Twenty-First Symposium on Naval Hydrodynamics (1997) / Chapter Skim
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Coupled Hydrodynamic Impact and Elastic Response
Coupled Hydrodynamic Impact and Elastic Response
Pages 424-437
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The motion of the masses are coupled through the hydrodynamic boundary value problem to form a time dependent hydroelastic impact model. Results applicable to shock mitigation and shock isolation studies are given.
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From page 425... ...
will be used to estimate the impact load acting on a rigid, constant deadrise section. The paper will discuss the significant difficulties associated with solving the fully nonlinear boundary value problem when time dependent spray sheets are present and assumptions based upon similarity flows are no longer valid.
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The integral equation formed from the cylinder kinematic boundary condition is of the Carleman type, which is inverted analytically to produce the contour vortex strength, tic =- 2Vs in O |
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1, 1 1~ 1 ~ ~ = VJ et(t) ~ 1 1 ~ ___U 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Nondimensional Time Figure 4: Impact force ~ F 3 and jet head velocity for a prescribed time varying impact velocity (a)
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These types of tests have been shown to be vital to the ongoing CSS shock mitigation program, and are expected to be useful in conjunction with future shock mitigation investigations. Within the CSS program, the data have been used for initial, qualitative evaluation and identification of shock mitigation concepts, for calibrating and validating the 2-D vertical water-entry model, and for validating evolving 2-D cylinder impact theories The application to qualitative evaluation and identification of shock mitigation concepts is justified in part by the observed similarity between at-sea measurements of high-speed-vessel acceleration time histories, and those of boat-segment drop tests for approximately similar hull geometries and vertical entrance velocities.
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experimental acceleration and the dashed line represents the theoretical equivalent. Generally, the unfiltered data of the constant deadrise section without strakes indicates mechanical frequency components at about 20 Hz and 60 Hz superimposed on the basic hydrodynamic shock record.
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The impact of the hull segment with strakes as shown in Figure 6 is significantly different from the time history shown in Figure 5 An examination of the geometry of the strakes and the boat velocities at water entry indicates that multiple peaks in the acceleration time histories are caused by the strakes entering the water. The drop tests indicate that strakes shorten the duration but increase the amplitudes of the shocks.
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From page 432... ...
single degree-of-freedom impact Before the dynamic effect of an interior oscillating mass is examined, the infinite spring constant case is investigated first. This will provide a basis for describing the effects of mass and stiffness parameter variations on the maximum acceleration, the maximum impact force, and the times to maximum impact force and chines wetting.
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As a approaches zero, which would be the case for lightly loaded hulls, chine wetting does not occur. When a is large, the maximum acceleration coincides with the time of chine wetting.
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1 0. 1 5 0.2 0.25 0.3 0.35 Nondimensional Time Figure 12: Impact acceleraiion iime histories for three spring constants.
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Important factors in the determining the severity of impact accelerations are hull geometry, hull weight, initial impact velocity, and ratios of internal/external masses and system internal stiffness. Generally, light craft experience high accelerations, but low impact forces and heavier, better riding craft experience large local structural loads but low accelerations.
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1 0. 1 5 0.2 0.25 0.3 0.3 5 Nondimensional Time Figure 14: Impact acceleration time histories for three spring constants.
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From page 437... ...
and Kang, C.-G, "Hydrodynamic Impact Loads on Three-Dimensional Bodies." 16th Symposium on Naval Hydrodynamics University of California, Berkeley, 1986.
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