Overview
Opening the windows on a highway drive versus keeping them closed and running the AC is a common fuel-saving debate. This investigation used Computational Fluid Dynamics (CFD) to settle it quantitatively: I built a 3D model of a 2020 Hyundai Santa Fe, created open- and closed-window versions in Blender, and ran a digital wind tunnel in SimFlow (an OpenFOAM-based CFD platform) at five velocities spanning city to highway speeds (5–40 m/s).
Method
- Geometry: 2020 Hyundai Santa Fe STL model, edited in Blender 4.2.3 to produce matched open-window and closed-window versions with a fixed ~3.86 m² frontal area.
- Solver setup: k–ω SST turbulence model, moving-wall ground plane to replicate real driving conditions, no-slip car surface, slip tunnel walls.
- Test matrix: 5 velocities (5, 10, 20, 30, 40 m/s) × 2 window states × 3 iteration counts (500 / 1000 / 1500) to check convergence — 30 runs total.
Results
| Configuration | Avg. Drag Coefficient (C_d) | Uncertainty |
|---|---|---|
| Closed windows | 0.2807 | ± 0.0014 |
| Open windows | 0.2909 | ± 0.0011 |
Opening the windows raised the drag coefficient by about 3.6% and the gap held steady across every tested speed.
At highway speed (40 m/s), the open-window configuration demanded about 1.57 kW more power than closed windows with AC — roughly 5.66 MJ of extra energy over one hour of driving.
Takeaway
Even a small geometric change — cracking the windows — measurably increases aerodynamic drag, and that penalty grows fast with speed (drag ∝ v², power ∝ v³). At city speeds the difference is negligible; at highway speeds, running the AC with the windows up is the more fuel-efficient choice.
Full write-up (methodology, uncertainty propagation, CFD boundary conditions, and evaluation): [link to PDF if hosted].