*3.5. Sand Trap with Rakes and Ribs*

It was hypothesised that the higher turbulence induced by the rakes would increase settling speed for larger sediment sizes. By studying the particle tracks in the simulation results, it can be seen from Figure 8 that sediments with a diameter larger than 1 mm tend to settle earlier in the sand trap compared with the geometries without the rakes in the diffuser. However, it can be observed that sediments smaller than 1 mm tend to remain suspended for longer when rakes are included. These smaller sediments have the potential to cause erosion damage on the turbine blades, and it is therefore desired to prevent these from escaping the sand trap.

**Figure 8.** Sand trap with ribs and v-shaped rakes, symmetry plane at t = 1000 s. (**a**) Velocity contour. Flow over the rakes is accelerated, while flow going through the rakes slows down and becomes turbulent. (**b**) Vorticity contour. High vorticity appears immediately downstream of rakes and remains throughout the sand trap. (**c**) Turbulence kinetic energy contour. Rakes induce higher levels of turbulence than can be seen in models without rakes. Turbulence has not dissipated before the flow exits the sand trap

The large circulation zones, which also occur in the models without rakes, can be seen clearly in Figure 9. Sediments are seen to become trapped in these circulation zones in particle track plots. The flow is separated as it passes the rakes, where flow going over is accelerated, while flow going through decelerates and becomes turbulent. Vorticity and turbulence are induced by the vortex shedding at the rakes. The highest levels of turbulence are observed between the two rows of rakes. A large turbulent wake is established downstream of the rakes and remains until the outlet. In the present work, this has been shown to decrease sand trap efficiency. The flow downstream of the diffuser when rakes are included is seen to be more turbulent than when rakes are omitted. As the turbulence does not dissipate before exiting the sand trap, this causes more turbulent flow to enter the penstock.

If the height of the rakes was to be increased so that they reach the crown of the tunnel, it could affect the settling characteristics in multiple ways. One possibility is that increasing the height of the rakes would cause an earlier onset of turbulence and vorticity, which again carries small diameter sediments further. From the results in the present work, it is believed that this would lead to an increase in head loss and a decrease in sand trap efficiency. Another possibility is that the flow would no longer be divided into high- and

low-velocity zones downstream of the rakes. Instead, a general reduction in absolute flow velocity would occur. This could mean that the flow becomes more uniform, which might be beneficial for sand trap efficiency. In both cases, increasing the flow obstructing area is likely to increase head loss.

**Figure 9.** Extended view of v-shaped rakes in symmetry and horizontal planes at t = 1000 s. (**a**) Velocity contours. The large circulation zones, which can also be seen in the models without rakes, appear at the entrance of the diffuser. These zones are observed trapping sediments. Flow is separated going past the rakes. Flow going over is accelerated, while flow going through is decelerated. Velocity is highest between rakes. (**b**) Vorticity contours. Vorticity and turbulence is induced by the vortex shedding at the rakes. (**c**) Turbulence kinetic energy contours. The highest levels of turbulence are observed between the two rows of rakes. A large turbulent wake is established downstream of the rakes, which has been shown to decrease sand trap efficiency in the present work.
