The development of a computer program for simulating body-cable systems has made it possible to study the behaviour of towed bodies in au and under water. It has led to both a greater understanding of the underlying dynamic processes and a better appreciation of the requirements necessary to achieve satisfactory towing performances, in particular of airborne targets and oceanographic vehicles (called "fishes" in the current] argon)
Although the basic physics is the same in both fluid media, the ratio of the two densities, which can be as much as 3000 to 1, has a profound effect upon the dynamic response of the towed vehicle to disturbances applied to the tugged end of the cable
The object of this chapter is to describe these behavioural alternatives and reveal how they lead to quite difference criteria for assessing and improving towing performance in the two cases.
Figure 1 illustrates typical airborne and underwater towed configurations and provides definitive labels. In general the airborne requirement calls for a large horizontal separation between tug and target, the vertical separation (in most cases) being less important. In contrast the underwater requirement is usually to achieve a large vertical separation between ship and fish, often achieved by installing ballast in the fish to pull it down.
A description of the software code is given below, followed by a comparison of the physical parameters appropriate to towing in au and water.
Typical towed body shapes are shown in Figures 2 and 3. Airborne bodies tend to be slender and marine bodies bluff. The marine body shown in Figure 3 is better streamlined than many towed underwater vehicles
Some discussion of the equilibrium conditions is necessary before the dynamic characteristics are treated in the remaining part of the chapter.
These FORTRAN programs comprise a full three-dimensional simulation of a body towed through fluid by an inextensible flexible cable CBUS (cable-body underwater(Fig. 1 is available in full paper) simulator) models the behaviour of a cable-body moving in sea-water, CBAS (cable-body aerodynamic simulator) models the behaviour in a r and s derived from CBUS Both programs assume that the motion of the tug (ship or aircraft) is known and that the towed system does not influence the tug's motion. The last assumption is fully(Fig. 2 and Fig. 3 are available in full paper) justified in the underwater case, but may be doubtful in the case of an aircraft or drone towing a heavy target if the total towed mass exceeds 1/50th (say) of the aircraft mass Needless to say the mathematical difficulties of simulating a coupled tug-cable-body system in three dimensions are too formidable to be essayed at the present time.