A Wind Tunnel Study of Flow Characteristics near Model Swine Production and Manure Storage Facilities

One of the most significant and persistent environmental concerns regarding swine production is the transport of odor constituents (e.g. ammonia and hydrogen sulfide), trace gases (e.g. greenhouse gases like methane and nitrous oxide), and particulates from animal production and manure storage facilities. Local environmental conditions, especially wind speed and direction, vegetative cover, and topography affect the amount of odor and trace gas compounds transported from production facilities. Due to the large number of potential building arrangements and varying land cover and topography, studies on the transport of air quality constituents from and near buildings of various types (i.e. urban, suburban, and industrial) have often been conducted in wind tunnels. Wind tunnels offer the advantage of being able to make detailed measurements with scale models of actual buildings under controlled environmental conditions. Comprehensive measurement of air flow and air quality constituent transport at full scale swine production facilities requires a very large investment in sophisticated sensing equipment. Even when such resources are available, the data collected are only relevant as a case study, i.e. for one location under the conditions occurring during the measurement period. If modifying the layout of a production facility reduces the downwind air quality impacts and can be accomplished with no or a small increase in construction costs, then the producer will derive direct air quality benefits for the lifetime of the facility for a minimal one-time cost. The objective of this study was to determine how swine housing unit orientation and distance from manure storage facilities affect potential air quality constituent (odor and trace gas) transport.

The Air Quality of Agricultural Systems (AQAS) Research Unit at the National Soil Tilth Laboratory (NSTL) has a low-speed wind tunnel for environmental research applications. The AQAS wind tunnel has a centrifugal blower capable of producing a range of air velocities up to 15 m/s in a control section 46 cm tall, 122 cm wide, and 5.5 m long. Models of swine housing units and manure storage facilities (approximately 1/300th scale) were placed in the control section of the wind tunnel to simulate a wide array of possible arrangements. A trip fence, conical vortex generators, and array of surface roughness elements (LEGO blocks) were used to create a surface boundary layer within the control section. Evaporation of water from scale models of aboveground (circular tank) and earthen manure storage (rectangular lagoon) facilities are used to evaluate relative odor/trace gas transport under different building orientations and environmental conditions. The baseline condition will be the model manure storage facility without any housing unit arrays upwind. Evaporation measurements were made over a range of air velocities with data collected on 1-min intervals for a length of time long enough to establish equilibrium conditions at that velocity (~2 hrs).

Design factors to be evaluated are: 1) number of housing units, 2) orientation of the housing units with regard to wind direction, 3) distance between the housing units and the manure storage facility, and 4) local surface roughness and topography. Scale models of housing units were constructed of balsa wood following typical heights and roof pitches of commercial production facilities. Measurements were made with 1, 2, and 4 housing units of the same dimensions. These models will be oriented parallel to airflow, at a 30° angle to airflow, and perpendicular to airflow. All measurements were made with the housing unit arrays upwind from the manure storage models. Separation distance between the housing unit arrays and manure storage models vary by multiples of building height (h). Separation distances evaluated are 2h, 5h, and 10h. The manure storage models were also be scaled following standard industry dimensions. Evaporation from the manure storage models was measured using a linear variable digital transformer (LVDT), which can measure water level to ~0.01 mm accuracy.

A 3D hot film anemometer system (IFA 300, TSI Inc., Shorview, MN) was used to measured air flow characteristics in the lee of the model buildings and manure storage facilities. Model orientation and air velocity effects on turbulence characteristics in the vertical, lateral, and longitudinal axes are interpreted and compared with observed differences in evaporation from the model manure storage facilities.