Simulation of the effect of windbreaks on odour dispersion

Computational fluid dynamic (CFD) models are capable of defining best management practices associated with the wide use of natural windbreaks to help disperse livestock odours. The Fluent shear stress transport (SST) k – ω model was used to simulate odour dispersion downwind from natural windbreaks and to test the effect of tree characteristics (tree porosity, type and height, and windbreak distance from the odour source). The airflow inertial resistance of windbreaks was defined as proportional to the square of the tree diameter. The SST k – ω model was initially calibrated for air velocity recovery rate on the downwind side of the windbreak, using a two-dimensional simulation. Following calibration with field odour measurements, the model was used to compare the size and length of odour plumes developing downwind from windbreaks with different tree characteristics and odour source position. With the air velocity recovery rate calibration, the SST k – ω model predicted the wind velocity profile with an acceptable error of 4.5%. The calibration of the SST k – ω model for odour dispersion required the use of a function transforming the simulated odour mass concentration into simulated absolute hedonic tone. Once calibrated for odour dispersion, the SST k – ω model was able to accurately predict odour concentration downwind from the windbreak. By comparison, the SST k – ω simulations indicated that a dense (aerodynamic porosity of 0.2 compared to 0.66) and tall (9.2 m compared to 4.6 m) natural windbreak produced a shorter odour plume, but the odour concentration immediately downwind from the windbreak was much higher. For the same aerodynamic porosity, tree type or structure had limited effect on odour plume size. As compared to a source located further away (60 m), an odour source located close (15 m) to the windbreak produced a shorter plume because a closer source resulted in less dilution upwind from the windbreak and a greater mass of odourous air trapped downwind from the windbreak. Thus, a tall and dense natural windbreak, located no more than 15 m downwind from the odour source, minimises the length of the odour plume, but increases the intensity of the odour level immediately downwind from its position.