Seminar - Flow over shallow dimple arrays by Professor B.C. Khoo
日期：2018 年 01 月 04 日 ( 星期四)
Time：4:00 pm – 5:00 pm
Dimple arrays have been successfully used for heat transfer enhancement because they increase the heat transfer at a relatively smaller penalty in terms of pressure losses compared to traditional heat transfer devices just as fins and pins. These usually involve relatively deep dimples with dimple depth to diameter ratios of more than 10% to generate the increased flow mixing for heat transfer enhancement. Some studies have also shown that arrays of dimples can also be used for drag reduction, and these typically involve very shallow round-edged dimples with depth to diameter ratios of 5% or less. Maximum drag reduction obtained with such passive dimples are relatively small, with maximum drag reductions of about 5% for circular axisymmetric dimples. Flow visualization experiments as well as numerical simulations have revealed the presence of streamwise vortices within the dimple as well as regions of flow separation at the upstream portion of such dimples. The flow structure within the dimples as well as the size of the flow separation region was found to vary with the Reynolds number of the flow. This, together with the many parameters such as dimple depth to diameter ratio and the relative curvature at the dimple edge and within the dimple that affect the flow make the flow over the dimples relatively complicated to study. Nevertheless, having a better understanding of the flow mechanism causing the drag reduction would enable us to optimize the shape of the dimple and maximum the drag reduction obtained using such passive dimples. This is the main motivation of the present work on dimples.
Both experiments and numerical simulations have been carried out on shallow round-edged dimple arrays with dimple depth to diameter ratios of 5%. Pressure measurements have been carried out to quantify the drag reduction due to the dimple array in a turbulent channel flow for Reynolds numbers between 5,000 and 37,000, and hot-wire anemometry and Detached Eddy Simulations (DES) have been carried out to understand the flow over the dimples in greater detail. The study shows that the drag due to the dimple array reduces as the Reynolds number increases from 5,000 to 37,000. A drag increase is observed at Reynolds numbers below 13,000, while a drag reduction is observed above this Reynolds number. Similar streamwise vortices are observed with such shallow dimples as those observed in deeper dimples with depth to diameter ratios of 10%. These streamwise vortices generate spanwise flow components near the dimple surface, resulting in reduced skin friction similar to those observed with traverse wall or flow motions. The flow is stabilized when drag reduction is present and shifts in the power spectra of the streamwise velocity signal as well as reductions of the peaks in the terms of the turbulence energy budget is observed with the drag reduction.
Although streamwise vortices are also present at lower Reynolds numbers, a drag increase is observed at lower Reynolds numbers, together with a relatively large flow separation region. The large flow separation region results in a large form drag at low Reynolds numbers. As the Reynolds number increases, the flow separation region shrinks, resulting in reduced form drag at higher Reynolds numbers.
The results show that while the streamwise vortices generating spanwise flow near the surface can reduce the skin friction drag, form drag present within the three-dimensional dimples can be significant enough that an increase is observed in the total drag. To optimize the dimple shape and maximize drag reduction, both the skin friction and form drag should be reduced. One possible method to reduce this form drag is through the use of asymmetric dimples, where the deepest point within the dimple is shifted backward, resulting in a shallower wall gradient at the upstream portion of the dimple. This has been shown to reduce the flow separation at the upstream portion of the dimples for deeper dimples with depth to diameter of 10%.
Prof. Khoo graduated from the University of Cambridge with a BA (Honours, 1st Class with Distinction) in 1980 under the Overseas Merit Scholarship and President’s Scholarship. In 1984, he obtained his MEng from the NUS and followed by PhD from MIT in 1989. He joined NUS in 1989. From 1998 to 1999, he was seconded to the Institute of High Performance Computing (IHPC, Singapore) and served as the deputy Director and Director of Research. In 1999, BC returned to NUS and spent time at the SMA-I (Singapore MIT Alliance I) as the co-Chair of High Performance Computation for Engineered Systems Program till 2004. In the period 2005-2013, under the SMA-II, he was appointed as the co-Chair of Computational Engineering Program. In 2011-2012, BC was appointed the Director of Research, Temasek Laboratories, NUS. Since 2012, he has been the Director, Temasek Laboratories. Prof. Khoo serves on numerous organizing and advisory committees for International Conferences/Symposiums held in USA, China, India, Singapore, Taiwan, Malaysia, Indonesia and others. He is a member of the Steering Committee, HPC (High Performance Computing) Asia. He has received a Defence Technology Team Prize (1998) and the prestigious Royal Aeronautical Prize (1980, UK). Among other numerous and academic and professional duties, he is the Associate Editor of Communications in Computational Physics (CiCP) and Advances in Applied Mathematics and Mechanics (AAMM), and is on the Editorial Board of American Journal of Heat and Mass Transfer, Ocean Systems Engineering (IJOSE), International Journal of Intelligent Unmanned Systems (IJIUS), The Open Mechanical Engineering Journal (OME) and The Open Ocean Engineering Journal. His reserach interests include (i) Fluid-structure interaction; (ii) Underwater shock and bubble dynamics; and (iii) Compressible/Incompressible multi-medium flow. He is the PI of numerous externally funded projects including those from the Defense agencies like ONR/ONR Global and MINDEF (Singapore) to simulate/study the dynamics of underwater explosion bubble(s), flow supercavitation and detonation physics. His work on water circulation and transport across the turbulent air-sea interface has received funding from then BP International for predicting the effects of accidental chemical spills.