Dear Emilie,
1) The pressure field information is usually present in the mesh solution input
file, the script uses it without asking for it.
2) The script computes wall shear rate, if you want wall shear stress you have
to multiply wsr with the viscosity value.
3) The scale depends on the measurement units you used in your simulation (it
does not make any conversion).
4) In my opinion if you build vector velocity from components using vmtk script
or using Paraview (u*iHat + v*jHat + w*kHat) the final result is the same. I
don't know which method vmtk/Paraview use, maybe Luca knows it.
Best Regards
Simone
--
Simone Manini, Eng
Orobix Srl
via L.A. Muratori 3, 24123 Bergamo, Italy
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On Jun 6, 2013, at 10:31 PM, "Emilie Sauvage" <emilie.sauv...@uclouvain.be>
wrote:
> Hello Simone and Evan,
>
> Thank you for your reply.
> I tried the function "vmtkmeshwallshearrate", and it seems to work fine. I
> get a nice picture, but unfortunately I don't know what am I looking at.
> With paraview filters, I tried to approximate the following formula:
>
> The traction vector t is defined as
>
> t = [-pI + mu*(nabla(v) + nabla(v)^T]*n // n ... normal to surface
> // I ... unit tensor
> // mu ... viscosity
> // v ... velocity vector field
>
> and wall shear stress is given by
>
> wss = mag( t - dot(t,n)n )
>
> As you can see, it involves also pressure and viscosity of the fluid. I
> have several questions:
>
> 1) The script does not ask for pressure field. Why?
> 2) The script does not ask for the viscosity of the fluid. Why?
> 3) What is the unit in which the WSS is written to the output file? My old
> file obtained with paraview and the WSS file obtained with VMTK show
> somewhat similar color maps, but the scale is completely different.
> 4) To join the request of Evan, could you please please be so kind and
> give some details about HOW are the velocity gradients computed? I need to
> have an idea about the accuracy of the algorithm.
>
> Thank you again for your previous quick reply.
>
> Best regards.
>
> Emilie Sauvage.
>
>
>
>> Hello vmtk users,
>>
>> To piggyback on Emilie's query on computing velocity gradients, is
>> there documentation for either vmtk or Paraview on how they compute
>> gradients in general for unstructured meshes? For instance, do they
>> use RBF-FD methods? I couldn't seem to find anything about the
>> specifics.
>>
>> Thanks,
>> Evan Kao
>>
>> On 6/6/13, Simone Manini <simone.man...@orobix.com> wrote:
>>> Dear Emily,
>>> I think you can use vmtkmeshwallshearrate which computes wsr from a
>>> velocity
>>> field.
>>> It supports any kind of mesh files supported by vmtk.
>>> You need to have the velocity array from components (u v w) so you can
>>> use
>>> vmtkmeshvectorfromcomponents:
>>>
>>> vmtkmeshvectorfromcomponents -ifile input_file -vector Velocity
>>> -components
>>> u v w --pipe vmtkmeshwallshearrate -velocityarray Velocity -ofile
>>> output_file
>>>
>>> Remember that the output file will be a surface file.
>>>
>>> http://www.vmtk.org/VmtkScripts/vmtkmeshvectorfromcomponents
>>> http://www.vmtk.org/VmtkScripts/vmtkmeshwallshearrate
>>>
>>> I hope it helps,
>>> Best Regards
>>> Simone
>>>
>>> --
>>> Simone Manini, Eng
>>> Orobix Srl
>>> via L.A. Muratori 3, 24123 Bergamo, Italy
>>>
>>> orobix: www.orobix.com
>>> home: http://daron1337.github.com
>>> twitter: twitter.com/daron1337
>>> mobile: +39.328.60.22.017
>>> office: +39.035.0296480
>>> +39.035.42131
>>>
>>> "This message originates from Orobix Srl and its contents and
>>> attachments
>>> are privileged and confidential and are intended only for the
>>> individual(s)
>>> or entity(ies) named above. This message should not be forwarded,
>>> distributed or disclosed. Any unauthorized use, dissemination and
>>> duplication is strictly prohibited and may be unlawful. All personal
>>> messages express views solely of the sender, which are not to be
>>> attributed
>>> to Orobix Srl, and may not be copied or distributed without this
>>> disclaimer.
>>> If you are not the intended recipient or received this message in error,
>>> please delete this message and notify the sender by reply e-mail.
>>> Opinions,
>>> conclusions and other information in this message that do not relate to
>>> the
>>> official business of Orobix Srl shall be understood as neither given nor
>>> endorsed by it."
>>>
>>> On Jun 6, 2013, at 6:09 PM, "Emilie Sauvage"
>>> <emilie.sauv...@uclouvain.be>
>>> wrote:
>>>
>>>> Dear Luca,
>>>>
>>>> I would like to ask you if there is a way of computing wall shear
>>>> stress
>>>> from nodal velocity and pressure values. In case this is possible,
>>>> could
>>>> you tell me what input file formats are supported?
>>>> I'm asking this because the flow solver that I use does not output wall
>>>> shear stress. I used ParaView to compute it, but my supervisor
>>>> expressed
>>>> some doubts about how accurate is vtk/ParaView when computing gradients
>>>> of velocity from Finite Element data on unstructured grid. I posted a
>>>> question on vtk mailing list, but did not get any reply. Then I thought
>>>> that maybe VMTK can actually do this ...
>>>>
>>>> Thank you very much.
>>>>
>>>> Best regards,
>>>>
>>>> Emilie Sauvage
>>>>
>>>>
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>>>
>>
>
>
>
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