This thesis presents a 1D-3D coupled approach for tunnel ventilation modelling. A simplified 1D tunnel network model, based on the industry standard tool SES, is developed for testing coupling strategies. For a small tunnel network, air-flow rates are within 2% of those computed using SES. The open-source CFD code OpenFOAM is used to compute pressure losses within the developed coupling approach. Pressure loss calculations using the standard k - ε turbulence model are verified against the commercial code Star-CCM+ and validated against experimental data. In most cases the calculations are within the error bounds of the data. A Lagrange multiplier approach is used to treat defective flow-rate boundary conditions, which arise in multi-dimensional modelling. A novel implementation of this method within the finite-volume framework of OpenFOAM is constructed. The additional unknowns, introduced to the Navier-Stokes equations are solved within an adaption of the PISO solution, used by OpenFOAM. The velocity profiles produced using this method are in excellent agreement with the analytical solutions for laminar pipe flow. Furthermore, a first implementation of the Lagrange multiplier method with turbulence modelling incorporated shows that the approach is stable and accurate for simple and complex flow scenarios. A 1D-3D 'hybrid' coupling strategy has been developed that can be implemented as a 'black box' within the framework of the 1D aerodynamic model, and applicable to SES, by representing the CFD sections of the network as pressure losses. The proposed 1D-3D coupling strategy accurately reproduces full 3D simulations for the same scenario, and also reduces the simulation time. The tools developed in this work allow sophisticated methods to simulate the flow more accurately than 1D tools alone, whilst allowing for a large tunnel network to be modelled at a reasonable cost.