A Wave energy dissipated due to geometry

AI Thread Summary
The discussion centers on modeling energy dissipation in wave propagation through geometrical configurations, specifically comparing 2D and 3D scenarios. In 2D, wave energy density decreases with the inverse of the radius, while in 3D, it decreases with the inverse of the radius squared. The energy distribution for 2D is over the circumference of a circle, and for 3D, over the surface area of a sphere, leading to different intensity dependencies. The conversation also touches on the concept of dissipation, clarifying that mechanical waves cannot travel indefinitely due to energy conversion to other forms, typically heat. Participants seek further materials and equations to understand the relationship between geometry and energy dissipation more deeply.
miraboreasu
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Hey

Condition 1:
A 2D infinite plane and there is a circular hole in the middle. When t=0, an impulsive loading, P=f(t), is applied to the boundary of the circle(outward), so the wave will start at the boundary of the circle and propagate in the plane

Condition 2:
A 3D infinite plane and there is a circular hole in the middle. When t=0, an impulsive loading, P=f(t), is applied to the boundary of the sphere(outward), so the wave will start at the boundary of the sphere and propagate in space

During the wave propagation in the plane, assuming there is dissipation (actually I don't know what caused it, thermal maybe, but I think in the world, waves cannot travel endlessly, and I only need to know from the geometry perspective). How to model the energy dissipated regarding the geometry? Basically, the modeling for the energy dissipated during 2D propagation in a plane and 3D spherical propagation?

Thanks
 
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miraboreasu said:
but I think in the world, waves cannot travel endlessly
uh, no, EM waves (light for example) can travel endlessly (assuming they don't run into anything). I think you probably mean that MECHANICAL waves can't travel endlessly.
 
In 2D the energy density falls in proportion to the inverse of the radius.
In 3D the energy density falls in proportion to the inverse of the radius squared.
 
Further to post #3, the material has a finite thickness, so the initial intensity can be found. For 2D the energy is then distributed over the surface of a cylinder of increasing radius, giving a 1/2 pi H R dependency for the intensity. For the 3D case, the energy is distributed over the surface of a sphere of increasing radius, giving a 1/4 pi R^2 dependency. For the case of dissipated energy, this falls exponentially with distance, so in the case of microwaves passing through water vapour, for instance, we would simply apply a factor in decibels per metre.
 
Baluncore said:
In 2D the energy density falls in proportion to the inverse of the radius.
In 3D the energy density falls in proportion to the inverse of the radius squared.
Thanks, can you please provide more materials, so I can take look at the derivation? I think this is coming from area and volume, but I want to see the details,
 
tech99 said:
Further to post #3, the material has a finite thickness, so the initial intensity can be found. For 2D the energy is then distributed over the surface of a cylinder of increasing radius, giving a 1/2 pi H R dependency for the intensity. For the 3D case, the energy is distributed over the surface of a sphere of increasing radius, giving a 1/4 pi R^2 dependency. For the case of dissipated energy, this falls exponentially with distance, so in the case of microwaves passing through water vapour, for instance, we would simply apply a factor in decibels per
Thanks, can you please provide more materials about this dependency?
 
miraboreasu said:
Thanks, can you please provide more materials, so I can take look at the derivation? I think this is coming from area and volume, but I want to see the details,
As the wave energy radiates, it is distributed over an ever-increasing line in 2D, or area in 3D.

As the radius of a circle increases, so does the circumference.
C = 2π · r

As the radius of a sphere increases, the surface area increases as the square of the radius.
A = 4π · r²
 
phinds said:
uh, no, EM waves (light for example) can travel endlessly (assuming they don't run into anything). I think you probably mean that MECHANICAL waves can't travel endlessly.
Yes, sorry I miss this part, assume the mechanical wave in a 2D and 3D elasticity pane and space
 
Baluncore said:
As the wave energy radiates, it is distributed over an ever-increasing line in 2D, or area in 3D.

As the radius of a circle increases, so does the circumference.
C = 2π · r

As the radius of a sphere increases, the surface area increases as the square of the radius.
A = 4π · r²
Thanks, is there any equation or material showing how C and A in the energy dissipation term? Like the whole form.
 
  • #10
Hi @miraboreasu. There may be a language problem causing some confusion. Maybe this will help...

Dissipation = the wave's energy being converted to other forms of energy (usually heat).

For example, for an EM wave traveling through free (empty) space, there is no dissipation; the total energy in the wave remains constant. When the wave spreads out as it travels, its energy gets ‘spread out’ - but the total wave energy remains the same.

In simple geometry, a 2D plane is a flat surface (e.g. a table top) of zero thickness. (There are other meanings at more advanced levels.)

In simple geometry 3D, means ordinary space. (There are other meanings at more advanced levels.) Therefore we don’t refer to a ‘3D plane’ (unless maybe you are a mathematician working in 4 or more dimensions!).

A wave in 2D would be confined to a surface. The nearest real-life example I can think of is dropping a stone in a pond giving waves which spread outwards on the water’s surface.

A wave in 3D occupies space – e.g. sound waves when you speak, with the energy spreading out in all possible directions.

For a brief intuition about the inverse square law, you could read ‘The fable of the butter gun’ here: https://www.khanacademy.org/science...ce-and-electric-field/a/ee-inverse-square-law.

Edit: you might find it useful to do a little research of your own on the 'intensity' of a wave.
 
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