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10:45   Vortex Dynamics and Structure Formation 2 10:45 15 mins #10 TWO-DIMENSIONAL VORTEX STATISTICS OF THE WAKE FLOWOF CYLINDER IN CHANNEL FLOW Xiang Qiu, YuXian Xia, JiangHua Li, Quan Gao Abstract: The vortex dynamics of 2D turbulence is considerable different from the 3D turbulence due to the absent of the stretch effect of third direction[1]. This motivation leads to get involved in studies on the vortex dynamics of two-dimensional turbulence. Our work mainly focuses on the 2D vortex statistics of the wake flow of cylinder in channel bases on the Rortex[2], a new vortex definition (C.Q.Liu 2018). The boundary layer appears and the secondary vortex is generated due to instability of boundary layer in Fig.1. The secondary vortex has a strong vorticity core connected to the boundary layer at the wall. Together with the primary vortex the secondary vortex from a couple that moves away from the wall which disappears due to viscous dissipation . As results, the prime vortex rebound from wall which considered as a rebound phenomenon of the vortex. In the steady state of interaction, the vortex street and the induced and weaker boundary layer finally appear. The dynamics behaviors of the Rortex of the interaction of wake flow with solid wall is given in Fig.1. Actually, Rortex is a better choice to finely describe the vortex dynamics in two dimensional turbulence. It is found that the temporal and spacing vortex number density distribution n(A)  A􀀀2 in Fig.2, based on the Rortex vortex definition criterion, which is in fair agreement with the theory of Burgess and Scott[3, 4] at intermediate scales sufficiently far from the forcing and the largest vortex. 11:00 15 mins #340 NEW DIAGNOSTICS FOR TURBULENT VORTICES Robert M. Kerr, H.. Jane Bae, John D. Gibbon, Adrian Lozano-Durán Abstract: Vorticity underlies the dynamics of turbulent flows. While diagnostics of its relationship to the strain and its production are well-developed, the diagnostics of vorticity in isolation has been limited to qualitative graphical tools, its quantita- tive maximum ∥ω∥∞ and mean square Z (enstrophy). This presentation introduces two sets of diagnostics, topological numbers of vortex lines and ratios of vorticity moments, λm , and ends with connections between new results. 11:15 15 mins #52 Vortex interaction with a rough wall formed by a hexagonal lattice of posts Qianhui Li, Christoph Bruecker Abstract: The interaction between rough walls and the near wall flow and the coherent vortices (CV) embedded in it has attracted new attention in fluid mechanics recently. A prominent example of CVs in transitional and turbulent boundary layer (BL) flows is the structure of a hair-pin vortex impacting with the wall. An experimental study is reported which investigates the head-on collision of a vortex ring of diameter D (ReГ=3000) on a fakir-like surface, which is composed of slender circular posts protruding out of a planar layer. Lattices of the posts in hexagonal and random distribution (average porosity of ϵ=0.94 in the layer) are compared to each other with respect to the plain wall. Prior to impact, the vortex ring develops the early state of natural azimuthal instabilities of different mode numbers N=5-7 competing with each other. When impacting with the wall, the hexagonal lattice causes rapid growth of secondary vortex structures in a regular mode number N=6 in form of six lobes which are aligned with the orientations of preferential “pathways” in the lattice at the outer edge of the primary ring, see Figure 1. At the outer tip of the lobes, radial wall-jets are generated. Besides, it is observed that rotating the layer with the hexagonal lattice results in the same rotation of the secondary flow pattern, i.e. the jets’ orientation lock-in with the orientation of the lattice. Whereas the layer with the random distribution of the posts is not able to repeat this observation and no regular secondary flow pattern is seen. 11:30 15 mins #109 Flow transitions in collisions between vortex-rings and free surfaces K. W. B. Yeo, J. Y. Koh, J. Long, T. H. New Abstract: Vortex-rings colliding with surfaces have always been an intriguing flow scenario, despite the seemingly simple flow configuration and initial conditions. It typically involves generating a discrete, coherent vortex-ring and setting it to collide with the selected surface of choice under a certain translational velocity. As some of the experimental and numerical studies in the past have revealed, the resulting collision produces a surprisingly rich and complex flow behavior that involves a multitude of vortex structures. While the behavior is relatively well-established for vortex-ring collisions with “non-slip” solid flat surfaces, those associated with vortex-ring collisions with “slip” boundaries with a compliant interface remain less well-known. Hence, an experimental laser-induced fluorescence and time-resolved particle-image velocimetry study had been conducted to investigate the flow dynamics of vortex-rings colliding with free-surfaces, with an emphasis on the flow transitions towards incoherence and turbulence. 11:45 15 mins #128 Tracking vortex surfaces frozen in the virtual velocity in non-ideal flows - An extension of the Helmholtz vorticity theorem Yue Yang, Jinhua Hao, Shiying Xiong Abstract: It appears that vortex surfaces cannot be exactly tracked in the flows with viscous, baroclinic and non-conservative body forces, because Helmholtz’s vorticity theorem breaks down in non-ideal flows. However, we demonstrate that, if a globally smooth, virtual circulation-preserving velocity exists, Kelvin’s and Helmholtz’s theorems can be extended to some non-ideal flows [1]. Thus we can accurately track vortex surfaces frozen in the virtual velocity even in some non-ideal flows, based on the evolution of a vortex-surface field (VSF) [2,3]. For a flow with a viscous-like diffusion term normal to the vorticity, we obtain an explicit virtual velocity to accurately track vortex surfaces in time. This modified flow is dissipative but prohibits reconnection of vortex lines. If a globally smooth virtual velocity does not exist, an approximate virtual velocity can still facilitate the tracking of vortex surfaces in non-ideal flows. We use the approximate virtual velocity to track vortex surfaces in a magnetohydrodynamic Taylor-Green flow with strong non-ideal forces. Compared with the VSF evolution convected by the physical velocity, the conservation of vorticity flux is significantly improved in the VSF evolution convected by the approximate virtual velocity, and the spurious vortex deformation induced by the Lorentz force is eliminated. From the Lagrangian view, the VSF with the virtual velocity can be used to track vortex surfaces and investigate the vortex dynamics in strong non-ideal flows, such as in compressible flows, combustion and MHD turbulence. References [1] J. Hao, S. Xiong, and Y. Yang, Tracking vortex surfaces frozen in the virtual velocity in non-ideal flows, J. Fluid Mech., 863:513-544, 2019. [2] Yang and D. I. Pullin, On Lagrangian and vortex-surface fields in flows with Taylor-Green and Kida-Pelz initial conditions, J. Fluid Mech., 661: 446-481, 2010. [3] Y. Yang and D. I. Pullin, Evolution of vortex-surface fields in viscous Taylor-Green and Kida-Pelz flows, J. Fluid Mech., 685: 146-164, 2011. 12:00 15 mins #210 THE ANALYSIS OF RECIRCULATION ZONE DYNAMICS IN CAVITY FLOW Paulius Vilkinis, Nerijus Pedišius Abstract: The flow separation phenomena caused by abrupt expansion in channel geometry, such as backward-facing step is a point of interest in many engineering applications related to fluid mixing and heat transfer enhancement like combustors, heat sinks and microreactors [1]. Practically, this channel expansion is usually limited in length by forward-facing step, resulting whole geometry to have a shape of the cavity. The flow structure inside the cavity depends on cavity geometrical parameters and flow regime in the channel [2, 3]. The dynamics of recirculating zone behind backward-facing step is investigated experimentally and numerically at wide range of Reynolds numbers (Re = 1 – 100000). Different cavity length and depth ratios (L/h1 = 6–40) and channels expansion ratios (H/h = 1.25–5) are applied with the aim to analyze the dynamics and structure of recirculating flow in the cavity. Micro-particle image velocimetry system is used to obtain two-dimensional flow velocity fields, velocity profiles and to measure the length of recirculation zone in the cavity. Experimental measurements are supported with commercial CFD software FLUENT which provides additional information on pressure, turbulent kinetic energy and wall shear stress distribution along the cavity. Instantaneous experimental flow fields together with LES simulation provide information of recirculating flow structure in the cavity. Also, distribution of vorticity values in the cavity is analysed. The obtained results show influence of cavity length, its depth and channel expansion ratio on recirculation zone dynamics, peak value of recirculation zone length and transition to turbulent flow. As the cavity length is increased the peak value of recirculating zone length decreases. Transition to turbulent flow regimes occurs at higher Reynolds numbers for lower channel expansion ratios. In laminar flow regime recirculation zone length depends on both cavity depth and channel expansion ratio and increases until transitional flow regime is reached. In turbulent flow regime recirculation zone length is only a function of Reynolds number and reaches asymptotical value at fully developed turbulent flow regime. The obtain data suggest possible scaling for the reattachment length in laminar and turbulent flow regimes 12:15 15 mins #95 SPATIOTEMPORAL MEASUREMENT OF SUPERSTRUCTURES IN A TURBULENT BOUNDARY LAYER FLOW Daniel Schanz, Matteo Novara, Reinhard Geisler, Janos Agocs, Felix Eich, Matthew Bross, Christian J. Kähler, Andreas Schröder Abstract: We report on an experimental undertaking to detect superstructures in their full volumetric extent within a turbulent boundary layer and describe their temporal development while propagating from a region of zero pressure gradient (ZPG) into an adverse pressure gradient (APG). The flow was characterized by Lagrangian Particle Tracking (LPT) of Helium-filled soap bubbles (HFSBs) using the DLR implementation of the Shake-The-Box (STB) algorithm 12:30 15 mins #430 CALCULATION OF THE PRESSURE FIELD FROM THE HIGH SPEED 3C PIV DATA OF THE TURBULENT FLOW AND ITS EVALUATION BY USE OF THE COBRA PROBE Djordje S. Cantrak, James T Heineck, Laura K Kushner, Nettie Roozeboom, Novica Z Jankovic Abstract: The challenge in making empirical observations of fluid flow is the inability to directly access the instantaneous pressure field by the use of non-intrusive experimental methods. The approach to determining pressure fields from high-speed stereo particle image velocimetry (HSS PIV) data in relation to the implementation for turbulent incompressible flows is discussed in this paper. Deduced pressure fields from the experimental data, by use of the flow constitutive equations, are evaluated by use of the four-hole pressure (Cobra)probe. The time-mean pressure field obtained from velocity ensemble statistics is evaluated by the Cobra probe in approximately the same measuring points. Pressure distributions around the airplane model calculated from PIV experiments are of a fundamental importance for windtunnel investigations.The aim of this study is to determine the lowest resolution for the probe data necessary for a converged pressure solution derived from the PIV data. The wing tip vortex of the 3% scalesemi-span Common Research Model (CRM) is studied in the NASA Ames Fluid Mechanics Laboratory (FML) 32-by 48-in indraft wind tunnel. Test cases for various angles of attack are considered. The wind tunnel speed was approximately the same for all measurements: 50 m/s (Figure 1a). This corresponds to a chord Reynolds number ReC ≈ 2.68•105, where the characteristic length is chord length of 3.2". Temporal and spatial resolutions are verified for both experimental techniques -HSS PIV and Cobra probe. The pressure field is computed as either a solution to a Poisson equation orbased on direct integration of the pressure gradient term in the Navier-Stokes equation for incompressible flow(Figure 1b). Applied iterative schemes, while effective in limiting the influence of measurements errors, still generate a relatively high computational cost. The results show the potential of using PIV data for pressure determination. An additional aim is determining if the turbulence statistics derived from the pressure fluctuation data resulting from the PIV conversion match the same statistics derived from the velocity data. Figure 1. a) Total velocity of flow of and around the tip vortex at angle 4̊ of attack and b) calculated pressure field. Keywords: particle image velocimetry, pressure, Cobra probe, CRM.