# Need help with Drag Coefficient using ANSYS Fluent

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 January 26, 2020, 12:00 Need help with Drag Coefficient using ANSYS Fluent #1 New Member   rekein Join Date: Jan 2020 Posts: 1 Rep Power: 0 Hi all, I'm doing a project on drag reduction for trucks using ANSYS Fluent. I want to see how much the the drag coefficient of a truck is reduced by when attaching a trucksack on to the rear of the trailer. I have created 3D CAD models TWC Email of both the truck and the trucksack and have analysed them using ANSYS Fluent using the k-epsilon model. I am using the same mesh for both simulations and same solver settings in ANSYS Fluent. However I am not getting much change between the drag coefficient values (1-2%). I am expecting a drag coefficient reduction between 10-20%. I am using the frontal area of the truck in the reference values, which is why I may not be seeing a change in drag coefficient. The frontal area of the truck with and without the trucksack attached is identical as the trucsack is slightly smaller than the frontal projected area of the truck. How do I take the trucksack at the rear of the trailer into consideration when calculating the drag coefficient in ANSYS Fluent. Thanks Last edited by nereik; January 27, 2020 at 08:51. Reason: spelling

 January 27, 2020, 11:16 #2 New Member   Arnie Join Date: Mar 2017 Posts: 22 Rep Power: 4 Nereik, Its not just about the Frontal Area. Look at the Cd formula..the Drag Force plays a role and this is dependent on the shape of the object. For instance, you would not expect a sphere and a cube to have the same Cd (at similar Re numbers), would you ? PS. The k-e solver treats the near wall affects with a function. You should rather use a k-w SST to ensure you more accurately determine the wall affects. Just ensure that your Y+ is below 1 (or 2 max.) PSS. Where did you get the expected 10-20% Cd reduction using a trucksack ? From Marketing material or actual wind tunnel tests ? By the way, a small (positive) aerodynamic change to the front face of the truck has about one order of magnitude aerodynamic impact compared to any change at the rear end of the truck... use the advice or not ! Regards, Arnie Last edited by arnie333; January 27, 2020 at 13:07.