Propagation of weak shocks through a random medium

The propagation of weak shock waves (M_s = 1.007, 1.03 and 1.1) through a statistically uniform random medium has been investigated experimentally in a shock tube. The wave-from geometry, rise time and amplitude of initially plane shocks which have propagated through a random mixture of helium and refrigerant 12 are measured. The effect of shock propagation on the properties of the random medium is visualized with schlieren and shadow photography. The pressure histories of the distorted shock waves reflecting from a normal end wall are observed to be both peaked and rounded. In the rounded case the perturbed shock is found to be made up of a succession of weak, slightly curved fronts with a total effective rise time orders of magnitude greater than the classical Taylor thickness. The radius of curvature of the weakest shocks after propagating through the random medium is inferred from observations at two downstream stations to be about 7 times the integral scale of the gas inhomogeneities. It is concluded that the observed distortions of the wave fronts can best be explained in terms of random focusing and defocusing of the front by the inhomogeneities in the medium. A ray-tracing calculation has been used to interpret the experimental observations. It is found that geometrical considerations are sufficient to account for many of the effects observed on the shocks

High speed optical tomography for flow visualization

A novel optical architecture (based on holographic optical elements) for making high speed tomographic measurements is presented. The system is designed for making density or species concentration measurements in a nonsteady fluid or combustion flow. Performance evaluations of the optical system are discussed, and a test phase object was successfully reconstructed using this optical arrangement

Wind induced dispersion and algal growth in shallow lakes

The objectives of this paper are (1) to present experimental measurements of wind induced, vertical dispersion in Lake Brielle, The Netherlands, and (2) to examine the utility of a euphotic—dark zone model for investigation of the effects of dispersion and sedimentation on algal growth and phosphate cycling in shallow lakes. A tracer technique was used to measure dispersion (D) caused by a moderate wind of 4 m/s. The resulting values ranged between 4 and 6 cm2/s and compared well with that predicted from theory. The latter objective was accomplished by comparing the results of a simple, two compartment model with those using a more complex, but more descriptive, finite difference model which accounts for the algal concentration gradient induced by growth, dispersion and sedimentation. Simulations showed good agreement in computation of the average euphotic and dark zone algal concentrations between the two models for D ranging from 0 to 6 cm2/s and for a sedimentation rate constant, Ks, of 0.002 and 0.02 m/h.\ud \ud The results of simulations suggest that rapid algal growth occurs most readily in calm weather when wind induced dispersion in negligible. Dispersion may also influence the rate of particulate phosphate accumulation in the sediment (due to the settling out of algal cells) and the soluble phosphate profile in the overlying water column. Field data is needed to verify the relationship between algal growth and wind induced dispersion