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Drag coefficient_cylinder

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Old   August 25, 2014, 08:30
Default Drag coefficient_cylinder
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Stas
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Hello,
Currently Iím writing thesis on dimpled surfaces applied to the objects to reduce drag. FloEFD Solidworks was chosen as software to simulate flow of air over the objects. Firstly I decided to try simple geometry objects such as sphere and flat plate (both have smooth surfaces) as simulations for them were introduced in the manual. The conditions of air were chosen by the default offered by software. (air properties at 20 degrees Celsius). A sphere of d=50mm at 0.003m/s velocity gave me drag coefficient approx. equal to 4.11 which was correct. However, at the velocity 3m/s it provided drag coefficient equal to 0.17, instead of 0.5 as it was supposed to be according to the table introduced in (Fundamentals of Fluid Mechanics by Munson). When the flow over a flat plate (200mmX20mmX10mm) was simulated drag coefficient was around 1.3 which satisfied the conditions.
However, when the flow over a smooth cylinder is simulated the results from software very significantly with the values from the book at certain Reynolds numbers. The cylinder is 155mm in diameter and 500mm long. At the air temperature 20 degrees Celsius Reynolds number is 31400, providing drag coefficient around 0.4 while the real drag coefficient from the book and other sources says that the drag coefficient should be around 1.1-1.2 at that Re number.
I was trying to set the level of initial mesh to the max (8). Also, set up the mesh size manually to a very small size. However, it did not result in the improvements of the results.
The method that I used was to define a global goal as the force in the direction x and then use that global goal into equation goal to obtain the value of drag by using a standard formula (Cd=Fd/(p*U^2*D*L)) with added values of the air and geometry parameters. The velocity in the direction x was set manually as an initial condition and was 3.2199. I also tried to enlarge computational domain but it did not improve the results.
Would anyone please tell me about the possible mistakes I could have made and particular setting of the software I should pay attention to. Also, could anyone run the same simulation and let us know if you are getting the same errors as me.
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Old   August 26, 2014, 08:27
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Boris Marovic
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Hi Stas,

Please have a look at the PDF that comes with the software called Technicalreference.pdf
You will find there are already such simulations done with the flat plate as well as the cylinder under the Validation Examples section.
These models also come with the software pre-set for calculation. You can find them also in the installation folder somewhere. I don't know the location for SW but for FloEFD.

Boris
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