×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Are you an
Engineering professional?
Join Eng-Tips Forums!
• Talk With Other Members
• Be Notified Of Responses
• Keyword Search
Favorite Forums
• Automated Signatures
• Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

#### Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

# wing airflow - a different "view" via CFD

## wing airflow - a different "view" via CFD

(OP)
Consider the "visuals" of the following:

1/ "Standard" wind tunnel wing "view" c/w streamlines. - At low AoA the air is leaving the trailing edge at an angle of -say- 15 degrees below HORIZONTAL

2/ A helicopter rotor "disk" rotating in the horizontal plane. - At low blade AoA the airflow is moving "only" VERTICALLY STRAIGHT DOWN

So why the difference?

In 1/ the air is moving over the stationary WING

In 2/ the wing is "cutting" through the stationary AIR

Why in "all" textbooks is option 1/ chosen? Maybe this was the "standard" and "only" option before CFD visuals.

Why not use CFD to promote the visuals of option 2 which in my opinion is the more "truthful" option since in the end lift is the reaction force to air being "forced" vertically downward by the "wing" shape.

Replies continue below

### RE: wing airflow - a different "view" via CFD

Either model will give you the same information.  Since wind tunnels were the way to get data in the golden days, it definitely follows that 1/ was the option of choice.  Air leaving the trailing edge of any wing is imparted a vertical velocity, whether the wing is moving and the air is stationary, or vice-versa.  The angle of downwash is a bit more meaningful in the wind tunnel as:

e = arctan(downwash velocity/wind velocity)

When you consider the air to be still, and the aircraft to be moving, the equation loses meaning unless you re-name the wind velocity to be "flight velocity".  Many textbooks do this interchangeably without much thought.  Since CFD is still being corroborated with wind tunnel tests these days, new textbooks will probably continue coming out using the wind-tunnel view.

STF

### RE: wing airflow - a different "view" via CFD

(OP)
Some descriptive "DOWNFLOW" here by Boeing experts

http://www.amasci.com/wing/straub.txt

BUT still not a single diagram (CFD or other) showing DOWNFLOW posted in response yet (three months).

#### Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

#### Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Close Box

# Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

• Talk To Other Members
• Notification Of Responses To Questions
• Favorite Forums One Click Access
• Keyword Search Of All Posts, And More...

Register now while it's still free!