The works presented here in order to obtain the “Habilitation à Diriger des Recherches” (HDR), are dedicated to the activities I have been carrying out at ONERA since 2009 as a research engineer. They reflect the two steps in my career at ONERA. First of all at the DAAA department in Châtillon where my topic was optimization and gradient computation by an adjoint method. It was also the opportunity for me to experience high order numerical methods like isogeometric analysis. Then, at the DMPE department in Toulouse where I have been focusing on icing activities, following the emergence of new certification rules for supercooled large droplets (SLD) or ice crystal icing for instance. In a context supported by French industry and European funding, I have been involved in many research subjects related to icing, such as complex wall impacts (SLD, ice crystals), thin liquid film runback and ice protection systems. My contribution consists in the coding of the 2D icing suite IGLOO2D and the derivation of models integrated in the code, notably through the supervision of interns and PhD students.