[Getdp] Obtaining quantities during postprocessing

[Timothy Atherton]

I have a problem to be solved in Getdp that is very simple from the  
point of view of the differential equation concerned --- it is merely  
Laplace's equation with a scalar potential phi. Indeed my mesh and  
solution are already obtained.

However, I need to compute from the calculated field phi a rather  
complicated quantity. Defining by analogy to electrostatics E = -  
Grad phi, I wish to know (E . Grad) E or more explicitly:

{ Ex * Dx (Ex) + Ey * Dy(Ex) + Ez * Dz(Ex), Ex * Dx (Ey) + Ey * Dy 
(Ey) + Ez * Dz(Ey), Ex * Dx (Ez) + Ey * Dy(Ez) + Ez * Dz(Ez) }

where by Dx() I mean the x derivative and by Ex I mean the x  
component of E and similar.

This is quite a complicated quantity so I tried to evaluate simpler  
parts in the Postprocessing part of the .pro file. In particular, I  
discovered I could not take multiple derivatives of fields:

e.g. I could write in the postprocessing->Quantity clause

{ Name hdhy ; Value { Local {[ ********* ] ; In Domain ; Jacobian  
JVol ; } } }

where the asterisks were
{- Grad phi}
or
CompX[{Grad phi}]

but not
{CompX[-Grad phi]} ---- so I guess you can't use component  
expressions inside these field quantity brackets

and moreover not
{Grad CompX[-Grad phi]} or anything similar

and surprisingly not even
{Div Grad phi}

So.... is there any way to calculate this sort of quantity (involving  
higher derivatives) without exporting the solution to an application  
such as Mathematica and constructing an interpolating function.

I'd be very grateful for any advice.

Dr Tim Atherton
Dept of Physics
Case Western Reserve University