The flow of particles around the tube under gravity is simulated and there are two problems as follows :
I notice that the flow of particles is not smooth, but there is an obvious vibration. I wonder if this is normal and can be improved by adjusting the parameters.
The tube wall is set to no slip boundary condition, but the particles always enter the tube. I increase the normal spring constant from 800 to 2000, but it does not work.
2000 N/m may still be too low for the spring constant. Please increase to say 10,000 N/m or more to see if this makes a difference. You may also need to increase the frequency of the neighbor search call.