Solve third order, non-linear, ODE with indefinite BC's?

In summary, the conversation is about solving a third order, non-linear, homogeneous, constant coefficient ODE with indefinite boundary conditions. The equation is the Falkner-Skan equation and the person is looking for a general or analytical solution. They have access to Maple but would like to solve it analytically without a computer first. The conversation ends with the suggestion to try solving it numerically in a limited range using Mathematica.
  • #1
Topher925
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I have a third order, non-linear, homogeneous, constant coefficient ODE that I need to solve but have no idea how to do it. To make matters worse, one of the boundary conditions are indefinite. Here's the equation,

y''' + y*y'' - y'^2 + 1 = 0

and the BC's
y(0) = 0
y'(0) = 0
y'([tex]\infty[/tex]) = 1

Does this equation have a general or analytical (non numerical) solution? I have access to Maple, just not at the moment, and would like to try and solve it analytically without computer first. Any suggestions on how to solve it? Can it be solved?
 
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  • #2
Topher925 said:
I have a third order, non-linear, homogeneous, constant coefficient ODE that I need to solve but have no idea how to do it. To make matters worse, one of the boundary conditions are indefinite. Here's the equation,

y''' + y*y'' - y'^2 + 1 = 0

and the BC's
y(0) = 0
y'(0) = 0
y'([tex]\infty[/tex]) = 1

Does this equation have a general or analytical (non numerical) solution? I have access to Maple, just not at the moment, and would like to try and solve it analytically without computer first. Any suggestions on how to solve it? Can it be solved?

It's called the Falkner-Skan equation but maybe you know that already. Maybe a Google can help you. Wikipedia says it can't be solve analytically. Here's my attempt to solve it numerically in a limited range, say (0,10) in Mathematica:

Code:
myend = 10;
sols = Map[
   First[NDSolve[{y'''[t] + y[t] y''[t] - (y'[t])^2 + 1 == 0, 
       y[0] == y'[0] == 0, y'[myend] == 1}, y, t, 
      Method -> {"Shooting", 
        "StartingInitialConditions" -> {y[0] == 0, y'[0] == 0, 
          y''[0] == #}}]] &, {1.7, 1.75, 1.8}];
Plot[Evaluate[y[t] /. sols], {t, 0, myend}, 
 PlotStyle -> {Black, Blue, Green}, AspectRatio -> 4/3]

And that code returns [itex]y''(0)\approx 1.233[/itex]
 
Last edited:
  • #3
see attached... it solves falkner skan in maple..

any questions, fire away
 

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  • HT6.m
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1. How do I determine the boundary conditions for a third order non-linear ODE?

In order to determine the boundary conditions for a third order non-linear ODE, you need to first identify the independent and dependent variables in the equation. Then, you can use the initial conditions of the problem or any other known values to determine the boundary conditions.

2. What method can be used to solve a third order non-linear ODE?

There are several methods that can be used to solve a third order non-linear ODE, including variation of parameters, Laplace transforms, and numerical methods such as the Runge-Kutta method. The most appropriate method to use will depend on the specific equation and boundary conditions.

3. Are there any special considerations when solving a third order non-linear ODE compared to a lower order ODE?

Yes, solving a third order non-linear ODE can be more complex than solving a lower order ODE. This is because the equation may not have a general solution and may require numerical methods to approximate the solution. Additionally, the boundary conditions may also be more difficult to determine.

4. Can I use any software to solve a third order non-linear ODE?

Yes, there are many software programs available that can solve third order non-linear ODEs. Some popular options include MATLAB, Mathematica, and Maple. However, it is important to understand the underlying principles and techniques involved in solving these equations before relying solely on software.

5. What is the significance of the indefinite boundary conditions in a third order non-linear ODE?

The indefinite boundary conditions in a third order non-linear ODE mean that there are multiple possible solutions that satisfy the given conditions. This can make it more challenging to find the exact solution and may require additional information or assumptions to be made.

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