moving boundary layers...... A piston moving in a cylinder driven by a motor force the differential pressure between the two piston's surfaces is about 20 mbar my question is about the leak through the piston/cylinder clearance , some experiments shows that the amount of leak is higher when the piston is moving upward compare to the amount of leak when the piston moves downward ………. why????????? Thanks

The high pressures produced at the piston surface when it goes upwards surely enhances the loss. When the piston goes downwards a decreasing of pressure is being produced locally at the piston surface. So that, the losses are smaller.

What do you think?