Left atrial appendage occlusion devices (LAAO) are a feasible alternative for non-valvular atrial fibrillation (AF) patients at high risk of thromboembolic stroke and contraindication to antithrombotic therapies. However, optimal LAAO device configurations (i.e., size, type, location) remain unstandardized due to the large anatomical variability of the left atrial appendage (LAA) morphology, leading to a 4–6% incidence of device-related thrombus (DRT). In-silico simulations can be used to estimate the risk of DRT and identify the critical parameters, such as suboptimal device positioning. However, simulation outcomes depend a lot on a series of modelling assumptions such as blood behaviour. Therefore, in this work, we present fluid simulations results computed on two patient-specific LA geometries, using two different commercially available LAAO devices, located in two positions: 1) mimicking the real post-LAAO intervention configuration; and 2) an improved one better covering the pulmonary ridge for DRT prevention. Different blood modeling strategies were also tested. The results show flow re-circulations at low velocities with significant platelet accumulation in LAA-deep device positioning uncovering the pulmonary ridge, potentially leading to thrombus formation. In addition, assuming Newtonian blood behaviour may result in an overestimation of DRT risk.