Three-dimensional Flow Regime Research Articles

Three-dimensional wake transition behind an elliptic cylinder near a moving wall

Three-dimensional flow past an elliptic cylinder with an aspect ratio of 0.5 near a moving bottom wall is investigated numerically for gap ratios of G/D=0.1,0.2,0.3, and 0.4 (where G denotes the gap between the cylinder bottom and the moving wall, and D is the major-axis length of the cylinder) with Reynolds numbers (Re) ranging from 100 to 200 (based on a constant inlet velocity and the major-axis length of the cylinder); the transition between two- and three-dimensional flow regimes is described in detail. For G/D=0.4, the flow is first two-dimensional with a Kármán vortex street followed by a two-layered wake, then it evolves into a three-dimensional flow regime with near-wake and far-wake elliptic instabilities of vortex pairs; for Re≥180, the near-wake elliptic instability disappears (i.e., the near wake becomes two-dimensional) while the far-wake elliptic instability persists. For G/D=0.3, the flow is first two-dimensional without the development of the two-layered wake, then it evolves into a three-dimensional flow regime with streamwise vorticity pairs propagating periodically in the spanwise direction; this propagation becomes irregular for Re≥160. For G/D=0.2, the flow is first two-dimensional as for G/D=0.3, then it becomes three-dimensional, exhibiting a behavior of modified mode C instability; for Re≥140, this flow exhibits a chaotic behavior. For G/D=0.1, the flow is first three-dimensional and steady without vortex shedding, and then develops into an unsteady flow with a dominating upper shear layer in the near-wake and a chaotic wake structure farther downstream.

Numerical Study of Magnetic Field Influence on Three-Dimensional Flow Regime and Combined-Convection Heat Exchange Within Concentric and Eccentric Rotating Cylinders

Abstract The forced and natural flows of fluid within an annulus caused by the rotation of cylinders and temperature differences of the inner and outer walls are observed in various engineering applications. In this research, the laminar flow regime and mixed convection inside a ring-shaped horizontal concentric and eccentric space for an incompressible fluid are studied in the existence of an axial magnetic field. The present work is the first effort to investigate the influence of a magnetic field on flow and combined-convection heat exchange characteristics within an annulus with a cold outer cylinder and an inner hot cylinder. Here, the properties of the flow and heat transfer characteristics are studied using the finite volume method. Numerical procedures are mainly investigated for recognizing the influence of Hartmann number (in the range of 0 ≤ Ha ≤ 100), as the representative of the magnetic force, on velocity components, Nusselt number, streamlines, and isothermal lines. One of the notable effects is that when Ha number increases, it will reduce the vorticity of the fluid and buoyancy forces. As a result, streamlines and isothermal lines can be seen more constant as regular concentric circles. A rise in Ha number decreases the range of local Nu number variation for both cylinders. The average Nu number for the outer and inner cylinders has different trends when Ha number increases. Taking concentric cylinders as an example, this parameter for the inner and the outer cylinders increases and decreases by about 1.2 and 1.6, respectively.

Investigation of unsteady flow regime in a T-shaped channel by means of Time-Resolved PLIF and PIV techniques

T-shaped channels are widely used as micro-mixers in microfluidic systems for chemical and biochemical engineering applications. The mixing efficiency of T-shaped channels relies on three-dimensional flow structure which is dependent on Reynolds number. The most favourable flow regime with relation to the mixing efficiency is an unsteady engulfment flow regime, which has a periodic nature. In the present work, the spatial and temporal scales formed in a T-shaped channel are studied at Reynolds number corresponding to unsteady three-dimensional flow regime. The combination of PLIF and PIV techniques with the high temporal resolution was used in the T-channel cross-sections to investigate the evolution of velocity and concentration fields.

Experimental investigation of three-dimensional flow regimes in a cross-shaped reactor

Flow regimes in a cross-shaped reactor with square cross sections of two inlets and two outlets were investigated at 20 ≤ Re ≤ 500, where Re is the Reynolds number. Visualization images on cross sections were obtained by planar laser induced fluorescence, and several flow regimes were identified. Results show that, with increasing Re, a symmetric segregated flow, a steady engulfment flow, an unsteady engulfment flow, and an unsteady symmetric flow emerge in turns. First, the symmetric segregated flow is formed at Re < 48. At 48 ≤ Re < 300, the flow becomes asymmetric and a spiral vortex is formed in the center of the outlet chamber, which is called steady engulfment flow. At 300 ≤ Re ≤ 400, the unsteady engulfment flow occurs and a periodic oscillation is established. With a further increase in Re, the flow regains symmetry to a large extent and is characterized by axial oscillation of the impingement plane in the outlet chamber. For steady engulfment flow, an interesting three-dimensional vortical structure was observed, which rotates around the center axis of the outlet chamber along both outlet channels. For unsteady engulfment flow, the periodic oscillation is characterized by vortex merging and breakup. The flow mechanisms of both steady and unsteady engulfment flows were discussed.

Investigation of three-dimensional flow regime and mixing characteristic in T-jet reactor

Three-dimensional flow regimes in the T-jet reactor were experimentally studied by planar laser induced fluorescence (PLIF) at 50 ≤ Re ≤ 560. The visualization images on cross-sections at different Reynolds numbers were obtained to investigate the engulfment flow regime and its influence on the mixing. With unsteady engulfment flow, periodic vortex merging in the impingement plane is observed and characterized. Results show that the merged vortex passes through the channel and causes periodic oscillation in the chamber. The effect of the confinement of the chamber wall on the engulfment flow regime was discussed. Large eddy simulation is also used to investigate the variation of pressure and velocity of the oscillation caused by the unsteady engulfment flow. Time-evolution and average of Intensity of Segregation (IOS) were quantitatively analyzed to reveal the mixing mechanism. Results show that the oscillation of vortex merging in the unsteady engulfment flow regime significantly improves the mixing performance.

Unsteady processes in a natural convective plume

The paper presents the results of an experimental and numerical study of the structure of a plume formed over a heated disk under conditions of conjugate heat transfer in a range of small and average Grashof numbers. Numerical simulation was performed using the Ansys Fluent code, where the problem of the flow of a compressible gas in conditions of conjugate heat transfer in an unsteady formulation was solved under the assumption of laminar flow regime. One of the main goal of this paper is to compare results of modelling of the plume in assumption of axisymmetric and three-dimensional flow regime. As a result of a comparison of the results of physical and numerical experiments, there is a conclusion about a good qualitative concurrence of the flow structure in a range of a small Grashof numbers.

Impact of Wind on Storm-Water Pond Hydraulics

AbstractA storm water pond’s effectiveness to treat particulate bound pollutants in urban runoff depends on its hydraulic behavior. Departures from the ideal plug flow, such as short-circuiting and mixing, pose hindrances to pond design. The goal of this study was to assess the importance of wind on storm water pond hydraulics. High resolution acoustic velocity measurements were conducted in a 0.3 ha, 2 m deep pond during dry weather. Results suggest that winds drive a turbulent, three-dimensional flow regime, including a lateral circulation and vertical exchange flows. A fully developed surface layer representing 1/3d of the water depth, with drift scaling as 0.004 on wind speed at 10 m above ground, was observed in the downwind section of the pond. Wind-induced vertical mixing, short-circuiting, and basin scale mixing were estimated to occur faster than the typical nominal residence time in storm water ponds. Wind is therefore an important hydraulic driver in small water systems, which may potentially r...

On the transition to turbulence of a viscoplastic fluid past a confined cylinder: A numerical study

Three-dimensional direct numerical simulations of a Bingham fluid flowing past a confined circular cylinder have been used in order to investigate viscoplastic effects in the wake-transition regime. The case of a cylinder confined in a plane channel with a fixed blockage ratio (ratio of the cylinder diameter to the channel height) of 0.2 has been studied, for values of the Bingham number and Reynolds numbers (based on the cylinder diameter and bulk flow velocity) in the range 0<Bn≤5 and 150⩽Re⩽600, respectively. The critical Reynolds number for the onset of three-dimensional flow regime has been shown to increase linearly with Bingham number, at least in the range of parameters considered. Two distinct modes of three-dimensional instability (modes A and B) have been identified in the flow, and the influence of viscoplastic effects on the evolution of these instabilities have been described. Significant deviations in the structure of the far wake from the Newtonian case have been observed and associated with a disruption of underlying processes that are known to operate in the Newtonian wake. Despite these structural differences, it has been shown that the evolution of the Strouhal number with Reynolds number in the viscoplastic fluid exhibits two discontinuous changes that are associated with the onset of the different instabilities in the flow, a behavior that mirrors the behavior in Newtonian fluids.

Primary oscillatory instability in a rotating disk–cylinder system with aspect (height/radius) ratio varying from 0.1 to 1

Three-dimensional instability of axisymmetric flow in a rotating disk–cylinder configuration is studied numerically for the case of low cylinders with the height/radius aspect ratio varying between 1 and 0.1. A complete stability diagram for the transition from steady axisymmetric to oscillatory three-dimensional flow regime is reported. A good agreement with the experimental results is obtained. It is shown that the critical azimuthal wavenumber grows with the decrease of the aspect ratio, reaching the value of 19 at the aspect ratio 0.1. It is argued that the observed instability cannot be described as resulting from a boundary layer only. Other reasons that can destabilize the flow are discussed.

New patterns in high-speed granular flows

We report on new patterns in high-speed flows of granular materials obtained by means of extensive numerical simulations. These patterns emerge from the destabilization of unidirectional flows upon increase of mass holdup and inclination angle, and are characterized by complex internal structures, including secondary flows, heterogeneous particle volume fraction, symmetry breaking and dynamically maintained order. In particular, we evidenced steady and fully developed ‘supported’ flows, which consist of a dense core surrounded by a highly energetic granular gas. Interestingly, despite their overall diversity, these regimes are shown to obey a scaling law for the mass flow rate as a function of the mass holdup. This unique set of three-dimensional flow regimes raises new challenges for extending the scope of current granular rheological models.

Non-dimensionalization and three-dimensional flow regime map for fluidization analyses

This article is on the dimensional analysis and the classification of fluidization from the viewpoint of numerical analysis. At first, the governing equations used in the DEM (Discrete Element Method) and CFD (Computational Fluid Dynamics) coupling model were non-dimensionalized with the method of Hellums and Churchill (1964). From the resulting dimensionless equations, it was concluded that the five dimensionless numbers, i.e. Re: Reynolds number, Ar: Archimedes number, Ga: Galilei number, Fr: Froude number and ρ⁎: ratio of particle density divided by fluid one, can be derived and hydrodynamically dominant on the fluid behaviors. Further, these can illustrate the dimensionless numbers proposed in the previous studies. Secondary, a three-dimensional flow regime map of homogeneous, bubbling and turbulent fluidizations was proposed with these dimensionless numbers using the DEM–CFD simulations. Finally, the plane of Reynolds number, Remb at the minimum bubbling fluidization velocity, umb in the map can be proposed and expressed as, Remb=0.263ρ⁎−0.553Ar0.612. umb can be estimated using this equation for various conditions.

A non-linear observer for unsteady three-dimensional flows

A method is proposed to estimate the velocity field of an unsteady flow using a limited number of flow measurements. The method is based on a non-linear low-dimensional model of the flow and on an expansion of the velocity field in terms of empirical basis functions. The main idea is to impose that the coefficients of the modal expansion of the velocity field gives the best approximation of the available measurements, while at the same time satisfying the non-linear low-order model as closely as possible. Practical applications may range from feedback flow control to the monitoring of the flow in non-accessible regions. The proposed technique is applied to the flow around a confined square cylinder, both in two- and three-dimensional flow regimes. Comparisons are provided with existing linear and non-linear estimation techniques.

Three-Dimensional Flow Regimes and Heat Transfer Characteristics in Surgical Operating Theatre

Three-Dimensional Flow Regimes and Heat Transfer Characteristics in Surgical Operating Theatre

Spatio-temporal behaviour in an enclosed corotating disk pair

We present a numerical investigation of the flow between corotating disks with a stationary outer casing – the enclosed corotating disk pair configuration. It is known that in such a geometry, axisymmetric and three-dimensional flow regimes develop depending on the value of the rotation rate. The three-dimensional flow is always unsteady flowing to its wavy structure in the radial-tangential plane. Axisymmetric regimes exhibit first a pitchfork bifurcation, characterized by a symmetry breaking with respect to the inter-disk midplane, before a Hopf bifurcation is established. The regime diagrams for these bifurcations are given in the (Re, G)-plane, where Re(= Ωb2/ν) is the rotational Reynolds number and G(= s/(b−a)) is the gap ratio. For values of G smaller than a critical limit Gc ∼ 0.26, there exists a range of rotation rates where the motion becomes time-dependent before bifurcating to a steady symmetry breaking regime. It is shown that for G [ges ] Gc the transition to unsteady three-dimensional flow occurs after the pitchfork bifurcation, and the flow structure is characterized by a shift-and-reflect symmetry. The transition to three-dimensional flow is consistent with experimental observations made by Abrahamson et al. (1989) where multiple solutions develop (known as the intransitivity phenomenon) with the presence of quasi-periodic behaviour resulting from successive vortex pairings. On the other hand, for smaller values of gap ratio, the three-dimensional flow shows a symmetry breaking. Finally, it is found that the variation of torque coefficient as a function of the rotation rate is the same for both the axisymmetric and three-dimensional solutions.

Low-Frequency Variability in a GCM: Three-Dimensional Flow Regimes and Their Dynamics

Abstract General circulation models (GCMs) can be used to develop diagnostics for identifying weather regimes. The author has looked for three-dimensional (3D) weather regimes associated with a 10-yr run of the U.K. UGAMP GCM with perpetual January boundary conditions; 3D low-pass empirical orthogonal functions (EOFs), using both the 500- and 250-mb streamfunctions (ψ) have been computed. These EOFs provide a low-order phase space in which weather regimes are studied. The technique here is an extension to 3D of that of Haines and Hannachi. They found, within the 500-mb ψ EOF phase space, two local minima of area-averaged ψ-tendency (based on barotropic vorticity dynamics), which were identified as ±Pacific–North America (PNA). In this work, the author demands that both the flow and its tendency be within the phase space spanned by the 3D EOFs. The streamfunction tendency is computed from the two-level quasigeostrophic potential vorticity equation and projected onto the EOF phase space. This projection pro...

BIPERIODIC WAVES IN SHALLOW WATER

The propagation of waves in shallow water is a phenomenon of significant practical importance. The ability to realistically predict the complex wave characteristics occurring in shallow water regions has always been an engineering goal which would make the development of solutions to practical engineering problems a reality. The difficulty in making such predictions stems from the fact that the equations governing the complex three-dimensional flow regime can not be solved without linearizing the problem. The linear equations are solvable; however, their solutions do not reflect the nonlinear features of naturally occurring waves. A recent advance (1984) in nonlinear mathematics has resulted in an explicit solution to a nonlinear equation relevant to water waves in shallow water. This solution possesses features found in observed nonlinear three-dimensional wave fields. The nonlinear mathematical formulation referred to above has never been compared with actual waves, so that its practical value is unknown. The purpose of the present investigation was to physically generate three-dimensional nonlinear waves and compare these with exact mathematical solutions. The goals were successfully completed by first generating the necessary wave patterns with the new U.S. Army Engineer Waterways Experiment Station, Coastal Engineering Research Center's (CERC) directional spectral wave generation facility. The theoretical solutions were then formed through the determination of a unique correspondence between the free parameters of the solution and the physical characteristics of the generated wave.

A herringbone bedform pattern of possible Taylor-Görtler type flow origin seen in sonographs

Side-scan sonar records collected in a shallow arctic lagoon (2–2.5 m depth) reveal a herringbone pattern of current-aligned linear reflectors with branching diagonals. The major longitudinal reflectors have no detectable relief (<20 cm), are spaced 5–10 m apart, and may represent current-aligned helical cell boundaries preserved in the silty fine sand of the lagoon floor. The pattern suggests a three-dimensional flow regime of the Taylor-Görtler type.