Page 95 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

where
Vehicle drag 5 1=2 3 σAV2Cd;
σ 5 density of seawater/gravitational acceleration, where density of seawater is 64 lb/ft3
(1035 kg/m3) and gravitational acceleration is 32.2 ft/s2 (9.8 m/s2);
A 5 characteristic area on which Cd (the drag coefficient) is non-dimensionalized. For an ROV, A is defined as the cross-sectional area of the front or the vehicle. In some cases, the ROV volume raised to the 2/3 power is used;
V 5 velocity in feet per second—1 knot 5 1.689 ft/s 5 0.51 m/s;
Cd 5 nondimensional drag coefficient. This ranges from 0.8 to 1 when based on the cross- sectional area of the vehicle.
Total drag of the system is equal to the vehicle drag plus the tether drag (Figure 3.21).
In the case of cables, the characteristic area, A, is the cable diameter in inches divided by 12, times the length perpendicular to the flow.
The Cd for cables ranges from 1.2 for unfaired cables, 0.5
0.6 for hair-faired cable, and 0.1
0.2 for faired cables (although ROV cables are not typically faired).
Since the ROV’s tether is typically the highest drag item on the ROV system, an understanding of the concept of vortex shedding (Figure 3.22) around a cylindrical item (e.g., a tether
3.5 Design theory 83
FIGURE 3.21
Component drag at constant speed.
Vehicle drag Tether length
(A)
(B)
FIGURE 3.22
Vortex shedding (a) symmetrical and (b) asymmetrical.
Total drag
Total drag
Tether drag