Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session S01: Flow Instability: Vortex-Dominated Flows |
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Chair: Peter Schmid, Imperial College of London Room: 2A |
Tuesday, November 26, 2019 10:31AM - 10:44AM |
S01.00001: Stability analysis of turbomachinery stages using a sliding-plane formalism Anton Glazkov, Miguel Fosas de Pando, Peter Schmid, Li He At present, much attention is focused towards the optimisation of contemporary aero-engine technology to meet more stringent emission regulations. Multiple scales, thermal effects and acoustics alongside complex geometries in relative motion, such as in rotor-stator configurations, however, still remain significant challenges for detailed understanding and efficient manipulation of these flows. The development of high-order solvers and adjoint-based techniques are therefore of fundamental interest, enabling, for example, stability analyses, control and optimisation. In this study we present a new framework for a time-domain sliding-plane treatment with direct and adjoint functionality, showcasing it with our fifth-order compressible flow solver, and illustrating test cases validating this approach and confirming its robustness. We then perform a linear stability analysis on a representative single-passage, subsonic compressor geometry, and use modal and non-modal techniques to illustrate vortex shedding within a laminar separation bubble and at the trailing edge. We extend this case to a rotor-stator geometry that makes use of the linear and adjoint calculations to compute sensitivities within the flow. [Preview Abstract] |
Tuesday, November 26, 2019 10:44AM - 10:57AM |
S01.00002: Triad interactions induced by vortex shedding in a free jet in air. Margherita Dotti, Preben Bucchave, Clara M. Velte The non-linear processes caused by the convection term in the Navier-Stokes equation are of fundamental importance for both the understanding of turbulence and modelling turbulent flows. These so-called triad interactions in Fourier space were investigated by measuring a single Fourier mode injected into the initial part of a round, free jet in air. We studied the development of the measured velocity power spectrum as a function of the downstream distance. Furthermore, the development of the downstream velocity was also calculated by means of a simple, one-dimensional computer model. The comparison between the measured power spectra and the computational ones showed a good agreement between them, allowing us to draw some interesting conclusions regarding the fundamental non-linear processes in turbulence. [Preview Abstract] |
Tuesday, November 26, 2019 10:57AM - 11:10AM |
S01.00003: Transition to vortex shedding and its impact on heat and solute transport in membrane filtration systems Jincheng Lou, Jacob Johnston, Nils Tilton We perform direct numerical simulations (DNS) to investigate the hydrodynamic stability of flow over a cylinder confined in a planar channel with a permeable wall. More specifically, we investigate how cylinder configuration influences transition to laminar vortex shedding, and how this impacts thermal and concentration boundary layers forming on the permeable wall. This flow plays an important role in modern desalination processes that use permeable membranes to remove solutes from feed solutions. The efficiency of these processes are reduced by the formation of thermal and solute boundary layers on the membrane surfaces. This has motivated considerable interest in disrupting these layers using nearby obstacles that generate vortex shedding. We show that while vortex shedding can indeed increase system efficiency by increasing transmembrane flow, it also generates recirculation zones on the membrane surface that lead to solute accumulation and precipitation. Such precipitation is known to damage membranes when treating complex feed solutions. The coupled momentum, energy, and mass transport equations are solved using a finite-volume method with recent advances in immersed boundary methods to enforce no-slip and no-flux conditions to second order spatial and temporal accuracy. [Preview Abstract] |
Tuesday, November 26, 2019 11:10AM - 11:23AM |
S01.00004: Controlled symmetry breaking and vortex dynamics in intersecting flows Amy Shen, Noa Burshtein, Simon Haward Vortices are a ubiquitous feature in complex flows and turbulence, but their dynamics are challenging to study due to their typically transient nature. Here, we perform a detailed study of the vortex dynamics and interactions associated with a symmetry-breaking flow instability at a 4-way intersection. By precisely controlling the flow rate above a critical value, we are able to induce the merging of two co-rotating vortices into a single structure and similarly to induce a single vortex to split into two. Using quantitative flow velocimetry, both processes are recorded with high spatial and temporal resolution. We find that both the merging and the splitting of vortices are exponential processes, with a rate that depends on the imposed flow rate. The vortex dynamics in our system are intimately connected with the symmetry-breaking transition and are affected by the degree of vortex confinement, which we control by varying the aspect ratio of the microfluidic device. We show how the confinement affects the fundamental nature of the flow transition, which varies from super through subcritical as the aspect ratio is increased. Our results are relevant to understand and predict flow transitions and vortex dynamics in flow intersections, particularly in confined environments. [Preview Abstract] |
Tuesday, November 26, 2019 11:23AM - 11:36AM |
S01.00005: Vorticity dynamics for a spatially developing liquid jet within a co-flowing gas William A. Sirignano, Arash Zandian, Fazle Hussain A three-dimensional transient round liquid jet with coaxial outer gas flow is simulated and analyzed via $λ_2$ vortex dynamics. Two surface-deformation types separate at an indentation of the jet stem. Local vorticity explains the deformations in the recirculation zone behind the cap that affect the cap dynamics. The Kelvin-Helmholtz (KH) instability dominates the deformation region upstream of the cap (UR), unaffected by the behind-the-cap region (BCR). Different three-dimensional UR atomization mechanisms are delineated on a gas Weber number ($We_g$) versus liquid Reynolds number ($Re_l$) map, consistent with temporal studies and limited experiments, in a frame moving with the liquid velocity to portray better the similarity, avoiding the common misuse of velocity ratio. Vorticity distributions show periodic vortex development and surface deformation in the UR, with lost periodicity closer to the BCR. For practical density ratios and early times in the process, axial vorticity is mainly generated by baroclinicity while streamwise vortex stretching becomes more important later only at lower relative velocities with reduced pressure gradients. Pressure and viscous forces cause azimuthal acceleration. Azimuthal viscous forces are important even at high $Re_l$. [Preview Abstract] |
Tuesday, November 26, 2019 11:36AM - 11:49AM |
S01.00006: Inertial flow past moderately yawed cylinders Mohammed Kharrouba, Jean-lou Pierson, Jacques Magnaudet The flow past a finite-end yawed cylindrical particle is studied numerically. Three dimensionless parameters govern the problem when the flow is steady and uniform: the aspect ratio $\frac{L}{D}$ where $L$ is the length of the cylinder and $D$ its diameter, the yaw angle $\theta$ which is the angle between the cylinder axis and the inlet velocity, and the Reynolds number based on $D$. Particular attention is paid to the effect of these parameters on the particle wake and hydrodynamic loads. The aspect ratio is prescribed in the range $[2;10]$, the yaw angle in the range $[0;30]$, and the Reynolds number in the range $[0;400]$. Various types of vortex patterns are observed, including steady shedding of two counter-rotating vortices, periodic shedding of counter-rotating vortices and unsteady shedding of hairpin-shaped vortices. Results show that the dynamical regime and time evolution of the loads change drastically with the yaw angle $\theta$. The wake is found to be unsteady in the range $Re\in[360;400]$ at small yaw angles $(\theta\leq30°)$ with $\frac{L}{D}=2$. We propose a drag law valid for low and high Reynolds numbers in the case of a cylinder aligned with the flow. [Preview Abstract] |
Tuesday, November 26, 2019 11:49AM - 12:02PM |
S01.00007: Hysteresis and Bistable Behavior in Low Reynolds Number Flow Over a Cylinder With a Slanted Afterbody Fernando Zigunov, Prabu Sellappan, Farrukh Alvi A cylinder with a slanted afterbody is a bluff body that has a wake pattern similar to aircraft fuselage wakes. Recent work (Bulathsinghala et. al, 2017; Zigunov et. al, 2019) improved our understanding of the vortex-dominated regime of this wake, where a pair of counter-rotating vortices is formed. Steepening the slant angle above a critical value causes the vortex-dominated wake to transition to a fully-separated ("stalled") wake, with a subsequent increase in unsteadiness and a significant drop in form drag. The current investigation shows the full three-dimensional flow topology of the two wake states, reconstructed through stacked stereoscopic particle image velocimetry (S-SPIV). The wake state transition is found to be a function of Reynolds number for a fixed slant angle and the wake presents a hysteretical behavior when the Reynolds number is slowly varied. The hysteresis effect is further detailed in this investigation through fully time-resolved, high-speed PIV, and physical insight into the flow instability mechanisms that contribute to this effect is presented. [Preview Abstract] |
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