15:00
15 mins
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The role of high vorticity structures in development of Kolmogorov turbulent spectra in inviscid flow
Dmitry Agafontsev, Evgenii Kuznetsov, Alexey Mailybaev
Abstract: This work is aimed for understanding nonlinear mechanisms at early stages of turbulence, when the flow is not yet affected by viscosity. Based on numerical simulations of the 3D incompressible Euler equations with generic large-scale initial conditions, we show that the exponential growth of vorticity developing in thin vortex sheets (pancake structures) leads to formation of Kolmogorov energy spectrum in fully inviscid flow. This direct observation yields the decoupling of the finite-time blowup problem from the Kolmogorov theory of turbulence. We demonstrate that the pancake structures have self-similar dynamics and evolve according to the scaling law W(t) ~ l(t)^(-2/3) for the local vorticity maximums W(t) and the transverse pancake scales l(t). Then, we argue that the energy spectrum requires an increasing number of such structures developing densely through the Kolmogorov range of wavenumbers, in good agreement with numerical data.
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15:15
15 mins
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The turbulence structure of 3D separation (Stall Cells) over an airfoil
Marinos Manoleosos, Spyros Voutsinas
Abstract: The flow over airfoils that experience separation of the trailing edge type becomes three-dimensional at angles of attack around maximum lift and Stall Cells (SCs) form. SCs are large scale coherent structures of separated flow that consist of two counter-rotating vortices. In the present study the turbulence structure of a SC over a rectangular wing is investigated using Stereo PIV measurements. It is found that the turbulence characteristics of the flow are highly anisotropic and that the Boussinesq approximation is invalid. High values of normal Reynolds stresses in the SC vortices and the separation shear layer region indicates a wandering motion of the former and a flapping motion of the latter. Based on the available data the relation between Reynolds stresses, their production terms and the mean flow gradients is examined. It is found that at the centre of the SC, between the two vortices, the flow characteristics resemble those of a double shear layer while at the vortex region the effect of the vortices leads to double peaks in production terms.
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15:30
15 mins
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On the optimal vorticity function of vortex rings
Ionut Danaila, Bartosz Protas
Abstract: We present here an original approach to reconstruct the vorticity distribution inside an axisymmetric vortex ring from some incomplete and possibly noisy measurements of the corresponding velocity field. The idea is to formulate an inverse problem for identifying the structure of the vorticity distribution and solve this problem by a suitable numerical optimization algorithm. The novelty of the present study is that the vorticity function is reconstructed in a very general form with no assumptions other than smoothness and the behaviour at the endpoints of its domain of definition. This is fundamentally different from classical approaches reducing the reconstruction problem to fitting a small number of variables parameterizing the vorticity distribution of a given vortex-ring model.
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15:45
15 mins
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Space-time reconstruction of finely resolved velocities of turbulent flows from low resolution measurements
Linh Van Nguyen, Jean-Philippe Laval, Pierre Chainais
Abstract: A model is developed to reconstruct the High-Temporal-High-Spatial (HTHS) resolution velocities of a wall bounded turbulent flow combining two sources of measured data: the Low-Temporal-High-Spatial (LTHS) and the High-Temporal-Low-Spatial (HTLS) resolution measurements. Direct Numerical Simulation (DNS) database of a turbulent channel flow is used. LTHS and HTLS are subsampled (in time or space) from the fully resolved data. A fusion model is constructed in Bayesian framework using a Maximum A Posteriori (MAP) estimation. By maximizing the posterior probability of desired HTHS conditional on LTHS and HTLS, the model estimates the most probable fields knowing the measurements. Reconstructed velocities are compared to the reference to qualify the fusion model and to those by conventional Linear Stochastic Estimation (LSE) and interpolation methods to demonstrate its robustness.
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16:00
15 mins
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CYCLONIC VORTEX IN A ROTATING LAYER WITH ISOLATED HEAT SOURCE
Andrei Sukhanovskii, Anna Evgrafova, Elena Popova
Abstract: Formation of cyclonic vortex from isolated heat source in a rotating fluid layer was studied experimentally and numerically. It was shown that structure of the vortex strongly depends on main governing parameters such as Grasshof number and Rossby number. PIV measurements were carried out for fluids with different values of Prandtl number (from 40 to 250). Numerical simulations were done using CFD software FlowVision.
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16:15
15 mins
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HIGH-RESOLUTION NUMERICAL ANALYSIS OF TURBULENT FLOW IN STRAIGHT OPEN DUCTS WITH RECTANGULAR CROSS-SECTION
Yoshiyuki Sakai, Markus Uhlmann, Genta Kawahara
Abstract: Turbulent secondary flow in straight open ducts with rectangular cross-section are studied numerically by means of pseudo- spectral direct numerical simulation (DNS). Similarly to the corresponding closed duct flows, the mean streamwise vorticity pattern in turbulent open duct flows were found to be the statistical footprint of the most probable locations of the quasi-streamwise vortices. A major difference between the two configurations was found in the vicinity of mixed-boundary corners where noticeably persistent vortical structures exist.Turbulent secondary flow in straight open ducts with rectangular cross-section are studied numerically by means of pseudo- spectral direct numerical simulation (DNS). Similarly to the corresponding closed duct flows, the mean streamwise vorticity pattern in turbulent open duct flows were found to be the statistical footprint of the most probable locations of the quasi-streamwise vortices. A major difference between the two configurations was found in the vicinity of mixed-boundary corners where noticeably persistent vortical structures exist.
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