15th European Turbulence Conference 2015
August 25-28th, 2015, Delft, The Netherlands

Invited speakers:

Prof. Marc Brachet. Ecole Normale Superieure, Paris, France

Prof. Peter G. Frick, Institute of Continuous Media Mechanics, Perm, Russia

Prof. Bettina Frohnapfel,  Karlsruher Institut fur Technology, Germany

Prof. Andrea Mazzino, Dipartimento di Fisica, University of Genova, Italy

Prof. Bernhard Mehlig. Department of Physics, University of Gothenburg, Sweden

Prof. Lex Smits, Mechanical and Aerospace Engineering, Princeton University, USA

Prof. Chao Sun Physics of Fluids, University of Twente, The Netherlands

Prof. Steve Tobias, Applied Mathematics, University of Leeds, United Kingdom

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15:00   Atmospheric turbulence 2
Chair: Arkady Tsinober
15 mins
The maximum sustainable heat flux in stably stratified channel flows
Judith Donda, Ivo Van Hooijdonk, Arnold Moene, GertJan van Heijst, Herman Clercx, Bas van de Wiel
Abstract: In analogy to the nocturnal atmospheric boundary layer a flux-driven, cooled channel flow is studied using Direct Numerical Simulations (DNS). Here, in particular, the mechanism behind the collapse of turbulence at large cooling rates is analyzed. In agreement with earlier studies, the flow laminarizes at large cooling rates. The mechanism for the cessation of turbulence is understood from a Maximum Sustainable Heat Flux hypothesis, which is here tested against simulations. In stratified flow the maximum heat flux that can be transported downward by turbulence at the onset of cooling is limited to a maximum, which, in turn, is limited by the initial momentum of the flow. If the heat extraction at the surface exceeds this maximum, near-surface stability will rapidly increase, which further hampers efficient vertical heat transport. This positive feedback eventually causes turbulence to be fully suppressed by the intensive density stratification. It is shown that the collapse in the DNS-simulations is successfully predicted by the MSHF-theory. Apart from formal analysis, also a simplified methodology is presented, which is more useful in practice for prediction of regime-transitions in atmospheric field observations.
15 mins
Turbulence in mixed-phase clouds
Benjamin Devenish, Kalli Furtado
Abstract: Mixed-phase clouds play an important role in the correct determination of the Earth's radiative balance but are not as well understood as warm clouds. It has been known for some time that turbulence is important in generating and sustaining mixed-phase clouds and here we present a simple model of a turbulent mixed-phase cloud by coupling a simple adiabatic mixed-phase cloud model with a Lagrangian stochastic model for the vertical velocity. We demonstrate that this model can indeed generate and maintain both liquid water and ice and reproduces qualitatively many features of mixed-phase clouds.
15 mins
Kanwar Nain Singh, Sreenivas K.R.
Abstract: Clouds are one of the major sources of uncertainty in climate prediction .The present work is to study the dynamics of orographic clouds. We considered a planar turbulent wall jet with off source volumetric heating as an appropriate low order fluid-dynamical model for studying the turbulence and entrainment in orographic-clouds. Entrainment was found to reduce drastically with the off-source volumetric heat addition. We found that the reduction of entrainment started at a bulk-Richardson number which was an order of magnitude smaller than that seen in earlier experiments [1] on free standing cumulus clouds. This is consistent with observations in real orographic clouds which develop as a thin sheet rising along the mountain slope. We have developed a novel scanning tomography technique for getting concentration field in turbulent shear flows. This technique is being applied on cloud-like flow to understand the entrainment characteristics. Detailed results will be presented at the conference.
15 mins
Implicit large-eddy simulation of the stratocumulus-topped boundary layer: a grid sensitivity study
Jesper G. Pedersen, Szymon P. Malinowski, Marta K. Kopeć
Abstract: A range of implicit large-eddy simulations of the stratocumulus-topped boundary layer is performed to study the influence of grid resolution on selected parameters including liquid water path and second and third moments of vertical velocity fluctuations. The simulations are based on two sets of aircraft measurements, which are also used to evaluate the results of the simulations. The specific case presented here indicates that simulations with a grid aspect ratio accounting for the anisotropic nature of the turbulence near the surface and at the top of the boundary layer lead to better agreement with measurements than simulations with an isotropic grid.
15 mins
Anna Gorska, Szymon Piotr Malinowski, Jacob Peter Fugal
Abstract: We analyze mixing at the top of laboratory analogs of convective clouds: stratocumulus and cumulus to investigate entrainment of environmental air into the cloud. We retrieve two components of air velocity using Particle Image Velocimetry technique. Suitable image processing allows to determine cloud–clear air interface. Using velocity differences between cloudy and clear sides of the interface we calculate entrainment / detrainment rates.
15 mins
Moist Rayleigh-Benard Convection
Prasanth Prabhakaran, Florian Winkel, Alexei Krekhov, Holger Nobach, Eberhard Bodenschatz
Abstract: We report the observations from turbulent thermal Rayleigh-Benard convection experiment with a two-phase liquid-vapor binary mixture. Evaporation/condensation processes in a turbulent convection are accompanied by the formation of cloud like structures above the liquid-vapor interface. We also found that a liquid film condensation on the underside of the top plate results in regular hexagonal patterns of falling droplets.
15 mins
Turbulent entrainment in a shearless mixing layer at the edge of a cloud
Paul Götzfried, Raymond A. Shaw, Jörg Schumacher
Abstract: Three-dimensional direct numerical simulations which combine the Eulerian description of temperature, vapor content and velocity with a Lagrangian ensemble of cloud water droplets are used to study the turbulent entrainment and subsequent mixing of clear air with a cloudy air filament. The study is conducted in a shearless mixing layer setup which is adjusted to realistic conditions at a cumulus cloud boundary. The magnitude of turbulent velocity fluctuations in- and outside the cloud can be varied independently. We find that the evolution of the cloud water droplet ensemble depends slightly only on the contrast of turbulent velocity fluctuations in- and outside the cloud filament. The buoyancy feedback on the flow via the evaporating droplets causes a transient amplification of all fluctuations before the turbulence eventually decays. We study the evolution of the probability density functions of droplet size as well as of supersaturation, temperature and vorticity at the droplet positions.