Prof. em. Yasushi Takeda
(Hokkaido University, Japan)

Date: June 6
Title: New Role of Visualization in Fluid Dynamics

Abstract:
Modern fluid mechanical study has been revolutionalized as multi variable observation of the flow field, namely, 2C2D by PIV and 1C1D+T by UVP. Especially UVP gives spatio-temporal data of the flow field, from which quantitative investigation of flow transition to turbulence can be realized for better physics on fluid flow. Such a fundamental change of methodology has enabled a parallel collaboration of theoretical, numerical and experimental investigations of this classical physics, with visualization at the center.

Prof. Hui Hu
(Martin C. Jischke Professor and Director, Iowa State University, U.S.A.)

Date: June 7
Title: Visualization of Wind Turbine Icing Phenomena with Advanced Flow Diagnostic Techniques

Abstract:
Wind turbine icing represents the most significant threat to the efficiency and integrity of wind turbines operating in cold climates. By leveraging the Icing Research Tunnel available at Iowa State University (ISU-IRT), a comprehensive experimental study was conducted to elucidate the underlying physics of the important micro-physical processes pertinent to wind turbine icing phenomena and explore novel anti-/de-icing strategies for wind turbine icing mitigation.  A suite of advanced flow diagnostic techniques, which include molecular tagging velocimetry and thermometry (MTV&T), digital image projection (DIP), and infrared (IR) imaging thermometry, were developed and applied to quantify the transient behavior of wind-driven surface water film/rivulet flows, unsteady heat transfer and dynamic ice accreting process over the surfaces of wind turbine blade models. The potentials of various bio-inspired coatings, including lotus-inspired superhydrophobic coatings and pitcher-plant-inspired Slippery Liquid-Infused Porous Surfaces (SLIPS), for wind turbine icing mitigation are evaluated under various icing conditions (i.e., ranged from dry rime icing to wet glaze icing conditions). A novel, hybrid anti-/de-icing strategy that combines minimized electro-heating at the blade leading edge and an ice-phobic coating to cover the blade surface was developed for wind turbine icing mitigation. In comparison to the conventional strategy of brutally heating the mass blade surface to keep the blade ice-free, the hybrid strategy was demonstrated to be able to achieve the same anti-/de-icing performance with substantially less power consumption (i.e., up to ~90% power saving). Our recent field campaign in a 50 MW mountainous wind farm to investigate the effects of icing events on the degradation of multi-megawatt (1.5MW) wind turbines by using a Supervisory Control and Data Acquisition (SCADA) system and an Unmanned-Aerial-Vehicle (UAV) equipped with a high-resolution digital camera is also introduced briefly.

Prof. Smith Eiamsa-ard
(Mahanakorn University of Technology, Thailand)

Date: June 8
Title: Heat transfer enhancement with passive techniques

Abstract:
In this lecture, a comprehensive review of heat transfer enhancement techniques is presented. In general, enhancing heat transfer can be divided into active and passive techniques. Active methods required external power input for the enhancement while passive methods can be implemented without requiring additional power input. Passive methods include surface coatings, modified surfaces for amplifying fluid mixing, insert devices for inducing swirl flow and promoting fluid turbulence. The passive methods show advantages over active methods attributed to their low cost and easy manufacturing and installation.

The major devices applied in passive techniques are twisted tapes because of their promising heat transfer efficiency and performance. The devices are highly recommended for fabricating compact heat exchangers and upgrading existing heat exchangers.  Twisted tapes functionalize as swirl generators/turbulators which facilitate fluid mixing and thus heat transfer between the fluid flow and tube surface.  The application of swirl generators/turbulators results in both desired heat transfer enhancement and undesired friction loss penalty. Numerous modified >swirl generators/turbulators have been proposed to optimize both effects in order to maximize the thermal performance which relates to energy saving and economic issues. Further development of proper geometric swirl generators and turbulators is a challenging task and is still ongoing.

Prof. Nan Jiang
(Tianjin University, China)

Date: June 9
Title: Characteristics of Multi-Scale Coherent Structures in Turbulent Boundary Layer Over Superhydrophobic Surface for Drag Reduction

Abstract:
A comparative experiment by time-resolved particle image velocimetry (TRPIV) of the turbulent boundary layer (TBL) over a smooth surface and an anisotropy superhydrophobic (SH) surface was carried out in an open-surface recirculating water channel at Reτ,smooth = 650. The wall friction velocity is fitted well from the velocity of the viscous sublayer calculated by the Single-pixel resolution ensemble correlation (SPEC). After that, a drag reduction rate of 17%, a slip velocity of 0.0119 m/s, and a slip length of 90.8µm are obtained over the SH surface. Comparing with the SH surface, the declining 2-order statistics in the near-wall region also indicates a significant drag reduction over the SH surface.

The continuous spatial wavelet transform is employed to unfold the longitudinal fluctuating velocity into multi-scale components in streamwise direction at all wall-normal positions. The distributions of fluctuating kinetic energy across all the spatial scales and all the wall-normal positions are obtained from the multi-scale wavelet coefficients. The turbulent structures are detected by zero-crossing wavelet coefficients. The conditional sampling scheme and spatial phase-locked method are employed to obtained the multi-scale typical structures at different wall-normal positions. The universality of multi-scale coherent structures is confirmed that their vorticity presents as a quadrupole alternating positive and negative along the longitudinal direction and wall-normal direction while their streamlines are represented as a dynamic system composed of a saddle point and focal points.

The features of the multi-scale coherent structures over the SH and smooth surfaces at different wall-normal positions are visualized by the conditional sampling and spatial phase-locked average to extract the topology of physical quantities such as the velocity fluctuation, vorticity, and Reynolds stress. The results indicates that the suppression of coherent structure in the near-wall region is the key mechanism of drag reduction in TBL over superhydrophobic surface.