Search

The present study discusses the results pertaining to eduction of coherent structures and assesses their contribution in promoting mixing between low and high momentum fluid streams to enhance thermal convection. The analysis has been carried out numerically in a periodic parallel-plate-channel containing built-in longitudinal vortex generators on the bottom wall. The Reynolds number of the investigation is 6000 based on the channel height and bulk mean velocity. The computational geometry resembles heat exchanger surfaces on which the vortex generators create strong longitudinal vortices that enhance heat transfer on the plane channel wall. At high Reynolds numbers, the enhancement of convective heat transfer near the wall largely depends on the large scale turbulent structures (coherent structures) that disappear and reappear frequently and exhibit typical correlated scales in time or space. Proper orthogonal decomposition (POD) is an efficient tool to identify these coherent turbulent structures from instantaneous flow field data. Despite geometrical simplicity, the complex swirling motion behind the vortex generators demands highly accurate computational method, like the large eddy simulation (LES) of turbulence for capturing the nuances of flow. The instantaneous velocity data at a cross-flow plane have been assembled over a long time-window and analyzed deploying POD technique to extract the large- and small-scale structures based on the turbulent kinetic energy of the structures. Reconstructions of the velocity field using different combinations of eigenmodes have yielded different patterns of the flow. Large-scale structures have been visualized using first few energetic modes, while remaining less energetic modes have provided small-scale structures. The technique enables us to identify coherent structures hidden in the random flow field by filtering the small scale effects. The analysis yields as large as 15 dominant POD modes to cumulatively capture as low as 25% of the total turbulent kinetic energy of the flow, indicating broad range of energy spectrum that shares a large number of POD modes and thereby concludes highly complex turbulent fields with numerous spatial and temporal turbulent scales. Reconstructed velocity fields considering the leading dominant modes reveal a cut-off of 43% turbulent kinetic energy to clearly expose the coherent structures, which are consistent with published literature on similar turbulent channel flows. Additionally, quadrant analysis on the reconstructed velocity fields has clearly revealed that sweep and ejection modes are the only dominating Reynolds-stress-producing events for the current flow of interest, where the vortices lift the low-momentum fluid away from wall through the ejection mechanism. The high-momentum fluid impinges on the wall in sweep mode.

The objective of the present investigation is to assess the performance of a plate-fin heat exchanger with an emphasis on acquiring fundamental understanding of the relation between local flow behavior and heat transfer augmentation mechanism. Numerical simulations are performed in a rectangular channel containing built-in longitudinal vortex generators on the bottom wall arranged periodically both in the streamwise and spanwise directions. Two types of vortex generators, namely, rectangular winglet pair (RWP) and delta-winglet pair (DWP) with two different flow arrangements, common-flow-up (CFU) and common-flow-down (CFD) have been explored to assess the influence of shape and flow arrangements on heat transfer enhancement. The basic mechanisms of flow structure and heat transfer characteristics have been examined with the help of secondary velocity vectors, streamlines, and temperature contours. Additionally, the mechanism of the local heat transfer augmentation has been explained using a novel concept called the field synergy principle. The performance of the vortex generators has been compared based on integral quantities such as Nusselt number, pressure loss, performance evaluation factor and domain averaged synergy angle. The computations reveal enhanced mixing of fluid between the wall layer and the core due to strong secondary flows produced by vortex generators. The performance analysis indicates that the RWP is more effective in terms of heat transfer enhancement as compared to DWP. The field synergy analysis has shown that the sites with higher Nusselt number are associated with smaller synergy angle or better coordination between the velocity vector and the temperature gradient.

A subgrid-scale model pertaining to Large Eddy Simulation (LES) was developed by modifying the standard Smagorinsky model in order to take into account the inhomogeneities of the mean flow. According to this model, the magnitude of mean strain-rate is subtracted from the magnitude of the resolved strain-rate tensor for the calculation of eddy-viscosity. In this work, we perform large eddy simulation of turbulent channel flows at low and moderate Reynolds numbers. The predicted results compare well with the DNS data and results due to dynamic LES closure. The focus of this study has been on the assessment of capabilities of the model pertaining to the description of flow physics for the Reynolds numbers of interest. Also, the results are intended to establish the dominant effects of shear-length-scale near the wall. The simulations highlight other statistical features and turbulence characteristics too in order to broaden the applicability of the model.