Author(s): Waman Inamdar

Abstract: Underbody diffusers and side skirts are among the few aerodynamic devices that can generate rear-axle downforce on a sedan without adding visible appendages, yet their combined effect on the downforce-to-drag balance is poorly mapped for production-representative geometries. This research used steady-state RANS CFD (k-omega SST turbulence model) to sweep five diffuser angles (5, 8, 11, 14, and 17°) across three side-skirt heights (30, 50, and 70 mm) on a simplified sedan body at 140 km/h, producing a 15-point parametric map of lift and drag coefficients. The baseline sedan without a diffuser or skirts had Cl = +0.068 (net lift) and Cd = 0.318. Adding the diffuser alone at 14° reduced Cl to -0.061 (net downforce) but raised Cd to 0.324. Combining the 14° diffuser with the 70 mm side skirt achieved the strongest downforce (Cl = -0.118) while actually lowering drag below the baseline (Cd = 0.308) a rare simultaneous improvement in both metrics. The interaction mechanism was confirmed by flow-field visualisation: the tall skirt sealed the underbody edges, prevented lateral air leakage, and allowed the diffuser to build a stronger low-pressure region beneath the rear overhang. Beyond 14° the diffuser stalled, and downforce fell by 9 to 11% at 17° regardless of skirt height. The optimal configuration 14° diffuser with 70 mm side skirt improved the rear-axle downforce-to-drag ratio (?Cl/Cd) by a factor of 4.7 relative to the diffuser-only case, offering a practical route to enhanced high-speed stability on production sedans without the fuel-economy penalty typically linked to aerodynamic add-ons.

DOI: https://www.doi.org/10.22271/27078205.2026.v7.i2a.85

Pages: 37-41 | Views: 143 | Downloads: 62

Download Full Article: Click Here