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Convergence for Cd and Cl of rocket

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Old   April 26, 2022, 14:26
Default Convergence for Cd and Cl of rocket
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Alex
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Hi all,

I'm attempting to calculate the Cd and Cl of a rocket for a Uni project at a number of different velocities (200,400,600 up to 1200 m/s) but I cannot achieve convergence and therefore reliable results.

I'm using an RMS criteria of 1e-4 and I'm using the K-Omega SST model with energy equation ticked. I'm running a pressure based simulation with the missile shape boolean subtracted from a cylinder and sliced in half length-ways. The surface area of the rocket facing the oncoming flow is then doubled and input into the reference values.

I have some reference data so I know what sort of result to expect, however my results are way off the mark. I've tried pressure based and density based. I've tried a range of velocities (in case transonic flow was causing any issues) but the result is the same. I've double, triple checked all my parameters, watched all the YT tutorials but I can't get this thing to converge! Is there something obvious I'm doing wrong? Any help is greatly appreciated. Many thanks in advance.

For reference I'm running this particular one at 300 m/s at 1000m altitude.
Cl [expected] = 0.016
Cl [result] = 0.0323
Almost double!

My setup


My residuals and Cl graph after running the sim for 500 iterations
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Old   April 28, 2022, 07:39
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Quote:
Originally Posted by al531246 View Post
Hi all,

I'm attempting to calculate the Cd and Cl of a rocket for a Uni project at a number of different velocities (200,400,600 up to 1200 m/s) but I cannot achieve convergence and therefore reliable results.

I'm using an RMS criteria of 1e-4 and I'm using the K-Omega SST model with energy equation ticked. I'm running a pressure based simulation with the missile shape boolean subtracted from a cylinder and sliced in half length-ways. The surface area of the rocket facing the oncoming flow is then doubled and input into the reference values.

I have some reference data so I know what sort of result to expect, however my results are way off the mark. I've tried pressure based and density based. I've tried a range of velocities (in case transonic flow was causing any issues) but the result is the same. I've double, triple checked all my parameters, watched all the YT tutorials but I can't get this thing to converge! Is there something obvious I'm doing wrong? Any help is greatly appreciated. Many thanks in advance.

For reference I'm running this particular one at 300 m/s at 1000m altitude.
Cl [expected] = 0.016
Cl [result] = 0.0323
Almost double!

My setup


My residuals and Cl graph after running the sim for 500 iterations
Check your reference values in fluent. Cl and Cd are caluclated by refrence values(length, surface area, ref-temperature and others) that you provide to Fluent. Checking forces on body with your experimental result can give you an idea if your sim is accurate.
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Old   April 28, 2022, 11:06
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Lorenzo Galieti
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The oscillations are consistent with how much your residuals are (not) dropping down
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