Date of Award
1996
Degree Type
Dissertation
Degree Name
Doctor of Philosophy
Abstract
The problem of polluted waste and thermal discharge into the environment has received considerable attention in recent years. The discharge of warm or polluted water into a water environment results in the formation of a shear layer that is capable of mixing with ambient water. The emphasis of this research is to study the turbulence characteristics of surface jets in shallow, deep, and flowing ambient fluid; and to examine the effect of the free surface as a moving boundary, with particular attention given to the development, growth, and similarity properties of surface jets.;Similarity was found to exist for both mean and turbulent quantities when the data were normalized by the excess surface velocity, excess surface temperature, and the velocity half-width ({dollar}\delta).{dollar} The free surface adjusted the pressure gradient and maintained self-similarity in the present experiments. This trend of self-similarity of velocity profiles deviated from the Gaussian distribution in the fully-developed region of surface jets. The self-similar near Gaussian velocity profiles have not been recognized in previous studies. The temperature profiles were linear and wider than the velocity profiles, while the thickness of the buoyancy profiles was smaller during winter in cold climate regions.;The growth of a surface jet was examined for self-preservation using a linear relation {dollar}(\delta\propto x){dollar} according to the classical similarity theory for jets. The co-flowing ambient current reduced the growth rate from 0.075 in stagnant ambient water to 0.054 for a small ambient speed ratio. An initial wake accelerated the transition from a self-preserving jet {dollar}(\delta\propto x){dollar} in the near field to a self-preserving wake {dollar}(\delta\propto x\sp{lcub}0.5{rcub}){dollar} in the far field. The mean jet depth obtained using digital imaging and LDA techniques correlated well suggesting a constant ratio of {dollar}\beta=h/\delta=2.3.{dollar} The excess surface velocity, excess surface temperature, and the ambient velocity decayed proportional to {dollar}(x\sp{lcub}-0.5{rcub}).{dollar} It can be concluded that the higher decay rates of the velocity scale were associated with loss of momentum, breakdown of the jet structure and reduction in entrainment.;The integral model showed that the change of surface jet momentum in an open channel varies with the magnitude and direction, of the ambient velocity and bed friction. The jet in zero ambient current had practically constant momentum. The surface jets in a positive current, {dollar}U\sb{lcub}a{rcub},{dollar} and negative gradient, {dollar}dU\sb{lcub}a{rcub}/dx<0,{dollar} gain momentum. Both back-flow and initial wake resulted in a loss in jet momentum.;The maximum turbulence intensity occurred at a depth {dollar}\eta=y/\delta=1.0{dollar} as opposed to 0.7 in plane free jets. The turbulence intensity, {dollar}u\sp\prime\sb{lcub}m{rcub}/\Delta U\sb{lcub}s{rcub},{dollar} remained constant in the fully developed region, indicating that both the mean and turbulent fields scale with the surface velocity. The results showed a drop from {dollar}u\sp\prime\sb{lcub}m{rcub}/\Delta U\sb{lcub}s{rcub}=0.2{dollar} for surface nonbuoyant jets to a value of {dollar}u\sp\prime\sb{lcub}m{rcub}/\Delta U\sb{lcub}s{rcub}=0.17{dollar} for surface thermal jets. This drop in the turbulence intensity is attributed to the effect of buoyancy forces.;The entrainment coefficient was found to be practically constant and equal to 0.04 in the self-similar region of surface jets in both a stagnant and a co-flowing ambient current. A weak irrotational ambient current had a minimal effect on the entrainment coefficient. On the other hand, both initial wake and buoyancy considerably reduced the magnitude of the entrainment coefficient. Finally, the study of thermal jet under winter condition indicates that the densimetric Froude number may alter the entrainment function.
Recommended Citation
Zaghloul, Ahmed M., "Turbulence Structure Of Plane Surface Jets In A Current" (1996). Digitized Theses. 2667.
https://ir.lib.uwo.ca/digitizedtheses/2667