The Theory Research Of Longitudinal Motion Stability Of High-speed Planing Craft

The porpoising is the motion of pitch coupling heave which can cause the structuredamages of high speed planing craft. Because it has the strongly nonlinear effective such asthe knuckle flow separation when the high speed craft is planing in the calm water, applyingthe nonlinear theory to numerical simulation the porpoising is difficult. The porpoisingcritical parameters of high speed craft is numerical simulated by nonlinear2D+t theory basedon simple Green function. The main work is as following:The nonlinear boundary element method based on simple Green function for the waterentry of a two-dimensional body is developed. The numerical difficulty including thetreatment of the intersection of free surface and body surface, thin jet, iteration of nonlinearfree surface condition, sawtooth instability of free surface, accurate solution of the pressure ofbody surface is solved. The effect of the iteration time step, the size of computational domain,the number of elements on the free surface and the way of the grid on the numerical results isdiscussed. A flow separation is introduced to numerical simulated the water entry of a bodywith a knuckle point. The free surface, pressure distribution, max pressure cofficients andvertical hydrodynamic force of eight wedges without knuckle and two wedges with knuckleare predicted and compared with the other theory results.The nonlinear boundary element method for the radiation problem of the two-dimensionalbody is developed. The far radiation condition is discussed. The effect of iteration time step,the number of elements on the free surface, the size of computational domain, the length ofdamping region and the cycles of smooth on the numerical results are discussed. The addedmass, damping coefficient, second order mean force, second and three order harmonic forceof a wedge, cylinder, ship bow-flare section with shallow and deep draft are calculated andcompared with the experiment results and the other theory results.The nonlinear2D+t theory for plaing craft advancing in the calm water with constantspeed is developed. The free surface and the section vertical force longituidal distribution ofthe planing craft are calculated. The effect of the deadrise angle, trim angle and the advancingspeed on the max section vertical force are discussed. The hydrodynamic lift coefficients ofdifferent advancing speed are calculated and compared with the Savitsky formula. The nonlinear2D+t theory based on the simple Green function for the high speed shipadvancing in the calm water with constant speed and with forced oscillation is developed.The heave and pitch hydrodynamic coefficients of the WigleyI and WigleyIV with differentadvancing speeds and oscillation frequencies are predicted and compared with experimentresults, the frequency domain linear theory results and three-dimensional frequency domainlinear theory results. The criterion of porpoising is given by using linear stability analysis andRouth-Hurwitz stability rule. The porpoising of palning craft with different deadrise anglesand load coefficients by using the criterion of porpoising and nonlinear2D+t theory andcompared with the experiment results and the Savitsky formula. The effect of loadcoefficients, pitch radiua of gyration and deadrise angle on the porpoising critical trim angle,gravity center position and the wet length is discussed.The above research indicates that using the nonlinear boundary element method based onthe simple Green function can predict accurate results of the pressure distribution and freesurface of the body entrying in the calm water. Using the flow separation model caneffectively solve the the water entry of the body with a knuckle point. Using the nonlinear2D+t theory can accurately predict the hydrodynamic coefficients of high speed shipadvancing in the calm water with constant speed and with forced oscillation. Using thenonlinear2D+t theory and the Routh-Hurwitz stability rule can effectively predict theporpoising of the planing craft. The hydrodynamic coefficients of planing craft haverelationship with deadrise angle, trim angle and advancing speed. The load coefficients, pitchradiua of gyration, deadrise angle and gravity center longitudinal position are the mainparameters that influence the porpoising of planing craft.