Peter Hamlington | University of Colorado, Boulder (original) (raw)

Papers by Peter Hamlington

This paper provides a detailed analysis of momentum, angular momentum, vorticity, and energy budg... more This paper provides a detailed analysis of momentum, angular momentum, vorticity, and energy budgets of a front developing (i.e., frontogenesis) between upper ocean submesoscale eddies in a Large Eddy Simulation (LES). The LES solves the wave-averaged, or Craik-Leibovich, equations in order to account for the Stokes forces that result from interactions between nonbreaking surface waves and currents, and resolves both submesoscale eddies and boundary layer turbulence down to 4.9 m x 4.9 m x 1.25 m grid scales. It is found that the submesoscale frontogenesis differs from traditional frontogenesis theory due to four effects: Stokes forces, momentum and kinetic energy transfer from submesoscale eddies to frontal secondary circulations, resolved turbulent stresses, and unbalanced torque. In the energy, momentum, angular momentum, and vorticity budgets for the frontal overturning circulation, the Stokes shear force is a leading-order contributor, typically either the second or third largest source of frontal overturning. These effects violate hydrostatic and thermal wind balances during the submesoscale frontogenesis. The effect of the Stokes shear force becomes stronger with increasing alignment of the front and Stokes shear and with a nondimensional scaling. The Stokes shear force and momentum transfer from submesoscale eddies significantly energize the frontal secondary circulation along with the buoyancy.

Renewable Energy, Apr 2015

As interest in marine renewable energy increases, observations are crucial for understanding the ... more As interest in marine renewable energy increases, observations are crucial for understanding the environments that prospective turbines will encounter. Data from an acoustic Doppler velocimeter in Puget Sound, WA are used to perform a detailed characterization of the turbulent flow encountered by a turbine in a tidal strait. Metrics such as turbulence intensity, structure functions, probability density functions, intermittency, coherent turbulence kinetic energy, anisotropy invariants, and a new scalar measure of anisotropy are used to characterize the turbulence. The results indicate that the scalar anisotropy magnitude can be used to identify and parameterize coherent, turbulent events in the flow. An analysis of the anisotropy characteristics leads to a physical description of turbulent stresses as being primarily one- or two-dimensional, in contrast to isotropic, three-dimensional turbulence. A new measure of the anisotropy magnitude is introduced to quantify the level of anisotropic, coherent turbulence in a coordinate-independent way. These diagnostics and results will be useful for improved realism in modeling the performance and loading of turbines in realistic ocean environments.

Proceedings of SPIE, Jan 1, 2010

Signatures of turbulence in atmospheric laser propagation. [Proceedings of SPIE 7687, 76870M (201... more Signatures of turbulence in atmospheric laser propagation. [Proceedings of SPIE 7687, 76870M (2010)]. Peter E. Hamlington, Elaine S. Oran. Abstract. The signatures of turbulence in atmospheric laser propagation are examined ...

AIAA Proceedings.[np]. 22-25 Jun, Jan 1, 2009

A new Reynolds stress anisotropy closure that includes nonlocal and nonequilibrium effects in tur... more A new Reynolds stress anisotropy closure that includes nonlocal and nonequilibrium effects in turbulent flows has been obtained from a recently proposed nonlocal anisotropy formulation. This formulation is based on a new nonlocal derivation of the rapid pressurestrain correlation, which rigorously accounts for nonlocal effects on the anisotropy due to spatial variations in the mean velocity gradient tensor. The present nonlocal and nonequilibrium anisotropy model is obtained as a quasi-linear solution to the anisotropy transport equation, and directly replaces the classical local equilibrium Boussinesq closure in standard two-equation turbulence models. This allows straightforward implementation of the present approach in existing computational frameworks for solving the Reynolds averaged Navier-Stokes equations. Here we present the first assessment of the model in inhomogeneous flows -where nonlocal effects are expected to be important -by comparing with results from direct numerical simulations of turbulent channel flow.

Bulletin of the American Physical …, Jan 1, 2011

Characterizing the intermittency of velocity gradient and scalar gradient fields in turbulent rea... more Characterizing the intermittency of velocity gradient and scalar gradient fields in turbulent reacting flows is important for developing a better understanding of the interactions between turbulence and flames. Here we examine intermittency in premixed reacting flows using ...

The computational formulation of a new nonequilibrium Reynolds stress closure is presented along ... more The computational formulation of a new nonequilibrium Reynolds stress closure is presented along with preliminary validation results for both homogeneous and inhomogeneous turbulent flow problems of practical engineering importance. The new nonequilibrium closure, which has been rigorously derived elsewhere, 1 replaces the classical Boussinesq hypothesis appearing in many current two-equation turbulence models with a comparably simple representation for the Reynolds stresses, thereby allowing straightforward implementation in existing computational frameworks. The new nonequilibrium closure has been extended to include a rigorously derived realizable eddy viscosity, and theoretical details of the closure are evaluated through fundamental tests of periodically and impulsively sheared homogeneous turbulence. The full computational formulation of the nonequilibrium closure is outlined for both k-and k-ω model frameworks. Finally, preliminary inhomogeneous flow results are presented using the k-ω framework for turbulent flow over a flat-plate and the interaction of an impinging oblique shock wave with a turbulent boundary layer.

Based on the physics underlying turbulence anisotropy in the equilibrium and nonequilibrium limit... more Based on the physics underlying turbulence anisotropy in the equilibrium and nonequilibrium limits, a new physically-based, fully-realizable, nonequilibrium k − RANS model has been developed. The model is based on an effective strain rate tensor that accounts for the strain history to which the turbulence has been subjected. This new model is applied to four distinctly different test cases for which the nonequilibrium history integral can be evaluated analytically. Results obtained from this new closure model show dramatically improved agreement with experimental and computational data when compared with the standard k − (SKE) model, without the need to vary any model parameters. The introduction of a nonequilibrium effective strain rate allows this new model to be applied within a similar framework as currently used for two-equation eddy viscosity models, thereby permitting relatively simple implementation in existing CFD codes.

... Author(s): M. Kate Bole; Dan Bole; Sean Anklam; Justin Janaskie. Tunable multi-wavelength res... more ... Author(s): M. Kate Bole; Dan Bole; Sean Anklam; Justin Janaskie. Tunable multi-wavelength resonance-Raman detection of ... Author(s): Andre Jackson; Bryan Nash; Agustin Ifarraguerri. Laser induced fluorescence lifetime characterization of Bacillus endospore species using time ...

Bulletin of the American Physical …, Jan 1, 2010

The interactions between turbulence and flames in premixed, stoichiometric hydrogen-air combustio... more The interactions between turbulence and flames in premixed, stoichiometric hydrogen-air combustion are studied as a function of turbulence intensity by analyzing the coupled dynamics of the vorticity, strain rate, and scalar (reactant mass fraction) gradient. The ...

Bulletin of the American Physical …, Jan 1, 2008

A direct Biot-Savart integration is used to decompose the strain rate into its local and nonlocal... more A direct Biot-Savart integration is used to decompose the strain rate into its local and nonlocal constituents, allowing the vorticity alignment with the local and nonlocal strain rate eigenvectors to be investigated. These strain rate tensor constituents are evaluated in a turbulent flow using data from highly resolved direct numerical simulations. While the vorticity aligns preferentially with the intermediate eigenvector of the combined strain rate, as has been observed previously, the present results, for the first time, clearly show that the vorticity aligns with the most extensional eigenvector of the nonlocal strain rate. This, in turn, reveals a significant linear contribution to the vortex stretching dynamics in turbulent flows.

Bulletin of the American …, Jan 1, 2010

Bulletin of the American Physical Society, Jan 1, 2005

APS Bulletin of the American Physical Society. 2005 58th Annual Meeting of the Division of Fluid ... more APS Bulletin of the American Physical Society. 2005 58th Annual Meeting of the Division of Fluid Dynamics. Sunday–Tuesday, November 20–22, 2005; Chicago, IL. ...

A method for implementing the Brayton cycle to provide a practical rotary ramjet engine/generator... more A method for implementing the Brayton cycle to provide a practical rotary ramjet engine/generator based on a fundamentally "inside-out" approach is presented. Such a rotary ramjet turbogenerator eliminates the need for the conventional bladed compressor and turbine used in traditional gas turbine engines. It replaces these with a single inside-out supersonic circumferential rotor having integrated varying-area shaped ceramic ramjet channels in its radially inward surface, in which compression, combustion and expansion occur. The inside-out design places all rotating parts under compressive centrifugal loading, allowing ceramic fabrication of the high-temperature ramjet channel sections to enable operation at increased combustor exit temperatures with substantially reduced cooling requirements. The increased combustor temperature leads to increased maximum power and increased thermal efficiency when operating at maximum power. Furthermore, in this inside-out approach the centripetal acceleration acting on the flow in the ramjet channels causes high-density reactants to be forced into the radially inward-facing combustor, and low-density combustion products to be forced out of the combustor. This greatly assists in mixing and flameholding without the need for vortex generators or similar devices. The advantages of the inside-out approach over previous rotary ramjet systems become increasingly important as the rotor radius is decreased, since the centripetal acceleration then becomes increasingly large.

Physical Review E, Jan 1, 2009

A new fundamentally-based formulation of nonlocal effects in the rapid pressure-strain correlatio... more A new fundamentally-based formulation of nonlocal effects in the rapid pressure-strain correlation in turbulent flows has been obtained. The resulting explicit form for the rapid pressure-strain correlation accounts for nonlocal effects produced by spatial variations in the mean-flow velocity gradients, and is derived through Taylor expansion of the mean velocity gradients appearing in the exact integral relation for the rapid pressure-strain correlation. The integrals in the resulting series expansion are solved for high-and low-Reynolds number forms of the longitudinal correlation function f (r), and the resulting nonlocal rapid pressure-strain correlation is expressed as an infinite series in terms of Laplacians of the mean strain rate tensor. The new formulation is used to obtain a nonlocal transport equation for the turbulence anisotropy that is expected to provide improved predictions of the anisotropy in strongly inhomogeneous flows.

Physics of Fluids, Jan 1, 2009

A detailed parametric study is presented for the dynamics of initially isotropic homogeneous turb... more A detailed parametric study is presented for the dynamics of initially isotropic homogeneous turbulence subjected to periodic shear with magnitude S and frequency . The study is based on a quasianalytical solution to the anisotropy transport equation, which is shown to provide results for the shear anisotropy in this flow that agree well with direct numerical simulation ͑DNS͒, and for some key aspects of the dynamics agree better than do the most widely used second-order moment closures. The present analytical approach allows a more detailed parametric study than is practical via DNS, and provides direct insights into the parametric origins of the resulting dynamics. The long-time limit form of the shear anisotropy provides an analytical expression for the phase lag. The general solution also provides simple scalings in the full equilibrium limit as well as in the quasiequilibrium and saturated nonequilibrium regimes. The transition to the saturated nonequilibrium regime is shown to occur over a narrow range of around a critical frequency cr , for which an analytical expression is also obtained. The fundamental change in the dynamics of the turbulence kinetic energy k͑t͒ and the turbulence relaxation time scale ⌳͑t͒ as increases beyond cr is additionally addressed.

Physics of Fluids, Jan 1, 2008

From consideration of turbulence anisotropy dynamics due to spatial or temporal variations in the... more From consideration of turbulence anisotropy dynamics due to spatial or temporal variations in the mean strain rate, a new Reynolds stress closure for nonequilibrium effects in turbulent flows has been developed. This closure, formally derived from the Reynolds stress anisotropy transport equation, results in an effective strain rate tensor that accounts for the strain rate history to which the turbulence has been subjected. In contrast to prior nonequilibrium models that have sought to address nonequilibrium effects via changes in the eddy viscosity, the present approach accounts for nonequilibrium effects in the fundamental relation between the anisotropy tensor and the strain rate tensor. The time-local form of the nonequilibrium closure can be readily implemented in place of the classical equilibrium Boussinesq closure on which most existing computational frameworks are currently based. This new closure is applied here to four substantially different classes of nonequilibrium test problems. Results show dramatically improved agreement with experimental and computational data, without the need to vary any model parameters, when compared with the standard equilibrium closure and with various prior nonequilibrium closures.

Physics of Fluids, Jan 1, 2008

A direct Biot-Savart integration is used to decompose the strain rate into its local and nonlocal... more A direct Biot-Savart integration is used to decompose the strain rate into its local and nonlocal constituents, allowing the vorticity alignment with the local and nonlocal strain rate eigenvectors to be investigated. These strain rate tensor constituents are evaluated in a turbulent flow using data from highly-resolved direct numerical simulations. While the vorticity aligns preferentially with the intermediate eigenvector of the combined strain rate, as has been observed previously, the present results for the first time clearly show that the vorticity aligns with the most extensional eigenvector of the nonlocal strain rate. This in turn reveals a significant linear contribution to the vortex stretching dynamics in turbulent flows. PACS numbers: 47.27.-i,47.32.C-,47.27.De

Physical Review E, Jan 1, 2008

Local and nonlocal contributions to the total strain rate tensor Sij at any point x in a flow are... more Local and nonlocal contributions to the total strain rate tensor Sij at any point x in a flow are formulated from an expansion of the vorticity field in a local spherical neighborhood of radius R centered on x. The resulting exact expression allows the nonlocal (background) strain rate tensor S B ij (x) to be obtained from Sij (x). In turbulent flows, where the vorticity naturally concentrates into relatively compact structures, this allows the local alignment of vorticity with the most extensional principal axis of the background strain rate tensor to be evaluated. In the vicinity of any vortical structure, the required radius R and corresponding order n to which the expansion must be carried are determined by the viscous lengthscale λν. We demonstrate the convergence to the background strain rate field with increasing R and n for an equilibrium Burgers vortex, and show that this resolves the anomalous alignment of vorticity with the intermediate eigenvector of the total strain rate tensor. We then evaluate the background strain field S B ij (x) in DNS of homogeneous isotropic turbulence where, even for the limited R and n corresponding to the truncated series expansion, the results show an increase in the expected equilibrium alignment of vorticity with the most extensional principal axis of the background strain rate tensor.

This paper provides a detailed analysis of momentum, angular momentum, vorticity, and energy budg... more This paper provides a detailed analysis of momentum, angular momentum, vorticity, and energy budgets of a front developing (i.e., frontogenesis) between upper ocean submesoscale eddies in a Large Eddy Simulation (LES). The LES solves the wave-averaged, or Craik-Leibovich, equations in order to account for the Stokes forces that result from interactions between nonbreaking surface waves and currents, and resolves both submesoscale eddies and boundary layer turbulence down to 4.9 m x 4.9 m x 1.25 m grid scales. It is found that the submesoscale frontogenesis differs from traditional frontogenesis theory due to four effects: Stokes forces, momentum and kinetic energy transfer from submesoscale eddies to frontal secondary circulations, resolved turbulent stresses, and unbalanced torque. In the energy, momentum, angular momentum, and vorticity budgets for the frontal overturning circulation, the Stokes shear force is a leading-order contributor, typically either the second or third largest source of frontal overturning. These effects violate hydrostatic and thermal wind balances during the submesoscale frontogenesis. The effect of the Stokes shear force becomes stronger with increasing alignment of the front and Stokes shear and with a nondimensional scaling. The Stokes shear force and momentum transfer from submesoscale eddies significantly energize the frontal secondary circulation along with the buoyancy.

Renewable Energy, Apr 2015

As interest in marine renewable energy increases, observations are crucial for understanding the ... more As interest in marine renewable energy increases, observations are crucial for understanding the environments that prospective turbines will encounter. Data from an acoustic Doppler velocimeter in Puget Sound, WA are used to perform a detailed characterization of the turbulent flow encountered by a turbine in a tidal strait. Metrics such as turbulence intensity, structure functions, probability density functions, intermittency, coherent turbulence kinetic energy, anisotropy invariants, and a new scalar measure of anisotropy are used to characterize the turbulence. The results indicate that the scalar anisotropy magnitude can be used to identify and parameterize coherent, turbulent events in the flow. An analysis of the anisotropy characteristics leads to a physical description of turbulent stresses as being primarily one- or two-dimensional, in contrast to isotropic, three-dimensional turbulence. A new measure of the anisotropy magnitude is introduced to quantify the level of anisotropic, coherent turbulence in a coordinate-independent way. These diagnostics and results will be useful for improved realism in modeling the performance and loading of turbines in realistic ocean environments.

Proceedings of SPIE, Jan 1, 2010

Signatures of turbulence in atmospheric laser propagation. [Proceedings of SPIE 7687, 76870M (201... more Signatures of turbulence in atmospheric laser propagation. [Proceedings of SPIE 7687, 76870M (2010)]. Peter E. Hamlington, Elaine S. Oran. Abstract. The signatures of turbulence in atmospheric laser propagation are examined ...

AIAA Proceedings.[np]. 22-25 Jun, Jan 1, 2009

A new Reynolds stress anisotropy closure that includes nonlocal and nonequilibrium effects in tur... more A new Reynolds stress anisotropy closure that includes nonlocal and nonequilibrium effects in turbulent flows has been obtained from a recently proposed nonlocal anisotropy formulation. This formulation is based on a new nonlocal derivation of the rapid pressurestrain correlation, which rigorously accounts for nonlocal effects on the anisotropy due to spatial variations in the mean velocity gradient tensor. The present nonlocal and nonequilibrium anisotropy model is obtained as a quasi-linear solution to the anisotropy transport equation, and directly replaces the classical local equilibrium Boussinesq closure in standard two-equation turbulence models. This allows straightforward implementation of the present approach in existing computational frameworks for solving the Reynolds averaged Navier-Stokes equations. Here we present the first assessment of the model in inhomogeneous flows -where nonlocal effects are expected to be important -by comparing with results from direct numerical simulations of turbulent channel flow.

Bulletin of the American Physical …, Jan 1, 2011

Characterizing the intermittency of velocity gradient and scalar gradient fields in turbulent rea... more Characterizing the intermittency of velocity gradient and scalar gradient fields in turbulent reacting flows is important for developing a better understanding of the interactions between turbulence and flames. Here we examine intermittency in premixed reacting flows using ...

The computational formulation of a new nonequilibrium Reynolds stress closure is presented along ... more The computational formulation of a new nonequilibrium Reynolds stress closure is presented along with preliminary validation results for both homogeneous and inhomogeneous turbulent flow problems of practical engineering importance. The new nonequilibrium closure, which has been rigorously derived elsewhere, 1 replaces the classical Boussinesq hypothesis appearing in many current two-equation turbulence models with a comparably simple representation for the Reynolds stresses, thereby allowing straightforward implementation in existing computational frameworks. The new nonequilibrium closure has been extended to include a rigorously derived realizable eddy viscosity, and theoretical details of the closure are evaluated through fundamental tests of periodically and impulsively sheared homogeneous turbulence. The full computational formulation of the nonequilibrium closure is outlined for both k-and k-ω model frameworks. Finally, preliminary inhomogeneous flow results are presented using the k-ω framework for turbulent flow over a flat-plate and the interaction of an impinging oblique shock wave with a turbulent boundary layer.

Based on the physics underlying turbulence anisotropy in the equilibrium and nonequilibrium limit... more Based on the physics underlying turbulence anisotropy in the equilibrium and nonequilibrium limits, a new physically-based, fully-realizable, nonequilibrium k − RANS model has been developed. The model is based on an effective strain rate tensor that accounts for the strain history to which the turbulence has been subjected. This new model is applied to four distinctly different test cases for which the nonequilibrium history integral can be evaluated analytically. Results obtained from this new closure model show dramatically improved agreement with experimental and computational data when compared with the standard k − (SKE) model, without the need to vary any model parameters. The introduction of a nonequilibrium effective strain rate allows this new model to be applied within a similar framework as currently used for two-equation eddy viscosity models, thereby permitting relatively simple implementation in existing CFD codes.

... Author(s): M. Kate Bole; Dan Bole; Sean Anklam; Justin Janaskie. Tunable multi-wavelength res... more ... Author(s): M. Kate Bole; Dan Bole; Sean Anklam; Justin Janaskie. Tunable multi-wavelength resonance-Raman detection of ... Author(s): Andre Jackson; Bryan Nash; Agustin Ifarraguerri. Laser induced fluorescence lifetime characterization of Bacillus endospore species using time ...

Bulletin of the American Physical …, Jan 1, 2010

The interactions between turbulence and flames in premixed, stoichiometric hydrogen-air combustio... more The interactions between turbulence and flames in premixed, stoichiometric hydrogen-air combustion are studied as a function of turbulence intensity by analyzing the coupled dynamics of the vorticity, strain rate, and scalar (reactant mass fraction) gradient. The ...

Bulletin of the American Physical …, Jan 1, 2008

A direct Biot-Savart integration is used to decompose the strain rate into its local and nonlocal... more A direct Biot-Savart integration is used to decompose the strain rate into its local and nonlocal constituents, allowing the vorticity alignment with the local and nonlocal strain rate eigenvectors to be investigated. These strain rate tensor constituents are evaluated in a turbulent flow using data from highly resolved direct numerical simulations. While the vorticity aligns preferentially with the intermediate eigenvector of the combined strain rate, as has been observed previously, the present results, for the first time, clearly show that the vorticity aligns with the most extensional eigenvector of the nonlocal strain rate. This, in turn, reveals a significant linear contribution to the vortex stretching dynamics in turbulent flows.

Bulletin of the American …, Jan 1, 2010

Bulletin of the American Physical Society, Jan 1, 2005

APS Bulletin of the American Physical Society. 2005 58th Annual Meeting of the Division of Fluid ... more APS Bulletin of the American Physical Society. 2005 58th Annual Meeting of the Division of Fluid Dynamics. Sunday–Tuesday, November 20–22, 2005; Chicago, IL. ...

A method for implementing the Brayton cycle to provide a practical rotary ramjet engine/generator... more A method for implementing the Brayton cycle to provide a practical rotary ramjet engine/generator based on a fundamentally "inside-out" approach is presented. Such a rotary ramjet turbogenerator eliminates the need for the conventional bladed compressor and turbine used in traditional gas turbine engines. It replaces these with a single inside-out supersonic circumferential rotor having integrated varying-area shaped ceramic ramjet channels in its radially inward surface, in which compression, combustion and expansion occur. The inside-out design places all rotating parts under compressive centrifugal loading, allowing ceramic fabrication of the high-temperature ramjet channel sections to enable operation at increased combustor exit temperatures with substantially reduced cooling requirements. The increased combustor temperature leads to increased maximum power and increased thermal efficiency when operating at maximum power. Furthermore, in this inside-out approach the centripetal acceleration acting on the flow in the ramjet channels causes high-density reactants to be forced into the radially inward-facing combustor, and low-density combustion products to be forced out of the combustor. This greatly assists in mixing and flameholding without the need for vortex generators or similar devices. The advantages of the inside-out approach over previous rotary ramjet systems become increasingly important as the rotor radius is decreased, since the centripetal acceleration then becomes increasingly large.

Physical Review E, Jan 1, 2009

A new fundamentally-based formulation of nonlocal effects in the rapid pressure-strain correlatio... more A new fundamentally-based formulation of nonlocal effects in the rapid pressure-strain correlation in turbulent flows has been obtained. The resulting explicit form for the rapid pressure-strain correlation accounts for nonlocal effects produced by spatial variations in the mean-flow velocity gradients, and is derived through Taylor expansion of the mean velocity gradients appearing in the exact integral relation for the rapid pressure-strain correlation. The integrals in the resulting series expansion are solved for high-and low-Reynolds number forms of the longitudinal correlation function f (r), and the resulting nonlocal rapid pressure-strain correlation is expressed as an infinite series in terms of Laplacians of the mean strain rate tensor. The new formulation is used to obtain a nonlocal transport equation for the turbulence anisotropy that is expected to provide improved predictions of the anisotropy in strongly inhomogeneous flows.

Physics of Fluids, Jan 1, 2009

A detailed parametric study is presented for the dynamics of initially isotropic homogeneous turb... more A detailed parametric study is presented for the dynamics of initially isotropic homogeneous turbulence subjected to periodic shear with magnitude S and frequency . The study is based on a quasianalytical solution to the anisotropy transport equation, which is shown to provide results for the shear anisotropy in this flow that agree well with direct numerical simulation ͑DNS͒, and for some key aspects of the dynamics agree better than do the most widely used second-order moment closures. The present analytical approach allows a more detailed parametric study than is practical via DNS, and provides direct insights into the parametric origins of the resulting dynamics. The long-time limit form of the shear anisotropy provides an analytical expression for the phase lag. The general solution also provides simple scalings in the full equilibrium limit as well as in the quasiequilibrium and saturated nonequilibrium regimes. The transition to the saturated nonequilibrium regime is shown to occur over a narrow range of around a critical frequency cr , for which an analytical expression is also obtained. The fundamental change in the dynamics of the turbulence kinetic energy k͑t͒ and the turbulence relaxation time scale ⌳͑t͒ as increases beyond cr is additionally addressed.

Physics of Fluids, Jan 1, 2008

From consideration of turbulence anisotropy dynamics due to spatial or temporal variations in the... more From consideration of turbulence anisotropy dynamics due to spatial or temporal variations in the mean strain rate, a new Reynolds stress closure for nonequilibrium effects in turbulent flows has been developed. This closure, formally derived from the Reynolds stress anisotropy transport equation, results in an effective strain rate tensor that accounts for the strain rate history to which the turbulence has been subjected. In contrast to prior nonequilibrium models that have sought to address nonequilibrium effects via changes in the eddy viscosity, the present approach accounts for nonequilibrium effects in the fundamental relation between the anisotropy tensor and the strain rate tensor. The time-local form of the nonequilibrium closure can be readily implemented in place of the classical equilibrium Boussinesq closure on which most existing computational frameworks are currently based. This new closure is applied here to four substantially different classes of nonequilibrium test problems. Results show dramatically improved agreement with experimental and computational data, without the need to vary any model parameters, when compared with the standard equilibrium closure and with various prior nonequilibrium closures.

Physics of Fluids, Jan 1, 2008

A direct Biot-Savart integration is used to decompose the strain rate into its local and nonlocal... more A direct Biot-Savart integration is used to decompose the strain rate into its local and nonlocal constituents, allowing the vorticity alignment with the local and nonlocal strain rate eigenvectors to be investigated. These strain rate tensor constituents are evaluated in a turbulent flow using data from highly-resolved direct numerical simulations. While the vorticity aligns preferentially with the intermediate eigenvector of the combined strain rate, as has been observed previously, the present results for the first time clearly show that the vorticity aligns with the most extensional eigenvector of the nonlocal strain rate. This in turn reveals a significant linear contribution to the vortex stretching dynamics in turbulent flows. PACS numbers: 47.27.-i,47.32.C-,47.27.De

Physical Review E, Jan 1, 2008

Local and nonlocal contributions to the total strain rate tensor Sij at any point x in a flow are... more Local and nonlocal contributions to the total strain rate tensor Sij at any point x in a flow are formulated from an expansion of the vorticity field in a local spherical neighborhood of radius R centered on x. The resulting exact expression allows the nonlocal (background) strain rate tensor S B ij (x) to be obtained from Sij (x). In turbulent flows, where the vorticity naturally concentrates into relatively compact structures, this allows the local alignment of vorticity with the most extensional principal axis of the background strain rate tensor to be evaluated. In the vicinity of any vortical structure, the required radius R and corresponding order n to which the expansion must be carried are determined by the viscous lengthscale λν. We demonstrate the convergence to the background strain rate field with increasing R and n for an equilibrium Burgers vortex, and show that this resolves the anomalous alignment of vorticity with the intermediate eigenvector of the total strain rate tensor. We then evaluate the background strain field S B ij (x) in DNS of homogeneous isotropic turbulence where, even for the limited R and n corresponding to the truncated series expansion, the results show an increase in the expected equilibrium alignment of vorticity with the most extensional principal axis of the background strain rate tensor.