Hello, everyone!
I'm working on flow instability and just a newbie. Now, I try to employ spectral method - collocation method to solve Orr-Sommerfeld(OS) equation. I've already solved the eigenvalue problem of Poiseuille flow. And I want to move to Boundary layer instability., during which I come across a problem concerning coordinates transformation. The problem is mainly a mathematical one.
Specificlly, OS equation is as follows.
Mean velocity profile of Poiseuille flow is
.
What I concerns is c, wave speed, also eigenvalue. I treat OS equation as a general eigenvalue problem, which leads OS equation to be of the form
, where both A and B are operators.
By some eigenvalue solver, eigenvalues can be solved. Mathematically, eigenvalues are determined by operators A and B. Ok, I finish the above.
Then I consider coordinate transformation. Take a simple example as translation:
. It translates field from
to
. Since the field is translated, operators A and B should be changed to be defined on the new field. Specifically, partial differential operator
and velocity profile
should be changed. In this case, coordinate transformation is just translation. So, partial differential operator is unchanged and velocity profile becomes
. All in all, what changes in this eigenvalue problem
is only velocity profile. It seems operator indeed changes, which results in different eigenvalues from original ones. Then there exists contradiction.
I mean, mathematically, different operators in eigenvalue problem will lead to different eigenvalues. Meanwhile, physically, coordinate translation leads to the same eigenvalues. In fact, I do get same eigenvalues numerically by spectral method - collocation method.
So, where I am wrong from the above?