Reza Miraghaie

SYMMETRY BREAKING IN FREE SURFACE CYLINDER FLOWS

The flow in a stationary open cylinder driven by the constant rotation of the bottom endwall is unstable to three-dimensional perturbations for sufficiently large rotation rates. The bifurcated state takes the form of a rotating wave. Two distinct physical mechanisms responsible for the symmetry breaking are identified, which depend on whether the fluid depth is sufficiently greater or less than the cylinder radius. For deep systems, the rotating wave results from the instability of the near-wall jet that forms as the boundary layer on the rotating bottom endwall is turned into the interior. In this case the three-dimensional perturbations vanish at the air/water interface. On the other hand, for shallow systems, the fluid at radii less than about half the cylinder radius is in solid-body rotation whereas the fluid at larger radii has a strong meridional circulation. The interface between these two regions of flow is unstable to azimuthal disturbances and the resulting rotating wave persists all the way to the air/water interface. The flow dynamics are explored using three-dimensional Navier-Stokes computations and experimental results obtained via digital particle image velocimetry

Symmetry breaking to a rotating wave in a lid-driven cylinder with a free surface: Experimental observation

A systematic experimental investigation of the flow in an open cylinder, driven by the constant rotation of the bottom endwall, shows that axisymmetry is spontaneously broken via a supercritical Hopf bifurcation to a rotating wave with azimuthal wave number 4. The physical mechanism responsible for the symmetry breaking is shown to be due to the instability of the shear layer that is produced by the boundary layer on the bottom rotating endwall being turned into the interior by the stationary sidewall. Comparison with other experiments and numerical studies (restricted to axisymmetric subspaces) sheds new light on disparate observations in the literature and helps distinguish between spontaneous and forced (via imperfections) symmetry breaking.

Determination of surface shear viscosity via deep-channel flow with inertia

Results of an experimental and computational study of the flow in an annular region bounded by stationary inner and outer cylinders and driven by the rotation of the floor are presented. The top is a flat air/water interface, covered by an insoluble monolayer. We develop a technique to determine the surface shear viscosity from azimuthal velocity measurements at the interface which extends the range of surface shear viscosity that can be measured using a deep-channel viscometer in the usual Stokes flow regime by exploiting flow inertia. A Navier{Stokes-based model of bulk flow coupled to a Newtonian interface that has surface shear viscosity as the only interfacial property is developed. This is achieved by restricting the flow to regimes where the surface radial velocity vanishes. The use of inertia results in an improved signal-to-noise ratio of the azimuthal velocity measurements by an order of magnitude beyond that available in the Stokes flow limit. Measurements on vitamin K1 and stearic acid monolayers were performed, and their surface shear viscosities over a range of concentrations are determined and found to be in agreement with data in the literature.

Measurement and computation of hydrodynamic coupling at an air/water interface with an insoluble monolayer

The coupling between a bulk vortical flow and a surfactant-influenced air/water interface has been examined in a canonical flow geometry through experiments and computations. The flow in an annular region bounded by stationary inner and outer cylinders is driven by the constant rotation of the floor and the free surface is initially covered by a uniformly distributed insoluble monolayer. When driven slowly, this geometry is referred to as the deep-channel surface viscometer and the flow is essentially azimuthal. The only interfacial property that affects the flow in this regime is the surface shear viscosity, μ s , which is uniform on the surface due to the vanishingly small concentration gradient. However, when operated at higher Reynolds number, secondary flow drives the surfactant film towards the inner cylinder until the Marangoni stress balances the shear stress on the bulk fluid. In general, the flow can be influenced by the surface tension, σ, and the surface dilatational viscosity, K s , as well as μ s

Flow induced patterning at the air–water interface

Patterns on the air–water interface of a swirling cylinder flow are produced via hydrodynamic symmetry-breaking instability of the bulk flow. The patterns are rotating waves breaking the axisymmetry of the system and are longitudinal at the free surface (i.e., not surface deforming). Qualitative observations and quantitative measurements of velocity and vorticity are provided. Three-dimensional Navier–Stokes computations identify the symmetry-breaking mode responsible for the waves. These waves are then used to pattern Langmuir monolayers at concentrations sufficiently below saturation.

MHD Effects on Heat Transfer in a Molten Salt Blanket

Abstract Heat transfer in closed channel flows of molten salts (MS)s, such as FLiBe or FLiNaBe, has been considered under specific reactor conditions. MHD effects have been accessed for two blanket concepts: self-cooled MS blanket, and dual-coolant MS blanket. The effect of heat transfer degradation due to turbulence reduction by a magnetic field in the First Wall channels of the self-cooled blanket was analyzed with the K-ε model of turbulence. In the dual-coolant blanket, the MS flow is laminar. A 2-D MHD code was used to calculate the laminar velocity profile first. Then, the temperature field was calculated using a 3-D temperature code. Reasonable interface temperatures below the material limit of 550°C, and low heat escape from the breeder zone have been demonstrated. Model limitations and the ways of their improvement are also discussed.

Turbulent velocity profile measurement in circular pipe flow using particle image velocimetry technique

Experimental study of turbulent flow in circular pipe is carried out by using PIV technique as a preliminary experiment for studying thermo fluid properties of molten salt, Flibe in particular, which is a high Prandtl number fluid. At this first stage, water is used as working fluid to obtain reference data. Measurements of the fully developed turbulent flow in a circular pipe at Re = 5286 are carried out using PIV system and near wall flow structure and turbulent statistics as well as averaged flow field are obtained. Average velocity profile has good agreement with the DNS data.