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Hi Chi-Tuan,

What I mean is, example I have the following field data:

@0.2H, the velocity is 0.2m/s, direction = 20 degree
@0.4H, the velocity is 0.5m/s, direction = 60 degree
@0.6H, the velocity is 0.8m/s, direction = 45 degree

depth average velocity = (0.2+0.5+0.8)/3 = 0.5 m/s

how about direction?

Hi Angie,

With your example: as told, you must calculate depth averaged velocity along x and y first:
u = (0.2*cos(20 deg)+0.5*cos(60 deg)+0.8*cos(45 deg))/3 = 0.33 m/s
v = (0.2*sin(20 deg)+0.5*sin(60 deg)+0.8*sin(45 deg))/3 = 0.36 m/s

Then depth averaged velocity magnitude = sqrt(u^2+v^2) = 0.49 m/s (slightly different from 0.5 m/s).
and direction = atan2(v,u) = 46.8 degrees (be careful, the order of arguments for atan2 with excel - x,y - is the reverse of atan2 with Fortran - y,x).

Hope this helps,

Chi-Tuan

Thanks for the very clear explanation!

Very appreciate~ =)

Hi chi-Tuan,

I have try through the solution you give it to me. But I could not get the answer of average current direction of 46.8 degree.

I use the following command in excel:

=MOD(DEGREES(ATAN2(B2,A2)),360) where B2 = v, A2 = u, but its came out the answer is 42.5 degree

Kindly correct me if I get any misunderstanding. Very appreciate!

Hi Angie,

With your case, in Excel, I have used the formula:
=ATAN2(A2,B2)*180/pi() Watch out the order of arguments!

You can find enclosed the Excel sheet I have used.





Regards,

Chi-Tuan