Bulletin of the American Physical Society
74th Annual Meeting of the APS Division of Fluid Dynamics
Volume 66, Number 17
Sunday–Tuesday, November 21–23, 2021; Phoenix Convention Center, Phoenix, Arizona
Session F24: Vortex Dynamics and Vortex Flows: General II |
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Chair: Banafsheh Seyed-Aghazadeh, U Mass Darmouth Room: North 224 B |
Sunday, November 21, 2021 5:25PM - 5:38PM |
F24.00001: Secondary lock-in observation of vortex-induced vibration of a two-degree-of freedom circular cylinder oscillating in the crossflow direction Banafsheh Seyed-Aghazadeh, Naumi Noshin Chowdhury, Seyedmohammad Mousavisani, Hamed Samandari We have experimentally studied vortex-induced vibration (VIV) of a flexibly-mounted circular cylinder, when it was given two degrees of freedom to oscillate in its first two natural modes in the crossflow direction. Dynamic response of the system was studied for a reduced velocity range of U* = 4.0 - 28, corresponding to a Reynolds number range of Re = 1,030-6,930. The natural frequency ratio between the first two modes was varied from 1.3-3.0 and the VIV response was studied for oscillations of the cylinder at its first and second mode, as well as the reduced velocity ranges over which transition between modes occurs. Flow evolution around the cylinder was qualitatively and quantitatively studied using a hydrogen bubble imaging and a time resolved volumetric particle tracking velocimetry technique, respectively. The VIV response of the system consisted of oscillations at a combination of the first two natural modes of the system with varying contributions from each mode. While the lock-in range at the first mode excitation stayed the same for all frequency ratios, as the frequency ratio increased, the lock-in range was extended to higher reduced velocities. By increase in frequency ratio, the reduced velocity range over which multi-modal oscillations were observed, decreased. |
Sunday, November 21, 2021 5:38PM - 5:51PM |
F24.00002: Force generation by the wake of an axisymmetric cone at high angles of attack Al Shahriar, Rajan Kumar, Kourosh Shoele The contribution of different flow features in the wake of axisymmetric elongated bodies has been investigated here by relating the localized surface pressure to coherent flow structures. Direct numerical simulation is used to solve the flow over the axisymmetric cone for a wide range of angles of attack. For higher accuracy and resolution near the body, immersed boundary method with pseudo-body-conformal grids is employed. It is found that two near-wake stable primary vortices form in the separated shear layer, which induces reverse flow in the wake and initiates another two strong secondary vortical structures on the surface. For a higher angle of attack, the primary vortices become asymmetric and hence induce side forces. Using an extension of force portioning approach, we study the role of major vortical structures on the force generation and pressure distribution on the cone surface and find the asymmetry's origin. |
Sunday, November 21, 2021 5:51PM - 6:04PM |
F24.00003: Impact of Cycle-to-Cycle Variation in Near-Blade Hydrodynamics on Cross-flow Turbine Performance Abigale Snortland, Owen Williams, Brian L Polagye Cross-flow turbines are a promising technology for harvesting kinetic energy from wind and water currents. The hydrodynamics are complex, and rapid changes in angle of attack lead to phase and rotation-rate dependent dynamic stall and periodic vortex shedding. It is well known that dynamic stall is both extremely sensitive to changes in inflow and operational conditions and is stochastic in nature. Previous work has shown that the duration/severity of flow reversal and detachment during dynamic stall appear critical to average performance. This work aims to understand the impact of cycle-to-cycle variations of these dynamics on turbine forcing. This analysis is of particular interest because phase-averaging is a common approach for the processing of experimental flow fields containing missing data points and measurement noise. However, this relies on the assumption that cycle-to-cycle variations are negligible. Two-component, planar particle image velocimetry data is examined for this purpose. Flow field variations for optimal (maximum power generation) and sub-optimal rotation rates are investigated. Conditionally averaged flow fields, based on hierarchical clustering with a PCA preprocessor, highlight hydrodynamic differences which are smoothed out when phase-averaging. |
Sunday, November 21, 2021 6:04PM - 6:17PM |
F24.00004: Inviscid damping of an elliptical vortex in an external strain flow Pakorn Wongwaitayakornkul, James R Danielson, Noah C Hurst, Daniel H Dubin, Clifford M Surko A 2D elliptical vortex undergoes rotation or nutation around its equilibrium when subjected to an applied external strain flow. When a small non-uniform peripheral vorticity is present, the amplitude of vortex oscillatory motion is observed to damp toward the non-axisymmetric elliptical equilibrium. This damping mechanism is studied experimentally by imposing ExB drift motion of an electron plasma in a Penning-Malmberg trap, which is analogous to the dynamics of a rotating vortex in a 2D inviscid, incompressible fluid. The external strain flow is generated by applying voltages to the sectors of the cylindrical boundary of the trap. The amount of peripheral vorticity is controlled by varying the plasma fill time. Without strain, a perturbed vortex with a smoothly decreasing profile exterior to the core experiences critical-layer damping. While external strain does not significantly change the measured decay rate, it reduces the rotation frequency of the mode about equilibrium. At large strain, the trapping oscillations are reduced by particles near the cat's eye going over the separatrix. These results are compared to particle-in-cell simulations and available theoretical results. |
Sunday, November 21, 2021 6:17PM - 6:30PM Not Participating |
F24.00005: Axial vortex breakdown topology in the Vogel-Escudier flow Manjul Sharma, A Sameen Vogel-Escudier flow is analyzed using three-dimensional numerical simulations, a flow inside a cylinder with a rotating endwall. The flow exhibits an axial vortex and serves as a model problem to study the axial vortex breakdown phenomenon. The breakdown of the axial vortex occurs at some critical values of Reynolds number ($Re$) and the aspect ratio of the cylinder. Apart from the vortex breakdown, the flow exhibits rich dynamics, which supports the appearance of rotating azimuthal waves when the symmetry of the flow breaks. The flow supports a single azimuthal mode at lower $Re$ but can support multiple azimuthal modes at higher $Re$. A map with the classification of the various flow states in the parameter space of the Reynolds number and the aspect ratio is proposed. The topology of these states is demarcated on the map. The flow states and the associated perturbations are analyzed and will be presented in detail. |
Sunday, November 21, 2021 6:30PM - 6:43PM Not Participating |
F24.00006: Vortex breakdown in a thermally stratified flow Manjul Sharma, A Sameen, R Vishnu Vortex breakdown is computationally studied by applying an unstable temperature gradient to the axial vortex model known as `Vogel-Escudier' flow. The rotating top plate and the stationary bottom plate are kept at constant but different temperatures. Two main parameters controlling the flow are Reynolds number ($Re$) and Rayleigh number ($Ra$). Based on the combinations of these parameters, the flow is either dominated by convection or rotation. The dynamics of the flow in both regimes are distinctively different. For a sophisticated classification of the flow, it is decomposed into the $rz-$plane and the out-of-the-plane components. The energy content of the two components is analyzed. Using this approach, the flow is categorized in (i) convection dominated, (ii) rotation dominated, and (iii) transitional regime. The analysis of the flow perturbations and thermal plumes reveal rich dynamics exhibited by the flow. The effect of the temperature is also studied on the vortex breakdown bubble and the role of the azimuthal vorticity. |
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