A neural network is trained for, and may be used in, the simulation of the fluid flow through a domain around an object geometry. A first training process for the neural network includes training (902) the network on a first set of encodings of pre-computed computational fluid dynamics (CFD) simulations for object geometries and associated boundary conditions. The first training process uses a first loss function that evaluates an error between the network output and the pre-computed CFD simulations. A second training process is then carried out which includes training (905) the network on a second set of encodings of object geometries and associated boundary conditions. The second training process uses a second loss function that evaluates an error between the network output and a set of fluid dynamics conditions.