Subgrid length-scales for large-eddy simulation of stratified turbulence

Abstract The stretched-vortex subgrid-scale (SGS) model is extended to enable large-eddy simulation of buoyancy-stratified turbulence. Both stable and unstable stratifications are considered. The extended model retains the anisotropic form of the original stretched-vortex model, but the SGS kinetic energy and the characteristic SGS eddy size are modified by buoyancy subject to two constraints: first, the SGS kinetic energy dynamics is determined by stationary and homogeneous conditions, and second, the SGS eddy size obeys a scaling analogous to the Monin–Obukhov similarity theory. The SGS model construction, comprising an ensemble of subgrid stretched-vortical structures, naturally limits vertical mixing but allows horizontal mixing provided the alignment of the SGS vortex ensemble is favorable, even at high nominal gradient Richardson numbers. In very stable stratification, the model recovers the z-less limit, in which a vortex-based Obukhov length controls the SGS dynamics, while in very unstable stratification, the model recovers the free-convection limit, in which a vortex-based Deardorff velocity controls the SGS dynamics. The efficacy of the present SGS model is demonstrated by simulating the canonical stationary and homogeneous, stratified sheared turbulence at high Reynolds numbers and moderately high Richardson numbers. In the postprocessing, the SGS dynamics of the stretched-vortex model is further interrogated to yield predictions of buoyancy-adjusted one-dimensional SGS spectra and SGS root-mean-square velocity-derivative fluctuations.

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Sampling error in aircraft flux measurements based on a high-resolution large eddy simulation of the marine boundary layer

Atmospheric Measurement Techniques ◽

10.5194/amt-14-1959-2021 ◽

2021 ◽

Vol 14 (3) ◽

pp. 1959-1976

Author(s):

Grant W. Petty

Keyword(s):

Boundary Layer ◽

High Resolution ◽

Large Eddy Simulation ◽

Random Error ◽

Marine Boundary Layer ◽

Sampling Error ◽

Track Length ◽

Length Scales ◽

Eddy Simulation ◽

Large Eddy

Abstract. A high-resolution (1.25 m) large eddy simulation (LES) of the nocturnal cloud-topped marine boundary layer is used to evaluate random error as a function of continuous track length L for virtual aircraft measurements of turbulent fluxes of sensible heat, latent heat, and horizontal momentum. Results are compared with the widely used formula of Lenschow and Stankov (1986). In support of these comparisons, the relevant integral length scales and correlations are evaluated and documented. It is shown that for heights up to approximately 100 m (z/zi=0.12), the length scales are accurately predicted by empirical expressions of the form If=Azb. The Lenschow and Stankov expression is found to be remarkably accurate at predicting the random error for shorter (7–10 km) flight tracks, but the empirically determined errors decay more rapidly with L than the L-1/2 relationship predicted from theory. Consistent with earlier findings, required track lengths to obtain useful precision increase sharply with altitude. In addition, an examination is undertaken of the role of uncertainties in empirically determined integral length scales and correlations in flux uncertainties as well as of the flux errors associated with crosswind and along-wind flight tracks. It is found that for 7.2 km flight tracks, flux errors are improved by factor of approximately 1.5 to 2 for most variables by making measurements in the crosswind direction.

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Spectral structure of stratified turbulence: direct numerical simulations and predictions by large eddy simulation

Theoretical and Computational Fluid Dynamics ◽

10.1007/s00162-012-0259-9 ◽

2012 ◽

Vol 27 (3-4) ◽

pp. 319-336 ◽

Cited By ~ 12

Author(s):

Sebastian Remmler ◽

Stefan Hickel

Keyword(s):

Large Eddy Simulation ◽

Numerical Simulations ◽

Direct Numerical Simulations ◽

Spectral Structure ◽

Stratified Turbulence ◽

Eddy Simulation ◽

Large Eddy

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