This PhD dissertation consists of a detailed study of the mixing between a turbulent crossflow and an incoming jet in a T-junction. It is focused on the complex flow structures that emerge in such flows. Special attention is also paid to the flow field in the near-wall regions. In the first part of the thesis we describe the governing system of equations and present the algorithm that we have implemented for the numerical treatment of those. In the second part we report on wall-resolved and experimentally validated Large-Eddy Simulations (LES) of constant-density flows. The bulk Reynolds number of the crossflow is 15,000. In the presentation of the results we elaborate on the main features of the flow, namely, the shear layers that emanate from the corners of the entry of the jet, and the large recirculation bubble downstream the incoming jet. In the third part of this dissertation we examine turbulent heat transfer in gas-gas flows via LES. The bulk Reynolds number is again set at 15,000. Emphasis is placed on the role of the temperature as an active scalar and on the effects of thermal stratification to the emerging structures in the flow field. In the fourth part we perform a Direct Numerical Simulation of turbulent heat transfer in liquid- liquid flows. The bulk Reynolds number is set at 6,000. Herein we present an extensive analysis of the instantaneous features of the flow field. We also compute the budget of the Turbulent-Kinetic-Energy which provides additional information about the turbulence generation mechanisms in the flow of interest. (FSA - Sciences de l'ingénieur) -- UCL, 2017