Impact of pressure gradients on fish scales in a jet fish pump

Pressure gradients cannot be avoided in jet fish pumps due to their contracting and expanding sections. Pressure gradients are an important source of fish injuries, particularly for damage to fish scales. The distribution of pressure on the surface of a fish passing through a jet fish pump was studied experimentally and also by numerical simulation. A theoretical model of the forces on fish scales caused by pressure gradient was established. The results showed that the pressure on the surface a fish firstly decreases and then increases as fish passes through a jet pump. Surface pressures reach a minimum when the fish is in the throat. Pressure gradient is also a key factor for damaging fish scales. Axial pressure gradients were greater at the inlet and outlet of the throat than in other parts of the pump and radial pressure gradients increased with radial position. Also, the greater the flow rate the higher both axial and radial pressure gradients produced. Radial position is therefore an important factor for both pressure gradients and determining the force on fish scales. The forces caused by pressure gradients on scales increase with radial distance and with flow rate ratio. Thus, this study specifies the effects of pressure gradients on fish scales in a jet fish pump and discusses the relative influencing factors. These results could provide a basis for improved jet fish pump designs.