## 4 Discussion  
The CFD simulations done in this work illustrate the impact of various parameters on the mean residence time of feed pellets in a cylindrical fish tank (Figure 1), which might be quite significant concerning the sustainability of such aquaculture systems. For example, the impact of the pellet characteristic size (diameter) is illustrated on Figure 8, and it is clear that with a smaller size, the mean residence time increases. A similar trend can be observed for lower flow rates, see Table 4.  
Another important parameter affecting the mean residence time of the feed pellets in a fish tank is their injection position. It is usually located at the liquid surface, and the exact position could be a part of an optimization procedure in CFD simulations. In this particular case of the cylindrical fish tank with an outlet at the bottom center, the smallest residence time is at the center (it is 17.0 s for TM0 pellets, and 21.3 s for TM75), and it increases when moving towards the outer edge of the tank in the radial direction (75.5 s for TM0, 75.3 s for TM75). The largest difference between the two types of pellets is at the center, up to 25%. However, there might exist an optimum (maximum) residence time with respect to the angular coordinate, see Figure 11. The maximum difference along the perimeter at 3/4 tank radius is 8%. Of course, the results could be different for different geometrical and operating parameters. For example, if we switched the inlet and outlet, that is, if the water inlet was at the bottom of the tank, we could substantially change the flow pattern and affect the residence time of pellets as well as their trajectories. Once the methodology described in this paper is well established, this could be a relatively easy task.  
Concerning the feed pellet properties which were determined in the experimental part of this work, they can change substantially with the time spent in water (see [11] describing the expansion rate of feed pellets, for example). Implementing such dependency might not be an easy task in ANSYS Fluent solver; a user-defined function (UDF) would have to be created based on some model describing experimental data. Moreover, other parameters of the pellets could change; for example, their resistance to higher shear stresses will probably decrease with larger residence time in the water so that they could disintegrate into smaller particles of quite different properties than at the beginning. Because the amount of pellets used for fish feeding in RAS systems is relatively very small, they were modeled as completely passive in our CFD simulations, that is, we assumed the so called one-way coupling between the fluid and solid (pellets) phases. The feeding is usually done several times per day with a small amount of pellets. Therefore, the effect of pellets settling to the flow field is very low. With a larger amount of pellets, two-way coupling could be used, and consequently, the impact of particles on the flow field, turbulence quantities and other variables could be investigated.  
This article can be served as a basis for the next simulating of feed particle transport and modeling of zones of sedimentation and zones of uniform distribution. This can be utilized in the feed production industry, as this sector is constantly testing novel feed ingredients, which influence feed pellets' properties. The CFD-based methodology developed in this study can also be used in the optimization of tank design and recirculating aquaculture technology. Future studies should aim to effect inlet modification, outlet modifications, and aeration equipment to optimize feed particle distribution within rearing tanks.