gerbil 0 Posted March 1, 2012 Share Posted March 1, 2012 Hello All, I have a question related to the fascinating topic of "Boundary Layer Theory". Take for example the simple equation mx'' + kx' + cx = 0 where k and c are finite constants and (m -> 0), (The derivative sign (') is by t) x(0) = 0 x'(0) = A > 0. In order to achieve a comprehension on what's going on near the point t = 0 when (m -> 0), it is suggested to take the change of variables t* = t/m. By that the former equation becomes, x'' + kx' + mcx = 0 (Now, the derivative sign (') is by t*). Now, moving into the limit or alternatively noticing that mcx is negligible, we can drop it and we get, x'' + kx' = 0 and now the solution is fairly easy. Books call this solution: "The inner solution" of the equation, since this solution describes the solution inside a boundary layer. My question is as follows: The idea of taking the change of variables t* = t/m, is just a mathematical trick or does it have a meaning (e.g. streching of the time scale, etc.)? If it has a meaning, i'd like to know its intuition. Best, Miki Link to post Share on other sites

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