## Fluids

Topics relate to Fluent, CFX, Turbogrid and more

#### Operating Density in Fluent

• Burak Onder
Subscriber

Hi,

I'm new for Ansys Fluent and i can't find something clear about operating density. It's always changing results when studying with fluid thermodynamics. For Which problems and how should i use this? It could be good if anyone can also send or show some theory documents about this.

Thanks.

• raul.raghav
Subscriber

Burak, as far as I understand the operating density is crucial for multiphase and free convection flows. Essential any flow scenario sith gravity enabled will use the operating density for calculating the local pressure in the fluid domain. This is used to avoid round-off errors during the calculation. And for the pressure equation that is solved, refer Eqn 13-3 in Fluent Operating Density.

If the single phase flow has a constant density, you don’t have to worry about the operating density value and can use the default that Fluent automatically calculates. However, if density is a function of temperature or pressure, then you have to define operating density depending on how the temperature and pressure values at the boundaries (or outside the domain).

• jmccasli
Ansys Employee

Hello Burak,

Here is what the Fluent User Guide says regarding the operating density. I hope this helps.

Regards,

Jeremy

• Raef.Kobeissi
Subscriber

Operating density is important for natural convection in fluid (fluids with different densities) and it is also important if you want to study the hydrostatic effect in fluid domains.
Imagine you have a tank filled with water, the height of the tank is 3 meters. in order to include the hydrostatic effect, you need to enable operating density, put the value at 0 and make sure you have a correct pressure reference point.

Regards

• mossaied2
Subscriber

is the operating density important for an evaporation problem of a flowing fluid (water to vapor) in a pipe?

• DrAmine
Ansys Employee
Yes . Either set it to zero and five the correct outside pressure at pressure boundaries or at least the one if the light phase (the latter is not always a good way). You need to review what Fluent is doing when gravity is enabled.
• irineupetri
Subscriber

Hi.

operating density is very important in multiphase flow, but is negligible in simpler flows.

Good luck