The high turbulence level of the incoming flow on a turbine has a significant impact on its performances, leading to a decrease in the electrical power output. Therefore, perturbations carried in the wake of an upstream turbine may also have a significant impact on the downstream one. This problematic has already been addressed but with simplified approaches such as the Blade Element Momentum theory giving interesting results regarding power extraction but lacking some precision regarding definition of the structure of the wake and its effect on load variation on the turbine blades. To get a better understanding of the effect of unsteady asymmetric flow on a downstream turbine, fully transient simulations designed to study the effect of the wake of an upstream turbine on a downstream one were performed with a RANS k-? SST turbulence model using ANSYS-CFX. Three different configurations were considered, namely the downstream turbine aligned with the upstream one, offset by 0.5D, and offset of 1D. A 10D clearance between both turbines was used. A horizontal axis tidal turbine (HATT) was used for the study. Results show that when fully in-line, the downstream turbine sees reduction in power coefficient by more than 69%, and temporal variation of this coefficient having a relative amplitude of more than 30%.