Date of Award:


Document Type:


Degree Name:

Doctor of Philosophy (PhD)


Civil and Environmental Engineering

Committee Chair(s)

William Rahmeyer


William Rahmeyer


Gilberto Urroz


J. Paul Tullis


Roland Jeppson


Michael O'Neill


The effects of natural aeration and fall height upon velocity dissipation and modelling were studied. Underwater velocities and void ratios (proportion of air volume to total volume) were measured along the jet centerline at various depths in the receiving pool under 1- and 2-inch diameter vertical circular free-falling jets. Maximum air bubble penetration depths were also measured. In the tests, the nozzle velocities were designed so that the two differently sized models would have equivalent Froude numbers for direct comparison of respective underwater parameters. Air entrainment-the natural development of air bubbles within a pool receiving a free-falling jet-causes voids to exist within the underwater jet flowfield. These voids lead to dissipation of underwater downward momentum flux and velocity. The measured velocities and void ratios were analyzed in context with previous research concerning the effects of natural aeration upon scour.

In general, higher jet Reynolds numbers and fall heights caused higher natural aeration rates and void ratios levels. Thus, these factors increased dissipation of underwater velocities. Fall height allowed for more radical transformations in jet diameter and velocity between nozzle and pool. Reynolds number enhanced jet expansion from air resistance, and gravity caused jet contraction. The overall Reynolds number effect diminished with scale.

By applying underwater jet diffusion theory and momentum flux analysis, a model was developed to predict underwater velocities along the jet centerline in the pool under free-falling jets for water depths up to 25 jet diameters. The correlation coefficient (r2) between the predicted velocities and measured velocities was 0.963. Methods of estimating centerline void ratios at any depth, centerline jet velocity entering pool, and maximum air bubble penetration depths were also developed.

Comparisons of measured underwater velocities and previous scour data from jets designed in Froude similitude were made. In cases of lower Froude numbers and higher fall heights, the smaller scale jets produced significantly larger relative velocities and scour depths. Froude similitude should be maintained if the difference inthe void ratios between scales remains small. Froude modelling maybe applied if certain sediment sizes and nozzle Reynolds numbers are exceeded in the model.