Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session G18: Flow Instability: General |
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Chair: Ryan Keedy, Sandia National Laboratories Room: D135 |
Monday, November 21, 2016 8:00AM - 8:13AM |
G18.00001: Numerical and experimental investigation of flow instabilities in the presence of a viscosity gradient Ryan Keedy, Alberto Aliseda Laboratory experiments were performed to understand the effect of viscosity ratio on the development of the round jet when a miscible liquid is injected into another stagnant ambient liquid. Altering the viscosity of the injected liquid jet resulted in noticeable changes in the turbulent/non-turbulent interface in the jet's developing region, including the instability wavelength. The change in the formation of structures at the interface is apparent even when several key non-dimensional numbers ($Pe$, $Re$) associated with the flow are kept constant. Large, coherent structures in the turbulent jet resulting from the shear instability of the interface may affect the downstream development of the self-similar profile. Hence, it is important to examine and understand the characteristics of the shear layer instability in order to better understand the role that a viscosity gradient plays in turbulent jet development. The spatial stability equations for a flow in which viscosity varies arbitrarily as a function of scalar concentration are presented. These equations are evaluated at various viscosity ratios and the predicted instability frequencies are compared to experimental results in the range of $\mu_{jet}/\mu_{amb}=0.5-2$ and $Re\approx 10^4$. [Preview Abstract] |
Monday, November 21, 2016 8:13AM - 8:26AM |
G18.00002: On long-time algebraic and exponential instabilities found in linear dispersive flows Nathaniel Barlow, Kristina King, Paula Zaretzky, Michael Cromer, Steven Weinstein A physically-motivated class of partial differential equations that describes the response of a system to disturbances is examined. Morphological differences are identified between system responses that exhibit algebraic growth and the more typical case of exponential growth. Specifically, the propagation characteristics of the response are examined in the context of spatio-temporal hydrodynamic stability theory. One key attribute of predicted algebraically growing solutions is the prevalence of transient growth in almost all of the response, with the long-time growth occurring asymptotically at precisely one wave speed. [Preview Abstract] |
Monday, November 21, 2016 8:26AM - 8:39AM |
G18.00003: Morphological instabilities during the rapid solidification of three component systems Anthony Altieri, Stephen Davis, Peter Voorhees Rapidly solidifying binary mixtures of a major component and a dilute solute are known to be subject to morphological instabilities. The stability of ternary mixtures is not well understood. A linear stability analysis of ternary mixtures of one major component and two dilute solutes is performed. The growing cellular and oscillatory instabilities, present in binary systems, are investigated for ternary systems. The effect of thermal and concentration gradients, surface energy, interface kinetics, and nonequilibrium thermodynamics on the morphological instabilities, and the use of the second solute to stabilize the system, are considered. [Preview Abstract] |
Monday, November 21, 2016 8:39AM - 8:52AM |
G18.00004: Effect of Discrete Roughness on Transition on a Sharp Cone at an Angle of Attach at Mach 6 Eric Matlis, Thomas Corke, Michael Semper Experiments were performed to investigate passive discrete patterned roughness for transition control on a sharp right-circular cone at a 6$^{\circ}$ angle of attack at Mach 6.0. The angle of attack was set to produce a mean cross-flow velocity component in the boundary layer over the cone in which the cross-flow instability is the dominant mechanism of turbulent transition. The focus is transition control which is based on exciting less-amplified stationary cross-flow modes that suppress the growth of the more-amplified cross-flow modes, and thereby delay transition. The passive roughness consisted of an azimuthal array of micron-size indentations (dimples) at an axial location that was just upstream of the first linear stability neutral growth branch for cross-flow modes. Both critical and sub-critical azimuthal mode numbers of roughness were examined. The receptivity of the stationary cross-flow modes to the roughness was evaluated using Silicone-oil surface flow visualization. The visualization images were post-processed using a pixel-intensity based spectral analysis. Of particular interest was the effect that higher (conventional) tunnel acoustic levels had on the roughness receptivity. [Preview Abstract] |
Monday, November 21, 2016 8:52AM - 9:05AM |
G18.00005: Subcritical Hopf bifurcations in low-density jets Yuanhang Zhu, Vikrant Gupta, Larry K. B. Li Low-density jets are known to bifurcate from a steady state (a fixed point) to self-excited oscillations (a periodic limit cycle) when the Reynolds number increases above a critical value corresponding to the Hopf point, $Re_{H}$. In the literature, this Hopf bifurcation is often considered to be supercritical because the self-excited oscillations appear only when $Re > Re_{H}$. However, we find that under some conditions, there exists a hysteretic bistable region at $Re_{SN} < Re < Re_{H}$, where $Re_{SN}$ denotes a saddle-node bifurcation point. This shows that the Hopf bifurcation can also be subcritical, which has three main implications. First, low-density jets could be triggered into self-excited oscillations even when $Re < Re_{H}$. Second, in the modeling of low-density jets, the subcritical or supercritical nature of the Hopf bifurcation should be taken into account because the former is caused by cubic nonlinearity whereas the latter is caused by square nonlinearity. Third, the response of the system to external forcing and noise depends on its proximity to the bistable region. Therefore, when investigating the forced response of low-density jets, it is important to consider whether the Hopf bifurcation is subcritical or supercritical. [Preview Abstract] |
Monday, November 21, 2016 9:05AM - 9:18AM |
G18.00006: Pattern-Formation in Moist Turbulent Rayleigh-Benard Convection Prasanth Prabhakaran, Stephan Weiss, Alexei Krekhov, Holger Nobach, Eberhard Bodenschatz We report experiments on droplet-condensation patterns in turbulent Rayleigh-Benard convection, where a horizontal fluid layer is heated from below and cooled from above. We use compressed Sulphur Hexafluoride (SF6) as the working fluid for pressures and temperatures in the liquid/vapor coexistence region. The vapor evaporating from the liquid pool above the heated bottom-plate undergoes film condensation on the cooled top-plate. We observe a finite wavelength instability of the condensed liquid film, which is in stark contrast to the well-known long-wavelength Rayleigh Taylor instability. In the non-linear stationary state, droplets periodically fall into the liquid pool below. Under appropriate conditions, we observe hexagonal patterns with a well-defined wavelength. By varying the pressure and temperature, and with it the evaporation/condensation rates we investigate the influence of these parameters on the observed patterns. [Preview Abstract] |
Monday, November 21, 2016 9:18AM - 9:31AM |
G18.00007: Hydrodynamic instabilities and concentration polarization coupled by osmotic pressure in a Taylor-Couette cell Denis Martinand, Nils Tilton This study addresses analytically and numerically the coupling between hydrodynamic instabilities and osmotic pressure driven by concentration polarization. The configuration consists of a Taylor-Couette cell filled with a Newtonian fluid carrying a passive scalar. Whereas the concentric inner and outer cylinders are membranes permeable to the solvent, they totally reject the scalar. As a radial in- or outflow of solvent is imposed through both cylinders, a concentration boundary layer develops on the cylinder where the solvent exits, until an equilibrium steady state is reached. In addition, the rotation of the inner cylinder is used to drive centrifugal instabilities in the form of toroidal vortices, which interact with the concentration boundary layer. By means of the osmotic pressure, concentration polarization is found to promote or hinder the hydrodynamic instabilities, depending on capacity of the vortices and diffusion to increase the concentration field at the membrane. The results obtained by analytical stability analysis agree with dedicated Direct Numerical Simulations. [Preview Abstract] |
Monday, November 21, 2016 9:31AM - 9:44AM |
G18.00008: Dynamics of a single flexible filament in a flowing soap film. Chaonan Chen, Shunshan Feng, Tong Zhou The interactions between flexible plates and surrounding fluids like two-dimensional flag-in-wind problems are important physical phenomena. Here we use a spandex filament with one end fixed flapping in gravity-driven soap film device which can be regarded as a quasi-two-dimensional flow tunnel. A silk filament had been used previously to demonstrate three stable dynamical states: stretched-straight, flapping, and bistable states. The similar phenomena occured for a spandex filament while the bifurcation conditions seem to be different compared with a silk filament, as the critical filament length is longer and critical inflow velocity is higher than that for a silk filament. In the experiment, we considered some representative parameters (filament length, inflow velocity, and bending stiffness of the filament) to study their effects on the stability of the filament and its bifurcation conditions. An interface-tracking ALE finite element method was then conducted to reproduce the experiment and investigate more details about effects of these parameters. which are significant to reveal the underlying mechanism of flag-in-wind problem. [Preview Abstract] |
Monday, November 21, 2016 9:44AM - 9:57AM |
G18.00009: Mode competition and destabilization of microfluidic channel flows by the Coriolis force Saunak Sengupta, Sandeep Saha, Suman Chakraborty Understanding flow stability in inertial microfluidics is very important due to its increased application in medical and chemical engineering. On a steady rotating platform centrifugal actuation drives fluid flow but Coriolis force can destabilize the flow and enhance mixing in a short span. We investigate the role of Coriolis force in micro-mixing and the structure of the roll-cells formed in rotating channel flow using linear stability theory. We conduct a parametric study at different rotation numbers, Reynolds number, axial and spanwise wavenumbers. Our results reveal existence of multiple competing unstable modes (Types I to IV) due to Coriolis force: Types I and II have been reported in literature and are responsible for the formation of evenly-spaced roll-cells. We find new instabilities (Types III and IV) which contribute to the formation of twisted roll cells. The existence of the instabilities is clearly demarcated on a regime map to assist future experiments to identify them. The kinetic energy budget has been analyzed to gain insight into the mechanism of energy transfer by Coriolis force from the mean flow to the perturbations. We make a qualitative comparison of roll-cells predicted by linear stability with previously reported experiments. [Preview Abstract] |
Monday, November 21, 2016 9:57AM - 10:10AM |
G18.00010: Flow Simulations of The Dynamics of a Perturbed Solid-Body Rotation Flow Shixiao Wang, Chunjuan Feng, Feng Liu, Zvi Rusak DNS is conducted to study the 3-D flow dynamics of a base solid-body rotation flow with a uniform axial velocity in a finite-length pipe. The simulation results describe the neutral stability line in response to either axisymmetric or 3-dimensional perturbations in a diagram of Reynolds number ($Re$, based on inlet axial velocity and pipe radius) versus the incoming flow swirl ratio ($\omega$). This line is in good agreement with the neutral stability line recently predicted by the linear stability theory of Wang {\it et al.} (2016). The Wang \& Rusak (1996) axisymmetric instability mechanism and evolution to an axisymmetric breakdown state is recovered in the simulations at certain operational conditions in terms of $Re$ and $\omega$. However, at other operational conditions there exists a dominant, 3-dimensional spiral type of instability mode that agrees with the linear stability theory of Wang {\it et al.} (2016). The growth of this mode leads to a spiral type of flow roll-up that subsequently nonlinearly saturates on a rotating spiral type of vortex breakdown. The computed time history of the velocity components at a certain point in the flow is used to describe 3-dimensional phase portraits of the flow global dynamics and its long-term behavior. [Preview Abstract] |
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