
57Physical Foundation of Interior Ballistics
shear stress and deformation rate. Another distinction has to be made between uids with
respect to the density. If the density is considered constant in a uid or solid that we model,
we call this material incompressible. If the density varies, we must analyze the problem with
the assumption of compressible material. This has many ramications. The most signicant
ramication is that if the material is incompressible, then the energy equation is decoupled
from the momentum equation and we can solve them independently [25]. This makes prob-
lem solving much simpler. We do not have this luxury when t ...