Stationary Enclosure Research Articles

Two modes of oscillatory instability in the flow between a pair of corotating disks

The mechanism of pattern formation in the flow between two corotating disks in a stationary enclosure and its transition are investigated experimentally, and the experimental results are confirmed by numerical simulation. Particle image velocimetry and laser-Doppler-velocimetry measurements are achieved in the experiment, and the data acquired are examined by using Fourier decomposition analyses and applying the bifurcation theory. Two distinct modes of oscillatory instability are identified as origins that break the axisymmetry of the flow; one originates from the detached shear layer and the other from the outer boundary layer along the enclosure. The former instability is found to lead to the polygonal patterns, which have been reported in previous papers, while the latter instability induces a flow with a shift-and-reflect symmetry, having small spiral vortices along the outer boundary. A mixed-mode polygonal flow, which is surrounded by small vortices is observed at large Reynolds numbers. We identify two transition routes to the mixed-mode flow by adopting two different driving processes of the disk rotation.

Rayleigh-Bénard Convection in Open and Closed Rotating Cavities

Buoyancy effects can be significant in the rotating annular cavities found between compressor discs in gas-turbine engines, where Rayleigh numbers above 1012 are common. In some engines, the cavity is “closed” so that the air is confined between four rotating surfaces: two discs and inner and outer cylinders. In most engines, however, the cavity is “open” and there is an axial throughflow of cooling air at the center. For open rotating cavities, a review of the published evidence suggests a Rayleigh–Bénard type of flow structure, in which, at the larger radii, there are pairs of cyclonic and anti-cyclonic vortices. The toroidal circulation created by the axial throughflow is usually restricted to the smaller radii in the cavity. For a closed rotating annulus, solution of the unsteady Navier–Stokes equations, for Rayleigh numbers up to 109, show Rayleigh–Bénard convection similar to that found in stationary enclosures. The computed streamlines in the r-θ plane show pairs of cyclonic and anti-cyclonic vortices; but, at the larger Rayleigh numbers, the computed isotherms suggest that the flow in the annulus is thermally mixed. At the higher Rayleigh numbers, the computed instantaneous Nusselt numbers are unsteady and tend to oscillate with time. The computed time-averaged Nusselt numbers are in good agreement with the correlations for Rayleigh–Bénard convection in a stationary enclosure, but they are significantly higher than the published empirical correlations for a closed rotating annulus.

Numerical Predictions of Thermal Convection in a Rectangular Enclosure with Oscillating Wall

ABSTRACT Effects of wall vibration on natural convection in a rectangular enclosure containing air are investigated in this report. In practical applications, the wall vibration driven by an external force leads to periodic variations in the flow and thermal fields within the rectangular enclosure, and hence results in different features from those in a stationary enclosure. Two types of thermal boundary condition are considered, involving a heated-vertical-walls and a heated-horizontal-walls situations. The solution method is based on a two-stage pressure-correction scheme, which is applied to determine the absolute pressure, density, temperature, and velocity components of the compressible flow in enclosures. The vibration frequency of the wall is varied from 1 to 50 Hz and the vibration amplitude is ranged between 1.4 % and 5.6 % of enclosure length. In this study, Rayleigh number is fixed at 105. For the parameter ranges considered, results show that a maximum amplitude of 36.73 in Nusselt number oscillation is attained due to the wall vibration.

Flow Characteristics of Rotating Disks Simulating a Computer Hard Drive

ABSTRACT A computational effort was undertaken to study the fluid flow inside a computer hard drive. Disk arrangements with multiple arms were considered inside a stationary enclosure. The results indicated large differences between circumferential velocities. In some cases, the slopes of the circumferential velocities were close to those represented by the solid-body rotation and the analytical solution for a thin fluid gap with one stationary and one rotating wall. The circumferential velocities were strongly affected by the arm length. The rotational speed and arm length had an increasing effect on the viscous power dissipation of the disks.

NUMERICAL ANALYSIS OF NATURAL CONVECTION IN AN ENCLOSURE WITH ROTATIONALLY PRODUCED ARTIFICIAL GRAVITY

Buoyancy-induced flow and heat transfer in an artificial gravitational field produced by rotation is examined. A comparison is made between such flows and those produced in a stationary enclosure exposed to a terrestrial gravitational field. The computational results show that the structure of the buoyancy-induced flow fields is different for the two cases. Natural convection in an artificial gravity field is examined in terms of the ratio of artificial gravity buoyancy forces to viscous forces. Unlike a stationary enclosure in a terrestrial gravitational field, in an artificial gravity field there is no bifurcation in the Nusselt number behavior.

Rotary acoustic sand-core shakeout

An acoustically enclosed apparatus for shaking out sand-cores from metal castings. A plurality of percussive shakeout stations are spaced around the circumference of a turntable. The stations are separated one from the other and from their surroundings by an acoustical enclosure which rotates with the turntable and within a stationary enclosure. The castings are loaded and unloaded through outwardly facing access openings in the rotatable enclosure as they become registered with an access port in the stationary enclosure. Shaking out is effected while the access openings are out of registry with the access port. The sand collector beneath the stations attenuates the sound emitted from beneath the station.

Flow and heat transfer in a rotating enclosure with axial throughflow

An analysis is made of the fluid flow and heat transfer processes in a circular cylindrical enclosure rotating about its own axis. A coolant is passed through the enclosure, entering and leaving through centrally located apertures in the end walls. This configuration is intended as a model of rotating enclosures in devices such as gas turbines and air compressors. The Navier-Stokes and energy equations were solved by a finite-difference formulation which can accommodate either steady or transient conditions. Buoyancy forces associated with the rotational body forces were included in some cases. All solutions were performed for laminar flow. For the parameter ranges investigated it was found that rotation inhibited the recirculating motion within the enclosure and thereby decreased the heat transfer relative to that for the stationary enclosure. Buoyancy further reduced the heat transfer owing to the break up of residual circulatory motions in the outer portion of the enclosure. Still stronger buoyancy brought about a slight increase in the heat transfer. The coolant flow was confined to a corridor adjacent to the axis of the enclosure, and there was no mixing between the coolant and the fluid in the enclosure proper.