Shock wave directivity from cylindrical charges with different casings: case 1 and 2

Directional effects, directivity, in underwater shock waves close to cylindrical charges with different casing configurations are studied experimentally and numerically. The charges are composed of melt cast TNT with length-to-diameter ratio 1:1 and nominal weights of 10 kg. Two casing configurations are studied: no confinement, and with a thin aluminium casing. Pressure is measured at three different radii from the charge centre, and in three different directions. The ALE method in LS-DYNA is used by FOI in the simulations, while the ?SPH method in IMPETUS Afea is used in the simulations by SAAB Dynamics AB. The results indicate that both ALE and ??SPH are feasible options for simulating underwater shocks for the cases studied in this report. ?SPH introduces significant oscillations in the pressure profile following the passage of the shock. The effect of the oscillations on predicting the interaction with a structure should be investigated further. The directivity effects result in 5-10% increase in peak pressure at 70 cm from the centre of the charge, while the impulse and energy flux density instead decreases by a similar factor. At 120 cm from the centre of the charge, the directivity effect is almost negligible. Comparing experimental results and simulations, there are acceptable differences given the uncertainties in the experimental setup. Only minor differences as result of the different casings are obtained.