- #1
Wuberdall
- 34
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Hi,
I am having a little trouble understanding a minor step in a paper by [V. Zimin and F. Hussain][1].
They define a collection of divergence-free vector wavelets as
$$\mathbf{v}_{N\nu n}(\mathbf{x}) = -\frac{9}{14}\rho^{1/2}_N \mathbf{e}_\nu\times\nabla_\mathbf{s}\bigg(\frac{\cos(s)-\cos(2s)}{s^2}\bigg),$$
where
$$ \rho_N = \frac{7\pi}{9}2^{3N} \qquad\text{and}\qquad \mathbf{s} = \pi2^N(\mathbf{x}-\mathbf{x}_{Nn}).$$
Here does $N$ denote the "scale" of the wavelet.
Zimin and Hussain then write the Galerkin projection of Navier-Stokes equation onto this divergence-free vector wavelet projection. The Laplacian term in NSE become
$$ \sum_{\mu}\sum_{m}A_{N\mu m}\int \mathbf{v}_{N\nu n}\cdot\Delta\mathbf{v}_{N\mu m}\,d^3\mathbf{x}. $$
But, when I try to write down this Galerkin projection I find that the Laplacian term becomes
$$ \sum_{M}\sum_{\mu}\sum_{m}A_{M\mu m}\int \mathbf{v}_{N\nu n}\cdot\Delta\mathbf{v}_{M\mu m}\,d^3\mathbf{x}. $$
This is almost, identical to Zimin and Hussain. But I have a additional sum over the scale-index $M$, and I can not seem to get rid of it.
NB. My feeling is, that this has nothing to do with the explicit form of the divergence-free vector wavelets. But something to do with the fact that they are wavelets (and thus an orthonormal basis) and perhaps (but not so much) that they are divergence-free.
[1]: http://dx.doi.org/10.1063/1.868669
I am having a little trouble understanding a minor step in a paper by [V. Zimin and F. Hussain][1].
They define a collection of divergence-free vector wavelets as
$$\mathbf{v}_{N\nu n}(\mathbf{x}) = -\frac{9}{14}\rho^{1/2}_N \mathbf{e}_\nu\times\nabla_\mathbf{s}\bigg(\frac{\cos(s)-\cos(2s)}{s^2}\bigg),$$
where
$$ \rho_N = \frac{7\pi}{9}2^{3N} \qquad\text{and}\qquad \mathbf{s} = \pi2^N(\mathbf{x}-\mathbf{x}_{Nn}).$$
Here does $N$ denote the "scale" of the wavelet.
Zimin and Hussain then write the Galerkin projection of Navier-Stokes equation onto this divergence-free vector wavelet projection. The Laplacian term in NSE become
$$ \sum_{\mu}\sum_{m}A_{N\mu m}\int \mathbf{v}_{N\nu n}\cdot\Delta\mathbf{v}_{N\mu m}\,d^3\mathbf{x}. $$
But, when I try to write down this Galerkin projection I find that the Laplacian term becomes
$$ \sum_{M}\sum_{\mu}\sum_{m}A_{M\mu m}\int \mathbf{v}_{N\nu n}\cdot\Delta\mathbf{v}_{M\mu m}\,d^3\mathbf{x}. $$
This is almost, identical to Zimin and Hussain. But I have a additional sum over the scale-index $M$, and I can not seem to get rid of it.
NB. My feeling is, that this has nothing to do with the explicit form of the divergence-free vector wavelets. But something to do with the fact that they are wavelets (and thus an orthonormal basis) and perhaps (but not so much) that they are divergence-free.
[1]: http://dx.doi.org/10.1063/1.868669