Large eddy simulations of aircraft contrails [electronic resource] : parametric study and reduced order modeling
Keywords: Mechanical Engineering.

Soot particles emitted by aircrafts at cruise altitudes act as nuclei for any excess water vapor in the ambient to freeze on to. This results in the formation of visible contrails that, ambient conditions permitting, persist and spread for hours to alter the local cloud cover. Given the ever increasing air traffic, these large number of aircraft cirrus have a sizable, yet uncertain, net impact on the total radiative balance of the planet making it prudent to study the physics of contrails. A major challenge in quantifying the climate impact of contrails is the weak understanding of the physics of contrail evolution due to the high dimensionality of the problem. Here, we present results from a parameter sensitivity study of contrails using a high fidelity Large Eddy Simulation (LES) model and then develop a simple predictive reduced order model for early persistent contrails that encapsulates our findings from the parametric study. Using the LES model, we explore the impact of ambient parameters such as ice supersaturation, stratification, turbulence intensity, shear and ambient temperature, and aircraft parameters such as wingspan, weight, soot emission index, initial particle size distribution, engine efficiency and engine type. The impact of asphericity of ice particles is also explored using a probabilistic model developed for this purpose and compared with an existing asphericity model. Given the large parameter space and the resulting wide spread of results for the primary quantities of interest - namely, surviving fraction of initial ice particles, plume ice mass and mean particle size - an appropriate dynamic normalization was developed based on our physical understanding of the problem to achieve an excellent collapse of the data. The primary drivers of the survival rate are the soot emission index and the treatment of the Kelvin effect. Reduction in the emission index by an order of magnitude increases the survival rate from 20% to 90%, while suppressing the Kelvin effect results in the survival of nearly all ice particles. The plume dynamics, that drives the total ice mass accrued, is a consequence of the interaction between the ambient turbulence, stability and the impact of the weight of the aircraft. An increase in weight by 33% results in an increase in the rate of accretion of ice mass by a factor of ~1.45 while increasing the turbulence intensity by an order of magnitude increases the ice mass accretion rate by a factor of ~2. Following this, a reduced order model was developed for the aforementioned dynamic quantities used for normalizing the data based on simple force and energy balance and dimensional and scaling arguments. This model can predict the quantities of interest within reasonable confidence (maximum 10% error observed) thus achieving orders of magnitude reduction in computational cost for predicting properties of early persistent contrails. This model is amenable to implementation within General Circulation Models (GCMs) that use subgrid parameterization of early contrails to predict long time horizon impact of contrails within larger climate simulations.