Building Simulation - Evidence based Building Simulation

Evidence Based Building Simulation - Low Carbon Design - Mean Wind Pressure Coefficients MWPC

The use of evidence based building simulation is more important than ever within the construction industry because it is one way that building designers can avoid expensive prototypes and ensure a low carbon building design.

Evidence based building simulation relies on data that is specific to a building and its site instead of relying upon generic data or rules of thumb. Evidence based building simulation is vital and to proceed without it, is a high risk route.

The knowledge and understanding gained from the analysis of mean wind pressure coefficients (MWPC) also enables effective remedial work for projects with post-occupancy issues.

Building Simulation has been constructing large, complex evidence based simulation models for many years and is of the view that there is no substitute for building and site-specific mean wind pressure coefficients (MWPC) if the design team is serious about delivering a low energy building.

The fees associated with a full scale physical wind tunnel test and evidence based building simulation are easily recouped by modifications made to the design which avoid unnecessary capital expenditure, unnecessary energy consumption or costly under-performance issues.

Wind tunnel testing is a physical testing method that is used to analyse the wind pressure field around a building. The pressure field around a building is influenced by surrounding buildings and local topology upstream of the wind. The pressure field results are used to develop the MWPCs which are needed to predict how entrances, openings and natural ventilation schemes will impact on a building’s energy consumption.

The key element of the evidence based building simulation model is the air flow network. This network links the internal zones with each other and the external environment. The network describes the size of openings that exist between the zones and between the zones and outside. The user must indicate the ease with which air can pass through these openings.

The air-flow network needs a set of MWPC for each entrance and each natural ventilation opening. The presence of mechanical ventilation can also be represented in the air-flow network.

The building model with the airflow network containing the MWPCs is then simulated with real weather data to produce 8760 hours of evidence based data that is specific to the building design and the site. Analysis of this data to look for further opportunities to reduce energy consumption and to optimise components takes place at an early design stage.

The analysis is different for every building and is guided by the outcome desired by the design team. This in-depth analysis and value engineering exercise is entirely dependent upon a comprehensive, holistic model that would not exist without the MWPCs unique to the building and the site.

Gloucester Quays Designer Outlet Centre

The malls are entirely naturally ventilated and are unheated. The centre has a very distinctive “gull wing” roof shape and the design required the supply ventilation turrets to work without dominating the roof profile.

The wind tunnel model had over-sized ventilation turrets so that pressure tappings could be placed at 2m, 4m and 6m above roof level. Analysis of this data indicated a significant improvement in the mean wind pressure coefficients on windward faces at 4m compared to the tappings at 2m high whilst there was only a marginal difference between the data at 4m and 6m. This indicated that the optimal height was 4m and this was the data set used in the initial dynamic thermal model. The wind tunnel data was also used to optimise the orientation of the wind turrets to maximise the natural ventilation forces for no additional capital expenditure.

The outlets for the natural ventilation scheme proved to be sub-optimal and the design of the outlets was reassessed using the wind tunnel to ensure that for all wind directions the outlets extracted air from the centre. This meant that the natural ventilation louvres could also be used as natural smoke extracts.

White City Shopping Centre, London

The centre is mechanically ventilated and so it would be easy to overlook the benefit of a wind tunnel test to the design team. However, the wind tunnel test results when combined with the dynamic thermal model were able to show which entrances were likely to suffer from the ingress of air during winter, mid-season and summer.

The data was sufficiently detailed that the draught analysis at each entrance could estimate the frequency and quantum of air movement at the entrances. Where possible the design was modified to address the problem of draughts and below design temperatures.

The analysis was also able to identify the frequency of air flowing out of an entrance and therefore the amount of energy wasted when over-door heaters were operating at these entrances could be assessed. As a result of this understanding, a simple energy saving sensor turns the over-door heaters off when the air-flow regime is such that an entrance has air leaving from the centre.

The evidence based building simulation model indicated that the proposed under-floor heating was only required for fewer than 20 hours a year. If the wind driven infiltration through the entrances had not been known, it would have been a riskier decision to remove this costly element from the scheme thereby saving an estimated £600,000.

Bluewater Shopping Centre, Kent

The centre was located within a chalk pit bowl that generated unusual wind effects around the centre. A wind tunnel model was constructed and the results used in the dynamic thermal model.

An objective was to naturally ventilate the car parks and therefore the mean wind pressure coefficients were also used in computational fluid dynamic (CFD) models of the car parks.

It was observed in both the physical (wind tunnel) and computational (CFD) models that the light wells within the car parks were a key element to achieving natural ventilation. Using the CFD model the design of these openings was refined to enable Carbon Monoxide levels to meet the Building Regulations and only 10% of the car park was installed with mechanical ventilation that only operates at peak periods. This saved in the region of £750,000.

Conclusion

The wind tunnel test data when combined with the building simulation model provides evidence based simulation results which avoid design mistakes and remove redundant capital items. The costs of the wind tunnel model and the building simulation model are recovered many times over by savings in capital and energy. In spite of this, many projects still rely on generic data or rules of thumb.

The importance of evidence based performance information at an early design stage ensures that any project can have a successful, low carbon outcome. It is a false economy to include “just in case” systems or to save fees by using generic input data during simulation. A far better use of resources is to allocate some of the “just in case” contingency costs to evidence based building simulation and to confidently pocket the difference!

Extract from a paper presented by Building Simulation at the the 2009 Building Simulation Conference in Glasgow. A copy of the paper covering nine large projects where evidence based building simulation has been used successfully.

A copy of the full paper can be obtained from Catherine Simpson C.Eng, C.Env, FCIBSE, FEI, Principal Director of Building Simulation Limited.