The initial effect of the impact force patterns on the object for achieving stably grasping is dependent on starting of the movement of a robot hand by taking an optimal preshape. These impact force patterns generated from the changes of the momenta obtained from the fingers lead to motion tendencies of the object. Determining optimal preshape of hand is closely related with the continuum between the initial position of hand preshape and posture of the hand at the contact points on the object. In this paper, we propose a new model for determining continuity momentum of fingers landing on the object based on the fluid dynamics. The commercial computational fluid dynamic (CFD) software package program, Fluent, is used to simulate the compressible fluid flows. The kinematic model of robot fingers and object geometry is created as a preprocessing of fluent. Various scenarios of simple hand preshaping have been applied to the simulation. The experimental results give an idea about how different preshape hand effects to detect the region of contacts while approaching onto an object.