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http://hdl.handle.net/11375/151
2019-07-23T11:02:59ZA Multiresolution Model for the Simulation of Transient Heat and Mass Transfer
http://hdl.handle.net/11375/16847
Title: A Multiresolution Model for the Simulation of Transient Heat and Mass Transfer
Authors: Alam, Jahrul M.; Kevlahan, Nicholas K.-R.; Vasilyev, Oleg V.; Hossain, Zahangir
Abstract: The development of an efficient computational methodology for transient heat and mass transfer applications is challenging. When a solution is localized on the fraction of a com- putational domain, an appropriate adaptive mesh method could minimize computational work. In this article, we propose a novel adaptive-mesh multiresolution algorithm for the transient momentum and energy equations. The nonlinear dynamics between the velocity and temperature fields are modeled by solving the coupled system of equations simul- taneously, where the rate of convergence has been optimized so that computational cost remains proportional to the number of grid points. Numerical experiments have exhibited good agreements with benchmark simulation data.2012-05-11T00:00:00ZNon-Gaussianity and coherent vortex simulation for two-dimensional turbulence using an adaptive orthogonal wavelet basis
http://hdl.handle.net/11375/16845
Title: Non-Gaussianity and coherent vortex simulation for two-dimensional turbulence using an adaptive orthogonal wavelet basis
Authors: Farge, Marie; Schneider, Kai; Kevlahan, Nicholas
Abstract: We decompose turbulent flows into two orthogonal parts: a coherent, inhomogeneous, non-Gaussian component and an incoherent, homogeneous, Gaussian component. The two components have different probability distributions and different correlations, hence different scaling laws. This separation into coherent vortices and incoherent background flow is done for each flow realization before averaging the results and calculating the next time step. To perform this decomposition we have developed a nonlinear scheme based on an objective threshold defined in terms of the wavelet coefficients of the vorticity. Results illustrate the efficiency of this coherent vortex extraction algorithm. As an example we show that in a 2562 computation 0.7% of the modes correspond to the coherent vortices responsible for 99.2% of the energy and 94% of the enstrophy. We also present a detailed analysis of the nonlinear term, split into coherent and incoherent components, and compare it with the classical separation, e.g., used for large eddy simulation, into large scale and small scale components. We then propose a new method, called coherent vortex simulation CVS, designed to compute and model two-dimensional turbulent flows using the previous wavelet decomposition at each time step. This method combines both deterministic and statistical approaches: i Since the coherent vortices are out of statistical equilibrium, they are computed deterministically in a wavelet basis which is remapped at each time step in order to follow their nonlinear motions. ii Since the incoherent background flow is homogeneous and in statistical equilibrium, the classical theory of homogeneous turbulence is valid there and we model statistically the effect of the incoherent background on the coherent vortices. To illustrate the CVS method we apply it to compute a two-dimensional turbulent mixing layer.1999-01-01T00:00:00ZNonlinear RDT theory of near-wall turbulence
http://hdl.handle.net/11375/16844
Title: Nonlinear RDT theory of near-wall turbulence
Authors: Nazarenko, S.; Kevlahan, N.K.-R.; Dubrulle, B.
Abstract: A WKB method was recently used to extend rapid distortion theory (RDT) to initially inhomogeneous turbulence strained by irrotational mean flows [S.V. Nazarenko, N. Kevlahan, B. Dubrulle, J. Fluid Mech. 390 (1999) 325]. This theory takes into account the feedback of turbulence on the mean flow, and it was used by Nazarenko et al. to explain the effect of strain reduction caused by turbulence observed by Andreotti et al. [B. Andreotti, S. Douady,Y. Couder, in: O. Boratav, A. Eden, A. Erzan (Eds.), Turbulence Modeling and Vortex Dynamics, Proceedings of a Workshop held at Istanbul, Turkey, 2–6 September 1996, pp. 92–108]. In this paper, we develop a similar WKB RDT approach for shear flows. We restrict ourselves to problems where the turbulence is small-scale with respect to the mean flow length-scale and turbulence vorticity is weak compared to the mean shear. We show that the celebrated log-law of the wall exists as an exact analytical solution in our model if the initial turbulence vorticity (debris of the near-wall vortices penetrating into the outer regions) is statistically homogeneous in space and shortly correlated in time. We demonstrate that the main contribution to the shear stress comes from very small turbulent scales which are close to the viscous cut-off and which are elongated in the stream-wise direction (streaks). We also find that anisotropy of the initial turbulent vorticity changes the scaling of the shear stress, but leaves the log-law essentially unchanged.2000-01-01T00:00:00ZWKB theory for rapid distortion of inhomogeneous turbulence
http://hdl.handle.net/11375/16843
Title: WKB theory for rapid distortion of inhomogeneous turbulence
Authors: Nazarenko, S.; Kevlahan, N.K.-R.; Dubrulle, B.
Abstract: A WKB method is used to extend RDT (rapid distortion theory) to initially inhomoge- neous turbulence and unsteady mean flows. The WKB equations describe turbulence wavepackets which are transported by the mean velocity and have wavenumbers which evolve due to the mean strain. The turbulence also modifies the mean flow and generates large-scale vorticity via the averaged Reynolds stress tensor. The theory is applied to Taylor’s four-roller flow in order to explain the experimentally observed reduction in the mean strain. The strain reduction occurs due to the formation of a large-scale vortex quadrupole structure from the turbulent spot confined by the four rollers. Both turbulence inhomogeneity and three-dimensionality are shown to be important for this effect. If the initially isotropic turbulence is either homogeneous in space or two-dimensional, it has no effect on the large-scale strain. Furthermore, the turbulent kinetic energy is conserved in the two-dimensional case, which has impor- tant consequences for the theory of two-dimensional turbulence. The analytical and numerical results presented here are in good qualitative agreement with experiment.1999-01-01T00:00:00Z