In the pursuit of enhanced energy efficiency and extended driving range for electric vehicles, reducing aerodynamic drag has become a critical focus. For electric SUV models, achieving a low drag coefficient (Cd) is particularly challenging due to their larger frontal area and complex flow structures around the wheels. This study investigates the flow field characteristics around the wheels of a low-drag electric SUV, employing computational fluid dynamics (CFD) simulations and wind tunnel testing to evaluate the impact of various wheel rim designs and front wheel deflectors. Our analysis aims to provide insights into how strategic design elements can minimize aerodynamic resistance without relying on fully enclosed aerodynamic rims, which are often costly and less versatile for mass-market electric SUV applications.
The importance of aerodynamic optimization for electric SUV cannot be overstated, as it directly influences energy consumption and overall performance. Under the “dual carbon” goals, manufacturers are pushing the boundaries of automotive aerodynamics, with many modern electric SUV models achieving Cd values below 0.26. A significant portion of aerodynamic drag originates from the wheel arches and rotating wheels, where turbulent flows can lead to substantial energy losses. By examining the flow field around the wheels, we can identify key factors that contribute to drag reduction and apply them to the development of future electric SUV designs.
In this work, we utilize Reynolds-Averaged Navier-Stokes (RANS) simulations to model the external flow field of an electric SUV, complemented by experimental validation in a full-scale wind tunnel. We compare three conventional rims with varying levels of open area against an ideal fully enclosed aerodynamic rim, all integrated with a front bumper and wheel deflector system. The results demonstrate that with proper flow guidance, conventional rims can achieve drag coefficients comparable to those of idealized designs, highlighting the potential for cost-effective aerodynamic improvements in electric SUV production.
The governing equations for fluid flow are based on the principles of mass and momentum conservation. For an incompressible, steady-state flow, the continuity equation and momentum equations are expressed as follows. The mass conservation equation is given by:
$$ \text{div} \, \mathbf{u} = 0 $$
where $\mathbf{u}$ represents the velocity vector of the fluid. The momentum conservation equation for each direction $i$ (where $i = 1, 2, 3$ corresponding to $x$, $y$, and $z$ directions) is:
$$ \frac{\partial (\rho u_i)}{\partial t} + \text{div} (\rho \mathbf{u} u_i) = \text{div} (\mu \, \text{grad} \, u_i) – \frac{\partial p}{\partial x_i} + S_i $$
In these equations, $\rho$ denotes the fluid density in kg/m³, $t$ is time in seconds, $p$ is the pressure in Pa, $\mu$ is the dynamic viscosity in kg/(m·s), and $S_i$ is a source term accounting for additional forces. For turbulent flow modeling, we employ the standard $k$-$\varepsilon$ model within the RANS framework, which is widely used for automotive applications due to its balance between accuracy and computational efficiency. This approach allows us to capture the mean flow characteristics around the electric SUV, including the complex interactions between the rotating wheels and the surrounding air.
Our simulation setup involves a detailed 3D model of the electric SUV, with overall dimensions of 4690 mm in length, 1936 mm in width, and 650 mm in height, and a wheelbase of 2845 mm. The computational domain is a rectangular volume measuring 66 m × 12 m × 10 m, sufficiently large to minimize boundary effects. We generate approximately 6 million surface mesh elements using triangular cells, with a minimum cell size of 4 mm. The volume mesh consists of 30 million trimmed cells, and a boundary layer mesh with 5 layers is applied near the vehicle surface, with a total thickness of 8 mm and a growth rate of 1.2 to resolve near-wall flows accurately.
Boundary conditions are defined to replicate real-world driving scenarios. The inlet is set as a velocity boundary condition with a speed of 120 km/h, while the outlet is a pressure outlet. The ground plane is modeled as a moving wall with the same velocity as the inlet to simulate the relative motion between the vehicle and the road. The side walls of the domain are treated as slip walls, and the wheels are assigned rotating wall conditions with an angular velocity of 92.7 rad/s to account for their spin. Steady-state simulations are run for 5000 iterations, and the average drag coefficient over the final 1000 steps is used for analysis, ensuring convergence and reliability of the results.
Wind tunnel tests are conducted at the China Automotive Engineering Research Institute Wind Tunnel Center, following the CSAE standard T/CSAE 146—2020 for automotive aerodynamic testing. The tests are performed at a speed of 120 km/h, matching the simulation conditions, to validate the CFD predictions. The test vehicle is equipped with an active grille shutter system maintained in the closed position to minimize drag, and various rim configurations are evaluated to assess their aerodynamic performance.
The wheel rims tested include three conventional designs—R19a, R20a, and R20b—with open areas, and one fully enclosed ideal aerodynamic rim, R19b. The specifications are 235/50 R19 for R19a and R19b, and 245/45 R20 for R20a and R20b. Additionally, a front wheel deflector is installed on the vehicle, with key dimensions including a height of 41 mm, length of 188 mm, front width of 133 mm, and rear width of 259 mm. This deflector is designed to guide airflow away from the wheels, reducing turbulence and drag.
The simulation results for the drag coefficients of the different rim configurations are as follows: R19a at 0.250, R20a at 0.252, R20b at 0.258, and R19b at 0.245. Wind tunnel measurements, however, show remarkably consistent values across all rims, with Cd values of approximately 0.249 for each configuration. This indicates that the conventional rims perform nearly identically to the ideal aerodynamic rim in terms of drag reduction, with differences of less than 0.3 counts. The close agreement between simulation and experiment, with a maximum deviation of 3.5%, validates the use of CFD as an effective tool in the aerodynamic development of electric SUV models.
To further analyze the performance, we examine the surface pressure distributions and total pressure iso-surfaces around the wheels. The front bumper and wheel deflector effectively shield the wheels from direct airflow, as evidenced by the minimal positive pressure on the y-oriented surfaces of the rims. This design prevents significant flow impingement on the rotating wheels, thereby reducing turbulent energy losses. The following table summarizes the drag coefficients from both simulation and wind tunnel tests for each rim type:
| Rim Type | Simulation Cd | Wind Tunnel Cd | Difference (counts) |
|---|---|---|---|
| R19a | 0.250 | 0.2491 | 0.9 |
| R20a | 0.252 | 0.2490 | 3.0 |
| R20b | 0.258 | 0.2494 | 8.6 |
| R19b | 0.245 | 0.2493 | -4.3 |
Note: 1 count = 0.001 in Cd value. The small differences highlight the effectiveness of the aerodynamic elements in homogenizing the flow field.
We also investigate the impact of the front wheel deflector by simulating the vehicle without this component. The removal of the deflector results in an average increase in drag coefficient of about 8 counts across all rim types, with values rising to 0.260 for R19a, 0.261 for R20a, 0.264 for R20b, and 0.253 for R19b. This underscores the critical role of the deflector in managing wheelhouse flows. The pressure distributions show increased positive pressure on the wheel surfaces and reduced negative pressure around the wheel arches, indicating greater flow intrusion into the wheel region. The total pressure iso-surfaces (where total pressure equals zero) become more pronounced and distorted without the deflector, reflecting higher energy dissipation and turbulent interactions.
The aerodynamic benefits of the front bumper and deflector can be quantified using the drag reduction ratio, $\Delta C_d$, defined as the difference in drag coefficient with and without the deflector. For a typical electric SUV configuration, this ratio can be expressed as:
$$ \Delta C_d = C_{d,\text{without}} – C_{d,\text{with}} $$
where $C_{d,\text{with}}$ is the drag coefficient with the deflector installed, and $C_{d,\text{without}}$ is without it. In our case, the average $\Delta C_d$ is approximately 0.008, demonstrating a significant improvement. This reduction is achieved by streamlining the flow around the wheels, which minimizes the vorticity and separation zones. The vorticity magnitude, $\omega$, in the wheel wake region can be described by:
$$ \omega = \frac{1}{2} \left| \nabla \times \mathbf{u} \right| $$
Lower vorticity values observed with the deflector indicate a more attached flow, contributing to the low drag coefficients. Additionally, the pressure coefficient $C_p$ on the wheel surfaces is calculated as:
$$ C_p = \frac{p – p_{\infty}}{\frac{1}{2} \rho U_{\infty}^2} $$
where $p$ is the local pressure, $p_{\infty}$ is the freestream pressure, and $U_{\infty}$ is the freestream velocity. The deflector helps maintain a more uniform $C_p$ distribution, reducing local pressure peaks that drive drag.
Further analysis of the flow field involves examining the velocity profiles and turbulence kinetic energy (TKE) around the wheels. The TKE, denoted as $k$, is derived from the $k$-$\varepsilon$ model and represents the intensity of turbulent fluctuations. In regions near the wheels, high TKE values are associated with increased drag, but the deflector mitigates this by promoting smoother flow transitions. The equation for TKE in the RANS context is:
$$ \frac{\partial (\rho k)}{\partial t} + \text{div} (\rho \mathbf{u} k) = \text{div} \left( \frac{\mu_t}{\sigma_k} \text{grad} \, k \right) + P_k – \rho \varepsilon $$
where $\mu_t$ is the turbulent viscosity, $\sigma_k$ is a model constant, $P_k$ is the production term, and $\varepsilon$ is the dissipation rate. Our simulations show that with the deflector, TKE levels in the wheel wake are reduced by up to 15%, correlating with the lower drag measurements.
The consistency in wind tunnel results across all rim types suggests that the front bumper and deflector effectively decouple the rim design from the overall aerodynamic performance. This is a key insight for electric SUV development, as it allows for greater flexibility in wheel styling without compromising on drag. The following table compares the pressure-related parameters with and without the deflector for the R19a rim configuration:
| Parameter | With Deflector | Without Deflector | Change |
|---|---|---|---|
| Average $C_p$ on Wheel Surface | -0.15 | 0.05 | +0.20 |
| Max TKE (m²/s²) | 8.2 | 12.5 | +4.3 |
| Drag Coefficient $C_d$ | 0.249 | 0.260 | +0.011 |
These data highlight how the deflector alters the flow dynamics, leading to improved aerodynamic efficiency. The reduction in TKE and more negative $C_p$ values indicate better flow attachment and lower energy losses.
In conclusion, our study demonstrates that through careful design of the front bumper and wheel deflector, a low-drag electric SUV can achieve Cd values as low as 0.249, even with conventional open rims. The aerodynamic elements effectively guide airflow around the wheels, minimizing the impact of rim geometry on overall drag. This approach provides a practical pathway for enhancing the energy efficiency of electric SUV models without resorting to expensive fully enclosed rims or complex suspension systems. Future work could explore optimizing these components further or extending the analysis to other vehicle regions, such as the underbody or rear spoiler, to achieve additional gains in aerodynamic performance for the next generation of electric SUV vehicles.
The methodologies and findings presented here offer valuable guidance for automotive engineers focused on reducing aerodynamic drag in electric SUV designs. By leveraging CFD simulations and wind tunnel testing, manufacturers can iterate quickly and cost-effectively, ultimately contributing to the broader goals of energy conservation and emission reduction in the transportation sector. As the electric SUV market continues to grow, such aerodynamic innovations will play a pivotal role in maximizing driving range and consumer appeal.