Internal & external flow

[valR7UZK]

Can you analyze both internal and external flow in the same run? I know in Solidworks sim flow that is not possible.

Example would be racecar cooling ducts or a muffler blowing gases towards the rear wing

[bob.schlosser]

This is possible.  It's all in how you set up your model.  If the air material is a single body it will treat it as such and allow flow anywhere.  If you have two separate bodies (think a sealed enclosure) it will treat each body as a distinct volume which will only interact with each other through heat transfer.  If it purely a flow simulation they will not interact, but will solve independently.  

[valR7UZK]

Not sure I understand how this solution applies to let say the interaction between a muffler and the wing behind it. There is airflow hitting the wing and creating downforce, In addition the muffler positioned below the wing will exhaust gas flow at 100 mph with the engine at 8000 RPM. This gas flow will also hit the wing and will create additional downforce if the airflow is under 100 mph. This is called "blown wing". I'd like to position the wing to optimize the effect while not burning it. 

[bob.schlosser]

I haven't set up a simulation in this way exactly, but I would start with the following. 

It sounds like you already know what the exhaust cfm and temperature is due to testing or some educated estimate.  I would set up your car in a wind tunnel type of air volume.  For instance if the car is traveling from right to left, the left inlet of the air volume has boundary conditions of ambient temp and 100 mph (speed of the car).  Right outlet surface of the air volume has pressure = 0.  This is very similar to the tutorial example.

The exhaust part is a bit trickier.  Since it sounds like you don't need to solve for the internal air flow of the engine and exhaust I would simplify it by putting a cfm boundary condition on the air intake of the car and a cfm condition and temperature boundary condition on the exhaust.  If you know the air speed and cross section area of the exhaust you could calculate cfm.  You may also need a good geometry representation of the air inlet and exhaust to help direct the airflow depending on how critical you need to be of the results.  

In this set up there isn't a second air volume.  You are treating the car as a black box that you don't know what is happening on the inside.  You would be forcing the mass balance in the car manually by your boundary conditions.  Normally this is a bad practice as your simulation needs the freedom to have very small mass balance errors due to the nature of FEA.  I think you can do this here because you are allowing he entire system to still solve with these errors on the outlet p=0 side of the wind tunnel.

[valR7UZK]

I've done mostly external flow simulation in Solidworks for over a year. I tried to use boundary conditions on the exhaust but got bogus results. 

I am doing an evolution to see if I should switch to Autodesk cfd. In your solution I can set cfm for the output of the exhaust and the cfm of the air hitting the car the volume being the size of the wind tunnel in solidworks terminology.

Did I get it right?

[bob.schlosser · Accepted solution]

I don't know what you mean by the size of the wind tunnel.

In any cfd package your wind tunnel dimensions should be many times that of the car.  I am referring to the intake vents and exhaust vents of the car itself.  Overall your wind tunnel must maintain a mass balance.  The car would do the same (ignore the combustion of the fuel) and need to maintain the relationship that the mass of air on the intake must equal the mass of air leaving the exhaust.  You end up having internal boundary conditions to your model which can cause problems.  You will have to try and see what you get and if this works.