Isentropic flow is assumed when the flow is at high velocity through a small orifice. This holds for the flow through the two ring gaps at all times. It is also true for the flow over the rings when the ring lifts off of the seat. For example, if the top ring were "airborne", there would be a small opening connecting zones 1 and 2 and another small opening between zones 2 and 3. The flow in this case will move quickly because of the lower restriction between zones 1 and 3, so it is assumed to be isentropic.

The governing equations for this flow are as follows:

(9–4)

for the mass flow rate where C _d is the discharge coefficient, A _g is the cross-sectional area available to the flow, ρ is the local gas density, c is the local speed of sound and η is defined as

(9–5)

where γ is the ratio of specific heats (usually 1.4) and the subscript, ‘i’, on the pressure terms denotes the region where the properties are calculated, with acceptable values running from 1 to 4.

Equation 9–4 is equation (8) and Equation 9–5 is equation (2) in Namazian and Heywood [62] .