Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session E28: Flow Instability: Nonlinear Dynamics and Global Modes II |
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Chair: Roman Grigoriev, Georgia Institute of Technology Room: Georgia World Congress Center B316 |
Sunday, November 18, 2018 5:10PM - 5:23PM |
E28.00001: Spatiotemporal intermittency of global helical modes in low-density jets David D.W. Ren, Larry K.B. Li In previous experiments, we demonstrated that low-density jets issuing from long injectors can exhibit global instability in the form of self-excited helical oscillations. We found that decreasing the non-dimensional injector length $L_t^{-1}$ can cause the jet to transition from a global axisymmetric mode to a global helical mode. In the present study, we quantify the statistical properties of this transition by modelling the jet as a superposition of axisymmetric and helical eigenmodes whose amplitudes vary independently in time. Depending on $L_t^{-1}$ and the density ratio $S$, we find that the joint probability distribution of these modal amplitudes shows a statistical preference for axisymmetric or helical modes. We present evidence of intermittency using scaling laws for the ‘laminar’ phases and for the spectral density. We also demonstrate how the intermittency varies with spatial location, $L_t^{-1}$, and $S$. Finally, we use dynamic mode decomposition to extract the dominant frequencies of the axisymmetric and helical modes, yielding a universal frequency scaling for these modes in low-density jets. |
Sunday, November 18, 2018 5:23PM - 5:36PM |
E28.00002: Precessing vortex core oscillations in a swirled jet - a weakly non-linear analysis Kiran Manoharan, Mark Frederick, Sean Clees, Jacqueline O'Connor, Santosh Hemchandra Swirled jets at high swirl number (S), show the formation of a central recirculation zone (CRZ) due to vortex breakdown. At high S values, CRZ precession about the streamwise axis generates a precessing vortex core (PVC) with a characteristic precession frequency, fPVC. This occurs due to an internal feedback mechanism in the hydrodynamic field that results in coherent self-sustained flow oscillations. We study the emergence of a PVC in a swirled jet (Re ~ 44000) where S is varied as a parameter for fixed net mass flow rate. Time resolved velocity field measurements are obtained using stereoscopic particle image velocimetry (sPIV) at 5 kHz. Wavelet and spatial cross spectral analysis of the data shows that a helical limit cycle flow oscillation emerges due to PVC formation at S=Sc=0.61. Global linear stability analysis using time averaged flow fields from sPIV accurately predicts fPVC at S=Sc. Predictions of fPVC for S > Sc from a weakly non-linear extension of linear theory agree well with experimental values. This confirms the fact that self-excited helical global modes can result in the emergence of PVCs through a supercritical Hopf bifurcation. |
Sunday, November 18, 2018 5:36PM - 5:49PM |
E28.00003: Recurrence analysis of a globally unstable jet subjected to transverse acoustic forcing Abhijit Kumar Kushwaha, Marek Mazur, Nicholas A Worth, James R Dawson, Larry K. B. Li In this experimental study, we take a dynamical systems approach to investigating the synchronization dynamics of a globally unstable low-density jet subjected to longitudinal and transverse acoustic forcing. On increasing the forcing amplitude, we find evidence of two classical routes to lock-in -- the saddle-node route and the torus-death route -- depending on the frequency detuning and the longitudinal/transverse nature of the forcing. Using cross-recurrence plots (CRPs) and cross-recurrence quantification analysis (CRQA), we show that the degree of complexity in the purely transversely forced jet is higher than that in the purely longitudinally forced jet. We quantify these differences in complexity using CRQA of the geometrical structures in the CRPs. We find that, compared with longitudinal forcing, transverse forcing produces lower values of the recurrence rate and determinism in the lead up to lock-in. This information could be useful for the design of open-loop control strategies for globally unstable jets. |
Sunday, November 18, 2018 5:49PM - 6:02PM |
E28.00004: Instability and Breakdown of a Low Viscosity Jet Ian Wright, Vinod Srinivasan Mixing of a liquid jet into a surrounding fluid of different properties is a commonly encountered phenomenon in natural and engineering systems. Studies in core annular flows have shown that viscosity contrast can lead to absolute instability; however free shear flows with viscosity contrasts have not received significant attention. Here we study the breakdown process of a round jet that is miscible with and has a viscosity at or below that of the ambient fluid. Viscosity ratios M (ambient-to-jet) ranging from 1 to 37 are investigated in a constant density environment for jet Reynolds numbers varying from 500 to 2000. Fluorescent dye imaging and PIV measurements are used to document the jet breakdown process. Consistent with linear stability theory calculations, a configuration with a low viscosity core and high viscosity ambient favors the strong growth of helical modes with wavelengths of the order of the jet diameter. The wavelength of the most unstable mode is reported as a function of the Reynolds number, viscosity ratio and non-dimensional boundary layer thickness at the nozzle exit. |
Sunday, November 18, 2018 6:02PM - 6:15PM |
E28.00005: Noise-induced triggering in low-density jets Yuanhang Zhu, Minwoo Lee, Vikrant Gupta, Larry K.B. Li We study the effect of white Gaussian noise on a low-density axisymmetric jet in the hysteretic bistable regime produced by a subcritical Hopf bifurcation. We find that noise can induce triggering, causing the jet to transition from a linearly stable fixed point to a stable periodic limit cycle. When the noise amplitude increases, the Hopf and saddle-node points move towards each other, decreasing the width of the bistable region. This decrease is approximately linear, which we attribute to the near-linear behavior of the amplitude of the unstable limit cycle between the Hopf and saddle-node points. When the noise amplitude is sufficiently high, the bistable region vanishes altogether, causing the subcritical bifurcation to appear like a supercritical bifurcation. We conclude by showing that these triggering dynamics can be accurately reproduced with a noise-driven van der Pol (VDP) oscillator. |
Sunday, November 18, 2018 6:15PM - 6:28PM |
E28.00006: Abstract Withdrawn |
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