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t

transition from laminar to turbulent boundary layer

w

wall condition

o

jet exit condition

Superscripts

(ˉ)

time-averaged mean property

(ˉ)′

time-averaged rms fluctuating property

INTRODUCTION

An experimental investigation concerning aspects of the generation of sprays by the bow waves (or bow sheets) of ships is described. This flow is important as a representative spray formation process of the marine environment, which contributes to the structure of ship-generated waves and the electromagnetic scattering properties (e.g., the photographic and radar signatures) of vessels. The overall objectives of the investigation were to make new measurements of several properties associated with the sprays produced by bow sheets, emphasizing transitions at the free surface of attached turbulent bow sheets (or turbulent wall jets). This work included measurements of the onset location of roughened liquid surfaces, and the properties (drop sizes and location) of the onset of primary drop breakup (turbulent primary breakup) along the liquid surface. Finally, the new measurements were interpreted and correlated using phenomenological theories.

Bow sheet/spray flows are complex and involve a number of turbulence/surface interactions and spray formation mechanisms. This complexity has prevented complete understanding of bow sheet/spray flows; nevertheless, there is general agreement about the qualitative features and spray forming mechanisms of bow sheets (13). In particular, flows associated with chutes, spillways, plunge pools, hydraulic jumps, open water waves and jets exhibit similar features of spray formation. In general, the mechanism appears to involve the propagation of vorticity (especially turbulence) to the liquid surface, or its development along the surface, with the subsequent appearance of a turbulence-wrinkled interface between the liquid and gas and eventually the formation of drops due to turbulent primary breakup at the liquid surface.

An important issue concerning the transitions of turbulent bow sheets is the origin of the turbulence near the liquid surface, e.g., whether this turbulence mainly is caused by motion along the bow surface or whether it mainly results from aerodynamic forces at the liquid surface. This issue was partly addressed during the present study by observing round water jets injected into still air at normal temperature and pressure (NTP), with large jet Reynolds numbers (ReD >120,000) and a variety of passage configurations. In all cases, a large contraction (roughly 100:1 and shaped according to Smith and Wang (4)) followed by boundary layer removal, was used to generate a uniform nonturbulent flow. This flow then entered round constant diameter passages having various lengths in order to study the effect of turbulence developed in the passage on liquid jet properties. Some typical pulsed shadowgraphs of the flow near the jet exit for short and long passages are illustrated in Fig.1. For the short passage, L/d=0.15, the flow remains essentially uniform and nonturbulent at the jet exit; this

Fig. 1. Pulsed shadowgraphs of round nonturbulent and turbulent liquid jets in still air.



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