Exhaust plumes emanating from smoke stacks at power plants or other industrial facilities can have adverse impacts on local aviation during periods of calm winds. Adverse impacts can be exacerbated if the temperature is low or the atmosphere is unstable. While low oxygen concentrations and elevated temperatures inside the plume can be detrimental to slow-flying or hovering helicopters, the turbulence generated from the upward motion of the plume is the main potential hazard to light, fixed-wing aircraft at low altitudes.
The Exhaust Plume Analyzer consists of three main parts: a convective flow model describing the mean flow of the plume, two aircraft response models judging the required vertical gust to achieve severe turbulence or aircraft upset, and a turbulence model computing the probability of experiencing a gust capable of causing severe turbulence or aircraft upset.
These aircraft response models require detailed aircraft parameters; therefore, the Exhaust Plume Analyzer provides aircraft parameters for four aircraft types representing light-sport aircraft, light General Aviation (GA) aircraft, business jets, and large jets. Furthermore, advanced users interested in modeling additional aircraft have the option to provide parameters for a user-defined aircraft type. Because the behavior of an exhaust plume depends greatly on the local weather conditions, the Exhaust Plume Analyzer also provides the option for the user to query an external MITRE server for hourly atmospheric conditions at a specific location to accurately portray the likelihood of a severe turbulence or an aircraft upset event.
An exhaust plume can be described as a vertical turbulent buoyant jet consisting of two main parts: a momentum-dominated region (jet region) and a buoyancy-dominated region (plume region). After comparing several models to experimental data (both laboratory and full-scale), the Spillane1 model was found to be the most accurate for these purposes. It can accurately describe the mean flow behavior of the exhaust plume in its various regions for a single stack or multiple aligned stacks.
Two aircraft response models were adopted to determine how aircraft are affected by vertical gusts created by exhaust plumes. The Exhaust Plume Analyzer uses the Gust Loads Formula to predict the vertical gust required to experience severe turbulence, which is defined as a 1g vertical acceleration per the National Oceanic and Atmospheric Administration (NOAA). In addition, the Exhaust Plume Analyzer uses the Houbolt2 roll model to determine the vertical gust that would cause aircraft upset (a bank angle of 45 degrees) if the vertical gust was concentrated on the tip of the wing, and the flight crew or Flight Management System (FMS) did not take corrective action.