When a large vehicle such as an airship is decelerated, the flow induced by its passage through the au, prior to the deceleration manoeuvre, imposes significant forces on the vehicle as it is brought to rest These forces are of the same order as the force required to decelerate a body in a vacuum having the same mass as the airship.
Lamb (1924) analysed the kinetic energy imparted to an infinite flow field as a result of accelerating ellipsoids from rest Current airship design methods are based on the work of Munk (1922), which assumes that the deceleration process is the inverse of acceleration in that an equivalent amount of kinetic energy is extracted from the external flow field This theory, however, is based on the assumption that the flow can at all times be treated as quasi-steady, and this will become progressively less true as the rate of deceleration is increased For example, in the limit, if the airship is stopped instantaneously, the theory implies that the external fluid is simultaneously brought to rest at all points In practice, however, simple flow visualization shows that an afterflow persists for some tune after the airship has itself been brought to rest
The present investigation was designed to measure the force imposed on a model airship by the externally induced flow field both during, and subsequent to, the deceleration process, and to compare this with the predicted values To simplify the investigation the motion of the airship was restricted to a vertical translation, and water was used as the working fluid in order to give reasonable Reynolds number compatibility with the 1/100 scale model available.
Munk (1922) developed a theory describing the deceleration process, basing his analysis on the work of Lamb (see Lamb, 1924) A modified version of his analysis is presented here to highlight some of the implicit assumptions and to clarify some points in the development
The total kinetic energy in an incompressible fluid field is Sven by (The Formula is available in full paper)
The experimental rig is shown in Figure 3 A 1/100 scale model airship was used The model was driven downwards through water using a hydraulic ram consisting of a cylinder in which ran a 10 3 mm diameter piston rod which was attached to the model (Fig. 2 is available in full paper) airship via a strain gauged force measurement section. The aluminium piston at the top of the rod was sealed against the cylinder wall by two neoprene "0"rings. The system was driven by a hydraulic pump having a capacity of 83 litres/min at a pressure of 17 2 bar.