# lRef and Aref

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 January 31, 2018, 14:15 lRef and Aref #1 New Member   mohammad amin adibipoor Join Date: Jul 2017 Posts: 5 Rep Power: 8 Hello foamers I wonder what is lRef and Aref for a semi submerged rectangular body with 0.8*0.02*0.4 (x*y*z) dimensions (interacting with regular wave) in calculating cL and cD in forceCoeffs?

 January 31, 2018, 14:41 #2 New Member   Join Date: Dec 2015 Posts: 16 Rep Power: 10 Hi amin.adibipoor, Tobias Holzmann gives a very good explanation on his YouTube Channel. Please watch the Suzanne Series. https://youtu.be/5iO89p0qso4 Good Luck Q.E.D. amin.adibipoor likes this.

June 23, 2018, 05:11
#3
New Member

Iman Sabahi
Join Date: May 2018
Posts: 17
Rep Power: 8
Quote:
 Originally Posted by Q.E.D. Hi amin.adibipoor, Tobias Holzmann gives a very good explanation on his YouTube Channel. Please watch the Suzanne Series. https://youtu.be/5iO89p0qso4 Good Luck Q.E.D.
Pardon me for replying on a rather old thread, I'm not trying to hijack it or anything but I think that the explanation in the above video may cause a false perspective for some people.

Watching this video ( https://youtu.be/5iO89p0qso4 ), I understood that for a typical 3D wing(with angle of attack=0) Aref will be equal to (max_thickness)*(span), and for a 2D airfoil it'll just be the (max_thickness), because that'll be the projected areal or line on the inlet(at least that's exactly what the guy explaining in the video says.)

But several other threads suggest otherwise. Examples being:
What should be the reference area for a 2d Airfoil Analysis
Characteristic Length of a 2D NACA Airfoil
liftDrag
forces: What are magUInf, lRef, Aref?

The correct answer seems to be:
aRef:
Keep in mind that aRef is required to calculate the drag, so it must somehow relate to the contact surface between flow and the shape.
Now imagine a flow in the x+ direction(we're talking 3D here). It's obvious that there are infinite number of planes along the x direction, i.e. without any "x" in their normal vectors. Now some of these infinite number of planes, will have a cross section with our arbitrary shape(like a cylinder or wing) in the flow. For some planes this cross section area is smaller, and for some others it is larger. For any arbitrary shape being put in this flow, the largest cross section of that shape and any plane which is in the direction of the flow(has no "x" in its normal vector) is the aRef.

For 2D cases, imagine the same setup but the profile(airfoil section, cylinder..) is in the x-y plane, or the Z-length of the profile is uniformly 1. In this case, the normal vectors of those planes has no x and z in them, i.e. they're parallel to the x-z plane. so aRef will be the largest area which is created by cross sectioning the arbitrary shape, with any plane parallel to x-z. But because the case is 2D and in Z direction the distance is "1", another way to say the definition is: aRef will be the longest horizontal line between any two points of the shape, if you look at the x-y plane view(your eye is looking in the z- direction).

lRef:
Keep in mind that lRef is required to calculate the moment.
for 2D cases, because in Z direction the distance is only "1", lRef seems to be equal to aRef.

but in 3D cases, remember the above definition for aRef, and imagine that same plane which was in x direction, and had the largest cross section area with the arbitrary shape. in that cross section, the longest line in the x direction will be the lRef.

I understand that these definitions may seem "too much", but this is the most accurate definition i could come up with.
Please feel free to correct me wherever you see fit.
Iman

Last edited by i.sabahi; June 23, 2018 at 05:19. Reason: added some details and explanations