Changes between Version 7 and Version 8 of palm

Timestamp:

Dec 14, 2017 9:40:43 AM (7 years ago)

Author:

maronga

Comment:

--

palm

@@ -17 +17 @@
 
 Following Monin-Obukhov similarity theory (MOST) a constant flux layer can be assumed as boundary condition between the surface and the first grid level where scalars and horizontal velocities are defined. In PALM we assume that MOST can be also applied locally and we therefore calculate local fluxes, velocities, and scaling parameters. This scheme involves calculation of the Obukhov length ''L'', which can be either done based on variables of the previous time step ("circular"), via a Newton iteration method, or via a look-up table for the stability parameter ''z/L''.
+
+== Topography parameterization ==
+The Cartesian topography in PALM is generally based on the mask method ([#briscolini1989 Briscolini and Santangelo, 1989]) and allows for explicitly resolving solid obstacles such as buildings and orography. The implementation makes use of the following simplifications:
+
+ 1. the obstacle shape is approximated by (an appropriate number of) full grid cells to fit the grid, i.e., a grid cell is either 100% fluid or 100% obstacle,
+
+ 2. the obstacles are fixed (not moving).
+
+Topography is realized in 3-D, e.g., overhanging structures as for example bridges, ceilings, or tunnels, are allowed, i.e. topography does not necessarily be surface-mounted. If no overhanging structures are present, the 3-D obstacle dimension reduces to a 2.5-D topography format, which is conform to the Digital Elevation Model (DEM) format (DEMs of city morphologies have become increasingly available worldwide due to advances in remote sensing technologies).
+In case of overhanging structures, however, 3-D topography information is required to mask obstacles and their faces in PALM. 
+
+The model domain is then separated into three subdomains:
+
+ A. grid points in free fluid without adjacent surfaces, where the standard PALM code is executed,
+
+ B. grid points next to surface that require extra code (e.g., surface parametrization), and
+
+ C. grid points within obstacles, where the standard PALM code is executed but multiplied by zero. 
+
+Additional topography code is executed in grid volumes of subdomain B. The faces of the obstacles are always located where the
+respective surface-normal velocity components ''u'', ''v'', and ''w'' are defined so that the impermeability boundary condition can be implemented by setting the respective surface-normal velocity component to zero.
+
+In case of 5th-order advection scheme, the numerical stencil at grid points adjacent to obstacles would require data which is located within the obstacle. 
+In order to avoid this, the order of the advection scheme is successively degraded at respective grid volumes adjacent to obstacles, i.e., from the 5th-order to 3rd-order at the second grid point above/beside an obstacle and from 3rd-order to 1st-order at grid points directly adjacent to an obstacle.
+
+Simulations with topography require the application of MOST between each surface and the first computational grid point outside of the topography. 
+For vertical and horizontal downward-facing surfaces, neutral stratification is assumed for MOST.
+
+In the PALM core, buildings are primarily realized as obstacles that react to the flow dynamics via form drag
+and friction forces by assuming a constant flux layer between the building surface and the adjacent air volume. A simple thermodynamic coupling is also possible by prescribing surface fluxes of sensible (and latent heat) at any of the building surface grid elements. 
+
+The technical realization of the topography and treatment of surface-bounded grid cells is be outlined in Section [wiki:doc/tec/topography topography implementation]. 
 
 == Parallelization and scaling==