define underwater explosion by incident wave

hi

i 've modeled a submerged cylinder subjected to underwater explosion.

there is an example of this problem in Abaqus Example Documentation.

 "9.1.3 RESPONSE OF A SUBMERGED CYLINDER TO AN UNDERWATER EXPLOSION

SHOCK WAVE"

i have modeled the problem and analysed  but the U1 Translation Graph for Endcaps is different to original example graph.

the velocity of endcaps during explosion is negative , but for my analys its positive !

i think the problem is related to Amplitude of Incident wave definition.

please help on this problem

thanks alot