Dear all, I am interested about studying the rotational cylinder in free flow (for incompressible flow). Looking around the Nektar's manual I realized that there is an option which, I think, allows to compute it, the MovingBody along with the Mapping (10.6.3 - 10.6.5) from Nektar++'s manual v4.4.0. I am using the example: the CylFlow_Mov_mapping as a reference one. However, I can not completely understand how it works. (I took the example from the 4.3.5 version) First of all, that example is for a rotational cylinder or is it for a cylinder moving up and down on a periodic motion? Tanking into account the first option, The mapping function computes the new coordinates for the specified boundary, However, I do not understand why the function is specified in that way: <FUNCTION NAME="Mapping"> <E VAR="x" VALUE="x" /> <E VAR="y" VALUE="y+A*cos(omega*t)" /> </FUNCTION> The value of the parameter A is not the radius of the cylinder, so it is not deifying a rotation over its center. Also, the new /x/ coordinate is the same as before, so the cylinder just moved upwards or downwards. Secondly, what is the MappingVel functions used for? I think it is used for specify the velocity on the new coordinates. However, the way it is defined, I think, it does not define a new velocity coordinates. What I am misleading? A part from that, I would also compute the aerodynamic forces on the moving surface, so I inserted the following: <FORCE TYPE="MovingBody"> <PARAM NAME="OutputFile">DragLift.frc</PARAM> <PARAM NAME="OutputFrequency">10</PARAM> <PARAM NAME="Boundary"> B[0] </PARAM> </FORCE> and also: <MAPPING TYPE="Translation"> <COORDS>MovingBody</COORDS> <VEL>MappingVel</VEL> </MAPPING> However, the file DragLift.frc is not generated. Hope I have well explained and clarified all my inquiries. Thank you very much, Guillermo Suarez.
Hi Guillermo, The Mapping is used when the position of the body changes, like in the up and down motion of the test case you mentioned. If you have a rotating cylinder at a fixed location, I believe you can just use a non-zero boundary condition for the velocity (there is no need for using a Mapping or the MovingBody forcing in this case). For example, if the angular velocity is Omega and the cylinder center is at (xc,yc), you could define the boundary condition for the cylinder as <REGION REF="0"> <D VAR="u" VALUE="-Omega * (y-yc)"/> <D VAR="v" VALUE=" Omega * (x-xc)"/> <N VAR="p" USERDEFINEDTYPE="H" VALUE="0"/> </REGION> You can then obtain the forces on the cylinder using the AeroForces filter. I hope this helps you. Cheers, Douglas ________________________________ From: nektar-users-bounces@imperial.ac.uk <nektar-users-bounces@imperial.ac.uk> on behalf of Guillermo <guillermo180395@gmail.com> Sent: 09 September 2017 18:27:06 To: nektar-users Subject: [Nektar-users] Nektar - Mapping Dear all, I am interested about studying the rotational cylinder in free flow (for incompressible flow). Looking around the Nektar's manual I realized that there is an option which, I think, allows to compute it, the MovingBody along with the Mapping (10.6.3 - 10.6.5) from Nektar++'s manual v4.4.0. I am using the example: the CylFlow_Mov_mapping as a reference one. However, I can not completely understand how it works. (I took the example from the 4.3.5 version) First of all, that example is for a rotational cylinder or is it for a cylinder moving up and down on a periodic motion? Tanking into account the first option, The mapping function computes the new coordinates for the specified boundary, However, I do not understand why the function is specified in that way: <FUNCTION NAME="Mapping"> <E VAR="x" VALUE="x" /> <E VAR="y" VALUE="y+A*cos(omega*t)" /> </FUNCTION> The value of the parameter A is not the radius of the cylinder, so it is not deifying a rotation over its center. Also, the new x coordinate is the same as before, so the cylinder just moved upwards or downwards. Secondly, what is the MappingVel functions used for? I think it is used for specify the velocity on the new coordinates. However, the way it is defined, I think, it does not define a new velocity coordinates. What I am misleading? A part from that, I would also compute the aerodynamic forces on the moving surface, so I inserted the following: <FORCE TYPE="MovingBody"> <PARAM NAME="OutputFile">DragLift.frc</PARAM> <PARAM NAME="OutputFrequency">10</PARAM> <PARAM NAME="Boundary"> B[0] </PARAM> </FORCE> and also: <MAPPING TYPE="Translation"> <COORDS>MovingBody</COORDS> <VEL>MappingVel</VEL> </MAPPING> However, the file DragLift.frc is not generated. Hope I have well explained and clarified all my inquiries. Thank you very much, Guillermo Suarez.
participants (2)
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Guillermo
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Serson, Douglas