Human-Powered Vehicle Ground Clearance Study

The goal of this study wasa to determine the optimal ground clearance height for the fairing's geometry.

Process

Based on the mesh independence study, the mesh M2 was used, with around 1.7M cells, to conduct this study. The fairing geometry was first parametrized in SolidWorks, and 9 ground clearance configurations were created and exported as STEP files to be used in ANSYS Fluent. The ride heights ranged from 20 mm to 60 mm in 5 mm increments. The objective was to determine how the distance between the vehicle and the ground influenced aerodynamic drag and the surrounding flow field. Each configuration was simulated for around 300 iterations, with drag coefficients averaged over the final 100 iterations to reduce the influence of numerical fluctuations and improve the reliability of the comparison.

Results and Analysis

The results revealed a nonlinear relationship between ground clearance and aerodynamic drag, with the lowest CdA occurring at 40 mm. Drag decreased as clearance increased from 20 mm to 40 mm, before increasing again toward 60 mm. A second-order polynomial trendline produced an R² value of 0.9956, indicating a strong fit to the simulated data. The 40 mm configuration achieved approximately 1.1% lower CdA than the 20 mm and 60 mm configurations.

Although 40 mm produced the lowest measured drag, the differences between 35 mm, 40 mm, and 45 mm were relatively small. This suggests an optimal operating range of approximately 35–45 mm, with the exact minimum subject to numerical uncertainty.

Flow Visualization and Conclusions

Velocity magnitude and total pressure contours were examined to investigate how ground clearance influenced airflow around and underneath the vehicle. The results suggest that aerodynamic performance depends on a balance between restricting underbody airflow at low clearances and the changing flow field at higher clearances. This can be seen in the 20mm, 40mm, and 60mm ground clearance velocity contour plots below.

Ultimately, the study identified 40 mm as the best-performing ground clearance among the simulated configurations, providing a quantitative basis for subsequent vehicle design decisions. The relatively small differences near the minimum also demonstrated the importance of considering numerical uncertainty when optimizing aerodynamic performance.

20mm

The narrow ground clearance restricts underbody airflow, potentially increasing local velocity gradients and skin-friction losses. This restriction may also contribute to unfavorable pressure recovery and increased drag.

40mm

The intermediate clearance provides a favorable balance between underbody flow restriction and wake development. Improved pressure recovery and reduced viscous losses may contribute to the lowest measured CdA.

60mm

The larger clearance allows more airflow beneath the vehicle, potentially increasing downstream mixing and wake-related pressure losses. These effects may outweigh reductions in near-ground shear, resulting in increased drag.

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