Mechanical positions at Arup take one of two basic trajectories (with some degree of overlap between the two): HVAC and CFD. HVAC-related roles can be filled by students pursuing mechanical engineering degrees or architectural engineering degrees (with architectural engineering students probably having an advantage here). CFD is more likely to just be mechanical engineering students. I primarily worked on the HVAC side and found that important skills include heating, cooling, and ventilation system sizing; a basic familiarity with ASHRAE documents; and familiarity with vocabulary and practices used in the construction/design industry. Proficiency with energy modeling software, Revit, and Microsoft Office software are definitely advantageous (though I was not proficient in Revit and it worked out alright).