COMPARACIÓN DE DOMINIOS

As discussed above in Section 5.2, the two-phase fluid approach allowed the representation of one discrete particle size per simulation, with one phase representing the air and the second fluid phase the discrete spherical particles of defined density and diameter. Results of the CFD simulation predictions for the individual discrete particle size deposition rates are presented and discussed below. A base case model configuration is used to investigate the influence of particle size on the in-pit deposition rates. While the influence of variations in meteorological conditions such as wind speed, atmospheric stability and wind direction are investigated for a limited number of up to two discrete particle sizes.

As mentioned above, the total particulate deposition rate on to a number of defined in-pit surfaces was calculated and written to the operation run log file for each simulation time step. The summation of the deposition for the individual in-pit surfaces provided an estimate of the total in-pit deposition. An alternative means of determining the deposition rate is through a mass balance of the second phase fluid or particle flux across the domain. The source mass flux of particulate or dust released from the road and excavation areas was known. Integration across the domain enabled the determination of the mass flux downwind of the pit. As deposition was only allowed on in-pit surfaces, continuity of mass enabled the determination of the total in-pit deposition as the difference between the mass of particulate released into the pit and the mass passing through the domain downwind of the pit.

For simulations that provided a steady state solution to the flow (i.e. those not at an oblique angle to the domain boundaries) the two approaches converged to a similar prediction of deposition rates and thus pit retention, although this generally took several hours of simulated real time to achieve.

66 Deposition within the pit was initially investigated for the base case model

configuration of a north wind neutral boundary layer with U10m = 5 m/s. The

simulations were conducted for four discrete particle sizes, with an emission source area factor of 1.226 applied to the predicted deposition estimates for each individual particle size to account for the additional area of the roads and excavation areas. At each discrete particle size, the percentage pit retention was determined from the predicted deposition rate and the known in-pit dust emission rate.

The variation of the predicted pit retention with particle size under the base case neutral boundary layer is presented in Figure 7.2. The third order polynomial presented in the figure gives a good representation up to the maximum particle size simulated (50 µm). As expected the pit retention rate is seen to increase with particle size. Extrapolation above 50 μm indicates all particles above about 60 μm are deposited and thus are retained within the pit. Pit retention levels for the smallest particles (2.5 µm) are predicted to be in the order of 0.5%.

y = -0.00000601x3+ 0.00065828x2- 0.00182144x + 0.00585155 R2= 1.00000000 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0 10 20 30 40 50 60 70 Particle Diameter (µm) Pit R e ten ti on (% )

Neutral, U10m = 5m/s, North Poly. (Neutral, U10m = 5m/s, North)

Figure 7.2: Predicted pit retention variation with discrete particle size for the pit with the base case neutral boundary layer, north winds, U10m = 5

m/s.

Figure 7.3 summarises the influence of atmospheric stability on the pit retention. Simulations were performed with particle diameters of 10 and 30 μm for both the stable and unstable atmospheric flow cases for comparison with the base case neutral simulations. Again, the investigation was performed for northerly winds only.

Again, the results suggest that pit retention may increase very slightly in the stably stratified atmospheric conditions, most likely due to a slight decrease in the pit surface velocities. This may be due to the lower wind velocity in the lower sections

of the atmospheric flow or a reduction in dispersion from the surface due to stratification of the flow along the pit surface. However, the influence is only slight, and clearly the stratified approach flow simulated did not result in any significant change in the flow or dust behaviour within the pit. Again, findings may differ with lower ambient velocities and it maybe worthwhile attempting the simulations at wind speeds of the order 0.6 m/s. This would emulate the 2 m/s neutral flow scenario under equivalent stable conditions.

Retention is slightly increased for the unstable boundary layer simulations. Again, it should be remembered that the wind profile was of a reduced velocity and consequently the increase in pit retention may be a direct result of the lower wind speeds.

The simulations provide no conclusive evidence that atmospheric stability will significantly influence deposition rates within the pit and thus pit retention.

y = -0.00000601x3+ 0.00065828x2- 0.00182144x + 0.00585155 R2= 1.00000000 0.1% 1.0% 10.0% 100.0% 0 10 20 30 40 50 60 70 Particle Diame te r (µm) P it R e te n ti o n (% )

Neutral, U10m = 5m/s, North Stable, U10m =1.5m/s, North Unstable, U10m = 3m/s, North Poly. (Neutral, U10m = 5m/s, North)

Figure 7.3: Predicted pit retention variation with atmospheric stability for the pit under northerly winds.

The influence of wind direction on the pit retention is summarised in Figure 7.4. Four additional wind directions were investigated, with simulations performed for two discrete particle sizes of 10 and 30 μm for the south and east winds and for only the larger particle size for the northeast and southeast winds. The predicted rates are compared with the full range of particle sizes simulated for the base case configuration to provide perspective on the degree of variation of pit retention due to wind direction.

68 the order of 40% lower than for the other wind directions. Interestingly, the

southerly wind direction was the scenario for which the flow structure within the pit varied most significantly from the typical. The recirculation established in only the upwind half of the pit, with the flow passing directly across the northern half of the pit. Further discussion on the flow structure is presented in Section 6.1.5, however it appears the difference in flow structure has resulted in less interaction of the dust plumes with the surface of the pit and consequently less deposition and thus pit retention.

Although the flow behaviour for the oblique angled winds (northeast and southeast) is also observed to differ from that observed for winds perpendicular to the domain boundaries, the variation in flow structure did not significantly affect pit retention. The fact that for the flow structures developed in both situations, flow is observed to pass from one side to the other along the surface of the pit is thought to have resulted in dust having virtually equivalent opportunity to deposit, apart from the case of the southerly winds.

y = -0.00000601x3+ 0.00065828x2- 0.00182144x + 0.00585155 0.1% 1.0% 10.0% 100.0% 0 10 20 30 40 50 60 70 Particle Diameter (μm) Pi t R e te n ti o n (% )

Neutral, U10m = 5m/s, North Neutral, U10m = 5m/s, South Neutral, U10m = 5m/s, East Neutral, U10m = 5m/s, Northeast Neutral, U10m = 5m/s, Southeast Poly. (Neutral, U10m = 5m/s, North)

Figure 7.4: Predicted pit retention variation with wind direction with the base case neutral boundary layer, U10m = 5 m/s.

The influence of ambient wind speed on deposition and thus pit retention was also investigated. A comparison of pit retention under neutral northerly winds for wind speeds of between U10m = 2 and 10 m/s is presented in Figure 7.5. Again the

simulations were performed for the two discrete particle sizes of 10 and 30 μm for the U10m = 2 and 10 m/s configurations for comparison with the four discrete

particle sizes simulate for the base case U10m = 5 m/s scenario. The results

demonstrate conclusively that the pit retention increases with a reduction in wind speed. The relative magnitude of the variation in pit retention with wind speed is observed to be consistent across the range of particle sizes investigated. The simulations predict all particles larger than about 40 ∀m will be deposited within the

pit at the lower wind speed. At the higher wind speed, a proportion of particles of up to about 70 μm aerodynamic diameter will be released from the pit.

y = -0.00000601x3+ 0.00065828x2- 0.00182144x + 0.00585155 0.1% 1.0% 10.0% 100.0% 0 10 20 30 40 50 60 70 Particle Diameter (µm) P it Re te n tio n (% )

Neutral, U10m = 5m/s, North Neutral, U10m = 10m/s, North Neutral, U10m = 2m/s, North Poly. (Neutral, U10m = 5m/s, North)

Figure 7.5: Predicted pit retention variation with wind speed under neutral boundary layer northerly winds.