This is the case when the flow is relatively slow. This occurs when the flow is into or out of regions 2 and 4, with the ring sealing one of the upper or lower surfaces of the groove in the piston. In essence, these two zones have one inlet unless the ring is not touching a surface, and therefore offer higher resistance to the flow.

In Ansys Forte, the isothermal flow is also considered to be laminar due to the small gaps in the rings. The governing equation in this case reduces to

(9–6)

where A is the area perpendicular to the mass flow, h is the distance between the ring and the ring groove, the subscripts u and d represent upstream and downstream pressure, W _r is the width of the ring (the distance from the wall to where the ring touches zone 2), μ is the gas viscosity, R is the ideal gas constant, and T is the temperature of the gas.

Equation 9–6 is reproduced from (9b) in Namazian and Heywood [62] .