D.G.E. Grigoriadis

Three dimensional flow around a circular cylinder confined in a plane channel

This paper presents two- and three-dimensional direct numerical simulations of the flow around a circular cylinder placed symmetrically in a plane channel. Results are presented in the Reynolds number range (based on the cylinder diameter and centerline velocity) of 10 to 390 for a blockage ratio (ratio of the cylinder diameter to the channel height) of 0.2. The aim of this work was to investigate in detail the confinement effect due to the channel’s stationary walls on the force coefficients and the associated Strouhal numbers, as well as on the generated flow regimes. Present results suggest a transition from a 2-D to a 3-D shedding flow regime between Re = 180 and Re = 210. This transition was found to be dominated by mode A and mode B three dimensional instabilities, similar to those observed in the case of an unconfined circular cylinder. This is the first time that the existence of the two modes, and of naturally occurring vortex dislocations, has been confirmed via full 3-D simulations for the case...

Spatial Variability and Source-Receptor Relations at a Street Intersection

A wind tunnel study of dispersion at a simple urban intersection comprising two perpendicular streets is described. Concentration and flow field measurement were undertaken to determine the importance of the exchange of pollutants between the streets and to investigate source-receptor relationships at the intersection. The results showed that only in a symmetrical situation were exchanges negligible and that small departures from symmetry, brought about in the experiments through an off-set in the street alignment or a change of orientation relative to the wind, were sufficient to establish significant exchanges. The results also showed that significant structure appeared in the concentration fields in the streets as a result. Examples are shown where concentrations on one side of a street are entirely due to emissions from the perpendicular street, whereas on the opposite side concentrations depend on emission upwind in the same street as the receptor. The results imply that exchanges between street systems are likely to be the norm in practice and that the consequences of such exchanges are not confined to the immediate vicinity of the intersection.

Three-dimensional modelling of concentration fluctuations in complicated geometry

The strong fluctuating component in the measured concentration time series of a dispersing gaseous pollutant in the atmospheric boundary layer, and the hazard level associated to short-term concentration levels, demonstrate the necessity of calculating the magnitude of turbulent fluctuations of concentration using computational simulation models. Moreover the computation of concentration fluctuations in cases of dispersion in realistic situations, such as built-up areas or street canyons, is of special practical interest for hazard assessment purposes. In this paper, the formulation and evaluation of a model for concentration fluctuations, based on a transport equation, are presented. The model is applicable in cases of complex geometry. It is included in the framework of a computational code, developed for simulating the dispersion of buoyant pollutants over complex geometries. The experimental data used for the model evaluation concerned the dispersion of a passive gas in a street canyon between 4 identical rectangular buildings performed in a wind tunnel. The experimental concentration fluctuations data have been derived from measured high frequency concentrations. The concentration fluctuations model is evaluated by comparing the models predictions with the observations in the form of scatter plots, quantile-quantile plots, contour plots and statistical indices as the fractional bias, the geometrical mean variance and the factor-of-two percentage. From the above comparisons it is concluded that the overall model performance in the present complex geometry case is satisfactory. The discrepancies between model predictions and observations are attributed to inaccuracies in prescribing the actual wind tunnel boundary conditions to the computational code.

Efficient treatment of complex geometries for large eddy simulations of turbulent flows

Abstract Incompressible turbulent flow over a backward facing step at Reh=5100 is investigated by large eddy simulations (LES). The ratio of the oncoming boundary layer thickness δ to the step height h was set to 1.2. Additionally channel flows at various Reτ numbers are presented for the validation of the numerical code. The results are compared with existing DNS and experimental databases. The present study focuses on different procedures for LES of engineering problems in complex geometries using structured rectangular grids. Two different methods that are able to treat complex geometrical configurations are implemented, examined and compared; namely the domain decomposition approach based on Schur’s complement and the immersed boundary method. In the present study both methods make use of a fast direct Poisson’s pressure solver based on a heavily modified version of the public domain package FISHPAK . The latter was optimised and fully parallelised for shared memory architectures, for solutions on rectangular grids stretched in one or two directions. The resulting code reaches performances of 1.0 μs/node/iter, allowing low cost computations on grids of the order of million points. The main objective of the present study was to investigate the potential of different methods for LES in complex geometrical configurations like bluff body flows and wakes. One of the main findings is that careful selection of numerical methods and implementation techniques can lead to accurate and very efficient codes, where the geometric complexity does not lead to algorithmic or numerical complexity.

Immersed boundary method for the MHD flows of liquid metals

Wall-bounded magnetohydrodynamic (MHD hereafter) flows are of great theoretical and practical interest. Even for laminar cases, MHD simulations are associated with very high computational cost due to the resolution requirements for the Hartmann and side layers developing in the presence of solid obstacles. In the presence of turbulence, these difficulties are further compounded. Thus, MHD simulations in complex geometries are currently a challenge. The immersed boundary (IB hereafter) method is a reliable numerical tool for efficient hydrodynamic field simulations in arbitrarily geometries, but it has not yet been extended for MHD simulations. The present study forms the first attempt to apply the IB methodology for the computation of both the hydrodynamic and MHD fields. A consistent numerical methodology is presented that is appropriate for efficient 3D MHD simulations in geometrically complicated domains using cartesian flow solvers. For that purpose, a projection scheme for the electric current density is presented, based on an electric potential correction algorithm. A suitable forcing scheme for electric density currents in the vicinity of non-conducting immersed surfaces is also proposed. The proposed methodology has been first extensively tested for Hartmann layers in fully-developed and developing channel and duct flows at Hartmann numbers Ha=500-2000. In order to demonstrate the potential of the method, the three-dimensional MHD flow around a circular cylinder at Reynolds number Re=200 is also presented. The effects of grid resolution and variable arrangement on the simulation accuracy and consistency were examined. When compared with existing numerical or analytic solutions, excellent agreement was found for all the cases considered. The proposed projection and forcing schemes for current densities were found capable of satisfying the charge conservation law in the presence of immersed non-conducting boundaries. Finally, we show how the proposed methodology can be used to extend the applicability of existing flow solvers that use the IB concept with a staggered variable arrangement.

Three-dimensional numerical simulations of magnetohydrodynamic flow around a confined circular cylinder under low, moderate, and strong magnetic fields

This paper presents three-dimensional direct numerical simulations of liquid metal flow around a circular cylinder placed symmetrically in a rectangular duct, under a wide range of magnetic field intensities. Results are presented for values of the Hartmann number (based on the duct width) in the range of 0 ⩽ Ha ⩽ 1120, and the Reynolds number (based on the cylinder diameter and centerline velocity) in the range 0 ⩽ Rec ⩽ 5000. The generated flow regimes and the associated critical values of parameters are investigated in detail through full three-dimensional simulations. The effect of the magnetic field on the wake structure is discussed in relation to the possible mechanisms for the generation or suppression of vortices, and to previous attempts to model magnetohydrodynamic flows using simplified two-dimensional models. Present results reveal a non-monotonic dependance of the critical Reynolds number for the onset of vortex shedding, with respect to the Hartmann number. For certain combinations of Ha an...

Laminar Free Convection in a Square Enclosure Driven by the Lorentz Force

A numerical study is presented of laminar free convection flow driven by magnetic forces. An external magnetic field with one spatially varying component is applied to an electrically conducting fluid in a square enclosure. This magnetically-driven flow is controlled by the intensity and the wave number of the applied magnetic forcing. In addition, when the enclosure is heated laterally in a non-zero gravity environment, the resulting buoyant forces may contribute or resist the magnetically-driven fluid motion. The present results show that a strong magnetic field can even reverse the buoyant flow. The circulation intensity of the flow and the heat transfer from the sidewalls is increased with increasing magnetic field or with decreasing magnetic Reynolds number. The wave number of the magnetic forcing is also an important parameter that determines the vortex patterns and, consequently, the convection heat transfer.

Diagnostic Properties of Structure Tensors in Turbulent Flows

The behavior of turbulence structure tensors based on Large-Eddy Simulations (LES) in a wide range of turbulent channel flows is presented. The structure tensors provide significant physical information on the character of turbulent flows, since they provide an accurate description of the energy containing turbulence structure. LES is ideally suited for their computation since these tensors are quantities representing the larger – energy containing – turbulent scales. The basic aims of the present work are to (i) demonstrate the diagnostic properties of structure tensors in turbulent flows, (ii) report turbulence quantities which would be useful to develop and assess structure-based turbulence models, (iii) demonstrate the capability of LES to accurately compute structure tensors in a variety of flows. Structure tensors have been computed in the presence of complicated physical phenomena like frame rotation or MHD effects. Comparisons with available DNS solutions, confirm the capability of LES to accurately predict such quantities with fundamental significance in turbulent flows.

Large Eddy Simulation of Flow over Dunes Laden with Inertial Particles

Waves and currents are the basic mechanisms that set sediment in motion affecting the morphology of erodible beds in coastal flows.

Natural Convection in Ventilated Building Facades Using LES

Understanding the buoyancy-driven flow in gas gap enclosures can provide significant physical insight into a variety of technological applications ranging from cooling of electronic equipment to energy related components and systems.