Wind Tunnel Modelling of Aerodynamic Baffle Arrays for Aircraft Exhaust Plume Control

Abstract

Local air quality is one of the factors constraining the development of airports. In countries of the European Union where new, stricter regulations for emissions of nitrogen oxides (NOx) have been introduced since 2010, the limits of mean annual concentrations are already exceeded at certain ground monitoring locations of large airports. This research project investigates the possibility of abating the aircraft exhaust plume at take-off by placing an array of aerodynamic windbreaks (‘baffles’) in the runway end safety area close to the aircraft starting position. The undertaken experimental investigation comprised sub-scale wind tunnel tests and full-scale field trials with a BAe 146-301 aircraft, performing take-off and landing cycles at Cranfield Airport. The initial wind tunnel experiments investigated the effect of a solid baffle row, placed in the path of a buoyant nozzle jet, on the development of the plume downstream. Using flow visualisation, the positive effect of the baffle row of promoting buoyant rise of the plume away from the ground was demonstrated successfully without the presence of wind tunnel flow. The investigation highlighted the importance of the distance of the baffles relative to the jet source on their effectiveness. In the presence of wind tunnel flow, the baffles caused an increased vertical spread of the plume downstream, but the plume was not observed to separate from the ground. In preparation of the field trials, the spatial arrangement of the baffle array was investigated by means of wind tunnel drag measurements, performed with a skinfriction balance. The experiments focused on key parameters such as the baffle slope angle and row spacing, favouring a configuration of three rows of baffles of increasing height. Based on the wind tunnel measurements, full-scale baffle prototypes were designed and manufactured at Cranfield University and were deployed in the field trials. Lidar and point sampler measurements during the field trials suggested that the plume had risen away from the ground on one occasion when the aircraft was located close to the baffles. A positive effect was shown in terms of reduced concentrations downstream of the baffles. This result was not replicated when the aircraft was further away from the baffles. The subsequent wind tunnel experiments focused on replicating the field trials at 1:200 scale in Cranfield’s Atmospheric Boundary Layer Wind Tunnel. The aircraft was represented at sub scale with a single stationary nozzle while the jet speed and buoyancy were modelled using similarity parameters such as the Froude number and the ratio of ambient and jet density. Mean concentration measurements were performed using a Flame Ionisation Detector method releasing methane as tracer gas. The effect of the baffles was observed to be mainly local in terms of reduced concentrations close the ground due to their sheltering effect. A more prolonged effect was found to be the increase of the plume’s vertical spread resulting in an increase in mean concentrations away from the ground.