1.8. Stacktip Downwash

There are a couple of small artifacts to this dispersion, these are the non-idealities one of them is called as stacktip downwash, now this is not important for ambient pollutions, this is important for in the place in the source, for example a factory and you have a stack you have a chimney which is releasing emissions.

Stacktip downwash refers to a phenomenon that occurs when emissions from a chimney or stack are adversely affected by nearby environmental conditions. More specifically, when exhaust gases rise from a stack, they can be pulled back towards the ground due to a low-pressure area created by wind blowing around the stack. This effect is particularly pronounced when the exit velocity of the gases from the stack is lower than the wind speed. As a result, the pollutants may accumulate near the base of the stack rather than dispersing into the atmosphere.

Imagine you are blowing air through a straw into a swimming pool. If you blow the air gently, it may not make it to the surface and instead spreads out below the water while getting pushed back by the water currents. Similarly, if the exhaust gases from a stack aren't strong enough to rise above the influence of the wind, they can be pulled back down and can accumulate right around the stack, increasing exposure for anyone nearby.

The reason for this is the same reason why particles get adsorbed with it is, the wind is here. The streamline of the wind flows around the stack when that happens there is a low pressure region that is created here right behind the stack because its low pressure if anything comes in this region, it gets sucked into this low pressure region and circulates there.

When wind flows around a stack, it alters the air pressure around it. This alteration creates a low-pressure area at the back of the stack. If the exhaust gases are not moving quickly enough to overcome the wind's influence, they can be drawn back toward this low-pressure zone. As a result, rather than dispersing away from the stack, they circulate back into the area near the ground, which can pose health risks for people working near the emission source.

Think of a bicycle rider speeding through a windy day. If the rider isn't pedaling fast enough against the wind, he might find it hard to move forward, almost as if the wind is pushing him backward. Just like the rider, if the emissions from the stack do not have enough velocity, they cannot break out of the wind's pull and may instead fall back into the low-pressure region created behind the stack.

So to avoid that people have a recommendation that the stack velocity the gas velocity coming from a stack must be at least 1.5 times greater than the wind speed at that point.

To mitigate the effects of stacktip downwash, it is recommended that the exiting gas velocity from stacks is at least 1.5 times the speed of the wind at that location. This higher velocity helps ensure that the emissions rise well above any low-pressure zones created by the wind, thus allowing for better dispersion into the atmosphere and reducing potential health hazards for nearby individuals.

Consider a hot air balloon. For it to rise high enough in the air, it must produce enough hot air inside. If the temperature difference isn't sufficient, the balloon will not lift off. Similarly, stacks need to eject gases at a higher velocity than the wind to ensure that they rise high enough into the atmosphere and avoid downwash.

The other type of artifacts is called the Building Downwash. This is more serious, this affects people away from the source, so there is a streamline.

Building downwash is a significant issue as it affects air quality beyond the immediate vicinity of the emission source. When buildings obstruct wind flow, they cause air to diverge and could generate low-pressure areas (called wakes) behind them. This can lead to the stagnation of exhaust plumes from nearby stacks, resulting in increased ground-level concentrations of pollutants, which could endanger public health.

Imagine air flowing over a large rock in a stream. Instead of flowing smoothly, the water becomes turbulent and may form an eddy behind the rock where debris collects. Similarly, buildings create 'eddies' in the air flow, capturing exhaust emissions instead of letting them disperse, which is why it’s critical to consider building placements and heights when designing industrial stacks.

If your stack releases right on the roof of a building and there is a height of this wake there is this some distance is the length of the wake. Depending on the size of the building this dimensions form and the velocity of the air.

To effectively mitigate building downwash, stack emissions must be designed to extend above any obstruction caused by the building. This means that emissions should rise high enough to break free from any stagnant air created by the building's wake. The height of the stack needs to account for this wake length to ensure that the emissions are properly dispersed and do not accumulate in harmful concentrations around or near the building.

Think of trying to throw a ball over a tall fence. If the ball doesn't go high enough, it will either bounce off the fence or fall right in front of it. Similarly, if emissions from a stack don’t rise above the height created by the building’s wake, they will not escape effectively and may linger, causing a buildup in concentration right around the building.

Stack on top of a building, which is not extending into the building. There a lot of examples you can go around see; Dispersion, stacktip downwash, building downwash, small sources.

In urban settings, various sources contribute to stacktip downwash and building downwash issues. For instance, many buildings house emission sources such as kitchens and diesel generators with stacks that often fail to rise sufficiently above surrounding obstacles. This failure leads to high pollutant concentrations accumulating in populated areas, worsening air quality and health concerns.

Imagine if every home in your neighborhood had a barbecue going, but the smoke from the grills is trapped in the yard by high fences. Instead of dissipating into the open air, the smoke builds up, which can be a risk to the health of those in the yard. Similarly, diesel generators and exhaust systems that do not properly rise above the urban structures can lead to air pollution that poses health risks.

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