# High Cm values in 2D analysis of VAWT

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 August 19, 2016, 11:31 High Cm values in 2D analysis of VAWT #1 New Member   Join Date: Aug 2016 Posts: 3 Rep Power: 8 Hi, I am new to fluent. I am trying to perform 2D analysis of Vertical Axis Wind Turbine. I have a few doubts regarding the setup in fluent. I am using sliding mesh technique to perform the analysis. The following are the specs of the wind turbine: Dia. of Rotor = 0.5m Height of blade = 1m (2D analysis) chord length = 200mm wind vel. = 5m/s I am trying to perform analysis for various rotor speed to determine the optimum TSR. To determine the power coefficient i am using the formula: Cp = TSR*Cm where Cm is the torque coefficient i am finding from the software. In the setup, the reference values have been set to the following values: Area = Dia.*Height = 0.5m^2 density = 1.225 Kg/m^3 Depth = 1m Length = radius of rotor = 0.5m vel. = 5m/s The Cm values i am retrieving from the software is very high. I am not able to identify my mistake in the setup. Are the reference values i am using correct or is there any other source of error for the Cm values? What other values in the setup may cause this error? Is there any document available that can guide me through the process? Kindly help me rectify this error. Thanks and regards, Pranav

 August 21, 2016, 19:29 #2 Senior Member   Have a nice time! Join Date: Feb 2016 Location: mech.eng.ahmedmansour@gmail.com Posts: 291 Rep Power: 9 Have you solved the problem?

August 22, 2016, 10:51
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Quote:
 Originally Posted by Ahmed Saeed Mansour Have you solved the problem?
The high Cm values were resolved by setting appropriate axes for every blade domain. However i would like to know if the net moment coefficient is the sum of the Cm obtained for individual blades and also what are the correct set of reference values to be used?
Regards,
Pranav

 August 22, 2016, 11:22 #4 Senior Member   Have a nice time! Join Date: Feb 2016 Location: mech.eng.ahmedmansour@gmail.com Posts: 291 Rep Power: 9 If you select all the airfoils when you set the monitor of the cm, it will be the sum of the cm of each airfoil, another thing that worked for us about the high value of the cm " First of all, the moment center is 0,0...to get the moment of the turbine...then the relation between the reference area and the cm is reverse...if the cm is too high, you have to increase the reference area...I think it is the surface area of the all airfoils ...For each airfoil you may calculate it by multiplying the chord and the span...as an initial guess, then you may increase them until you reach an acceptable limit..the moment center is the origin...the force multiplied by the rotor radius...You have to try this and find out many references if you still have the problem.

 August 22, 2016, 11:28 #5 New Member   Join Date: Aug 2016 Posts: 3 Rep Power: 8 Thank you for your reply, The high Cm issue is resolved by using the method that you suggested. When i select all three airfoils for the Cm value, the resultant Cp value i am obtaining is crossing the Betz's limit (0.593). I'm using the relation Cp = TSR*Cm. How do i resolve this issue? Regards, Pranav

 August 22, 2016, 11:33 #6 Senior Member   Have a nice time! Join Date: Feb 2016 Location: mech.eng.ahmedmansour@gmail.com Posts: 291 Rep Power: 9 Try to change the reference are to .8 not 0.5 and the reference length is the chord and create a monitor for the total cm and check out the results...you have to validate the results with some experimental data to know the best settings

 October 12, 2017, 02:53 Cm value is continuously increasing with increasing TSR in 2D Unsteady analysis #7 New Member   Join Date: Oct 2017 Posts: 2 Rep Power: 0 Dear Friends, I am trying to simulate three bladed VAWT considering free stream velocity 6.0 m/s, having chord length of airfoil 5 cm, H/D ratio is 1, diameter of rotor 50 cm considering 2D Unsteady analysis using fluent. But, i found that my Cm value is increasing with increase in TSR and never decrease which leads to increase in Cp. But the trend of Cp VS TSR is after reaching a pick point Cp should decrease with increase in TSR. But in my case Cp it is continuously increase with increase in TSR which is wrong. I need help, can anyone tell me where is the mistake. Thanks.

 November 7, 2018, 13:15 Measure bubble size. #8 New Member   pun Join Date: Nov 2018 Posts: 1 Rep Power: 0 Is there any tutorial to measure the average diameter of outlet.

 November 7, 2018, 17:44 #9 Senior Member   Have a nice time! Join Date: Feb 2016 Location: mech.eng.ahmedmansour@gmail.com Posts: 291 Rep Power: 9 What outlet?