Drag Coefficient Convergence/Accuracy Problems

cfdkktr · May 26, 2020, 09:42

Hello,

I am trying to get C_D vs Mach distribution for both finned and finless artillery projectiles with zero angle of attacks. The range for Mach number is from 0 to 4. I am trying to compute the C_D value for every Mach number with increment of ~0.1-0.25. However, I am experiencing quite significant convergence and/or accuracy problems for most of the cases. Although I done research and tried many different mesh and solution setup combinations, the problem is remaining the same.

For this kind of cases what do you suggest as a general technique?

What turbulence model should be implemented?

Which points should be considered as important while meshing? Is a different mesh for each Mach number a must? How to determine the correct y+ values? What geometric detail should be neglected during modelling?

What can be done to get solution converges faster in terms of both calculation time and number of iterations?

I would be very grateful for any help, thank you!

vinerm · May 27, 2020, 04:36

For 0-4 Mach, there are three regimes and all required different settings.

1. 0-0.3 is incompressible regime and you can work with constant density fluid. Beyond that, fluid has to be compressible.

2. 0.3 - 1 is compressible but subsonic regime. Actually, 0.9 to 1.1 is considered transonic regime but this one does not really require anything special

3. 1 - 4 is supsersonic regime and would have shocks

As far as modeling is concerned, it has four regimes. Mach 1-2 can be worked with pressure based solver, but beyond that you need density based solver. You will also have to look at Re and the ratio of Ma and Re because that defines Kn. For low Re, laminar flow will work but may require transient simulation. For very high Re, you can work with Euler equations (inviscid). So, there is no single set of conditions that will work with all. You have to setup three to four regimes, each with separate type of mesh, material, boundary and operating conditions, as well as, physical and numerical models. You can use density based solver for all cases but that will be very time-consuming and difficult to converge. Similarly, you can use very fine mesh for all cases but would be an overkill for low Re and low Ma flows.

mahdi-united · August 19, 2020, 10:15

hi my dear friend
thanks for all helps in past

in simulation of projectile motion, how to calculate Re number?
i need to set the one of the turbulent models and i don't know which one is appropriate
mach number is varied as 0.1 to 3 in my project

i read your post about different regimes and different setups in projectile motion.

May 26, 2020, 09:42	Drag Coefficient Convergence/Accuracy Problems	#1
cfdkktr New Member Join Date: May 2020 Posts: 1 Rep Power: 0	Hello, I am trying to get C_D vs Mach distribution for both finned and finless artillery projectiles with zero angle of attacks. The range for Mach number is from 0 to 4. I am trying to compute the C_D value for every Mach number with increment of ~0.1-0.25. However, I am experiencing quite significant convergence and/or accuracy problems for most of the cases. Although I done research and tried many different mesh and solution setup combinations, the problem is remaining the same. For this kind of cases what do you suggest as a general technique? What turbulence model should be implemented? Which points should be considered as important while meshing? Is a different mesh for each Mach number a must? How to determine the correct y+ values? What geometric detail should be neglected during modelling? What can be done to get solution converges faster in terms of both calculation time and number of iterations? I would be very grateful for any help, thank you!

May 27, 2020, 04:36	Mach number	#2
vinerm Senior Member Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,946 Blog Entries: 1 Rep Power: 35	For 0-4 Mach, there are three regimes and all required different settings. 1. 0-0.3 is incompressible regime and you can work with constant density fluid. Beyond that, fluid has to be compressible. 2. 0.3 - 1 is compressible but subsonic regime. Actually, 0.9 to 1.1 is considered transonic regime but this one does not really require anything special 3. 1 - 4 is supsersonic regime and would have shocks As far as modeling is concerned, it has four regimes. Mach 1-2 can be worked with pressure based solver, but beyond that you need density based solver. You will also have to look at Re and the ratio of Ma and Re because that defines Kn. For low Re, laminar flow will work but may require transient simulation. For very high Re, you can work with Euler equations (inviscid). So, there is no single set of conditions that will work with all. You have to setup three to four regimes, each with separate type of mesh, material, boundary and operating conditions, as well as, physical and numerical models. You can use density based solver for all cases but that will be very time-consuming and difficult to converge. Similarly, you can use very fine mesh for all cases but would be an overkill for low Re and low Ma flows. __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.

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August 19, 2020, 10:15		#3
mahdi-united Senior Member mahdi rostami Join Date: Jan 2020 Posts: 155 Rep Power: 6	hi my dear friend thanks for all helps in past in simulation of projectile motion, how to calculate Re number? i need to set the one of the turbulent models and i don't know which one is appropriate mach number is varied as 0.1 to 3 in my project i read your post about different regimes and different setups in projectile motion.