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Bug in actuationDiskSource????

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Old   July 6, 2018, 07:36
Default Bug in actuationDiskSource????
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Amadeo Moran
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Dear community,

I was trying to perform a simulation of an actuator disk and reviewing the source code C++ in "actuationDiskSourceTemplates.C" there is a line with the calculation of T (total thrust).

Code:
scalar T = 2.0*upRho*diskArea_*mag(upU)*a*(1 - a);
if mag() calculates the magnitude of a field, is there is a bug? because the thrust should be:

Code:
scalar T = 2.0*upRho*diskArea_*sqr(mag(upU))*a*(1 - a);
Probably I am considering something wrong because is not a new implementation and have been tested a lot but if anybody can help I will appreciate it very much.

Thanks and regards from Spain!
amg.

Last edited by a.morang; July 9, 2018 at 04:06.
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Old   July 7, 2018, 05:11
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Michael Alletto
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I think this is correct since mag calculates the magnitude and not the magnitude squared


see, e.g. http://foam.sourceforge.net/docs/Gui...mmersGuide.pdf
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Old   July 9, 2018, 04:13
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Quote:
Originally Posted by mAlletto View Post
I think this is correct since mag calculates the magnitude and not the magnitude squared


see, e.g. http://foam.sourceforge.net/docs/Gui...mmersGuide.pdf
sqr is the squared:

Code:
sqr(mag(U)) == mag(U)*mag(U)
for it my question... In the original code U is not squared and should be because if not the units are not correct unless:

Code:
mag(U) = Ux^2 + Uy^2 + Uz^2
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Old   July 9, 2018, 10:28
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Michael Alletto
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The units are correct since the complete calculation of the force is:



scalar T = 2.0*upRho*diskArea_*mag(upU)*a*(1 - a);
forAll(cells, i){Usource[cells[i]] += ((Vcells[cells[i]]/V())*T*E) & upU;}
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Old   July 10, 2018, 04:07
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Amadeo Moran
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Quote:
Originally Posted by mAlletto View Post
The units are correct since the complete calculation of the force is:



scalar T = 2.0*upRho*diskArea_*mag(upU)*a*(1 - a);
forAll(cells, i){Usource[cells[i]] += ((Vcells[cells[i]]/V())*T*E) & upU;}
Do you mean that in C++:

Code:
upU & upU == upU^2
Thanks.
amg.
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Old   July 10, 2018, 06:03
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yes. The dot product of two vectors is equal to the magnitude squared:


u & u = ux*ux + uy*uy + uz*uz
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Old   July 11, 2018, 13:14
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Quote:
Originally Posted by malletto View Post
yes. The dot product of two vectors is equal to the magnitude squared:


U & u = ux*ux + uy*uy + uz*uz
thanks you very much for your time!!
Regards!
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