Changes between Version 3 and Version 4 of doc/tec/noncyclic

Timestamp:

Mar 12, 2013 5:32:55 PM (12 years ago)

Author:

fricke

Comment:

--

doc/tec/noncyclic

@@ -55 +55 @@
 So far, we have experience with gravity waves in case of cold air outbreaks, which grow in amplitude up to quite extreme values, if no damping is applied.
 In the respective simulations, we used typical values for [../../app/inipar/#pt_damping_factor pt_damping_factor] of 0.05 and for [../../app/inipar/#pt_damping_width pt_damping_width] of 25 km in order to prevent the gravity waves from growing.
+
+=== Outflow boundary ===
+
+At the outflow, an open boundary condition is needed to ensure that disturbances of the mean flow can exit the model domain without effecting the flow upstream.
+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/#bc_lr bc_lr] and [../../app/inipar/#bc_ns bc_ns]).
+For the radiation condition, the Sommerfeld radiation equation is solved at the outflow
+{{{
+#!Latex
+ \partial_t \psi  + c_{\psi} \partial_n \psi  = 0 \; , \quad (5)
+}}}
+which considers flow disturbances propagating with the mean flow and by waves.
+Here ψ is the transported quantity and  $\partial_n$ is the derivative normal to the outflow boundary.
+In PALM, based on 5, the radiation boundary condition is realized in two ways as follows.
+