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In aftertreatment system design, flow uniformity is of paramount importance as it affects aftertreatment device conversion efficiency and durability. The major trend of downsizing engines using turbochargers means the effect of the turbine residual swirl on the flow needs to be considered. In this paper, this effect has been investigated experimentally and numerically. A swirling flow rig with a moving-block swirl generator was used to generate swirling flow in a sudden expansion diffuser with a wash-coated diesel oxidation catalyst (DOC) downstream. Hot-wire anemometry (HWA) was used to measure the axial and tangential velocities of the swirling flow upstream of the diffuser expansion and the axial velocity downstream the monolith. With no swirl, the flow in the catalyst monolith is highly non-uniform with maximum velocities near the diffuser axis. At high swirl levels, the flow is also highly nonuniform with the highest velocities near the diffuser wall. An intermediate swirl level exists where the flow is most uniform. To gain further insight into the mechanisms controlling flow redistribution, numerical simulations have been performed using the commercial CFD code STARCCM+. With no swirl, the central jet transverses the diffuser, and a drastic flow redistribution takes place near the monolith face due to its high resistance. Immediately downstream of the sudden expansion, the flow separates from the diffuser wall forming a separation zone around the central jet. Increasing swirl reduces the size of this separation zone, and eventually leads to the formation of the central recirculation zone characteristic of high swirl flows. At intermediate swirl levels, the size of the wall separation zone is reduced considerably, while the axial adverse pressure gradient is insufficient to cause a central recirculation. Such a flow regime occurs at relatively low swirl levels (𝑆 ~ 0.23). This may have positive implications for aftertreatment system design with low residual swirl levels from the turbine, which might be tuned by adjusting the distance between the turbine and the catalyst or employing guide vanes. The findings can be directly transferred to other aftertreatment systems with a catalyst or particulate filter. Moreover, swirling flows with an obstruction or a high resistance device downstream (e.g. a heat exchanger or filter) are present in many other applications such as cooling flows, combustion and turbomachinery. Therefore the results are relevant to a much wider research and industrial community.