Simulating the surface temperature of an aircraft - which approach is sufficient?

CellZone · February 22, 2023, 08:16

Hello,

I would like to simulate the temperature on the surface of an aircraft, flying with mach number = 1 through air.

To my understanding, there are two possibilities now:

1. Assume that the wall of the aircraft is adiabatic. The question for me is, what will a CFD solver like fluent return as temperature on the surface? Is it the stagnation temperature on all surfaces of the aircraft or will it return a kind of recovery temperature which will be calculated?

2. Assume that the aircraft is a solid (with mass, heat capacity and heat transfer coefficient) and do a steady state simulation. Do you think there will be a big difference im comparison to 1) ? Since steady state condition is reached, heat flux should be zero and the surface temperature = fluid temperature (near the wall). Or is this wrong? If not, then this should be the same as 1) ?

Edit: Sorry , in steady-state the the heat flux is not zero. Steady state the heat flux with respect to time is zero. But what I wrote with "surface temperature = fluid temperature (near the wall)" for steady state is not true.

Thank you for your thoughts!

CellZone

LuckyTran · February 22, 2023, 14:54

Keep in mind you are talking about a very marginal difference but since you are indeed asking...

1. If walls are adiabatic then it returns the adiabatic wall temperature which will be slightly less than the stagnation temperature because some of the heat is conducted away from the fluid near the surface. Stagnation temperature assumes the fluid is brought to rest isentropically. In reality there is some heat transfer taking place as the fluid is brought to rest.

Recovery temperature only applies to when you stick a sensor into the flow to measure something and additional processes take place that are not really related to the original flow. In CFD you don't need to worry about recovery temperature.

2. Airplanes are never steady state unless they're parked in a hanger. If one decides to model a flying craft using a steady state simulation, then there are tons of models on top of models to be applied that would make the original question irrelevant because the models being applied would need to account for those differences. Unless you are talking about a novel all-electric aircraft.

CellZone · February 23, 2023, 05:42

Firstly: Thank you LuckyTrain for your help, I really appreciate!

Quote:

Keep in mind you are talking about a very marginal difference but since you are indeed asking...

Why do you think the difference is "marginal" ?

To my understanding, when I use the approach of "adiabatic wall" , what I simulate is the temperature of the hot air around the aircraft.

When I simulate using a solid (with heat conduction inside the solid) the temperature of the surface of the aircraft is different than that from the hot air around the aircraft.

So there should be a big difference, especially when the aircraft has a lower temperature at the beginning and the hot air is heating up the aircraft?

LuckyTran · February 26, 2023, 14:10

Because your original question was about the difference between surface temperature and recovery temperature, and the recovery factory for most sensors is very high (e.g. 90%).

Yes surface temperatures changes during a flight envelope, but you're asking about a steady state simulation. That means you have to give the right heat flux to get the right surface temperature. If you do it poorly then of course you get a different result. If you do it well then you get the right temperature.

February 22, 2023, 08:16	Simulating the surface temperature of an aircraft - which approach is sufficient?	#1
CellZone Member Join Date: Apr 2016 Posts: 91 Rep Power: 10	Hello, I would like to simulate the temperature on the surface of an aircraft, flying with mach number = 1 through air. To my understanding, there are two possibilities now: 1. Assume that the wall of the aircraft is adiabatic. The question for me is, what will a CFD solver like fluent return as temperature on the surface? Is it the stagnation temperature on all surfaces of the aircraft or will it return a kind of recovery temperature which will be calculated? 2. Assume that the aircraft is a solid (with mass, heat capacity and heat transfer coefficient) and do a steady state simulation. Do you think there will be a big difference im comparison to 1) ? Since steady state condition is reached, heat flux should be zero and the surface temperature = fluid temperature (near the wall). Or is this wrong? If not, then this should be the same as 1) ? Edit: Sorry , in steady-state the the heat flux is not zero. Steady state the heat flux with respect to time is zero. But what I wrote with "surface temperature = fluid temperature (near the wall)" for steady state is not true. Thank you for your thoughts! CellZone

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February 22, 2023, 14:54		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,754 Rep Power: 66	Keep in mind you are talking about a very marginal difference but since you are indeed asking... 1. If walls are adiabatic then it returns the adiabatic wall temperature which will be slightly less than the stagnation temperature because some of the heat is conducted away from the fluid near the surface. Stagnation temperature assumes the fluid is brought to rest isentropically. In reality there is some heat transfer taking place as the fluid is brought to rest. Recovery temperature only applies to when you stick a sensor into the flow to measure something and additional processes take place that are not really related to the original flow. In CFD you don't need to worry about recovery temperature. 2. Airplanes are never steady state unless they're parked in a hanger. If one decides to model a flying craft using a steady state simulation, then there are tons of models on top of models to be applied that would make the original question irrelevant because the models being applied would need to account for those differences. Unless you are talking about a novel all-electric aircraft.

February 26, 2023, 14:10		#4
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,754 Rep Power: 66	Because your original question was about the difference between surface temperature and recovery temperature, and the recovery factory for most sensors is very high (e.g. 90%). Yes surface temperatures changes during a flight envelope, but you're asking about a steady state simulation. That means you have to give the right heat flux to get the right surface temperature. If you do it poorly then of course you get a different result. If you do it well then you get the right temperature.