It's great to see that you're using computational fluid dynamics (CFD) to model particle transport in a cube. Based on your description, it seems like you're on the right track, but there are a few things you should consider when estimating the number of particles leaving the cube.
First, it's important to note that when you set the front surface as a velocity inlet and the back surface as a pressure outlet, you're creating a flow through the cube, which means that particles are being transported through the cube by the flow. Therefore, it's not enough to simply estimate the number of particles leaving through the outlet surface. You also need to consider the number of particles that are being transported through the cube by the flow.
To estimate the number of particles leaving the cube, you can follow a similar approach to the first case. You can calculate the total number of particles in the cube using a volumetric report of the custom field function (UDS*cell volume), but this time, you should subtract the total number of particles in the cube at the beginning of the simulation from the total number of particles in the cube at the end of the simulation. This will give you the net change in the number of particles in the cube, which is equal to the total number of particles that have left the cube.
Alternatively, you can use a particle tracking method to track the movement of individual particles through the cube. This method involves injecting a large number of particles into the cube and tracking their movement using CFD. You can then count the number of particles that exit the cube during a given duration to estimate the number of particles leaving the cube. Get more Information from https://dreamyworldinfo.com/gepco-online-bill-gepco-online-bill-check-dreamyworldinfo/