Unsteady wind effects in natural ventilation design.

Unsteady wind effects can be important in natural ventilation, but their treatment requires knowledge of instantaneous surface pressure distributions which are extremely difficult to obtain. The paper describes a theoretical investigation aimed at determining the effects of unsteadiness and, perhaps more important, the conditions for which it may be worth accounting for them in the design process. For generality the study uses nondimensional parameters. The effects of unsteadiness on both mean and instantaneous flow rates are covered. The latter effect is particularly important for natural ventilation stacks, where flow reversal is undesirable. It is concluded that unsteady effects can be important, but they are restricted to a relatively narrow range of conditions. A simple procedure for calculating the mean flow rates when the unsteady effects are large has been derived. In its simplest form the procedure does not require knowledge of the instantaneous wind pressures. For instantaneous flow rates it is shown that substantial reductions in the occurrence of flow reversal can be achieved by the use of long ducts, but the siting of openings is also important.