Next: Reverse transition from turbulent
Up: 3.3.1 Is the flow
Previous: A note on numerical
= 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 overshooting or undershooting
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.
Next: Reverse transition from turbulent
Up: 3.3.1 Is the flow
Previous: A note on numerical
Contents
Genick BarMeir  www.potto.org
copyright Dec , 2006
The pdf version is also available here