[FLASH-USERS] 2D disk models

[Tomasz Plewa]

Peter -

Keeping all forces together is a good first approximation. It might be  
done somewhat better if more force-specific information is available  
than for gravity.

Tomek
-- 
School of Computational Science, DSL 443       ph:  850.644.1959
Florida State University                       fax: 850.644.0098
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Peter Woitke wrote:

> Dear Tomasz,
>
>> The simplest recipe is to advect the gas specific angular momentum. This
>> is quite similar to advecting mass scalars (individual species) and you
>> can use one of the mass scalars for that purpose. (I hope mass scalars
>> are supported in the official 2.5 version.) Once you know actual
>> distribution of specific angular momentum, centrifugal force can be
>> easily calculated.
>
> thank you very much for this comment! In fact, in (r,z,phi) cylinder
> coordinates with axisymmetry (d./dphi=0), the phi-component of the
> equation of motion is equivalent to
>
>   D/Dt(rho L3) = 0                           (1)
>
> where D/Dt means the Lagrangian derivative, r=sqrt(x^2+y^2), and
>
>   L3 = r vphi,                               (2)
>
> i.e. the angular momentum density rho*L3 in an invariant of the flow. So I
> just have to define and initialize the mass scalar L3 and FLASH will solve
> Eq.(1) automatically for me. The z-comonent of the equation of motion is
> already correct, even with phi-rotation. Only the r-component needs a
> modification, namely the additional source term +rho vphi^2/r (the
> centrigugal force) on the r.h.s.
>
> I will implement this by a modified gravity module which uses L3 as:
>
>   gr = -GM/(r^2+z^2)^(3/2)*r + L3/r^3        (3)
>   gz = -GM/(r^2+z^2)^(3/2)*z                 (4)
>
>> From my experience with radiation pressure as additional force,
> I can confirm that mass scalars work properly with Flash 2.4.
> A modified gravity module might be more accurate, since it is
> called several times during the hydro-integration, than an additional
> force term which would be treated by operator splitting, wouldn't it?
>
> Thank you,
>
>   Peter Woitke
>
>
> ============================================
> Dr. Peter Woitke
> --------------------------------------------
> School of Physics & Astronomy
> University of St Andrews
> North Haugh
> St Andrews KY16 9SS
> Scotland
> UK
> --------------------------------------------
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> Tel.:  (+44) 1334 463068
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> ============================================
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