Speakers: Dr Qi Gao Beijing University of Aeronautics and Astronautics, Dr Chong Pan, Beijing University of Aeronautics and Astronautics, Dr Hao Guo, Beijing University of Aeronautics and Astronautics
Research Centre: Turbulence and Flow Centre Research Group
Speaker: Dr Qi Gao
Title: 3D flow visualization and tomographic particle image velocimetry for vortex breakdown over a non-slender delta wing
Volumetric measurement for the leading-edge vortex (LEV) breakdown of a delta wing has been conducted by three-dimensional (3D) flow visualization and tomographic particle image velocimetry (TPIV). The 3D flow visualization is employed to show the vortex structures, which was recorded by four cameras with high resolution. 3D dye streaklines of the visualization are reconstructed using a similar way of particle reconstruction in TPIV. Tomographic PIV is carried out at the same time using same cameras with the dye visualization. Q criterion is employed to identify the LEV. Results of tomographic PIV agree well with the reconstructed 3D dye streaklines, which proves the validity of the measurements. The time-averaged flow field based on TPIV is shown and described by sections of velocity and streamwise vorticity. Combining the two measurement methods sheds light on the complex structures of both bubble type and spiral type of breakdown. The breakdown position is recognized by investigating both the streaklines and TPIV velocity fields. Proper orthogonal decomposition is applied to extract a pair of conjugated helical instability modes from TPIV data. Therefore, the dominant frequency of the instability modes is obtained from the corresponding POD coefficients of the modes based on wavelet transform analysis.
Dr. Gao got his Bachelor's degree in the Zhejiang University in 2001, Master degree in the Tsinghua University in 2005 and Ph.D degree in the University of Minnesota in 2011. He was an Assistant Professor in the Institute of Fluid Mechanics at Beijing University of Aeronautics and Astronautics from 2011 to 2013. He has been promoted to an Associate Professor in 2013. Dr. Gao's research is broadly in the area of experimental fluid mechanics. Current projects focus on developing techniques of volumetric velocimetry and their applications. Studies on turbulent boundary layers, cardiovascular flows and flow control are highly interested.
Speaker: Dr Chong Pan
Title: On the Spatial Organization of Hairpin Packets in a Turbulent Boundary Layer at low to moderate Reynolds number
The primary aim of the present study is to characterize the vortex organization of the hairpin packets in wall-bounded turbulence. The streamwise—wall-normal plane of a smooth-wall turbulent boundary layer with zero-pressure-gradient in the range of Reτ=342~1113 was measured by two-dimensional Particle Image Velocimetry (PIV). POD analysis, in together with correlation analysis of the time coefficients of the resulted POD modes, showed that the first four modes share both geometrical similarity and dynamic coherency, and jointly depict the downstream convection of the same large-scale structures, i.e. the hairpin packets. A simple indicator was proposed to extract the inclined interface of the hairpin packet by POD-based low-order filtering. Linear stochastic estimation further showed that small-scale prograde spanwise vortices in the full-rank fields prefer to reside on the large-scale inclined interface in the low-order filtered fields. This promoted a two-point conditional correlation analysis, which yielded a clear signature of the spatial organization of multiple hairpin heads in a statistical sense. The averaged geometric parameters, i.e. the inclination angle and the streamwise spacing of hairpin vortices in one packet, were then scrutinized. It was found that the vortices’s streamwise spacing follows inner-scaling, and presents a detached behavior beyond certain wall-normal height, which seems to be scaled by and is in accordance with the wall-normal position that the quick drop of the streamwise turbulent fluctuation begins.
Dr. Chong Pan obtained his Ph.D degree at Beihang University in 2009. He is now an associated professor in Institute of Fluid Mechanics, Beihang University. His research interests are focused on experimental investigation of flat-plate boundary layer transition, shear layer instability and turbulent structure. He has published over 20 Journal papers on J. Fluid Mech.,Phy. Fluids, Exp. Fluids, J. Turbulence and so on. He is now a member of Chinese Aerodynamics Research Society, and has won a nomination for National Excellent Doctoral Dissertations of China in 2011.
Speaker: Dr Hao Guo
Title:Experimental study of the interaction of the streaks in a laminar boundary layer
Wind tunnel experiments are carried out to examine the interaction of up- and down-stream streaks which are generated by two rows of vortex generators, and to shed some light on the physical mechanisms of streak interaction in a real flow. It is found that, instead of direct breakdown, streaks of higher amplitude (increments up to 63% is observed) are formed due to the consecutive transportation of the low speed fluid by the vortices. The stream-wise vortices play an important role in the interaction process. When the vortices are coincident well in the interaction, the new streaks tend to be very narrow with strong shear layer which would damp the streak soon, and vice versa for the vortices with finite span-wise shift. Interaction of streaks generated by counter rotating vortices tends to result in new streaks with much higher amplitude and extended distribution of secondary instability. Transition process induced by the streaks can be greatly promoted via secondary instabilities which undergo relative high amplification rate. Interaction of streaks can finally cause laminar-turbulent transition far earlier than the streaks without interaction.
Dr. Guo is currently an Associate Professor at the Institute of Fluid Mechanics at Beihang University in China. He obtained his B.Eng. in Structural Engineering in 1997 from Peking University, China; and earned his Ph.D. in Physics from the Chinease University of Hong Kong in 2007, Hong Kong. He joined the Institute of Fluid Mechanics at Beihang University in 2008 as an Assistant Professor. Dr. Guo’s research interests include turbulent thermal convection, wall turbulent flows, flow control, aeroacoustics, etc. He has published more than 20 SCI journal papers
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When and where
10.30amFriday 18th November 2016