After the F1-75 and W13 sidepods simulations, I did the RB18 as well. Sadly, due to a lack of frontal images showing more about the frontal beam-wing geometry, I didn't want to invest the time to simulate these as well. You can look the results and comparisons in detail here -->
viewtopic.php?p=1038370#p1038370
In short, the design is very streamlined, low drag and conditions the rear tyre flow in some way. However, in my view it's aimed more at conditioning the flow between the rear tyre and diffuser wall, as even with my modelling this area has a very clean and fully attached airflow.