8 Replies Latest reply on Mar 28, 2011 4:29 PM by Brandon Spires


    Filip Tejszerski

      This part-time project started over two years ago, just as an exercise to practice 3D modelling. The concept to be modelled was a design for a car inspired by aviation themes. The full story can be found here : http://www.local-motors.com/designProcessDet.php?c=4525


      To cut to the chase - after modelling the design in Solidworks, I began experimenting with FlowXpress. Finding it to be extremely interesting I just had to get my hands on the full version of Flow Simulation and see how the B7 performed at its intended, albeit virtual, speed of 500km/h.


      Thanks to Solidtec Australia I was able to play around with the software and generated some fascinating results.

      See images below. There is also a video of the animated flow simulation results here  http://www.youtube.com/watch?v=kIilEHSubmY


      I am no aeronautical engineer, but can see how the design of the car body affects drag, turbulence and lift forces thanks to the visual nature of Flow Simulation results. What I would like to know though, is how to generate a coefficient of drag figure for the B7. Is it possible to do with this software?

        • Re: B7 Hi-SPEED HYBRID
          Sebastian Johansen



          Unfortunately I don't have an answer to your questions... But I'm curious on what software you used to render the images?


          Also, do you have any thoughts on what type of engine to use, or is this just a design study of the bodywork?


          Kind Regards



          • Re: B7 Hi-SPEED HYBRID
            Bill McEachern

            You can do that with an equation goal - make a goal for the drag force and just rearrange the equation Fd=Cd*Aprojected*rho*v^2 for Cd and put all that into an equation goal.....

              • Re: B7 Hi-SPEED HYBRID
                Joe Galliera

                The 1/2 was missed to account for when the velocity potential is integrated in Bernoulli's equation.  http://en.wikipedia.org/wiki/Drag_equation


                F_d = (1/2)*rho*V^2*C_d*A



                F_d is the force on the body in the direction of drag

                rho is density of freestream fluid

                V is velocity of freestream

                C_d is coefficient of drag

                A is the projected area on a plane perpendicular to the direction of drag




                C_d = 2*F_d/(A*rho*V^2)

                  • Re: B7 Hi-SPEED HYBRID
                    Joe Galliera

                    I've worked up an example using a tractor-trailer truck (also known as a semi, a lorry or a prime mover) model from 3D Content Central in SW Flow Sim 2010 SP3.1, see attached.


                    You would have to make sure that your computational domain (CD) is sufficiently large, especially in the rear of the body, to capture the form (or pressure) drag, and the mesh is sufficiently fine to capture the friction drag.  Even with this, you should be leery of getting very accurate results as compared to real world, but for use in comparison the results should be good.


                    I added a moving ground solid body to capture the ground effects.  The CD should be buried into the ground just a bit so that the condition is captured by the solver.  The tires and wheel assembly should be modeled as rotating to take in their effect, so I also added this to the model.  If the wheels are axi-symmetric in nature, as it appears in your car model, then you could more simply add this effect as a moving/rotating wall boundary condition instead of a local rotating region.  Also it is easier to define local coordinate systems about each axle to define the rotation without further trouble.  I put an Excel spreadsheet in the zip file in order to calculate the RPM of the tires based on the tire's diameter (in inches) and the speed of the vehicle (in MPH); any other units like km/h would have to be converted first before being used.  (I'm in the US.)


                    I did a quick sketch and made a planar surface about the frontal area of the truck to estimate its projected area, but since we're using it for comparison sake (see above), then it should be fine.


                    Next I switched the units to SI to make sure that all the units  cancel themselves out.  And finally among adding other goals need to compute the drag coefficient, Cd, I made an equation goal using the formula in the previous post.


                    Some good tricks to use when defining the equation goal is that you can select any previously defined goal or boundary condition to use in the equation.  Next, the program will automatically set the equation goal's unit type based on the first goal that you had selected to add to the equation, in this case most likely force, so you will need to change the unit to "No units" (type n on the keyboard to jump to that value in the list) since Cd is without units.


                    There is a lot of detail in this post, but just follow the basic principals set up in the attached example, and you'll be up and running in no time.

                      • Re: B7 Hi-SPEED HYBRID
                        Filip Tejszerski

                        wow what an answer! my head is bursting all ready. so i guess a cd figure is possible, but seems so complicated. you mean there is no magic button for this??

                        remember, i am in the infancy stage of this software - haven't even experimented with many of the parameter inputs available.

                        i will have a close look at what is suggested here, and try it. would it be possible for you to list the procedure step by step ? it may help more people than just me:)