ALBERT

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.

Description: Current experimental and computational techniques must be improved in order to advance the prediction capability of the longitudinal vortical flows shed by underwater vehicles. The generation, development, and breakdown mechanisms of the shed vortices at high Reynolds numbers are not fully understood. The ability to measure hull separated vortices associated with vehicle maneuvering does not exist at present. The existing point-by-point measurement techniques can only capture approximately the large 'mean' eddies but fail to meet the dynamics of small vortices during the initial stage of generation. A new technique, which offers a previously unavailable capability to measure the unsteady cross-flow distribution in the plane of the laser light sheet, is called Particle Displacement Velocimetry (PDV). PDV consists of illuminating a thin section of the flowfield with a pulsed laser. The water is seeded with microscopic, neutrally buoyant particles containing imbedded fluorescing dye which responds with intense spontaneous fluorescence with the illuminated section. The seeded particles in the vortical flow structure shed by the underwater vehicle are illuminated by the pulse laser and the corresponding particle traces are recorded in a single photographic frame. Two distinct approaches were utilized for determining the velocity distribution from the particle traces. The first method is based on matching the traces of the same particle and measuring the distance between them. The direction of the flow can be identified by keeping one of the pulses longer than the other. The second method is based on selecting a small window within the image and finding the mean shift of all the particles within that region. The computation of the auto-correlation of the intensity distribution within the selected sample window is used to determine the mean displacement of particles. The direction of the flow is identified by varying the intensity of the laser light between pulses. Considerable computational resources are required to compute the auto-correction of the intensity distribution. Parallel processing will be employed to speed up the data reduction. A few examples of measured unsteady vortical flow structures shed by the underwater vehicles will be presented.