UNSTEADY HYDRODYNAMIC FORCES ON PARTIALLY SUBMERGED PROPELLERS

在所有可能用于高速船舶的候选推进器中，部分浸没式通风螺旋桨（PSVP）理论上具有最高的推进效率。原因有二：1)在50年代初，Marshall Tulin发明并提出了L/D非常高的叶型形状，如10 ~ 15；2)由于PSVP的性质，传动轴和轮毂在水面以上，因此不受水动力阻力的影响。然而，上述性质也是PSVP结构的一个关键缺陷；在每一次旋转中，每片叶片都进入水面并离开水面，造成巨大的机械振动，从而损坏PSVP及其支撑系统。正是由于这个原因，迄今为止，psvp的成功使用速度仅为50节。土林的叶片截面需要有一个锋利的前缘和薄的叶片厚度，以实现高的L/D值。在这种恶劣的条件下，它自然不是一个最好的，坚固的刀片。在本文利用二维叶片进入水箱的研究工作中，发现了以下事实：1)叶片的振荡特性与分离的自由流的波的振荡特性相匹配，因此这些波不是毛细波或重力波，而是由叶片的振荡产生的波。2)基于典型叶片几何形状、叶片或波浪频率以及叶片进入速度的斯特劳哈尔数在所有测试情况下保持不变。虽然需要对许多其他参数进行更详细的实验，但似乎存在这种类型的入水叶片的特定斯特劳哈尔数。后一事实在设计船舶PSVP推力系统时很重要；通过考虑PSVP的斯特劳哈尔数与螺旋桨轴，轮毂和/或船尾结构的固有频率，可能很有可能避免海船的严重振动问题。

Among all other candidate thrusters possibly used for high-speed sea crafts, a partially submerged ventilated propeller (PSVP) is said to have the highest propulsive efficiency theoretically. There exist two reasons: 1)a blade profile shape having a very high L/D such as 10〜15 was invented and proposed by Marshall Tulin in the early 50's, and 2)owing to the nature of PSVP, the propeller shaft and hub are above the water surface, and therefore are not subject to the hydrodynamic drag. However, the above nature also acts as a critical deficiency on the structure of PSVP ; at every rotation each blade enters into water surface and exits, causing a substantial mechanical vibration and thus damage to PSVP and its supporting system. It is for this reason that PSVPs are successfully used only up to 50 knots to date. The Tulin's blade section needs to have a sharp leading edge and thin blade thickness to achieve a high L/D value. It is naturally not a best, sturdy blade for use under this type of harsh conditions.During the research work conducted herein with use of two-dimensional blades entering into water tank, the following facts have been found. 1)The blade oscillatory characteristics matches with that of the waves of the separated free stream so that these waves are not of capillary or gravity type but those generated by the oscillation of the blade. 2)The Strouhal number based on the typical blade geometry, blade or wave frequency, and the blade entry velocity remains the same for all the cases tested. Although more elaborate experiments over many other parameters are needed, it seems that there exists the specific Strouhal number for this type of water entry blade. The later fact is important in designing a PSVP thrusting system for a sea craft ; it may be quite possible to avoid a severe vibration problem of the sea craft by considering the Strouhal number of the PSVP against the natural frequency of the propeller shaft, hub and/or the stern structure.