IM is an important parameter in the fort.15 file that defines numerical model formulation and dimension. Among other things, IM specifies whether ADCIRC is 2DDI or 3D, solution of the governing equations is implicit or explicit in space, and whether the model formulation is barotropic or baroclinic.
Shortcut IM Values
The most common combination of options used for simulation can be specified through the shortcut values
Six-digit IM Codes
For fine-grained control of various options in simulations six-digit codes for IM can be specified. Each digit represents a specific option regarding the formulation of certain terms or integration methods in the GWCE or momentum equations. The available options for each digit are specified below:
|Value||Digit 1: 2DDI/3D, Lateral Stress in GWCE||Digit 2: Advection in GWCE||Digit 3: Lateral Stress in Momentum||Digit 4: Advection in Momentum||Digit 5: Area Integration in Momentum||Digit 6: GWCE mass matrix|
|1 (default)||2DDI, Kolar-Gray flux-based||Non conservative||Integration by parts, velocity-based||Non conservative||Corrected||Consistent (semi-implicit), barotropic|
|2||2DDI, 2-part flux-based||Conservative form 1||Integration by parts, flux-based||Conservative form 1||Original||Lumped (explicit), barotropic|
|3||2DDI, 2-part velocity-based||Conservative form 2||Integration by parts, velocity-based symmetrical||Conservative form 2||-||Lumped (explicit), baroclinic (not yet implemented in ADCIRC release version)|
|4||2DDI, 2-part flux-based symmetrical||-||Integration by parts, flux-based symmetrical||-||-||-|
|5||2DDI, 2-part velocity-based symmetrical||-||2 Part, velocity-based (not implemented)||-||-||-|
|6||3D, Kolar-Gray flux-based||-||2 Part, flux-based (not implemented)||-||-||-|
A common code combination is IM = 111112, which uses default options (same as IM = 0), but simulates in explicit mass-lumping mode. This is a useful alternative to the (default) semi-implicit consistent GWCE mass matrix mode, which requires a matrix solve increasing computational time and memory compared to the explicit mass-lumping mode, which as about twice as fast and scales to fewer grid nodes per computational core.
- K.M. Dresback, R.L. Kolar, R.A. Luettich, Jr., On the Form of the Momentum Equation and Lateral Stress Closure Law in Shallow Water Modeling, in: Estuar. Coast. Model., American Society of Civil Engineers, Reston, VA, 2005: pp. 399–418. doi:10.1061/40876(209)23.
- S. Tanaka, S. Bunya, J.J. Westerink, C. Dawson, R.A. Luettich, Scalability of an Unstructured Grid Continuous Galerkin Based Hurricane Storm Surge Model, J. Sci. Comput. 46 (2011) 329–358. doi:10.1007/s10915-010-9402-1