Modeling of the subgrid-scale term of the filtered magnetic field transport equation

Accurate subgrid-scale turbulence models are needed to perform realistic numerical magnetohydrodynamic (MHD) simulations of the subsurface flows of the Sun. To perform large-eddy simulations (LES) of turbulent MHD flows, three unknown terms have to be modeled. As a first step, this work proposes to use a priori tests to measure the accuracy of various models proposed to predict the SGS term appearing in the transport equation of the filtered magnetic field. It is proposed to evaluate the SGS model accuracy in term of "structural" and "functional" performance, i.e. the model capacity to locally approximate the unknown term and to reproduce its energetic action, respectively. From our tests, it appears that a mixed model based on the scale-similarity model has better performance.Comment: 10 pages, 5 figures; Center for Turbulence Research, Proceedings of the Summer Program 2010, Stanford Universit

Large-Eddy Simulation and Conjugate Heat Transfer Around a Low-Mach Turbine Blade

International audienceDetermination of heat loads is a key issue in the design of gas turbines. In order to optimize the cooling, an exact knowledge of the heat flux and temperature distributions on the airfoils surface is necessary. Heat transfer is influenced by various factors, like pressure distribution, wakes, surface curvature, secondary flow effects, surface roughness, free stream turbulence, and separation. Each of these phenomenons is a challenge for numerical simulations. Among numerical methods, large eddy simulations (LES) offers new design paths to diminish development costs of turbines through important reductions of the number of experimental tests. In this study, LES is coupled with a thermal solver in order to investigate the flow field and heat transfer around a highly loaded low pressure water-cooled turbine vane at moderate Reynolds number (150,000). The meshing strategy (hybrid grid with layers of prisms at the wall and tetrahedra elsewhere) combined with a high fidelity LES solver gives accurate predictions of the wall heat transfer coefficient for isothermal computations. Mesh convergence underlines the known result that wall-resolved LES requires discretizations for which y+ is of the order of one. The analysis of the flow field gives a comprehensive view of the main flow features responsible for heat transfer, mainly the separation bubble on the suction side that triggers transition to a turbulent boundary layer and the massive separation region on the pressure side. Conjugate heat transfer computation gives access to the temperature distribution in the blade, which is in good agreement with experimental measurements. Finally, given the uncertainty on the coolant water temperature provided by experimentalists, uncertainty quantification allows apprehension of the effect of this parameter on the temperature distribution

Effect of turbulence on the wavefront of an ultra high intensity laser beam

International audienceUltra high intensity lasers face thermal management issues that limit their repetition rates. The key challenge is to efficiently evacuate the heat deposited in the amplifier by the optical pumping without impacting the output laser beam quality. The amplifier can have a multi-slab geometry where the laser beam crosses successive amplifying slabs and the cooling channels that separate them. The present work investigates numerically how a cryogenic cooling of the multi-slab amplifier by turbulent channel flows may affect the wavefront of the laser beam. To this end, Large Eddy Simulations representative of the amplifier cooling are performed using TrioCFD, a code developed by the CEA. First, validation simulations are carried out for heated channel flows, allowing comparisons to Direct Numerical Simulation results from the literature. Then Large Eddy Simulations of an open turbulent channel flow cooling two slabs are conducted using conjugated heat transfer between the solid and the fluid. The phase distortions, mean and fluctuations, induced by the inhomoge-neous and turbulent temperature field are computed directly from the LES. The Sutton model is widely used in aero-optic studies ; its validity and applicability to this problem is then examined. Finally, the effect of an increased optical heating of the slabs is investigated. It is the first time that TrioCFD is used to address the question of the impact of the cooling of laser amplifiers, and it has proven to be a valuable tool for this application

Integrated exploration of turbulent cross-helicity effect: theory, observation, modeling and numerical simulations of the solar convection zone

Cross helicity (velocity–magnetic-field correlation) is one of the basic quantities in magnetohydrodynamics. In the presence of large-scale vortical motion, if there is positive (or negative) cross helicity in turbulence, a contribution to the electromotive force parallel (or anti-parallel) to the large-scale vorticity arises. By considering the evolution equation of turbulent cross helicity, the mechanisms that create turbulence with cross helicity are investigated. In order to analyze cross helicity-related transport and crosshelicity generation mechanisms, three kinds of numerical simulations are performed: (i)direct numerical simulations of the local Kolmogorov flow with an imposed magnetic field, (ii) global direct simulation of a rotating spherical fluid shell, and (iii) large-eddy simulation of the local compressible flow near the sunspot region. Through these simulations, the importance of the cross-helicity effect is validated. The cross-helicity generation mechanisms relevant to the incompressible and compressible cases are also discussed