In patients diagnosed with transcatheter aortic heart valve thrombosis (THVT) after implantation of a transcatheter aortic valve (TAVI), the accumulation of a thin layer of thrombus on the prosthetic leaflets leads to reduced leaflet mobility and can increase the risk for stroke and valve degeneration (Rosseel et al., 2019). The comparison of aortic root morphology of patients with and without THVT in the Bern TAVI Registry showed that patients with THVT tend to have a larger ascending aorta (AAo). In order to find a relation between geometrical aortic dimensions and flow features leading to the development of thrombus in case of THVT, we perform highly-resolved fluid-structure interaction (FSI) simulations in a comparative study. Two generic parameterized models of an aortic root with a biological tissue valve model are created of which one has aortic dimensions found in THVT patients and the other reflects aortic dimensions of the general TAVI population. The interaction of blood flow and valve tissue is modelled with a high-fidelity FSI solver using a modified Immersed Boundary Method based on variational transfer (Nestola et al., 2019). Blood flow motion is simulated with a high-order Navier-Stokes solver (Henniger et al., 2010) which is coupled to a structural solver for aorta and valve motion solving the full elastodynamic equations in their weak form. Aortic wall and valve stent are modelled linear-elastic while we apply a Holzapfel-Gasser-Ogden material model for the fiber-reinforced biological leaflet tissue. During peak systole, numerical results reveal similar peak jet velocities for both cases, but wider jet velocity profiles with less retrograde flow along the aortic wall for the THVT model within in the AAo. In the sinus of the THVT model, we observe a lower total systolic turbulent dissipation rate and lower flow velocity magnitudes which indicate less fluid motion in this region. In contrast, the total systolic turbulent dissipation rate in the AAo is higher for the THVT compared to the control model. Less retrograde flow towards and less flow motion in the sinus may indicate less washout of the sinus which, in combination with higher turbulent dissipation in the AAo, could promote the formation of thrombus in the THVT model. The observation of blood flow patterns promoting thrombus formation in larger ascending aortas could explain the relation between aortic root morphology and THVT.