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Article Date: 15th January 2014

Breathing Buildings - The Science of Water Bath Modelling

Natural Ventilation - Ventilation Systems - Passive Ventilation - Comfort Control

Breathing Chimney

Water bath modelling of ventilation flows in buildings is an extremely powerful tool at the design stage of a building. What other technique for example enables you to easily view in three dimensions a transient flow pattern in a building? Computational fluid dynamics (CFD) output is currently really limited to a 2D visualisation because of the fact most people view the output on a 2D screen. Furthermore, if you are using a numerical scheme to determine the flows and observe an oscillatory flow pattern, then you would be right to wonder whether the oscillations are a function of a numerical instability, or whether they are really a physical instability.

The purpose of water bath modelling is to help inform the building designer of the potential building-scale flow patterns which can develop. The technique involves constructing a Perspex model scaled section of a building which represents the ventilation area of interest together with the ventilation openings, introducing source(s) of buoyancy to represent the heating or cooling loads within the building, and immersing the model in a large glass water bath. The water bath can be quiescent or else can itself be linked to a flume so that a background current of water passes round the building to simulate the effects of wind on a building.

Once the sources of buoyancy are turned on, a ventilation flow will commence. This can be visualised using tracer dye released into the model or using a light source through the tank with an image projected onto parchment paper on the side of the water tank.

The source of buoyancy can be heat, in the form of hot plates or wire heating elements located within the tank, or saline solution. If saline solution is used as a source of positive buoyancy, the laboratory model needs to be inverted. The critical things to consider in the design of a laboratory experiment are the following:

Heat transfer: heat transfer in most buildings is dominated by fluid flows through the building. If heat is used as a source of buoyancy in the experiment, it is helpful if the heat transfer is dominated by advection rather than conductive losses through the walls of the tank.

Reynolds number: most air flows in a building are turbulent. It is helpful if the flows in the experimental building and through its openings are turbulent, as the flows will more closely represent the flows in the actual building. The flows will still be informative if the flows through the openings are laminar, but any interpretation of the resistance to flow through the openings needs to account for the different flow regimes

Rayleigh number: convection tends to dominate the air flows in an occupied space and the transfer of heat from lower to higher level, rather than conduction. It is helpful if the Rayleigh number, which is a ratio of convective heat transfer to conductive heat transfer, is sufficiently high that convection dominates in the experiment

Boundary layer flows: Not everything in the laboratory scale experiment can readily represent the flows in the building. Boundary layer flows can be rather different due to the different viscosities of the fluids and we do not usually use water bath modelling to infer what boundary layer flows might arise in an actual building

In summary, water bath modelling is helpful in terms of gaining insights into the potential building scales flows. Once these are known, appropriate mathematical modelling can be undertaken in order to understand temperature variations through a building and over time. This is one of the key points - it helps avoid lots of time-consuming, irrelevant CFD simulations. Instead, if CFD is subsequently used on a project, it can be focused and add real value to the design process! So yes, we love CFD, but only when it is used intelligently, and after insights from techniques such as water bath modelling have been gleaned.

Shaun Fitzgerald
Managing Director

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