斜航船体まわりの流場と流体力の計測 : 第2報,タンカー船型

抄録

In the previous report, measured results of wake flow and surface pressure distribution on a simple and fine hull form were presented with some theoretical investigations. In order to confirm and extend the knowledge obtained about the nature and structure of the wake flow and its relation to the hydrodynamic forces on the ship model in oblique towing condition, similar experiments were conducted on a full and practical ship model. It was a tanker model with bluff and bulbous bow, wide beam and long parallel middle body. The following five different kinds of experiments were carried out, when the model was towing with constant drift angle and constant forward speed. (1) measurement of lateral force and yaw moment acting on the model (2) measurement of pressure distribution on the hull (3) observation of surface streamlines and trailing vortices by means of dye tracer method (4) measurement of velocity and vorticity in the wake flow by means of 5-hole pitot tube and rotor-type vortexmeters (5) visualization of limiting streamlines on the hull by meams of oil film method Though the oil film method was applied on a small model (L=0.8m) in a circular water channel, the others were carried out with a large model (L=4m) in towing tanks. After the examination of these experimental results, they were compared with those for the simple and fine hull form reported in the previous paper. They were also compared with the results of potential flow calculation. Through these experimental and theoretical investigations on the wake flow around the full tanker model and the hydrodynamic forces and moment on it, the followings were obtained. (1) There observed three or four groups of trailing vortices in the wake flow. They are the main causes of hydrodynamic forces acting on a ship in manoeuvring motion. The first vortex initiates at bilge part near shoulder of the model (S.S. 8) in the leeward side. It collects the vorticity of the separated flow at the bilge in the parallel middle body, and it concentrates in a small region. On the other hand other group of vortices initiate in the aft body, which have comparable amount of vorticty in the total as the first one, spread in wider range and rotate slowly. (2) Measured pressure distribution corresponds well to the potential flow calculation in the fore body. Longitudinal distribution of lateral force obtained by surface pressure integration brings useful information about the contribution of each flow component on the lateral force and yaw moment. In the fore body, potential component is dominant, and the vortex flow component which is defined as the difference between measured total value and potential flow component, is dominant in the middle body. In the aft body, both of the components have almost the same contributions for the lateral force and yaw moment. (3) Non-linear characteristics of total lateral force to the variation of drift angle mostly comes from the difference in the lateral force distribution in the middle part, which is related to the growth of trailing vortex from shoulder part of the model. (4) Behaviour of stern vortex influences on the directional stability of the model, because linear part of lateral force is caused by the stern vortex. It is presumed by the investigation of the strength change for the stern vortex to the drift angle variation and its contribution on the lateral force distribution in the aft body. (5) Comparision between the results of full and fine ship models shows that the difference in the proportion of the potential flow component to the vortex flow component is quite important to explain the difference in the nature of hydrodynamic forces and flow structure around model. Finaly it was proved that precise flow measurement is useful not only to understand the structure of the wake but also to explain the nature and characteristics of the hydrodynamic forces on the model in manoeuvring motion.