The Experts below are selected from a list of 216 Experts worldwide ranked by ideXlab platform
Eric C. Kerrigan - One of the best experts on this subject based on the ideXlab platform.
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Tollmien-Schlichting Wave Cancellation by Feedback Control
Bulletin of the American Physical Society, 2015Co-Authors: Hari Vemuri, Jonathan Morrison, Eric C. KerriganAbstract:Tollmien-Schlichting waves are naturally occurring primary instabilities that enter the boundary-layer because of environmental noise, surface imperfections etc. via the receptivity mechanism. Amplification of TS waves is one of the paths to turbulence, which can be delayed by actively interfering with the linear stage of their growth. The picture below shows the calculated streamwise Disturbance Velocity of a growing TS wave initiated by a twodimensional source on a flat plate model.
Shaodong Zhang - One of the best experts on this subject based on the ideXlab platform.
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Propagation and reflection of gravity waves in a meridionally sheared wind field
Journal of Geophysical Research, 2008Co-Authors: Kaiming Huang, Shaodong ZhangAbstract:[1] Although the properties of gravity waves propagating in a vertically sheared flow have been extensively studied, the effects of a horizontally sheared wind on gravity wave propagations were seldom reported. In this paper, according to the linear theory, the characteristics of gravity wave reflection in a meridionally sheared zonal background wind field are discussed, which are evidently different from those of wave reflected by a vertically sheared flow. By numerical simulations, we present the whole process of a gravity wave packet reflection in a meridionally sheared wind. When the wave reaches the reflecting level, the zonal Disturbance Velocity in the evanescent region shows an evanescent wave configuration, which is consistent with the Airy function form predicted by the linear theory; while the meridional Disturbance Velocity exhibits rather different wave structures. The wave number spectra of zonal and meridional Disturbance velocities also show different characteristics in the reflection process. The energy center of the wave packet is reflected in a position nearer than the reflecting level predicted by the linear theory. While the wave propagates along (against) the meridionally sheared wind, the meridional perturbation kinetic energy and total wave energy decrease (increase), whereas, the zonal perturbation kinetic energy increases (decreases); and an energy exchange between the wave potential and wave kinetic energies can be observed. Earth rotation has a slight influence on the energy transfer between the wave and the background flow. If the wind shear beyond the reflecting level isn't strong enough, part components of the incident wave may steadily penetrate across the reflecting level, and yields a transmission wave. A large amplitude gravity wave propagating in a meridionally sheared wind can obviously induce a mean flow, which strengthens slightly the wave reflection, indicating that the transmission rate slightly decreases with the increasing amplitude of the incident wave. This differs from the reflection of waves in a vertical sheared wind field, in which the mean flow induced by the large amplitude wave significantly enhances the wave transmission effect. In the meridionally sheared flow, the transmission rate seems to mainly depend on the sheared wind and the parameters and spectral components of the incident waves.
Hari Vemuri - One of the best experts on this subject based on the ideXlab platform.
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Tollmien-Schlichting Wave Cancellation by Feedback Control
Bulletin of the American Physical Society, 2015Co-Authors: Hari Vemuri, Jonathan Morrison, Eric C. KerriganAbstract:Tollmien-Schlichting waves are naturally occurring primary instabilities that enter the boundary-layer because of environmental noise, surface imperfections etc. via the receptivity mechanism. Amplification of TS waves is one of the paths to turbulence, which can be delayed by actively interfering with the linear stage of their growth. The picture below shows the calculated streamwise Disturbance Velocity of a growing TS wave initiated by a twodimensional source on a flat plate model.
François Feuillebois - One of the best experts on this subject based on the ideXlab platform.
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Far-field Disturbance flow induced by a small non-neutrally buoyant sphere in a linear shear flow
Journal of Fluid Mechanics, 2010Co-Authors: Evgeny S. Asmolov, François FeuilleboisAbstract:The Disturbance flow due to the motion of a small sphere parallel to the streamlines of an unbounded linear shear flow is evaluated at small Reynolds number using the method of matched expansions. Decaying laws are obtained for all Velocity components in a far inviscid region and viscous wakes. The z component (in the direction of the shear-rate gradient) of the Disturbance Velocity is cylindrically symmetric in the inviscid region. It decays with the distance r from the sphere like r -5/3 , while the y component (in the direction of vorticity) decays like r -4/3 . The widths of two viscous wakes, located upstream and downstream of the sphere, grow with the longitudinal coordinate x as γ ω ∼ z ω ∼ |x| 1/3 . The maximum x and z components of the Velocity are located in the wake cores; they scale like |x| -2/3 and |x| -1 respectively. For two particles interacting through their outer regions, the migration Velocity of each particle is the sum of the Velocity of an isolated particle and of a Disturbance Velocity induced by the other one. Particles placed in the normal or transversal directions repel each other. When each particle is located in a wake of the other one, they may either attract or repel each other.
Donald L. Koch - One of the best experts on this subject based on the ideXlab platform.
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Flow of power-law fluids in fixed beds of cylinders or spheres
Journal of Fluid Mechanics, 2012Co-Authors: John P. Singh, Sourav Padhy, Eric S. G. Shaqfeh, Donald L. KochAbstract:AbstractAn ensemble average of the equations of motion for a Newtonian fluid over particle configurations in a dilute fixed bed of spheres or cylinders yields Brinkman’s equations of motion, where the Disturbance Velocity produced by a test particle is influenced by the Newtonian fluid stress and a body force representing the linear drag on the surrounding particles. We consider a similar analysis for a power-law fluid where the stress $\boldsymbol{\tau} $ is related to the rate of strain $ \mathbisf{e} $ by $\boldsymbol{\tau} = 2m \mathop{ \vert \mathbisf{e} \vert }\nolimits ^{n\ensuremath{-} 1} \mathbisf{e} $, where $m$ and $n$ are constants. In this case, the ensemble-averaged momentum equation includes a body force resulting from the nonlinear drag exerted on the surrounding particles, a power-law stress associated with the Disturbance Velocity of the test particle, and a stress term that is linear with respect to the test particle’s Disturbance Velocity. The latter term results from the interaction of the test particle’s Velocity Disturbance with the random straining motions produced by the neighbouring particles and is important only in shear-thickening fluids where the Velocity Disturbances of the particles are long-ranged. The solutions to these equations using scaling analyses for dilute beds and numerical simulations using the finite element method are presented. We show that the drag force acting on a particle in a fixed bed can be written as a function of a particle-concentration-dependent length scale at which the fluid Velocity Disturbance produced by a particle is modified by hydrodynamic interactions with its neighbours. This is also true of the drag on a particle in a periodic array where the length scale is the lattice spacing. The effects of particle interactions on the drag in dilute arrays (periodic or random) of cylinders and spheres in shear-thickening fluids is dramatic, where it arrests the algebraic growth of the Disturbance Velocity with radial position when $n\geq 1$ for cylinders and $n\geq 2$ for spheres. For concentrated random arrays of particles, we adopt an effective medium theory in which the drag force per unit volume in the medium surrounding a test particle is assumed to be proportional to the local volume fraction of the neighbouring particles, which is derived from the hard-particle packing. The predictions of the averaged equations of motion are validated by comparison with simulations of randomly distributed hydrodynamically interacting cylinders.
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critical bacterial concentration for the onset of collective swimming
Journal of Fluid Mechanics, 2009Co-Authors: Ganesh Subramanian, Donald L. KochAbstract:We examine the stability of a suspension of swimming bacteria in a Newtonian medium. The bacteria execute a run-and-tumble motion, runs being periods when a bacterium on average swims in a given direction; runs are interrupted by tumbles, leading to an abrupt, albeit correlated, change in the swimming direction. An instability is predicted to occur in a suspension of ‘pushers’ (e.g. E. Coli , Bacillus subtilis , etc.), and owes its origin to the intrinsic force dipoles of such bacteria. Unlike the dipole induced in an inextensible fibre subject to an axial straining flow, the forces constituting the dipole of a pusher are directed outward along its axis. As a result, the anisotropy in the orientation distribution of bacteria due to an imposed Velocity perturbation drives a Disturbance Velocity field that acts to reinforce the perturbation. For long wavelengths, the resulting destabilizing bacterial stress is Newtonian but with a negative viscosity. The suspension becomes unstable when the total viscosity becomes negative. In the dilute limit ( nL 3 ≪ 1), a linear stability analysis gives the threshold concentration for instability as ( nL 3 ) crit = ((30/ C ℱ( r ))( D r L / U )(1 + 1/(6τ D r )))/(1−(15