Saturday, 20 January 2018
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After a brief introduction of turbulent free convection, the talk will deal with two specific types of flow: convection over horizontal surfaces and axially homogeneus turbulent convection in tubes. The emphasis will be on the physics of the two flows, flux scaling and spectra.
Sunday, 21 January 2018
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In classical turbulence helicity is known to deplete nonlinearity and can alter the evolution of turbulent flows. In quantum turbulence its role is not fully understood. First, we will summarize recent published results [PRA 95, 053636 (2017)] on the free decay of a zero-temperature helical quantum turbulent flow, studied by direct numerical simulations of the Gross-Pitaevskii equation (GPE) at high spatial resolution. Finally, using the spectrally-truncated GPE, we will present results on finite temperature effects on the superflow dual cascade of energy and helicity.
Monday, 22 January 2018
Quantized vortex reconnections are central in the dissipation of quantum turbulence. We visualized vortex reconnections in superfluid 4He using sub-micron frozen air tracers. Compared to previous work, the fluid was almost at rest leading to fewer, straighter, slower-moving vortices. This condition allowed us to observe the propagation of Kelvin waves and to characterize the influence of the inter-vortex angle on the evolution of the recoiling vortices. The agreement of the experimental data to the analytical and numerical models suggests that the dynamics of the reconnection of long straight vortices on the scale of these experiments can be described by the self-similar solutions of the local induction approximation or Biot-Savart equations. This observation suggests also that the distortion induced by the two approaching vortices to locally arrange themselves in an antiparallel configuration can be neglected. Moreover, the small deviations from the 1/2 scaling of the inter-vortex separation distance as function of time, provide evidence that the correction factors depend on the influence of neighboring vortices.
Helicity is a conserved quantity that arises in ideal fluid flows and ideal magnetohydrodynamic magnetic fields. I will first review the background theory of Helicity in those two cases, a famous paper by Finn and Antonsen, and another by Keith Moffatt. I will follow by covering some basic phenomenology of quantized vortices, reconnection, and Kelvin waves, and background of our visualization studies in superfluid helium. These topics lead into a discussion of what has been done, what we know, and what is predicted about Helicity dynamics. Some observations about the untangling of vortices via reconnection lead to predictions regarding the Helicity we are exploring experimentally. Some puzzles and questions about the role of invariants like the Helicity in the Gross-Pitaevskii (nonlinear Schrodinger) equation play a role in thinking about this phenomenon.
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In this presentation we provide several examples of application of basic turbulent boundary layer concepts to improve our understanding of the basic flow structure in large wind farms.
Most relevant turbulent transport processes in wind farms occur at scales ranging from 10-1000 meters, the upper limit being $10^{13}$ A or about $10^{-13}$ LY, squarely near the geometric middle of the tremendous range encompassed by this conference. The ongoing rapid expansion of wind energy as a cost-effective renewable source of energy calls for an expanded understanding of associated turbulent flow phenomena. We review the basic flow structure and turbulence in the wind turbine array boundary layer (WTABL), also distinguishing between the developing and fully developed WTABL. We perform a series of Large Eddy Simulations that represent the turbines as actuator disks. Salient LES results are synthesized in order to develop simplified analytical models needed for wind farm design and optimization. We show how such tools may help responding to the important question whether large wind farms will increase or decrease surface fluxes (such as evaporation) from the ground.
Direct numerical simulations of compressible mixing layers are performed using finite volume gas kinetic method. The primary effect of compressibility on mixing layers is the inhibition of spreading rate of the mixing layer. It is observed that this is due to reduced levels of turbulence at higher convective Mach numbers. Lower levels of turbulent kinetic energy are attributed to the reduced production of TKE as compressibility effects increase. The reason for this is attributed to the pressure-strain rate terms. The effect of compressibility on flow topologies are also studied. Further, a statistical analysis conditioned on local flow topology relates how compressibility brings about changes in turbulent statistics.
This talk will describe and assess S. Chandrasekhar's work in fluid dynamics from about 1948 to 1960 and include remarks on his interactions during these years with leaders in the field. A brief remark on Chandra's philosophy will follow.
Tuesday, 23 January 2018
Compressible flows in many engineering applications are often turbulent, such as those involved in high-speed propulsion or aerodynamic applications. Other applications, such inertial confinement fusion, and astrophysical phenomena also involve flows in the compressible turbulence regime. In this talk we will revisit the studies undertaken during the last 25 or so years, which attempt to characterize the effects of compressibility on turbulence. We will delineate the effects on the processes active in the energy-bearing range of turbulence, on the turbulent cascade, and on the dissipative scales, and on far-field acoustic wave radiation. Additionally we will separate variable-density and variable–property effects, which dominate the turbulence behavior in supersonic and hypersonic turbulent boundary layers from intrinsic compressibility effects. Modeling challenges, both physical and computational will also be discussed.
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Ambrish Pandey, Janet D. Scheel and J\"org Schumacher
Turbulent Rayleigh-B\’{e}nard convection displays a large-scale order in the form of rolls and cells on lengths larger than the layer height once the fluctuations of temperature and velocity are removed. These turbulent superstructures are reminiscent of the patterns close to the onset of convection. They are analyzed by numerical simulations of turbulent convection in fluids at different Prandtl number ranging from 0.005 to 70 and for Rayleigh numbers up to 1e+7. For each case, we identify characteristic scales and times that separate the fast, small-scale turbulent fluctuations from the gradually changing large-scale superstructures. The characteristic scales of the large-scale patterns, which change with Prandtl and Rayleigh number, are also found to be correlated with the boundary layer dynamics, and in particular the clustering of thermal plumes at the top and bottom plates. Our analysis suggests a scale separation and thus the existence of a simplified description of the turbulent superstructures in geo- and astrophysical settings.
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After a brief introduction to the importance of scaling, normal and anomalous, we discuss our search for normal scaling from cosmic microwave background radiation to circulation around closed contours. Circulation, in particular, has not received as much attention in turbulence as it deserves, though it plays an important role in fluid dynamics. No special knowledge of the subject is expected.
Wednesday, 24 January 2018
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In this talk I will provide an overview of the recent developments on turbulent convection, namely:
- Using pseudospectral simulations of turbulent thermal convection at very high resolution (4096^3) and high Rayleigh number (Ra = 1.1 × 10^{11}) with unit Prandtl number, we conclude that convective turbulence exhibits behaviour similar to fluid turbulence, that is, Kolmogorov’s k^{−5/3} spectrum with forward and local energy transfers, along with a nearly isotropic energy distribution. The energy transfer diagnostics provide a very useful diagnostics in this investigation.
- The viscous dissipation rate in turbulent convection is not U^3/d, but it is (U^3/d)Ra^{-0.20}. Also, the viscous dissipation in the bulk dominates those in the boundary layers. Using these results, we present a new formulae for the scaling of Reynolds and Nusselt numbers.
Reference:
- M. K. Verma, A. Kumar, and A. Pandey, Phenomenology of buoyancy-driven turbulence: recent results, New J. Phys. 19, 025012 (2017).
- S. Bhattacharya, A. Pandey, A. Kumar, and M. K. Verma, Complexity of viscous dissipation in turbulent thermal convection, arXiv:1801.01701 (2018).
- M. K. Verma, Physics of Buoyant Flows: From Instabilities to Turbulence, World Scientific, Singapore (March 2018)
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Inertial droplets in a turbulent flow are centrifuged out of vortical regions and preferentially occupy regions of strain. We ask how this affects collision and droplet growth and the relative velocity of impact. We show that there is a special radius around a vortex, and droplet dynamics is completely different on either side. The preferential concentration results in inhomogeneous phase change, and hence inhomogeneous temperature, resulting in buoyancy effects which modify the turbulence spectrum. This is work done with S Ravichandran, Jason Picardo, Lokahith Agasthya and Samriddhi Sankar Ray.
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A rich variety of features emerges when two substances mix with the aid of turbulence. This talk is a summary of a few major themes on the topic, from the universal to the anomalous. No special knowledge of the subject is expected.