Large Eddy Simulation of early stage aircraft contrails.

摘要

Aircraft contrails at cruise altitudes present a unique problem in atmospheric sciences due to their location where other anthropogenic sources of particulate emissions are absent, and due to their potential impact on the Earth's radiative balance. The scale separation between contrails and atmospheric motions contributes to the large uncertainties in contrail models used in global atmospheric calculations. We study the early-stage contrail growth using high-fidelity Large Eddy Simulations (LES) to better characterize the early contrail-scale processes.; We develop an extended Boussinesq approximation to improve the traditional Boussinesq approximation for inertia-dominated flows with buoyancy. We develop an Eulerian-Lagrangian ice microphysics model and couple it to the flow solver. Verification and validation of the model and the numerical method are discussed. The choice of the number of Lagrangian computational particles required to adequately represent particle statistics is studied.; We investigate the behavior of two dimensional aircraft wakes containing exhaust jets. A mechanism is identified by which the jet-induced baroclinic vorticity influences the wake dynamics. The jets strengthen the primary vorticity and reduce the primary vortex pair spacing, causing accelerated wake descent.; Three-dimensional LES of contrails are carried out for the first 20 second duration of contrail life, starting at 1 second behind the aircraft. The initial conditions at 1 s correspond to a typical large subsonic aircraft. Three different regimes of early contrail growth are found, which are controlled by jet humidity, jet turbulence and ice particle-induced perturbations, and ambient relative humidity with respect to ice (RHi) respectively. Sensitivity of ice properties to atmospheric temperature and relative humidity, initial ice nuclei size and number density is characterized. Ambient temperature and RHi are found to be critical in determining the rate of ice growth. Particle size distribution is skewed to higher radii for higher RHi. The total ice area, ice particle size, and number density are found to be sensitive to the initial ice nuclei number density, but the total ice mass remains insensitive to it at least in the regime considered. Ice growth is found to be relatively insensitive to the initial particle size for the size range considered.