scales=domain.dealias

[Jack]

Hello,

In Dedalus (v2.2006), should one always use "scales = domain.dealias" when defining a field to insert into the equation? Or does it not make any difference? I am confused with the use of scales. For example a force or an initial condition for an IVP. 

I have seen in the methods paper example code https://github.com/DedalusProject/methods_paper_examples/blob/6f08b60361b721c20d0cf044f9b611104b4b1491/cylindrical_stokes_flow/simulation.py#L158

scales=domain.dealias

is used. But in the example code of 2D rayleigh-Benard (https://github.com/DedalusProject/dedalus/blob/514fbc39bb4c0c599ae7ce4c4edd843537e3e665/examples/ivp/2d_rayleigh_benard/rayleigh_benard.py#L87), it is not. 

Particularly, one can compare

    gshape = domain.dist.grid_layout.global_shape(scales=1)

vs

gshape = domain.dist.grid_layout.global_shape(scales=domain.dealias)[:2]

in 

https://github.com/DedalusProject/dedalus/blob/514fbc39bb4c0c599ae7ce4c4edd843537e3e665/examples/ivp/2d_rayleigh_benard/rayleigh_benard.py#L87

vs

https://github.com/DedalusProject/methods_paper_examples/blob/6f08b60361b721c20d0cf044f9b611104b4b1491/quasigeostrophic_flow/simulation.py#L122

Thanks, 

Jack

[Keaton Burns]

Hi Jack,

In general it’s fine to either set initial conditions with scales=1 or scales=dealias.  The dealias scales are what are used when evaluating operators.  It can therefore be easier to use these scales if you’re computing some operations on fields and using these to set the values of other fields, or are saving data during the main loop, etc.  However it’s also fine to always use scales=1, you just have to remember to change the scales to 1 for fields that are from or have been used in evaluating some operator.

Best,

-Keaton

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