scope:

This guideline describes a computational model which can be used to calculate a time-average, threedimensional flow field over a flat surface in the region of obstacles (for example buildings). The typical dimensions of the zone lie between a few decameters and a few kilometers.

Typical applications of the guideline:

• provision of a three-dimensional flow field for the subsequent propagation calculation
• measurement network planning in the inner city zone for flow measurements
• investigations into possible changes in the mean wind field caused by building measures Building measures can limit the ventilation in an entire urban district or in a town, or zones can arise in which the flow velocity is extremely high. These problems must be taken into account during planning.
• determination of the wind field for the purpose of calculating energy requirements
• Modern calculations of energy requirements for buildings should take account of the
• wind field and the variations in the wind field caused by building measures. wind field calculations as a basis for human biometeorological investigations (see also VDI 3787 Part 2)

The thermal sensitivity of humans depends via their energy balance on, e. g., the flow velocity. If the latter rises, the thermal sensitivity is influenced positively (decrease in thermal stress in summer) or, negatively (increase in low-temperature stimulus in winter).

Input variables for the model are the undisturbed vertical profile of the wind speed, the register of obstacles and/or buildings and land use. It can be assumed in general that the flow is determined by dynamic influences within the built-up area, and that stability is of subordinate importance.

Diagnostic models are economical in terms of storage space and computer time, and so large fields with a high spatial resolution (to the extent this is sensible) can be calculated on a PC.

The application of the guideline requires expert knowledge.

A computer program relating to this guideline and including the associated user manual can be obtained as an example for a diagnostic microscale wind field model from the following address: http:// www.vdi.de/dmw. The computer program is one possible programming approach to converting the exemplary implementation, described in the annex, of a diagnostic flow model.

The initialisation method described here is not invariant against rotation on the buildings in the computational grid and against a potentially different kind of digitalisation of the obstacles.

Another implementation without these limitations is described in [16].