[MissCFD]

Hello,

I don't understand why in initial conditions the definition of k is:
k , where v is the turbulent velocity scale

instead of:
k , where is the velocity initial guess

What does turbulent velocity mean exactly ? Is that the velocity at which the turbulent eddy turns ? If yes, it doesn't represent the same velocity that averaged flow-field, isn't it ?

[fluid23]

No. You are over complicating it.


The turbulent velocity scale is just some characteristic velocity of your flow. Most commonly an inlet velocity or freestream velocity. Something that helps scale the turbulence parameters for your initial conditions and is representative of your flow in a global sense.

[MissCFD]

I'm not sure I understand very well because in options of initial conditions there are two velocities that we have to need ... ''Turbulent Velocity Scale'' and ''Velocity'' so these are two different velocities , isn't it ? Or ''Turbulent Velocity Scale'' must correspond to inlet velocity magnitude ?

Originally Posted by MBdonCFD

No. You are over complicating it.


The turbulent velocity scale is just some characteristic velocity of your flow. Most commonly an inlet velocity or freestream velocity. Something that helps scale the turbulence parameters for your initial conditions and is representative of your flow in a global sense.

[fluid23]

You don't have to have the same velocity defined for both. They can (and in most cases probably should) be the same, but they let you be flexible. With the velocity value you are actually setting your initial velocity everywhere in your domain. With the turbulent velocity scale you are just giving the turbulence model a way to estimate its initial inputs given the way you chose to specify your turbulence. Basically, it will go and calculate k and e (for k-e model) given your intensity and length scale, or whatever values you define. You aren't actually setting the velocity with this value, just helping the turbulence model calculate your initial values for k and e.

Make sense?

[spartan42]

Originally Posted by MissCFD

I'm not sure I understand very well because in options of initial conditions there are two velocities that we have to need ... ''Turbulent Velocity Scale'' and ''Velocity'' so these are two different velocities , isn't it ? Or ''Turbulent Velocity Scale'' must correspond to inlet velocity magnitude ?

You do not need both conditions. You are just giving some initial conditions to help reach convergence quicker. From the same method you can choose K+Epsilon/ Intensity + Length Scale/ or Intensity + Viscosity Ratio. Depending on what you have you can choose from those 3 Methods. If you choose K+Epsilon then you can just use the formulas that you found to calculate TKE and TDR. For your velocity just use the magnitude. Again is an estimation, its just to initialize the problem.