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Assing a sinusoidal body force F(x,y,z,t) in comsol 4.3

Posted Oct 18, 2012, 10:31 a.m. EDT Geometry, Materials, Modeling Tools & Definitions, Parameters, Variables, & Functions Version 4.2, Version 4.2a, Version 4.3, Version 4.3a 3 Replies

Alexia Giannoula
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Dear all,

I am trying to simulate shear wave propagation in tissue using a PDE wave equation (I manually defined the equations).

In my geometry, I have a 3D box (imaging it is tissue) and in the center of the box (point(0,0,0)) there should be a sinusoidal force of the form:

F(x,y,z,t) = force(x,y,z)cos(2pi*df*time)

which means that at every point (xi,yi,zi) in the neighborhood of point(0,0,0) a force is applied that depends on time.

My question is how to define that in Comsol ?

what I did so far, was to define an Interpolation function force(x,y,z), where I load the values of the force at time t=0 from a txt file.
I also define a pfunction = pforce(t) = cos(2pi*df*t)

and then in the physics, I select point (0,0,0) and I assign it a Point Source, where I give it the value:

pforce(t)*force(x,y,z).

I am thinking that by this way, in every point around the center, I will be having a sinusoidal force excitation.

Does this seem correct? or if not, can you suggest another way of doing so?

Many thanks

Alexia
3 Replies Last Post Oct 19, 2012, 6:23 a.m. EDT
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago Oct 18, 2012, 1:27 p.m. EDT
If you apply the force at a Point as you described it will only be applied at that one point. The x, y and z coordinates of the point (0, 0, 0) will be used to evaluate the “force” function.
If you apply the force at a Point as you described it will only be applied at that one point. The x, y and z coordinates of the point (0, 0, 0) will be used to evaluate the “force” function.
Alexia Giannoula
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Posted: 1 decade ago Oct 19, 2012, 3:57 a.m. EDT
Thank you very much for the response.

you are right, I understand that's wrong to define a point source like this.

Would a correct way be to define as the "source" of the PDE equations (for the domain that corresponds to the 3D box) the same function, pforce(t)*force(x,y,z)? If I did that, I assume it would mean that all points xi,yi,zi within the extents of the force distribution would act as a source?

another way that I thought, was to leave the "source" term for this domain = 0, and then go to PDE -> right click -> add Source and there to define the equation above?

which way do you think is correct? or are they both equivalent?


finally, another question, is there any way that I can define that this force has a direction along the z-axis? how can I do that by using PDE general form?

thanks in advance for your help

Alexia
Thank you very much for the response. you are right, I understand that's wrong to define a point source like this. Would a correct way be to define as the "source" of the PDE equations (for the domain that corresponds to the 3D box) the same function, pforce(t)*force(x,y,z)? If I did that, I assume it would mean that all points xi,yi,zi within the extents of the force distribution would act as a source? another way that I thought, was to leave the "source" term for this domain = 0, and then go to PDE -> right click -> add Source and there to define the equation above? which way do you think is correct? or are they both equivalent? finally, another question, is there any way that I can define that this force has a direction along the z-axis? how can I do that by using PDE general form? thanks in advance for your help Alexia
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Oct 19, 2012, 6:23 a.m. EDT
Hi

try to avoid applying loads or constraints on anything lower than "Boundaries" (<1 dimension below your "Domain" entity check the doc or see

www.comsol.eu/products/multiphysics/glossary/

for the definitions)

This would avoid "singularities". Then when you do transient analysis do not forget to first set our initial conditions correctly (i.e. solve a stationary case first) and then apply temporal BC loads such that you know how many "modes" you excit:e use smooth step functions with corresponding "fine" time stepping

--
Good luck
Ivar
Hi try to avoid applying loads or constraints on anything lower than "Boundaries" (

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