# Why use "Fourtakas (full)" in DDT?

Hello!

Under examples/other/freedraw mode there is a case called "CaseFrBeach". It outputs this warning:

"1. The selected DDT '(Fourtakas et al 2019 (full)' needs several boundary layers when h/dp>1.5"

Why do I use "full" option for DDT in this case and how do I know I have enough boundary layers?

Kind regards

## Comments

Hi Asalih3d,

The density diffusion term (DDT) of Fourtakas et al., 2019 has been implemented in v5.0 of DualSPHysics. This version of the diffusion term has advantages over the traditional we have been using in DualSPHysics in the past (which is still present in the solver as DDT1).

You have two options, either apply the diffusion term only to particles which do not interact with the fluid (that is 2

haway from the boundary, DDT2) or apply the term in the whole (full DDT3) domain. Clearly the latter will produce much more accurate pressure field near the boundary free from numerical noise . Nevertheless, a severely truncated kernel with high density boundary particles from DBC will impact the accuracy of this diffusive term.For soothing lengths coefficients smaller than h/dp<1.5 the DDT works sufficiently well - even when only one line of boundary particles is used, for larger kernel support you may need to use more than one line of wall particles, this will help with the kernel truncation and most importantly the large density of the boundary particles.

The kernels we use in DualSPHysics have a support of 2

hthus, you can easily calculate how many particles are within the radius of your kernel and use the appropriate number of boundary particle lines.The message is simply a warning/reminder for the user.

Happy DualSPHysics-v5-ing!!!

Regards,

George

Thanks for the reply @gfourtakas !

That makes sense, also the help file suggests heavily to use DDT3, so I will keep doing that. It would be nice though if the warning would explain (or just mention) that the criteria to look out for is; 2h <= x*Dp, where x is number of layers - since this is mentioned very nicely in the helpfile.

Kind regards and thank you for the indepth explanation, made me understand it