[Ryuzaki]

Hi,

I am simulating compressible flow with buoyancy in Fluent. I receive this warning

"Warning: For compressible (ideal and real) gas models with buoyancy, it is recommended that you use a specified operating density value of zero: define/operating-conditions/operating-density? yes 0."

Can someone explain what exactly means? I mean what if I dont set operating density to zero?

[LuckyTran]

Two important points to keep in mind
1) Fluent (and pretty much all reputable CFD solvers) work in gauge pressures and for buoyancy problems gauge densities.

2) If you do not set a reference density, Fluent computes an average and uses that as a reference. Using an average as a reference reduces numerical truncation errors and is the best practice for choosing a gauge from a numerical standpoint. It is user friendly when the user forgets to specify a gauge but becomes user hostile when the user doesn't understand the consequences of it.


If you set your reference density to something that isn't zero, you better be prepared to deal with everything in gauges. One of the most difficult aspects is making sure you remember to remove the hydrostatic contribution to the pressure at every location when you specify "pressure" on a boundary for example. If you know what this operating density is (because you set it) then this is pretty straightforward. Now if you let Fluent determine the operating density on its own, then you actually have no clue (you can find out later) what even is the gauge. The solver will still solve, but it will be the solution to a condition that most likely will not be the intended conditions because what you think is P=1 atm isn't actually P=1atm because you forgot to account for the gauge.


The operating density doesn't have to be 0, 0 isn't even the best reference for numerical reasons. But 0 is the easiest one for users that don't know what this option does (just like setting operating pressure to 0 makes everything absolute pressures and easier for newbies to learn). Once you understand what the operating pressure and density does, you can set it to whatever you like =)

[Ryuzaki]

Dear LuckyTran, Thank you very much for your detailed explanation.

After your explanation, the following question arose in my mind.

For example, I am modeling an aircraft cruising at 0.6 mach at 5000 ft conditions. During modeling, I enter the operating pressure value for the altitude value of 5000 ft. Since I enter the operating pressure, I enter 0 in the gauge pressure parts in the boundary conditions and do my analysis in this way. Is there a problem here?

My second quention is that,

For example, let my density be 1.056 kg/m3 for 5000 ft conditions. Should I enter this value in operating density? Or should I enter this value as 0 and leave everything to the equation of state?

(All in my simulations carried out with bouyancy.)

thnx in advance..

[LuckyTran]

Rather than actually try to tempt you to do it one way or another. You should just do it and plot the resulting "pressure" field. They'll be look different. Choose the one that is more within your capacity to interpret. You need to convince yourself.