For a school project, I am going to be running simulations comparing drag forces against a car model with varying external accesories. Before I move to complex geometries, I am trying to prove the validity of the CFD results with known values. Since there is no way to use hand calculation/known drag force coefficents on anything but the most basic of shapes, thats what I have started with.

I have been using a smooth sphere, with varying diameters and flow velocities. I am calcuating reynolds number based on these parameters, and also the drag force coefficent based on the drag force experianced by the model. I have plotted the different Cd Vs. Re values on a graph, and have been comparig them to text book values for smooth spheres. I am attaching an excel with the data I have gathererd so far and a plot of accepted values. My results are quite different than what they should be and I am trying to figure out what parameters in my model/calculation error which could be casuing this error.

Current Setup:

V= 0.1 to 30 m/s

D= .0625 to 1 m

Boundary condition of a real wall with 0 roughness

Air at 293K, 1.2041 kg/m3, 0% humidity, 0.1% turbulence

Computational Domain Size: 5D downstream, 2D upstream, 1.5D Surrounding

Surface Goal: normal force in direction of flow

Hi Tom, you're in luck, you problem is in the validation examples that are included in flow. Have you checked them out? They are in the technical reference.

Also, have you taken a look at the solidworks KB? there are a bunch of really good articles that will help you set your expectation about the results for drag/lift and recommendations.

Also, this topic was discussed recently. Check this forum post out:

https://forum.solidworks.com/message/359647#359647

To get really accurate results, you will need a lot of cells and you may need a transient analysis depending on the reynolds number. Also the computational domain will need to be appropriately sized. All are discussed in the tutorial/validation problem in the software documentation.

a couple suggestions:

1. for your simulation and your physical/hand calcs, write down all the assumptions, know how the differences may affect your comparison.

2. variance testing, check comp domain, mesh and setup parameter changes and how they affect the results

3. outputs. you've chosen normal force, why not force? double check the definition in the help.