CFD Question/Result

[larb_10]

Hey,

Quick question I am not quite getting the results I was expecting from a simulation I have been running. See below an image of the model sectioned:

I have set it up as a steady state analysis, and have prescribed velocites on two surfaces. I have also created the inverse model in order to show the airflow as required. See the image below:

My question then is why am I getting results like this,

I would have expected the air to travel 'up" the duct more, or have spread more. Do I need to add some time steps or more iterations in order for the simulated pressure to build inside and therefore the airflow to spread out properly? Or should I add an internal pressure initially?

Any help appreciatted.

[John_Holtz]

Hi,

I am not entirely clear what the flow path is. Are there two paths to the outlet or just one?

Regardless, the flow rate is inversely proportional to the resistance (or something like that). So the largest volume of flow takes the shortest path, and the remaining takes longer paths in decreasing amounts. So the results you show look reasonable to me.

But here are two modeling issues that you need to investigate:

1) Do the walls in the original model have thickness, or is it just a surface model? If it is a surface model, then the inverse model of the fluid just has "slits" where the walls are located, instead of having a gap of X thickness. I have been cautioned in the past that "0 thickness walls" or obstructions may not work as expected. So please check the results and loads & constraints in the Results environment to make sure the walls have 0 velocity conditions on them.

2) Is there some place in the model for air to "leak" in or out? Or are the only places for the air to enter and exit at the inlet and outlet where you have the prescribed velocity? If the latter, you should change one of the velocities to an inlet/outlet condition. From the documentation "Help > Autodesk Simulation > Setting Up and Performing the Analysis > Setting Up Part 2 > Fluid Flow Analyses > Loads and Constraints > Prescribed Velocities"

Because prescribed velocities lead to a mass flow and the mass flow in must equal the mass flow out, it is best to NOT fully prescribe the velocities on all boundaries of a model. Leave one face unrestrained and let the processor calculate the velocity on the unrestrained face. For example, the velocity profile in and out of a straight pipe may be identical in an ideal situation, but in an analytical situation, there may be some variation. Therefore, specify the inlet conditions but not the outlet condition, or vice versa.

[larb_10]

Hey,

There is only one path to the outlet around the bottom fixture. There is however an arrangment of holes up the back of the mock up for the air to disperse and flow out of and down to the outlet.

And yes the original model does have thickness. The walls are only like 1mm thick though, but they are not surfaces. Should I still check to make sure they have 0 velocity conditions?

And no there is no where else for the air to leak out, I have changed the condition on the outlet to be an outlet and not a prescribed velocity. I am running the simulation now, will post images up once I have some results.

See below an image from the front of the mockup showing the air spreading out the bottom but not flowing up the rear of the mockup. 

[larb_10]

Just got the results of the revised setup see image below.

I guess my main issue is that in real life we in fact get the most airflow out of the top of the rear ducting and not the bottom of the ducting as is being shown here. I am not sure if the simulation is not agreeing because of a problem with the turbulence settings or something like that? I just want it to match the real world results we have had more closely.

[Joey.X]

hi, arb_10

I am trying to answer your questions from your multiple posts.

- If user applies no boundary conditions (BCs) on exterior surfaces, ASIM will treat them as wall (i.e., zero velocity). 

- Check the mesh density in narrow junction interfaces between chambers, if it has no (or very few) interior fluid nodes, the air flow cannot go through.  BTW, it is suggested to turn on boundary layer mesh for fluid flow analysis and remember turn off boundary layer mesh from the open inlets/outlets and the surfaces between parts.