INFO_INPUT.md

Enabling input files

Depending on the pre-processor flags considered, different input files are required. In the following, descriptions of the input files are provided. Important premises:

• FluTAS strictly employs dimensional inputs based on the SI units. The user should choose the input (physical) quantities to match the desired dimensionless parameters of the study.

  1. Example A: for a channel flow configuration, the user should input the forcing bulk velocity (or the imposed pressure gradient) $U_b$, the height of the channel $l_z$, the dynamic viscosity $\mu$, and the density of the fluid $\rho$ to match the desired Reynolds number, i.e. $Re = \rho U_bl_z/\mu$;
  2. Example B: for a heat transfer problem, the user should input the dynamic viscosity $\mu$, the specific heat capacity $c_p$, and the thermal conductivity $k$ to match the desired Prandtl number, i.e. $Pr=\mu c_p/k$.

  The above reasoning applies to any other physical dimensionless parameter of the problem under consideration.

• Line position is essential when constructing the input files. As a reference, use and, if necessary, modify the provided templates available in the examples folder.

The input file dns.in must be always provided. It contains:

• itot,jtot,ktot : grid points in each direction
• lx,ly,lz : domain dimensions in each direction
• gr : stretching parameter (SUPPORTED only in single-phase)
• cfl,dt_input : CFL and employed constant time-step. Note that dt_input is used only if constant_dt is true
• constant_dt : prescribe a constant dt (T) or constant CFL (F)
• time_scheme, space_scheme_mom: scheme for the time and space discretization of the momentum equation
• rho_sp,mu_sp : density and dynamic viscosity of the fluid (relevant only single-phase, i.e.,!defined(_USE_VOF). If the flag _USE_VOF is employed during the compilation, the values of rho_sp and mu_sp are overwritten by the corresponding ones defined in vof.in for phase 2)
• inivel, is_noise_vel, noise_vel: type of initialized velocity field. If the logical variable is_noise_vel is true, a random noise of absolute maximum magnitude noise_vel is superimposed on the initial velocity field
• is_wallturb : initializes velocity conditions for faster turbulence transition in wall-bounded flows
• wallturb_type : initial condition for triggering turbulent transition in wall-bounded flows
• bulk_ftype : type of forcing to sustain the flow in canonical wall-bounded domains
• nstep,time_max,tw_max : stopping criteria, i.e. maximum number of time-steps, maximum simulation time, maximum wall-time
• stop_type(1),stop_type(2),stop_type(3) : enables stopping criteria for the simulation
• restart, num_max_chkpt, input_chkpt, latest : restart is a logical variable, which is true when the user wants to restart the simulation from a dump file (restarting files are printed in data/restart_dir/restart_subdir_???). num_max_chkpt is the maximum number of saved checkpoint files for restarting. input_chkpt is a user-chosen checkpoint file from which the simulation should be restarted (if available). If the logical variable latest is true, the simulation restarts from the latest available/saved checkpoint and, therefore, the variable input_chkpt is not used
• icheck,iout0d,iout1d,iout2d,iout3d,isave : set the time-step frequency employed to: i) perform numerical stability checks, i.e., time-step restriction and velocity divergence (icheck), ii) print zero, one, two, three dimensional outputs (iout0d,iout1d,iout2d,iout3d) and iii) save the restarting files (isave)
• cbcvel(0,1,1),cbcvel(1,1,1),cbcvel(0,2,1),cbcvel(1,2,1),cbcvel(0,3,1),cbcvel(1,3,1): U velocity BC type
• cbcvel(0,1,2),cbcvel(1,1,2),cbcvel(0,2,2),cbcvel(1,2,2),cbcvel(0,3,2),cbcvel(1,3,2): V velocity BC type
• cbcvel(0,1,3),cbcvel(1,1,3),cbcvel(0,2,3),cbcvel(1,2,3),cbcvel(0,3,3),cbcvel(1,3,3): W velocity BC type
• cbcpre(0,1 ),cbcpre(1,1 ),cbcpre(0,2 ),cbcpre(1,2 ),cbcpre(0,3 ),cbcpre(1,3 ): pressure BC type
• bcvel(0,1,1), bcvel(1,1,1), bcvel(0,2,1), bcvel(1,2,1), bcvel(0,3,1), bcvel(1,3,1): U velocity BC value
• bcvel(0,1,2), bcvel(1,1,2), bcvel(0,2,2), bcvel(1,2,2), bcvel(0,3,2), bcvel(1,3,2): V velocity BC value
• bcvel(0,1,3), bcvel(1,1,3), bcvel(0,2,3), bcvel(1,2,3), bcvel(0,3,3), bcvel(1,3,3): W velocity BC value
• bcpre(0,1 ), bcpre(1,1 ), bcpre(0,2 ), bcpre(1,2 ), bcpre(0,3 ), bcpre(1,3 ): pressure BC value
• is_forced(1),is_forced(2),is_forced(3) : choose the direction along which a velocity (bulk_ftype='cfr') or pressure gradient (bulk_ftype='cpg') is imposed to sustain the flow
• gacc_x,gacc_y,gacc_z : gravity acceleration
• bvel_x,bvel_y,bvel_z : value of the imposed velocity
• dpdl_x,dpdl_y,dpdl_z : value of the imposed pressure gradient
• is_outflow(0,1),is_outflow(1,1),is_outflow(0,2),is_outflow(1,2),is_outflow(0,3),is_outflow(1,3) : set if the physical boundary is an outflow (T) or not (F)
• dims_in(1),dims_in(2): number of CPU cores or GPUs per parallelized direction. Note that FluTAS employs a two-dimensional parallelization with pencils aligned along the non-parallelized direction. Three options are available:
  1. Pencils oriented along the x direction. The pre-processor flag _DECOMP_X controls this option. In this case, a number of dims_in(1) and dims_in(2) processors are distributed along the y and the z directions, respectively. This option should be typically preferred both for CPUs and GPUs runs since it minimizes the number of all-to-all operations in the Poisson solver. For GPUs runs, the option _DECOMP_X allows to employ a slab-decomposition only;
  2. Pencils oriented along the y direction. The pre-processor flag _DECOMP_Y controls this option. In this case, a number of dims_in(1) and dims_in(2) processors are distributed along the x and the z directions, respectively. For GPUs runs, the option _DECOMP_Y allows to employ a slab-decomposition only;
  3. Pencils oriented along the z direction. The pre-processor flag _DECOMP_Z controls this option. In this case, a number of dims_in(1) and dims_in(2) processors are distributed along the x and the y directions, respectively. For GPUs runs, the option _DECOMP_Z allows both a slab and a pencil decomposition.
• nthreadsmax: maximum number of threads for OpenMP (UNTESTED)

The input file vof.in is required when _USE_VOF is used. It contains:

• rho1,rho2 and mu1,mu2 : density and dynamic viscosity for phase 1 and 2
• inivof : type of vof initialization
• nbub : number of bubbles/droplet initialized if inivof='bub'. Note that if nbub>1 the file bub.in is required with the center coordinates and radii of each emulsion, droplet or bubble. Otherwise, if nbub=1, this information is provided in the next line.
• xc(1), yc(1), zc(1), r(1) : center and radius of the single emulsion, droplet or bubble present in the domain
• cbcvof(0,1 ),cbcvof(1,1 ),cbcvof(0,2 ),cbcvof(1,2 ),cbcvof(0,3 ),cbcvof(1,3 ) : VoF BC type
• bcvof(0,1 ),bcvof(1,1 ),bcvof(0,2 ),bcvof(1,2 ),bcvof(0,3 ),bcvof(1,3 ) : VoF BC values
• sigma : surface tension coefficient
• late_init, i_late_init : if late_init is true, the VoF field is initialized at time-step i_late_init instead of at istep=0. This option is typically useful when the user wants to run a precursor single-phase simulation and add the disperse phase at later stage.

Note that by convention phase 1 and 2 represent the dispersed and the continous phase, respectively.

The input file bub.in is required when _USE_VOF is used, inivof='bub' and bub>1 in vof.in. It contains:

• xc(i), yc(i), zc(i), r(i) : center and radius for the i-th bubble/droplet. This line should be repeated for as many elements as nbub. The user can either manually provide the center coordinates and radius of each droplet or, alternatively, generate a random distribution of droplets with equal radius using the script randomDroplet.py available here.

The input file forcing.in is required when _TURB_FORCING is used. It contains:

• turb_type: type of turbulence forcing. Currently available: Arnold-Beltrami-Childress ('abc') and Taylor-Green Vortex ('tgv')
• u0_t : initial velocity magnitude
• f0_t : intensity of the forcing term
• k0_t : wavenumber at which energy is injected
• abc_x, abc_y, abc_z : values of A, B and C coefficients for ABC forcing
• add_noise_abc : add disturbances to the initial condition to trigger transition

The input file heat_transfer.in is required if _HEAT_TRANSFER is used. It contains:

• initmp, is_noise_tmp, noise_tmp: type of initialized temperature field. If the logical variable is_noise_tmp is true, a random noise of absolute maximum magnitude noise_tmp is superimposed on the initial temperature field
• tl0,tg0: initial liquid and gas temperature
• cp1,cp2: specific heat capacity at constant pressure of phase 1 and 2
• cv1,cv2: specific heat capacity at constant volume of phase 1 and 2
• kappa1,kappa2 : thermal conductivity of phase 1 and 2
• cbctmp(0,1 ),cbctmp(1,1 ),cbctmp(0,2 ),cbctmp(1,2 ),cbctmp(0,3 ),cbctmp(1,3 ): temperature BC type
• bctmp(0,1 ),bctmp(1,1 ),bctmp(0,2 ),bctmp(1,2 ),bctmp(0,3 ),bctmp(1,3 ): temperature BC value
• tmp0,beta1_th,beta2_th: if _BOUSSINESQ is enabled, the code requires also to provide a reference temperature (e.g. the arithmetic mean between top and bottom wall), the thermal expansion coefficient for phase 1 and phase 2

Note that in case the app single_phase is chosen and heat transfer effects are included, i.e.,HEAT_TRANSFER=1, the input file heat_transfer_sp.in must be provided together with dns.in. In particular, heat_transfer_sp.in contains:

• initmp, is_noise_tmp, noise_tmp: type of initialized temperature field. If the logical variable is_noise_tmp is true, a random noise of absolute maximum magnitude noise_tmp is superimposed on the initial temperature field
• tmp0: initial temperature
• cp_sp: specific heat capacity at constant pressure
• cv_sp: specific heat capacity at constant volume
• kappa_sp : thermal conductivity of the fluid
• cbctmp(0,1 ),cbctmp(1,1 ),cbctmp(0,2 ),cbctmp(1,2 ),cbctmp(0,3 ),cbctmp(1,3 ): temperature BC type
• bctmp(0,1 ),bctmp(1,1 ),bctmp(0,2 ),bctmp(1,2 ),bctmp(0,3 ),bctmp(1,3 ): temperature BC value
• beta_th: if _BOUSSINESQ is enabled, the code requires also to provide the thermal expansion coefficient of the fluid