
Re: B7 HiSPEED HYBRID
Sebastian Johansen Jun 1, 2010 4:57 AM (in response to Filip Tejszerski)Hi!
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
Sebastian

Re: B7 HiSPEED HYBRID
Filip Tejszerski Jun 1, 2010 11:09 PM (in response to Sebastian Johansen)hello, maxwell render was used via solidworks.


Re: B7 HiSPEED HYBRID
Bill McEachern Jun 1, 2010 8:51 AM (in response to Filip Tejszerski)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 HiSPEED HYBRID
Joe Galliera Jun 1, 2010 3:41 PM (in response to Bill McEachern)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
where,
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
Thus,
C_d = 2*F_d/(A*rho*V^2)

Re: B7 HiSPEED HYBRID
Joe Galliera Jun 1, 2010 6:06 PM (in response to Joe Galliera)I've worked up an example using a tractortrailer 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 axisymmetric 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.

Cd_for_truck.zip 7.3 MB

Re: B7 HiSPEED HYBRID
Filip Tejszerski Jun 1, 2010 10:46 PM (in response to Joe Galliera)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:)

Re: B7 HiSPEED HYBRID
Joe Galliera Jun 2, 2010 4:01 PM (in response to Filip Tejszerski)Filip, it is a generalized flow code, so there is no "magic button," but I do like the suggestion. I will submit this as an enhancement request, and I recommend that you do the same.

Re: B7 HiSPEED HYBRID
Brandon Spires Mar 28, 2011 4:29 PM (in response to Joe Galliera)Im working on a project similar to this one. I understand manipulating the drag force equation but i dont understand how solidworks incorporates form drag and friction drag into the drag coefficient value if you are only inputting the frontal area.




