Dynamic Large Eddy Simulations of the Ekman Layer Based on Stochastic Analysis

2016 ◽  
Vol 17 (2) ◽  
pp. 77-98 ◽  
Author(s):  
Ehsan Kazemi ◽  
Stefan Heinz
Keyword(s):  
Large Scale ◽  
Ekman Layer ◽  
Small Scale ◽  
Smagorinsky Model ◽  
Mean Velocity ◽  
Turbulence Structures ◽  
Large Eddy ◽  
Scale Structures ◽  
Les Model ◽  
Small Scale Structures

AbstractLarge eddy simulation (LES) of the neutrally stratified turbulent Ekman layer is performed. In particular, we compare three LES models with direct numerical simulation (DNS), which was validated against existing DNS. The models considered are a standard nondynamic LES model, the Smagorinsky model (SM), a standard dynamic LES model, the stabilized dynamic Smagorinsky model (DSM), and a new linear dynamic model (LDM), which was derived from a realizable stochastic turbulence model. The following conclusions are obtained. The SM does not represent an appropriate model for the flow considered. Mean velocity and turbulence intensities are poorly predicted. With respect to instantaneous fields, the SM provides a tilting of turbulence structures in the opposite direction as seen in DNS. The stabilized DSM also suffers from significant shortcomings. First, its behavior depends on the wall distance. Close to the wall, it produces acceptable turbulence structures. Away from the wall, it suffers from the same shortcomings as the SM. Second, it incorrectly describes the effect of grid coarsening. The new LDM is free from the disadvantages of the SM and stabilized DSM. Its predictions of both mean and instantaneous velocity fields agree very well with DNS. The relevant conclusion is the following. The use of a dynamic LES method represents a mean for correctly simulating large-scale structures (means and stresses), but it does not ensure a correct simultaneous simulation of small-scale structures. Our results indicate that a dynamic method designed in consistency with a realizable stress model can correctly simulate both large-scale and small-scale structures.

scholarly journals Large-eddy simulation of large-scale structures in long channel flow

2010 ◽  
Vol 661 ◽  
pp. 341-364 ◽  
Author(s):  
D. CHUNG ◽  
B. J. McKEON
Keyword(s):  
Large Scale ◽  
Small Scale ◽  
Mean Velocity ◽  
Large Scale Structures ◽  
Eddy Simulation ◽  
Filter Width ◽  
Half Height ◽  
Large Eddy ◽  
The Mean ◽  
Scale Structures

We investigate statistics of large-scale structures from large-eddy simulation (LES) of turbulent channel flow at friction Reynolds numbers Reτ = 2K and 200K (where K denotes 1000). In order to capture the behaviour of large-scale structures properly, the channel length is chosen to be 96 times the channel half-height. In agreement with experiments, these large-scale structures are found to give rise to an apparent amplitude modulation of the underlying small-scale fluctuations. This effect is explained in terms of the phase relationship between the large- and small-scale activity. The shape of the dominant large-scale structure is investigated by conditional averages based on the large-scale velocity, determined using a filter width equal to the channel half-height. The conditioned field demonstrates coherence on a scale of several times the filter width, and the small-scale–large-scale relative phase difference increases away from the wall, passing through π/2 in the overlap region of the mean velocity before approaching π further from the wall. We also found that, near the wall, the convection velocity of the large scales departs slightly, but unequivocally, from the mean velocity.

scholarly journals Super-resolution emulator of cosmological simulations using deep physical models

2020 ◽  
Vol 495 (4) ◽  
pp. 4227-4236 ◽  
Author(s):  
Doogesh Kodi Ramanah ◽  
Tom Charnock ◽  
Francisco Villaescusa-Navarro ◽  
Benjamin D Wandelt
Keyword(s):  
Large Scale ◽  
Initial Conditions ◽  
Computational Cost ◽  
Physical Modelling ◽  
Real Space ◽  
Physical Models ◽  
Small Scale ◽  
Cosmological Simulations ◽  
Scale Structures ◽  
Small Scale Structures

ABSTRACT We present an extension of our recently developed Wasserstein optimized model to emulate accurate high-resolution (HR) features from computationally cheaper low-resolution (LR) cosmological simulations. Our deep physical modelling technique relies on restricted neural networks to perform a mapping of the distribution of the LR cosmic density field to the space of the HR small-scale structures. We constrain our network using a single triplet of HR initial conditions and the corresponding LR and HR evolved dark matter simulations from the quijote suite of simulations. We exploit the information content of the HR initial conditions as a well-constructed prior distribution from which the network emulates the small-scale structures. Once fitted, our physical model yields emulated HR simulations at low computational cost, while also providing some insights about how the large-scale modes affect the small-scale structure in real space.

scholarly journals Additive Manufacture of Small-Scale Metamaterial Structures for Acoustic and Ultrasonic Applications

Micromachines ◽  
2021 ◽  
Vol 12 (6) ◽  
pp. 634
Author(s):  
Alicia Gardiner ◽  
Paul Daly ◽  
Roger Domingo-Roca ◽  
James F. C. Windmill ◽  
Andrew Feeney ◽  
...  
Keyword(s):  
Large Scale ◽  
Acoustic Properties ◽  
Small Scale ◽  
Additive Manufacture ◽  
Acoustic Metamaterials ◽  
Acoustic Metamaterial ◽  
Multi Scale ◽  
Advantages And Disadvantages ◽  
Scale Structures ◽  
Small Scale Structures

Acoustic metamaterials are large-scale materials with small-scale structures. These structures allow for unusual interaction with propagating sound and endow the large-scale material with exceptional acoustic properties not found in normal materials. However, their multi-scale nature means that the manufacture of these materials is not trivial, often requiring micron-scale resolution over centimetre length scales. In this review, we bring together a variety of acoustic metamaterial designs and separately discuss ways to create them using the latest trends in additive manufacturing. We highlight the advantages and disadvantages of different techniques that act as barriers towards the development of realisable acoustic metamaterials for practical audio and ultrasonic applications and speculate on potential future developments.

scholarly journals Modelling turbulent boundary layer flow over fractal-like multiscale terrain using large-eddy simulations and analytical tools

2017 ◽  
Vol 375 (2091) ◽  
pp. 20160098 ◽  
Author(s):  
X. I. A. Yang ◽  
C. Meneveau
Keyword(s):  
Large Scale ◽  
Wind Farm ◽  
Ground Surface ◽  
Small Scale ◽  
Subgrid Scale ◽  
Mean Velocity ◽  
Roughness Model ◽  
Roughness Elements ◽  
Large Eddy ◽  
Scale Roughness

In recent years, there has been growing interest in large-eddy simulation (LES) modelling of atmospheric boundary layers interacting with arrays of wind turbines on complex terrain. However, such terrain typically contains geometric features and roughness elements reaching down to small scales that typically cannot be resolved numerically. Thus subgrid-scale models for the unresolved features of the bottom roughness are needed for LES. Such knowledge is also required to model the effects of the ground surface ‘underneath’ a wind farm. Here we adapt a dynamic approach to determine subgrid-scale roughness parametrizations and apply it for the case of rough surfaces composed of cuboidal elements with broad size distributions, containing many scales. We first investigate the flow response to ground roughness of a few scales. LES with the dynamic roughness model which accounts for the drag of unresolved roughness is shown to provide resolution-independent results for the mean velocity distribution. Moreover, we develop an analytical roughness model that accounts for the sheltering effects of large-scale on small-scale roughness elements. Taking into account the shading effect, constraints from fundamental conservation laws, and assumptions of geometric self-similarity, the analytical roughness model is shown to provide analytical predictions that agree well with roughness parameters determined from LES. This article is part of the themed issue ‘Wind energy in complex terrains’.

scholarly journals Line profile variability in spectra of hot massive stars

2014 ◽  
Vol 9 (S307) ◽  
pp. 113-114
Author(s):  
Alexander Kholtygin ◽  
Natallia Sudnik ◽  
Viacheslav Dushin
Keyword(s):  
Line Profile ◽  
Large Scale ◽  
Massive Stars ◽  
Special Astrophysical Observatory ◽  
Small Scale ◽  
Astronomy Observatory ◽  
Large Scale Structures ◽  
Scale Structures ◽  
Optical Astronomy ◽  
Small Scale Structures

AbstractWe report the results of our study of the fast line profile variability (LPV) (hours – few days) in the spectra of bright OB and WR stars. All spectra were obtained with 6-m and 1-m telescope of Russian Special Astrophysical Observatory (SAO) and 1.8-m telescope of Bohyunsan Optical Astronomy Observatory, Korea (BOAO). We detected both the stochastic LPV, connected with the formation of small-scale structures in the stellar wind and the regular LPV induced by the large-scale structures in the wind.

scholarly journals Large-scale amplitude modulation of the small-scale structures in turbulent boundary layers

2009 ◽  
Vol 628 ◽  
pp. 311-337 ◽  
Author(s):  
ROMAIN MATHIS ◽  
NICHOLAS HUTCHINS ◽  
IVAN MARUSIC
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Amplitude Modulation ◽  
Large Scale ◽  
Wall Turbulence ◽  
Small Scale ◽  
Supporting Evidence ◽  
Scale Structures ◽  
Small Scale Structures ◽  
Near Wall

In this paper we investigate the relationship between the large- and small-scale energy-containing motions in wall turbulence. Recent studies in a high-Reynolds-number turbulent boundary layer (Hutchins & Marusic, Phil. Trans. R. Soc. Lond. A, vol. 365, 2007a, pp. 647–664) have revealed a possible influence of the large-scale boundary-layer motions on the small-scale near-wall cycle, akin to a pure amplitude modulation. In the present study we build upon these observations, using the Hilbert transformation applied to the spectrally filtered small-scale component of fluctuating velocity signals, in order to quantify the interaction. In addition to the large-scale log-region structures superimposing a footprint (or mean shift) on the near-wall fluctuations (Townsend, The Structure of Turbulent Shear Flow, 2nd edn., 1976, Cambridge University Press; Metzger & Klewicki, Phys. Fluids, vol. 13, 2001, pp. 692–701.), we find strong supporting evidence that the small-scale structures are subject to a high degree of amplitude modulation seemingly originating from the much larger scales that inhabit the log region. An analysis of the Reynolds number dependence reveals that the amplitude modulation effect becomes progressively stronger as the Reynolds number increases. This is demonstrated through three orders of magnitude in Reynolds number, from laboratory experiments at Reτ ~ 103–104 to atmospheric surface layer measurements at Reτ ~ 106.

scholarly journals Stochastic Modelling of Small-Scale Perturbation

Water ◽  
2020 ◽  
Vol 12 (10) ◽  
pp. 2950
Author(s):  
Franco Flandoli ◽  
Umberto Pappalettera
Keyword(s):  
Large Scale ◽  
Stochastic Modelling ◽  
Small Scale ◽  
Reduction Procedure ◽  
Full Generality ◽  
Scale Perturbation ◽  
Stratonovich Noise ◽  
Scale Structures ◽  
Small Scale Structures ◽  
Transport Type

In this paper we propose a stochastic model reduction procedure for deterministic equations from geophysical fluid dynamics. Once large-scale and small-scale components of the dynamics have been identified, our method consists in modelling stochastically the small scales and, as a result, we obtain that a transport-type Stratonovich noise is sufficient to model the influence of the small scale structures on the large scales ones. This work aims to contribute to motivate the use of stochastic models in fluid mechanics and identifies examples of noise of interest for the reduction of complexity of the interaction between scales. The ideas are presented in full generality and applied to specific examples in the last section.

Box-fault systems and ramps: atypical associations of structures from the eastern shoulder of the Kenya Rift

1980 ◽  
Vol 117 (6) ◽  
pp. 579-586 ◽  
Author(s):  
P. S. Griffiths
Keyword(s):  
Rift Valley ◽  
Large Scale ◽  
Small Scale ◽  
Structural Pattern ◽  
Kenya Rift ◽  
Fault Systems ◽  
Scale Structures ◽  
Small Scale Structures

SummaryBox-fault systems are striking arrangements of short faults that are associated with complex groups of elongate, tilted fault blocks (ramps). They occur within the faulted eastern shoulder of the Rift Valley, close to the equatorial bend in its alignment. This association of small-scale structures is shown to be integrated within the large-scale, asymmetrical structural pattern of the equatorial bend.

scholarly journals Generating small-scale structures from large-scale ones via optical near-field interactions

2007 ◽  
Vol 15 (19) ◽  
pp. 11790 ◽  
Author(s):  
M. Naruse ◽  
T. Yatsui ◽  
H. Hori ◽  
K. Kitamura ◽  
M. Ohtsu
Keyword(s):  
Large Scale ◽  
Near Field ◽  
Small Scale ◽  
Scale Structures ◽  
Small Scale Structures
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