open TELEMAC-MASCARET The mathematically superior suite of solvers

Hi

I am using the DRAGFO subroutine to model subsea structures over a large area, and have a question related to the area, AIRE calculated in DRAGFO:

I define in DRAGFO the coordinates of the polygon to apply the drag over as:

NSOM = 4
XSOM(1) = -236035.D0
XSOM(2) = -226397.D0
XSOM(3) = -227424.D0
XSOM(4) = -236625.D0
YSOM(1) = 2062147.D0
YSOM(2) = 2060662.D0
YSOM(3) = 2056982.D0
YSOM(4) = 2058883.D0

In Blue Kenue the attribute table for this polygon gives a total area of 3.4E7 m^2.
I checked this with the value of AIRE calculated in DRAGFO (after the quasi-bubble loop), which gives AIRE=1.09E7 m^2.

Please can someone explain why the difference between the two is so large (-68%)?

I am using a mesh resolution of 250 m.

Cheers

Danny

Hello,

I enclose a small program computing your area. If I am not wrong the real area is rather -3.4E7, so it seems that Blue Kenue is not sensitive to a polygon with points given clockwise, but Telemac is because the positivity of areas is used to know whether a point is inside or outside the polygon. So please give the points in a counterclock wise way and I hope it will be better. I'll add wherever I can in the programs that the points must be given in such a way.

With best regards,

Jean-Michel Hervouet

Hi JMH,

I tried using counter-clockwise coordinates as follows...

XSOM(1) = -236625.D0
XSOM(2) = -227424.D0
XSOM(3) = -226397.D0
XSOM(4) = -236035.D0
YSOM(1) = 2058883.D0
YSOM(2) = 2056982.D0
YSOM(3) = 2060662.D0
YSOM(4) = 2062147.D0

... but I still get the same problem, AIRE is calculated as 1.09E7 m^2. why could this be?

Cheers

Danny

Hello,

No more idea... Are you in parallel (in that case your print should be after the line AIRE=P_DSUM(AIRE)), have you quasi-bubble velocities, is your polygon partially inside the domain ?

Regards,

JMH

Hi JMH,

On inspection I have found that the reason is that I am using spherical coordinates. If I turn spherical coordinates off Telemac calculates approx. the same area as Blue Kenue.

Would I be correct in saying that when spherical coordinats are on, Telemac calculates the correct area as it accounnts for spherical coordinates?

Presumably Blue Kenue calculates the area incorrectly as it does not account for spherical coordinates. (I.e. it assumes a flat surface)

Cheers

Danny

Hello,

Yes, there is a factor 1/cos(latitude) in areas if spherical coordinates are not taken into account. We also recently found that there is an approximation of about 5% in domains of 1000 km in Telemac, and there will be a much better implementation in the enxt version.

With best regards,

Jean-Michel Hervouet

Hi JMH,

Ok so to convert an area without spherical coordinates (A) to an area with spherical coordinates (A_s), the calculation would be:

A = A_s * (1/coslatitude)

Is this correct? Also is latitude in degrees or radians?

I will use this to check the area I am getting from AIRE in dragfo is approx. correct.

Cheers

Danny

Hello,

Hum, I wrote too fast, this should be A = A_s * (1/coslatitude)**2 since we have here an area and not a distance.

You do not have to worry about radians or degrees, as there is a component of mesh structure which gives you directly cos(latitude) : MESH%COSLAT%R.

With best regards,

Jean-Michel Hervouet

Hi JMH,

I have just done a quick check of the areas and this makes sense now,

Thanks again,

Danny

Hi All,

I am having a similar issue using a modified version of the DRAGFO routine in that the area of my polygon (refer to - BK area.png) is ~40000m2, however AIRE calculated by TELEMAC is 26239m2 (refer to - EnergyExt.out.5748733), can you help?

I have tried applying coordinates both clockwise and counter-clockwise, starting at the NW corner of my polygon and I am not using spherical coordinates. I am running on a single processor now, however the result is the same on multiple processors. I have not included the quasi-bubble loop either (refer to - TECdrag_4.f) so why is the applied area so different?

thanks in advance...
M