Trans RINA, Vol 153, Part A4, Intl J Maritime Eng, Oct-Dec 2011 PROPELLER CAVITATION AND INDUCED VIBRATION
C Leontopoulos, S K Lee and L Karaminas, American Bureau of Shipping, Greece (DOI No: 10.3940/rina.ijme.2011.a4.222) SUMMARY
The demand to increase the efficiency of propellers has led to optimized propeller blade designs finding their way into the construction of high-powered commercial vessels, such as containers or LNG carriers and certain categories of passenger vessels, to mention but a few. It has become increasingly common to see the propeller tip rotate closer to the hull surface, sweeping the thick turbulent boundary layer attached to the hull, causing fluid structure interaction. At the same time, increasing the loading on marine propellers can lead to problems, such as noise, hull vibration, and cavitation. The degree above which, such phenomena as propeller cavitation can be the main perpetrators for intensive vibration during operation, their diagnosis and the solutions to mitigate this risk, such as the use of vortex generators, are discussed here, taking into account cost and longevity of the vessel as well as the involvement of classification rules.
NOMENCLATURE q
ρ Z
ω Rp Vqz
the harmonic order (blade passing frequency) fluid density(kg/m3) No of blades
angular velocity (rad/s) distance from the field point (m)
the qzth magnitude of the harmonic component of the cavity volume (m3)
RMS Root Mean Square 1
INTRODUCTION
The unsteady inflow to the propeller blade while passing through a non-uniform ship wake can cause alternating pressure
profiles on the propeller blade surface. A
decrease in blade pressure to a level below vapour pressure causes the water to boil locally on the propeller blade, so that the phenomenon of cavitation occurs.
Propeller cavitation, as a form of hydrodynamic instability, has been emerging as a serious problem for some types of commercial vessels, such as containers and LNG vessels for both the ballast and –less so- for the fully loaded condition, causing erosion of the propeller blade, pressure fluctuations on the stern hull, underwater noise and structural vibration. These problems highlight the need for Computational
detailed flow analysis based on both Fluid Dynamic analyses (CFD) and
reliable experimental measurements (model tank tests or cavitation tunnel tests) when designing the geometrical shapes of modern propellers. Literature survey shows that propeller cavitation studies are still very current research topics in most related technological institutes and universities. Experience has shown that a direct correlation between excitation forces predicted on the basis of model tank tests and pressures measured on full- scale ships is not always successful because the latter are themselves heavily influenced by the hull vibrations. In other words, one has to predict or measure not only the pressure fluctuations onto the hull but involve the “hull impedance” concept, see Gent W.V.[1], in order to obtain more
accurate strength of the cavitation depends on the total volume of
cavitation and its dynamic behaviour, which, in turn, involves matching the timing of the actual pressure pattern with the pressure pattern observed in the model tank test. Any difference in the timing of these two patterns, i.e., between the tank test and in the real full- scale, can cause underestimation of the cavitation strength of the values of the maximum pressure pattern that the hull of the vessel is subjected to, see Vrijdag A et al, [2].
With respect to the CFD analysis of the wakefield, the prediction of the propeller pressure patterns depends very much on the “wake quality”. If the wake quality is good and predictable, this allows the designer to push further on propeller energy-saving design under the condition of appropriate control for propeller cavitation. If the wake quality is low, the risk of propeller-induced vibration will be higher and wake smoothing devices such as vortex generators or equalizing ducts may have to be adopted to remedy the problems, see Lee, S.K. et al [3], Liao, M, Wang, S, Propeller-Induced Hull Vibration – Analytical Methods, ABS Technical Papers 2006, Li, JY, et al [4].
There have been a number of recent cases cited by classification societies where high vibration levels at the aft end of newly built vessels have been observed when assessed against industry guidelines. Nearly all propellers can more or less be subjected to some degree of cavitation or hydrodynamic instability under
various
wakefield conditions. Although these phenomena are to be addressed at an early design stage of the propeller to ensure that its geometry minimizes such problems, there is, unfortunately, no clear design criteria or limits above which one can guarantee that propeller cavitation will not occur and will not cause severe vibrations during the operational life of the vessel, let alone the case of propeller blade self-erosion.
predictions. Furthermore, the
When a specification contract for a newly designed vessel is signed between two parties, such as shipyard and owner, most of the times, there is no specific requirement regarding the amount of acceptable propeller cavitation that the vessel can sustain.
Classification societies have yet to introduce rules or limits, other than
©2011: The Royal Institution of Naval Architects
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