[mshahriar]

Hello everyone, I want to simulate Rayleigh-Taylor instability where I inject CO2 in water from the top interface and it moves through the water in a finger sort of formation (picture attached).

I created a geometry, where I selected the top-edge as mass flow inlet. I use an arbitrary mass flow kg/s. The other edges are set up as wall.

When I run the simulation, the CO2 moves in a square shape (it fills up the whole surface gradually). I can't get the finger sort of movement pattern. I found this on the article, "To observe the non-linear behavior, i.e., the fingering behavior it was necessary to disturb the interface. Therefore in the numerical simulations, we start with a wavy perturbation on the top interface, i.e.,

c(x; z = 0;t = 0) = 1 + 0.01sin(2*pi*x/0.083)

I don't have any experience on writing UDF. But seems like I need to change the inlet condition for time t=0, to a varied sinusoidal mass flow. How can I do that? Any sort of suggestion or help is greatly appreciated.

(Side note: I can't find out how to provide concentration value of CO2 in mol/l. Instead I am doing everything as mass flow. Will that be any problem)

[Micael]

Not so long ago (a couple of releases ago) you would have indeed written a UDF, but now you can achieve that very well with Named Expressions.

[mshahriar]

Thank you for replying Micael, can you kindly explain a little bit?

I am fairly new to Ansys Fluent and it would help me a lot if you give a detailed answer or guide me to some resources that can help.

[LuckyTran]

Searched the manual or google "Named Expressions" in Fluent. There's a fair number of youtube videos as well.