Side Length Of Cylinder Research Articles

Aerodynamics and Wake Flow Characteristics of a Four-Cylinder Cluster

The aerodynamic behaviour and wake flow of a cluster of two-dimensional sharp-edged bluff bodies exhibits extremely complex unsteady phenomena in both near and far fields. Due to the high cost of wind-tunnel experiments and numerical simulations, a complete understanding of wake flows and a description of their characteristics are lacking. This paper presents large-eddy simulations (LES) in different flow/wind directions for a cluster of 2 times 2 aligned square cylinders, at a separation distance in streamwise and cross-wind directions equal to cylinder side length, and at Reynolds number {Re}=22,000 based on the single cylinder side length D. The case at 0^circ incidence shows an evident channel-type flow in the along-wind street/gap, and at its exit an irregularly pulsing jet with an intense shedding of large vortices. The wavelet analyses of the side force/lift coefficient and instantaneous velocities in the wake show that the characteristic length and time scales of the large vortical structures in the far-field wake are close to the cluster size 2D; this is the so called ‘cluster effect’. The cluster effect increases monotonically as the flow incidence angle increases. At a large incidence angle in the near-field wake, the cylinder-scale flow structures are much weaker compared to the cluster-scale structures. At the incidence angle of 45^circ, the overall wake flow and the aerodynamic characteristics are well scaled by the scale approximately equal to 2D. Nevertheless, the interaction between cylinders significantly affects the aerodynamics performance of the individual cylinders. The drag and lift coefficients of the individual cylinders differ substantially from each other in the cluster, and are significantly different from observations on a single isolated cylinder too.

Large-Eddy Simulation of Turbulent Flow Around a Finite-Height Wall-Mounted Square Cylinder Within a Thin Boundary Layer

In this paper, large-eddy simulation (LES) has been performed to investigate the turbulent wake behind a wall-mounted square cylinder. The flow features a relatively high cylinder aspect ratio of 4, a Reynolds number of 12,000 (based on the free-stream velocity and cylinder side length), and a thin developing boundary layer with a thickness of only 18 % of the obstacle height. These characteristics of the flow impose challenges to LES for accurate simulation of large energetic eddies induced by the cylinder and their intense interactions with the thin developing boundary layer. The coherent flow structures around the cylinder have been studied based on the instantaneous and time-averaged resolved velocity and pressure fields. The local kinetic energy transfer between the resolved and subgrid scales and the streamwise evolution of the subgrid-scale viscosity have been investigated to provide physical insights into the subgrid-scale dynamics. The LES predictions of the flow statistics have been validated against a set of recently reported wind-tunnel experimental data.

A smoke visualization study of the flow over a square cylinder at incidence and tandem square cylinders

An experimental investigation of flow around a square cylinder placed at various incidence angles (0–45) with respect to the approach flow is reported (Re d = 2,033, 6,776, 7,575, 8,246). Also flow over two square cylinders in a tandem arrangement at zero incidence is studied for different gap spacing values ranging from 1d to 6d, where d is cylinder side length (Re d = 2,033). A low-speed vertical smoke tunnel is used to visualize flow patterns. For single cylinder, the flow patterns, stagnation and separations points, separation angle, attachment point, transient and turbulence lengths of the shear layer, wake length, frequency and Strouhal number for various incidence angles are determined quantitatively using flow visualization and image digitization process. It is found that a critical value appears around α = 15° for aforementioned parameters due to occurrence of the reattachment of the separated shear layer on the lateral face of the cylinder.For tandem cylinders, the effect of gap spacing between cylinders on the flow patterns, separation angle, wake length, and transient and turbulence lengths of the shear layer of the upstream cylinders is investigated. The results showed that there are three flow patterns when the gap spacing values are varied from 1d to 6d. Also it is observed that the aforementioned parameters for the upstream cylinder become approximately similar to those for single cylinder due to reduction in the flow interference of the cylinders at higher gap spacing values, i.e., 5d and 6d.

Influence of Flexible Spacers on Liquid-Crystalline Self-Assembly of T-Shaped Bolaamphiphiles

T-shaped bolaamphiphiles composed of a biphenyl rigid core, a semiperfluorinated lateral chain, two polar 1,2-diol groups in the terminal positions and flexible alkyl spacers connecting the polar groups with the biphenyl core have been synthesized and investigated by polarizing microscopy, DSC and X-ray scattering. The influence of spacer length and position of the spacer on the self-assembly in liquid-crystalline phases was studied. A series of four different columnar phases (Col(hex)/p6mm, Col(rec)/p2gg, Col(squ)/p4gm and Col(squ)/p4mm), representing liquid-crystalline honeycomb structures composed of cylinders having hexagonal, pentagonal, and square cross section, were found on increasing the spacer length. It is also shown that introduction of aliphatic spacers in the backbone of the T-shaped bolaamphiphiles replaces the Col(rec)/c2mm phase made up of rhombic cylinders with the Col(squ)/p4mm phase composed of square cylinders. It also causes the 2d lattice of pentagonal cylinders to increase the symmetry from Col(rec)/p2gg to Col(squ)/p4gm. A temperature-dependent second-order phase transition between these two pentagonal cylinder structures was observed for the first time. Beside these effects on cylinder shape and phase symmetry the flexible spacer units also lead to reduced phase transition temperatures and allow adjustment of cylinder side length to envelop a wider range of side-chain sizes. Electron density maps suggest that this may involve sacrificing some of the hydrogen bonds.

COMPUTATIONAL SIMULATION AND EXPERIMENTAL VALIDATION OF DOUBLE SQUARE CYLINDERS FLOW IN STAGGERED ARRANGEMENT

Experimental and numerical study of the flow field around two finite square cylinders in staggered arrangement is the objective of the present work. Two similar cylinders having the same square cross-section of 50 mm x 50 mm with blockage ratio of 16.7% were considered in different arrangements. The staggered arrangement with different angular positions was considered. The turbulent incompressible Navier-Stokes equations are solved numerically using finite element method. The static pressure distributions on the two-cylinder surfaces were estimated and measured in all cases with different apart-distances between the two cylinders and consequently the pressure coefficient variations were calculated. In addition, the experiments were developed for three Reynolds numbers of 13.5, 9.5 and 6.8x104 (based on the cylinder side length) to investigate their effect on the turbulent flow. The upstream cylinder is not affected greatly for different angular positions and apart-distances. This effect is significant in the case of side-by-side, where the flow stream equally affects the two cylinders. Up to four cylinder-widths apart-distance, the upstream cylinder affects the downstream cylinder. The effect is changed from maximum value in tandem arrangement to minimum one for the case of side-by-side configuration. Moreover, where the upstream side of the downstream cylinder is affected by changing the Reynolds number, the other sides do not vary. The highest considered Reynolds number produces the lowest drag and side force coefficient. On the other hand, the computational results are in close agreement with the measured ones. The numerical visualization presents clearly the flow behavior around the cylinders and was validated by the measured and published data.

Numerical simulation of bubbly flow around two tandem square-section cylinders by vortex method

The air—water bubbly flow around two tandem square-section cylinders is simulated with the vortex method proposed by the authors in a prior study. The flow was experimentally investigated by Shakouchi et al. The side length of cylinder s is 30 mm, and the distance between the cylinders is 2s. The water Reynolds number is 10 000, and the air volumetric flow ratio to whole fluid is 0.03. A pair of Karman vortices appear behind the first cylinder. The strength of the Karman vortex downstream of the second cylinder is larger than that for the single-cylinder. The strength of the Karman vortex reduces due to the bubble entrainment. The pressure reduction on the rear surface of the first cylinder is relaxed when compared with that on the single-cylinder. Therefore, the present simulation can predict the reduction of the drag force acting on the first cylinder, which was clarified by the corresponding experimental study.

Three-dimensional MHD flows in rectangular ducts with internal obstacles

This paper presents a numerical simulation of the magnetohydrodynamic (MHD) liquid metal flow around a square cylinder placed in a rectangular duct. In the hydrodynamic case, for a certain parameter range the well-known Kármán vortex street with three-dimensional flow patterns is observed, similar to the flow around a circular cylinder. In this study a uniform magnetic field aligned with the cylinder is applied and its influence on the formation and downstream transport of vortices is investigated. The relevant key parameters for the MHD flow are the Hartmann number M, the interaction parameter N and the hydrodynamic Reynolds number, all based on the side length of the cylinder. The Hartmann number M was varied in the range 0 [les ] M [les ] 85 and the interaction parameter N in the range 0 [les ] N [les ] 36. Results are presented for two fixed Reynolds numbers Re = 200 and Re = 250. The magnetic Reynolds number is assumed to be very small. The results of the numerical simulation are compared with known experimental and theoretical results. The hydrodynamic simulation shows characteristic intermittent pulsations of the drag and lift force on the cylinder. At Re = 200 a mix of secondary spanwise three-dimensional instabilities (A and B mode, rib vortices) could be observed. The spanwise wavelength of the rib vortices was found to be about 2–3 cylinder side lengths in the near wake. At Re = 250 the flow appears more organized showing a regular B mode pattern and a spanwise wavelength of about 1 cylinder side length. With an applied magnetic field a quasi-two-dimensional flow can be obtained at low N ≈ 1 due to the strong non-isotropic character of the electromagnetic forces. The remaining vortices have their axes aligned with the magnetic field. With increasing magnetic fields these vortices are further damped due to Hartmann braking. The result that the ‘quasi-two-dimensional’ vortices have a curvature in the direction of the magnetic field can be explained by means of an asymptotic analysis of the governing equations. With very high magnetic fields the time-dependent vortex shedding can be almost completely suppressed. By three-dimensional visualization it was possible to show characteristic paths of the electric current for this kind of flow, explaining the action of the Lorentz forces.

Effects of end-plate dimensions on hydrodynamic forces on circular cylinders oscillating sinusoidally in water.

The hydrodynamic forces generated by water-square cylinder interaction have been investigated experimentally. A square cylinder model of 50mm side length and 200mm span length, and five kinds of end-plates of 100, 200, 300, 400, 500mm width were used. The cylinder supported by two end-plates was oscillated horizontally in standing water by mechanical oscillation machinery. The force was measured by means of a quartz force transducer. Period parameters ranging from 0.2 to 3.0 and Reynolds numbers from 5000 to 40000 were covered in the investigation. The results have shown that the force amplitude coefficients based on the acceleration of two-dimensional square cylinder are obtained at end-plate dimensions of greater than 10 times the square cylinder side length, and the force amplitude coefficients and the force wave forms depend on the period parameter but not on the Reynolds number.