This model based on numerical simulations based on the following assumptions: 1) the flow is turbulent, 2) turbulence was assume to be isentropic homogeneous every where (k-k- model), 3) un-specified boundary conditions at the free interface (how they solve it with this kind of condition?), and 4) unclear how they dealt with the ``corner point'' in which plunger premiter in which smart way is required to deal with zero velocity of the sleeve and known velocity of plunger.
Several other assumptions implicitly are in that work18such as no heat transfer, a constant pressure in the sleeve etc.
According to their calculation a jet exist somewhere in the flow field. They use the k-k- model for a field with zero velocity! They claim that they found that the critical velocity to be the same as in Garber's model. The researchers have found same results regadless the model used, turbulent and laminar flow!! One can only wonder if the usage of k-k- model (even for zero velocity field) was enough to produce these erroneous results or perhaps the problem lays within the code itself19.