Unconfined Cylinder Research Articles

Onset of laminar separation and vortex shedding in flow past unconfined elliptic cylinders

This article presents the numerical studies on predicting onset of flow separation and vortex shedding in flow past unconfined two-dimensional elliptical cylinders for various Axis Ratios (AR) and a wide range of Angles of Attack (AOA). An efficient Cartesian grid technique based immersed boundary method is used for numerical simulations. The laminar separation Reynolds number (Res) that marks separation of flow from surface and the critical Reynolds number (Recr) which represents transition from steady to unsteady flow are determined using diverse methods. A stability analysis which uses Stuart-Landau equation is also performed for calculating Recr. The shedding frequency (Stcr) that corresponds to Recr is calculated using Landau constants. The simulated results for circular cylinder are found to be in good agreement with the literature. The effects of AR and AOA on Res, Recr, and Stcr are studied. It is observed that the Res, Recr, and Stcr exhibit a direct/inverse relationship with AR depending upon the given AOA. Correlations of Res, Recr, and Stcr with respect to AR and AOA are proposed with good accuracy.

Forced convection in power-law fluids from an asymmetrically confined heated circular cylinder

In this work, the influence of asymmetric confinement on momentum and heat transfer characteristics from a heated circular cylinder submerged in power-law liquids has been investigated numerically. The detailed flow and temperature fields are visualized in terms of the streamline and isotherm contours. Due to the asymmetry of the flow field, in addition to the drag force, there is also a net lift force acting on the cylinder. Furthermore, shear-thinning characteristics tend to modulate the influence of blockage and asymmetry on the rate of heat transfer. All else being equal, shear-thinning behaviour facilitates heat transfer. On the other hand, depending upon the degree of asymmetry, the heat transfer may increase or decrease thereby suggesting a non-monotonic relationship between Nusselt number and asymmetric placement of the cylinder. The results reported herein embrace the following ranges of conditions: Reynolds number, 0.1 ⩽ Re ⩽ 100 ; power-law index, 0.3 ⩽ n ⩽ 1 ; Prandtl number, 1 ⩽ Pr ⩽ 100 ; blockage ratio, β = 0.2, 0.3 and 0.4, and gap ratio, 0.25 ⩽ γ ⩽ 1 . The range of Reynolds number is such that the flow is likely to be in the steady and time-periodic flow regime, at least for an unconfined cylinder.

Strengthening fire-damaged concrete by confinement with fibre-reinforced polymer wraps

Extensive research has shown that fibre-reinforced polymer (FRP) wraps are very effective for strengthening concrete columns for increased axial and flexural load and deformation capacity, and this technique is now widely used around the world. The study reported in this paper extends the FRP confinement technique to strengthening fire-damaged circular concrete columns. An experimental programme was undertaken to study the compressive strength and stress–strain behaviour of both unconfined and FRP-confined plain concrete cylinders after being heated to various elevated temperatures for up to four hours and cooled to room temperature. The results show that FRP confinement is highly effective for enhancing the load-carrying capacity of even severely fire-damaged concrete columns. The results also yield insights into the mechanics of FRP confinement of concretes of similar composition but with varying compressive strengths. A modified version of a pre-existing confinement model is proposed for use in designing FRP strengthening schemes for fire-damaged concrete columns.

Three dimensional flow around a circular cylinder confined in a plane channel

This paper presents two- and three-dimensional direct numerical simulations of the flow around a circular cylinder placed symmetrically in a plane channel. Results are presented in the Reynolds number range (based on the cylinder diameter and centerline velocity) of 10 to 390 for a blockage ratio (ratio of the cylinder diameter to the channel height) of 0.2. The aim of this work was to investigate in detail the confinement effect due to the channel’s stationary walls on the force coefficients and the associated Strouhal numbers, as well as on the generated flow regimes. Present results suggest a transition from a 2-D to a 3-D shedding flow regime between Re = 180 and Re = 210. This transition was found to be dominated by mode A and mode B three dimensional instabilities, similar to those observed in the case of an unconfined circular cylinder. This is the first time that the existence of the two modes, and of naturally occurring vortex dislocations, has been confirmed via full 3-D simulations for the case of a confined circular cylinder in a channel. A discontinuity in the variation of the Strouhal number St, and of the base pressure coefficient Cpb, with Re was also observed. This was found to be associated with the onset of mode A instability and the development of vortex dislocations, and parallels what occurs in the unconfined case, but previous studies could not confirm its existence in the confined case. Furthermore, by analyzing the mechanisms affecting the shape and evolution of these instabilities, it is demonstrated that they are significantly affected by the confinement only in the far wake.

Fiber reinforced cement-based composite system for concrete confinement

This paper reports on a feasibility study to develop a reversible and potentially fire-resistant fiber reinforced cement-based matrix (FRC) composite system for concrete confinement applications. The first part of this study aimed at selecting a candidate FRC system from different fiber (including glass and basalt) and inorganic matrix combinations on the basis of: constructability, by verifying the workability and ease of installation on concrete cylinders; structural performance, by evaluating strength and deformability enhancement in confined concrete cylinders tested under uniaxial compression; and compatibility, by examining the quality of the concrete–FRC interface and the level of fiber impregnation using scanning electron microscope images. In the second part of this study, the selected FRC system was further assessed through compression tests of additional concrete cylinders confined using different FRC reinforcement ratios, where both axial and in-plane (radial) deformations were measured to assess confinement effectiveness. The feasibility of making the application reversible was investigated by introducing a bond breaker between the concrete substrate and the composite jacket in a series of confined cylinders. The prototype FRC system produced a substantial increase in strength and deformability with respect to unconfined cylinders. A superior deformability was attained without the use of a bond breaker. The predominant failure mode was loss of compatibility due to fiber–matrix separation, which points to the need of improving fiber impregnation to enable a more efficient use of the constituent materials. Semi-empirical linear and nonlinear models for compressive strength and deformation in FRC-confined concrete are also presented.

Compression strength of concrete cylinders reinforced with carbon fiber laminate

Composite laminates made from carbon fibers and epoxy resin (CFRP) have been obtained and used as a strengthening of concrete in the form of cylinders. The laminates were made with the use of high-modulus and high-strength carbon fibers. Different types of laminates (1D, 2D) of different thickness were wrapped and glued to the surface of concrete cylinders. The studies were focused on the compressive strength improvement of the cylinders. Compressive strengths and axial strains of unconfined and CRFP-confined concrete cylinders were compared. Compressive strength of concrete cylinders was considerably improved by wrapping the cylinders with the high-strength carbon laminates in the hoop direction.

Confining jackets for concrete cylinders using NiTiNb and NiTi shape memory alloy wires

This study used prestrained NiTiNb and NiTi shape memory alloy (SMA) wires to confine concrete cylinders. The recovery stress of the wires was measured with respect to the maximal prestrain of the wires. SMA wires were preelongated during the manufacturing process and then wrapped around concrete cylinders of 150 mm×300 mm (ϕ×L). Unconfined concrete cylinders were tested for compressive strength and the results were compared to those of cylinders confined by SMA wires. NiTiNb SMA wires increased the compressive strength and ductility of the cylinders due to the confining effect. NiTiNb wires were found to be more effective in increasing the peak strength of the cylinders and dissipating energy than NiTi wires. This study showed the potential of the proposed method to retrofit reinforced concrete columns using SMA wires to protect them from earthquakes.

FRP tube encased rubberized concrete cylinders

In this study, FRP tube encased waste tire rubber modified concrete cylinders were investigated. Four batches of confined and unconfined concrete cylinders with a diameter of 101.6 mm and a height of 304.8 mm were prepared. Each batch contained three confined cylinders and three unconfined cylinders. The total number of effective cylinders prepared was 24. Batches 1–3 were made of rubberized concrete. In Batch 1, 15% by volume of coarse aggregate was replaced by waste tire fibers or stripes; in Batches 2 and 3, 15% by volume of sand and 30% by volume of sand were replaced by crumb rubbers, respectively. Batch 4 was a control batch with conventional plain concrete. Uniaxial compression tests were conducted on all the cylinders per ASTM C39. Strain gages were installed on the encased rubberized concrete cylinders to obtain local strain distributions. The compressive strength and strain at the peak load were compared with the predictions by available design-oriented confinement models. It is found that the FRP tube encased rubberized concrete cylinders have higher confinement effectiveness, ductility, and elastic regions than FRP tube encased conventional concrete cylinders. Waste tire fiber modified concrete performs better than crumb rubber modified concrete with a lower cost. Instead of volume contraction, FRP encased rubberized concrete cylinders experience volumetric expansion. The current design-oriented confinement models cannot consistently predict the compressive strength and strain of the encased cylinders. An 1-D coupon test cannot uniquely determine the hoop tensile strength of the FRP tubes which are subjected to a 2-D stress condition.

Concrete laterally confined with fibre-reinforced polymers (FRP): experimental study and theoretical model

This paper presents the findings of an experimental and analytical study of concrete cylinder behaviour when wrapped in fibreglass or carbon fibre-reinforced polymers (FRP). Compression testing was conducted on normal (30 MPa) and high (70 MPa) strength confined and unconfined concrete cylinders measuring 150 x 300 mm. The stress-strain relationship was evaluated in both cases. The findings showed that strength and ductility rose with FRP confinement. The experimental findings were used to develop an analytical model for predicting the stress-strain behaviour of FRP-confined concrete. A comparison of the experimental and analytical results revealed that the model can satisfactorily predict the stress-strain behaviour and ultimate compressive strength of the concretes studied.

Two-dimensional unsteady flow of power-law fluids over a cylinder

The unsteady flow of incompressible power-law fluids over an unconfined circular cylinder in cross-flow arrangement has been studied numerically. The two-dimensional (2-D) field equations have been solved using a finite volume method based solver (FLUENT 6.3). In particular, the effects of the power-law index ( 0.4 ⩽ n ⩽ 1.8 ) and Reynolds number ( 40 ⩽ Re ⩽ 140 ) on the detailed kinematics of the flow (streamline, surface pressure and vorticity patterns) and on the macroscopic parameters (drag and lift coefficients, Strouhal number) are presented in detail. The periodic vortex shedding and the evolution of detailed kinematics with time are also presented to provide insights into the nature of flow. The two-dimensional flow transits from steady to unsteady behaviour at a critical value of the Reynolds number Re∼(40–50) and the von-Karman vortex street is observed beyond the critical Reynolds number ( Re). Obviously, both the lift coefficient and Strouhal number values are zero for the steady flow, but their values increase with the increasing Reynolds number ( Re) in the unsteady flow regime. For highly shear-thickening fluids ( n=1.8), the flow becomes unsteady at Re=40 and unsteadiness in the flow appears at Re=50 for all values of power-law index ( n). As expected, the evolution of the kinematics and vortex shedding show a complex dependence on the flow parameters near the transition in the flow. For a fixed value of the Reynolds number ( Re), the drag coefficient increases and lift coefficient decreases with increasing value of the power-law index ( n). For a fixed value of the power-law index ( n), the drag coefficient gradually increases with the Reynolds number ( Re). Similar to the drag coefficient, lift coefficient also shows a complex dependence on the power-law index ( n) near the transition zone. The value of the Strouhal number ( St) decreases with the increasing value of the power-law index ( n) at a fixed value of the Reynolds number ( Re).

Numerical Modeling of Concrete Cylinders Confined with CFRP Composites

This article presents the results of numerical analyses of concrete cylinders (i.e., columns) confined in composites jackets. The specimens were subjected to uniaxial compression and the analysis was carried out using a non-linear finite element method. Various parameters such as wrap thickness, fiber orientation, concrete strength, and interfacial bonding were considered. Three different wrap thicknesses (0.9, 1.8, and 2.7mm), fiber orientation of 0°, ±15°, ±30°, and ±45° with respect to the hoop direction, and concrete strength values ranging from 20 MPa to 40 MPa were investigated. The finite element analysis results were in good agreement with experimental data presented by other researchers. The numerical analysis results demonstrated significant enhancement in the compressive strength, stiffness, and ductility of the CFRP-wrapped concrete cylinders compared to unconfined concrete cylinders. The stress—strain response of cylinders was greatly affected by analysis parameters.

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)

Sedimenting sphere in a variable-gap Hele-Shaw cell

We have measured the trajectory and visualized the wake of a single sphere falling in a fluid confined between two closely spaced glass plates (a Hele-Shaw cell). The position of a sedimenting sphere was measured to better than 0.001d, wheredis the sphere diameter, for Reynolds numbers (based on the terminal velocity) between 20 and 330, for gaps between the plates ranging from 1.014dto 1.4d. For gaps in the range 1.01d–1.05d, the behaviour of the sedimenting sphere is found to be qualitatively similar to that of an unconfined cylinder in a uniform flow, but our sedimenting sphere begins to oscillate and shed von Kármán vortices forRe>200, which is far greater than theRe= 49 for the onset of vortex shedding behind cylinders in an open flow. When the gap is increased to 1.10d–1.40d, the vortices behind the sphere are different – they are qualitatively similar to those behind a sphere sedimenting in the absence of confining walls. Our precision measurements of the velocity of a sedimenting sphere and the amplitude and frequency of the oscillations provide a benchmark for numerical simulations of the sedimentation of particles in fluids.

Steady flow of power-law fluids across an unconfined elliptical cylinder

The momentum transfer characteristics of the power-law fluid flow past an unconfined elliptic cylinder is investigated numerically by solving continuity and momentum equations using FLUENT (version 6.2) in the two-dimensional steady cross-flow regime. The influence of the power-law index ( 0.2 ⩽ n ⩽ 1.8 ) , Reynolds number ( 0.01 ⩽ Re ⩽ 40 ) and the aspect ratio of the elliptic cylinder ( 0.2 ⩽ E ⩽ 5 ) on the local and global flow characteristics has been studied. In addition, flow patterns showing streamline and vorticity profiles, and the pressure distribution on the surface of the cylinder have also been presented to provide further physical insights into the detailed flow kinematics. For shear-thinning ( n < 1 ) behaviour and the aspect ratio E > 1 , flow separation is somewhat delayed and the resulting wake is also shorter; on the other hand, for shear-thickening ( n > 1 ) fluid behaviour and for E < 1 , the opposite behaviour is obtained. The pressure coefficient and drag coefficient show a complex dependence on the Reynolds number and power-law index. The decrease in the degree of shear-thinning behaviour increases the drag coefficient, especially at low Reynolds numbers. While the aspect ratio of the cylinder exerts significant influence on the detailed flow characteristics, the total drag coefficient is only weakly dependent on the aspect ratio in shear-thickening fluids. The effect of the flow behaviour index, however, diminishes gradually with the increasing Reynolds number. The numerical results have also been presented in terms of closure relations for easy use in a new application.

Steady Flow of Power Law Fluids across a Circular Cylinder

AbstractThe momentum equations describing the steady cross‐flow of power law fluids past an unconfined circular cylinder have been solved numerically using a semi‐implicit finite volume method. The numerical results highlighting the roles of Reynolds number and power law index on the global and detailed flow characteristics have been presented over wide ranges of conditions as 5 ≤ Re ≤ 40 and 0.6 ≤ n ≤ 2. The shear‐thinning behaviour (n &lt; 1) of the fluid decreases the size of recirculation zone and also delays the separation; on the other hand, the shear‐thickening fluids (n &gt; 1) show the opposite behaviour. Furthermore, while the wake size shows non‐monotonous variation with the power law index, but it does not seem to influence the values of drag coefficient. The stagnation pressure coefficient and drag coefficient also show a complex dependence on the power law index and Reynolds number. In addition, the pressure coefficient, vorticity and viscosity distributions on the surface of the cylinder have also been presented to gain further physical insights into the detailed flow kinematics.

A numerical study of the steady forced convection heat transfer from an unconfined circular cylinder

Forced convection heat transfer from an unconfined circular cylinder in the steady cross-flow regime has been studied using a finite volume method (FVM) implemented on a Cartesian grid system in the range as 10 ≤ Re ≤ 45 and 0.7 ≤ Pr ≤ 400. The numerical results are used to develop simple correlations for Nusselt number as a function of the pertinent dimensionless variables. In addition to average Nusselt number, the effects of Re, Pr and thermal boundary conditions on the temperature field near the cylinder and on the local Nusselt number distributions have also been presented to provide further physical insights into the nature of the flow. The rate of heat transfer increases with an increase in the Reynolds and/or Prandtl numbers. The uniform heat flux condition always shows higher value of heat transfer coefficient than the constant wall temperature at the surface of the cylinder for the same Reynolds and Prandtl numbers. The maximum difference between the two values is around 15–20%.

Effect of power-law index on critical parameters for power-law flow across an unconfined circular cylinder

The conditions for the formation of a wake and for the onset of wake instability for the flow of power-law fluids over an unconfined circular cylinder are investigated numerically by solving the continuity and momentum equations using FLUENT (version 6.2). The effect of power-law index on the critical Reynolds numbers, Strouhal number and drag coefficient has been presented over a wide range of power-law index ( 0.3 ⩽ n ⩽ 1.8 ) thereby establishing the limits of the flow without separation and the steady symmetric flow regimes, respectively. While both the shear-thinning ( n < 1 ) and the shear-thickening ( n > 1 ) seem to lower the value of the critical Reynolds number denoting the onset of wake instability as compared to that for Newtonian fluids, the effect is seen to be more prominent for shear-thickening fluids than that for shear-thinning fluids. The corresponding values of the critical Strouhal number ( St c ) and drag coefficient have also been presented for the critical values of the Reynolds number. Included here are also a series of streamline plots showing the onset of asymmetry and of the time-dependent flow regime.

Dead Zones in LX‐17 and PBX 9502

AbstractPin and X‐ray corner turning data have been taken on ambient LX‐17 and PBX 9052, and the results are listed in tables as an aid to future modeling. The results have been modeled at 4 zones/mm with a reactive flow approach that varies the burn rate as a function of pressure. A single rate format is used to simulate failure and detonation in different pressure regimes. A pressure cut‐off must also be reached to initiate the burn. Corner turning and failure are modeled using an intermediate pressure rate region, and detonation occurs at high pressure. The TATB booster is also modeled using reactive flow, and X‐ray tomography is used to partition the ram‐pressed hemisphere into five different density regions. The model reasonably fits the bare corner turning experiment but predicts a smaller dead zone with steel confinement, in contradiction with experiment. The same model also calculates the confined and unconfined cylinder detonation velocities and predicts the failure of the unconfined cylinder at 3.75 mm radius. The PBX 9502 shows a smaller dead zone than LX‐17. An old experiment that showed a large apparent dead zone in Composition B was repeated with X‐ray transmission and no dead zone was seen. This confirms the idea that a variable burn rate is the key to modeling. The model also produces initiation delays, which are shorter than those found in time‐to‐detonation.

Steady Flow of Power-law Fluids Across a Square Cylinder

The two-dimensional flow of power-law fluids over an isolated unconfined square cylinder has been investigated numerically in the range of conditions 1 ≤ Re ≤ 45 and 0.5 ≤ n ≤ 2.0. The global quantities such as wake length, drag coefficients and the detailed kinematic variables like stream function, vorticity and so on, have been calculated for the above range of conditions. In particular, the effects of Reynolds number and of the power-law index have been investigated in the steady flow regime. The shear-thinning fluid behaviour increases the drag above its Newtonian value whereas the shear-thickening behaviour reduces the drag below its Newtonian value. However, as the value of the Reynolds number is gradually increased, the role of power-law index diminishes. Similarly, the wake size is shorter in shear-thinning fluids than that in Newtonian fluids under otherwise indentical conditions.

Experimental study of FRP confined concrete cylinders

In this study, two types of fiber reinforced polymer (FRP) confined concrete cylinders were prepared. One was FRP jacketed concrete cylinders; the other was FRP tube encased concrete cylinders. A total of 24 jacketed cylinders and 15 encased cylinders were prepared. For the jacketed cylinders, six fiber orientations and two FRP wall thicknesses were used; for the encased cylinders, four batches of concrete with normal to high strength were used and both bonded and unbonded interfacial conditions were considered. It is found that insufficiently confined cylinders behave similar to unconfined cylinders. FRP cannot confine the concrete core until the concrete is damaged (cracked or crushed) due to the larger transverse Poisson’s ratio and lower axial stiffness of FRP. The rate of increase in confinement effectiveness decreases nonlinearly as confinement ratio increases. A considerable deviation is found between the prediction by existing design-oriented confinement models and test results.