Investigation On The Added-mass For Structures Based On Dynamic Analysis

Vibration problems are very common in practical engineering,some of them may be beneficial or useful for human beings,but usually they are harmful for the structures,which may lead to malfunction and accuracy reduction of engineering machinery.Moreover,additional dynamic stress will exert on the structure and shorten its longevity,sometimes even results in disastrous accident.To effectively reduce the harm caused by vibration,it requires for accurate prediction of the dynamic characteristics of structures.It is not easy in many cases due to the interaction with fluids.When structures vibrate in fluids,it will result in disturbances to the field,and the hydrodynamic pressure will exert on the surface of structures in turn.This lead to the decrease in natural frequency,differences in mode shapes and increases in equivalents damping coefficient.The dynamic characteristic of structures vibrating in fluids is investigated through theoretical,numerical and experimental ways.Based on the assumption that fluids are irrotational,inviscid and incompressible,the velocity potential of fluid domain is obtained.Pressure on the interaction area is exerted on the structure,which leads to the governing equation for Fluid-Structure vibration.The equation reveals that,the impact of fluids mainly lies in the equivalent mass addition to the structure,the added mass.For rigid body which is infinite in length,the added mass is mainly related to fluid width,distance to the free surface and the bottom.As for the one-dimensional elastic beams,slenderness ratio,submergence ratio and mode shapes will have great impact on the added mass.Furthermore,finite difference method is employed to solve the fluid domain,and the influence of fluid-width,fluid-depth and submergence ratio on added mass is analyzed through numerical computation.Thus,based on Lewis' method,the fast prediction formula for added mass of underwater vehicle is proposed.For cantilever beams submerged in fluids,the mode shape is assumed to be the superposition of mode shapes in vacuum,and by using Rayleigh-Ritz method,its natural frequency and mode shapes is obtained.Furthermore,the influence of slenderness,fluid depth and order of modes are discussed and the fast prediction equation is proposes based on the results.To validate the Effectiveness of prediction formula,modal identification methods using strain gauges,which is applicable to underwater structures modal test is deduced based on traditional theories.a simplified experiment is conducted and the results are roughly in line with the trend seen from the prediction value,and the error is within an acceptable range.Moreover,based on the strain gauge test theory,the vibration characteristics of the cantilever beams under the excitation form fluids are studied.In this thesis,dynamic characteristic of structures vibration in fluids is revealed,and the influence of various parameters on added mass is investigated.Base on the results from numerical study,fast prediction formulas to accommodate engineering applications are proposed.Additionally,the method based on strain gauge test proposed in this thesis will be helpful to study the hydrodynamic characteristics of more complex structures.