[Ryuzaki]

Hello,
I want to perform CFD analysis of airfoil for 0.7 Mach under 10,000 feet conditions. In the atmospheric condition table, the static pressure value for 10,000 feet is 69,681 Pa. This value represents the static pressure of stagnant air at an altitude of 10,000 feet. In the equation P_total = P_static + P_dynamic, for a stagnant environment, P_dynamic is 0, so the value 69,681 is essentially equal to the P_total value at that altitude.

In my analysis, when I use the initial value of P_static as 69,681, the total pressure within the domain increases in the far-field. The reason for this is the presence of a freestream throughout the domain. Therefore, the P_dynamic value comes into play. Increasing the P_total value in the far-field doesn't seem correct to me because there is no freestream there in real life.

The main issue bothering me here is whether to keep the P_total value in the far-field constant or not. In reality, there is no freestream in the far-field at that altitude, so P_total equals P_static there. At the end of my simulation, to see the P_total value in the far-field as 69,681 Pa, I calculate the P_static value using isentropic relations based on the Mach number. When I use this value as the initial condition, I can observe the P_total value in the far-field as 69,681 at the end of the analysis.

Can you help me determine which method is correct?

[LuckyTran]

I am too lazy to check that your numbers are correct, but assuming that they are:

The static and total pressure of stagnant air at (any) altitude is indeed 69,681 Pa.

But in reality and in numerics, stagnant air does not have an airfoil flying through it at Mach 0.7. Stagnant air is stagnant. Once you have the moving airfoil, you have to reconsider what happens.

With an airfoil, the stagnation pressure stays the same 69,681 Pa, but the static pressure upstream of the airfoil will decrease (because it starts moving) and adjust to whatever it needs to be. And this is why you get right results when you back-calculate it yourself. If you measure the static and total pressure in front of an airfoil in a wind tunnel or even on an airplane, you will see the same thing.

[Ryuzaki]

First of all, thank you for your valuable explanation and time,

In this case, should I fix the total pressure value or the static pressure value in my CFD analysis?

For example, my total and static pressure value for stationary air is 69681 Pa. When I send flow into my flow field at, say, 220 m/s, I now also have a dynamic pressure. In this case, will my P_total value be updated to 69681+P_dynamic? Or should I define it to the domain by calculating the P_static value so that P_total (69681) = P_static + P_dynamic?