Please consider a U-tube filled with an incompressible fluid as in the attached figure. Piston P divides the fluid in two segments. When P moves, the fluid particles on immediate vicinity of either face (points marked 1 and 2) will have same velocity. Does this mean, they may considered to be the same point on a streamline ? Or, can the piston be neglected and the entire fluid mass be considered continuous from fluid mechanics point of view ?