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Vortex Generators

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Vortex Generators

Old 27th Nov 2011, 17:33
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Vortex Generators

Greetings for the Season!

I have a question. first story is - Vortex Generators (VGs) are used to energize the airflow which re-fixes the boundary layer on top of the wing. This ideally can be thought of a sprinkler effect - laymen's thought! in Boeing 757 there are 14 VGs on each wing. The 14 VGs energizers the boundary layer

Second story is - Boeing 737NGs have 10 VGs just before the windshield. Boeing says that reduces the cockpit noise by 3-4 dbs.

Now my question is if the VGs are used to energize the airflow, what is the explanation of them using to reduce the cockpit noise. to my understanding the 10 VGs energizes the airflow more! and the energized airflow hit the windshield creating more noise? Should they call it "NOT" Vortex Generators but something else?

Hoping an answer that could straighten my so confused eyeballs!

thanks

Code0
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Old 27th Nov 2011, 20:13
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Making the boundary layer of the airflow deliberately turbulent can help it follow/stick to areas of high camber that might otherwise cause much greater turbulence.

Turbulator - Wikipedia, the free encyclopedia
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Old 28th Nov 2011, 10:25
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Yeah I quite understand the purpose of the VGs on top of the wing. But what is the connection of reducing the cockpit noise on 737NGs having 10 Vgs on the nose ?

Thanks for your reply anyways!
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Old 28th Nov 2011, 11:26
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Greetings code0,

I think they help the airflow to re-attach around the "sharp" angular edges of the front & side windows. The noise is mostly from the turbulent boundary layer that detaches at those sharp edges.
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Old 28th Nov 2011, 23:32
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Pretty much every aircraft has a lot of camber around the nose/cockpit area, both sides and the roof. Possible exception being concord with the nose up.
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Old 1st Dec 2011, 17:00
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Greetings rudderrudderrat! i think that explanation could somewhat satisfy my soul! thanks! but what is the exact aerodynamic answer for that? I wish i could find a 737NG type rated guy somewhere around here! wold he know? or any Boeing personal ?

appreciate ur feedback, it help a lot!
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Old 1st Dec 2011, 18:39
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737NG rated people won't know to be honest. Most don't even know that some aircraft don't have the generators. Your best option is to go to the 737 tech site and there is a paragraph on the subject or even email Chris Brady. I do find a fairly minor but noticeable difference between the aircraft that have them and the aircraft that do not.
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Old 2nd Dec 2011, 11:15
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code0, whats so hard to understand, as previously stated the VG allow smooth air to flow over the windscreen as opposed to turbulent air. With air swirling around the windows you'll get a lot more noise than if its flowing smoothly over the surface. Just imagine driving along on a very windy day in your car as opposed to on a calm day. When its windy you'll get a lot more noise inside the cabin.

P.S. have you ever heard what a vortex sounds like?
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Old 2nd Dec 2011, 15:50
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Thanks a heap! i already email Chirs! thanks for ur lead waiting for reply!
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Old 2nd Dec 2011, 17:23
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greetings imnotwhoyouthinkiam

thanks for your reply! eye opener! but do u think there is a turbulent airflow over the nose that much? and that can be drastically reduced by 10VGs?

i get ur example driving in a windy day! but just having 10VGs can the noise be reduced considerably?

thanks for the post!

Code 0
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Old 3rd Dec 2011, 09:46
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3 db is nothing, probably less the sound of a needle dropping on a tiled floor.

What is the maximum ammount of Decibals a Human ear can withstand without taking damage? - Yahoo! Answers


0 (N) BEGINNING OF HEARING, A MOSQUITO 10 FEET AWAY, THE EAR DRUM MOVES LESS -REF.1.1990
THAN 1 / 100 THE LENGTH OF AN AIR MOLECULE (N)
3.5 (P) 1E-10 METERS OF MERCURY = 0.0000000001
METERS OF MERCURY
10 (P) ABSOLUTE SILENCE, ATT-BELL LABORATORY QUIET ROOM
13 (P) ORDINARY LIGHT BULB HUM
15 (N) A PIN DROP FROM A HIEGHT OF 1 CENTIMETER AT A DISTANCE OF 1 METER -REF.1.2003
30 (P) TOTALLY QUIET NIGHTIME IN DESERT-IMPOSSIBLE ANYWHERE NEAR CITY
35 (P) ANECHOIC HEARING TEST ROOM -REF.1. 1998
40 A WHISPER, A NORMAL CONVERSATION IS 60 DB, NORMAL SOUND 70 DB -REF.1.1983
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Old 3rd Dec 2011, 10:06
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Reducing the noise by 3 decibels is equal to halving the total energy of the sound.
Human ears more than 85db causes damage and more than 140db causes pain. Give or take a few.
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Old 3rd Dec 2011, 10:47
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A 3dB difference in noise level is observable, but not very noticeable. You'll easily hear a 3dB drop in sound, but telling which of two cockpits has a 3dB higher background noise level is harder, but generally doable.

With noise reduction schemes the aim is to get over 6dB reduction, so that all people say: it is quieter here.
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Old 3rd Dec 2011, 17:22
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So does anyone have any documentation on why the VGs are on the nose, or could it be they just wanted to pull the air down next to the windshield to bring up the pressure so ice would come off quicker, rain would be wicked away faster....

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Old 3rd Dec 2011, 20:54
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Originally Posted by RainingLogic
So does anyone have any documentation on why the VGs are on the nose .
Fuselage - ATA53

Scroll down to the bottom line of "EYEBROW WINDOWS" which says
"Notice the 10 small vortex generators above the radome. These reduce the cockpit noise from the windshield by 3dB." WHICH IS WHAT Code0 was asking at the beginning

Originally Posted by code0
...........Second story is - Boeing 737NGs have 10 VGs just before the windshield. Boeing says that reduces the cockpit noise by 3-4 dbs............... .
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Old 4th Dec 2011, 15:02
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Re: code0

My understanding is that the flow from the nose is rather smooth. The vortex generators then add energy/turbulence to the airflow/boundary layer, to stop it from detaching at the relatively sharp window edges and forming big eddies that dissipate energy and make lots of noise: the vortex generators introduce small-scale vortices into the flow, and the flow then instead follows the contours of the windows, dissipating less energy and being quieter.
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Old 4th Dec 2011, 19:33
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but what is the exact aerodynamic answer for that?
The term 'exact aerodynamic answer' is, in itself, an oxymoron!

The fact is, as measured during flight test with a Decibel meter, repeated on more than one aircraft probably, with the VG's location being varied somewhat, 10 VG's mounted in that specific location on the glare shield reduced the noise by 3 or 4 db, as empitically proven. Game over, go solve the next problem.

Establishing the 'why' is not likely to reduce the cost or improve the performance, or even be useful on another airframe of different configuration!
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Old 5th Dec 2011, 19:20
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I'm sticking with VGs pulling air down to the windows to help the wipers and such...volume reduction might be an added benefit but not the purpose for engineering VGs and their placement, it's a process, and a cost, that doesn't seem justified just to make the cockpit quieter.
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Old 10th Dec 2011, 13:45
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Rough area calculation - area effecting the VGs



Greetings! and Thanks for the ideas!

As the main reason here is "reducing" the cockpit noise, I did a rough mapping of the surface area which is subjected to RAF. Please note that I have based the BS & WL as per to the 737-100/200 AMM

pls refer the image above

A - This is the lateral area - total length of the two No.1 windshields (about 66.14 inches)

B - The hypotenuse is the length of the over nose area + little over the eye brow windows
BS 130 is at the start of nose radome and the BS 259.50 is at the very end of the window No.3 (129.5 inches)
The height is from the WL 208.1 to the top of the fuselage which is at WL 306.5. Since I did not have the information (at this time) the WL that intersect the BL 259.50, I assumed it and calculated about 80% from the WL 208.1, which is 78.7 inches.

The hypotenuse = 151.5 inches

C - is the calculation of the area in 3D space. The pink area is the upper lobe area of the fuselage which meets the RAF before anything! which is about 10059 square inches (about 69.8 square feet)

The green area is the area behind the 10 VGs, which "I" think is subjected for the noise reduction! assuming the green area is about 50% of the pink, which comes to about 5029.8 square inches (about 35 square feet) - please let me know if I have missed anything or have miss-calculated anything!, I will correct it and re-post the image.

Hens what is the practical / technical / aerodynamic answer for Boeing's theory of reducing cockpit noise by placing 10VGs effecting an area of about 35 square feet ? And why other airliners such as (direct comparable) A320 family, Embraer 190 series, Bombadier CRJs and hand full of other's don't have it ?

Last edited by code0; 27th Dec 2011 at 15:30.
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