These beta command objects are available as an option to control the solution settings for the SPH solver in Explicit Dynamics analyses. Although the default options are usually sufficient for many applications, these command objects may be used to overcome any numerical problems exhibited by an analysis, such as tensile instability. For further information, see Command Objects in Explicit Dynamics.
To access the command objects, turn on Beta in the Workbench options.
| SPHSETTINGS,DENSITYCALC,CONTINUITYEQ | Calculate the density using the continuity equation (Default) |
| SPHSETTINGS,DENSITYCALC,KERNELSUM | Calculate the density using a kernel sum |
| SPHSETTINGS,SMOOTHINGLENGTH,CONSTANT | Use a constant smoothing length (Default) |
| SPHSETTINGS,SMOOTHINGLENGTH,VARIABLE | Use a variable smoothing length |
| SPHSETTINGS,SMOOTHINGFUNCTION,CUBICSPLINE | Use the cubic B-spline as the weighting function (Default) |
| SPHSETTINGS,SMOOTHINGFUNCTION,QUINTICSPLINE | Use a quintic spline as the weighting function |
| SPHSETTINGS,VISCOSITYOPTION,SPHWITHSHEARCORRECTION | Use the Monaghan type artificial viscosity (Default) |
| SPHSETTINGS,VISCOSITYOPTION,STANDARD | Use the Neumann and Richtmyer artificial viscosity |
Density Calculation
Two alternative methods for calculating density have been implemented—the continuity equation is the default method. Calculating the density by kernel sum is activated using the beta command object shown in the table above. The default option is the most common found in the literature, and it works well for a wide range of applications. However, under certain circumstances, inconsistencies in the SPH equations may result in negative densities and numerical issues. In such cases, the density calculation by kernel sum may overcome such problems.
Variable Smoothing
The accuracy and robustness of the SPH processor depends on the quality (in particular, the number) of the local neighboring particles. In expansive flow of material, the distance between SPH particles increases. If this distance exceeds twice the smoothing length of the particles, the two particles will no longer interact. This loss of interaction may be unphysical and is commonly described as 'numerical fracture'.
In an attempt to reduce the problem of numerical fracture, an option to add a variable smoothing length has been included as a beta functionality, which can be activated by a command object. When activated, as particles separate and their density decreases, their smoothing length increases so that interaction with neighboring particles is maintained.
Since changes in the smoothing length result from changes in density, the variable smoothing option works well for isotropic flows. However, for many continuum dynamics applications, anisotropic flow is observed. For example, in plastic flow of metals, the volume (hence density) is almost constant while the deformations are grossly anisotropic. The current implementation of variable smoothing does not work well in such situations and numerical fracture is often unavoidable.
SPH Artificial Viscosity
There are two types of artificial viscosity available for the SPH solver. The default option is the Monaghan-type artificial viscosity, which is described in Damping Controls. This is the recommended formulation to use. The Neumann and Richtmyer form of artificial viscosity is the second option. Refer to the table above to use the respective command objects.