particles/Main.cs

using Godot;
using System;

public class Main : Node
{

    public void StartSimulation(int seed, int nParticles)
    {
        var particleSimulationPackedScene = GD.Load<PackedScene>("res://ParticleSimulation/ParticleSimulationScene.tscn");
        var particleSimulationScene = particleSimulationPackedScene.Instance<ParticleSimulationScene>();
        particleSimulationScene.Name = "Simulation";
        AddChild(particleSimulationScene);
        particleSimulationScene.Initialize(seed, nParticles);
        GetNode<Control>("MainMenu").Hide();
        //OS.WindowResizable = false;
    }

    public void ExitToMenu()
    {
        GetNode("Simulation").QueueFree();
        GetNode<Control>("MainMenu").Show();
    }
    
}