5.2.1. Straight roof geometry
Fig. 8a displays the volume flow rates for the straight roof geometry
cases (A2 e reference case e, A2x2_OR1 and A2x2_OR0.5)
and shows that the double-span leeward roof with an opening ratio
of 1 (A2x2_OR1) reaches a slightly higher volume flow rate (increase
of 1.4%) than the single-span reference case (A2). The case
A2x2_OR0.5 however reaches a 25.6% higher volume flow rate than
A2. This increase of volume flow rate with decreasing opening ratio
(larger outlet openings) was already reported by Karava et al. [39]
and can be attributed to the lower overall flow resistance as a
result of the larger outlet opening (window opening area), which
leads to more pressure equalization between the indoor and outdoor
environment near the outlet. The increase is in line with what
one could expect based on the orifice equation for cross-ventilation
flow. If one would assume (hypothetical situation) that the two
leeward openings are actually one opening, then Eq. (7) can be used
to calculate the wind-driven ventilation flow through two openings
that are in series, based on the pressure coefficients on the windward
(CP,W) and leeward facade (CP,L), the total discharge coefficient
CD,total and the equivalent window opening area A.
Q ¼ CD;totalAUref
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
CP;W CP;L
q
(7)
In which CD,totalA can be calculated using Eq. (8) (e.g. [39]).