3.3.1.1 Additional note on numerical simulation

= 0.5

Figure: Two streams of fluids into a medium

The solution of momentum equation for certain situations may lead to unstable solution. Such case is the two jets with deferent velocity flow into a medium and they are adjoint (see Figure ). The solution of such flow can show that the velocity field can obtain unstable solution for which the flow moderately change to became like wave flow. However, in many cases this flow is turnout to be full with vortexes and such. The reason that this happened is the introduction of instabilities. Numerical calculations intrinsically are introducing instabilities because truncation of the calculations. In many cases, these truncations results over-shooting or under-shooting of the nature instability. In cases where the flow is unstable, a careful study is required to make sure that the solution did not produce an unrealistic solution in which larger or smaller than reality introduced instabilities. An excellent example of such a poor understating is a work made in EKK company [#!poro:ekk!#]. In that work, the flow in the shot sleeve was analyzed. The nature of the flow is two dimensional which can be seem by all the photos taken by numerous people (staring from the 50's). The presenter of that work explained that they have used 3D calculations because they want to study additionally the instabilities perpendicular to the flow direction. The numerical ``instability'' in this case are larger than real instabilities and therefore, the numerical results shows phenomena not exist in reality.