August 24, 2023 at 9:51 am

Loic Ivaldi

Ansys Employee

Hello,

Let's do the math.

The timestep is defined as dt = L / c with L the characteristic length of the element and c the speed of sound.

in 1D, L is the length of the element and c = (Young's modulus / density )^1/2

squaring both sides, you have dt^2 = L^2 * density / Young's modulus.

Taking L=1 and Young's modulus= 1 => dt^2 = density

By doing so the timestep is a function of the density only (and again this is a simplification).

The dt on your screenshot 7.12e-10 is the dt after scaling by the TSSFAC=0.9 parameter. Before this scaling, the timestep is 7.91e-10.

Squaring it gives dt^2 = 6.26e-19 = density

Now what you want is to reach ~5% of mass increase. the formula is :

((new density / density) - 1) * 100

if you take new density = 6.58e-19 you have ((6.58e-19 / 6.26e-19) - 1) * 100 = ~5%

taking the square root of new density gives you the new dt : dt = new density^(1/2) = 8.11e-10

Don't forget this will be the added mass at the beginning of the simulation : meaning thatthe added mass can still increase afterwards as the simulation carries on.

Also you want to scale your time from 1s to 0.0001s so maybe what you want to achieve is too much. maybe you can try with something like 0.01s and see how it works out.

Best Regards,

Loïc