10:30
Atmospheric turbulence 3
Chair: Berengere Dubrulle
10:30
15 mins
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Large-eddy simulation of a separated flow with a sub-filter scale model based on the integral length-scale
Alexandre Silva Lopes, Ugo Piomelli, José M.L.M. Palma
Abstract: A new sub-filter scale model for large-eddy simulations, which uses a length-scale proportional to the integral scale of the turbulence instead of the grid resolution to parametrize the modelled stresses, will be assessed in the prediction of the flow of a boundary-layer over a rough surface, which includes separation and reattachment.
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10:45
15 mins
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Large eddy simulations of weakly heated stratocumulus top boudary layer
Marta Kopec, Szymon Malinowski, Zbigniew Piotrowski
Abstract: Performing Large Eddy Simulations (LES) of marine stratocumulus in the weakly heated boundary layer is an opportunity to evaluate the relative importance of radiative cooling and of a wind shear in cloud top region on cloud structure. It is shown that cooling due to longwave radiation influences the convective circulation in the atmospheric boundary layer and counteracts dilution caused by the wind shear.
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11:00
15 mins
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Streamwise turbulent intensity under unstable atmospheric stratification explained by a spectral budget
Tirtha Banerjee, Marcelo Chamecki, Gabriel Katul
Abstract: Because of its non-conformity to Monin-Obukhov Similarity Theory (MOST), the effects of thermal stratification on scaling laws describing the stream-wise turbulent intensity $\sigma_u$ normalized by the turbulent friction velocity ($u_*$) continues to draw research attention. The streamwise turbulent intensity happens to be of utmost importance as a direct measure of the intensity of turbulence and an analytical model able to predict its nature would be considered useful for a copious number of practical applications- ranging from industrial pipe flow to air pollution modeling among many. A spectral budget method used previously by \cite{Banerjee2013} was demonstrated as a suitable workhorse to analytically explain the `universal' logarithmic scaling law exhibited by $\sigma_u^2/u_*^2$ for neutral conditions as reported in different high Reynolds number experiments. In the present work \cite{Banerjee2014}, that theoretical framework has been expanded to assess the variability of $\sigma_u/u_*$ under unstable atmospheric stratification. At least three different length scales- the distance from the ground ($z$), the height of the atmospheric boundary layer ($\delta$), and the Obukhov length ($L$) are all found to be controlling parameters in the variation of $\sigma_u/u_*$. Analytical models have been developed and supported by experiments for two limiting conditions: $z/\delta<0.02$, $-z/L<0.5$ and $0.02 <0.5$. Under the first constraint, the turbulent kinetic energy spectrum is predicted to follow three regimes: $k^0$, $k^{-1}$ and $k^{-5/3}$ divided in the last two-regimes by a break-point at $kz=1$, where $k$ denotes wavenumber. The $\sigma_u/u_*$ is shown to follow the much discussed logarithmic scaling reconciled to Townsend's attached eddy hypothesis $\sigma_u^2/u_*^2= B_1-A_1 log(z/\delta)$, where the coefficients $B_1$ and $A_1$ are modified by MOST for mildly unstable stratification. Under the second constraint, the turbulent energy spectrum tends to become quasi inertial, displaying a $k^0$ and a $k^{-5/3}$ with a breakpoint predicted to occur $0.3
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11:15
15 mins
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Measurements of turbulence at stratocumulus top
Szymon P. Malinowski, Imai Jen-La Plante, Katarzyna Karpińska, Herman Gerber, Djamal Khelif
Abstract: Using $\sim$1m resolution airborne data from research flights we divide stratocumulus top region into sublayers and characterize properties of turbulence in each sublayer. Results indicate, that there are no clear differences of turbulence properties between thermodynamically different "calassical" and "non-classical" stratocumulus regimes, but there are clear signs of turbulence ainsotropy in stably stratified sublayers in the cloud top region.
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11:30
15 mins
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In-cloud Measurements of Drop Dynamics
Jan Molacek, Haitao Xu, Steffen Risius, Eberhard Bodenschatz
Abstract: We here present the results of a measurement campaign at the Umweltforschungsstation Schneefernerhaus (UFS) research station at an elevation of 2650 m, on the mountain top of Zugspitze, Germany. We measured the three-dimensional motion of the water droplets inside a turbulent cloud using three high-speed stationary cameras. We show the statistics of droplet acceleration and velocities, and their dependence on the flow parameters.
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11:45
15 mins
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Convective Ripening and Rainfall
Michael Wilkinson
Abstract: This paper discusses the evolution of the droplet size distribution for a liquid-in-gas aerosol contained in a Rayleigh-B\'enard cell. It introduces a non-collisional model for broadening the droplet size distribution, termed \lq convective ripening'. The paper also considers the initiation of rainfall from ice-free cumulus clouds. It is argued that while collisional mechanisms cannot explain the production of rain from clouds with water droplet diameters of $20\ \mu {\rm m}$, the non-collisional convective ripening mechanism gives a much faster route to increasing the size of the small fraction of droplets that grow into raindrops.
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12:00
15 mins
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ENERGY TRANSFER IN ROTATING STRATIFIED TURBULENT FLOWS
Raffaele Marino, Annick Pouquet, Duane Rosenberg, Pablo Mininni
Abstract: When including rotation and stratification non-linear interactions in turbulent
flows occur both at small scales, with turbulent mixing and dissipation, as
well as at large scales where accumulation of energy can take place through
an inverse cascade. We present a characterization of the energy transfer
across the scales in rotating and/or stratified turbulent flows showing how the
spectra and fluxes in the Fourier space depend on the relative strength
between rotation and stratification.
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12:15
15 mins
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Exploitation of homogeneous isotropic turbulence models for optimization of turbulence remote sensing
Albert Oude Nijhuis, Oleg Krasnov, Christine Unal, Herman Russchenberg, Alexander Yarovoy
Abstract: Homogeneous isotropic turbulence (HIT) models are compared,
with respect to optimization of turbulence remote sensing.
HIT models have different applications
such as load calculation for wind turbines (Mann, 1998)
or droplet track modelling (Pinsky and Khain, 2006).
Details of vortices seem of less relevance for modelling `realistic measurements',
where the single purpose is to retrieve the eddy dissipation rate (EDR).
Without the need for modelling the vortices, a faster and simpler approach might be favorable.
The cascade turbulence model (CTM) is suggested.
The CTM solution is scale invariant and a fast solution for one-dimensional HIT modelling.
In this presentation modelled radar measruments for scanning mode (rotating antenna)
are compared for different HIT models.
The consequences for turbulence remote sensing optimization are discussed.
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