10:30
Instability and Transition 1
Chair: Dan Henningson
10:30
15 mins
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A new scenario of turbulence theory and an application to pipe turbulence
Tsutomu Kambe
Abstract: A new general scenario of turbulence theory is proposed and applied to pipe-flow turbulence. The theory supports tranverse traveling waves which suffer energy loss by internal friction (Joule-like effect). Its predictions are consistent with some characteristic features of pipe turbulence found in recent experiments: (i) existence of traveling waves and their important function
and (ii) existence of two large scales (LSM and VLSM), both in turbulent state. In fact, predicted waves are characterized by two scales (wave-length and damping-length). Bulk energy dissipation is expressed unexpectedly in a form analogous to a model of eddy-viscosity.
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10:45
15 mins
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EFFECTS OF INITIAL CONDITIONS AND MACH NUMBER IN THE EVOLUTION OF RICHTMYER-MESHKOV INSTABILITIES
Brandon Wilson, Ricardo Mejia-Alvarez, Kathy Prestridge
Abstract: We present an experimental study of the effects of shock intensity and initial conditions on the evolution of Richtmyer-Meshkov Instabilities (RMI). This study is carried out in a vertical shock tube with a single interface of sulfur-hexafluoride and air. We use combined particle image velocimetry (PIV) and planar laser induced fluorescence (PLIF) to obtain simultaneous measurements of velocity and density. These measurements enable us to determine single- and multi-point statistics of vector, scalar, and combined fields. We use these statistical descriptors to study the evolution of turbulence mixing in RMIs under different Mach numbers and initial conditions.
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11:00
15 mins
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NEW EXACT COHERENT STATES IN CHANNEL FLOW
Darren WALL, Masato NAGATA
Abstract: Three spatially periodic travelling wave exact coherent states are presented
for channel flow. Two of the flows, which are asymmetric with respect to the channel centreplane, are derived by homotopy from solutions for channel flow subject to a spanwise rotation investigated by Wall & Nagata (2013). The third flow satisfies a half-turn rotational symmetry about a point on the channel centreplane, and turns out to be the flow from which one of the asymmetric flows bifurcates in a symmetry breaking bifurcation. One of the asymmetric flows is found to substantially reduce the value of the lowest Reynolds number at which exact solutions are known to exist down to 665.
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11:15
15 mins
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Acoustic radiation due to scattering of T-S wave by the mean-flow distortion induced by steady local suction
Ming Dong, Xuesong Wu
Abstract: Substantial sound waves can be generated by boundary-layer instability modes when the latter are scattered by a rapid mean-flow distortion. This is a rather generic mechanism and operates when an oncoming T-S wave is scattered by a steady local suction slot. This paper focuses on this problem by extending a recently developed Local Scattering Theory (Wu & Dong, J. Fluid Mech. submitted), where a so-called transmission coefficient, defined as the ratio of the T-S wave amplitude downstream of the scatter to that upstream, is introduced to characterize the effect of a local scatter on boundary-layer instability and transition. As in the earlier work, the mathematical formulation is based on triple-deck formulism, but in order to accommodate the acoustic far field, which was not considered in the paper mentioned, the unsteady terms in the upper deck, which play a leading-order role in radiation, are retained, and the influence of the radiated sound on the near-wall perturbation is included. The upper deck equation for the pressure is the Helmholtz equation rather than the Laplace equation. This leads to a modified pressure-displacement relation, which is coupled with the linearized boundary-layer equations in the lower deck. Discretization of the whole system formulates a generalized eigenvalue problem, which is solved numerically. It is found that suction suppresses oncoming T-S waves, and this effect increases with the suction velocity and the slot width. The directivity is ndependent of the flow parameters only when the Mach number is low. The intensity of the radiated sound in general increases with the frequency, the suction velocity and the width of the suction slot. Interestingly, for O(1) suction velocities, the radiated sound is very weak, indicating that the gain of stabilizing effect does not cause aeroacoustic penalty.
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11:30
15 mins
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EMERGENCE OF OBLIQUE TS MODE DUE TO LONGITUDINAL WALL OSCILLATION IN 2D CHANNEL FLOW
Takashi Atobe
Abstract: Stabilizing effects of wall oscillation on two dimensional (2D) Tollmein-Schlichting (TS) waves and oblique TS waves developing in 2D channel flow are numerically investigated using the Floquet method. It was shown from the previous study[1] that the wall oscillation mostly has stabilizing effect and then TS mode is occasionally more stable than the oblique mode. In the present study the characteristics of the stability are investigated in detail. Then it is cleared that the exchange of the most unstable mode form the 2D TS to the oblique TS can occur at a certain value of Reynolds number.
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11:45
15 mins
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TURBULENT ANNULAR PIPE FLOW IN SUBCRITICAL TRANSITION REGIME: EFFECT OF RADIUS RATIO ON STRUCTURES
Takahiro Ishida, Takahiro Tsukahara
Abstract: Direct numerical simulations are performed for annular Poiseuille flows with various radius ratios η (= rin/rout) ∈ [0.1, 0.8] in subcritical transition regime of Reτ ∈ [48, 150]. Because the flow system of the annular Poiseuille flow can be regarded as the Hagen-Poiseuille flow (when η ≈ 0) and the plane Poiseuille flow (η ≈ 1), the (dis-)similarity and the connection between them are discussed. We found the occurrences of localized structure like the turbulent puff in the Hagen-Poiseuille flow at low η and of helical wave like the turbulent stripe pattern in the plane Poiseuille flow at high η. The switching point between helical wave and localized structure by changing η is caught in the region η ∈ [0.1, 0.3].
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12:00
15 mins
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VARIABLE DENSITY MIXING UNDER VARIABLE MEAN PRESSURE GRADIENT
Denis Aslangil, Daniel Livescu, Arindam Banerjee
Abstract: Turbulent mixing of a heterogeneous mixture of two incompressible, miscible fluids with different densities is investigated by using Direct Numerical Simulations (DNS). The mixing occurs in response to stirring induced by buoyancy-generated motions, in a triply periodic (1024^3) domain subjected to acceleration, g. The acceleration starts as positive and is reversed (g<0) or changed to neutral (g=0) during the flow evolution. These are unit problems that aim to mimic the core of the mixing layer of acceleration driven Rayleigh-Taylor (RT) and shock-driven Richtmyer-Meshkov (RM) instabilities and are also useful for verification and validation of mix models. The flow starts from rest and, for g>0, there is an initial growth of turbulence followed by turbulence decay as the fluids become molecularly mixed. The acceleration is changed at the peak of the Favre averaged turbulent kinetic energy (TKE). When compared to the constant gravity case, acceleration reversal causes a faster decay of total kinetic energy of the system and replaces larger structures with well-mixed small structures. The transition of energy from large to small scales and the dependence of mixing on the acceleration switch are investigated for Atwood numbers ranging from 0.05 to 0.9. In addition, the decay law for the g=0 case, corresponding to anisotropic, variable density turbulence decay and relevant to the RM problem is also examined.
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12:15
15 mins
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Dynamics and influence of the buoyancy frequency in unstably stratified homogeneous turbulence
Alan Burlot, Benoit-Joseph Gréa, Fabien Godeferd, Claude Cambon
Abstract: We investigate unstably stratified homogeneous turbulence through direct numerical simulation and a spectral model based on a eddy-damped quasi-normal Markovian closure. In this study, we take into account and evaluate the importance of an unsteady buoyancy frequency expressing the evolution of a mixing zone submitted to Rayleigh-Taylor instability.
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