Sunday, September 30, 2012

Propeller and hull wake resistance

In ship vibration, the propeller is normally the trouble source which can cause an excessive ship vibration problem. The consequences of excessive vibration in the ship's stern area can be very detrimental to the crews' comfort. Deterioration of the structural members can be accelerated as a result of fatigue caused by long term cyclic vibration. Excessive vibration can damage or adversely impact the performance of the ship’s mechanical and electrical equipment. Prolonged exposure to vibration can contribute to crew discomfort and increasing the opportunities for human error.

Increased flexibility of the hull girder of larger, and particularly longer, ships with a fine, underwater form can significantly increase susceptibility to vibration. Moreover, as the weight and distribution of steel within ship structures are optimized as shipbuilders attempt to control production and material costs, the probability for vibration-related troubles, particularly in the stern section of the vessel, increases. As the demand for higher service speeds for many of these vessels also increases, attendant increases in the propulsive power are required. This translates into higher loads on propellers, which in turn lead to greater propeller excitation and an increase in the risk of vibration and vibration-induced failures. These create an intense, fluctuating pressure impact on the ship’s hull.

There is now the availability of modern propeller design, moderate amount of sheet cavitation is often unavoidable in order to maintain the required propulsion efficiency. Reconciling the challenges posed by these conflicting technical and operational demands is essential if further improvements in the speed-power-size ratio are to be realized, particularly for large vessels.

To study and predict propeller-induced hull vibration is not simple. It is a synthetical analysis involving methodologies of many topics such as Computational Fluid Dynamics (CFD), Finite Element Method (FEM), and fluid cavitation dynamics. In propeller induced hull vibration assessment, the prediction of stern flow is central to the problem of unsteady propeller loads, cavitation, and propeller-induced hull pressure. The solution to these problems requires detailed knowledge of the turbulent stern flow (including thick and perhaps separated boundary layers), bilge vorticity, and propeller/hull interaction. Common in ship design, the technology for these predictions was mainly based on regression and empirical formulae. At best, the use of ship flow codes was restricted to potential flow calculation augmented by boundary layer predictions to approximate viscous effects. Propeller calculations were performed using empirically generated effective wakes, and the propeller’s interaction with the hull was approximated with a thrust deduction coefficient.

The use of CFD in ship hydrodynamics has increased in the marine field. This is due to continuous advances in computational methods together with the increase in performance and affordability of computers. Also, due to the emergence of many unconventional propulsor designs such as tractor PODs, tip plate propellers, and propellers with wake equalizing ducts/spoilers, empirical methods based on the historical databases developed for conventional propeller designs become questionable in the innovative designs. More sophisticated analyses based on direct simulation using CFD and FEA methods are required to associate with the model tests for propeller-induced vibration studies.

Nowadays, with advances in CFD techniques, more comprehensive analyses can be performed for propeller/hull interaction flow problems. It has been demonstrated that CFD simulation, particularly using RANS-based methods, can provide more flow details in understanding the complex propeller/hull interaction process. This paper provides an overview of the methodologies and the state-of-the-art computational analysis tools that ABS has developed in order to more accurately estimate propeller-induced hull vibration.

The sequence of comprehensive analyses in the integrated simulation system is summarized as follows:

1. Bare hull wake field (nominal wake) simulation
2. Simulation of wake field under propeller-ship hull interaction (effective wake)
3. Propeller performance analysis (thrust and torque coefficients, K T and K Q )
4. Propeller cavitation analysis (cavity patterns on propeller blades)
5. Hydrodynamic loading assessment (pressure on blades, propeller induced hull pressure and bearing forces/moments)
6. FEM analysis for vibration and stress on ship hull, shaft and propeller blades

Resistance and propulsion summary - With courtesy from Newcastle UponTyne University
Resistance Propulsion

Global vibration on container vessel - courtesy of Delta Marine Engineering

GlobalVibration Container Vessel

Lloyds Register Ship vibration - Courtesy of Llyods Register
LR ShipVibrationNotes

Vibration Analysis and Noise Rev1

Vibration Analysis

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