Modeling software is only as good as its simulation engine. But the engine is the least visible and most difficult part of any modeling software product for users to evaluate.
This note examines the characteristics of SIM, the simulation engine at the heart of Wallingford Software's InfoWorks CS, explaining how it works compared to other approaches and why it is the best available for wastewater and stormwater modeling.
The main benefits of SIM are the high level of accuracy with which it models wastewater and stormwater hydraulics and the wide range of flow conditions to which it can be applied. The primary reason for this is the use of true dynamic modeling for its flow calculations.
There are two approaches to modeling fluid flow in open pipes - the steady state/extended period approach, or dynamic modeling. Steady state modeling looks at 'snapshots' of the system, moving a hydrograph or flow rate value forward in time but not representing the change in flow pattern that occurs in real life. As its name implies, steady state modeling is not designed to model changing flow conditions such as peaks. To address this, extended period modeling changes the hydrograph through time according to a static look-up table of average attenuation, in effect taking a series of snapshots and interpolating between them. This is a step towards reflecting the true flow, but again it is based on simplistic assumptions. In contrast, a dynamic approach accurately reflects the state of the system continuously over time, and over the full range of operating conditions. A dynamic model calculates at every timestep of the simulation, second by second if required, what will happen next on the basis of the exact conditions of the depth and speed of fluid flow in that section of the network, plus the effects of any backwater in the system.
Examination of steady state models shows they do not show the attenuation of flow that takes place in real life, whereas dynamic analysis shows this lower peak, and a greater spread of the event over time, as shown in the diagram below.
The errors in the steady state results in the diagram, which is typical of steady state analysis, are:
- a 28% over-estimate of the level of the peak - an error that would be carried through into an overestimate of the required pipe sizes in a capital planning study
- an underestimate of the period of flow - an error that would be carried through into an underestimate of storage requirements.
There are further reasons for the greater accuracy that SIM provides relative to steady state models:
- Computations are based on the industry leading methods for solving the St Venant equations - unquestionably the best representation of fluid flow in pipes.
- Different calculations are used for gravity and pressurized pipes. SIM represents each of these with the appropriate equations, and automatically switches based on the flow conditions.
- Controls such as weirs are modeled from their true dimensions rather than relying on simplified representations of hydraulic behavior defined in terms of a head/discharge relationship.
- Optimum timesteps are calculated and used throughout the simulation, changing from short timesteps when the hydraulic processes are complex to longer steps for dry weather flows.
- Reverse flow conditions due to backing up from a downstream constriction or river ingress can only be represented with a fully dynamic model.
- SIM is inherently mathematically stable. Other engines are prone to instability, producing misleading results, but SIM is recognized as the most stable in the industry, even with very large models and/or models with complex flow control structures.
- Purpose - SIM was developed specifically for the analysis of wastewater and stormwater networks. It can be applied in the associated disciplines of wastewater treatment works and simple river channels.
- Speed - SIM is independently benchmarked as the fastest engine on the market, twice as fast as the next best, and many times faster than others.
- Control - The modeler has far greater control of a simulation's progress in terms of simulation tolerance, diagnostic feedback and general 'health' of the simulation.
- Continuous in-house development - SIM was first developed by Wallingford Software in the early 1990´s, and continues to progress to match precisely the development directions of InfoWorks CS.
The key criteria for evaluating the simulation engine of wastewater and stormwater modeling software are accuracy and serviceability, in that order. SIM, the engine at the heart of Wallingford Software's InfoWorks CS, is the best of breed on both counts. The benefits of this are:
- Network plans that are realistic and not over-sized
- Storage plans that are realistic and not under-sized
- Accurate analyses of pollution spills and flooding - steady state analysis cannot address these
- Operational planning based on an undestanding of the true performance of the network.
Good modeling - modeling that can be used as a basis for investment and operational decisions - requires an accurate, robust and versatile simulation engine. And that means a dynamic model such as Wallingford Software´s InfoWorks CS.