Advanced Building Simulation

2.4.2 Uncertainty in wind pressure coefficients
Uncertainty in building simulation 43
2.4.2.1 Introduction
To simulate natural ventilation flows in buildings, the wind pressure distribution over
the building envelope is required. In the design of low-rise buildings, wind tunnel
experiments are scarcely employed to measure these wind pressures. Instead, techniques
are used which predominantly rely on inter- or extrapolation of generic
knowledge and data, for example, wind pressure coefficients, previously measured in
wind tunnel studies and full-scale experiments. Due to the complexity of the
underlying physics, this is a process, which may introduce considerable uncertainty.
In the crude uncertainty analysis reported in the previous paragraph, the quantification
of this uncertainty did not go beyond the appraisal done by the analyst performing
the study. However, the uncertainty in the wind pressure coefficients can
more adequately be quantified by experts in the field of wind engineering. These
experts are acquainted with the complexity of the underlying physics and hence best
suited to interpolate and extrapolate the data they have available on the subject and
assess the uncertainties involved. The next section reports on an experiment in which
expert judgment was used to quantify the uncertainties in the wind pressure difference
coefficients in the case at hand.
2.4.2.2 Principles of an expert judgment study
In an expert judgment study, uncertainty in a variable is considered as an observable
quantity. Measurement of this quantity is carried out through the elicitation of
experts, namely people with expertise in the field and context to which the variable
belongs. These experts are best suited to filter and synthesize the existing body of
knowledge and to appreciate the effects of incomplete or even contradictory experimental
data. The uncertain variables are presented to the experts as outcomes of
(hypothetical) 3 experiments, preferably of a type the experts are familiar with. They
are asked to give their assessments for the variables in terms of subjective probabilities,
expressing their uncertainty with respect to the outcome of the experiment.
Combination of the experts’ assessments aims to obtain a joint probability distribution
over the variables for a (hypothetical) decision-maker, DM, who could use the
result in his/her decision-problem. The resulting distribution, which is referred
to as the DM, can be interpreted as a “snapshot” of the state-of-the-knowledge,
expressing both what is known and what is not known.
To meet possible objections of a decision-maker to adopt the conclusions of an
expert judgment study, which are based on subjective assessments, it is important that
a number of basic principles are observed. These include the following:
● Scrutability/accountability: all data, including experts’ names and assessments,
and all processing tools are open to peer review.
● Fairness: the experts have no interest in a specific outcome of the study.
● Neutrality: the methods of elicitation and processing must not bias the results.
● Empirical control: quantitative assessments are subjected to empirical quality
controls.