Question about the Tau Method

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MinYu Li

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Nov 5, 2025, 11:32:51 AM (3 days ago) Nov 5
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Hi everyone,

I am working with the following set of partial differential equations.

iαu + Dv = 0
-iωu + iαUu + U'v = -iαp
-iωv + iαUv = -Dp

where D = d/dy

This set of partial differential equations requires two boundary conditions. According to the documentation, applying these two conditions involves adding two tau terms. However, I am unsure which equations the tau terms should be applied to in my case.

Based on the example in the documentation, should I add the tau terms to each y-differential equation, that is, the first and third equations?

Any guidance would be greatly appreciated!
Minyu Li

Keaton Burns

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Nov 5, 2025, 11:38:48 AM (3 days ago) Nov 5
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Hi Minyu,

Yes I would try doing everything here implicitly (move the pressure terms to the LHS) and add a tau term to each equation with a Chebyshev derivative, and then add your two boundary conditions (presumably Dirichlet conditions on v?).

Best,
-Keaton


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MinYu Li

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Nov 5, 2025, 10:22:53 PM (2 days ago) Nov 5
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Hi Keaton,

Thanks for your response, it was really helpful.

Best regards,  
Minyu
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