Possible Solution for a Moving Cylinder of Charge Injected into a Fluid

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Discussion Overview

The discussion focuses on a moving boundary problem involving a charged cylinder injected into a fluid, specifically addressing the mathematical formulation of the problem and potential solutions to the associated partial differential equation (PDE).

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant seeks references for solving a moving boundary problem described by a specific PDE involving a cylinder of charge.
  • Another participant suggests shifting to moving coordinates to simplify the boundary value problem, proposing a new form of the equation.
  • A different participant expresses difficulty with the suggested method, noting that the moving origin complicates the problem further.
  • Another participant mentions that in a 1D or symmetric 3D case, the solution may involve hyperbolic or trigonometric functions, contingent on the initial conditions of the field.
  • One participant indicates interest in calculating the electric and magnetic fields from the moving charged cylinder, mentioning that the 1D case has been solved and they are now exploring the 2D case, which leads to a damped wave equation.
  • This participant is specifically interested in the solution of the potential \varphi outside the cylinder.

Areas of Agreement / Disagreement

Participants express differing views on the effectiveness of the proposed methods for solving the problem, indicating that there is no consensus on the best approach or solution at this time.

Contextual Notes

Some participants highlight the challenges posed by the moving boundary and the implications for the mathematical treatment of the problem, including the dependence on initial conditions and the complexity introduced by the moving origin.

Who May Find This Useful

This discussion may be of interest to researchers and students working on moving boundary problems, fluid dynamics, or electromagnetic fields, particularly in the context of charged objects in motion.

hunt_mat
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Does anyone know any good references for a moving boundary problem? I am looking at a cylinder of charge being injected into a fluid, the PDE is:
[tex] -\nabla^{2}\varphi +a\frac{\partial^{2}\varphi}{\partial t^{2}}+b\frac{\partial\varphi}{\partial t}=0[/tex]
I want [itex]\varphi =\varphi_{0}[/itex], a constant on the moving boundary [itex]x=v_{0}t[/itex]
Can anyone suggest some possible solutions?
 
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Let me start by saying that I could be way off here as I haven't worked with moving boundaries before. However, why don't you shift to moving co-ordinates such that [itex]\eta = x - v_0 t[/itex], then your boundary value problem reduces to

[tex]-\nabla^{2}\varphi(\eta) + v_0\{av_0 - b\}\varphi(\eta) =0\;,[/tex]
[tex]\varphi(0) = \varphi_0\;.[/tex]
 
I tried this method before and I didn't get really far with it. You are still left with the problem of the origin moving away from you and that doesn't really help you much.
 
hunt_mat said:
I tried this method before and I didn't get really far with it. You are still left with the problem of the origin moving away from you and that doesn't really help you much.
I'm not sure I follow. In 1D (or a symmetric case in [itex]\mathbb{R}^3[/itex] which reduces to 1D), you will be left with a family hyperbolic or trigonometric functions, depending on the sign. The remaining constant can be determined by the initial distribution of the field - I assumed that this is given.
 
Last edited:
Initially, I am interested in a calculating the electric and magnetic fields from a cylinder of charge moving at a speed v_0 from a plane at x=0. The 1D case has been solved and some very nice solutions have been obtained and now my colleague and I are interested in the 2D case. We reduced the problem down to a damped wave equation which I thought was rather nice.

I am interested in the solution of [itex]\varphi[/itex] outside of the cylinder.
 

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