(ISTFD 2023)

27-29 July 2023, Nanjing, China


Prof. Boo Cheong Khoo

Temasek Laboratories,

Department of Mechanical Engineering,

National University of Singapore, Singapore

Professor BC Khoo has been the Director of Temasek Laboratories since 2012. He graduated from the University of Cambridge with a BA (Honours, 1st Class with Distinction). In 1984, he obtained his MEng from the NUS and followed by PhD from MIT in 1989. He joined National University of Singapore (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, he 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, he was appointed the Director of Research, Temasek Laboratories, NUS. BC 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, Singapore) and the prestigious Royal Aeronautical Prize (1980, UK). Among other numerous and academic and professional duties, he is on the Editorial Board of Advances in Aerodynamics (AIA), International Journal of Thermofluid Science and Technology, Ocean Systems Engineering (IJOSE), International Journal of Intelligent Unmanned Systems (IJIUS), The Open Mechanical Engineering Journal (OME) and The Open Ocean Engineering Journal. 

In research, his interests are in:

(i)Fluid-structure interaction

(ii)Underwater shock and bubble dynamics

(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 transport across the turbulent air-sea interface has received funding from the then BP International for predicting the effects of accidental chemical spills. Qatar NRF has funded study on internal sloshing coupled to external wave hydrodynamics of (large) LNG carrier. He has published over 470 international journal papers, and over 400 papers at international conferences/symposiums. He has presented at over 138 plenary/keynote/invited talks at international conferences/symposiums/meetings. His H-index according to Web of Science stood at 54.

Title: Drag Reduction and Enhanced Heat Transfer on Flow over Dimples in Turbulent Channel Flows

Abstract:Dimples have been used effectively for many heat transfer enhancement applications due to the relatively high heat transfer efficiency. The flow over dimples are relatively complex, and flow visualization experiments have shown the presence of a region of separated flow at the upstream portion of the dimple, particularly for deeper dimples. This region of flow separation generally reduces as the Reynolds number increases. At sufficiently high Reynolds numbers, the region of separated flow can shrink until it is eliminated for dimples of moderate depth to diameter ratios. For very shallow dimples with depth to diameter ratios of less than 5%, no flow separation is observed, even at low Reynolds numbers.

Both hot-wire velocimetry and numerical simulations via DNS and DES of dimple arrays in fully developed turbulent channel flows were conducted to study the flow further. The dimples studied consists of shallow smooth edge dimples of various shapes, including axisymmetric circular dimples, skewed circular dimples, elliptical dimples, teardrop-shaped dimples as well as diamond-shaped dimples. These dimples all have a fixed spanwise width, and a depth to spanwise width ratio of 5%.

These studies show the presence of two high speed streaks and streamwise vortices within these dimples. Increasing drag reduction below that of the flat plane channel flow was generally observed for most of these dimples as the Reynolds number increases. At low Reynolds numbers, a drag increase is observed. The presence of spanwise flow at the dimple surface at the location of the high speed streak regions is accompanied by reductions in the Reynolds stress u’u’, showing reduced turbulence production and increased flow stability at these locations. Such reductions in turbulence production accompanying spanwise flow motions near the wall is also observed for drag reducing flows involving spanwise wall oscillations or near wall streamwise vortices, suggesting a similar mechanism for drag reduction at work with the flow over these dimples.

For circular axisymmetric dimples, flow separation is a commonly observed flow feature occurring over the upstream edge of the dimple. The flow then reattaches within the dimple depression, forming a region of recirculating flow within the dimple depression. This separated flow lowers heat transfer enhancement and increases drag due to the dimples. Moving the deepest position of the dimple downstream by skewing the axisymmetric dimple reduces the wall slope at the upstream portion of the dimple, resulting in reduced separated flow, and enhancing heat transfer and increasing the drag reduction observed with these dimples. Reduced vortices emitted from these dimples shown by the DNS with the deepest point shifted downstream also suggests reduced turbulent energy production at the upstream portion. Shifting the deepest point too far back however, results in a steep downstream dimple wall. The flow impingement onto the steep dimple wall results in increased form drag, raising the overall drag if the deepest point is shifted too far back. At the same time, heat transfer is not significantly increased because of the elimination of the flow recirculation when the deepest point is shifted sufficiently far back.

A numerical study conducted for deeper circular skewed dimples with depth to diameter ratio of 15% and with its deepest point shifted downstream however shows that the previous trend of drag variation seen with shallow dimples with depth to diameter ratio of 5% with movement of the deepest point is not the same for such deeper dimples. For circular dimples with depth to diameter ratio of 15%, shifting the deepest point from 0.2 diameters upstream of the dimple center to 0.2 diameters downstream of the dimple center is not accompanied by any reduction in drag as is the case with dimples with depth to diameter ratio of 5%. For these deeper dimples, the upstream and downstream wall slopes are already relatively high such that a shift in the deepest point downstream does not reduce the flow separation at the upstream portion significantly. However, the shift in the deepest point downstream increases the already steep downstream wall slope, so that flow impingement is increased and this results in a drag increase instead when the deepest point is shifted downstream. 

To avoid raising the downstream wall slope that results in increased drag as the deepest point is shifted downstream, the circular dimple can be modified to an elliptical, teardrop or diamond shape. Streamwise elongation of the dimple allows the reduction of both the upstream and downstream wall slopes. This reduces both the flow separation at the upstream portion of the dimples, as well as the flow impingement at the downstream portion of the dimple. This combination results in significantly increased drag reduction for these non-circular dimple shapes. Among these, the diamond dimples show the greatest drag reduction, with the drag being almost 8% less than that of the plane channel flow. At the same time, heat transfer is also increased above that of the plane channel flow for all these dimple shapes.

Analysis of the heat transfer efficiency in terms of the volume goodness factor highlights the flow parameters that are significant for heat transfer enhancement by the dimples. For heat transfer, deeper dimples with depth to diameter ratios of more than 10% perform best for circular axisymmetric dimples. However, when the deepest point is shifted so that the dimples are skewed, the volume goodness ratio can be further increased as the dimple depth increases. Modification of the dimple shape to an elliptical, teardrop or diamond shape also changes the heat transfer efficiency. A general observation that can be made is that dimple geometries with higher drag reduction tends to have lower heat transfer efficiency. However, even with dimples that show a lower drag compared to a plane channel flow, it can also have enhanced heat transfer compared to it, showing the usefulness of dimples for heat transfer applications as well.