Page 92 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

80 CHAPTER 3 Design Theory and Standards
FIGURE 3.18
Ideal form with skin surface detail.
Full current
Relatively calm next to vehicle
Vehicle
Current flow
Turbulent boundary layer
Hull structure skin
Vehicle
distance from the surface, i.e., it exhibits a certain gradient. The velocity gradient across the flow produces friction between successive fluid layers which we call vicious friction.
The Reynolds number is a dynamic factor for fluid flow and comes into place for determining the flow characteristics around the vehicle (which directly affects the drag equation). The three modes of flow around a body are as follows:
1. laminar—smooth flow over the body
2. transient—approaching the critical Reynolds number where laminar transitions to turbulent 3. turbulent—disorganized flow over the body
With that simple introduction in place, a more analytical discussion can now be considered.
The drag dynamics of submerged vehicles were worked out during manned submersible research done in the 1970s by the office of the Oceanographer of the Navy. According to Busby
(1976):
Skin friction is a function of the viscosity of the water. Its effects are exhibited in the adjacent, thin layers of fluid in contact with the vehicle’s surface, i.e., the boundary layer (Figure 3.18). The boundary layer begins at the surface of the submersible where the water is at zero velocity relative to the surface. The outer edge of the boundary layer is at water stream velocity. Consequently, within this layer is the velocity gradient and shearing stresses produced between the thin layers adjacent to each other. The skin friction drag is the result of stresses produced within the boundary layer. Initial flow within the boundary layer is laminar (regular, continuous movement of individual water particles in a specific direction) and then abruptly terminates into
Exploded View