# Negative Drag coefficient naca 0012 airfoil using fluent 15

 Register Blogs Members List Search Today's Posts Mark Forums Read

 March 2, 2015, 21:36 Negative Drag coefficient naca 0012 airfoil using fluent 15 #1 New Member   Lucas Vinicius Fernandes Coelho Join Date: Mar 2015 Posts: 3 Rep Power: 11 Hi, I'm new in CFD and I have a problem, in my simulation the drag coefficient of a naca 0012 is negative, as I increase the speed angle of attack, the drag coefficient decreases and becomes negative, I don't now why, I'm pretty sure that my velocity components are OK. I'm use the k-omega model for a Re=3e06. I need help. And I want to know if fluent have a way to rotate only the airfoil or the mesh, without I need to use the velocity components to change the angle of attack. Thanks p.s.: Sorry for my english

 March 7, 2015, 11:01 #2 New Member   Jesus Ramirez Join Date: Aug 2014 Location: Colombia Posts: 13 Rep Power: 11 I a similar problem. Initially I performed a naca 0012 simulation on OpenFOAM and I got good results of Cd. However when I just modified the profile the Cd became negative. Is there any physical reason for this?

 March 9, 2015, 11:51 #3 New Member   Anthony McCarthy Join Date: Nov 2014 Location: United Kingdom Posts: 21 Rep Power: 11 Is it definitely negative coefficient you are recording or are you referring to a negative force? If it is the latter this is correct

 March 9, 2015, 12:40 #4 New Member   Join Date: Mar 2015 Posts: 13 Rep Power: 11 Within the Drag Monitor you have to modify Force Vector (on the bottom of the Drag Monitor window) which is by default X=1 and Y=0. If your velocity is parallel to the X-axis then this is OK (X=1 and Y=0) but if you change AoA (velocity vector) you also need to modify this Force Vector within Drag Monitor so X=cos(AoA) and Y=sin(AoA) (for positive AoA). Drag force should be calculated as parallel to the free stream velocity. Also don't forget to modify Lift Monitor accordingly (lift force should be calculated perpendicular to free stream velocity). Try to change (if you already didn't) and see if it helps. Best regards, CFDBuddha

 March 9, 2015, 12:54 #5 New Member   Anthony McCarthy Join Date: Nov 2014 Location: United Kingdom Posts: 21 Rep Power: 11 Also as an extension to what cfdbuddha pointed out, manually calculating your co-efficients for the forces leads to less confusion when changing your x-y for different angles of attack

 March 13, 2015, 14:18 #6 New Member   Lucas Vinicius Fernandes Coelho Join Date: Mar 2015 Posts: 3 Rep Power: 11 Thanks CFDBuddha, but now I have another problem, the Cd must be something like 0.015 to 10 degrees, but my Cd is to high, something like 0.027, how I can correct this?

 March 13, 2015, 14:26 #7 Member   VS Join Date: Nov 2012 Posts: 86 Rep Power: 13 Do you use the exact same conditions as for the Cd value you are trying to acquire? What simulation or test gave you that value of Cd? Regards VS

 March 13, 2015, 14:35 #8 New Member   Lucas Vinicius Fernandes Coelho Join Date: Mar 2015 Posts: 3 Rep Power: 11 Yes, for 1 , 2 , 3 degrees its ok, but for high degrees, the Cd increases. All the Cl are OK

 March 13, 2015, 14:43 #9 Member   VS Join Date: Nov 2012 Posts: 86 Rep Power: 13 It makes sense for the low AoA, but for higher AoA, many unsteady phenomena could cause differences in your calculations. Is your time integration time-accurate or steady state? You should use the exact same simulation specifications as the for the Cd you are trying to acquire. Also, keep in mind that there may be fluctuations to your Cd value, even for the steady-state case. You should first average over your simulation cycles and then compare. Regards, VS

 Tags fluent 15, negative drag