Browsing by Author "Hahn, Marco"
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Item Open Access High resolution methods for incompressible, compressible and variable density flows.(2004-01-01T00:00:00Z) Drikakis, Dimitris; Hahn, Marco; Patel, Sanjay; Shapiro, EvgeniyThe uid dynamics community has dealt with a number of numerical challenges since the 1950's. These include the development of numerical methods for hyperbolic conservation laws with particular interest in capturing shock wave propagation and related phenomena, solution algorithms for the solution of the incompressible Navier-Stokes equations - a numerical challenge arises here due to the absence of the pressure term from the continuity equation - methods/ techniques for the acceleration of the numerical convergence, modelling of turbulence and grid generation techniques. Within each of those areas di erent numerical approaches have been pursued by various researchers aiming to achieve higher accuracy and ef ciency of the numerical solution. Continuous interest exists in relation to the development of accurate and e cient numerical methods for the computation of instabilities, transition and turbulence. It has been observed for more than a decade that high-resolution methods can be used in (under-resolved) turbulent ow computations without the need to resort to a turbulence model, but this approach has only recently gained some theoretical support and structural explanation for the observed results [3, 15]. Because of this there is a necessary overlap between the classical modelling of turbulence and its computation through high resolution methods [5]. These methods are currently used to simulate a broad variety of complex ows, e.g., ows that are dominated by vorticity leading to turbulence, ows featuring shock waves and turbulence, and the mixing of materials [23]. Such ows are extremely dif cult to practically obtain stably and accurately in under-resolved conditions (with respect to grid resolution) using classical linear (both second and higherorder accurate) schemes. Further, new applications at micro-scale, e.g. micro uidics, microreactors and lab-on-a-chip, have raised a number of challenges for computational science methods. In this paper we provide a brief overview of highresolution methods in connection with some of the above problems. An extensive description of these methods for incompressible and low-speed Flows can be found in [5].Item Open Access Implicit large-eddy simulation of low-speed separated flows using high-resolution methods(Cranfield University, 2008-03) Hahn, Marco; Drikakis, DimitrisMost flows of practical importance are governed by viscous near-wall phenomena leading to separation and subsequent transition to a turbulent state. This type of problem currently poses one of the greatest challenges for computational methods because its characteristics covers a wide range of physical processes that often place contradictory requirements on the numerics employed. This thesis seeks to investigate the physics of complex, separated flows pertinent to aeronautical engineering and to assess the performance of variants of the Implicit Large-Eddy Simulation approach in predicting this type of problem realistically. For this purpose, different numerical solution strategies based on high-resolution methods, distinguished by their order of accuracy, are used in precursor simulations and one selected approach is applied to a fully three-dimensional wing flow. In order to isolate the development from laminar to turbulent flow after separation has occurred, the prototype Taylor-Green Vortex is considered. Here, the behaviour of the numerical schemes during the linear, non-linear and fully turbulent stages in the flow evolution is tested for different grid sizes. It is found that the resolution power and the likelihood of symmetry breaking is increasing with the order of accuracy of the numerical method. These two properties allow the flow to develop more realistically on coarse grids if higher order schemes are employed. In the next step, flow separation from a gently curved surface is included. The fundamental study of a statistically two-dimensional channel flow with hill-type constrictions demonstrates the basic applicability of ILES to problems featuring massive separation. Without specific wall-treatment, high-resolution methods can improve prediction of the detachment location when compared to classical Large-Eddy Simulations. Finally, an ILES simulation of three-dimensional flow over a swept wing geometry at moderate angle of incidence is presented. The results are in excellent agreement with experiment in the fully separated and turbulent region and they are more accurate than a classical hybrid RANS/LES approach, using a grid twice the size, over the majority of the wing. This outcome will probably settle the dispute that has erupted in the past over the applicability of ILES to complex, wall-bounded flows.Item Open Access Implicit Large-Eddy Simulations of Wall-Bounded Turbulent Flows.(2007-01-01T00:00:00Z) Drikakis, Dimitris; Hahn, Marco; Malick, Zeshan; Shapiro, EvgeniyThe paper presents application of Implicit Large Eddy Simulation(ILES)to wall- bounded turbulent flows. A characteristics-based scheme in conjunction with total variation diminishing(TVD)Runge-Kutta time stepping and slope-limiting variants,for the com-pressible flow case, have been employed. Results are pre- sented for an incompressible lid-driven cavity flow,using an incompressible solver, and low-Mach number flows over a hill and around a delta wing, using a compressible solver. Good agreement with experimental data and numerical results using classical LES has been obtained. Future ILES developments are also discussedItem Open Access Large eddy simulation using high-resolution and high-order methods(Royal Society, 2009-07-31T00:00:00Z) Drikakis, Dimitris; Hahn, Marco; Mosedale, Andrew; Thornber, BenRestrictions on computing power make direct numerical simulation too expensive for complex flows; thus, the development of accurate large eddy simulation (LES) methods, which are industrially applicable and efficient, is required. This paper reviews recent findings about the leading order dissipation rate associated with high-resolution methods and improvements to the standard schemes for use in highly turbulent flows. Results from implicit LES are presented for a broad range of flows and numerical schemes, ranging from the second-order monotone upstream-centered schemes for conservation laws to very high-order (up to ninth-order) weighted essentially non-oscillatory schemes.