow states. While significant prior work exists on the effects of elasticity and shear thinning of polymer solutions on TC flow, the effects of changes in polymer chain conformation on these properties and the resultant changes in the TC flow states have not been extensively studied. Here, we have explored the effects of changing polymer chain conformation of polyelectrolyte solutions on laminar and turbulent TC flow by using a quasi-static ramp protocol to vary the inner cylinder rotation rates. The ionic strength of the cationic polyacrylamide (CPAM) solution was varied to modify the equilibrium polymer equilibrium conformation and solution rheological properties. In general, as the increasing solution ionic strength increases polymer chain flexibility and decreases elasticity and degree of shear thinning, there is a shift toward more Newtonian-like flow behavior. Additionally, the effects of co- and counter-rotation of the cylinders on the stability of flow states were observed as a function of solution ionic strength, and phase diagrams of the resulting flow states were mapped as a function of the inner and outer cylinder Reynolds numbers. It was found that co-rotation of the cylinders stabilizes some of the polymer-influenced flow states, while counter-rotation de-stabilizes flow states.

Optimal position of square cylinder vortex generator in channel is not a function of ordinate only – A heat transfer and optimization study

Wall heat transfer enhancement of a channel by introducing an adiabatic square cylinder as a vortex generator is investigated numerically and reported. An open-source computational fluid dynamics toolbox, OpenFOAM is used for the present numerical investigation where a base solver icoFOAM is modified by incorporating the energy equation for analyzing the heat transfer characteristics. The cylinder position; axial (in the direction of flow) and transverse (normal to the flow direction) both the locations are varied to analyze the effect of these parameters on the wall heat transfer performance of the channel. In addition, the effect of change of channel length and height along with Reynolds number on the heat transfer characteristics are investigated. The present result indicates that the position of an adiabatic cylinder for enhancing the wall heat transfer in a channel may not be presumed a priori. It is observed that axial and transverse location of cylinder has significant influence on the enhancement of heat transfer rate. An optimum location of cylinder is observed based on maximum Nusselt number. Further, the Nusselt number is noticeably affected by the channel length and height along with Reynolds number. Thus, the location of a cylindrical vortex generator in a channel needs to be optimized for maximum thermal performance and cannot be ascertained a priori. For this purpose, a correlation of thermal performance with cylinder location, channel dimension, and Reynolds number is presented in the present study, and then the optimized location of the adiabatic cylinder is found for maximum thermal performance by using Particle Swarm Optimization (PSO).

Higher-Order Turbulence Around Different Circular Cylinders Using Particle Image Velocimetry

Abstract Wakes and higher-order turbulence around circular cylinders of different diameters are investigated using particle image velocimetry measurements. The cylinder Reynolds number (Red) is defined by the depth-averaged velocity, cylinder diameter, and kinematic viscosity. The influence of Red on third-order moment of velocity fluctuations, turbulent kinetic energy (TKE) flux, TKE budget, and turbulent length scales are presented. The intermittency factor (IF) and quadrant analysis around the cylinders are discussed for Red values ranging from 12,600 to 21,000. The energy budget mostly near free surface upstream and near-bed downstream is changed due to Red. Stream-wise and transverse TKE fluxes decreased significantly with Red, but not the vertical flux. Stream-wise skewness changed sign from positive to negative at a certain critical distance downstream, while vertical skewness showed reversed effect with Red. Negative stream-wise skewness and positive vertical skewness indicate the occurrence of ejection events. On the downstream, the production and dissipation rates increased with Red, whereas an opposite trend was observed for dissipation only on the upstream. Increase of Red led to an increase in the Kolmogorov length-scale much higher on the upstream than the downstream, whereas it showed depleting effect on Taylor's length-scale on the upstream and an increasing effect on the downstream. Ejection and sweep events were more prominent in the downstream near the cylinder than upstream with higher values of shear stress on the downstream. The sum of the shear stress contributions of all quadrant events increased consistently with an increase in Red near the cylinder.

Three-Dimensional Mixed Convection in a Rectangular Enclosure Containing a Circular Cylinder

This study carried out a three-dimensional, unsteady, numerical simulation to investigate the effects of the cylinder radius, Reynolds number, three-dimensionality and unsteadiness on the mixed convection in a rectangular enclosure containing a cold circular cylinder. The variations in the cylinder radius in the range of 0.1 to 0.3 times the length of the enclosure and the Reynolds number in the range of 100 to 1000 were considered. The flow and thermal structures reached three different regimes depending on the radius of cold cylinder and Reynolds number. The patterns in the flow and thermal fields were categorized into three regimes: steady two-dimensional convection, steady three-dimensional convection, and unsteady three-dimensional convection. The temperature distribution, vortical structure, and Nusselt number for a heated bottom wall were extensively analyzed. The transition of flow regime, flow instability and the heat transfer performances were influenced by the presence of cylinder. In addition, the three-dimensional flow characteristics were quantified using orthogonal enstrophy, and the unsteady characteristics were investigated by carrying out the frequency analysis of Nusselt number. As the three-dimensionality in flow and thermal fields increases and the flow regime changes to the three-dimensional unsteady convection regime, the difference in the results of two and three-dimensional simulations also increases.

Non-Stokesian flow through ordered thin porous media imaged by tomographic-PIV

The 3D flow-fields in a staggered and cubic arrangement of mono-radii cylinders are investigated using tomographic-PIV. The cylinder Reynolds-number is in the range of approx 10 to approx 800 giving an almost complete overview of the transition region. Two pore-scale effects are discovered. The first, visible in the cubic packing, is a spatially alternating lateral velocity field, which has a significant impact on the pressure drop and transversal dispersion. The second effect, present in the staggered array, is an example of a disturbance propagation effect that takes place in the laminar steady region; this manifests as a peculiar and complex flow-pattern. In accordance with other studies, it is shown that Darcy’s law can, from an engineering point of view be valid far beyond the limit for Stokesian flow.Graphic abstract

Flow over rectangular cylinder: Effects of cylinder aspect ratio and Reynolds number

Numerical simulations of two- and three-dimensional unconfined flows over rectangular cylinders are conducted at Reynolds number Re = 30 – 200. The cylinder cross-sectional aspect ratio AR (length-to-width) is varied as 0.25, 0.5, 1.0, 2.0, 3.0 and 4.0. The focus is given on how AR and Re influence the flow structure and associated aerodynamic parameters. The first critical Reynolds number (Recr1) associated with the transition from the steady flow to the two-dimensional unsteady flow is determined for each AR and found to linearly increase with increasing AR. The same observation is made for the second critical Reynolds number (Recr2) where the two-dimensional unsteady flow metamorphoses into the three-dimensional unsteady flow. A larger Re is required for a larger AR to have the onset of vortex shedding or the transition from two-dimensional flow to the three-dimensional. Three distinct scenarios of the flow separation and reattachment are identified in the ranges of Re and AR examined. The physical insight into the flow dependence on AR and Re is provided. Both AR and Re play a role in the formation of separation bubbles on the cylinder side surfaces. Two distinct mechanisms of the separation bubble formation are imparted. The detailed flow structures are linked to the mean and fluctuating forces and Strouhal numbers. The vortex strength beefs up with increasing Re but diminishes with AR, as do the normal and shear stresses and fluctuating forces.

Solid-flow interactions of a large aspect ratio cylinder in a shallow open channel

The complex wake created by an emergent slender circular cylinder in a shallow open channel flow is studied at a cylinder Reynolds number (ReD) of 3300 and at a subcritical Froude number of 0.58 in water. Methodical laser Doppler velocimetry measurements were taken in a very fine grid in three horizontal planes at near bed, mid-depth, and near free surface both upstream and downstream of the cylinder. Demarcation of the entire wake region starting from the first point of the onset of disturbed flow upstream to undisturbed flow downstream entirely based on experiments successfully added a novel contribution to the research on wake flow over a single cylinder. The proximity of the bed and free surface has a significant effect on the structure of the wake compared to the mid-depth. The size, shape, and development of the recirculation region behind the cylinder vary in the vertical direction from the bed to free surface. The longest wake closure point was found in the near bed region and the shortest was found near the free surface. The new model developed pertinent to longitudinal velocity deficit can be used as a velocity deficit scale for the entire far wake region for shallow wakes at mid-depth and near free surface. Urms/Us profiles along the transverse direction initially show a double peak behavior for all three levels, and well away from the cylinder downstream, the double peak becomes broader and less distinct, which is attributed to the effect of the separating shear layers. It is also noted that the wake characteristics of slender cylinders are significantly different from those of wakes generated by cylinders with small aspect ratios.

Pulsatile flow of power-law fluids over a heated cylinder: Flow and heat transfer characteristics

This study examines the momentum and heat transfer aspects of the laminar forced-convection induced by a pulsatile flow of power-law fluids past a heated circular cylinder by numerically solving the time-dependent momentum and energy equations over the ranges of dimensionless parameters as the cylinder Reynolds number based on the mean velocity (0.1 ≤ Re ≤ 40), Prandtl number (0.7 ≤ Pr ≤ 100), power-law index (0.3 ≤ n ≤ 1.4), frequency (0 ≤ ω* ≤ π) and amplitude (0 ≤ A ≤ 0.8). The detailed kinematics of the flow and temperature fields is visualized in terms of instantaneous streamlines and isotherm contours, especially adjacent to the cylinder. Further detailed insights are developed by examining the distribution of the pressure coefficient and local Nusselt number along the surface of the heated cylinder at different instants of time during the course of a periodic cycle. Finally, the overall gross characteristics are reported in terms of the time average drag coefficient and Nusselt number. It is possible to achieve varying levels of enhancement in the overall mixing of fluid and heat transfer under appropriate conditions of the amplitude of velocity, Reynolds number and power-law index. This is explained via the dynamics of the separated flow regions in terms of their growth and collapse during the course of a pulsation cycle. The fluid shear-thinning behavior promotes heat transfer in line with that seen in non-pulsating flows and shear-thickening behavior impedes it with reference to the corresponding value in Newtonian fluids otherwise under identical conditions.

Hydrodynamic characteristics in a sheet flow upstream water flow of a circular cylinder

Sheet flow featured with shallow depth on vegetated slopes plays a key role on the dynamics of soil and water loss, yet the hydrodynamic characteristics of sheet flow pasting a vegetation stem simplified by an emergent cylinder have not been revealed. Laboratory flume experiments were conducted to investigate potential effects of a vegetation stem on velocity components, flow vortexes, and shear stress from time-averaged and time-resolved perspectives. Flow fields on the upstream flow of the cylinder at the symmetry plane were captured by using a high precision Particle Image Velocimetry (PIV, 63 pixel/mm) system. Four flow conditions with flow depths from 0.4 to 0.57 cm and cylinder Reynolds number from 2440 to 3806 were selected to fully evaluate the sheet flow condition. Time-averaged hydrodynamic features were analyzed in terms of streamlines, streamwise velocity, wall-normal velocity, and vorticity. Time-resolved features of two velocity components were then analyzed. Joint probability density functions of the two velocity components exhibited asymmetrical bimodal, indicating two preferred flow states occurred frequently, namely, backflow event and downflow event. Subsequently, analyses by linear stochastic estimation showed that the backflow event was induced by a reverse upstream flow starting from the leading edge of the cylinder and penetrating primary horseshoe vortex, which was motivated by the intermediate-flow mode proposed in previous open channel flow. Meanwhile, the downflow event was induced by a portion of fluid that was unable to penetrate the primary horseshoe vortex and then vertically impinged the flume bed, which was motivated by the downflow mode proposed in this study. As the critical hydrodynamic parameter for local scouring, shear stress was finally sketched. It was suggested that soil control measures should be implemented around the vegetation stem with a radius of 0.1D (D is the cylinder diameter), where the maximum shear stress mostly occurs. The newly defined flow mode could provide deeper insights into the mechanisms of sheet flow and promote the practice of soil erosion control on vegetated hillslopes.

Flow Around Three Side-by-Side Square Cylinders and the Effect of the Cylinder Oscillation

Abstract In this study, the flow field over three square cylinders (SCs) arranged side by side is investigated in a low-speed wind tunnel. The experiments are performed with three similar SCs for Reynolds number (Re) 295. The influences of spacing ratio on the wake size, drag coefficient, and flow interference of the cylinders are reported with the hotwire anemometry, particle image velocimetry (PIV), and the flow visualization techniques. Special attention is paid to the oscillation given to the middle cylinder and its effect on flow structure and related forces. The spacing ratio (s/D) ranges from 1.5 to 3, whereas the forcing frequency ratio ranges from 0.5 to 2 with amplitude of 10% of the cylinder width. It is observed that the spacing influences the flow structure, and the vortex shedding mechanism strongly. A secondary frequency appears in the flow field for spacing ratio s/D = 2 and 3. Depending upon the spacing ratios, the flow pattern is seen to be asymmetric biased, symmetric biased, and weakly interactive. The wake interaction decreases with increase in spacing ratios. With the oscillations, the wake becomes more unstable and complex. Additional wake oscillation frequency appears in the power spectra. With an increase in spacing ratios, the drag coefficient decreases, whereas with oscillations, higher drag force is observed compared to a stationary cylinder. A correlation is developed between the time-averaged drag coefficient with cylinder spacing and Reynolds number.

Evolution of Turbulent Horseshoe Vortex System in Front of a Vertical Circular Cylinder in Open Channel

A turbulent horseshoe vortex (HV) system around a wall-mounted cylinder in open channel is characterized by random variations in vortex features and an abundance of vortex interactions. The turbulent HV system is responsible for initiating the local scour process in front of the cylinder. The evolution of the turbulent HV system is investigated statistically and quantitatively with time-resolved particle image velocimetry. The cylinder Reynolds numbers of the flow are 8600, 10,200, and 13,600, respectively. A novel vortex tracking method was proposed to obtain the variations in position, size, and strength of the primary HV (PHV) which dominates the system most of the time. Relationships between the various features of the PHV during its evolutionary process were obtained through correlation analyses. Results show that the dimensionless mean lifespan of the PHV is about 5.0. Statistically, the downstream movement of the PHV toward the cylinder is accompanied with its bed-approaching movement and decreasing in size, and the opposite is true. The circulation strength of the PHV decreases and increases dramatically in the region downstream of its time-averaged position when the PHV approaches and departs from the cylinder, respectively. Meanwhile, mechanisms responsible for the generation, movement, variation, and disappearance of the PHV are re-investigated and enriched based on its interactions with vortices in the separation region and structures in the incoming flow. The obtained change trends of the features of the PHV and the underlying mechanisms for its evolution are valuable for predicting and controlling the initial stage of the local scour in front of cylinders.

Slit Aspect Ratio Effect on Flow Instability and Heat Transfer Characteristics of Wavy Vortex Flow

The slit aspect ratio effect on the flow distribution, shear stress at the cylinder and heat transfer characteristics of wavy vortex flow under variable conditions was investigated by changing the rotating Reynolds number of the inner cylinder and temperature gradient between the inner and outer cylinder walls. The accuracy of numerical simulation was validated by comparing the simulation results with the experimental results. The flow field distribution in the concentric cylinder models with four slit aspect ratios was studied. The results of the wavy vortex flow indicated that the slit depth has a remarkable impact on the heat transfer process of the flow in the annulus. It can be found that the wall shear stress at the inner cylinder experiences a significant decrease with slit depth increasing. The wall shear stress at the inner cylinder of model h* = 0.75 was the largest among four models. Furthermore, jet-like flow and heat transfer enhancement became stronger as shear stress increased. The model with 0.75 slit aspect ratio had the largest heat transfer augmentation among four models.