How does increasing source height relative to building height influence downwash probability?

How does increasing source height relative to building height influence downwash probability?

• A. Downwash probability is independent of relative heights.
• B. Downwash is more likely when the source height H is near or below building heights.
• C. Downwash is less likely when the source is much taller than nearby buildings.
• D. Downwash only occurs at very high wind speeds.

Answer: (C)

The key idea is that downwash is driven by how much the exhaust plume interacts with the turbulent wake caused by nearby buildings. When the source height is near or below building heights, the plume stays within that wake and encounters strong vertical and horizontal eddies that can tilt and pull it downward toward the ground, increasing downwash probability. If you raise the source height so it’s much taller than surrounding buildings, the plume tends to rise above the main wake region and experiences less of that downward deflection, so the chance of downwash drops. It isn’t independent of height, and downwash isn’t limited to very high wind speeds—the interaction with building-induced turbulence at lower or moderate wind speeds can still produce downwash.

The key idea is that downwash is driven by how much the exhaust plume interacts with the turbulent wake caused by nearby buildings. When the source height is near or below building heights, the plume stays within that wake and encounters strong vertical and horizontal eddies that can tilt and pull it downward toward the ground, increasing downwash probability. If you raise the source height so it’s much taller than surrounding buildings, the plume tends to rise above the main wake region and experiences less of that downward deflection, so the chance of downwash drops.

It isn’t independent of height, and downwash isn’t limited to very high wind speeds—the interaction with building-induced turbulence at lower or moderate wind speeds can still produce downwash.