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Article Date: 18th December 2013

Breathing Buildings - Water Bath Modelling at Different Building Scales

Natural Ventilation - Ventilation Systems - Building Ventilation - Passive Ventilation

Breathing Perspex Model

Water bath modelling is a powerful way of examining the various flow regimes that may occur within a naturally ventilated building. It involves creating a scaled Perspex model of part or all of the building, along with its ventilation openings. This model is submerged in water and heat gains on each floor are created by supplying power to hot wires representing the locations of heat sources on each floorplate. With proper scaling, the large scale motions of buoyancy-driven flows are representative of those in a real building, and can show up features such as transient or oscillatory flow regimes that would require very significant effort to replicate in computational fluid dynamics. It also allows the experimenter to try out numerous different scenarios (such as changing sizes of individual, or multiple ventilation openings, or changing the heat gains in different areas of the building). Within minutes, the outcomes become observable in 3D, and videos or images of the resulting flows (visualised with dye) can provide a clear explanation to the client of complex flow phenomena.

The scaling of the building is crucial to a successful representation of the flow. It's always best to consider which features of the building are necessary to demonstrate the flow. Where spaces are individually naturally ventilated, it is often easiest to create a few, simple, small models. However, where spaces are interconnected, a larger model may be required to properly represent the interactions that result from air moving between the various spaces.

Once these decisions have been made, the next consideration is maintaining the aspect ratios of the floorplate depth, breadth and height. When buildings become deep plan with a large number of storeys interlinked, there is a limit to the minimum height of a floor due to the practicalities of the size of hot wires. This can then lead to large floorplate models and the need to use larger water tanks.

We have two water tanks so as to be able to cater for both simpler and more complex models. Our smaller tank is of dimensions 750mm by 750mm by 500mm and our larger one is 2000mm by 2000mm by 1000mm. This allows us to carry out a wide range of experiments including buildings with deep plan floorplates, atria and many storeys, as well as the simpler spaces where scaling permits using the smaller tank.

The small water tank is great, and is definitely easy to work with. It has lent itself to really cost effective experiments and visualisations for spaces from atria in schools, offices and healthcare buildings through to theatres, and even a dog's home. but the real excitement comes when we get to use the large tank to look at something really complex.

Recently we have carried out a water bath experiment that made full use of the larger tank and was logistically the most challenging model we have yet worked on. Housed in our manufacturing facilities, we had up to 4 tonnes of water to contend with to model a large multi-storey office building and when running the model, we had to be careful not to exceed the power that our industrial unit's supplies could muster. The results threw up some interesting findings about the directions of flow, and our client brought in specialist camera equipment including underwater cameras to capture video showing the stack effect in action. It promises to be a fantastic building delivering a world-class level of thermal comfort.

Our MD, Shaun has recently blogged on these pages in more detail about the science of water bath modelling so if you are interested in the topic, it's a good read. Hopefully this blog gives a bit of an insight into the practicalities and benefits of the modelling and our capabilities to model a wide range of buildings, from single rooms right up to the most complex schemes!

David Hamlyn
Consulting Engineer

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