Low Aspect Ratio Cylinder Research Articles

Exotic liquid crystalline phases in monolayers of vertically vibrated granular particles

ABSTRACTVibrated monolayers of granular particles confined into horizontal cavities form a variety of fluid patterns with orientational order that resemble equilibrium liquid-crystal phases. In some cases, one can identify nematic and smectic patterns that can be understood in terms of classical statistical mechanics of hard bodies. Low aspect ratio cylinders project as rectangles and form uniaxial, or 2-atic, and tetratic, or 4-atic, nematic phases. Other polygonal particles may exhibit different liquid-crystal phases, in general p-atic phases, of higher symmetries. We give a brief summary of theoretical work on rectangles and triangles, and provide some experimental results on vibrated monolayers. In the case of equilateral triangles, the theory predicts an exotic triatic phase, or 6-atic phase, with six-fold symmetry and three equivalent directors. The right-angled triangles exhibit a 4-atic phase with strong octatic (8-atic) correlations. Experiments on cylinders show 4-atic textures and, even more remarkable, geometric frustration caused by confinement excites topological defects, which seem to follow the same topological rules as standard liquid crystals. Some of our findings can be understood with the help of simulations of hard particles subject to thermal equilibrium, although standard Density-Functional Theories fail to account for the correct equilibrium phases in some cases.

End effects in low aspect ratio Taylor-Couette flow.

Taylor-Couette flow with a low aspect ratio cylinder suffers from end effects due to the finite-span of the gap between the cylinder sides and the secondary flow in the region below the inner cylinder. We experimentally explore these end effects by varying the cylinder aspect ratio between 6.67 and 40 for a range of wall gap widths and bottom gap heights. For these geometries, end effects (i.e. non-ideal Taylor-Couette flow) can be substantial due to both features of the finite-span and the bottom secondary flow. In some cases, the finite-span effects extended between 20% and 30% of the way into the Taylor-Couette flow region, and the secondary flow at the bottom accounted for nearly half of the total measured torque. By taking these effects into consideration, our high aspect ratio results agreed well with those obtained by Taylor (Taylor 1936 Proc. R. Soc. Lond. A 157, 546-564. (doi:10.1098/rspa.1936.0215)) at considerably higher aspect ratios. This article is part of the theme issue 'Taylor-Couette and related flows on the centennial of Taylor's seminal Philosophical Transactions paper (part 1)'.

Experimental study on the pressure distribution of a low aspect-ratio circular cylinder and interference effects of a square cylinder in downburst flows

Wind loads on a surface-mounted finite-height circular cylinder are strongly influenced by the flow field structure. Based on the stationary and moving downburst flows simulated by an impinging jet device, this paper studies the wind pressure distribution in a circular cylinder with a low aspect ratio of AR = 2 at different radial positions and discusses the interference effects of other cylinders. The results indicate that the Reynolds number has a significant effect on the mean wind pressure coefficient on the crosswind side. The maximum absolute value of the mean coefficients on the leeward and crosswind sides appears at r/Djet = 1.0. In general, the interference effect of the square cylinder closely spaced on the fluctuating coefficients of each side is the largest when it is in the upstream position in tandem. The pressure distribution on the crosswind side is more easily affected by the interference cylinder. Under the action of a moving downburst, the instantaneous pressure coefficients on all sides of the cylinder have both positive and negative peaks, and the maximum overshooting variation is close to three. The overshooting effect lasts in a shorter time and has a greater impact on the structure when the downburst moving speed is faster.

Axial variability of pattern formation in Rayleigh-Bénard convection: MRI velocimetry in a low aspect ratio cylinder

Natural convection is a fundamental hydrodynamic instability and a template process for the study of pattern formation. It is also an important mode of energy transport in natural geophysical and environmental systems, as well as technological applications. In solar heating devices, pasteurization processes, industrial mixing, and pebble-bed nuclear reactors, low aspect ratio systems of greater depth than width are widely used for their favorable convection properties. Here, magnetic resonance imaging was used to quantify the axial spatial variation of flow patterns in a narrow cylinder. Fluid velocity maps were acquired in multiple transverse and longitudinal planes. It is shown that the flow patterns vary significantly as a function of the height above the heated bottom boundary. The large scale circulation varies among predicted azimuthal mode patterns. Azimuthal 180o reorientation of the upward and downward flow cells was observed over short axial distances. The complex circulation patterns observed suggest acquisition of velocity data in only one plane could lead to mischaracterization of flow patterns in low aspect ratios systems, an aspect of the current literature which is lacking quantification. A preliminary computational fluid dynamics model in STAR-CCM+, corroborated by the experimental results, provided velocity and temperature data for the full fluid column height. Together these results indicate the need for further investigation into circulation characteristics in long narrow cylinders. Enhanced understanding of these rapid axial variations in flow pattern will allow better modeling of heat and mass transport in low aspect ratio natural convection flows.

Investigation of drag, lift and torque for fluid flow past a low aspect ratio (1:4) cylinder

Non-spherical objects exist in many multiphase applications but there is very limited data available on the fluid forces acting on these different shapes. This work derives correlations for drag, lift and torque coefficients for a single low aspect ratio 1:4 cylinder (tablet shaped object) placed at different incidence angles to the approach flow using direct numerical simulations (DNS). An Immersed Boundary Method (IBM) is used for deriving the force coefficients for 10 ≤ Re ≤ 300 and incidence angles 0° ≤ θ ≤ 90°. The method is validated by comparisons to existing data on spheres, to results from a body conforming grid, and through a grid independency study. The flow around the cylinder remains steady till Re = 200 θ ≤ 30°. The general correlation of Hölzer and Sommerfeld [1] for non-spherical particles, over predicts the drag coefficient for 0° < θ < 90° and the degree of over prediction increases with Reynolds number. The lift coefficient has a symmetric distribution about the maximum value at θ = 45° at Re = 10 but develops asymmetries in θ as Reynolds number increases. Contributions of pressure force and shear to drag and lift for each case is presented to provide a detailed view of the hydrodynamic forces on the object. The trends in the torque coefficient are similar to the lift coefficient reaching a maximum value near 45°. Correlations based the Reynolds number and incidence angle are developed for drag, lift and torque coefficients.

A Magnus Wind Turbine Power Model Based on Direct Solutions Using the Blade Element Momentum Theory and Symbolic Regression

A model of the power coefficient of a mid-scale Magnus wind turbine using numerical solutions of the Blade Element Momentum Theory and symbolic regression is presented. A direct method is proposed for solving the nonlinear system of equations which govern the phenomena under study. The influence of the tip-speed ratio and the number, aspect ratio, and the angular speed of the cylinders on the turbine performance is obtained. Results show that the maximum power coefficient is on the order of 0.2, which is obtained with two low aspect ratio cylinders, a dimensionless cylinder speed ratio of 2, and a turbine tip-speed ratio between 2 and 3. The predicted power coefficient at low tip-speed ratio suggests that a Magnus turbine may be adequate in the urban environment.

Offset Turbulent Jets in Low-Aspect Ratio Cavities

The flow induced by a round turbulent offset jet in a low-aspect ratio cylinder is investigated experimentally, with applications to degassing of U.S. Strategic Petroleum Reserves (SPR). Particle image velocimetry and flow visualization are used for flow diagnostics. The measurements include the jet penetration (mixing) depth l, jet spreading rate, and the mean velocity/vorticity fields for different offset positions Δ. With the introduction of offset, the flow patterns change drastically. For 0 &lt; Δ/D &lt; 0.2 the jet deflects toward the wall while precessing (as in the axisymmetric case), for 0.2 &lt; Δ/D &lt; 0.4 the jet hugs the wall but with an oscillating tail, and for 0.45 &lt; Δ/D the jet appears as a wall jet. In all cases, the jet is destroyed at a certain distance (mixing or penetration depth) from the origin. This mixing depth takes its lowest value for 0 &lt; Δ/D &lt; 0.2, with l ≈ (3.2–3.6)D, becomes maximum at Δ/D = 0.4 with l ≈ 5.2D, and drops to l ≈ 4.5D when the jet is close to the wall. Recommendations are made for suitable Δ/D values for optimal operation of SPR degassing.

Interaction of a synthetic jet with the flow over a low aspect ratio cylinder

This paper discusses the interaction of a single synthetic jet with the flow over a finite span low aspect ratio cylinder. The synthetic jet was located at the mid-span of the cylinder, and was oriented such that the length of the jet orifice was parallel to the freestream direction. The investigation incorporated experimental techniques such as stereoscopic particle image velocimetry (SPIV) and hot-wire anemometry. The near wake of the cylinder was dominated by a non-negligible spanwise velocity, or downwash, from the cylinder's free-end that interacted with the near wake resulting in a highly three-dimensional flow field with a strong spanwise dependence. SPIV measurements within the near wake showed that, due to the actuation of the synthetic jet, the wake was vectored and narrowed, resulting in a redirection of the downwash behind the cylinder. Furthermore, the synthetic jet actuation resulted in a localized region of inward-directed flow at a spanwise location outboard of the jet orifice leading to a breakdown of the streamwise vorticity field within the near wake. Measurements obtained near the surface of the cylinder demonstrated that the synthetic jet created an elongated vortex ring that redirected the upwash flow away from the cylinder surface inboard of the jet orifice and entrained flow back towards the surface outboard of the orifice. The result was a three-dimensional separation line along the span of the cylinder that gave rise to the spanwise dependency seen in the near wake. Finally, it was shown, using a stability theory analysis that the near surface interaction of the synthetic jet with the separated mixing layer did not result in excitation of the mixing layer instability.

Modified Structure Function Model Based on Coherent Structures

In the present study, a modified Structure Function was introduced. In this modified Structure Function model, the coefficient of model was computed dynamically base on the coherent structure in the flow field. The ability of this Modified Structure Function was investigated for complex flow over a square cylinder in free stream and a low aspect ratio cylinder confined in a channel. The Results was compared with experimental data, standard structure function and Coherent Structure Model which use a dynamically method to computed the coefficient of Smagorinsky model based on coherent structure in the flow field. The results show that Modified Structure Function improved the stability and accuracy of simulation in comparison to Coherent Structure Model and standard structure function model

Experimental Comparison of Two Degrees-of-Freedom Vortex-Induced Vibration on High and Low Aspect Ratio Cylinders with Small Mass Ratio

Vortex-induced motion (VIM) is a specific way for naming the vortex-induced vibration (VIV) acting on floating units. The VIM phenomenon can occur in monocolumn production, storage and offloading system (MPSO) and spar platforms, structures presenting aspect ratio lower than 4 and unity mass ratio, i.e., structural mass equal to the displaced fluid mass. These platforms can experience motion amplitudes of approximately their characteristic diameters, and therefore, the fatigue life of mooring lines and risers can be greatly affected. Two degrees-of-freedom VIV model tests based on cylinders with low aspect ratio and small mass ratio have been carried out at the recirculating water channel facility available at NDF-EPUSP in order to better understand this hydro-elastic phenomenon. The tests have considered three circular cylinders of mass ratio equal to one and different aspect ratios, respectively L/D = 1.0, 1.7, and 2.0, as well as a fourth cylinder of mass ratio equal to 2.62 and aspect ratio of 2.0. The Reynolds number covered the range from 10 000 to 50 000, corresponding to reduced velocities from 1 to approximately 12. The results of amplitude and frequency in the transverse and in-line directions were analyzed by means of the Hilbert-Huang transform method (HHT) and then compared to those obtained from works found in the literature. The comparisons have shown similar maxima amplitudes for all aspect ratios and small mass ratio, featuring a decrease as the aspect ratio decreases. Moreover, some changes in the Strouhal number have been indirectly observed as a consequence of the decrease in the aspect ratio. In conclusion, it is shown that comparing results of small-scale platforms with those from bare cylinders, all of them presenting low aspect ratio and small mass ratio, the laboratory experiments may well be used in practical investigation, including those concerning the VIM phenomenon acting on platforms.

Experimental investigation on the aerodynamic loads and wake flow features of a low aspect-ratio circular cylinder

An experimental investigation on the flow features of the wake generated from a circular cylinder with finite height and placed vertically on a plane is presented. Through force measurements the mean drag coefficient is found to be roughly invariant by varying Reynolds number in a range between 6×10 4 and 11×10 4. As for the fluctuating forces, a dominant spectral component is clearly detected for the signals of the cross-flow force. A spectral contribution with roughly the same Strouhal number is detected from velocity signals acquired, through hot-wire anemometry, in proximity to the lateral wake boundary; its energy is found to decrease by moving the probe away from the wake and upwards. Simultaneous velocity measurements showed that these fluctuations can confidently be ascribed to an alternate vortex shedding. Subsequently, dynamic measurements of the pressure field over the lateral surface and the free-end of the model were carried out, which highlight that the spectral component connected to vortex shedding is found over the lateral surface, with maximum energy at an azimuthal position just before the separation of the shear layers. The fluctuating energy connected to vortex shedding decreases by moving towards regions immersed in the separated wake, and with increasing vertical coordinate; as a matter of fact, above about half model height an evident energy peak cannot be detected anymore. This feature highlights that a regular alternate vortex shedding occurs only for the lower half-span of the model and that the remaining part is dominated by the upwash generated by the flow passing over the free-end. From the spectral analysis of the pressure measurements carried out over the model free-end no evidence of the presence of the spectral component connected to the alternate vortex shedding is found, as expected. However, a significant fluctuating energy is observed at lower dominating frequencies.

Separated flow structures around a cylindrical obstacle in a narrow channel

A detailed experimental study is performed on the separated flow structures around a low aspect-ratio circular cylinder (pin-fin) in a practical configuration of liquid cooling channel. Distinctive features of the present arrangement are the confinement of the cylinder at both ends, water flow at low Reynolds numbers ( Re = 800, 1800, 2800), very high core flow turbulence and undeveloped boundary layers at the position of the obstacle. The horseshoe vortex system at the junctions between the cylinder and the confining walls and the near wake region behind the obstacle are deeply investigated by means of Particle Image Velocimetry (PIV). Upstream of the cylinder, the horseshoe vortex system turns out to be perturbed by vorticity bursts from the incoming boundary layers, leading to aperiodical vortex oscillations at Re = 800 or to break-away and secondary vorticity eruptions at the higher Reynolds numbers. The flow structures in the near wake show a complex three-dimensional behaviour associated with a peculiar mechanism of spanwise mass transport. High levels of free-stream turbulence trigger an early instabilization of the shear layers and strong Bloor–Gerrard vortices are observed even at Re = 800. Coalescence of these vortices and intense spanwise flow inhibit the alternate primary vortex shedding for time periods whose length and frequency increase as the Reynolds number is reduced. The inhibition of alternate vortex shedding for long time periods is finally related to the very large wake characteristic lengths and to the low velocity fluctuations observed especially at the lowest Reynolds number.

Tomographic and time resolved PIV measurements on a finite cylinder mounted on a flat plate

Tomographic and time resolved PIV measurements were performed to examine the 3D flow topology and the flow dynamic above the upper surface of a low-aspect ratio cylinder at Re ≈ 1 × 105. This generic experiment is of fundamental interest because it represents flow features which are relevant to many applications such as laminar separation bubbles and turbulent reattachment. At Re ≈ 1 × 105, laminar separation bubbles arise on the side of the cylinder. Furthermore, on the top of the cylinder a separation with reattachment is of major interest. The tomographic PIV measurement, which allows to determine all three velocity components in a volume instantaneously, was applied to examine the flow topology and interaction between the boundary layer and wake structures on the top of the finite cylinder. In the instantaneous flow fields the tip vortices and the recirculation region becomes visible. However, it is also observed that the flow is quite unsteady due to the large separation occurring on the top of the cylinder. In order to study the temporal behaviour of the separation, time resolved PIV was applied. This technique allows capturing the dynamic processes in detail. The development of vortices in the separated shear layer is observed and in addition regions with different dominant frequencies are identified.

Adhesion of Biologically Inspired Oil-Coated Polymer Micropillars

This paper presents the performance of a beetle foot-hair inspired polymer microfibrillar wet adhesive. Micropatterned poly(dimethylsiloxane) arrays of low aspect ratio cylinders are fabricated using optical lithography and molding techniques. A thin layer of viscous silicone oil is applied via stamping. The synthetic micropillar wet adhesive is characterized with a custom tensile adhesion measurement setup by varying the retraction speed, preload, contact time, oil viscosity and oil volume. An enhancement of adhesion up to 25% over the same micropattern without any applied liquid is demonstrated on a smooth substrate. A thicker silicone layer is shown to increase the adhesion of the micropillars by a factor of almost 6 on a 650 nm root-mean-square rough slightly polished aluminum surface. A model is also developed that combines surface tension and dynamic viscous forces to predict the adhesive performance. This model matched experimental data with reasonable tuning of only a single parameter. These results are used to propose further advancements in the design and fabrication of synthetic wet micropatterned adhesives.

２次元円柱，３次元球及び低アスペクト比円柱周り非対称流れの全体安定性解析

It is well known that the flow around a sphere becomes steady and non-axisymmetric when the freestream Reynolds number (based on the diameter) becomes greater than 210. However such steady asymmetric patterns cannot be found in the case of the two-dimensional flow around a circular cylinder. In this paper, the flow around a three-dimensional low aspect ratio cylinder is calculated, and it is found that the flow around a low aspect ratio cylinder also becomes steady and asymmetric under a certain range of freestream Reynolds numbers. The two-dimensional and three-dimensional incompressible Navier-Stokes equations are numerically solved for flows around these bodies, and the transition mode from steady symmetric flow to asymmetric flow is investigated by global stability analysis. These results show that in the two-dimensional case, the asymmetric non-oscillatory mode decays faster than the oscillatory modes. In the case of the asymmetric non-oscillatory mode of the two-dimensional flow, the region of its influence extends far downstream and reaches the outer boundary of the computational domain, while in the three-dimensional non-oscillatory mode the region of influence is observed only in the vicinity of the body. The similarities of the flow around a sphere and low aspect ratio cylinder are discussed from a viewpoint of the configuration of these transition modes.

Discussion: “Pressure and Vortex Shedding Patterns Around a Low Aspect Ratio Cylinder in a Sheared Flow at Transcritical Reynolds Numbers” (Rooney, D. M., and Peltzer, R. D., 1981, ASME J. Fluids Eng., 103, pp. 88–96)

Discussion: “Pressure and Vortex Shedding Patterns Around a Low Aspect Ratio Cylinder in a Sheared Flow at Transcritical Reynolds Numbers” (Rooney, D. M., and Peltzer, R. D., 1981, ASME J. Fluids Eng., 103, pp. 88–96)

Closure to “Discussion of ‘Pressure and Vortex Shedding Patterns Around a Low Aspect Ratio Cylinder in a Sheared Flow at Transcritical Reynolds Numbers’” (1981, ASME J. Fluids Eng., 103, p. 373)

Closure to “Discussion of ‘Pressure and Vortex Shedding Patterns Around a Low Aspect Ratio Cylinder in a Sheared Flow at Transcritical Reynolds Numbers’” (1981, ASME J. Fluids Eng., 103, p. 373)

Closure to “Discussion of ‘Pressure and Vortex Shedding Patterns Around a Low Aspect Ratio Cylinder in a Sheared Flow at Transitional Reynolds Numbers’” (1981, ASME J. Fluids Eng., 103, p. 95)

Closure to “Discussion of ‘Pressure and Vortex Shedding Patterns Around a Low Aspect Ratio Cylinder in a Sheared Flow at Transitional Reynolds Numbers’” (1981, ASME J. Fluids Eng., 103, p. 95)

Discussion: “Pressure and Vortex Shedding Patterns Around a Low Aspect Ratio Cylinder in a Sheared Flow at Transitional Reynolds Numbers” (Rooney, D. M., and Peltzer, R. D., 1980, ASME J. Fluids Eng., 103, pp. 88–95)

Discussion: “Pressure and Vortex Shedding Patterns Around a Low Aspect Ratio Cylinder in a Sheared Flow at Transitional Reynolds Numbers” (Rooney, D. M., and Peltzer, R. D., 1980, ASME J. Fluids Eng., 103, pp. 88–95)

Pressure and Vortex Shedding Patterns Around a Low Aspect Ratio Cylinder in a Sheared Flow at Transitional Reynolds Numbers

Model tests were performed in a wind tunnel to determine vortex shedding patterns induced around a circular cylinder by spanwise shear in transitional Reynolds number flow. In addition, mean and fluctuating pressure measurements were obtained. The introduction of shear in the upstream flow generated two distinct cells of vortex frequencies behind the cylinder in the transcritical regime, thereby documenting for the first time that the re-established high Reynolds number shedding closely parallels patterns already observed in subcritical flow. The two cell pattern did not permit any correlation between shear level and cell length to be found.