ISIS Free/Professional Flow Solvers
At the core of ISIS (Professional and Free) are its industry leading solvers that provide a comprehensive range of methods for simulating flows and levels in open channels, floodplains, reservoirs and estuaries.
Independent benchmarking and more than 30 years of extensive application and development of the ISIS solvers has made it extremely robust, fast and flexible and an ideal choice for most modelling problems.
It can be confidently applied to applications ranging from steep river flows to tidally influenced estuaries and it’s steady and unsteady solvers are suited to subcritical, supercritical and transitional flow regimes. It also has an integrated Muskingum-Cunge flow routing solver and a comprehensive range of hydraulic structures, including gates, abstractions and pumps operated by logical rules.
ISIS includes a wide range of diagnostic error checks and a comprehensive on-line help system, which are constantly being improved upon based on user feedback and the expanding capabilities of ISIS.
Steady State Solvers
The steady state simulation solver provides both direct steady-state and Pseudo-Timestepping methods to optimise run-time and enhance model stability.
The main steady state solution method is called the Direct Method and is applicable to in-bank flow regimes. It is a fast and very accurate solution method which requires very little initial data. It also incorporates an optional accurate supercritical and transcritical flow solver which has the capability of modelling hydraulic jumps and supercritical flow to a high degree of accurately.
The Pseudo-Timestepping method uses a Preissmann 4-point scheme and is often used to develop initial conditions for unsteady runs or where flows are initially out of bank.
Although easy to apply to simple systems, the steady state solver includes the full range of hydraulic structures and is able to model complex looped channels and transcritical flows in steep rivers. This arguably gives ISIS a wider range of application when compared to other steady state modelling software.
Unsteady Solver
The unsteady simulation solver employs the Preissmann implicit scheme (4-point Box scheme) to solve the equations for free surface flow, based on the Saint-Venant equations for flow in open channels. These are used in conjunction with the governing hydraulic equations for each hydraulic unit (structure).
These equations are inevitably a combination of empirical and theoretical equations, many of them non-linear. The non-linear equations are first linearised and the solution to the linear form of the problem is then found.
In order to carry out unsteady simulations an estimate of the initial conditions (flow and stage) are required at every model node. This is most often obtained by carrying out a steady state run at the proposed start time.
The ISIS run forms interface is used to set the simulation parameters and is designed to ease the use and re-use of these parameters and incorporates many features designed to improve ISIS model simulation performance and management capabilities. Major features are:
- fixed and adaptive timestepping
- dynamic linking to 2D solvers
- low flow options
- specify time-varying units externally from the data file
- use alternative initial conditions
- snapshot file output facility
- batch run facility
- setting of additional output variables
Hydraulic structures
A key strength of ISIS is the ability to model a wide range of hydraulic structures including all common types of bridges, sluices, culverts, pumps and weirs. Wherever possible, standard equations or methods are incorporated into the software so that the calculation of level and discharge relationships is fully handled by the software. For structures with automatic operation such as pumps and sluices ISIS allows the user to incorporate logical control rules.
ISIS modules
