Dear Emilie, Evan and Simone, as Simone was pointing out, the vmtk script computes wall shear rate, rather than WSS.
It actually does so through its definition (the contraction of the velocity gradient tensor with the surface normal), by *solely* by performing derivatives of the velocity field - so no pressure or viscosity involved, since we're talking about derivatives of the velocity field, and the units for the WSR are consistent with whatever units your velocity is in. A caveat: the mesh units must be consistent with the velocity units (mesh in mm, velocity in mm/s, mesh in cm, velocity in cm/s). If they aren't, you have to account for the scaling factor. Paraview computes gradients by taking partial derivatives using the shape functions of each element (which is in general linear), computing a derivative value for each element and obtaining a nodal value by averaging over the cell neighborhood. vmtk computes gradients by performing an L2 projection using a finite elements discretization. Intuitively the problem is: find the nodal values such that the integral over each element equals the integral obtained by evaluating derivatives within the element at each nodal point. I hope this provides some clarification, in any case I'm here for further information. Luca On Jun 6, 2013, at 10:31 PM, Emilie Sauvage 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 >>>> >>>> >>>> ------------------------------------------------------------------------------ >>>> How ServiceNow helps IT people transform IT departments: >>>> 1. A cloud service to automate IT design, transition and operations >>>> 2. Dashboards that offer high-level views of enterprise services >>>> 3. A single system of record for all IT processes >>>> http://p.sf.net/sfu/servicenow-d2d-j >>>> _______________________________________________ >>>> vmtk-users mailing list >>>> vmtk-users@lists.sourceforge.net >>>> https://lists.sourceforge.net/lists/listinfo/vmtk-users >>> >>> >> > > > > ------------------------------------------------------------------------------ > How ServiceNow helps IT people transform IT departments: > 1. A cloud service to automate IT design, transition and operations > 2. Dashboards that offer high-level views of enterprise services > 3. A single system of record for all IT processes > http://p.sf.net/sfu/servicenow-d2d-j > _______________________________________________ > vmtk-users mailing list > vmtk-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/vmtk-users ------------------------------------------------------------------------------ How ServiceNow helps IT people transform IT departments: 1. 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