Short Circular Cylinder Research Articles

Passive control of porous media on the aerodynamic forces and wake structures of wall-mounted short circular cylinders

This paper numerically investigates the aerodynamic forces and the three-dimensional wake characteristics of wall-mounted circular cylinders with and without porous media coatings using large eddy simulation at a Reynolds number of 3.2×104. Short cylinders with aspect ratios of 0.5, 1.0, and 3.0 are considered, with one end fixed to a bottom wall in the current work. The study focuses on aerodynamic coefficients, flow characteristics, and wake structures for cylinders both with and without porous coatings. The statistical results indicate that porous media significantly alter flow patterns behind the cylinders, suppress downwash flow from the free end, and reduce velocity fluctuations and turbulent kinetic energy within the wake. The porous coating enhances the leeward side's base pressure, leading to a reduction in drag on the cylinder surface. The analysis of flow structures reveals that the topology of the arch vortex behind solid cylinders is significantly dependent on the aspect ratio, whereas this dependency is negligible for porous cylinders. Porous coatings diminish the intensity of the tip and trailing vortices behind the cylinder. Finally, based on the time-averaged flow field, we proposed two conceptual models of topological correlation for wall-mounted short cylinders, both with and without porous coatings, which contributes to describing the geometric characteristics and interactions of vortex structures.

Near-wake flow and thermal characteristics of three side-by-side circular cylinders for large temperature differences using large-eddy simulation

The trailing edge of a gas turbine blade contains rows of short circular cylinders which extract the heat from blade, with the help of the coolant. Large temperature differences exist between the incoming coolant and the blade, and its effect on the flow and thermal characteristics need to be understood. To this end, in the present work, non-isothermal flow over three side-by-side circular cylinders is investigated for a Reynolds number of 3900 using large-eddy simulation (LES). The numerical solver used is validated for isothermal flow using reference experimental data. Large temperature differences, ranging from 25∘C to 300∘C, between the incoming flow and the surface of the cylinders are considered and their effect on the flow and thermal characteristics in the near wake region of cylinders are analyzed. The cylinder-surface and wake characteristics are analyzed using instantaneous and mean quantities. The surface characteristics of the cylinders are studied using Nusselt number, wall shear stress, and lift and drag coefficients. The wake characteristics are studied using Reynolds stresses, turbulent heat flux, and turbulence anisotropy invariant maps. Phase-averaging is used to analyze the unsteady flow and topology of the wake. It is observed that, compared to the middle cylinder the production of turbulence and diffusion of heat is higher behind the two outer cylinders, and their location remains stationed independent of the time. Convection and diffusion of heat takes place along the free shear layer of the wake of the middle cylinder. The vortex shedding behind the cylinders are noted to have multiple frequencies. Further, the vortices shed by the outer cylinders contain more energy and at higher-frequencies when compared to those shed by the middle cylinder.

Modeling of thermal stresses during hardening the product surface by thermal pulse

A particular solution of a linear variant of the dynamic thermal elasticity problem is considered in application to modeling the conditions of surface hardening of metal products by an energy pulse. The authors determined the equation of medium motion with the model of temperature pulse tested earlier for compatibility with special cases of the equations of parabolic and hyperbolic thermal conductivity. The problem of loading a flat plane of a short circular cylinder (disk) with a temperature pulse is presented. Pulse is a consequence of adopted structure of the volumetric power density of the heat flux, the time multiplier of which has the form of a single wave of the Heaviside function. Classical thermoelastic displacement potential and the method of its division into the product of independent variables functions were used to construct the thermal stress tensor. Differential equations for multiplier functions and their general solutions were found. Natural boundary conditions were set for the components of thermal stress tensor, and their tasks were solved. The obtained solutions are in the form of segments of functional series (the Bessel function in radial coordinate and the exponential function in axial coordinate). The article considers a numerical example of loading a disk made of 40KhN steel which has the mechanical properties sensitive to temperature treatment. Maple computer mathematics package was used in the calculations. Approximate solutions take into account the first 24 terms of the functional series. Estimation of the example makes it possible to explain the presence of stress peaks and stress intensity as a consequence of mutually inverse processes of temperature stress growth and reduction of elasticity coefficients with temperature rise. The numerical example warns against relying only on estimates of solutions to thermoelasticity problems without taking into account the plastic and viscous properties of the material.

Three-dimensional wake structures controlled by the flow issuing from a horizontal hole in a wall-mounted short cylinder

This paper examines the three-dimensional (3D) wake flow structures induced by a wall-mounted short circular cylinder. The structures are controlled by the flow issuing from a horizontal hole (HH) drilled from the front to the rear of the cylinder. The 3D velocity fields are instantaneously measured at a Reynolds number of 10,720 using high-resolution tomographic particle image velocimetry (Tomo-PIV) in a water tunnel. The 3D vorticity fields, Q criterion, and characteristics of arch-type and tip vortices are compared between the HH cylinders and a standard cylinder based on the measured instantaneous 3D velocity fields. A 3D W-type arch vortex appears behind the short HH cylinders. Compared with the standard cylinder, the 3D W-type arch vortex and large-scale vortices break down more rapidly in the wake of the HH cylinders. The flow from the HH may reduce the rear recirculation region to 15%, effectively controlling the wake structures of short cylinders. A 3D wavelet multiresolution analysis is used to decompose the 3D velocity fields given by the Tomo-PIV data. The large-scale streamwise vortices in the HH cylinder wakes are smaller than those in the standard cylinder wake, and this becomes more evident as the hole height increases. The intermediate-scale vortices are also controlled by the flow issuing from the hole.

Three-Dimensional Flow Structures Behind a Wall-Mounted Short Cylinder with a Rear Inclined Hole

Three-dimensional (3-D) flow structures around a wall-mounted short circular cylinder, into which was drilled a rear inclined hole (RIH) going from the rear surface to the top surface of the cylinder, were instantaneously measured at Reynolds number 10,720 in a water tunnel by high-resolution tomographic particle image velocimetry (Tomo-PIV). Based on the measured instantaneous 3-D velocity distribution, the 3-D vorticity field, the criterion, and characteristics of arch-type and tip vortices were compared between the RIH cylinders and a standard cylinder. A 3-D -type arch vortex appeared behind the standard and RIH cylinders, and the height of the RIH cylinders was higher than that of the standard cylinder. Compared to the standard cylinder, the 3-D -type arch vortex and large-scale vortices broke down slowly in the wake of the RIH cylinders. A 3-D orthogonal wavelet multiresolution technique was used to decompose the 3-D velocity fields from the Tomo-PIV data. The time-averaged large-scale structures of the RIH cylinders exhibited a stronger M-shape arch vortex, and a strong -type-shape arch structure was extracted from the time-averaged intermediate-scale structures. Meanwhile, the tip vortices simultaneously existed in the large- and intermediate-scale structures. The heights of instantaneous large-scale streamwise structures were proportional to the hole height of the RIH cylinders. Large-scale spanwise vorticity components increase and the strength of vortex shedding decreases when using an RIH cylinder.

The effect of base column on vortex-induced vibration of a circular cylinder with low aspect ratio

The end condition of a cylinder is known to influence its vortex-induced vibration (VIV) response. The design of a short circular cylinder with a concentric base column attached to its bottom is adopted by some floating structures for improved stability, but how the base column would affect the VIV of the cylinder has been rarely studied. In this paper, the VIV of an elastically mounted rigid cylinder with a base column and a low aspect ratio of 2 is investigated numerically by solving the Navier-Stokes equations. The numerical methods are validated against two existing VIV studies, including a 2D cylinder and a cylinder with a finite length. The impacts of the base column on the cylinder are analysed. It is found that although the free end effects associated with fluid flowing around the cylinder end are still present, attaching the base column leads to the expansion of the lock-in regime of the cylinder response. A relationship between non-dimensional response amplitude and lift coefficient is established, which takes into consideration the geometrical properties of the base column. By analysing the energy transfer from fluid to structure, the base column is also found to have significant damping effects on the cylinder response.

Effect of a horizontal hole on flow structures around a wall-mounted low-aspect-ratio cylinder

To control the vortex structures and separation region of a low-aspect-ratio cylinder wake, a passive flow control method that is to drill a horizontal hole from the front side surface to the rear side surface was proposed for a short circular cylinder having an aspect ratio H/D = 1 with height H and diameter D of 70 mm. The PIV measurements were performed at Reynolds number of 8570 in a circulation water tunnel in order to compare the flow characteristics between the controlling and no-controlling wakes. Furthermore, to study the position effect of the horizontal hole, three kinds of the hole positions having different height h from wall were tested. It was found that the jet flow from a horizontal hole leads to suppress the vortex formation and to reduce the rear separation region. The vorticity, Reynolds shear stresses and TKE were evidently suppressed by the jet flow of the horizontal hole in the rear recirculation zone, resulting in the reduction of the drag acted on the cylinder. Meanwhile, the instantaneous large-scale vortical structures of the rear recirculation zone were broken down into several small-scale vortices by the jet. As increasing the height of hole, the downwash flow is evidently suppressed and the upwash flow is increased. Spanwise vortex shedding is altered from symmetric to asymmetric arrangement.

Utilising flags to reduce drag around a short finite circular cylinder

ABSTRACTThis paper utilises flags to decrease the drag around a short finite circular cylinder. Wall-adapted large eddy simulation and two-way fluid–structure interaction methods were applied to resolve unsteady turbulent flow structure. The far-field Reynolds number of the current configuration based on the cylinder diameter was chosen to be 20,000. In addition, the length-to-diameter ratio of the cylinder was assumed to be L/D = 2 whereas the flexible flag had a width-to-diameter ratio of W/D = 1.5. The results were compared with the regular short finite circular cylinder and the rigid flagged cylinder in our previous work. The results indicate that utilising flags inside the near-wake region of the cylinder reduces the pressure drag. The physical mechanism of this drag reduction is presented.

Prediction of plane‐wise turbulent events to the Reynolds stress in a flow over scour‐bed

In this paper, we intend to quantify the contribution of turbulent events to the total Reynolds shear stresses , , and from the four different quadrants of three different planes (xy, xz, yz); and to make a comparative study among the planes in the scour geometry developed by short circular cylinder of fixed length with a fixed diameter placed over the sand bed transverse to the flow. We also intend to predict the magnitude of covariance terms , , and and their contributions in the four quadrants by making use of the conditional probability distribution of the Reynolds shear stresses , , and , which can be derived by applying the cumulant‐discard method to the Gram‐Charlier probability distribution of the two variables. This consideration motivates the work on the flow over the obstacle marks generated on sand bed using different short cylinders. The contributions of burst‐sweep cycles to the Reynolds shear stresses from the planes over and within the scour around the obstacle are computed using the quadrant analysis to identify the leading shear stress plane, which are responsible to form the scour geometry. It is discovered that the yz and xy‐ planes are much more important in the scouring regions, whereas xz‐ plane is important for the smooth surface. Using cumulant‐discard method (taking into account the cumulants of less than fourth order), it is shown that the qualitative behaviours of turbulent events agree well with experimental data. Thus, it is confirmed that even the third‐order probability distribution of the Reynolds stresses can describe the experimental results very well.

Experimental investigation of free convection from short horizontal cylinder to Newtonian and power-law liquids of large Prandtl numbers

This paper reports the results of an experimental investigation of free convection from electrically-heated short horizontal circular cylinder of small aspect ratio (L/d=8) to five liquids of large Prandtl numbers. Three of these fluids are Newtonian and two are weakly power-law shear thinning fluids. Experiments are conducted, under constant heat flux, at various bulk temperatures (40–175°C) and heat fluxes (0.66–104kW/m2) that correspond to heat generation from about 0.7 to 113MW/m3. Effects of oil bulk temperatures, oil properties and heat generation parameter, on the free convection results, are investigated.The results indicated that Newtonian fluids attain higher free convection heat transfer coefficient than the power-law fluids by 16% or more. Nusselt number (Nu=hd/k) of the laminar free convection data is correlated in terms of the power-law index and in classical and non-classical forms. Correlations cover Grashof number (Gr∗=gβqd4/kυ2) from 33 to 1.68×107, Rayleigh number (Ra∗=gβqd4/kυα) from 6.1×104 to 6.88×108, Prandtl numbers (Pr=cpμ/k) from 30 to 1850, Prandtl numbers ratio (Prb/Prw) from 1.03 to 133 and heat generation parameter from 0.001 to 0.009. The developed non-classical correlation that is based on the presented experimental data is the first correlation to consider the heat generation parameter. It predicts the experimental Nu within ±5% whereas the classical form correlation predicts Nu with uncertainty over ±20%.

Thermal flows around a fully permeable short circular cylinder

Secondary flows and corresponding endwall heat transfer around a fully permeable (porous) short circular cylinder differ from those around an impermeable (solid) cylinder. There exists a through flow which is portion of the mainstream entering the front of the cylinder and exiting at the rear of the cylinder, and the alteration of an adverse pressure gradient along the plane of symmetry that causes three-dimensional boundary layer separation (or forms horseshoe vortices). This study experimentally demonstrates how these distinctive features vary horseshoe vortices, wake patterns, and endwall heat transfer around the porous short circular cylinder. Furthermore, the fluidic effect of the extent of the through-flow that is determined by the permeability of the porous cylinder on downstream heat transfer is discussed.

Numerical characterization of three-dimensional bluff body shear layer behaviour

Three-dimensional bluff body aerodynamics are pertinent across a broad range of engineering disciplines. In three-dimensional bluff body flows, shear layer behaviour has a primary influence on the surface pressure distributions and, therefore, the integrated forces and moments. There currently exists a significant gap in understanding of the flow around canonical three-dimensional bluff bodies such as rectangular prisms and short circular cylinders. High-fidelity numerical experiments using a hybrid turbulence closure that resolves large eddies in separated wakes close this gap and provide new insights into the unsteady behaviour of these bodies. A time-averaging technique that captures the mean shear layer behaviours in these unsteady turbulent flows is developed, and empirical characterizations are developed for important quantities, including the shear layer reattachment distance, the separation bubble pressure, the maximum reattachment pressure, and the stagnation point location. Many of these quantities are found to exhibit a universal behaviour that varies only with the incidence angle and face shape (flat or curved) when an appropriate normalization is applied.

Investigation of turbulent flow past a flagged short finite circular cylinder

ABSTRACTThis study was conducted to investigate the flow structures of turbulent flow passing over a short finite cylinder in which a rigid flag is attached to the rear of the cylinder, in wake region. The length-to-diameter ratio of the cylinder was chosen to be L/D = 2, whereas the rigid flag had a width-to-diameter ratio of W/D = 1.5. Wall-adapted large-eddy simulation (LES-WALE) was used to resolve unsteady turbulent flow structures. The far field Reynolds number based on cylinder diameter was chosen to be 20,000. The results were compared with the regular case wherein no flag was attached to the cylinder. Results revealed that the flow pattern behind the cylinder with flag was totally different in comparison with the regular case one. However, top free end of the cylinder was not influenced by the flag in contrast with the wake region. At far downstream from the cylinder, most of the flow structures in both cases appeared the same. The horseshoe vortices in both cases appeared to be an unsteady phenomenon, with slightly different patterns. Moreover, in the case of flag attachment, the pressure coefficient was smaller than that of with no flag. Finally, it was shown that the main and secondary Strouhal numbers locations were different in both cases.

Gravity and Surface Tension Effects on the Shape Change of Soft Materials

Surface tension and gravity, whose influence on the deformation of conventional engineering materials is negligible, become important for soft materials that are typically used in the microfabrication of devices such as microfluidic channels. Although for soft materials the shape change due to these forces can be large, it is often neglected in the design processes. To capture conditions under which the influence of these forces is important, we propose a deformation map in which the shape change is captured by two dimensionless material parameters. Our idea is demonstrated by simulating the large deformation of a short circular cylinder made of a neo-Hookean material in frictionless contact with a rigid substrate. These simulations are carried out using a finite element model that accounts for surface tension and gravity. Our model integrates the two different approaches typically used to determine the shape change of solids and liquid drops in contact with a substrate.

Two classes of vibrations of a short circular cylinder

This paper gives a solution of the stationary dynamic problem of elasticity which describes two classes of natural nonaxisymmetric vibrations of a finite circular cylinder. In the particular case of axial symmetry, the resulting solution describes two well-known classes of axisymmetric vibrations: vibrations of the first class become longitudinal-transverse vibrations and vibrations of the second class become torsional vibrations. The existence of two classes of nonaxisymmetric vibrations is due to the boundary conditions at the ends. It is shown that as the length (height) of the cylinder increases, the effect of the boundary conditions at the ends on the frequency spectrum reduces, and the vibration frequencies of the two classes become similar and then identical.

Contributions of burst-sweep cycles to Reynolds shear stress over fluvial obstacle marks generated in a laboratory flume

This paper investigates the phenomena of turbulent bursting events, and to make a comparative study among the events in the scour geometry developed by short circular cylinders of fixed length with different diameters placed on the sand bed transverse to the flow. The distributions of turbulent kinetic energy (TKE), turbulent diffusion in both longitudinal and vertical directions at different locations of scour marks, and the contributions of burst-sweep cycles to the Reynolds shear stress are presented.

Three-dimensional vortex flow near the endwall of a short cylinder in crossflow: Uniform-diameter circular cylinder

Flow characteristics, around a short uniform-diameter circular cylinder in crossflow, are investigated experimentally. Extensive flow visualization using oil-lampblack and smoke-wire methods have been performed. Near-wake velocity measurements have been performed using a hotwire anemometer. Complex secondary flows are observed on and around the cylinder in crossflow. Multiple vortices are observed in the horseshoe vortex system near the cylinder–endwall junction. Based on this flow visualization and local mass transfer measurement results, a six-vortex secondary flow model has been proposed.

Flow-induced motion of a short circular cylinder spanning a rectangular channel

Abstract in Undetermined Flow-induced oscillation of an elastically supported circular cylinder subject to an incompressible fluid at Re=100 and 400 has been studied using three-dimensional simulations. The cylinder is either subjected to a uniform flow in an unconfined surrounding or confined by rectangular channel, i.e. the cylinder is confined in both the span-wise and the cross-stream directions. The cylinder surface is represented by a virtual boundary method which replaces a solid object in flow by additional force distribution to satisfy local boundary condition. The simulations have been performed for nondamped cylinder with low mass ratio. For the unconfined cylinder, the effects of Reynolds number are studied. The effects of confinement have been investigated by varying the extent of the channel (and thereby also the cylinder length) in the span-wise direction. The maximal amplitude is found to be decreased by the confinement. For very strong confinement (short cylinder) the motion of the cylinder is governed by the natural frequency of the structure even beyond synchronisation range. The upper boundary of the synchronisation range exhibits no dependency on the confinement, only on Reynolds number. (Less)

Stress Concentration Effects in Short Cylindrical Vessels With Holes Subjected to Tension: A Complete Account

Whenever regular geometric discontinuities are present the so called stress concentration factors concept is widely used in both analysis and design of loaded components especially when subjected to fatigue, frequently the working condition of vessels. However, recent observations suggest that the influence of member length on the magnitude of the stated factors was not considered in previous analyses. In this work, this observation was studied in the context of cylindrical vessels and it was found that in this case, as well, length could be a critical factor when computing stresses developed as a result of externally applied loads. Accordingly, the values of the finite element calculated theoretical stress concentration factors are computed, for the case of short circular cylinders with circular holes subjected to axial tension, in the context of elastic shell theory, and are presented in a fashion similar to existing published results. It is shown that significantly larger stress concentrations appear for shorter members. The transition length concept defining the threshold between long cylinders and short cylinders is discussed in the context of this study and reported as well.

Experiments on free convective heat transfer from side walls of a vertical square cylinder in air

This communication presents the results of experimental investigations on average natural-convection heat transfer from the isothermal vertical surfaces of a vertical short and slender square cylinder to air obtained with a lumped capacitance method. A validation experiment performed with the short circular cylinder showed very good agreement with the formula of McAdams [Heat Transmission, McGraw-Hill, New York, 1954] recommended for a flat isothermal plate. Results of average heat transfer measurements for side walls of the vertical square cylinder are approximated with the formula: Nu H =1.10 Ra H 0.2( H/ W) 0.123 in the range 1.8×10 4< Ra H <5.5×10 8 and 1< H/ W<21 (where: Nu H ––average Nusselt number, Ra H ––Rayleigh number, H and W––height and width of the square cylinder).