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Sep 18, 2013
09/13

by
Bernard J. Geurts

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Buoyancy effects in unstably stratified mixing layers express themselves through gravity currents of heavy fluid which propagate in an ambient lighter fluid. These currents are encountered in numerous geophysical flows, industrial safety and environmental protection issues. During transition to turbulence a strong distortion of the separating interface between regions containing `heavy' or `light' fluid arises. The complexity of this interface will be used to monitor the progress of the mixing....

Source: http://arxiv.org/abs/nlin/0202064v1

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44

Sep 18, 2013
09/13

by
Bernard J. Geurts; Darryl D. Holm

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A new formulation is derived for the commutator-errors in large-eddy simulation of incompressible flow. These commutator-errors arise from the application of non-uniform filters to the Navier-Stokes equations. As a consequence, the filtered velocity field is no longer solenoidal. The order of magnitude of the commutator-errors is compared with the divergence of the turbulent stresses. This shows that one can not reduce the size of the commutator-errors independently of the turbulent stress...

Source: http://arxiv.org/abs/nlin/0210037v1

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57

Sep 18, 2013
09/13

by
Bernard J. Geurts; Darryl D. Holm

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Mathematical regularisation of the nonlinear terms in the Navier-Stokes equations provides a systematic approach to deriving subgrid closures for numerical simulations of turbulent flow. By construction, these subgrid closures imply existence and uniqueness of strong solutions to the corresponding modelled system of equations. We will consider the large eddy interpretation of two such mathematical regularisation principles, i.e., Leray and LANS$-\alpha$ regularisation. The Leray principle...

Source: http://arxiv.org/abs/nlin/0504038v1

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54

Sep 20, 2013
09/13

by
Arkadiusz K. Kuczaj; Bernard J. Geurts

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A new computational framework for the simulation of turbulent flow through complex objects and along irregular boundaries is presented. This is motivated by the application of metal foams in compact heat-transfer devices, or as catalyst substrates in process-engineering. The flow-consequences of such complicated objects are incorporated by adding explicit multiscale forcing to the Navier-Stokes equations. The forcing represents the simultaneous agitation of a wide spectrum of length-scales when...

Source: http://arxiv.org/abs/physics/0601164v1

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67

Sep 18, 2013
09/13

by
Bernard J. Geurts; Darryl D. Holm

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We consider so-called Leray regularization of the convective contributions. This gives rise to a subgrid parameterization which involves both explicit filtering and (approximate) inversion. The Leray model also arises from the alpha-modeling strategy derived via Kelvin's circulation theorem. We study the dynamics associated with the Leray model in a turbulent mixing layer and compare predictions with filtered DNS results and findings due to dynamic (mixed) models. In particular, the kinetic...

Source: http://arxiv.org/abs/nlin/0202062v1

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48

Sep 18, 2013
09/13

by
Bernard J. Geurts; Darryl D. Holm

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The $\alpha$-modeling strategy is followed to derive a new subgrid parameterization of the turbulent stress tensor in large-eddy simulation (LES). The LES-$\alpha$ modeling yields an explicitly filtered subgrid parameterization which contains the filtered nonlinear gradient model as well as a model which represents `Leray-regularization'. The LES-$\alpha$ model is compared with similarity and eddy-viscosity models that also use the dynamic procedure. Numerical simulations of a turbulent mixing...

Source: http://arxiv.org/abs/nlin/0202012v1

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54

Sep 19, 2013
09/13

by
Bernard J. Geurts; Darryl D. Holm

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A new modeling approach for large-eddy simulation (LES) is obtained by combining a `regularization principle' with an explicit filter and its inversion. This regularization approach allows a systematic derivation of the implied subgrid-model, which resolves the closure problem. The central role of the filter in LES is fully restored, i.e., both the interpretation of LES predictions in terms of direct simulation results as well as the corresponding subgrid closure are specified by the filter....

Source: http://arxiv.org/abs/nlin/0206026v1

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54

Sep 18, 2013
09/13

by
Arkadiusz K. Kuczaj; Bernard J. Geurts; Detlef Lohse

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The response of turbulent flow to time-modulated forcing is studied by direct numerical simulations of the Navier-Stokes equations. The large-scale forcing is modulated via periodic energy input variations at frequency $\omega$. The response is maximal for frequencies in the range of the inverse of the large eddy turnover time, confirming the mean-field predictions of von der Heydt, Grossmann and Lohse (Phys. Rev. E 67, 046308 (2003)). In accordance with the theory the response maximum shows...

Source: http://arxiv.org/abs/physics/0509208v1

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49

Sep 23, 2013
09/13

by
Arkadiusz K. Kuczaj; Bernard J. Geurts; Darryl D. Holm

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Rotation modulates turbulence causing columnar structuring of a turbulent flow in case of sufficiently strong rotation. This yields significant changes in the flow characteristics and dispersion properties, which makes rotational turbulence modulation particularly relevant in the context of atmospheric and oceanic flows. Here we investigate the canonical flow of turbulence in a periodic box, subjected to rotation about a fixed vertical axis. As point of reference we consider direct numerical...

Source: http://arxiv.org/abs/0904.0713v1