Comparison of flow characteristics around an equilateral triangular cylinder via PIV and Large Eddy Simulation methods

The flow structures around an equilateral triangular cylinder, which is commonly used as a vortex shedder in the vortex flowmeter, were investigated experimentally and numerically. Flow characteristics such as vorticity contours, patterns of sectional streamlines, velocity vectors, velocity fields, Reynolds stress correlations, Strouhal numbers and drag coefficients were examined using the Particle Image Velocimetry (PIV) technique and the Large Eddy Simulation (LES) turbulence model. Experimental studies were performed in an open water channel for Re=2.9×10³, Re=5.8×10³ and Re=1.16×10⁴ based on the equilateral triangle edge. A sharp-tip corner of the cylinder with a triangle cross-section was exposed to the upstream side while the other two sharp-tip corners were placed on the downstream side. Numerical studies were also completed at Reynolds numbers in the range of 2.9×10³≤Re≤1.16×10⁵ to obtain the changes in the Strouhal numbers and drag coefficients. When the results of PIV and LES are considered in the same interval of Reynolds numbers, the maximum and minimum values of each flow pattern were nearly the same. The time-averaged patterns had considerable symmetry with respect to the axis line passing through the sharp-tip corner of the cross-section of the triangular cylinder. The Strouhal number was independent of the Reynolds number and was found to be approximately 0.22. The drag coefficient decreased with increasing Reynolds numbers while increasing the Power Spectral Density (PSD) and the vortex shedding frequency. For the same Reynolds numbers, the experimental and numerical results were in good agreement. Therefore, the LES turbulence model is recommended for applications of flow around this type of bluff body that is generally used in the design of vortex flowmeters to generate vortex shedding.     

Numerical investigation of effects of buoyancy around a heated circular cylinder in parallel and contra flow

Two-dimensional, steady, incompressible Navier-Stokes and energy equations are expressed in the stream function/vorticity formulation and solved numerically by finite difference method to study effects of buoyancy on fluid flow and heat transfer from a horizontal circular cylinder. The cylinder is exposed to approaching flow stream, for parallel (parallel flow) and opposing (contra flow) directions to the buoyant force. Two different thermal boundary conditions were considered at the cylinder surface: constant temperature (CT) and constant heat flux (CHF). The results elucidating the dependence of the flow and heat transfer characteristics on the Richardson number 0≤ Ri ≤ 2, Prandtl number 0 ≤ Pr ≤ 100 and Reynolds number 0 ≤ Re ≤ 40 are presented. Overall, for parallel flow regime, an increase in the Ri led to a raise in both Nusselt number and drag coefficient. However, for contra flow regime, these trends were reversed. For both regimes, the aforementioned behaviors were more pronounced for CT boundary condition than that for the CHF boundary condition.     

Drag reduction for flow past a square cylinder through corner chamfering

This study investigates the unsteady incompressible flow around a square cylinder with different chamfer ratios (CRs) using a commercial finite volume code, ANSYS Fluent. CR ranges from 0.0 (sharp square cylinder) to 0.5 (diamond cylinder) with variable increments. Detailed analysis of flow characteristics is conducted at Reynolds number (Re) = 2100. Additionally, simulation is extended to cover Re, i.e., Re = 100, 500, and 10000. The simulation results show that cylinder with CR = 0.1 outperforms all other cases by enabling a drag reduction of about 60 % at Re = 10⁴. Drag has an inverse relationship with the wake closure length. Time-averaged coefficient of pressure, streamlines, and vorticity contours are also discussed to better understand near-wake features and the physics of drag reduction.

     

Experimental and numerical study for the cross-flow around four cylinders in an in-line square configuration

The cross-flow around four cylinders in an in-line square configuration with the spacing ratio (L/D) 1.5, 2.5, 3.5 and 5.0 have been investigated experimentally using Laser Doppler Anemometer (LDA) and Digital Particle Image Velocimetry (DPIV). The experiments were carried out in a closed-loop wind tunnel with Reynolds number 1.128× 10⁴ to 1.982× 10⁴. Mean velocity distributions are obtained by LDA. The full field instantaneous and averaged velocity and vorticity components are measured by DPIV The present experimental study indicated that several distinct flow patterns exist. Distinct vortex shedding of the upstream cylinders was suppressed forL/D < 3.5 atRe=1.128× 10⁴. The flow patterns are affected by the spacing ratio andRe. In order to capture the details of the 3-D vortices structures and obtain all the instantaneous physical information, 3-D numerical simulations of the cross-flow around the four cylinders in an in-line square configuration with the spacing ratio 1.5 and 3.5, andRe=1.50× 10⁴ are carried out using large eddy simulation (LES). The numerical results are in good agreement with the experimental results. These results provided full field instantaneous information of the flow structures, velocity field and vorticity field of cross-flow around the four cylinders in an in-line square configuration.     

Fundamental studies on free stream acceleration effect on drag force in bluff bodies

This paper describes fundamental studies on free stream acceleration effect on drag force in bluff bodies. The flow with gradual velocity increase assumed an accelerated flow. The wind tunnel tests were conducted in order to investigate the difference of aerodynamic characteristics between non-accelerated flow and accelerated flow. The experimental models were a circular cylinder and a square cylinder. In an accelerated flow, the condition of free stream was an acceleration of about 3.6m/s². Experimental Reynolds number varied between form 4.0×10⁴ to 1.64×10⁵. The pressure distributions and the aerodynamic force were measured in both case of nonaccelerated flow and accelerated flow. In case of a circular cylinder, the drag of accelerated flow is lower than that of non-accelerated flow in the low Reynolds number regime. Then, it becomes higher than that of non-accelerated flow in the high Reynolds number regime. On the other hand, in case of a square cylinder, the drag of accelerated flow is higher than that of non-accelerated flow in the whole Reynolds number regime. If a separation point can be movable such as the circular cylinder, the additional momentum due to flow acceleration leads to delay separation and to decrease the drag in the low Reynolds number regime. If a separation point is nearly fixed such as a square cylinder, the additional momentum due to acceleration always affects to increase the drag than that of non-accelerated flow. Based on this research, it is expected that the roof shape of high speed train where the separation point is movable has the advantages to reduce the effect of accelerated flow such as strong crosswind/gust.     

Characteristics of flow past a circular cylinder with a rectangular groove

In the present study, features of the flow past a circular cylinder with single longitudinal groove pattern placed on its surface were investigated. Six different rectangular groove sizes were tested for angular position of the groove from the forward stagnation point of the circular cylinder within 0°≤θ≤150°. The particle image velocimetry (PIV) technique were employed to measure flow field downstream of the cylinder immersed in a uniform flow field with the Reynolds number, Re=5000. Time-averaged flow data such as vorticity, 〈ω〉 streamline, 〈Ψ〉, streamwise, 〈u′u′〉 and transverse, 〈v′v′〉 Reynolds normal stresses, turbulent kinetic energy, TKE and RMS of streamwise, u_rms and transverse, v_rms velocity components were obtained from PIV data to demonstrate flow features. Moreover, frequency of Karman vortex shedding was explored using single point spectral analysis. It is concluded that presence of the groove on a cylinder surface significantly affects the near wake flow structure and turbulence statistics. Karman vortex shedding frequency, f_k strongly depends on the groove size. Moreover, the shear layer instability is induced on the grooved side with additional frequencies.     

Flow characteristic of the horizontal cylinder placed on the plane boundary

In this study, the flow characteristic of the circular cylinder laid on the free surface is studied quantitatively using the particle image velocimetry technique. The flow phenomena in the upstream and downstream regions of the horizontal cylinder and free surface junctions increase the instability of the vortical flow structure significantly. Since the scour and burial process of the pipelines laid on the seabed depends mostly on the hydrodynamic behavior of the flow, the present investigation concentrates on the characterization of the flow structures using the time-averaged and instantaneous flow data. Distributions of velocity vectors, patterns of streamlines, vorticity contours and corresponding Reynolds stress correlations characterize the flow structure in detail.     

Numerical investigation of fluid flow past a square cylinder using upstream,downstream and dual splitter plates

A two-dimensional numerical study is carried out to analyze the drag reduction and vortex shedding suppression behind a square cylinder in presence of splitter plate arranged in upstream, downstream and both upstream and downstream location at low Reynolds number (Re = 160). Computations are performed using a Single relaxation time lattice Boltzmann method (SRT-LBM). Firstly, the code is validated for flow past a single square cylinder. The obtained results are compared to those available in literature and found to be in good agreement. Numerical simulations are performed in the ranges of 1 ≤ L ≤ 4 and 0 ≤ g ≤ 7, where L and g are the length of splitter plate and gap spacing between the splitter plate and main square cylinder, respectively. The effect of these parameters on the vortex shedding frequency, time-trace analysis of drag and lift coefficients, power spectra analysis of lift coefficient, vorticity contours visualization and force exerted on the cylinder are quantified together with the observed flow patterns around the main cylinder and within the gap spacings. The observed results are also compared with a single square cylinder without splitter plate. We found that at some combinations of L and g, the mean drag coefficient and Strouhal number reach either its maximum or minimum value. It is found that the drag is reduced up to 62.2 %, 13.3 % and 70.2 % for upstream, downstream and dual splitter plates, respectively as compared to a single square cylinder (without splitter plate). In addition, in this paper we also discussed the applications of SRT-LBM for suppression of vortex shedding and reduction of the drag coefficients.     

Effect of Reynolds numbers on flow past four square cylinders in an in-line square configuration for different gap spacings

In this paper two-dimensional (2-D) numerical investigation of flow past four square cylinders in an in-line square configuration are performed using the lattice Boltzmann method. The gap spacing g = s/d is set at 1, 3 and 6 and Reynolds number ranging from Re = 60 to 175. We observed four distinct wake patterns: (i) a steady wake pattern (Re = 60 and g = 1); (ii) a stable shielding wake pattern (80 ≤ Re ≤ 175 and g = 1); (iii) a wiggling shielding wake pattern (60 ≤ Re ≤ 175 and g = 3) and (iv) a vortex shedding wake pattern (60 ≤ Re ≤ 175 and g = 6). At g = 1, the Reynolds number is observed to have a strong effect on the wake patterns. It is also found that at g = 1, the secondary cylinder interaction frequency significantly contributes for drag and lift coefficients signal. It is found that the primary vortex shedding frequency dominates the flow and the role of secondary cylinder interaction frequency almost vanish at g = 6. It is observed that the jet between the gaps strongly influenced the wake interaction for different gap spacing and Reynolds number combination. To fully understand the wake transformations the details vorticity contour visualization, power spectra of lift coefficient signal and time signal analysis of drag and lift coefficients also presented in this paper.     

Passive control of circular cylinder wake in shallow flow

The control of vortex shedding of a circular cylinder in shallow water using a splitter plate located in the downstream of the circular cylinder was studied by employing particle image velocimetry (PIV) technique. Experiments were carried out in a water channel having a test section of 8000 mm × 1000 mm × 750 mm dimensions at a Reynolds number of 6250. The length of the splitter plate (L) was varied within the range of 0.5 ? L/D ? 2 with an increment of 0.5. The plate was submerged into water at different height ratios (h_p/h_w) such as 0.25, 0.5, 0.75 and 1.0. Mean velocity vector field, corresponding vorticity contours, streamline topologies and turbulent quantities were calculated using 300 instantaneous velocity vector field measured by PIV. As the ratio of h_p/h_w increases, the effect of the splitter plate on the suppression of the vortex shedding increases. Flow characteristics and examination of spectra indicate that Karman vortex shedding is attenuated pronouncedly for the cases of L/D ? 1 and h_p/h_w ? 0.75. The transverse Reynolds normal stress is more effective on the attenuation of turbulent kinetic energy than the streamwise Reynolds normal stress. The value of peak transverse Reynolds normal stress is reduced to 90% of that of the bare cylinder at most.     

An experimental study on swirling flow behind a round cylinder in the horizontal circular tube

An experimental study is performed for turbulent swirling flow behind a circular cylinder using 2-D PIV technique. The Reynolds number investigated are 10,000, 15,000, 20,000 and 25,000. The mean velocity vector, time mean axial velocity, turbulence intensity, kinetic energy and Reynolds shear stress behind the cylinder are measured before and behind the round cylinder along the test tube. A comparison is included with non swirl flow behind a circular and square cylinder. The recirculation zones are showed asymmetric profiles.     

Experimental study on the confined flow over a circular cylinder with a splitter plate

An experimental study was carried out to investigate the effect of a splitter plate on wake flows downstream of a circular cylinder symmetrically placed in a confined channel. A particle image velocimetry (PIV) measurement was applied to visualize the flow structure and analyze changes in the vortex shedding process. The control elements of the splitter plate length, L/D (D is the cylinder diameter) was varied from 0 to 1.5 and Reynolds number, Re_D was considered at 2400 and 3000. The experimental results showed that the splitter plate had an influence on stabilization of wake turbulences in a confined channel. For shorter splitter plate length of L/D=0.5 and 0.75 cases, flow structures were significantly modified and the vortex shedding frequency decreased as compared with bare cylinder cases. For longer splitter plate length of L/D=1, 1.25 and 1.5 cases, the generation of a secondary vortex was observed based on the snapshot proper orthogonal decomposition (snapshot POD) analysis. In addition, turbulent characteristics corresponding to turbulent kinetic energy (TKE) and Reynolds shear stress correlations took the lowest values and the dominant vortex shedding frequency disappeared. There was an optimal value of the splitter plate length at L/D=1 on suppression of velocity fluctuations. Moreover, the stabilizing effect of a splitter plate was more obvious at Reynolds number of Re_D=3000 than that at Re_D=2400.     

Numerical solution of flow past two circular cylinders in different arrangement rotating in various directions

The combined effects of different rotation types and the Reynolds number on the flow past two rotating circular cylinders about their axes in different arrangement (Side-by-side and tandem) were considered at a range of 520 ≤ Re ≤ 1570 and 0 ≤ ω ≤ 4 (ω is the rotational speed) at one gap spacing of L/D = 2 for a side-by-side arrangement, ω = 0, 2000 ≤ Re ≤ 21000 and L/D = 2 and 4/3 for tandem arrangement (L and D are the distance between the centers of two cylinders and the cylinder diameter, respectively). The results show that the variation of both rotation speed and Reynolds number have an important role in changing the pattern of vortex shedding. As the rotational speed further increases, the separation phenomenon in the boundary layers disappears at the attachment rotational speed. Regardless of Reynolds number, as ω increases, the lift decreases for up and down cylinders while the drag decreases for up cylinder and increases for down cylinder. Quantitative information is highlighted about the flow variables such as the pressure coefficient the Stanton number, the skin friction factor and wall viscous coefficient of the cylinders.     

A numerical study on mixed convection in a lid-driven cavity with a circular cylinder

A two-dimensional numerical simulation is carried out in this study to investigate mixed convection in a lid-driven cavity with an isothermal circular cylinder. The simulation is conducted at three Reynolds numbers of Re = 100, 500, and 1000 under a fixed Grashof number of Gr = 10⁵. The top wall of the cavity moves to the right at a constant velocity and is kept at a low temperature of T _c , whereas the stationary bottom wall is kept at a constant high temperature of T _h . The immersed-boundary method, which is based on the finite volume method, is adopted for the boundary of the circular cylinder that is present in the square cavity. The present study aims to investigate the effects of circular cylinder on fluid flow and heat transfer in a cavity at different locations. The fluid flow and heat transfer characteristics in the cavity strongly depend on the position of the circular cylinder as well as on the relative magnitude of the forced convection and the natural convection caused by the movement in the top wall of the cavity and the heating at the hot bottom wall, respectively.     

Direct measurement of bottom shear stress under high-velocity flow conditions

The objectives of the present research are to accurately measure bottom shear stress under high-velocity flow conditions. To achieve high-velocity flow conditions, a laboratory-scale flume has been specially built in which flow velocity can reach over 3 m s⁻¹. Also an instrument that can directly measure bottom shear stress has been developed and validated. Then, the flow resistance has been estimated by simultaneously measuring flow velocity and bottom shear stress. It appears that the shear stress is indeed proportional to velocity squared and also to Reynolds number. On the other hand, Manning's n value and the skin friction factor are more or less uniform across all experimental cases.     

Control of flow around a square cylinder at incidence by using a splitter plate

Passive control of vortex shedding behind a square cylinder at incidence has been conducted experimentally by using a stationary splitter plate for the Reynolds numbers of 3.0×10⁴. The splitter plate was located at the center of the rear face of the square cylinder in tandem. The width of the cylinder and the plate were both chosen to be 30 mm and the incidence angle of the square cylinder was rotated between 0° to 45°. In this study, the combined effects of the splitter plate and angle of incidence on the pressure distributions and vortex-shedding phenomenon were investigated. Vortex shedding frequency was obtained from velocity measurements and aerodynamic force coefficients acted on the cylinder were calculated from pressure distributions. Characteristics of the vortex formation region and location of the flow attachments, reattachments and separation were observed by using the smoke–wire flow visualization technique. For the case with the plate, there is a sudden jump in the Strouhal number in the vicinity of 13° which corresponds to a minimum value of the drag coefficient. At zero angle of incidence, Strouhal number and a drag coefficient of the square cylinder decreased about 20% by means of the splitter plate. Drag reduction was minimum at about 13° and reached its maximum value at about 20°.     

Effects of flexible plate attached to the rear of the cylinder on flow structure

In this experimental study, the flow structure in the wake flow region was investigated with the Particle image velocimetry technique (PIV) by attaching elastic plates at different lengths behind the cylinder. The flow structure occurred at the wake flow region altered depending on the length of the flexible matter. In this experiment, the strips with the lengths of 75, 90, 120, 135 and 180 mm were used to control instabilities. Diameter of the cylinder (D) is 60 mm and the water height (h _w ) is 600 mm. Reynolds number was kept constant as 5000 based on cylinder diameter. The images were captured at mid-height of the cylinder (h _m ) which is 250 mm. As a result of experimental studies, attached flexible strip suppressed vortex shedding occurred in the behind of the cylinder and it is observed that effect of the length flexible of the strip is pretty much. Maximum level of flow characteristics such as Reynolds stress, fluctuation velocities and turbulent kinetic energy were decreased with flexible splitter plate and shifted through the downstream region.     

Effect of circular cylinder location on three-dimensional natural convection in a cubical enclosure

This paper presents the results of immersed boundary method-based three-dimension numerical simulations of natural convection in a cubical enclosure with an inner circular cylinder at a Prandtl number of 0.7. This simulation spans three decades of Rayleigh number, Ra, from 10³ to 10⁶. The location of the inner circular cylinder is changed vertically along the centerline of the cubical enclosure. This study primarily focuses on the effects of both buoyancy-induced convection and the location of the inner circular cylinder on heat transfer and fluid flow in the cubical enclosure. In the range of Rayleigh numbers considered in this study, the thermal and flow fields eventually reach steady state, regardless of the location of the inner cylinder. When Ra is 10³, the end wall of the cubical enclosure has a negligible effect on the thermal and flow fields in the enclosure. However, in the range of 10⁴ ≤ Ra ≤ 10⁶, the effect of the end wall on heat transfer and fluid flow in the enclosure depends on both the location of the inner cylinder and the Rayleigh number. Detailed analysis results for the distribution of streamlines, isotherms, and Nusselt numbers are presented in this paper.     

The effect of low Reynolds number flows on pitot tube measurements

An investigation on the low Reynolds number effect on hemispherical-tipped Pitot tube measurements was performed by measuring the center-line velocity during the laminar flow of a Newtonian fluid in a 25 mm (1 in.) diameter vertical recirculating pipe loop. The primary objective of the study was to reconsider the available low Reynolds number Pitot tube data in the literature with modern instrumentation.Using the results of this experimental study, a correlation that accurately predicts the low Reynolds number Pitot tube behavior has been developed. The correlation accounts for an additional viscous term in the relationship for the pressure coefficient (C_p) which is not accounted for in Bernoulli's Equation. The correlation is semi-empirical and accurately fits experimental data gathered in this study, as well as a significant body of experimental data available in the literature. The correlation, which is based on a Pitot tube Reynolds number calculated using the opening diameter (d), has been shown to be provide more accurate predictions of C_p for a wide range of opening diameter to outer diameter ratios (0.22≤d/D≤0.6) than available correlations based on outer diameter.The transition Pitot tube Reynolds number, below which Bernoulli's Equation is no longer appropriate, was predicted to be approximately 35, compared to a value of 79 obtained from fitting data collected by Barker. The correlation developed in this study provides smoother transitions at both ends of the low Reynolds range. At the low end (Re<10) it converges with a Stokes Law’ analogy, while at the critical transition (Re~35) it converges asymptotically with Bernoulli's Equation. The correlation also accurately predicts the behavior of the pressure coefficient with Reynolds numbers between these ranges.     

Slip flow over micron-sized spherical particles at intermediate Reynolds numbers

A numerical investigation has been performed to identify the rarefaction effects on the flow structure of an isolated micron-sized spherical particle. An isothermal sphere in the slip flow regime 10^?3 ?? Kn ?? 10^?1 at intermediate Reynolds numbers (1 ?? Re ?? 50) is considered. The Navier-Stokes equations are solved by a control volume technique in conjunction with the velocity slip boundary condition. It was found that the wake region can shrink considerably as the Knudsen number increases. Furthermore, the skin friction and pressure drag coefficients decrease as the Knudsen number increases due to the reduction in normal velocity gradients and shrinkage of the wake region, respectively. Engineering correlations for predicting the total drag coefficient in the slip flow regime are presented.