Research On Fluid Noise Generation Mechanism Of Ship Flexible Rudder

Fluid noise is one of the main sources of ship noise，and with the significantreduction of machinary noise and propeller noise， fluid noise is becomingincreasingly prominent. This paper was focused on the2-D flow noise problemaround rudder. The unsteady flow phenoma around the rigid rudder and flexiblerudder were studied，then the generation mechanism of flow noise were analysed.With the interaction between fluid and rudder taken into account，The influence ofrudder flexible deformation on fluid sound source was analysed. The soundpropagation characteristics in space and the sound spectrum characteristicsgenerated by the flow around rudders were discussed as well，and the influence ofrudder deformation on far-field noise was further analysed.Firstly，the large-eddy simualtion method was applied to simulate the unsteadyflow around a NACA0018airfoil. In different attack angles the flow field structurenear rudder wall was captured relative accurately，and the simulation results agreedwell with experiment. Based on the unsteady flow field results，the FW-H equationwas used to caculate the sound level at monitor point in far field， and thecharacteristic frequency aditified in experiment was discerned in the soundfrequency plot. The flow noise algorithm was confirmed by the agreement betweensimualtion results and experimental results.Secondly，the unsteady flow fileds around a rigid rudder in different attackangles and inlet velocitise were simulated using large eddy simulation. The sheddedvortexse on suction side and pressure side left the rudder trailing edgequasi-periodically. In large attack angle conditions，the boundary layer near thesuction side of developed in a form of laminar flow，transition，turbulent flow. Thevortexes in turbulent boundary layer were found coalesed because of the speeddifference. The main frequencise of wall pressure fluctuation were below2000Hz，and with attack angle increasing the main fluctuation frequency decreased，while thefluctuation amplitates increased gradually. With the inlet velocity increasing boththe fluctuation frequencise and amplitudes increased.Then，the rudder-fluid coupling model was established，and based on theweak-coupling method and large eddy simulation method the unsteady flow fieldsaround the flexible rudder were simulated in different working conditions. Under theaction of diffrential pressure between two sides，the flexible rudder was deflected，meanwhile，the trailing edge vibrated constantly with very small amplitude excited by the shedding vortexse.0degree was found to be a special sttack anlge，in thiscondition vortex shedding was a process with strict period time，and the vortexshedding frequency agreed well with the wall pressure fluctuation frequency. Theflow separation was controlled by the rudder flexible deformation，thus the turbulentarea and the wake turbulence were reduced，moreover the supression effect of rudderflexible deformation on flow separation became more evident with attank angleincreasing，and the the pressure fluctuation amplitude was reduced apparently.Finally，on the basis of numerical unsteady flow field results，the far-fieldsound propagation generated by flow around rigid rudder and flexible was caculatedrespectively employing FW-H equation which the arbitrary movement of structureboundary in flow has been taken into account，and the surface dipole distribution，the propagation characteristic of far-field noise was acquired for rigid rudder andflexible rudder separetely， then they were comapared. The surface dipoledistribution in transition area was found to be strongest，and that in the turbuelntarea was stronger than that in laminar，and comapared with rigid rudder the surfacedipole source was reduced obviously by the flexible deformation of rudder. The flownoise propagation in space was presented as dipole characteristic，and overall soundpressure level in chord line direction was minmum. The spectrum characteristic offar-field noise was similar to that of pressure fluctuation.