1,721,053 research outputs found
A general strategy for the optimization of Runge-Kutta schemes for wave propagation phenomena
No full textSpace- and time-optimized schemes for computational aeroacoustics
No full textThe performances of high-order, highly efficient finite difference schemes with Runge-Kutta time integration are gauged for classical benchmark test cases of CAA. The validity of theoretical results related to the optimal performance of a given scheme and the devel- opment of space- and time- optimized schemes tailored for specific applications has been checked to verify the extension of the theory to practical problems. The results of the study show that most of the theoretical findings related to the optimal performance of numerical schemes do apply to real-world problems. The improvement of efficiency obtained with the use of optimized schemes is confirmed, even though the actual gain may sensitively depend upon the problems under consideration. The study has also confirmed problem related to the proper specification of numerical boundary conditions and boundary closures. © 2007 by Sergio Pirozzoli
Analysis of vortical structures in highly compressible turbulent boundary layers
No full textSan Antonio T
On the suitability of the immersed boundary method for the simulation of high-Reynolds-number separated turbulent flows
No full textDelayed detached eddy simulation based on the Spalart–Allmaras turbulence model is applied in conjunction with the immersed boundary method to high-Reynolds number turbulent flows in complex geometries. A fourth-order, finite-difference solver capable of discretely preserving the total kinetic energy in the limit of inviscid flow is adopted to solve the compressible Navier–Stokes equations and model-consistent, adaptive wall functions are employed to provide proper numerical boundary conditions at the fluid/solid interface. Numerical tests, performed for several configurations involving massively separated flows, demonstrate that computations at high Reynolds number typically occurring in flows of industrial relevance can be successfully carried out using the immersed boundary strategy, providing predictions whose accuracy is comparable to that of standard, body-fitted, structured and unstructured flow solvers
Scrutiny of buffet mechanisms in transonic airfoils
No full textWe carry out numerical simulations of transonic flow around a supercritical airfoil at M = 0.7, incidence angle 7 deg, and Reynolds number Re = 3000000, with the aim of elucidating the mechanisms responsible for large-scale shock oscillations (buffet) which are expected to occur under these conditions. An implicit large-eddy simulation (ILES) is carried out with an upstream laminar boundary layer. Companion Reynolds-Averaged Navier-Stokes (RANS) and detached-eddy simulations (DDES) are also carried out to ascertain the feasibility of the various buffet mechanisms. Estimations of the propagation velocities of coherent disturbances and of acoustic waves are obtained, to be inserted in standard predictive models. It appears that the dynamics underlying buffet is highly localized around the mean shock location
Velocity statistics in turbulent channel flow up to Reτ =4000
No full textThe high-Reynolds-number behaviour of the canonical incompressible turbulent channel flow is investigated through large-scale direct numerical simulation (DNS). A Reynolds number is achieved ( Reo = h/δv 4000, where h is the channel half-height, and δv is the viscous length scale) at which theory predicts the onset of phenomena typical of the asymptotic Reynolds number regime, namely a sensible layer with logarithmic variation of the mean velocity profile, and Kolmogorov scaling of the velocity spectra. Although higher Reynolds numbers can be achieved in experiments, the main advantage of the present DNS study is access to the full three-dimensional flow field. Consistent with refined overlap arguments (Afzal & Yajnik, J. Fluid Mech. vol. 61, 1973, pp. 23-31; Jiménez & Moser, Phil. Trans. R. Soc. Lond. A, vol. 365, 2007, pp. 715-732), our results suggest that the mean velocity profile never achieves a truly logarithmic profile, and the logarithmic diagnostic function instead exhibits a linear variation in the outer layer whose slope decreases with the Reynolds number. The extrapolated value of the von Kármán constant is k 0.41. A near logarithmic layer is observed in the spanwise velocity variance, as predicted by Townsend's attached eddy hypothesis, whereas the streamwise variance seems to exhibit a shoulder, perhaps being still affected by low-Reynolds-number effects. Comparison with previous DNS data at lower Reynolds number suggests enhancement of the imprinting effect of outer-layer eddies onto the near-wall region. This mechanisms is associated with excess turbulence kinetic energy production in the outer layer, and it reflects in flow visualizations and in the streamwise velocity spectra, which exhibit sharp peaks in the outer layer. Associated with the outer energy production site, we find evidence of a Kolmogorov-like inertial range, limited to the spanwise spectral density of u, whereas power laws with different exponents are found for the other spectra. Finally, arguments are given to explain the 'odd' scaling of the streamwise velocity variances, based on the analysis of the kinetic energy production term. © 2013 Cambridge University Press
The effect of large-scale turbulent structures on particle dispersion in wall-bounded flows
No full textThe effect of large-scale turbulent structures on the motion of heavy particles in wall-bounded turbulent flows is investigated by mining a direct numerical simulation database of particle-laden turbulent Couette flow, and comparing the results with a turbulent Poiseuille at similar friction Reynolds number. It is shown that the large-scale structures found in the core of the Couette flow have an influence on the turbophoretic mechanism, leading to different distributions of the particle concentration. The main differences in the two flows are observed in the spatial distribution of the suspended phase, which is found to be strongly dependent on the structure of the underlying streamwise velocity field. In addition to the standard particle streaks in the inner layer, spaced at 100 wall units, typical of the Poiseuille flow, in the Couette case particles with non-negligible inertia respond to the large-scale structures of the core, by organizing themselves into large-scale rows whose typical spanwise separation is of the order of 4-5 channel half-height. © 2012 Elsevier Ltd
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