Lars Davidson

Low‐Reynolds‐number flow around a square cylinder at incidence: study of blockage, onset of vortex shedding and outlet boundary condition

Calculations of unsteady 2D flow around a square cylinder at incidence (α=0°−45°) are presented. The Reynolds numbers are low (Re=45–200) so that the flow is presumably laminar. A von Karman vortex sheet is predicted behind the cylinders with a periodicity which agrees well with experiments. An incompressible SIMPLEC code is used with a non-staggered grid arrangement. A third-order QUICK scheme is used for the convective terms. The time discretization is implicit and a second-order Crank–Nicolson scheme is employed. At the outlet of the computational domain a convective Sommerfeld boundary condition is compared with a traditional Neumann condition. The convective boundary condition is shown to be more effective in reducing the CPU time, reducing the upstream influence of the outlet and thus reducing the necessary downstream extent of the domain. A study of the effects of spatial resolution and blockage is also provided. The onset of vortex shedding is investigated by using the Stuart–Landau equation at various angles of incidence and for a solid blockage of 5%. A number of quantities such as Strouhal number and drag, lift and moment coefficients are calculated.

Simulation of three-dimensional flow around a square cylinder at moderate Reynolds numbers

Direct numerical simulations of two-dimensional (2D) and 3-D unsteady flow around a square cylinder for moderate Reynolds numbers (Re=150–500) are performed, employing an implicit fractional step method finite-volume code with second-order accuracy in space and time. The simulations, which are carried out with a blockage ratio of 5.6%, indicate a transition from 2-D to 3-D shedding flow between Re=150 and Re=200. Both spanwise instability modes, A and B, are present in the wake transitional process, similar to the flow around a circular cylinder. However, seemingly in contrast to a circular cylinder, the transitional flow around a square cylinder exhibits a phenomenon of distinct low-frequency force pulsations (Re=200–300). For 3-D simulations, the Strouhal number and the mean drag coefficient are in general agreement with existing experiments. Between Re=300 and 500, an increase in the spanwise coupling of fluctuating forces is indicated. The influence of the spanwise aspect ratio using periodic boundary...

Large Eddy Simulation of Flow Past a Square Cylinder: Comparison of Different Subgrid Scale Models

Large eddy simulation of flow past a rigid prism of a square cross section with one side facing the oncoming flow at Re=2.2×10 4 is performed. An incompressible code is used employing an implicit fractional step method finite volume with second-order accuracy in space and time. Three different subgrid scale models: the Smagorinsky, the standard dynamic, and a dynamic one-equation model, are applied. The influence of finer grid, shorter time step, and larger computational spanwise dimension is investigated. Some global quantities, such as the Strouhal number and the mean and rms values of lift and drag, are computed. A scheme for correcting the global results for blockage effects is presented

Flow Around a Simplified Car, Part 1: Large Eddy Simulation

Large eddy simulations (LES) were made of flows around a generic ground vehicle with sharp edges at the rear end (an Ahmed body with a 25° angle of the rear slanted surface). Separation of the flow at the rear results in large regions with recirculating flow. As the separation is determined by the geometry, the Reynolds number effects are minimized. Resolution requirements of this recirculating flow are smaller than those in LES of wall attached flows. These two consequences of the geometry of the body are used to predict the experimental flow at relatively high Reynolds number. Recommendations are pre-sented for the preparation and realization of LES for vehicle flows. Comparison of the LES results with the experimental data shows good agreement. Copyright

Numerical simulation of unsteady low-Reynolds number flow around rectangular cylinders at incidence

Abstract Calculations of unsteady two-dimensional-flow around rectangular cylinders at incidence are presented. The Reynolds numbers are low ( ⩽ 200), so that the flow presumably is laminar. The results are in reasonable agreement with the indeed scarce experimental data available at these low Reynolds numbers. An incompressible SIMPLEC code is used employing non-staggered grid arrangement. A third-order QUICK scheme is used for the convective terms. The time discretization is implicit and a second-order Crank-Nicolson scheme is employed. The influence of the cylinder side ratio ( B A = 1−4) at various angles of incidence (α = 0°−90°) is investigated. A number of quantities such as Strouhal number, drag, lift, and moment coefficients are calculated. Time sequences of fully saturated flow are also provided.

Large-Eddy Simulation of the Flow Around a Bluff Body

Large-eddy simulations are made of the flow around a surface-mounted cube, showing that it is possible to obtain accurate results in a coarse grid simulation. The inadequate resolution is compensated for by the use of a dynamic one-equation subgrid-scale model. Two one-equation subgrid models are used here to model the subgrid-scale stress tensor. A series of time-averaged velocities and turbulent stresses are computed and compared with the experiments. Global quantities such as drag and lift coefficients and vortex shedding frequency are presented. The transfer of the turbulent energy was studied and the reverse transfer of energy (backscatter) was predicted. Coherent structures and other flow features were also examined

Zonal k–l based large eddy simulations

Abstract Zonal k – l based large eddy simulation (LES) approaches are presented. To reduce computational demands, near walls, Reynolds averaged Navier–Stokes (RANS) like modelling is used. The interface location for the differing models is either explicitly specified, or, based on length scale compatibility, allowed to naturally locate. With the latter approach the location is strongly grid controlled. When explicitly specified (based on turbulence physics grounds), to enhance results length scale smoothing is implemented. Using standard established LES and RANS model constants the zonal methods are shown to reproduce a satisfactory law of the wall. The approaches are implemented in both cell-vertex and cell-centred codes with similar results being found. Various other sensitivity studies are performed. These show that, as with standard LES, predictions are most sensitive to filter definition, first off wall grid node normal positions and temporal scheme order. For a non-isothermal periodic ribbed channel, the new zonal LES predictions are found to be significantly more accurate than those for an established RANS model and also LES.

Numerical Study of the Flow Around a Bus-Shaped Body

Flow around a simplified bus is analyzed using large-eddy simulation. At the Reynolds number of 0.21 × 10 6 , based on the model height and the incoming velocity. the flow produces features and aerodynamic forces relevant for the higher (interesting in engineering) Reynolds number. A detailed survey of both instantaneous and time-averaged flows is made and a comparison with previous knowledge on similar flows is presented. Besides the coherent structures observed in experimental and previous numerical studies, new smaller-scale structures were registered here. The mechanisms of formation of flow structures are explained and the difference between instantaneous and time-averaged flow features found in the experimental observations is confirmed. Aerodynamic forces are computed and their time history is used to reveal the characteristic frequencies of the flow motion around the body

Large Eddy Simulation for Turbulent Buoyant Flow in a Confined Cavity

Abstract A turbulent natural convection flow ( Ra =1.58×10 9 ) in a confined cavity with two differentially heated side walls was numerically investigated by means of large eddy simulation (LES). The mean flow in the cavity is characterized by stable thermal stratification and a relatively low turbulence level. The LES results for the mean flow quantities show good agreement with the experiment. This is particularly the case when the dynamic model is used. Nevertheless, there are some discrepancies in the prediction of turbulence statistics, particularly in the outer region of the near-wall flow where the boundary layer interacts with the recirculating core region. In the viscous/conductive sublayer of the boundary layer close to the heated/cooled vertical walls, the flow tends to form streak-like structures, which do not however emerge in the near-wall flow along the horizontal top and bottom walls. To resolve the flow structure near the vertical walls, sufficient grid resolution is required.

Hybrid LES-RANS: An approach to make LES applicable at high Reynolds number

The main bottle neck for using large eddy simulations (LES) at high Reynolds number is the requirement of very fine meshes near walls. Hybrid LES-Reynolds-averaged Navier-Stokes (RANS) was invented to get rid of this limitation. In this method, unsteady RANS (URANS) is used near walls and away from walls LES is used. The matching between URANS and LES takes place in the inner log-region. In the present paper, a method to improve standard LES-RANS is evaluated. The improvement consists of adding instantaneous turbulent fluctuations (forcing conditions) at the matching plane in order to provide the equations in the LES region with relevant turbulent structures. The fluctuations are taken from a DNS of a generic boundary layer. Simulations of fully developed channel flow and plane asymmetric diffuser flow are presented. Hybrid LES-RANS is used both with and without forcing conditions.