Changes between Version 21 and Version 22 of doc/tec/noncyclic

Timestamp:

Jan 18, 2021 10:50:07 AM (4 years ago)

Author:

raasch

Comment:

--

doc/tec/noncyclic

@@ -61 +61 @@
 For the scalar quantities, Neumann boundary conditions are used at the outflow boundary which is the simplest way.
 For the velocity components, a Neumann condition would require to be considered in the solution of the Poisson equation for perturbation pressure, which has not been realized so far, because it requires some technical effort. 
-Instead, PALM offers two types of radiation boundary conditions for the velocity components, which are not in conflict with the pressure solver (see [../../app/inipar/#use_cmax use_cmax], [../../app/inipar/#bc_lr bc_lr] and [../../app/inipar/#bc_ns bc_ns]).
+Instead, PALM offers a radiation boundary conditions for the velocity components, which are not in conflict with the pressure solver (see [../../app/inipar/#bc_lr bc_lr] and [../../app/inipar/#bc_ns bc_ns]).
 For the radiation condition, the Sommerfeld radiation equation is solved at the outflow
 {{{
@@ -69 +69 @@
 which considers flow disturbances propagating with the mean flow and by waves.
 Here ψ is the transported quantity and ∂,,n,, is the derivative normal to the outflow boundary.
-In PALM, based on (5), the radiation boundary condition is realized in two ways as follows.
+In general, the phase velocity c,,ψ,, needs to be calculated like described below (''Variable Phase Velocity''). Since r4845, PALM always uses the maximum phase velocity that is allowed by the CFL-criterion, i.e. for a Courant number of one (see section ''Constant Phase Velocity'' further below).
 
 ==== Variable Phase velocity ====
@@ -132 +132 @@
 }}}
 
-==== Constant Phase velocity ====
+==== Constant Phase velocity (method used in PALM since r4845) ====
 
 Setting c,,ψ,, = c,,max,, in (5) leads to a more simple radiation boundary condition (here e.g. for a left-right flow along positive x-direction):
@@ -141 +141 @@
 with ψ = {u,v,w}.
 This formulation of the radiation boundary condtions saves computational time compared to the formulation of equations (7) to (10).  
-Although, Orlanski (1976) suggested that this approach of radiation boundary condition leads to reflection for waves smaller than c,,max,, which may occur in complex geophysical flows, our simulations of a convective boundary layer with background wind have been stable so far.
+Although, Orlanski (1976) suggested that this approach of radiation boundary condition leads to reflection for waves smaller than c,,max,, which may occur in complex geophysical flows, our simulations of stable and convective boundary layers with background wind have shown no problems so far.
 
 === Mass flux correction ===