| 264 | | Since r2259 a synthetic turbulence generator is implemented in PALM to generate a turbulent inflow condition. The method is based on the work of [#xie2008 Xie and Castro (2008)] and [#kim2013 Kim et al. (2013)]. Unscaled turbulent motions ''u,,*i,,'' are calculated via a statistical method based on length scales along each direction and scaled with an amplitude tensor ''a,,ij,,'' based on a prescribed Reynolds stress tensor. The calculated turbulence is then added to a fixed mean profile ''U,,i,,'' at the inflow boundary: |
| | 264 | Since r2259 a synthetic turbulence generator is implemented in PALM to generate a turbulent inflow condition. The method is based on the work of [#xie2008 Xie and Castro (2008)] and [#kim2013 Kim et al. (2013)]. Unscaled turbulent motions ''u,,*j,,'' are computed based on length scales along each direction and the amplitude tensor ''a,,ij,,'' which in turn bases on the Reynolds stress tensor. The calculated turbulence is then added to the mean inflow data of the velocity components ''U,,i,,'': |
| 272 | | The amplitude tensor ''a,,ij,,'' depends on the Reynolds stress tensor ''R,,ij,,'' and is calculated using a Cholesky decomposition as suggested by [#lund1998 Lund et al. (1998)]. The unscaled turbulent motions ''u,,*i,,'', which are calculated on the 2D inflow plane, depend on the prescribed time scales ''T'' and length scales ''L'': |
| | 272 | The amplitude tensor ''a,,ij,,'' depends on the Reynolds stress tensor ''R,,ij,,'' and is calculated using a Cholesky decomposition as suggested by [#lund1998 Lund et al. (1998)]. The unscaled turbulent motions ''u,,*j,,'', which are calculated on the 2D inflow plane, depend on the prescribed time scales ''t,,ij,,'' and length scales ''l,,ij,,'': |
| 279 | | where ''Ψ'' denotes a part of the generated 2D signal which is correlated in space using ''L'' along the vertical and spanwise direction. Correlation along streamwise direction is assured via the time scale ''T'' which is estimated by ''L'' along streamwise direction and ''U''. |
| | 279 | where ''Ψ'' denotes a part of the generated 2D signal which is correlated in space using the turbulent length scales ''l,,ij,,'' along the vertical and spanwise direction. Correlation along streamwise direction is assured via the time scale ''t,,ij,,'' which is estimated by ''t,,ij,,'' along streamwise direction using the ''U,,i,,''. |
| 295 | | where ''u,,i,corrected,,'' is the corrected wind velocity at the inflow boundary, ''U,,b,,'' and ''U,,b,prescribed,,'' the instantaneous and prescribed bulk velocity at the inflow boundary, ''S'' the surface area of the inflow boundary, and ''u,,n,,'' the component of ''u,,i,,'' normal to the inflow boundary. |
| 296 | | |
| 297 | | Although the method does provide correlated turbulence, it only generates statistical correlation. Large coherent structures like a hexagonal pattern produced by convection cannot be covered by this method. So far, turbulence is only added to the three wind components. No perturbations are added to the subgrid-scale turbulent-kinetic energy and potential temperature. |
| | 295 | where ''u,,i,c,,'' is the corrected wind velocity at the inflow boundary, ''U,,b,,'' and ''U,,b,p,,'' the instantaneous and prescribed bulk velocity at the inflow boundary, ''S'' the surface area of the inflow boundary, and ''u,,n,,'' the component of ''u,,i,,'' normal to the inflow boundary. |
| | 296 | |
| | 297 | The required length- and time scales, as well as the Reynolds strees tensor can be either prescribed (method 1), 0if known from previous simulations or measurements, or they can be parametrized (method 2). |
| | 298 | Please note, time and length scales as well as the components of the Reynolds stress tensor depend on the height and the horizontal location, particularly over heterogeneous surfaces and model domains with large horizontal extensions. |
| | 299 | For the sake of simplicity, however, we consider only height dependent information of the Reynolds stress as well as length and time scales. |
| | 300 | |
| | 301 | '''Method 1:''' If these information is available, it can be provided via an ASCII file which contains all necessary information. |
| | 302 | For an example please see [attachment:STG_PROFILES:wiki:doc/app/iofiles example file]. |
| | 303 | This ASCII input file will be read automatically by PALM if the namelist parameter ''use_synthetic_turbulence_generator'' is set to ''True'' within the namelist ''stg_par''. Be sure that the input file is added to the list of input files in your ''.palm.iofiles'' like so: |
| | 304 | {{{ |
| | 305 | STG_PROFILES in:locopt d3#:d3r $base_data/$fname/INPUT _iprf |
| | 306 | }}} |
| | 307 | and named with the suffix ''_iprf'' in your ''INPUT'' directory. Please have look at the list of [wiki:/doc/app/iofiles#STG_PROFILES input and output files] for a detailed description of the input file. |
| | 308 | |
| | 309 | '''Method 2:''' In many cases detailed information about the Reynolds stress and turbulent length scales are not available, so that these information need to be parametrized. If no ASCII input file is provided in the input folder, this will be done automatically and the turbulence statistics at the inflow boundary will be estimated. |
| | 310 | Please note, the derived turbulence statistics will depend on the height above ground but not on the horizontal location. Parametrization of the Reynolds stress follows [#rotach1996 Rotach et al. (1996)]. |
| | 311 | An description of the applied turbulence parametrization will follow soon. |
| | 312 | |
| | 313 | At this point we emphasize that this method only generates turbulence which is statistically correlated. Large coherent structures such as e.g. hexagonal pattern as typically observed in a convective boundary layer, however, cannot be generated by this method. So far, turbulence is only added to the three wind components. No perturbations are added to the subgrid-scale turbulent-kinetic energy and potential temperature. |
| 300 | | |
| 301 | | The turbulence generator can be operated in two ways, with prescribed or with parametrized turbulence information. |
| 302 | | The turbulence generator requires information about the length and time scales of turbulence, as well as height-dependent information about the Reynolds stress. |
| 303 | | '''Method 1:''' If these information is available, e.g. from measurements or from a previous simulation, it can be provided via an ASCII file which contains all necessary information, see [attachment:STG_PROFILES:wiki:doc/app/iofiles example file], which will be read automatically by PALM if the namelist parameter ''use_synthetic_turbulence_generator'' is set to ''True'' within namelist ''stg_par''. Be sure that the input file is added to the list of input files in your ''.palm.iofiles'' like so: |
| 304 | | {{{ |
| 305 | | STG_PROFILES in:locopt d3#:d3r $base_data/$fname/INPUT _iprf |
| 306 | | }}} |
| 307 | | and named with the suffix ''_iprf'' in your ''INPUT'' directory. Please have look at the list of [wiki:/doc/app/iofiles#STG_PROFILES input and output files] for a detailed description of the input file. |
| 308 | | '''Method 2:''' In many cases, however, detailed information about the Reynolds stress and turbulent length scales are not available, so that these information need to be parametrized. If no ASCII input file is provided, this will be done automatically. Further, turbulence statistics will be adjusted automatically, so that the turbulence generator can account for non-stationary situations. Parametrization of the Reynolds stress follows [#rotach1996 Rotach et al. (1996)]. |
| 309 | | An description of the applied turbulence parametrization will follow soon. |
