# Convergence for Cd and Cl of rocket

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 April 26, 2022, 13:26 Convergence for Cd and Cl of rocket #1 New Member   Alex Join Date: Apr 2022 Posts: 1 Rep Power: 0 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

April 28, 2022, 06:39
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Void_CFD-user
Join Date: Feb 2022
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Quote:
 Originally Posted by al531246 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.

 April 28, 2022, 10:06 #3 Senior Member   Lorenzo Galieti Join Date: Mar 2018 Posts: 373 Rep Power: 12 The oscillations are consistent with how much your residuals are (not) dropping down

 Tags missile, rocket