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1

Thermal Boundary Condition Influence on Bi-Periodic Channel Flow with High Thermal Gradient

Aulery F., Baux Y., Toutant A., Bataille F.

Machine Dynamics Research

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2011

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Vol. 35, No. 1

19-28

EN

This study focuses on TLES (Thermal Large Eddy Simulation). We study the influence of thermal boundary conditions on mean and fluctuating profiles for a turbulent bi-periodic channel with a thermal gradient. TLES simulations show that mean profiles of longitudinal velocity, transversal velocities and their fluctuations are not affected by the therm 1 boundary conditions (flux or temperature imposed at wall). Profiles of mean temperature and mean vertical velocity are slightly changed by the thermal boundary conditions. The thermal gradient at wall is stronger and temperature slope in the middle of channel is lower with imposed temperature boundary conditions. Although there are the same in the middle of the channel, temperature fluctuations profiles are very different near to the wall according to thermal boundary conditions. In fact, with flux boundary conditions, the temperature fluctuation profile is similar close to the wall than in the middle of channel; with imposed temperature boundary conditions, theses profiles tend to zero close to the wall. This result is very important for industrial process because thermal fatigue due to thermal fluctuations at wall can damage it.

2

Spatially-averaged flow statistics within a canopy of large bluff bodies: Results from direct numerical simulations

Coceal O., Thomas T. G., Belcher S.

Acta Geophysica

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2008

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Vol. 56, no. 3

862-875

EN

The flow within a canopy of large bluff bodies is highly turbulent and spatially heterogeneous. Results from direct numerical simulations over groups of cubical obstacles are analysed using the double-averaging methodology. The obstacles occupy a significant fraction of the canopy space; this gives rise to substantial dispersive stresses within the canopy. The underlying bluff-body turbulent dynamics is different from typical canopy turbulence, and this is reflected in the double-averaged statistics. The spatially-averaged velocities, stresses and drag force depend significantly upon the layout of the obstacles. An ongoing challenge is to parameterise these spatially-averaged quantities in terms of the obstacle geometry and layout.

3

Modelling of near-wall turbulence with large-eddy velocity modes

Wacławczyk M., Pozorski J.

Journal of Theoretical and Applied Mechanics

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2007

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Vol. 45 nr 3

705-724

EN

In the paper, low-order modelling of the turbulent velocity field in the near-wall region is performed using the Proper Orthogonal Decomposition (POD) approach. First, an empirical eigenfunction basis is computed, basing on two-point velocity correlations. Next, the Galerkin projection of the Navier-Stokes equations on the truncated basis is performed. This results in a system of Ordinary Differential Equations (ODEs) for time-dependent coefficients. Evolution of the largest vortical structures in the near-wall zone is then obtained from the time dependent coefficients and eigenfunctions. The system applied in the present work consists of 20 ODEs, the reconstructed velocity field is two-dimensional in the pIane perpendicular to the main flow direction. Moreover, the filtering procedure associated with the POD method is discussed, the POD filter is derived and compared with LES filters.

PL

Przedmiotem pracy jest modelowanie turbulentnego pola prędkości w obszarze przyściennym za pomocą niskowymiarowego systemu dynamicznego, opartego o dekompozycję w bazie funkcji własnych POD (ang. Proper Orthogonal Decomposition). Empiryczna baza funkcyjna POD została wyznaczona z rozwiązania zagadnienia własnego, w którym obecne są dwupunktowe korelacje prędkości. Następnie, w wyniku projekcji Galerkina równań pędu na podprzestrzeń rozpiętą na tej bazie funkcyjnej, otrzymano układ równań różniczkowych zwyczajnych na zależne od czasu wspołczynniki. Na podstawie funkcji własnych oraz z wyznaczonych współczynników rozkładu uzyskano ewolucję w czasie charakterystycznych struktur wirowych w obszarze przyściennym. System dynamiczny rozpatrywany w pracy składa się z 20 równań różniczkowych zwyczajnych. Zrekonstruowane pole prędkości jest dwuwymiarowe (w płaszczyźnie prostopadłej do głównego kierunku przepływu). Ponadto w pracy dyskutowana jest procedura filtrowania związana z metodą POD. Wyprowadzony filtr POD porównano z formułą używaną w metodzie symulacji dużych wirów.

4

Wavelet analysis of near-wall region in turbulent channel flow

Wacławczyk M., Pozorski J.

Turbulence : international journal

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2002

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Vol. 8-9

113-130

EN

An experimentally obtained set of instantaneous velocity realisations in the near--wall region of the turbulent channel flow is analysed using the wavelet transform technique. Selected notions of the wavelet approach are recalled together with an example of signal analysis. Some ideas of mu Hi resolution analysis are originally explained. Results of the application of the wavelet multiresolution analysis to the velocity field are presented. Velocity fluctuation is decomposed into a nearly-Gaussian background flow and so-called coherent structures that are tentatively identified.