strange contour results of prescribed displacement

[Anonymous]

Hi, 

I am trying to conduct a simple linea static simulation on a part(Fig.1). The part is a small cubic truss structure made of ABS plastic.

After specifing a 1mm displacement on the top surface and fix the bottom, I run the simulation, it gives out a strange contour plot(Fig. 2). It seems that the deformed shape as well as the von mises stress are normal, only the red streamlines above the model are unexpected. Does anyone know what's wrong here? Thanks in advance!