Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session G23: Instabilities with Vortices, Wave Interactions, and Pulsatile FlowsInstabilities Vortexes
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Chair: Arnab Samanta, Indian Institute of Science Room: 710 |
Monday, November 20, 2017 10:35AM - 10:48AM |
G23.00001: Biglobal stability of vortex rings Naveen Balakrishna, Joseph Mathew, Arnab Samanta Shear layers of low-speed round jets are known to roll up via Kelvin-Helmholtz-like instability into vortex rings, which are then susceptible to azimuthal instability. Using an inviscid linear stability analysis, Widnall and Tsai (1977) found for a thin vortex ring the most unstable azimuthal mode to depend upon the core to ring radius ratio and the core vorticity distribution. In addition, the outside straining field deforms this ring core into the shape of an ellipse. In this work, both the azimuthal and elliptic instabilities are investigated using a linear biglobal stability framework. It is formulated as a standard eigensystem in cylindrical polar coordinates with Fourier decomposition along the azimuthal direction, while Chebyshev collocation points are used in other directions. We explore the role of viscosity on instability, which has not been studied before in detail. [Preview Abstract] |
Monday, November 20, 2017 10:48AM - 11:01AM |
G23.00002: Transient growth from the continuous spectrum of a high-speed rapidly-swirling jet Arnab Samanta, Gopalsamy Muthiah We investigate the possibility for short-time transient growths involving the helical modes of a rapidly-swirling, high-speed jet that has undergone a sub-critical transition via an axisymmetric vortex breakdown. The base flow is extracted from the time-averaged measurements, consisting of the recirculation bubble and its wake. A pseudospectrum analysis complements a local normal-mode based stability analysis in identifying the continuous spectrum, which is further split into a potential and freestream spectrum, where the non-orthogonality between the former type and the existing discrete stable modes is shown to be the main origin of strong transient growths in such flows. As the swirling flow develops post the bubble collapse, this potential mode spectrum widens, increasing the importance of transient growth inside the wake region. The local transient gains calculated at the wake confirms this, where strong growths far outstrips the exponential modal growth at shorter times, especially at the higher helical orders and smaller streamwise wavenumbers. These short-time transients are likely to be a necessary first-step toward the formation of a wavemaker region at the wake of such flows, leading to their eventual spiral breakdown. [Preview Abstract] |
Monday, November 20, 2017 11:01AM - 11:14AM |
G23.00003: Low frequency instabilities in the wake of a cantilevered circular cylinder. Robert Martinuzzi, Matthew Kindree, Maryam Shahroodi, Chris Morton The flow development over a cantilevered circular cylinder (AR $=$ 4) protruding through a thin laminar boundary layer was investigated experimentally (Re $=$ 6,800 -10,800) and numerically (Re $=$ 300). For both the experiment and simulation, the unsteady flow development was found to be dominated by two instabilities: (i) vortex shedding, and (ii) a low frequency instability centered on approximately one-quarter of the vortex shedding frequency. The low frequency phenomenon persisted only in the case of an incoming laminar boundary layer as was demonstrated experimentally when compared to an incoming turbulent boundary layer. Coherent flow structures were analyzed using proper orthogonal decomposition (POD). The low frequency instability was found to represent a spanwise motion in the cylinder's upper wake. POD also provided insight into the nature of vortex formation and associated coupling with the detected low frequency instability. The temporal coefficients obtained from POD analysis were used to construct a low order model which described the influence of the low frequency instability on the vortex shedding amplitude. [Preview Abstract] |
Monday, November 20, 2017 11:14AM - 11:27AM |
G23.00004: POD analysis of laminar flow in a two-dimensional 180-degree sharp bend with bypass. Koushik Chandramouli, Arul Prakash K A Proper Orthogonal Decomposition analysis on laminar flow physics in a 180-degree sharp bend with bypass is demonstrated. The unsteadiness in the flow field observed downstream of thebend and the bypass is captured. The data for POD analysis has been obtained by solving mass, momentum and energy equations in Cartesian framework using Streamline Upwind/Petrov-Galerkin Finite element method. A parameter called IOR (Inlet to Outlet Ratio) is defined based on the inlet and outlet domain heights, with a bypass in the divider at 3 different locations. The fluid flow involves steady, periodic unsteady and chaotic unsteadiness and POD is conducted for the transient cases. The presence of the bypass increases the interaction of the vortices with the fluid from the bypass. The simulated results demonstrate the understanding of the vortices interaction with the walls and each other and thus the enhancement in the heat transfer. [Preview Abstract] |
Monday, November 20, 2017 11:27AM - 11:40AM |
G23.00005: Nonlinear Evolution of Azimuthally Compact Crossflow-Vortex Packet over a Yawed Cone Meelan Choudhari, Fei Li, Pedro Paredes, Lian Duan Hypersonic boundary-layer flows over a circular cone at moderate incidence angle can support strong crossflow instability and, therefore, a likely scenario for laminar-turbulent transition in such flows corresponds to rapid amplification of high-frequency secondary instabilities sustained by finite amplitude stationary crossflow vortices. Direct numerical simulations (DNS) are used to investigate the nonlinear evolution of azimuthally compact crossflow vortex packets over a 7-degree half-angle, yawed circular cone in a Mach 6 free stream. Simulation results indicate that the azimuthal distribution of forcing has a strong influence on the stationary crossflow amplitudes; however, the vortex trajectories are nearly the same for both periodic and localized roughness height distributions. The frequency range, mode shapes, and amplification characteristics of strongly amplified secondary instabilities in the DNS are found to overlap with the predictions of secondary instability theory. The DNS computations also provide valuable insights toward the application of planar, partial-differential-equation based eigenvalue analysis to spanwise inhomogeneous, fully three-dimensional, crossflow-dominated flow configurations. [Preview Abstract] |
Monday, November 20, 2017 11:40AM - 11:53AM |
G23.00006: Nonlinear modeling of wave-topography interactions, shear instabilities and shear induced wave breaking using vortex method Anirban Guha Theoretical studies on linear shear instabilities as well as different kinds of wave interactions often use simple velocity and/or density profiles (e.g. constant, piecewise) for obtaining good qualitative and quantitative predictions of the initial disturbances. Moreover, such simple profiles provide a minimal model to obtain a mechanistic understanding of shear instabilities. Here we have extended this minimal paradigm into nonlinear domain using vortex method. Making use of unsteady Bernoulli's equation in presence of linear shear, and extending Birkhoff-Rott equation to multiple interfaces, we have numerically simulated the interaction between multiple fully nonlinear waves. This methodology is quite general, and has allowed us to simulate diverse problems that can be essentially reduced to the minimal system with interacting waves, e.g. spilling and plunging breakers, stratified shear instabilities (Holmboe, Taylor-Caulfield, stratified Rayleigh), jet flows, and even wave-topography interaction problem like Bragg resonance. We found that the minimal models capture key nonlinear features (e.g. wave breaking features like cusp formation and roll-ups) which are observed in experiments and/or extensive simulations with smooth, realistic profiles. [Preview Abstract] |
Monday, November 20, 2017 11:53AM - 12:06PM |
G23.00007: The Role of Flow Reversals in Transition and Relaminarization of Pulsating Flows Joan Gomez, Oleg Goushcha, Yiannis Andreopoulos Pulsating flows, such as the flows in cardiovascular systems, exhibit a cyclic behavior of the axial velocity. They are of particular interest because at different times of the cycle the flow is laminar or turbulent, depending on the local Reynolds number. An experiment was setup to replicate the cyclic motion of the fluid in a clear, rigid tube. The flow was driven by a piston-motor assembly controlled by a computer. The motion of the piston was programmed to induce a forward-only cyclic motion of the mean flow by adjusting the amplitude of the longitudinal velocity pulsation in relation to the mean velocity. Time-Resolved Particle Image Velocimetry (TR-PIV) techniques were used to acquire velocity data on the plane of a CW laser illumination sheet. Flow reversal occurs first near the walls and the corresponding strong shearing induces transition to turbulence where the rest of the flow remains laminar. The behavior of reversed flow was analyzed under various Reynolds and Womersley numbers.~~ [Preview Abstract] |
Monday, November 20, 2017 12:06PM - 12:19PM |
G23.00008: Adjoint-based shape optimisation of inkjet printhead with compliant boundaries Petr Kungurtsev, Matthew Juniper A drop-on-demand inkjet printhead is a narrow, ink-filled, channel with a piezoelement on the upper boundary and a nozzle on the opposite side. A high-frequency electric pulse is applied to deform the piezoelement, pushing a small amount of fluid through the nozzle to create a droplet. After the pulse ends, acoustic waves continue to propagate inside the channel. The printing speed depends on the rate at which droplets are expelled and therefore on the frequency of the applied pulse. As the pulse frequency increases and approaches the first natural mode, the amplitude of the acoustic wave increases rapidly. This leads to an unpredictable oscillatory behaviour inside the channel as the acoustic waves have not decayed before the start of the next pulse, and the print quality reduces. We demonstrate how to adjust the printhead geometry to increase the decay rate of acoustic oscillations, which should maintain print quality for higher frequencies. We have derived the decay rate sensitivity to the shape changes in Hadamard form, and modified the channel’s geometry to increase the decay rate of acoustic fluctuations. Also, the presence of the domain boundaries compliance is taken into account to consider their deformation under the influence of the oscillations propagating inside. [Preview Abstract] |
Monday, November 20, 2017 12:19PM - 12:32PM |
G23.00009: Pulsating Flows in a Tube with Expandable Wall Frank Raguso, Oleg Goushcha A mean axial fluid flow inside a cardiovascular system has a periodic behavior driven by a heart. In one period, the flow through aorta is accelerated to a Reynolds number associated with turbulent flow and decelerated to nearly stagnant condition. The cyclic pressure in the aorta also exerts time-dependent forces on the walls of the cardiovascular system. Since walls are not rigid, they can expand under fluidic pressure. It is of interest to examine the effect of expandable walls on the flow regime transition. To achieve this, an experimental apparatus has been set up. The periodic mean axial flow inside the tubes is driven by a motor-controlled piston programmed to induce a periodic flow. A time-resolved particle image velocimetry method has been used to calculate the flow velocity field in two tubes: (1) a rigid tube and (2) a flexible tube with expandable walls. The velocity fields from two tubes were comparted to identify any differences in flow transition mechanisms. [Preview Abstract] |
Monday, November 20, 2017 12:32PM - 12:45PM |
G23.00010: Extremely pulsatile flow around a surface-mounted hemisphere: synergistic experiments and simulations Ian A. Carr, Nikolaos Beratlis, Elias Balaras, Michael W. Plesniak Extremely pulsatile flow (where the amplitude of oscillation pulsation is of the same order as the mean flow) over a three-dimensional, surface-mounted bluff body gives rise a wealth of fluid dynamics phenomena. In this study, we extend our previous experimental work on extremely pulsatile flow around a surface-mounted hemisphere by performing a complementary direct numerical simulation. Results from the experiment and simulation will be presented and compared. After establishing the agreement between experiment and simulation, we will examine the morphology and dynamics of the vortex structures in the wake of the hemisphere, and the effects of extreme pulsatility. The dynamics of the arch-type recirculation vortex is of primary interest, in particular its upstream propagation due to self-induced velocity in the direction opposite to the freestream during deceleration. In addition to the velocity field, the surface pressure field throughout the pulsatile cycle will be presented. These synergistic experiments and simulations provide a detailed view into the complex flow fields associated with pulsatile flow over a surface-mounted hemisphere. [Preview Abstract] |
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