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AFT Blog

Welcome to the Applied Flow Technology Blog where you will find the latest news and training on how to use AFT Fathom, AFT Arrow, AFT Impulse, AFT xStream and other AFT software products.
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Secret Variable-g Feature in AFT Software? Yep, and It is Now Visible to All

When I wrote AFT Impulse 1 back in 1996, I slipped in an undocumented feature to make it capable of modeling the first waterhammer project I did in industry. Since then, this feature had a secret (i.e., undocumented) way of being enabled. AFT Impulse 8 will be released any day, and we have finally made this feature accessible to all.

In the User Options window, you can turn on "Enabled Variable g in System Properties" as shown below in Figure 1. 

Figure 1: User Options Window


Once enabled, the Variable G button is visible in System Properties and you can set your variable g table as shown below in Figure 2. 

Figure 2: System Properties Window

I ended up writing an ASME paper that discussed my first waterhammer project and you can find it on the AFT website here - Rocket Propellant Line Waterhammer Transients In a Variable-G Environment.

Figures 3 and 4 below show how AFT Impulse 8 predictions compare to flight data for AC-60 (taken as the g level varied, as discussed in the ASME paper). AC-60 launched in 1982 (when I was a second-year engineering student!). The data comparison below is pretty good!

If you look closely, AFT Impulse in Figs 3-4 does not agree perfectly with the simulation results in the ASME paper. I do not know exactly why, but suspect that AFT Impulse is better as it is based on Pressure, whereas my tool in the ASME paper was based on Piezometric Head – which does not lend itself as well to varying g levels.

In case you are curious, we have had customers in the aerospace industry inquire about this cool feature – and we told them how to enable it. Now it is there for all to see – and for you to use on your next rocket project! I am confident AFT Impulse is the only commercial waterhammer software that can do this trick.


Figure 3: Pressure at Booster 1 Pump Inlet for AC-60 Flight


Figure 4: Pressure at Booster Pump 2 Inlet for AC-60 Flight
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Monday, 15 August 2022
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