Investigation of unsteady separated flow and heat transfer using direct and large eddy simulations




Suksangpanomrung, Anotai

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This dissertation presents a numerical analysis of the separated flow and convective heat transfer around a bluff rectangular plate. This geometrically simple “prototype” configuration exhibits all the important features of complex separated and reattaching flow and has the advantage of well defined upstream conditions. The main objective of this work is the investigation of three-dimensional, high Reynolds number, unsteady separated flow using the large eddy simulation technique. However, two-dimensional and three-dimensional low and moderate Reynolds number simulations leading up to this are also of interest. A staggered grid, finite volume method is used in conjunction with a third order Runge-Kutta temporal algorithm. The linear system for pressure is solved by, depending on the case, either a direct method or an efficient conjugate gradient with preconditioning. Two spatial discretizations are used, QUICK and CDS. In order to avoid the numerical diffusion effect from QUICK and dispersive effect from CDS, a mixed discretization is also introduced at high Reynolds number (Red = 50,000). The two-dimensional steady and unsteady simulations are first presented. The predicted flow characteristics are in agreement with those reported in previous numerical studies. The two-dimensional unsteady simulations ( Red = 1,000) provide good insight into the overall dynamic features of separation process, onset of instabilities and pseudo-periodic pattern of vortex formation, pairing and shedding. The realism of the simulation is however constrained by the artificially high coherence of the flow imposed by two-dimensionality. The three-dimensional simulations provide a much improved representation of the flow. Three-dimensional instabilities are found to appear soon after the onset of the shear layer roll-up, and result in the rapid break-up of spanwise vortices. Convective heat transfer simulations highlighting the important role of large scale structures in enhancing turbulent transport are also presented. At high Reynolds number, Red = 50,000, simulations are performed with three subgrid scale models. The selective structure function model, which allows improved localization, yields excellent agreement of the mean flow statistics with available experimental data. The dynamics of the flow is investigated using wavelet transform analysis and coherent structure identification. Characteristic frequencies related to shear layer instability, flapping and vortex shedding are identified consistent with experimental observation. The flow in the reattachment region is highly intermittent and characterized by a complex quasi-cyclic growth and bursting of the separation bubble, and horseshoe structures are identified in the recovery region of the flow.



Heat, Simulation methods