Ram pressure correlations for aspirated cylinders
Scholz, Zachary James
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Design of automobile cooling systems involves tradeoffs in the sizing of grille openings to provide adequate cooling airflow and the tendency to reduce grille opening size to decrease vehicle cooling drag and produce aesthetically pleasing designs. Air that enters the cooling system of an automobile is driven by two major sources, the freestream dynamic pressure resulting from the forward motion of the vehicle and the internal vacuum created by the underhood fan. The flow fields associated with both sources must be considered when assessing the cooling performance of a new automobile design. The current investigation focuses on characterizing the external or dynamic pressure induced flow through a parameter known as the ram coefficient. The investigation utilized an aspirated cylinder in cross-flow as an idealized representation of an automobile front end with grille openings. The pressure distribution on the upstream side of the cylinder model includes a stagnation point and a significant surface pressure gradient similar to those of an actual automobile front end fascia. Various sized openings machined into the side of the cylinder model simulated the grille openings in an automobile. A flexible hose connecting one end of the cylinder to a shop vacuum provided a simulation of the cooling air flow induced by a radiator fan. The primary advantage of the cylinder model is a dramatic reduction in the number of experimental influences on the ram coefficient. The elimination of the various underhood components simplifies the investigation process down to the most basic components, yielding accurate, repeatable results. Primary results are that the cylinder does provide a useful representation of automobile front end. These results verify the general trends seen in previous full scale model tests. Additionally, it was found that ram coefficients for single openings are determined by opening size and location relative to the external surface pressure distribution. It was also found that ram coefficients for combinations of openings can be predicted from knowledge of the performance characteristics of the individual openings.