A new small-disturbance model for a steady transonic flow of moist air with
non-equilibrium and homogeneous condensation around a thin airfoil is presented.
The model explores the nonlinear interactions among the near-sonic speed of the
flow, the small thickness ratio and angle of attack of the airfoil, and the small amount
of water vapour in the air. The condensation rate is calculated according to classical
nucleation and droplet growth models. The asymptotic analysis gives the similarity
parameters that govern the flow problem. Also, the flow field can be described by
a non-homogeneous (extended) transonic small-disturbance (TSD) equation coupled
with a set of four ordinary differential equations for the calculation of the condensate
(or sublimate) mass fraction. An iterative numerical scheme which combines Murman
& Cole's (1971) method for the solution of the TSD equation with Simpson's integration
rule for the estimation of the condensate mass production is developed. The
results show good agreement with available numerical simulations using the inviscid
fluid flow equations. The model is used to study the effects of humidity and of energy
supply from condensation on the aerodynamic performance of airfoils.