13:30
Atmospheric turbulence 4
Chair: Joerg Schumacher
13:30
15 mins
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A TURBULENCE STATISTICAL ANALYSIS OF SIMULATIONS OF TROPICAL CYCLOGENESIS
Gregory P. King, Galina V. Levina, Michael T. Montgomery
Abstract: In a numerical study of tropical cyclogenesis, Montgomery et al [1] focused on the problem of how a mid-level mesoscale convective vortex (MCV), a frequent by-product of mesoscale convective systems during summertime conditions over tropical oceans, may be transformed into a surface-concentrated (warm core) tropical depression. The simulations demonstrated an upscale cloud/mesoscale mechanism for building the incipient tropical storm vortex. Within the convectively unstable and cyclonic-vorticity rich environment of the initial MCV embryo, horizontally small-scale warm-core updrafts possessing intense cyclonic vorticity in their cores (“vortical hot towers”; VHTs) emerged spontaneously as the dominant coherent structures. By continuity, downdrafts come bundled with updrafts. Updrafts and downdrafts are vortical and hence have helicity: cyclonic updrafts and anticyclonic downdrafts have positive helicity, while cyclonic downdrafts and anticyclonic updrafts have negative helicity.
Levina and Montgomery [2,3] calculated and analyzed helical characteristics of the Montgomery et al [1] numerical experiments. They demonstrated how the VHTs work to form a strong secondary circulation, generate helicity and provide the linkage of the tangential and overturning circulation. By means of a quantitative analysis, they suggested that the linkage provides a positive energy feedback between the two circulations. The results support the hypothesized model of a large-scale, helical-vortex instability that operates over the ocean in which sufficient moisture fluxes maintain convective instability. This perspective of large-scale helical development as articulated by these authors is complementary to and consistent with the rotating convection paradigm for tropical cyclone intensification by Montgomery and Smith [4]. The emphasis on helicity as an important characteristic of intensifying and sustaining large-scale vortex disturbances in the atmosphere due to energy transfer from small-scale helical convective turbulence (turbulent vortex dynamo) was proposed several years ago by Moiseev et al [5, 6].
Tropical cyclones are self-organized coherent structures, but also three-dimensional and turbulent. The self-organization is contrary to a classical three-dimensional turbulence interpretation. The coherence may be due to upscale energy transfer, as proposed in the turbulent vortex dynamo hypothesis, or some other reason. In order to advance understanding, we are investigating the Montgomery et al experiments using statistical tools from turbulence theory (namely, probability distributions of velocity and helicity increments and their moments). The statistics will be used to characterize tropical cyclogenesis and intensification and further appraise the hypothesis of the turbulent vortex dynamo.
References
[1] M. T. Montgomery, M. E. Nicholls, T. A. Cram, and A. B. Saunders, A vortical hot tower route to tropical cyclogenesis, J. Atmos. Sci., 63, 355–386 (2006). doi: 10.1175/JAS3604.1
[2] G. V. Levina and M. T. Montgomery, A first examination of the helical nature of tropical cyclogenesis, Doklady Earth Sciences, 434, Part 1, pp. 1285–1289 (2010). doi: 10.1134/S1028334X1009031X
[3] G. V. Levina and M. T. Montgomery, Tropical Cyclogenesis: a numerical diagnosis based on helical flow organization, Journal of Physics: Conference Series 544 012013 (2014); doi:10.1088/1742-6596/544/1/012013
[4] M. T. Montgomery and R. K. Smith, Paradigms of tropical cyclone intensification, Australian Meteorological and Oceanographic Journal, Bruce Morton Memorial Volume, 64, 37-66 (2014).
[5] S. S. Moiseev, R. Z. Sagdeev, A. V. Tur, G. A. Khomenko, and V. V. Yanovsky, A theory of large-scale structure origination in hydrodynamic turbulence, Sov. Phys. JETP, 58, 1149–1157 (1983).
[6] S. S. Moiseev, R. Z. Sagdeev, A. V. Tur, G. A. Khomenko, A. M. Shukurov. Physical mechanism of amplification of vortex disturbances in the atmosphere. Engl. transl. Sov. Phys. Dokl., 28, 925–928 (1983).
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13:45
15 mins
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Large deviations of planetary jets
Freddy Bouchet, J Brad Marston, Cesare Nardini, Tomas Tangarife
Abstract: Rare or extreme events are of great interest in climate and other systems. Few studies address these statistics from a
dynamical perspective. Classical statistical approaches, for instance closures or stochastic averaging usually describe
typical states or low order statistics only. Large deviation theory is a very interesting alternative to these classical
methods. It can in principle describe both typical fluctuations and extreme fluctuations. This allows us to discuss the
long time evolution of the jet. One goal is to predict the dynamics that may lead to change of regimes and change of
attractors in atmospheric jet dynamics.
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14:00
15 mins
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Rapid growth of Coalescing Droplets and Observation of Fine Structures in Turbulent Flow
Ewe-wei Saw, Jeremie Bec, Holger Homann, Samriddhi Sankar Ray, Bérengère DUBRULLE, François Daviaud
Abstract: I will present our results on size-growth dynamics of coalescing droplets in simulation of isotropic turbulent flow. In the short time limit, we observe very fast growth due to correlations of these droplets which can be related to the interaction between their inertia to turbulent advection (this work is done with colleagues at affiliations 1 \& 2). In a later part, I will describe our attempt to experimentally observe the intermittent fine structures of turbulence flow using high resolution Particle Image Velocimetry (PIV) technique (this work is done with colleagues at affiliation 3).
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14:15
15 mins
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JOINT SCALING ANALYSIS OF ATMOSPHERIC VELOCITY AND WIND POWER PLANT PRODUCTION
Olmo DURAN MEDINA, François G. SCHMITT, Rudy CALIF
Abstract: In a context of energy transition, wind energy is a source of clean energy with the potential of partially satisfying the growing demand. The main problem of this type of energy, and other types of renewable energy remains the discontinuity of the electric power produced in different scales, inducing large fluctuations also called intermittency. This intermittency of wind energy is inherent to the turbulent nature of wind. Here, we consider the relation between velocity and power output with two wind turbine databases. We focus on joint relations with Fourrier analysis, empirical mode decomposition (EMD), Time-dependent intrinsic correlation (TDIC). We also consider the causality using a new method of analysis of the causality between two time series.
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14:30
15 mins
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EXPERIMENTAL INVESTIGATION OF EFFECT OF HIGH TURBULENCE ON THE AERODYNAMICS OF LOW RE AIRFOIL
Abhishek Bhesania, Ravi Dodamani, Parag J. Deshpande
Abstract: Turbulence pertaining to low Re flows is a subject of interest due to its considerable effect on the aerodynamics of high-lift low Re airfoils. The formation of laminar separation bubble (LSB) on the upper surface differentiates low Re airfoils from the conventional airfoils. The LSB has several detrimental effects on the airfoil performance such as, high drag[1], drastic change in the pressure distribution with varying bubble length[2], losing control on the pitching moment stability[2] etc. The dynamics of the LSB is also depends largely on the freestream turbulence level present in the flow.
In the present study, high turbulence is created inside a low speed wind tunnel by inserting passive grid at the entry of the test section. The grid is designed based on the empirical relation given by Roach [3] and the turbulence level is varied from 4 % to 10 % by varying the mesh size and the rod dimensions. The grid is made in the form of parallel rods and two grid configurations; parallel horizontal and parallel vertical grid are derived by changing the orientation of the rods as shown in Fig.1. NACA 4415 airfoil is chosen in the present study to explore the influence of turbulence on the dynamics of LSB at various angles of attack and at Reynolds number of 120000 based on chord length and freestream velocity. The orientation of vorticity production in both the grid configurations will be orthogonal and subsequently it will be interesting to see their interaction with the vorticity present in the LSB. Fig. 2 shows the decay in turbulence level from 16 % at the exit of the grid to 4 % downstream of the grid at mid of the test section measured using hot-wire by traversing it along tunnel centerline. ESP scanner is used for the measurement of surface pressure distribution over pressure tapped NACA-4415 airfoil model. Fig. 3 shows typical Cp distribution plot for 40 angle of attack for the case of flow without grid turbulence. The extent of LSB from x/c = 0.4 to 0.7 is also marked on the plot. The influence of the grid turbulence on the Cp distribution is currently underway. However, Surface oil flow visualization over the airfoil is carried out with and without grid. Fig. 4 (a) shows clearly, the separation and reattachment lines indicating the presence of LSB on the upper surface of the airfoil for the nominally smooth flow (turbulence level less than 0.1 %). Fig 4 (b) shows remarkable change in the flow pattern due to presence of high turbulence in the flow due to grid. No distinct separation and reattachment lines are observed and spots of the oil flow are seen along the span of the airfoil as shown in Fig 4 (b).
Detailed characterization of the grid turbulence based on hot-wire measurements and study of its effect on the pressure distribution through ESP measurements are underway and the flow physics associated with interaction of LSB with high turbulence will be elucidated in the full length paper.
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14:45
15 mins
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Large and detached eddy simulation of separated flow over 3D hill geometries with surface roughness to mimic flows over complex terrains
RAMESH BALAKRISHNAN
Abstract: With the push to making wind power a significant contributor to the energy portfolio in the U.S. and Europe, there is considerable effort to deploy the currently available peta-scale computational resources to assess and improve well known simulation techniques, such as the large eddy simulation (LES) and detached eddy simulation (DES) techniques, to model the complex flows in wind farms, taken as a whole, as opposed to individual wind turbines. Simulating turbulent flows in wind farms, consisting of arrays of wind turbines, begins with the modeling and simulation of the atmospheric boundary layer (ABL) over complex terrain that is characterized by regions of separated flow with a high degree of turbulence anisotropy. Over the years there has been considerable work on applying LES and Reynolds Averaged Navier--Stokes (RANS) simulations over terrain geometries, such as the Askervein Hill, to understand turbulence closure models for flow over complex terrain. Such studies, however, have had limited success due to difficulties associated with the closure models in the near wall region of the flow. At the same time, turbulence simulations over \emph{canonical} geometries, such as the periodic and axisymmetric hills, have been shown to compare well with data obtained from laboratory scale experiments, where the inflow turbulence and boundary conditions are better characterized and defined respectively. In an effort to extend these canonical flows to be more representative of flows over complex terrain, this paper aims to present results of large and detached eddy simulations of separated flow over three dimensional hill geometries with roughness parametrization, with the objective of developing better closure models for flow over complex terrain.
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