ALBERT

Description: Direct numerical simulations of fully developed pressure-driven turbulent flow in a rotating channel have been performed. The unsteady Navier-Stokes equations were written for flow in a constantly rotating frame of reference and solved numerically by means of a finite-difference technique on a 128 x 128 x 128 computational mesh. The Reynolds number, based on the bulk mean velocity Um and the channel half-width h, was about 2900, while the rotation number Ro = 2 Ω/h/Um varied from 0 to 0.5. Without system rotation, results of the simulation were in good agreement with the accurate reference simulation of Kim, Moin & Moser (1987) and available experimental data. The simulated flow fields subject to rotation revealed fascinating effects exerted by the Coriolis force on channel flow turbulence. With weak rotation (Ro = 0.01) the turbulence statistics across the channel varied only slightly compared with the nonrotating case, and opposite effects were observed near the pressure and suction sides of the channel. With increasing rotation the augmentation and damping of the turbulence along the pressure and suction sides, respectively, became more significant, resulting in highly asymmetric profiles of mean velocity and turbulent Reynolds stresses. In accordance with the experimental observations of Johnston, Halleen & Lezius (1972), the mean velocity profile exhibited an appreciable region with slope 2Q. At Ro = 0.50 the Reynolds stresses vanished in the vicinity of the stabilized side, and the nearly complete suppression of the turbulent agitation was confirmed by marker particle trackings and two-point velocity correlations. Rotational-induced Taylor-Görtler-like counter-rotating streamwise vortices have been identified, and the simulations suggest that the vortices are shifted slightly towards the pressure side with increasing rotation rates, and the number of vortex pairs therefore tend to increase with Ro. © 1993, Cambridge University Press. All rights reserved.

Description: The presence and behaviour of vaporous cavities are of major importance in many modern industrial applications where heat transfer, boiling or cavitation are involved. Following a sudden depressurization of a superheated fluid, the bubble growth rate controls the generated transients and heat transfer. Most existing computer modelling and prediction codes are based on individual spherical-bubble-growth studies and neglect possible interactions and collective phenomena. This paper addresses this collective behaviour using a singular-perturbation approach. The method of matched asymptotic expansions is used to describe the bubble growth, taking into account its interaction with a finite number of surrounding bubbles. A computer program is developed and the influence of the various parameters is studied numerically for the particular case of a symmetrical equal-size-bubble configuration and a thermal-boundary-layer approximation. A significant influence of these interactions on bubble growth and heat transfer is observed: Compared to an isolated-bubble case, the growth rate of a bubble is reduced in the presence of other bubbles, and the temperature drop at its wall is smaller. As a result the heat loss due to bubble growth is smaller. These effects increase with the number of interacting bubbles. © 1985, Cambridge University Press. All rights reserved.