Author Topic: Filter factor in Digital Filtering Method  (Read 1617 times)

stan_lw

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Filter factor in Digital Filtering Method
« on: February 22, 2021, 08:37:07 PM »
I have a question regarding the filter factor for the Digital Filtering Method. If I correctly understood the theory Section 1.3 (please correct me if I am wrong), N is the filter width and n is a ratio between a length scale L and grid spacing Δx, i.e., n = L/Δx. It is also mentioned that N should be chosen such that N ≥ 2n. However, in Section 2.2, Table 2.2 specifies that the filterFactor entry should be ≥ 4, i.e., N/n ≥ 4 or N ≥ 4n. Any reason why you implement a larger filter factor (N/n ≥ 4) than required by the theory (N/n≥ 2)?

Also, I think there is a typo regarding the definition of n, where n should be L/Δx (unit-less) instead of LΔx (unit of meter squared) in the documentation.

ajaybh

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Re: Filter factor in Digital Filtering Method
« Reply #1 on: February 23, 2021, 06:27:47 PM »
Hey,


Let me have a look at this and get back to you asap.
Dr. Ajay B Harish,
Post-doc, Civil & Environmental Engineering
University of California, Berkeley
Web Page: https://sites.google.com/view/ajaybharish

pmackenz

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Re: Filter factor in Digital Filtering Method
« Reply #2 on: March 28, 2021, 02:55:08 AM »
The theoretical requirement is N/n>2 but larger values will increase accuracy.  On the other hand, very large values increase the computational effort for generating the inflow conditions.  N/n >=4 is a recommendation based on experience.