COMSOL navier-stoke equation help

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Discussion Overview

The discussion centers around the application of the Navier-Stokes equations in COMSOL for modeling boundary conditions. Participants explore the feasibility of discretizing boundary conditions into multiple segments and applying different conditions to each segment, as well as alternative methods for defining boundary conditions.

Discussion Character

  • Technical explanation
  • Exploratory

Main Points Raised

  • One participant inquires about the possibility of discretizing boundary conditions in COMSOL and applying different conditions to each segment.
  • Another participant confirms that it is possible to divide the boundary into multiple segments and apply different conditions, suggesting the use of a function of coordinates for more flexibility.
  • A participant mentions a preference for scripting in COMSOL and notes challenges with boundary indices in 2D and 3D geometries.
  • Further suggestions include using logical expressions and step functions to define boundary conditions based on coordinate dependencies, as well as the option to define boundary conditions in a tabular format for interpolation.
  • There is a mention of the possibility of re-defining geometry to partition existing boundaries for applying conditions directly.

Areas of Agreement / Disagreement

Participants generally agree that it is possible to apply different boundary conditions in COMSOL, but there are multiple approaches suggested, and the discussion remains exploratory without a definitive consensus on the best method.

Contextual Notes

Some limitations include the dependence on specific definitions of boundary conditions and the potential complexity of modifying geometries in COMSOL.

gganendra
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Hi everybody, newbie here...:smile:

im doing a modelling work with comsol and have a problem of solving using navier-stoke equation for the boundary. is it possible to discretize the boundary condition into several parts and apply different condition in each segment?

thank you beforehand,
 
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Hi gganendra,

yes - that is possible. Probably the easiest way is to do it like you described, divide the (original) boundary to multiple new boundaries and apply different conditions to each new segment. Alternatively, in Comsol you can define the boundary condition as a function of coordinates along the boundary (you can use either a global coordinate system, or then a local coordinate system going along the boundary) which gives you freedom to define pretty much whatever kind of bcs you desire (non-piece-wise mean).
 
oh so it is possible...because in comsol script (i prefer script rather than multiphysics, it gives more freedom), each boundary is given an indices (fem.bnd.ind field), and each indices apply to each line (2d, or face, 3d) for example in a rectangular geometry and i can't divide the line to several parts

okay then i will try the coordinate system... thank you
 
yeah, if you don't want to modify your model working with coordinate systems is pretty straightforward (or logical expressions -- there is nothing preventing writing the bc as a combination of (something>something) or if(cond,true,false) where the conditions are coordinate dependent. Step functions can also be a nice aid. And actually also defining the bc as a tabular expression [the ability to define functions in a tabular fashion which comsol then interpolates ] where the expression spans the boundary is a pretty elegant way of doing it). If you "partition" existing boundaries by re-defining the geometry then you can naturally apply your conditions straight on those, but that requires some "operating" on the model.
 

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