Wall Proximity Effects Research Articles

A numerical study on dynamic flows past three tandem inclined elliptic cylinders near moving wall

This numerical study focuses on the dynamic flows past three tandem inclined elliptic cylinders of equal spacing parallel to a moving wall using a lattice Boltzmann method. The gap ratio (G/D=0.6–2.5, where G and D are the gap between the wall surface and cylinder center and major axis, respectively), spacing ratio (L/D=1.5–10, where L is the distance of two adjacent cylinder centers), and inclination angle (α=±15°,±30°,±45°—the angle between normal vector and cylinder's major-axis) are explored at Reynolds number Re = 150 (based on D). The intended analysis links hydrodynamic coefficients, wake structures, and spectral analysis in parameter space of α−G/D−L/D to fluid mechanics. The flow is highly adjustable in this space, dividing into seven regimes: overshoot, continuous reattachment, alternative reattachment, wavy, meandering, quasi-coshedding, and coshedding, which are spatially classified into four modes due to flow interference: shear layer, primary, two-layered, and secondary vortex shedding modes. Transitions between adjacent modes determine three boundaries; and hydrodynamic coefficients differ substantially in parameter space. Due to shadowing, the upstream cylinder has a larger drag coefficient than the middle and downstream cylinders, reducing the drag coefficient of upstream cylinder and the lift coefficient of middle and downstream cylinders. α=±45° has the highest lift oscillation among the three cylinders and a small drag coefficient of the upstream cylinder. The moving wall's proximity effect increases the upstream cylinder's lift coefficient for α<0°, being negligible for high G/D across the full L/D range and stabilizing the lift oscillation of three cylinders.

Wall-proximity effects on vortex-induced vibrations of a circular cylinder

Vortex-induced vibrations (VIV) of an elastically supported circular cylinder (of diameter D) undergoing a wall interference are investigated for a mass-damping ratio 0.31, Reynolds numbers 1.77 × 103–1.24 × 104, and wall-cylinder gap height ratios e* = e/D = 0.1–1.6. Measurements of vibration responses and vortex shedding frequencies are done using a laser vibrometer and a hotwire anemometer, respectively, while particle image velocimetry techniques are employed to capture flow fields. This investigation aims to assimilate the effect of e* on vibration response, frequency response, VIV range, quality factor of VIV, added mass, added damping, lift force, and flow structure. A smaller e* is observed to increase the added mass, reduce the maximum vibration amplitude, and shorten the VIV range while enhancing the quality factor of VIV. The increase in added mass and the decrease in the maximum vibration amplitude in the VIV range are dramatic when e* is decreased from 0.6. Decreasing e* significantly impacts the flow structures and vortex shedding modes, resulting in intensified asymmetry in pressure distributions. The interference between the wall and the cylinder is further reflected by increasing time-mean lift forces and decreasing fluctuating lift forces with decreasing e* < 0.6.

Numerical simulation of flow around circular cylinder near a plane wall: effects of wall proximity, boundary layer and reynolds number

The cylindrical structures in close proximity to a solid surface have diverse and numerous applications within current engineering. While the flow dynamics around an isolated circular cylinder located in a uniform flow are reasonably well understood, this is not the case when the cylinder is positioned near a plane wall. This particular configuration is studied using computational fluid dynamics (CFD) through the Fluent code, which implements the finite volume method. For this purpose, a two-dimensional and transient flow is carry out using the SST-SAS turbulence model. The effects of changing the separation (G) between the cylinder (with diameter D) and the wall are analyzed for different Reynolds numbers and different boundary layer thicknesses. The lift and drag coefficients and the behavior of vortex shedding (Strouhal number, St) are examined. The numerical results are compared with values available in the literature from experimental wind tunnel tests, showing that both the drag and lift coefficients strongly depend on the separation ratio (G/D) and are affected by the boundary layer thicknesses.

Numerical study on compressible flow around a circular cylinder in proximity to the wall

In the present study, the compressible flow around a circular cylinder in proximity to the wall is investigated. By using direct numerical simulation approach, the flow behavior with the Reynolds number 100 and the Mach numbers 0.1–0.6 are examined. The numerical result shows that the compressibility effect would enhance the flow stability, whereas the strength of shedding vortices, the shedding frequency, and the fluctuation of aerodynamic force are reduced by increasing the Mach number. Through the examination of signed enstrophy and shear steepness in the cylinder boundary layer, it is found that the vortices shedding from the free-stream side and the wall side are concurrently inhibited by the wall when the cylinder is fully embedded in the wall boundary layer, and the strength of the wall-side vortices is relatively weaker. This imbalance would be intensified when the cylinder gets closer to the wall, and thus a reduction in the shedding frequency is produced. Based on this effect of wall proximity on frequency, the flow behavior is divided into three regimes, i.e., high-frequency, low-frequency, and completely suppression regimes. Finally, from the observation of mean forces, the trend of mean drag and lift force coefficients with regard to the Mach number and gap ratio is described as the scaling behavior in terms of two piecewise functions.

Effect of Wall Proximity and Surface Tension on a Single Bubble Rising near a Vertical Wall

Path instability of a rising bubble is a complex phenomenon. In many industrial applications, bubbles encounter walls, and the interactions between the bubbles and the wall have a significant impact on flow physics. A single bubble rising near a vertical wall was experimentally observed to follow a bouncing trajectory. To investigate the near-wall dynamics of rising bubbles, 3D numerical simulations were performed based on the volume of fluid (VOF) method using the open source solver OpenFOAM. The effect of wall proximity and surface tension on the bubble trajectory was investigated. Previous studies have focused on the near-wall rising dynamics of bubbles for higher Eotvos numbers (Eo) and varied the Galilei number (Ga). The physical properties of the flow were chosen such that the free-rising bubble lies in the rectilinear regime. The Ga number was fixed and the Eo number was varied to analyze its effect on the bubble’s rising trajectory. It was found that the presence of the wall increases the drag experienced by the bubble and induces an early transition from rectilinear to a planar zigzagging regime. We identify the maximum wall distance and the critical Eo number for the bubble to follow a bouncing trajectory. The amplitude, frequency and wavelength of the bouncing motion are independent of the initial wall distance, but they decrease with decreasing surface tension.

Effect of Wall Proximity and Shape on Bubble Enhanced Heat Transfer from a Vertical Heated Surface

&lt;div class="section abstract"&gt;&lt;div class="htmlview paragraph"&gt;Numerical simulation has been performed to study the heat transfer enhancement from the vertical heated wall surfaces with the help of rising bubbles due to the buoyancy force. The effect of wall proximity and bubble shapes are investigated for three wall shapes such as plane wall, wavy wall and triangular wall. Numerical solution is obtained by solving both the thermos-fluid governing equations and the Volume of Fluid (VOF) advection equation along with the Piecewise-Linear Interface Construction (PLIC) algorithm available in ANSYS-Fluent, an FVM based commercial CFD code. The results observed in the three types of wall geometries were showing the heat transfer differently for the 3 mm bubble. For the plane wall from the rise of the bubble to 0.3 seconds the temperature gradient is 10 K whereas for the curved and triangular wavy walls these gradients are 9.6 K and 17.23 K respectively. and after 0.6 seconds, this gradient is almost the same for all the wall shapes. In the latter two cases, the heat transfer by the effect of the wall was changing because of the alternating widening and narrowing channel. The research also discusses the varying distance of the injection of the bubble from the heated wall viz. 6 mm, 12 mm, 8 mm, 14 mm. It is observed that in addition to the well-known wake enhanced heat transfer by rising bubble, the contribution in the heat transfer enhancement by the wall modification from plane surface to wavy/triangular surfaces is substantial. It is also found that, as expected, this enhancement is significantly decreased as the wall proximity is increased.&lt;/div&gt;&lt;/div&gt;

Evaluating suitability of confined impinging jet reactor for exothermic reactions: Hydrodynamics, residence time distribution, and heat transfer

AbstractEffect of confinement (wall proximity) of a confined impinging jet reactor (CIJR) on the flow field, residence time distribution and heat transfer are explored, through experiments and CFD simulations. Hydrodynamic characteristics are evaluated for different parameters namely confinement, impinging jet velocity, temperature gradient, and so on. For 2 mm confinement, highest values of dispersion number and overall heat transfer coefficient are observed due to interaction of turbulent eddies followed by the effect of reactor wall proximity. For the CIJR having confinements above 10 mm, jet velocity need to be greater than 3 m/s to achieve both, excellent mixing efficiency and high heat removal rate. Empirical correlations for Dispersion and Nusselt numbers as a function of Rej and L/D are obtained, over a range of and , which correspond to jet velocity of 0.5–3 m/s. The present study gives a basis for designing CIJR suitable for rapid, homogeneous, exothermic reactions.

Triggering mechanics for transverse vibrations of a circular cylinder in a shear flow: Wall-proximity effects

Vortex-Induced Vibrations (VIVs) of a near-wall cylinder involve complex interactions between the structure and the boundary layers in the shear flow. Based on dimensional analyses, the wall-proximity effects on the triggering of transverse VIVs of a circular cylinder are physically investigated with flume observations. An accessorial low structural-damping VIV simulation device was developed and an upward-illumination Particle Image Velocimetry (PIV) system was employed in the physical modeling. Although the regular Kármán-like vortex-shedding could be suppressed for small gap-to-diameter ratios (e.g., e/D < 0.40) as reported in previous studies, the present power spectral analyses on the velocity fluctuations in the lee-wake indicate that the characteristic frequency can still be well identified, which could further trigger the vibrations of a low mass-damping cylinder. PIV measurements show that, as the cylinder approaches the bottom, the asymmetry of swirling-strengths in the lee-wake gets more remarkable, and the values of Strouhal number increase correspondingly. Four typical stages can be identified in the process of the vibration being triggered under the wall-free condition, including (a) the fully-stationary, (b) the intermittent-vibrating, (c) the sharp-jump, and (d) the upper-branch vibration stages. But, for e/D≤ 0.20, the intermittent-vibrating stage disappeared. During the sharp-jump stage, the peak jump-amplitude decreases dramatically as the cylinder approaches the bottom, which has negative correlation with the vibration frequency. The effects of the swirling-strength attenuation and the downward collisions on the vibration responses are discussed. The relationship between critical reduced velocity V rcr and e/D is finally established by fitting the experimental results. It is indicated that the wall-proximity effects could be significant and should be taken into account while evaluating the critical velocity for triggering transverse vibrations of the near-wall cylinder in the engineering practices.

Effect of wall proximity on the wake of a rotating and translating sphere

Effect of wall proximity on the wake of a rotating and translating sphere

Effect of wall proximity on the lateral thermocapillary migration of droplet rising in a quiescent liquid

A numerical study is performed to observe the effect of lateral wall proximity on the thermocapillary migration of a droplet. Three-dimensional simulations of the droplet with lateral wall proximity show that the droplet is pulled toward the wall for larger temperature gradients in the ambient and pushed away from the wall at smaller temperature gradients. Parametric studies carried out for migration of a droplet in the vicinity of a wall in a two-dimensional domain show that the droplet behavior is similar to the three-dimensional domain. At different temperature gradients, the final lateral distance of the droplet from the wall does not vary monotonically. The interaction of the temperature field at the leading and trailing ends of the migrating droplet with the wall explains the observed behavior. An extensive parametric study is performed to understand the effect of the Marangoni number, Reynolds number, and property ratios on droplet migration near the wall. Variation in each parameter influences the evolution of temperature both within the droplet and in the ambient fluid. The asymmetric interfacial temperature variation due to the asymmetric evolution of internal circulation within the droplet is correlated with the lateral migration of the droplet. The observations made in the present work reveal physical mechanisms that influence the thermocapillary migration behavior of a droplet near a wall.

Effects of Domain Wall Proximity on Nanoscale Polarization Switching in Relaxor‐Ferroelectric Single Crystals

Domain walls’ vibration and motion contribute significantly to the exceptionally large dielectric and piezoelectric response of ferroelectric materials. Yet, the specific length scales at which domain walls impact characteristic parameters remain largely unprobed. Previous studies examining correlation of domain wall proximity and functional response at the micrometer or submicrometer scales are often based on (locally or globally) “written” domains. The stability of such domains can be affected by many factors, resulting in convoluted effects of domain wall proximity and their stability when studying the local functional response. Herein, the effects of preexisting domain walls on the nanoscale polarization switching in a [001]‐cut relaxor‐ferroelectric single crystal are probed by piezoresponse force microscopy. It is found that domain wall proximity has limited impact on polarization switching for locations ⪝300 nm away. While a transition from a growth‐ to nucleation‐limited regime and/or change in dimensionality of domain growth is possibly observed, the effective impact on nucleation voltage does not exceed 25% variation. These results are consistent with the well‐documented pervasive chemical, polar, and structural heterogeneities present in relaxor‐ ferroelectrics and the resulting “soft” piezoelectric behavior.

Effects of wall proximity on the airflow in a vertical solar chimney for natural ventilation of dwellings

This study investigates performance of a vertical solar chimney, which absorbs solar energy and induces airflow for natural ventilation and cooling of dwellings, under effects of walls neighboring to its air channel. A computational fluid dynamics model was developed to predict induced flow rate and thermal efficiency of a vertical solar chimney with four types of nearby walls: a vertical wall to which the solar chimney was attached, a horizontal plate above the outlet of the air channel, a horizontal plate, and a horizontal wall below the inlet of the air channel. Examined factors included the heat flux in the air channel, the chimney height, the air gap, the distance of the walls, and the location of the heat source in the air channel. The results showed that effects of the wall proximity were modulated by the location of the heat source and the ratio G/ H between the air gap and the chimney height. Particularly, performance of the chimney was enhanced when the heat source was on the opposite side of the vertical wall and when G/ H was large.

Flat Wall Proximity Effect on Micro-Particle Sedimentation in Non-Newtonian Fluids

We investigate the sedimentation of colloidal micro-spheres and red blood cells (RBCs) in non-Newtonian fluid - silicone oil with different viscosities. We use digital holographic microscopy (DHM) to obtain volumetric information of the sedimenting micro-objects. Especially, the numerical refocusing feature of DHM is used to extract the depth information of multiple particles moving inside the fluid. The effects of proximity to a flat wall and the non-Newtonian behavior on the sedimenting micro-spheres and RBCs are studied by trajectory analyzing and velocimetry. We observe that for lower viscosity values the proximity effect is more pronounced. The variation rate of the particle falling velocities versus their distance to the flat wall decreases by increasing the viscosity of the fluid. For RBCs, however, the decreasing of the velocity variations have a smoother trend. The experimental results verify the theoretical prediction that, similar to Newtonian case, a correction factor in Stokes’ law suffices for describing the wall effect.

Asymmetric wake of a horizontal cylinder in close proximity to a solid boundary for Reynolds numbers in the subcritical turbulence regime

The near wake dynamics developed behind a horizontal cylinder with wall proximity effects are elucidated from laboratory experiments and Large-Eddy Simulations (LES). Fixed vertical gap to diameter (G/D) ratios of 0.5 and 1.0 were investigated for Reynolds numbers equal to 6,666, 10,000 and 13,333. The LES results agreed well with the experimental measurements for the time-averaged flow quantities and captured the upward flow motion developed over the lower half of the flow depth as a consequence of the near-wall effect. The presence of a narrow gap between the cylinder and the bed, i.e. G/D = 0.5, significantly influenced the dynamics of the vortex generation and shedding which, in consequence, led to an increasingly pronounced asymmetric wake distribution with increasing Reynolds number. In the wider gap case of G/D = 1.0, the wake remained relatively symmetrical, with reduced impact of ground proximity. Kelvin-Helmholtz instabilities developed in the upper and lower shear layers were shown to be decoupled as their instantaneous laminar-to-turbulent transition occurred at different downstream distances at any given time. Spanwise rollers were shown to form with an undulating pattern and presented irregularly located vortex dislocations. Furthermore, a ground-vortex induced during the early stages of the lower roller’s generation in the wake lifted off the ground and merged with the vortices to form a single vortical structure. For G/D = 0.5, a positive upwards force was present, and experimental and LES Strouhal number values ranged between 0.28–0.32, while computed drag coefficient values were lower than those typical for unbounded cylinder flows. As for G/D = 1.0, Strouhal numbers decrease to a 0.26–0.30 range whilst drag coefficient increases, further demonstrating the effects on the cylinder wake structure dynamics due to the proximity to a solid boundary.

Effect of wall proximity on the flow over a cube and the implications for the noise emitted

The flow over an object such as a cube and the resulting aerodynamic noise are affected by its proximity to a wall. To evaluate the effect of wall proximity on the aerodynamics induced by a cube, numerical investigations have been performed for the flow past the cube elevated to different heights above a solid surface, using the delayed detached eddy simulation method. A benchmark case of a wall-mounted cube in uniform flow is first studied, which gives commendable agreement with the available measurement results, validating the numerical methodology adopted. Subsequently, the cube is elevated to different heights above the ground. Detailed flow topologies around the cube affected by the elevated height are investigated. In addition, the effect of wall proximity on near-wall flow patterns and distributions of the surface pressure are also analyzed. After examining the flow features, the far-field noise emitted from the cube at different elevated heights is predicted by using the Ffowcs Williams-Hawkings acoustic analogy and some implications of the effect of wall proximity on the emitted noise are summarized. For the wall-mounted cube, the noise is greatest along the lateral direction. As the cube is lifted, the radiated sound in the vertical direction increases rapidly and peaks at one quarter of its side length above the ground. The noise induced by the cube tends to be broadband although broad peaks at a Strouhal number of around 0.1 are observed in the vertical and the lateral directions.

Flow of power-law fluid past a circular cylinder in the vicinity of a moving wall

In this paper, the flow of power-law fluid over a circular cylinder near a moving wall is simulated numerically using a finite volume method for different Reynolds numbers (Re = 1, 10, 40), gap ratios (G/D = 0.2–1.0), and power-law indices (n = 0.5–1.5). The effects of fluid inertia, wall proximity, and rheological property on the drag and lift coefficient, gap flow characteristics, and recirculation modes are studied. Possible mechanisms for the variation of the drag and lift coefficient and the change in recirculation mode are addressed. The results show that the decrease in G/D or Re results in the increase in drag and lift coefficient, while the effect of n is dependent on Re and G/D. For the drag coefficient, shear-thickening fluid is more sensitive to the change of G/D. The variation of the lift coefficient can be explained by the movement of angular position of the front stagnation point. The redistribution of the flow around the cylinder results in different recirculation modes. Five distinct modes are found and the critical value of G/D for the mode change decreases with the decrease in n and increase in Re. Variation of the relative vortex intensity and the relative flow intensity nearby leads to the change of recirculation mode.

Consistently formulated eddy-viscosity coefficient for k-equation model

ABSTRACTAn approach to devising a consistency formulation for (production-to-dissipation ratio) is proposed to obtain a non-singular (coefficient of eddy-viscosity) embedded in the one-equation model based on the turbulent kinetic energy k. The dissipation rate ε is evaluated with an algebraically prescribed length scale having only one adjustable coefficient, accompanied by an anisotropic function enhancing the dissipation in non-equilibrium flow regions. The model accounts for the distinct effects of low Reynolds number (LRN) and wall proximity. The stress-intensity ratio is formulated as a function of local variables without resorting to a constant . The parameters and entering the turbulence production prevents presumably the overestimation of in flow regions where non-equilibrium effects could result in a misalignment between turbulent stress and mean strain rate with a linear eddy-viscosity model. A comparative assessment of the present model with the Spalart–Allmaras (SA) one-equation model and the shear stress transport (SST) k–ω model is provided for well-documented simple and non-equilibrium turbulent flows. Finally, the current model provides a proposal to compute free shear flows.

Free Surface Characteristics of Flow around Two Side-by-Side Circular Cylinders

The three-dimensional free surface characteristics of flow around two equal diameter cylinders in a side-by-side arrangement were studied numerically. The flow fields were simulated with a three-dimensional finite volume method based on the RNG k-ɛ model for Reynolds number Re = 1.0 × 104. The volume-of-fluid method was applied to track air–water interfaces. Computations were performed for gap ratios of 1.25, 1.5, and 1.75 to examine the influence of the gap between two cylinders, and for distance to diameter ratios of 8.0 and 1.0 to study the wall proximity effects. The model was verified by comparing it with the other numerical and experimental results. The results indicated that the evolution of the free surface was periodic in time scale. A weak hydraulic jump occurs in the wake flow. Moreover, a significant difference between upstream and downstream free surface elevations exists in the vicinity of the cylinder. A runup in front of the cylinder and a ‘depression’ around the side edge were also observed. Computational results showed that the flow near the two cylinders was pushed outwards, and the flow between the cylinder and the wall was deflected inwards by the wall. The vortex structures on and near the free surface were closely correlated with the free surface. The shedding vortex far from the free surface was not affected.

Three-dimensional transient flows past plates translating near a wall

Large eddy simulations are conducted to investigate transient three-dimensional flows past arrays of finite plates translating in the vicinity of a wall. Flows around a single plate are examined for a wide range of wall distances at a Reynolds number based on plate height of 50,000. The drag coefficient decreases rapidly when the plate is placed very close to the wall; it reaches a maximum at an intermediate distance and it asymptotes to the value corresponding to unconfined flows as the gap between the wall and the plate is increased. The reduction in drag coefficient is directly related to the pressure drop caused by vortices generated upstream of the plate induced by the close proximity of the wall. Temporal and spatial characteristics of flows past arrays of yawed plates are examined for the wall distance ratio of 1.5 and the spacing ratios of 2.5 and 5. Drag coefficient of the plates is larger than those placed away from the wall. With suitable spacing the wall proximity effects result in an increase of drag force exerted on the plates. The present study aids in optimizing marine energy harvesting systems that have blades translating beneath ocean platforms or floating river docks.

Wall proximity effects on the flow past cylinder with flexible filament

The response of thin, elastic filament attached to a circular cylinder is numerically investigated under uniform flow at different wall proximities, GD (0.5, 0.6, 0.8, 1, 1.5, 2 and 2.5). The Reynolds number based on the diameter of the cylinder is 200. Numerical simulations were conducted for a range of reduced velocities Ur (3,4,5 and 6), where Ur = U∞fnD which appropriately captures the synchronization range of vortex-induced vibration(VIV) of the structure. Ur is varied by changing the density of the material. The force coefficients, the frequency of shedding/flapping are observed to be the function of GD and Ur. The positive vortices diffuses faster as GD decreases. The skewness in the amplitude attained by the flapping filament on either side of the cylinder causes unsteady shedding of vortices. Filament flaps in frequency that is closer to its natural frequency at higher GD and at lower GD, the frequency of flaps for all Ur is similar. For lower gap ratios, 0.5 and 0.6, the modulations in flapping amplitude is observed. The modulation is predominant as the Ur increases. During the growth and damping phase of the amplitudes in each modulation cycle, the vortex shedding is observed to be oblique towards the wall. The filament flaps in single mode shape. The frequency of the flapping depends on the Ur and GD. The frequency of the flapping is in resonance with shedding frequency.