Setup for turbulent inflow

Starting from version 3.5 PALM includes a so-called turbulence recycling method which allows a turbulent inflow with non-cyclic horizontal boundary conditions. The method follows the one described by Lund et al. (1998, J. Comp. Phys., 140, 233-258), modified by Kataoka and Mizuno (2002, Wind and Structures, 5, 379-392). The method is switched on by setting the initial parameter turbulent_inflow = .TRUE..

The turbulent signal A'(y,z) to be imposed at the left inflow boundary is taken from the same simulation at a fixed distance x_r from the inflow (given by parameter recycling_width): A'(y,z) = A(x_r,y,z) - A(z), where A(z) is the line average along y at x = x_r. The turbulent quantity A'(y,z) is then added to a mean inflow profile a(z) which is the horizontal (and temporal) average as calculated from a precursor run. This is done after each timestep.

So far (09/16/08), a turbulent signal is imposed to all prognostic variables except humidity (passive scalar) and salinity!

In order to avoid a continous growth of the turbulent layer with time, the height up to which turbulence is recycled is limited by parameter inflow_damping_height. By default, this height is calculated and taken from the precursor run.

The following tries to give a short recipe about how to create a proper setup for using the turbulence recycling method:

• A precursor run with cyclic horizontal boundary conditions has to be carried out in order to create a (quasi-) stationary state. Since the horizontal average from this precursor run is used as the mean inflow profile for the main run, the wall-normal velocity component must point into the domain at every grid point and its magnitude should be large enough in order to guarantee an inflow even if a turbulence signal is added.

• The main run requires from the precursor run the mean profiles to be used at the inflow. For this, the horizontally and temporally averaged mean profiles as provided with the standard PALM output are used. The user has to set parameters dt_dopr, averaging_interval, etc. for the precursor run appropriately, so that an output is done at the end of the precursor run. The profile information is then contained in the restart (binary) file created at the end of the precursor run and can be used by the main run. It is very important that the mean profiles at the end of the precursor run are in a stationary or quasi-stationary state, because otherwise it may not be justified to use them as constant profiles at the inflow. Also, turbulence at the end of the precursor run should be fully developed. Otherwise, the main run would need an additional spinup-time at the beginning to get the turbulence to its final stage.

• In order to read the binary data from the precursor run, parameter initializing_actions = 'cyclic_fill' has to be set in the main run.

• Horizontal boundary conditions have to be set to bc_lr = 'dirichlet/radiation' . The radiation boundary condition at the outflow requires that the wall-normal velocity component at the outflow must point out of the domain at every grid point and at every timestep. Special care has to be taken in case of obstacles near the outflow, because their recirculations may violate this condition.

• Because of the Dirichlet condition at the inflow, the flow does not require any further driving force in general. An external pressure gradient or a geostrophic wind can be used, but should have the same value as for the precursor run. Otherwise, the horizontally diverging wind field will induce vertically descending or ascending motions. An application of volume flow conservation (conserve_volume_flow = .TRUE.) should be done with care, because it may have the same effects.

• The main run allows to use a larger domain along x and y than in the precursor run.

• The distance between the inflow and the recycling plane should be large enough to allow for a sufficient advancement of the turbulent eddies (should be somehow correlated with the eddy turnover time). Otherwise, almost the same eddies will be recycled again and again, which will distort the turbulence statistics. Typically, the recycling width will be similar or equal to the domain size of the precursor run.