Tall Buildings and Wind Speed?

Male Answer Syndome strikes me again!

What I suspect you are experiencing is the disruption caused by the building. This will cause air to sweep up and down the face of the building. At the bottom you get rapidly moving air currents as a result. The same thing happens at the base of a mountain where you get what is known as ridge lift as air sweeps over the sides of the mountain and is pushed upward producing unstable air at the base.

As an aside, aircraft flying close to buildings experience very turbulent air as the air rushes up and down along the building making it tricky for pilots in fixed or rotary wing aircraft.

Ohh..... that's not at all what I remember the explanation being...

You're probably right though its just that I remember that there was explained some sort of mechanism that amplified the wind at the base of a tall structure...

I think the studies were done in an American city where pedestrians were being blown over by the wind gusts... They found that by modifying the building shape or maybe the spacing between buildings the effect was lessened....

I could be remembering totally incorrectly of course!!

John.

Hmm, I wonder if the modification was that everyone had to open their windows? :D

If you change the shape you can encourage more of the air to go upwards - so there is less need for speed-up between the buildings.

BTW, the force on you can rise as the square of the windspeed, so the perceived effect could be greater than you expect.

Bernoulli?

Electroman,

Wind shear around and over buildings and structures of all heights is a commonly experienced phenomenon. Think of it like this.... As you approach the building you experience a breeze wind of, say, velocity 3. (This is during a moment during which you pause, standing still...or, for sake of explanation, we can disregard your small walking velocity.) Now, the wind that impacts you (frontally) at speed 3, must also be approaching the far side of the building up ahead at velocity 3. However, before that wind that approached the building at velocity 3 reaches you, it must first make its way (must travel a longer distance in the same time) around the building on both sides, then merge back together on the near side of the building, and then continue on towards you where you sense its velocity of 3. But, on the far side of the building the wind's original progress at speed 3 must have been impeded when it encountered the building in its path and when it was deflected to either side of the building. So, in order to again be travelling at 3 when the wind encounters you as you approach the building, it must speed up to a velocity higher than 3 in order to resume its journey away from the building at a velocity of 3. This increase in velocity along (in this case) the sides of the building are a result of the building's having "sheared" the wind. Finally, as you come near and pass by one side of the building, you find yourself in a sheared portion of the wind stream--the portion of the wind stream that approached the building that is now rushing to travel the longer distance around the building to rejoin the wind stream rushing round the other side of the building, so it can continue from that point (now behind you) onward at a velocity of 3. As others have pointed out, the shearing effect will occur respecting wind making its way over the top of the building as well. Finally, in locales where numerous buildings are situated near to each other, the shearing produced by so many structures in close proximity can bring about turbulence (eddies and such) as the different wind shear effects interact with each other...and yet the net result will be the same: the breeze that encounters the group of buildings at a certain speed will also depart the group of buildings at the same speed.

I too remember hearing of an explanation and cant remember it clearly but I think it went something like this.

The effect is similar to the wing tip vortices on an aircraft. At the tip of an aircrafts wing the air flowing under the wing will be at a higher static pressure than that flowing over the upper surface. As a result a percentage will spill around the wingtip from the lower surface to the upper causing the wing tip vortex. The result of the vortex will be a decrease in the lift and consequently velocity difference, between upper and lower surface, at the wing tip compared with the airflow at the wing root.

Now with a short building a similar thing takes place with some of the air on high pressure side spilling over the top of the building to the low pressure size reducing the pressure differential and thus reducing the velocity around the building. With a tall building the same thing happens but this time at the top of the building and the effect decreases rapidly as you get closer to the ground. As a result at the ground the wind velocities experienced will be higher as they can't spill over the top of the building like they can with a short building.

I think that's the way it works, if I'm however wrong please feel free to correct me.

Ahhh yes that seems a more likely explanation...

Afterall if you place a block in an airstream you will generate vortices around the back of the block where the pressure will be less than at the front... With a tall building these would be magnified as the vortices from the top of the block wouldn't help equalise the pressure near the base...

I think?

As you said Masu, with aircraft wing tips those vortices are very powerful, in fact I know that at airports the distance between aircraft landing has to be kept above a certain distance due to an aircraft following a large aircraft being literally torn to bits in the vortex turbulence...

Thanks guys - Unless anyone wants to comment further of course?!!

John.

Those wing tip vortices can get really nasty an can linger for quiet a wile, I saw a piece of film once of a Lear Jet following behind and below a 747 in a test of wing tip vortex strength. The guy in the Lear Jet needed a new flying suit after the test. The vortex flipped the Lear jet and rolled it several times before he managed to get control again. The Airbus crash in Queens, New York was indirectly caused by getting caught in wake turbulence and the pilot ripping the tail fin off while he tried to regain control.

Masu (elecroman)

Read guest's post above yours and feel corrected.

The Building was designed for efficient vertical use of space and since this entails a square or rectangle and it is not at all aerodynamic where the flat shapes are resisting any wind pressure and the wind is forced to divert around and away from the vertical cube and hence the wind disturbances even at ground level.

Not all building are rectangular. I know of one skyscrapers that is circular and 3 that are octagonal. If it was just the fact that the building was square the wind speed around any square building would be the same. What we are trying to explain is not why it is windy near a building but rather why near a high rise building is the wind stronger than that near a low rise building.

There could be another effect at work here - the wind at ground level is usually much slower than higher up, and the tall building provides local coupling. If this is correct, the proportionate effect will be greater when the wind is relatively gentle, because turbulence is not bringing gusts to ground level unless there is a major obstacle like the building

There is another significant factor regarding this topic, especially regarding mountains and valleys. Maybe it applies to tall buildings also. The sunny side of a mountain, valley, or building will reach temperatures that cause the air to rise and in contrast the air on the opposite or shady side will be significantly cooler causing the air to drop towards the ground. This is especially dangerous to small aircraft flying at low altitudes over mountains and valleys. Could it also be a factor with tall buildings???

Yes

What loads of crap to a simple question seeking a simple answer! Windshear does not require explanation of all things physical and meteorological to understand. Air travelling a greater distance (around a building for example) during the same time span in which it would have travelled if the building were not there must increase speed to do so. It's that simple. Otherwise, all one would need to do in order to mitigate the effects of highwinds travelling across a countryside, would be to build a a group of buildings in the winds path--ridiculous when you thing logically about it, don't you see?

I'M SORRY YOU ARE SO BITTER WHEN FOLKS CHOOSE TO DISCUSS TOPICS IN WHICH THEY ARE INTERESTED. MAYBE YOU SHOULD POST A LIST OF THINGS THAT INTEREST YOU SO WE DON'T BORE YOU.

Whichever Guest wrote this.

"Read guest's post above yours and feel corrected."

The process that Guest describes in post #5 dose not explain why if you have a single high rise building the wind around it will be greater that the wind around a single low rise building.

Secondly wind shear refers specifically to the particular meteorological phenomenon where there is a turbulent boundary layer caused by winds at different but adjacent altitudes traveling in different directions or at different speeds. Wind shear is not what is causing the increased velocity of the wind as it is deflected and or channeled around or between buildings. Channeling can however cause quiet severe winds but doesn't explain the effect electroman is describing of a single tall building.

The guest that posted this

"There could be another effect at work here - the wind at ground level is usually much slower than higher up, and the tall building provides local coupling. If this is correct, the proportionate effect will be greater when the wind is relatively gentle, because turbulence is not bringing gusts to ground level unless there is a major obstacle like the building"

in post #11 is actually referring to wind shear and this very well could contribute to why the wind around tall buildings is stronger.

ietech wrote

"The sunny side of a mountain, valley, or building will reach temperatures that cause the air to rise and in contrast the air on the opposite or shady side will be significantly cooler causing the air to drop towards the ground."

in post #12 is describing two separate things. The falling air is katabatic wind but they normally take mountain ranges to form and need gullies to concentrate them and so I doubt that they could be the primary cause but it may contribute to the phenomena electroman described.

The rising air is a thermal and they have some interesting properties. To start with their vertical speed rarely exceeds 1,000 fee per minute (10Kts, 11.4mph, 18Km/h 5ms^-1) but they do have the tendency to rotate as they rise. Most people will have seen a vortex or whirlwind on a hot day pick up dust and papers hurling them into the atmosphere. What you have seen is a thermal forming and lifting off. They can create isolated pockets of quiet intense wind but they generally require fairly large areas to form and I doubt that a building is big enough.

Having said that both the katabatic wind and thermal formation could contribute in some way to the effect we are attempting to describe.

As for post #15 by person unknown calm down, we are just discussing what could cause the phenomenon of the wind around a skyscraper being stronger than the wind around a low rise building. Being a glider pilot I am fairly well versed in the movement of air and the causes of these movements. Itcan be a complex subject so I would be interested in your simple explanation for the phenomenon.

The process that Guest describes in post #5 dose not explain why if you have a single high rise building the wind around it will be greater that the wind around a single low rise building.

Actually the post explains it quite well; but one needs one's thinking cap on. Consider first that taller buildings, although tallness gives the appearance of slimness, are usually proportionately larger around the base--where our friend might find himself walking in the stiff breeze. A larger base implies that the deflected wind racing around the building must travel a greater distance to rejoin its sheared counterpart on the leeward side of the building. The greater distance means that the wind speed must be faster, proportionally as the distance travelled increases. Now take a case with all things, except height, being equal between a tall and short structure. Remember, though, that the wind will "attempt" to go not only laterally, but also vertically "around" the obstruction. The taller building, however, will present a greater obstacle to the wind's surmounting it than the shorter building. Since the path over the building is not so readily available with the taller, as opposed to the shorter, building, then more wind will be impelled to take the journey laterally around the taller building. More wind (particles) a stiffer breeze. As stated or implied previously, the effect takes place in spite of size of the obstruction--but with most wind impediments (such as a house, tree, boulder, delivery van,.....the effect is not so noticable if at all. It's only when one encounters very large obstacles that that wind shear velocity becomes so great as to be noticable...or, on a cold day, uncomfortable. Hopes this makes it clearer.

Guest in post #18 you stated

"Remember, though, that the wind will "attempt" to go not only laterally, but also vertically "around" the obstruction. The taller building, however, will present a greater obstacle to the wind's surmounting it than the shorter building. Since the path over the building is not so readily available with the taller, as opposed to the shorter, building, then more wind will be impelled to take the journey laterally around the taller building."

Which is pretty much what I said with;

"Now with a short building a similar thing takes place with some of the air on high pressure side spilling over the top of the building to the low pressure size reducing the pressure differential and thus reducing the velocity around the building. With a tall building the same thing happens but this time at the top of the building and the effect decreases rapidly as you get closer to the ground. As a result at the ground the wind velocities experienced will be higher as they can't spill over the top of the building like they can with a short building."

Back in post #6 isn't it, or am I missing something?

Masu,

Great! Then we have actually been in accord since post #5. Apologies for misconstruing the post to which I responded.

To the gentleman who stated that the air must travel faster around the sides in order to meet up in the back.

You are wrong here, there is no physical reason for this to be so. In if the air were deflected around as you say it would travel around the sides of the building more slowly than the freestream velocity not faster. The same explanation you have given has been used in slightly different form to explain how wings create lift, but it is incorrect.

For sake of argument say the building is the shape of a symetrical airfoil. Wind could come from the front and be deflected, travel around the sides and continue downstream. The air immediately downstream does not have to be the same speed as the incoming freestream velocity. It would likely be slower for some time downstream of the trailing edge, also turbulent.

Presumably all the air upstream air that is moving to cross the plane of the building goes somewhere. So you have the same mass of air crossing a plane (perpendicular to the original flow) that includes the building as you have crossing an upstream plane. So either some of the air must be compressed as it crosses the building plane, or it must move faster. I know of no case where there is significant average compression, therefore the air mostly moves faster - though there will doubtless be regions (even in the plane of the building) where it is slower. If the wind is sufficiently slow that viscosity is more important near the building than momentum, then I expect the wind would indeed be slower immediately by the building. But it would still be faster on average in the environs of the building than elsewhere.

Of course, this is only an answer to the point you raised here. The more extreme effects that are often observed are due to the building providing coupling of the fast-moving air at high altitudes (due to wind shear) towards the ground, as discussed by other contributors.

Fyz

Hello, english is foreign to me so sorry for spelling and maybe my grammar.

So, the answer here is very logic.

I think this phenomenun is most easy to explain like this.

Lets say you have a big pipe, radious 5 m and speed of water going in 10 m/s and after 100 m the pipe gets constricticted on radious of 2,5 m , so what will happen with speed ???

The answer is that it will increase to cca ~ 20 m/s because it will become more compressed becaouse the pipe has a shorter radious.

Maybe even more accessible way for me to explain is this - Have you ever hosed a lawnand put your thumb on the and so that wather would splash further ? If you did than you know what i'm talking abut , water got compressed and it got out of hose with much greater speed witch allowed it to go further .

Same thing is with your guys problem, it has a big skyscraper in it's way and it got compressed and got higher speed.

So now you think , but pipes it made of plastic and water can't go out, so why doesn't wind spread , well answer is, it does, but i also has it's boundries (borders or whatever you call it ) wind can't just go anywhere, it dipends on so many different factors, i don't have time to explain .

So i hope i helped :D

bye

Radius changed factor 2, area changes factor 4, so speed changes factor 4...