Abstract:
In response to rising fuel prices, limited availability, and environmental concerns, the automotive
industry has focused on reducing aerodynamic drag to reduce vehicle consumption of fuel and
carbon dioxide emissions. Buses are a common means of transportation, and understanding their
aerodynamics contributes to their increased efficiency and lower drag. This study aimed to lessen
the aerodynamic drag of the Bishoftu bus through varying windshield angles and utilizing base
cavity configurations. Computational fluid dynamics (CFD) analysis in ANSYS 19.2 was
conducted to evaluate the aerodynamic performance of vehicle models at various vehicle speeds.
The SOLIDWORKS 21 was used for the creation of a 3D CAD model of baseline model and 29
modified models. To verify the reference bus model's numerical results, wind tunnel tests were
also conducted. Through investigated speeds for the baseline model, an average error percentage
of 6.09% was observed between the Cd CFD findings and the wind tunnel test outcomes. This
suggests that the numerical and experimental data have a very precise connection. The average
drag coefficient reduction of 7.06% was achieved by base cavity application solely on the
baseline model with a 12º base cavity angle and 0.3 times the height of the vehicle (0.3H)
tapered base cavity depth. Additionally, the study examined the impact of windshield angle
variation on the aerodynamic characteristics of a baseline model, finding that variations resulted
in reduced stagnation pressure area and maximum average drag coefficient reduction of 17.12%.
Also, lower coefficients of lift and lift forces, indicating an increase in downward force were
exhibited through these modifications. Furthermore, the combined effect of windshield angle
variations and base cavity configurations was studied. The minimum average drag
coefficient(Cd) and drag force(Fd) values of 0.4779 and 1420.97 N were achieved respectively
by model 26 which has a 17º windshield angle,12º base cavity angle, and 0.3H tapered base
cavity length. This modified model reduced the drag coefficient by 25.22% and drag force by
26.6%, resulting in a maximum fuel consumption reduction of 3.66 lit/hr, and the highest
average CO2 saved annually was 48.35 tonne/year. Finally, Multi-objective optimization was
done through response surface methodology by giving precedence to the study's primary goal.
The optimum dimensions were identified as an 11.54º base cavity angle, 17º windshield angle,
and 0.3 times the height of the vehicle (0.3H) tapered base cavity length.
