OverviewOverviewControl of water flows is essential to a wide range of infrastructure:
Water flows are also central to understanding of the water environment, as flow quantities determine the transport and mixing of dissolved and suspended material through the water column.
Many organisations spend millions or hundreds of millions of dollars on making water flow where it is wanted, and stopping it from flowing elsewhere.
Unfortunately, present engineering design approaches can be highly conservative, so projects fail or cost far more than they should. Consistent improvements in design technique and environmental protection rely on advances in hydraulic modelling. This process will ensure that the designs used will successfully control the water under all the conditions that are required to be met.
Hydraulics
Design for all of these projects is based on the engineering discipline of hydraulics. This name derives from the Greek hudor + aulos = water + channel, so hydraulics deals with water in channels.
For thousands of years, engineers had to rely on previous similar projects as initial concepts, adjusting the design to the new conditions by a combination of experience and observation. This is still an important part of the engineering process, because many alternatives always exist, and it is not possible to reduce the range to a single adopted design by purely rational means.
Modelling
In the last hundred years, it has become recognized practice to construct a model of the proposed design to check that it will perform as intended before construction begins, and to test adjustments to improve that performance. This requires confidence that the constructed project will behave like the model.
Initially this confidence was based on an assumption that the project could be scaled down to a geometrically similar construction small enough to fit inside a laboratory. This has been found to be partially true, but with accuracy decreasing sharply as the model scale becomes smaller.
This led to laboratories becoming larger and more expensive, so that only major projects could be modelled economically. At the same time, computers were becoming more reliable and cheaper, so started to be competitive as an alternative approach to modelling. Through the development of computational hydraulics, it was shown that numerical models could be constructed which were free of the scale effects of laboratory models.
However, resolution limitations are inherent in approximating a continuous system using a grid of finite dimensions, and these have turned out to impose formidable obstacles to the search for complete numerical hydraulic models. For example, it is not known at present whether stable general numerical solutions to the Navier-Stokes equations (fundamental to Fluid Mechanics) even exist!
Yet numerical solutions for many standard hydraulic problems have been available for years. These were usually simplified by the assumption of steady flow, which is not changing with time.
Unfortunately, this simplification produces unrealistic model performance, as most real projects have to deal with actual flows, which do change in time. To allow for the resulting errors, most steady flow models are highly conservative, so that project designs cost far more than they should, or even fail to meet the project aims.
Many such tools are available, but most of them oversimplify the task to make it easy for simple computation methods to be used. To model the behaviour of the water you use some basic principles of physics – conservation of mass, conservation of energy, and conservation of momentum. The commonly used ones use the conservation of momentum, and work well in simple situations, but for a full and accurate model you must model the conservation of energy as well. AULOS is unique in the way that it does this, offering unsteady energy analysis that extends your solutions well beyond those given by momentum analysis.
A new analysis of the problems with unsteady flow models has been completed. This is based on the Full Hydraulic Equations, a complete set of unsteady energy, momentum and mass equations derived using Cell Integrals, a new method of linking the physical continuous system to a finite grid. This analysis determines the reasons for solution instability, and shows it is linked to limited areas of physical instability. Software can therefore be written using this new analysis to provide maximum accuracy with the computing resources available.
Although some deeply abstract physics has gone into developing AULOS, it is intended as a tool for design engineers working in both small scale and large scale real-world situations. For example, when you use AULOS to do unsteady energy analysis in a practical stormwater design situation, you may find that the peak upstream level is half a metre higher than that predicted by full dynamic momentum analysis. While this might not overtop freeboard allowances, it does affect the true value of the return period for overflow. A design done in this situation using the momentum (St Venant) equations could have a flood protection return period of perhaps only 10 years when the design code required 100 year protection.
In more complex situations you will find that you cannot model the system at all, or in a sensible time frame, without AULOS . An example of this is the modelling of systems that combine overland flow and piped or channelled flow. This is important in modelling designs that rely on overflows in extreme events, such as the Auckland City example described below. These designs use distributed surface ponding areas to reduce construction costs and to improve stormwater quality. By using AULOS, you can design realistically, by allowing for controlled inundation up to permitted limits. This can typically save 10% or more of the capital cost compared with conventional design of drainage projects, with the added assurance of closer conformance to design code specifications.
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