Defining Density

Bradley R · July 18, 2013, 16:45

Hi Everyone

I am new to fluent and I am currently busy with my honors project on a solar car. I am using air for my fluid domain with the temperature set to 298 K and pressure of 101325 Pa. In my boundary conditions I am using the pressure far field option. I am also using a density based solver and ideal gas law for air. When I set my reference values based on the pressure far field, it is giving me a density of 2.37 kg/m3 for the air. This means that I am getting a very high drag force and I'm not sure if this affects my Cd during computation?

I am also using k-epsilon as my solver. Is it necessary to check solver independence because my results with k-omega are quite different.

Why is it giving me such a high density as the reference value?

Also would you suggest using pressure based for speeds of 120km/h although air is compressible?

I would appreciate any ones help. Thank You

Bradley

blackmask · July 18, 2013, 21:50

Can the car run so fast that the compressibility come into play?

You can see that 120 km/h is approximately 33 m/s, or a Mach number around 0.1 at the temperature you give. In most cases the flow can be regarded to be incompressible as long as Mach number does not exceed 0.3. The exceptions can be some quasi one-dimensional flow where shock formed at low Mach number such as the movement of piston.

Why simply use the incompressible model?

BTW, do you know how does FLUENT handle the pressure far field condition? This type of b.c. is handy for some of wind tunnel experiments but I do not believe that you could figure out how to specify the parameters for this b.c. easily.

Bradley R · July 19, 2013, 03:00

Thank you for your response.

The reason I looked into density based is because there are quite a few curves on the underside of the car with the wheel bays and the sitting position. I thought that this may cause the air to compress. It is probably not significant.
Where would be the best place to find a solid reason which I can reference as to why I am using one over the other?

The pressure far field I have applied the flow in the one direction however I am also doing cross wind analysis so I will be applying a cross wind. How would I apply a velocity inlet and outlet? Would that be a better option and why if I may ask?

Bradley R · July 19, 2013, 03:19

Sorry I had forgotten. I actually tried pressure based first and I was getting divergence.

blackmask · July 19, 2013, 05:11

You should try to extend your computational domain to appropriate size and use the combination of velocity inlet + pressure outlet. That is a robust combination for incompressible outer-flow aerodynamics.

July 18, 2013, 16:45	Defining Density	#1
Bradley R New Member Bradley Rautenbach Join Date: Jul 2013 Posts: 3 Rep Power: 13	Hi Everyone I am new to fluent and I am currently busy with my honors project on a solar car. I am using air for my fluid domain with the temperature set to 298 K and pressure of 101325 Pa. In my boundary conditions I am using the pressure far field option. I am also using a density based solver and ideal gas law for air. When I set my reference values based on the pressure far field, it is giving me a density of 2.37 kg/m3 for the air. This means that I am getting a very high drag force and I'm not sure if this affects my Cd during computation? I am also using k-epsilon as my solver. Is it necessary to check solver independence because my results with k-omega are quite different. Why is it giving me such a high density as the reference value? Also would you suggest using pressure based for speeds of 120km/h although air is compressible? I would appreciate any ones help. Thank You Bradley

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July 18, 2013, 21:50		#2
blackmask Senior Member Join Date: Aug 2011 Posts: 421 Blog Entries: 1 Rep Power: 22	Can the car run so fast that the compressibility come into play? You can see that 120 km/h is approximately 33 m/s, or a Mach number around 0.1 at the temperature you give. In most cases the flow can be regarded to be incompressible as long as Mach number does not exceed 0.3. The exceptions can be some quasi one-dimensional flow where shock formed at low Mach number such as the movement of piston. Why simply use the incompressible model? BTW, do you know how does FLUENT handle the pressure far field condition? This type of b.c. is handy for some of wind tunnel experiments but I do not believe that you could figure out how to specify the parameters for this b.c. easily.

July 19, 2013, 03:00		#3
Bradley R New Member Bradley Rautenbach Join Date: Jul 2013 Posts: 3 Rep Power: 13	Thank you for your response. The reason I looked into density based is because there are quite a few curves on the underside of the car with the wheel bays and the sitting position. I thought that this may cause the air to compress. It is probably not significant. Where would be the best place to find a solid reason which I can reference as to why I am using one over the other? The pressure far field I have applied the flow in the one direction however I am also doing cross wind analysis so I will be applying a cross wind. How would I apply a velocity inlet and outlet? Would that be a better option and why if I may ask?

July 19, 2013, 03:19		#4
Bradley R New Member Bradley Rautenbach Join Date: Jul 2013 Posts: 3 Rep Power: 13	Sorry I had forgotten. I actually tried pressure based first and I was getting divergence.

July 19, 2013, 05:11		#5
blackmask Senior Member Join Date: Aug 2011 Posts: 421 Blog Entries: 1 Rep Power: 22	You should try to extend your computational domain to appropriate size and use the combination of velocity inlet + pressure outlet. That is a robust combination for incompressible outer-flow aerodynamics.