[Paraview] Computing a normal derivative of velocity

[Adriano Gagliardi]

'Generate Surface Normals' requires a vtkPolyData as input. Run 'Extract
Surface' which will give you a vtkPolyData, which you can generate surface
normals from.

Running GradientOfUnstructuredDataSet on a scalar will give you a vector.
You may have to compute the velocity magnitude first and store it as a
variable before trying to compute the gradient. You can do this using the
calculator with a function 'mag(v)' naming it something creative like say
'mag_v', then selecting mag_v as your input to the gradient computor.

Similarly, to compute the dot product of two vectors, in the calculator it
is simply '(Normals).(grad_mag_v)'.

===================================

Adriano Gagliardi MEng PhD
Business Sector Leader
Computational Aerodynamics
Aircraft Research Association Ltd.
Manton Lane
Bedford

Tel: 01234 32 4644
E-mail: agagliardi at ara.co.uk
Url: www.ara.co.uk 
-----Original Message-----
From: paraview-bounces at paraview.org [mailto:paraview-bounces at paraview.org]
On Behalf Of Martin Vymazal
Sent: 31 August 2010 19:18
To: paraview at paraview.org
Subject: [Paraview] Computing a normal derivative of velocity

Dear ParaView users and developers,

  I have an output file of a Finite Element code, which contains a fluid
velocity vector field. I would like to plot the values of dv/dn on the walls
of my domain. My idea was this: plot a gradient of velocity magnitude,
generate surface normals and then compute the scalar product (grad(v),n).
Unfortunately, I have several problems:  
when I plot the gradient of velocity
(Filters->GradientOfUnstructuredDataSet), I have a scalar field of gradient
magnitude (I guess), but no vector field, even though mathematically
speaking, grad(scalar) results in a vector. Could you tell me how to
generate a vector field where the vector components are
(grad(v)_x,grad(v)_y,grad(v)_z) ?

Second, I don't understand why Filters->Generate Surface Normals is disabled
when I load my data file (in Ensight format). The only way of plotting some
normals is using 'glyphs', but these glyphs are then not available in the
'Calculator' filter which I wanted to use to compute the final scalar
product (grad(v),n). One strange thing I also noticed is that the glyphs are
not plotted on a part of the domain boundary (which is smooth).

Could you please give me a hint how to obtain the plot of dv/dn ?

Thank you.

  Martin Vymazal

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