The ability of a second-moment closure to predict the three-componential mean flow which results ... more The ability of a second-moment closure to predict the three-componential mean flow which results when a plane Couette flow is subjected to moderate anticyclonic system rotation is explored. The counterrotating streamwise-oriented roll cells, which may occur by the imposition of a destabilizing Coriolis force field, are treated as an integral part of the steady mean motion governed by the Reynolds-averaged Navier-Stokes equations. Near-wall effects are accounted for by Durbin's elliptic relaxation approach, while system rotation only appears naturally in rotational stress-producing terms and in the mean intrinsic vorticity in the non-linear pressure-strain model.
The utilization of the Reynolds-averaged Navier-Stokes (RANS) approach continues to dominate comp... more The utilization of the Reynolds-averaged Navier-Stokes (RANS) approach continues to dominate computational predictions of turbulent flows in a wide range of disciplines. Despite the simplistic statistical treatment of the turbulence motion, RANS prevails as the primary computational tool in the vast majority of applications. The present study is motivated by the challenges posed in conjunction with computing transitional flows, in particular by the need to better understand the response of models that routinely are applied in complex turbulent flow computations where regions of very low turbulence levels exist. A dynamical systems analysis is employed in this study to shed light on the dynamical behavior of the most commonly used two-equation models, and to aid the identification of inherent limitations that may have practical consequences. It is demonstrated that some common forms of the K-e model can yield arbitrary steady state solutions in transitional flows that depend on numerical solution parameters such as initial conditions and solution methodologies. In particular models that utilizes a wall damping coefficient in the destruction term of the dissipation rate transport equation. A so-called null-cline analysis will be introduced as a useful tool to analyze the solution of models equations near critical points.
The objective of this paper is to assess the accuracy of low-order finite volume (FV) methods app... more The objective of this paper is to assess the accuracy of low-order finite volume (FV) methods applied to the v 2f turbulence model of Durbin (Theoret. Comput. Fluid Dyn. 1991; 3:1-13) in the near vicinity of solid walls. We are not (like many others) concerned with the stability of solvers -the topic at hand is simply whether the mathematical properties of the v 2f model can be captured by the given, widespread, numerical method. The v 2f model is integrated all the way up to solid walls, where steep gradients in turbulence parameters are observed. The full resolution of wall gradients imposes quite high demands on the numerical schemes and it is not evident that common (second order) FV codes can fully cope with such demands. The v 2f model is studied in a statistically one-dimensional, fully developed channel flow where we compare FV schemes with a highly accurate spectral element reference implementation. For the FV method a higher-order face interpolation scheme, using Lagrange interpolation polynomials up to arbitrary order, is described. It is concluded that a regular second-order FV scheme cannot give an accurate representation of all model parameters, independent of mesh density. To match the spectral element solution an extended source treatment (we use three-point Gauss-Lobatto quadrature), as well as a higher-order discretization of diffusion is required. Furthermore, it is found that the location of the first internal node need to be well within y + = 1. ‡ Even for uniform CVs the first internal node will be three times closer to the wall than the next internal node and the nodes will never be uniformly distributed in a cell-centred FV mesh.
ABSTRACT Widely-used forms of the -¡ turbulence model are shown to yield arbitrary steady-state c... more ABSTRACT Widely-used forms of the -¡ turbulence model are shown to yield arbitrary steady-state converged solutions that are highly dependent on numerical considerations such as initial conditions and solution procedure. These solutions contain pseudo-laminar regions of varying size. By applying a nullcline analysis to the equation set, it is pos-sible to clearly demonstrate the reasons for the anomalous behavior. In summary, the degenerate solution acts as a stable fixed point under certain conditions, causing the numerical method to converge there. The analysis also suggests a methodology for preventing the anomalous behavior in steady-state computations.
Two-and three-dimensional simulations of the flow in rotating rib-roughened ducts are carried out... more Two-and three-dimensional simulations of the flow in rotating rib-roughened ducts are carried out using several turbulence closures. One and two-equation models have been used together with the four-equation v 2 − f model. In addition, a modification of this model that systematically accounts for system rotation has been used. Results show that the v 2 − f model is superior to the others in predicting wall heat transfer and, for the rotating case, the modified model accurately accounts for the effect of the system frame rotation.
Theoretical aspects of modeling stratied turbulent flows subjected to rotation are con- sidered. ... more Theoretical aspects of modeling stratied turbulent flows subjected to rotation are con- sidered. The structural equilibrium behavior of second-moment closure (SMC) models is explored, guided by bifurcation analysis. It is shown that the ability of the models to pre- dict a critical gradient Richardson number in the absence of system rotation Ricr g 0:25 is largely dependent on the model
Late-time dynamics and morphology of a stratified turbulent shear layer are examined using 1) Rey... more Late-time dynamics and morphology of a stratified turbulent shear layer are examined using 1) Reynolds-stress and heat-flux budgets, 2) the single-point structure tensors introduced by , and 3) flow visualization via 3D volume rendering. Flux reversal is observed during restratification in the edges of the turbulent layer. We present a first attempt to quantify the turbulence-mean-flow interaction and to characterize the predominant flow structures. Future work will extend this analysis to earlier times and different values of the Reynolds and Richardson numbers.
The objective of the present work is to provide a hypothetical scenario for indoor dispersion of ... more The objective of the present work is to provide a hypothetical scenario for indoor dispersion of a highly toxic chemical which can be used as starting point for discussion, training and exercises for emergency services and responsible authorities. The evaporation and indoor dispersion of a highly toxic compound (sarin) has been simulated using a Computational Fluid Dynamics (CFD) approach. The possible consequences of this hypthetical scenario have been assessed, and some consequence management challenges discussed.
Evaporation from a pool of liquid sarin in a ventilation shaft has been calculated. Turbulent flo... more Evaporation from a pool of liquid sarin in a ventilation shaft has been calculated. Turbulent flow of air through the ventilation shaft is simulated with the large eddy simulation model, and the evaporation process simulated by passive transport of vapour by the airflow, and with the volume-of-fluid model.
Abstract In this work the original V2F model of Durbin (Theor. Comput. Fluid Dyn. 3: 113, 1991) ... more Abstract In this work the original V2F model of Durbin (Theor. Comput. Fluid Dyn. 3: 113, 1991) and the nonlinear V2F model of Pettersson Reif (Flow Turbul. Combust. 76: 241256, 2006) have been assessed for several adverse pressure gradient (APG) turbulent ...
ABSTRACT Direct numerical simulation data of an evolving Kelvin-Helmholtz instability have been a... more ABSTRACT Direct numerical simulation data of an evolving Kelvin-Helmholtz instability have been analyzed in order to characterize the dynamic and kinematic response of shear-generated turbulent flow to imposed stable stratification. Particular emphasis was put on anisotropy and shear-layer edge dynamics in the net kinetic energy decay phase of the Kelvin-Helmholtz evolution. Results indicate a faster increase of small-scale anisotropy compared to large-scale anisotropy. Also, the anisotropy of thermal dissipation differs significantly from that of viscous dissipation. It is found that the Reynolds stress anisotropy increases up to a stratification level roughly corresponding to Ri g ≈ 0.4, but subsequently decreases for higher levels of stratification, most likely due to relaminarization. Coherent large-scale turbulence structures are cylindrical in the center of the shear layer, whereas they become ellipsoidal in the strongly stratified edge-layer region. The structures of the Reynolds stresses are highly one-componental in the center and turn two-componental as stratification increases. Stratification affects all scales, but it seems to affect larger scales to a higher degree than smaller scales and thermal scales more strongly than momentum scales. The effect of strong stable stratification at the edge of the shear layer is highly reminiscent of the non-local pressure effects of solid walls. However, the kinematic blocking inherently associated with impermeable walls is not observed in the edge layer. Vertical momentum flux reversal is found in part of the shear layer. The roles of shear and buoyant production of turbulence kinetic energy are exchanged, and shear production is transferring energy into the mean flow field, which contributes to relaminarization. The change in dynamics near the edge of the shear layer has important implications for predictive turbulence model formulations.
ABSTRACT The presence of undetonated explosive residues following high order detonations is not u... more ABSTRACT The presence of undetonated explosive residues following high order detonations is not uncommon, however the mechanism of their formation, or survival, is unknown. The existence of these residues impacts on various scenarios, for example their detection at a bomb scene allows for the identification of the explosive charge used, whilst their persistence during industrial explosions can affect the safety and environmental remediation efforts at these sites. This review article outlines the theoretical constructs regarding the formation of explosive residues during detonation and their subsequent dispersal and deposition in the surrounding media. This includes the chemical and physical aspects of detonation and how they could allow for undetonated particles to remain. The experimental and computational research conducted to date is presented and compared to the theory in order to provide a holistic review of the phenomenon.
An inherent shortcoming of single-point closure models is in general the lack of information conc... more An inherent shortcoming of single-point closure models is in general the lack of information concerning turbulence structures. Kassinos phet al. (2001) have developed a set of single-point structure tensors in an attempt to rectify the incompleteness of single-point models. This novel approach is based on a a turbulence vector streamfunction which satisfies a Poisson equation, and hence contain nonlocal information,
The ability of a second-moment closure to predict the three-componential mean flow which results ... more The ability of a second-moment closure to predict the three-componential mean flow which results when a plane Couette flow is subjected to moderate anticyclonic system rotation is explored. The counterrotating streamwise-oriented roll cells, which may occur by the imposition of a destabilizing Coriolis force field, are treated as an integral part of the steady mean motion governed by the Reynolds-averaged Navier-Stokes equations. Near-wall effects are accounted for by Durbin's elliptic relaxation approach, while system rotation only appears naturally in rotational stress-producing terms and in the mean intrinsic vorticity in the non-linear pressure-strain model.
The utilization of the Reynolds-averaged Navier-Stokes (RANS) approach continues to dominate comp... more The utilization of the Reynolds-averaged Navier-Stokes (RANS) approach continues to dominate computational predictions of turbulent flows in a wide range of disciplines. Despite the simplistic statistical treatment of the turbulence motion, RANS prevails as the primary computational tool in the vast majority of applications. The present study is motivated by the challenges posed in conjunction with computing transitional flows, in particular by the need to better understand the response of models that routinely are applied in complex turbulent flow computations where regions of very low turbulence levels exist. A dynamical systems analysis is employed in this study to shed light on the dynamical behavior of the most commonly used two-equation models, and to aid the identification of inherent limitations that may have practical consequences. It is demonstrated that some common forms of the K-e model can yield arbitrary steady state solutions in transitional flows that depend on numerical solution parameters such as initial conditions and solution methodologies. In particular models that utilizes a wall damping coefficient in the destruction term of the dissipation rate transport equation. A so-called null-cline analysis will be introduced as a useful tool to analyze the solution of models equations near critical points.
The objective of this paper is to assess the accuracy of low-order finite volume (FV) methods app... more The objective of this paper is to assess the accuracy of low-order finite volume (FV) methods applied to the v 2f turbulence model of Durbin (Theoret. Comput. Fluid Dyn. 1991; 3:1-13) in the near vicinity of solid walls. We are not (like many others) concerned with the stability of solvers -the topic at hand is simply whether the mathematical properties of the v 2f model can be captured by the given, widespread, numerical method. The v 2f model is integrated all the way up to solid walls, where steep gradients in turbulence parameters are observed. The full resolution of wall gradients imposes quite high demands on the numerical schemes and it is not evident that common (second order) FV codes can fully cope with such demands. The v 2f model is studied in a statistically one-dimensional, fully developed channel flow where we compare FV schemes with a highly accurate spectral element reference implementation. For the FV method a higher-order face interpolation scheme, using Lagrange interpolation polynomials up to arbitrary order, is described. It is concluded that a regular second-order FV scheme cannot give an accurate representation of all model parameters, independent of mesh density. To match the spectral element solution an extended source treatment (we use three-point Gauss-Lobatto quadrature), as well as a higher-order discretization of diffusion is required. Furthermore, it is found that the location of the first internal node need to be well within y + = 1. ‡ Even for uniform CVs the first internal node will be three times closer to the wall than the next internal node and the nodes will never be uniformly distributed in a cell-centred FV mesh.
ABSTRACT Widely-used forms of the -¡ turbulence model are shown to yield arbitrary steady-state c... more ABSTRACT Widely-used forms of the -¡ turbulence model are shown to yield arbitrary steady-state converged solutions that are highly dependent on numerical considerations such as initial conditions and solution procedure. These solutions contain pseudo-laminar regions of varying size. By applying a nullcline analysis to the equation set, it is pos-sible to clearly demonstrate the reasons for the anomalous behavior. In summary, the degenerate solution acts as a stable fixed point under certain conditions, causing the numerical method to converge there. The analysis also suggests a methodology for preventing the anomalous behavior in steady-state computations.
Two-and three-dimensional simulations of the flow in rotating rib-roughened ducts are carried out... more Two-and three-dimensional simulations of the flow in rotating rib-roughened ducts are carried out using several turbulence closures. One and two-equation models have been used together with the four-equation v 2 − f model. In addition, a modification of this model that systematically accounts for system rotation has been used. Results show that the v 2 − f model is superior to the others in predicting wall heat transfer and, for the rotating case, the modified model accurately accounts for the effect of the system frame rotation.
Theoretical aspects of modeling stratied turbulent flows subjected to rotation are con- sidered. ... more Theoretical aspects of modeling stratied turbulent flows subjected to rotation are con- sidered. The structural equilibrium behavior of second-moment closure (SMC) models is explored, guided by bifurcation analysis. It is shown that the ability of the models to pre- dict a critical gradient Richardson number in the absence of system rotation Ricr g 0:25 is largely dependent on the model
Late-time dynamics and morphology of a stratified turbulent shear layer are examined using 1) Rey... more Late-time dynamics and morphology of a stratified turbulent shear layer are examined using 1) Reynolds-stress and heat-flux budgets, 2) the single-point structure tensors introduced by , and 3) flow visualization via 3D volume rendering. Flux reversal is observed during restratification in the edges of the turbulent layer. We present a first attempt to quantify the turbulence-mean-flow interaction and to characterize the predominant flow structures. Future work will extend this analysis to earlier times and different values of the Reynolds and Richardson numbers.
The objective of the present work is to provide a hypothetical scenario for indoor dispersion of ... more The objective of the present work is to provide a hypothetical scenario for indoor dispersion of a highly toxic chemical which can be used as starting point for discussion, training and exercises for emergency services and responsible authorities. The evaporation and indoor dispersion of a highly toxic compound (sarin) has been simulated using a Computational Fluid Dynamics (CFD) approach. The possible consequences of this hypthetical scenario have been assessed, and some consequence management challenges discussed.
Evaporation from a pool of liquid sarin in a ventilation shaft has been calculated. Turbulent flo... more Evaporation from a pool of liquid sarin in a ventilation shaft has been calculated. Turbulent flow of air through the ventilation shaft is simulated with the large eddy simulation model, and the evaporation process simulated by passive transport of vapour by the airflow, and with the volume-of-fluid model.
Abstract In this work the original V2F model of Durbin (Theor. Comput. Fluid Dyn. 3: 113, 1991) ... more Abstract In this work the original V2F model of Durbin (Theor. Comput. Fluid Dyn. 3: 113, 1991) and the nonlinear V2F model of Pettersson Reif (Flow Turbul. Combust. 76: 241256, 2006) have been assessed for several adverse pressure gradient (APG) turbulent ...
ABSTRACT Direct numerical simulation data of an evolving Kelvin-Helmholtz instability have been a... more ABSTRACT Direct numerical simulation data of an evolving Kelvin-Helmholtz instability have been analyzed in order to characterize the dynamic and kinematic response of shear-generated turbulent flow to imposed stable stratification. Particular emphasis was put on anisotropy and shear-layer edge dynamics in the net kinetic energy decay phase of the Kelvin-Helmholtz evolution. Results indicate a faster increase of small-scale anisotropy compared to large-scale anisotropy. Also, the anisotropy of thermal dissipation differs significantly from that of viscous dissipation. It is found that the Reynolds stress anisotropy increases up to a stratification level roughly corresponding to Ri g ≈ 0.4, but subsequently decreases for higher levels of stratification, most likely due to relaminarization. Coherent large-scale turbulence structures are cylindrical in the center of the shear layer, whereas they become ellipsoidal in the strongly stratified edge-layer region. The structures of the Reynolds stresses are highly one-componental in the center and turn two-componental as stratification increases. Stratification affects all scales, but it seems to affect larger scales to a higher degree than smaller scales and thermal scales more strongly than momentum scales. The effect of strong stable stratification at the edge of the shear layer is highly reminiscent of the non-local pressure effects of solid walls. However, the kinematic blocking inherently associated with impermeable walls is not observed in the edge layer. Vertical momentum flux reversal is found in part of the shear layer. The roles of shear and buoyant production of turbulence kinetic energy are exchanged, and shear production is transferring energy into the mean flow field, which contributes to relaminarization. The change in dynamics near the edge of the shear layer has important implications for predictive turbulence model formulations.
ABSTRACT The presence of undetonated explosive residues following high order detonations is not u... more ABSTRACT The presence of undetonated explosive residues following high order detonations is not uncommon, however the mechanism of their formation, or survival, is unknown. The existence of these residues impacts on various scenarios, for example their detection at a bomb scene allows for the identification of the explosive charge used, whilst their persistence during industrial explosions can affect the safety and environmental remediation efforts at these sites. This review article outlines the theoretical constructs regarding the formation of explosive residues during detonation and their subsequent dispersal and deposition in the surrounding media. This includes the chemical and physical aspects of detonation and how they could allow for undetonated particles to remain. The experimental and computational research conducted to date is presented and compared to the theory in order to provide a holistic review of the phenomenon.
An inherent shortcoming of single-point closure models is in general the lack of information conc... more An inherent shortcoming of single-point closure models is in general the lack of information concerning turbulence structures. Kassinos phet al. (2001) have developed a set of single-point structure tensors in an attempt to rectify the incompleteness of single-point models. This novel approach is based on a a turbulence vector streamfunction which satisfies a Poisson equation, and hence contain nonlocal information,
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