Three principle measures of reducing the vibration levels of the tank bulkheads were identified. 1. High propeller excitation, a result of poor flow around the aft ship, could be reduced by fitting vortex generators upstream of the propeller. These vortex generators will mix the high energy flow, away from the ship, with the low energy boundary layer flow. As such, it will increase propeller inflow velocities and reduce the cavitation and consequently the radiated hull pressures. An added significant benefit of this strategy would be the reduction of vibration levels in other parts of the ship. The optimum location, shape and size of vortex generator can be determined with Computational Fluid Dynamics. An example of such an optimization is shown in Figure 10 where the stagnant flow from a cooling water outlet is forced to outside the propeller inflow by an appropriately placed vortex generator. Cooling water outlet and original location of vortex generator Velocity deficit as a fraction of the free stream velocity Optimized location of vortex generator Slow flow moves into propeller disc and is deflected by the optimised vortex generator Fig. 10 The cooling water outlet causes slow (poor) inflow into the propeller plane and results in high pressure pulses. Optimization of the vortex generator location improves inflow conditions and reduces propeller excitation 2. The transmission of energy from the propeller through the aft peak tank could be reduced by increasing the local natural frequencies of the aft peak tank structure beyond the fourth and possibly the fifth blade rate; an increase in frequency of the order of 50%. It is expected that this would require a significant amount of steelwork. 3. The response of the bulkheads could be reduced by increasing the local natural frequencies of the water tank beyond the fourth and possibly fifth blade rates and or reduce the mobility. This also would require a significant amount of steelwork. Alternatively the amplitude of excitation could be reduced by fitting dampers to the tanks. These could possibly be mounted internally, spanning between the walls. Case Study 2 – Navigation bridge vibration A tanker, such as shown in Figure 11, suffered from high vibrations of the navigation bridge and in particular the bridge wings. At several shaft speeds vibration levels exceeded the ISO6954 - 2000 standard [1] and it was understood that the ship’s officers found it difficult to make chart corrections at these speeds. In addition, concerns existed that electronic components on the bridge deck might fail as a result of excessive vibration. The distribution of vibration energy of the navigation bridge wing during a shaft speed run up is shown in Figure 12. Vibration energy is confined to harmonics of shaft rate, in particular 6x and 8x shaft rate. The 6x shaft rate vibration coincided with the principal excitation frequency of the 6 cylinder diesel main engine whereas the 8x shaft rate coincided with the twice blade frequency of the four bladed propeller. Levels of engine vibration were within acceptable limits of [3] and accordingly, excitation from the main engine was not considered excessive. The maximum measured hull pressure excitation from the propeller was also within acceptable limits. 286
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