Exporting Fields

Event Handle

shockFLOW uses an Event Handle to capture moments in time during the simulation. These Events can be used to change flow properties, activate probes, or, primarily, to export "snapshots" of the simulation variables at a given timestep.

There are two Event types: events, and capture events. Events are the primary capture method, and measure time from the simulation start. Capture events behave similarly, but do not measure time until a triger (by, say, a shock passing through) is activated. Both are defined in the configuration file:

events:
  EXT:
    time: 300
capture_events:
  EXT:
    time: 300

Note that both require EXT events to be defined.

Exporting as HDF5

The solver's default export mode is HDF5 (Hierarchical Data Format version 5). To export in this mode, you simply need to define an extra event:

events:
  store_1:
    time: 200
  EXT:
    time: 300
capture_events:
  EXT:
    time: 300

This will export simulation data 200us after simulation start. Data will be stored in a output path directory, named "200.hdf5".

HDF5 File Contents

Each exported .hdf5 file contains a set of global attributes describing the simulation setup, plus a set of field datasets holding the actual simulation variables.

Attributes

Attribute	Type	Description
nx	unsigned long long	Number of grid cells in the x-direction
ny	unsigned long long	Number of grid cells in the y-direction
dx	double	Grid spacing in the x-direction
dy	double	Grid spacing in the y-direction
mx	double	Mach number in the x-direction (config->mx)
px	double	Reference/freestream pressure ratio (config->px)
res	double	Domain height / resolution parameter (params.height)
noseX	double	X-coordinate of the body/nose reference point
noseY	double	Y-coordinate of the body/nose reference point
P0	double	Reference (initial) pressure
T0	double	Reference (initial) temperature
rho0	double	Reference (initial) density
GasConstant	double	Specific gas constant used in the simulation
Gamma	double	Ratio of specific heats (γ)
origin	unsigned long long	Location/origin index of the simulation domain

Note: dy is currently written from the same source value as dx (params.dx) in the code. If your domain uses non-square cells, double-check this before relying on dy for anisotropic grids.

Fields

Field	Units	Reference State
density_gradient	kg/m⁴
density	kg/m³	Normalized against rho0 (×4)
vx	m/s	Normalized against v_ref
vy	m/s	Normalized against v_ref
temperature	K	T0

Exporting as VTK

The solver's second export mode is VTK (Visualization Toolkit). To export in this mode, you simply need to add a vtk flag to your event:

events:
  store_1:
    time: 200
    vtk : true
  EXT:
    time: 300
capture_events:
  EXT:
    time: 300

This will then export in a similar fashion to HDF5, but in VTK format ready to be used in, for example, ParaView. Note that this file format will use more storage space, and, generally, will be a bit slower. In the VTK format, however, attributes are not stored. Instead, raw fields are exported (in SI units):

Fields

Field	Units	Reference State
density_gradient	kg/m⁴	rho0
density	kg/m³	rho0 / dx
vx	m/s	v_ref
vy	m/s	v_ref
temperature	K	T0

Where v_ref is the reference sound speed derived from freestream conditions.