CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > CFX

Quantifying the swirl in CFX, Swirl Number/Swirl Angle

Register Blogs Community New Posts Updated Threads Search

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   March 15, 2012, 05:56
Default Quantifying the swirl in CFX, Swirl Number/Swirl Angle
  #1
Member
 
Raja_Bhai
Join Date: Feb 2012
Location: UK
Posts: 40
Rep Power: 14
tauqirnawaz is on a distinguished road
[Note: It might be the case that this information has already been shared on the CFX forum but this is an effort to bring it under one thread.]

Quantifying swirl becomes very important in some situations where you would like to reduce it or even sometimes when requirement of design is to increase the swirl.

Swirl Number:

The degree of swirl in the flow can be quantified by the dimensionless parameter, Sn, known as the swirl number which is defined as the ratio of the axial flux of angular momentum to the axial flux of axial momentum:



To calculate in the CFX, create the following CEL expression;

Swirl Number [non dimensional] = areaInt(Density*sqrt(Velocity v*Velocity v)*sqrt((Velocity u*Velocity u)+(Velocity w*Velocity w))*sqrt((X*X)+(Z*Z)))@Plane 1/(maxVal(sqrt((X*X)+(Z*Z)))@Plane 1*areaInt(Density*Velocity v*Velocity v)@Plane 1)

Where
areaInt = Area Integral
sqrt = Square Root
Velocity v = Velocity in mean flow direction i.e. Y-axis in this case
Velocity u = Velocity in X-axis
Velocity w = Velocity in Z-axis
maxVal = Maximum Value
Y-axis is perpendicular to Plane 1, while X-axis and Z-axis are parallel to the Plane1 in this case.

For centrifugal pump impeller design it should be between 0.05-0.1 for good suction performance or 0.01 for excellent suction performance.

Swirl Angle:
This is again very important for specifying the blade angles in centrifugal pumps.

Use following CEL expression;

Swirl Angle[radians] = atan2(sqrt(Velocity u^2+Velocity w^2), sqrt(Velocity v^2))
Where
atan2 = arctangent
sqrt = Square Root
Velocity v = Velocity in mean flow direction i.e. Y-axis in this case
Velocity u = Velocity in X-axis
Velocity w = Velocity in Z-axis
Y-axis is perpendicular to Plane 1, while X-axis and Z-axis are parallel to the Plane1 in this case

Then create a variable SwirlAngleVariable to calculate an area average over the plane, this would give you a value in degrees.
Attached Images
File Type: png user46827_pic359_1331478970.png (11.8 KB, 1912 views)

Last edited by tauqirnawaz; March 16, 2012 at 04:48.
tauqirnawaz is offline   Reply With Quote

Old   March 15, 2012, 22:50
Default
  #2
Senior Member
 
Safia
Join Date: Oct 2010
Location: Australia
Posts: 161
Rep Power: 16
happy is on a distinguished road
Quote:
Originally Posted by tauqirnawaz View Post
[Note: It might be the case that this information has already been shared on the CFX forum but this is an effort to bring it under one thread.]

Quantifying swirl becomes very important in some situations where you would like to reduce it or even sometimes when requirement of design is to increase the swirl.

Swirl Number:

Sn or Swirl number is defined as

To calculate in the CFX, create the following CEL expression;

Swirl Number [non dimensional] = areaInt(Density*sqrt(Velocity v*Velocity v)*sqrt((Velocity u*Velocity u)+(Velocity w*Velocity w))*sqrt((X*X)+(Z*Z)))@Plane 1/(maxVal(sqrt((X*X)+(Z*Z)))@Plane 1*areaInt(Density*Velocity v*Velocity v)@Plane 1)

Where
areaInt = Area Integral
sqrt = Square Root
Velocity v = Velocity in mean flow direction i.e. Y-axis in this case
Velocity u = Velocity in X-axis
Velocity w = Velocity in Z-axis
maxVal = Maximum Value
Y-axis is perpendicular to Plane 1, while X-axis and Z-axis are parallel to the Plane1 in this case.

For centrifugal pump impeller design it should be between 0.05-0.1 for good suction performance or 0.01 for excellent suction performance.

Swirl Angle:
This is again very important for specifying the blade angles in centrifugal pumps.

Use following CEL expression;

Swirl Angle[radians] = atan2(sqrt(Velocity u^2+Velocity w^2), sqrt(Velocity v^2))
Where
atan2 = arctangent
sqrt = Square Root
Velocity v = Velocity in mean flow direction i.e. Y-axis in this case
Velocity u = Velocity in X-axis
Velocity w = Velocity in Z-axis
Y-axis is perpendicular to Plane 1, while X-axis and Z-axis are parallel to the Plane1 in this case

Then create a variable SwirlAngleVariable to calculate an area average over the plane, this would give you a value in degrees.
Hi Raja_Bhai.....
As I know that the results from cfx is in x,y,z coordiantion, and the velocity component into the above expression is in r, tetha, z coordiante. your expresion can make unaccurate estiamtion of swrling no.

You know some times the horizontal plane which include the two component of the velocity, (in r and theta direction or in x&z direction ( considering Z is vertical) is inclined with angule. so the unaccurate estiamtion can come from.

If any one has any knowledge about this issue, please share it with us.

Best Regards
happy is offline   Reply With Quote

Old   March 15, 2012, 23:59
Default
  #3
Senior Member
 
Safia
Join Date: Oct 2010
Location: Australia
Posts: 161
Rep Power: 16
happy is on a distinguished road
Quote:
Originally Posted by tauqirnawaz View Post
[Note: It might be the case that this information has already been shared on the CFX forum but this is an effort to bring it under one thread.]

Quantifying swirl becomes very important in some situations where you would like to reduce it or even sometimes when requirement of design is to increase the swirl.

Swirl Number:

Sn or Swirl number is defined as

To calculate in the CFX, create the following CEL expression;

Swirl Number [non dimensional] = areaInt(Density*sqrt(Velocity v*Velocity v)*sqrt((Velocity u*Velocity u)+(Velocity w*Velocity w))*sqrt((X*X)+(Z*Z)))@Plane 1/(maxVal(sqrt((X*X)+(Z*Z)))@Plane 1*areaInt(Density*Velocity v*Velocity v)@Plane 1)

Where
areaInt = Area Integral
sqrt = Square Root
Velocity v = Velocity in mean flow direction i.e. Y-axis in this case
Velocity u = Velocity in X-axis
Velocity w = Velocity in Z-axis
maxVal = Maximum Value
Y-axis is perpendicular to Plane 1, while X-axis and Z-axis are parallel to the Plane1 in this case.

For centrifugal pump impeller design it should be between 0.05-0.1 for good suction performance or 0.01 for excellent suction performance.

Swirl Angle:
This is again very important for specifying the blade angles in centrifugal pumps.

Use following CEL expression;

Swirl Angle[radians] = atan2(sqrt(Velocity u^2+Velocity w^2), sqrt(Velocity v^2))
Where
atan2 = arctangent
sqrt = Square Root
Velocity v = Velocity in mean flow direction i.e. Y-axis in this case
Velocity u = Velocity in X-axis
Velocity w = Velocity in Z-axis
Y-axis is perpendicular to Plane 1, while X-axis and Z-axis are parallel to the Plane1 in this case

Then create a variable SwirlAngleVariable to calculate an area average over the plane, this would give you a value in degrees.
Hi again
does your celexpression is for the swirl coming from rotating configuration or even for the natural induced?
Regards
happy is offline   Reply With Quote

Old   March 16, 2012, 05:20
Default
  #4
Member
 
Raja_Bhai
Join Date: Feb 2012
Location: UK
Posts: 40
Rep Power: 14
tauqirnawaz is on a distinguished road
Quote:
Originally Posted by happy View Post
Hi again
does your celexpression is for the swirl coming from rotating configuration or even for the natural induced?
Regards
Safia,

I normally use these expressions for the calculation of swirl at pump suction (i.e. stationery frame of reference), to adjust the blade angles. Never used them for the rotating frames.

Last edited by tauqirnawaz; March 16, 2012 at 10:58.
tauqirnawaz is offline   Reply With Quote

Old   April 12, 2012, 05:03
Default
  #5
igo
New Member
 
federico ghirelli
Join Date: Mar 2009
Posts: 4
Rep Power: 0
igo is on a distinguished road
it seems to me that the swirl number as defined above is not dimensionless.
igo is offline   Reply With Quote

Old   April 12, 2012, 08:52
Default
  #6
Member
 
Raja_Bhai
Join Date: Feb 2012
Location: UK
Posts: 40
Rep Power: 14
tauqirnawaz is on a distinguished road
Quote:
Originally Posted by igo View Post
it seems to me that the swirl number as defined above is not dimensionless.
Did you check it in CFX or is it a guess?
tauqirnawaz is offline   Reply With Quote

Old   April 13, 2012, 06:05
Default
  #7
igo
New Member
 
federico ghirelli
Join Date: Mar 2009
Posts: 4
Rep Power: 0
igo is on a distinguished road
Hi,
no I didn't check it in CFX, i just looked at the expression.
Probably the correct expression includes a r^2 in the integrand on top:
Attached Images
File Type: jpg swirlNumber1.jpg (12.3 KB, 182 views)
igo is offline   Reply With Quote

Old   April 14, 2012, 00:03
Default correct expression
  #8
Senior Member
 
Safia
Join Date: Oct 2010
Location: Australia
Posts: 161
Rep Power: 16
happy is on a distinguished road
Quote:
Originally Posted by igo View Post
Hi,
no I didn't check it in CFX, i just looked at the expression.
Probably the correct expression includes a r^2 in the integrand on top:
yes, I agree with you I check with many references that r^2 instead of r. see Snegireve et al. (2004).
Regrads
happy is offline   Reply With Quote

Old   April 15, 2012, 19:24
Default
  #9
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,854
Rep Power: 144
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
Also be aware that there are different definitions of swirl. For internal combustion engines, swirl inside a combustion chamber is usually defined as the angular momentum of the gas divided by the angular momentum of the same gas if it was in solid body rotation at the crank angular velocity. This results in a unitless number as it is the ratioes of angular momentums.
ghorrocks is offline   Reply With Quote

Old   April 25, 2012, 07:10
Default
  #10
Member
 
Raja_Bhai
Join Date: Feb 2012
Location: UK
Posts: 40
Rep Power: 14
tauqirnawaz is on a distinguished road
Agreed the correct equation is

areaInt(Density*(sqrt(X^2+Z^2))^2*sqrt(Velocity v^2)*sqrt((Velocity u^2)+(Velocity w^2))*sqrt(X^2+Z^2))@Plane 1/(maxVal(sqrt(X^2+Z^2))@Plane 1*areaInt(Density*(sqrt(X^2+Z^2))*Velocity v^2)@Plane 1*1 [m])

Any comments!
Attached Images
File Type: png Picture1.png (2.1 KB, 1676 views)

Last edited by tauqirnawaz; April 25, 2012 at 09:49.
tauqirnawaz is offline   Reply With Quote

Old   April 25, 2012, 22:54
Talking questions
  #11
Senior Member
 
Safia
Join Date: Oct 2010
Location: Australia
Posts: 161
Rep Power: 16
happy is on a distinguished road
Quote:
Originally Posted by tauqirnawaz View Post
Agreed the correct equation is

areaInt(Density*(sqrt(X^2+Z^2))^2*sqrt(Velocity v^2)*sqrt((Velocity u^2)+(Velocity w^2))*sqrt(X^2+Z^2))@Plane 1/(maxVal(sqrt(X^2+Z^2))@Plane 1*areaInt(Density*(sqrt(X^2+Z^2))*Velocity v^2)@Plane 1*1 [m])

Any comments!
yes, where 1[m] comes from where you represent r as sqrt(X^2+Z^2)? as well as, I why you take square root of v^2 or u^2 instead of taking direct variable ( v or u)?
happy is offline   Reply With Quote

Old   April 26, 2012, 04:29
Default
  #12
Member
 
Raja_Bhai
Join Date: Feb 2012
Location: UK
Posts: 40
Rep Power: 14
tauqirnawaz is on a distinguished road
Quote:
Originally Posted by happy View Post
yes, where 1[m] comes from where you represent r as sqrt(X^2+Z^2)? as well as, I why you take square root of v^2 or u^2 instead of taking direct variable ( v or u)?
You have to divide the equation by 1[m] to make it dimensionless because when you take max value of the radius; the output you get is dimensionless in CFX. If you do not divide by the unit 1[m], the value of Swirl Number that you get is in meters.
Secondly you take root of squares of velocities to get absolute values, otherwise you might get a negative swirl number.
tauqirnawaz is offline   Reply With Quote

Old   April 26, 2012, 07:22
Default
  #13
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,854
Rep Power: 144
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
Quote:
Secondly you take root of squares of velocities to get absolute values
Then why not use the abs() function?

Most definitions of swirl numbers can be positive or negative so you get the direction of the swirl. If you want the absolute swirl number then you should calculate it including sign, and take the abs value once at the end.

Quote:
(sqrt(X^2+Z^2))^2
Why take the square root then square it again?

Your comment about units is puzzling. The equation seems to have the units cancelling giving a unitless number. If you have to put a divide by 1[m] at the end to get the correct units then you have a mistake in the CEL.
ghorrocks is offline   Reply With Quote

Old   April 26, 2012, 11:49
Default
  #14
Member
 
Raja_Bhai
Join Date: Feb 2012
Location: UK
Posts: 40
Rep Power: 14
tauqirnawaz is on a distinguished road
Quote:
Originally Posted by ghorrocks View Post
Then why not use the abs() function?

Most definitions of swirl numbers can be positive or negative so you get the direction of the swirl. If you want the absolute swirl number then you should calculate it including sign, and take the abs value once at the end.



Why take the square root then square it again?

Your comment about units is puzzling. The equation seems to have the units cancelling giving a unitless number. If you have to put a divide by 1[m] at the end to get the correct units then you have a mistake in the CEL.
Agree that abs() function would do the same thing. As regards square of radius, it comes from the formula quoted earlier and I have used sqrt(X^2+Z^2) to calculate radius. Also sqrt((Velocity u^2)+(Velocity w^2) to calculate radial velocity components.
tauqirnawaz is offline   Reply With Quote

Old   April 26, 2012, 19:45
Default
  #15
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,854
Rep Power: 144
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
Quote:
I have used sqrt(X^2+Z^2) to calculate radius
That is obvious. So why write "(sqrt(X^2+Z^2))^2" when this obviously becomes (X^2+Z^2)?

But my key point is about the units - your formula is almost certainly wrong if you have to add a divide by 1[m] at the end to get the units to check out.
ghorrocks is offline   Reply With Quote

Old   April 27, 2012, 08:03
Default
  #16
Member
 
Raja_Bhai
Join Date: Feb 2012
Location: UK
Posts: 40
Rep Power: 14
tauqirnawaz is on a distinguished road
Quote:
Originally Posted by ghorrocks View Post
That is obvious. So why write "(sqrt(X^2+Z^2))^2" when this obviously becomes (X^2+Z^2)?

But my key point is about the units - your formula is almost certainly wrong if you have to add a divide by 1[m] at the end to get the units to check out.
Thank you Ghorrocks, please find the corrected formula below;

areaInt(Density*(X^2+Z^2)*sqrt(Velocity v^2)*sqrt((Velocity u^2)+(Velocity w^2)))@Plane 1/(maxVal(sqrt(X^2+Z^2))@Plane 1*areaInt(Density*(sqrt(X^2+Z^2))*Velocity v^2)@Plane 1)
tauqirnawaz is offline   Reply With Quote

Old   April 27, 2012, 21:00
Default to know the direction of swirling flow
  #17
Senior Member
 
Safia
Join Date: Oct 2010
Location: Australia
Posts: 161
Rep Power: 16
happy is on a distinguished road
Quote:
Originally Posted by ghorrocks View Post
Then why not use the abs() function?

Most definitions of swirl numbers can be positive or negative so you get the direction of the swirl. If you want the absolute swirl number then you should calculate it including sign, and take the abs value once at the end.



Why take the square root then square it again?

Your comment about units is puzzling. The equation seems to have the units cancelling giving a unitless number. If you have to put a divide by 1[m] at the end to get the correct units then you have a mistake in the CEL.
As I read through fluid flow books, I learnt that the sign of vortisity (omiga) provides the researcers with the direction of swirling if it was anticlockwise or clockwise.
happy is offline   Reply With Quote

Old   April 28, 2012, 07:25
Default
  #18
Member
 
Raja_Bhai
Join Date: Feb 2012
Location: UK
Posts: 40
Rep Power: 14
tauqirnawaz is on a distinguished road
Quote:
Originally Posted by happy View Post
As I read through fluid flow books, I learnt that the sign of vortisity (omiga) provides the researcers with the direction of swirling if it was anticlockwise or clockwise.
So this means we can use Swirl Number to quantify the flow and Vortisity to predict the direction?
tauqirnawaz is offline   Reply With Quote

Old   April 29, 2012, 00:44
Default
  #19
Senior Member
 
Safia
Join Date: Oct 2010
Location: Australia
Posts: 161
Rep Power: 16
happy is on a distinguished road
Quote:
Originally Posted by tauqirnawaz View Post
So this means we can use Swirl Number to quantify the flow and Vortisity to predict the direction?
swirl no. use to know the strenght of swirling flow and yes the vorticity is useful to know swirling flow direction.

Last edited by happy; April 30, 2012 at 23:48.
happy is offline   Reply With Quote

Old   May 1, 2012, 00:43
Default
  #20
New Member
 
Join Date: May 2012
Posts: 2
Rep Power: 0
geno0624 is on a distinguished road
Quote:
Originally Posted by ghorrocks View Post
Also be aware that there are different definitions of swirl. For internal combustion engines, swirl inside a combustion chamber is usually defined as the angular momentum of the gas divided by the angular momentum of the same gas if it was in solid body rotation at the crank angular velocity. This results in a unitless number as it is the ratioes of angular momentums.
Mr Glen Horrocks, your comment above has confused me! I just read through a research paper titled "Swirl Control of Combustion Instabilities is a Gas Turbine Combustor" by C. Stone and and S. Menon - Proceedings of the Combustion Institute, Vol 29 / 2002, pg 155-160.

This paper calls out the exact (corrected) formula for swirl number that tariq wrote down in post# 10. Would this formula still be valid, given the alternate definition that you posted for swirl for a gas inside a combustion chamber??

I am a student and relatively new to CFD so please excuse my ignorance.
geno0624 is offline   Reply With Quote

Reply


Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
angle of attack CFX Arj CFX 17 October 2, 2015 08:07
Pros and Cons for CFX, CFdesign, COMSOL Val Main CFD Forum 3 June 10, 2011 03:20
Proper output of angle of attack in CFX post Kevin CFX 3 October 18, 2006 13:18
Angle measurement in CFX sham CFX 0 September 2, 2006 01:26
CFX 4.4 installation problem Pandu Sattvika CFX 1 December 1, 2001 05:07


All times are GMT -4. The time now is 17:17.