Ibrahim E Abdalla

Computational analysis and flow structure of a transitional separated-reattached flow over a surface mounted obstacle and a forward-facing step

Large-eddy simulation (LES) of transitional separating-reattaching flow on a two-dimensional square surface mounted obstacle and a forward facing step has been performed using a dynamic sub-grid scale model. The Reynolds number based on the uniform inlet velocity and the obstacle/step height is 4.5 × 103. The mean LES results for both the obstacle and step flow compare reasonably well with the available experimental and DNS data. The flow structures upstream of the surface-mounted obstacle (referred to hereafter as obstacle) and the forward-facing step (referred to hereafter as FFS) consist of unstable two-dimensional structures and coherent rib-shaped structures. These structures with the aid of 3D streamline visualisation strongly indicate that the upstream separation bubble is a closed one rather than an open one in the sense that there is little evidence to suggest that there is fluid injection from the upstream separation region into the downstream separated region for the two geometries. The spectra and time history for the velocities and pressure fields at locations immediately upstream of the obstacle and FFS (including the recirculation region) were analysed using both the Fourier and wavelet transforms and revealed the unsteady nature of the recirculation region upstream of the obstacle and FFS. The transition process has been elucidated using both 2D and 3D flow visualisation of the flow. In both geometries (obstacle and FFS), the separated boundary layer downstream of the leading edge shows 2D nature and roll-up shortly downstream of the separation line leading to 2D K-H rolls to be shed from the leading edge. Coherent structures such as the λ-shaped and rib-like vortices commonly associated with a flat plate boundary layer and also found in the separated-reattached flow of a blunt leading edge plate aligned horizontally to a flow are not common in the separated-reattached flow over the obstacle and FFS.

Numerical Study of a Separated-Reattached Flow on a Blunt Plate

Large-eddy simulation (LES) of transitional separatingreattaching flow on a flat plate with a blunt leading edge has been performed. The Reynolds number based on the uniform inlet velocity and the plate thickness is 6.5103. A dynamic subgrid-scale model is employed in the transitional flow case. The LES results compare reasonably well with the available experimental data. The entire transition process has been visualized by using the LES data, and large-scale vortical structures have been observed at different stages of transition. It is known that different vortex shedding frequencies exist, especially the so-called low-frequency flapping when there is a separation bubble. It is not clear whether all transitional and turbulent separatingreattaching flows have different vortex shedding frequencies. It is also not clear what the working mechanisms are behind the so-called low-frequency flapping as reported widely. These issues are addressed.

Large-eddy simulation of buoyancy-driven natural ventilation in an enclosure with a point heat source

Rising buoyant plumes from a point heat source in a naturally ventilated enclosure have been investigated using large-eddy simulation (LES). The aim of the work is to assess the performance and the accuracy of LES for modelling buoyancy-driven displacement ventilation of an enclosure and to shed more light on the transitional behaviour of the plume and the coherent structures involved. The Smagorinsky sub-grid scale model is used for the unresolved small-scale turbulence. The Rayleigh number, Ra is chosen to be in the range where spatial transition from laminar to turbulent flow takes place (Ra = 1.5 × 109). The plume properties (source strength and rate of spread) as well as the ventilation properties (stratification height and temperature of stratified layer) estimated using the theory of Linden et al. are found to agree reasonably well with the LES results. The variation of the plume width with height indicates a linear variation of the entrainment coefficient rather than a constant value used by Linden et al. for a fully turbulent thermal plume. Flow visualisation revealed the nature of the large-scale coherent structures involved in the transition to turbulence in the plume. The most excited modes observed in the velocity, pressure and temperature fields spectra correspond to Strouhal number in the range 0.3 ≤ St ≤ 0.55 which is in agreement with those observed by Zhou et al. for a turbulent forced plume. Excited modes less than thisvalue (St = 0.2) were observed and may be due to low-frequency motions felt throughout the flow.

Numerical study of thermal plume characteristics and entrainment in an enclosure with a point heat source

Abstract The structure of a buoyant plume above a point heat source in a ventilated enclosure has been investigated using large-eddy simulation (LES). The aim of the work is to assess the performance and the accuracy of LES for modelling buoyancy-driven displacement ventilation of an enclosure and investigate the role of coherent structures in the plume entrainment mechanism which is important in these flow types because, for example, entrainment determines the ventilation flow rate. The Smagorinsky subgrid-scale model is used for the unresolved small-scale turbulence. The Rayleigh number Ra is chosen in the range where spatial transition from laminar to turbulent flow takes place (Ra=1.5×109). The stratification height and temperature of the stratified layer deduced from the mean field of the LES data is in good agreement with the theory of Linden, Lane-Serff and Smeed (1990). The plume entrainment coefficient is in good agreement with experimental values determined by Morton, Taylor and Turner (1956), Rouse, Yih and Humphreys (1952), and Baines and Turner (1969). Instantaneously, the plume develops through expansion and contraction phases, where the expansion phase is associated with the existence of coherent large-scale structures leading to an outward stretching of the plume and the contraction occurs as a result of partial breakdown and/or loss of coherence of these structures. As a result, the instantaneous entrained mass (and thus the entrainment coefficient) at different heights below the mean interface height were found to fluctuate about a mean value. Visualization of the computed flow showed that the stretching mechanism of the large-scale structures, which governs the expansion-contraction behaviour of the plume, occurs in such a way that the coherent structure dominating the flow below the interface height takes a spiral shape.

Investigation of Vortex Shedding in Transitional Separating-Reattaching Flow on a Blunt Plate

It has been reported in transitional and turbulent separating-reattaching flows that different vortex shedding frequencies exist, in particular the so called low-frequency flapping [l, 21. In addition it has been reported that a higher-frequency shedding also exists by Tafti and Vanka [3]. It is quite debatable if these low and high frequency vortex shedding exist in all transitional and turbulent separating-reattaching flows. It is also not clear what the working mechanisms are behind the so called low-frequency flapping as reported by several researchers. The major purpose of the paper is trying to address these issues. Large-Eddy Simulation of the transitional flow over a blunt plate held normal to a uniform stream has been carried out to study the physics of separated boundary layer transition. The Reynolds number based on the uniform inlet velocity and the plate thickness is 6500. A dynamic subgrid-scale model is employed to compute the subgrid-scale stresses more accurately in the transitional flow case where the subgrid eddy-viscosity should be zero in the laminar region and starts to increase in the transition region and eventually to full turbulent value. Statistics of the LES are found to be in acceptable agreement with the available experimental data. Time histories of three velocity components and pressure have been taken at many selected points in the flow field, concentrated mainly in the separation bubble region. Detailed processing of these data has been done by Fourier analyses and comprehensive results will be presented. Transactions on Modelling and Simulation vol 33,