Wind & Pressure Analysis

A wind rose is the single most useful wind graphic in design. It plots, for a location, how often the wind blows from each compass direction, usually with the petals split into speed bands. A long petal to the west means wind comes from the west a large fraction of the time; a small calm circle at the centre shows the share of near-still hours. Read four things off it: the prevailing direction (the longest petal), how often it actually blows from there (a prevailing wind present only 15% of hours is a weak design basis), the calm fraction (high calm means you cannot rely on wind and should lean on stack ventilation), and seasonal variation - the summer rose can point a different way from the annual one, and it is the summer rose that matters for cooling ventilation.

The data comes straight from the EPW weather file you met in Module 1: it carries hourly wind speed and direction, and tools like Ladybug (in Grasshopper) or Climate Consultant draw the rose for you, filtered to any season or hour range you like. A common and costly mistake is designing to the annual prevailing wind when your ventilation need is in summer afternoons - always filter the rose to the hours you actually care about. Wind speed also rises with height and is slowed by surrounding buildings and trees, so a rose from an open airport station overstates the wind a low building in a dense neighbourhood will really see.

Wind ventilates by creating a pressure difference across the building, and the tool that quantifies it is the pressure coefficient, Cp. Cp is a dimensionless number - roughly between +1 and -1 - describing the pressure the wind produces on a particular face relative to the free wind. The windward face, where wind piles up, has a positive Cp (typically around +0.6 to +0.8). The leeward face and the side walls, where the wind separates and pulls suction, have negative Cp (often -0.3 to -0.7). The roof is usually strongly negative too.

The pressure on a face follows dP = Cp x (0.5 x rho x v-squared), where rho is air density and v the reference wind speed - which is just Bernoulli's principle: faster free wind, bigger pressures, and the effect scales with the square of wind speed, so doubling the wind quadruples the driving pressure. The flow through an opening is driven by the difference in pressure between its two ends - a windward inlet at +0.7 and a leeward outlet at -0.5 give a large driving Cp difference of 1.2, which is exactly why cross-ventilation works. Cp values depend on building shape, the angle the wind hits, and surrounding obstructions; airflow-network tools carry standard Cp tables for simple rectangular blocks, while complex or sheltered buildings need CFD or wind-tunnel data to get Cp right.

Now the payoff: turning wind and pressure into opening decisions. Placement follows the pressure map - put inlets on the positive-Cp (windward, summer-prevailing) face and outlets on the negative-Cp (leeward or side) faces, so the largest possible pressure difference drives the flow. Openings on two faces with similar Cp barely ventilate, because there is little difference to push the air. This is why orienting the building and its main openings to the summer wind is a first-order massing decision, not a facade afterthought.

Sizing balances the two openings. For a given wind, the flow rate rises with opening area but is limited by the smaller of the inlet and outlet (the bottleneck governs). A useful design rule: making the outlet somewhat larger than the inlet speeds the air up as it passes the occupants - valuable in warm-humid climates where air movement over skin is the comfort mechanism, not lower air temperature. Conversely a large inlet and pinched outlet slows the interior air. You size for the summer prevailing condition, then sanity-check the calm and off-wind hours, where stack ventilation (Lesson 7.1) must take over. India's Eco Niwas Samhita sets minimum openable-area fractions for residential ventilation; treat those as the floor, and use the wind and pressure analysis to do better than the minimum where comfort depends on it. All of this feeds the airflow-network model, which turns your Cp map and opening sizes into hour-by-hour air-change rates.

A quick numeric walk-through makes the chain concrete. Suppose a summer afternoon wind of 3 m/s reaches your building's windward face at roughly full strength. Air density (rho) is about 1.2 kg/m3, so the dynamic pressure of the free wind is 0.5 x rho x v-squared = 0.5 x 1.2 x 3-squared = 5.4 Pa. Now apply the pressure coefficients: a windward inlet at Cp = +0.7 sees +0.7 x 5.4 = +3.8 Pa, and a leeward outlet at Cp = -0.5 sees -0.5 x 5.4 = -2.7 Pa. The pressure difference driving flow across the pair is 3.8 - (-2.7) = 6.5 Pa - a modest but very usable driving force for cross-ventilation.

Two lessons fall straight out of the arithmetic. First, the square law: if the wind drops to 1.5 m/s, the dynamic pressure falls to a quarter (1.35 Pa) and the driving difference collapses to about 1.6 Pa - which is why light-wind hours ventilate so poorly and why you cannot lean on wind alone. Second, the driving pressure is genuinely small - single-digit pascals - so anything that adds resistance in the flow path (insect screens, tortuous routes, part-open sashes) eats a large fraction of it. This is also why the numbers should not be over-trusted as absolutes: real Cp values scatter with wind angle and surroundings, and the standard tables assume an isolated block. Use the calculation to understand direction and magnitude - is this a strong or a marginal ventilation situation? - and let the airflow-network model, fed with proper Cp data, turn it into hour-by-hour air-change rates.

Wind is not only a resource to capture; around and between buildings it can become a nuisance or a hazard, and the same analysis governs it. Tall buildings deflect fast upper-level wind down to the ground (the downwash effect), and gaps between buildings squeeze and accelerate flow (the channelling or venturi effect) - which is why the base of a tower or a narrow gap between two slabs can be uncomfortably or even dangerously windy while the street beside it is calm. Pedestrian wind comfort studies assess whether ground-level wind speeds stay within acceptable bands for sitting, standing and walking, using criteria such as the Lawson or NEN 8100 comfort classes, and flag any exceedance of safety thresholds.

These studies are classic outdoor CFD (or wind-tunnel) work: the domain is the building and its surroundings, the boundary is the wind rose and a wind profile that increases with height, and the output is a ground-level wind-speed map read against comfort criteria. The design responses are architectural - podiums, canopies, setbacks, screens and planting to break up downwash and channelling. For the ventilation designer the link is direct: the same wind that you invite into the building through openings is the wind that sweeps around it, and a serious project studies both. Placing an inlet, shaping a courtyard, positioning a tower on a windy site - all rest on the same wind and pressure analysis.