Unsteady mixed convection heat transfer in a 3D closed cavity with constant heat flux on the centre part of the bottom wall and isothermal sidewalls moving in the same vertical direction is investigated numerically in this research. The other remaining walls forming the geometry are kept stationary and adiabatic. This research is accomplished with different Reynolds number, Re = 5000, 10,000, 15,000 and 30,000. Numerical methodology based on the finite volume method is utilised. The simulations and analysis have been carried out by evaluating the performance of two turbulence methods, Unsteady Reynolds Averaged Navier-Stokes (URANS) and Large Eddy Simulation (LES), in terms of flow vectors, isotherm contours, turbulent kinetic energy, the average Nusselt number (Nu(av)) and the local Nusselt (Nu(local)) number along the hot part of the bottom wall. The results show that by increasing the Reynolds number leads to enhanced Nusselt number and turbulent kinetic energy of the fluid in the domain. Moreover, both LES and URANS solutions captured the existence of the two primary vortexes (clockwise and anticlockwise). However, the comparisons have demonstrated clearly the ability and accuracy of the LES method in predicting the secondary vortexes in the corners of the cavity. (C) 2016 Elsevier Ltd. All rights reserved.