Filtering

Filtering in Nek5000 is controlled by

We prefer to use HPFRT (high-pass filtering with relaxation term) in Nek5000. There is also an

HPFRT vs explicit filtering

Similarities

  • Both methods aims to extract energy from higher modes of the solution.

  • Filtering weight determines the strength of the filter

  • Filter cut-off ratio determines the what part of the spectrum is diffused numerically by the filter,

Differences

  • HPFRT introduces a term in the RHS of the governing equations, thereby not violating zero velocity divergence, or in other words behaving like a source or sink.

  • Explicit filtering is performed at the end of the time step, and very likely could result in velocity divergence.

Filtering as a sponge-layer

Filter weight can be varied within the domain, and this would allow the filter to behave like a sponge-layer if needed.