In recent years, the global energy crisis and environmental pollution have intensified, making the research and application of new energy vehicles, particularly Battery Electric Vehicles (BEVs), a core direction for the automotive industry’s transformation. However, compared to traditional internal combustion engine vehicles, electric cars face a significant challenge in terms of driving range. Industry statistics show that at high speeds, aerodynamic drag accounts for 30% to 40% of a vehicle’s total energy consumption, making it a key factor limiting the endurance of an electric car. Therefore, optimizing vehicle aerodynamic design to reduce drag is a crucial technical pathway for improving the energy efficiency of pure electric cars.
Among aerodynamic components, the rear spoiler is a key structure for managing the tail flow field, directly influencing the vehicle’s drag coefficient ($C_D$) and lift coefficient ($C_L$). While traditional spoilers are often designed based on styling aesthetics, there is a lack of systematic research into their aerodynamic characteristics for modern electric vehicles. Existing studies indicate that rear spoilers can control rear-end flow separation, suppress vortex generation, and effectively reduce pressure drag while enhancing driving stability. However, much of the existing research focuses on traditional fuel-powered vehicles. The aerodynamic study of rear spoilers for pure electric cars, especially SUV-type electric cars, remains relatively limited. The layout of the powertrain and mass distribution in an electric car differs significantly from traditional vehicles, leading to distinct flow field characteristics. Therefore, targeted optimization design for specific electric car models is necessary.
This study focuses on an SUV-type electric car and employs Computational Fluid Dynamics (CFD) simulation methods to investigate the impact of rear spoiler slotting on the vehicle’s aerodynamic performance. The core question addressed is whether introducing slots into a rear spoiler can further enhance the aerodynamic efficiency of an electric car, or if it leads to detrimental effects.
Numerical Simulation Methodology
To investigate this, a full-scale 1:1 three-dimensional model of a representative SUV electric car was developed. The main body dimensions are 4860 mm in length, 1950 mm in width, and 1600 mm in height. For computational efficiency, small-scale features such as side mirrors, door handles, and intricate grille details were simplified. A computational domain was established to accurately capture the flow structures around the electric car. The domain size was set to 4 times the vehicle height, 5 times the vehicle width, and 6 times the vehicle length. The inlet boundary was positioned 1 vehicle length upstream of the front bumper, and the outlet boundary was placed 3 vehicle lengths downstream of the rear bumper, ensuring minimal boundary interference.
The mesh generation process utilized a hybrid approach. A trimmed (Trim) cell mesh was used for the core volume, combined with prism layers at the vehicle surfaces to resolve the viscous boundary layer. Five prism layers with a total thickness of 8 mm were applied on all vehicle surfaces. To improve simulation accuracy in critical regions, local refinement boxes with cell sizes of 16 mm and 32 mm were implemented around the vehicle, particularly near the rear spoiler and wake region. The final volumetric mesh consisted of approximately 9.1 million cells. The standard k-ω turbulence model was selected for its good performance in capturing near-wall flow and separated flow characteristics expected in the spoiler wake and underbody regions of the electric car.
The boundary conditions were set to simulate highway cruising speed. The inlet velocity was set to 33.33 m/s (120 km/h). The outlet was defined as a pressure outlet with a gauge pressure of 0 Pa (atmospheric pressure). All vehicle surfaces were treated as no-slip walls. The simulations were performed for four distinct rear-end configurations to isolate the effect of the spoiler and its slots:
- Type I: Baseline electric car model without a rear spoiler.
- Type II: Electric car with a standard, solid (non-slotted) rear spoiler attached.
- Type III: Electric car with a rear spoiler featuring a single central square slot of 280 mm width.
- Type IV: Electric car with a rear spoiler featuring three evenly distributed square slots, each 140 mm wide.
The aerodynamic forces were monitored, and the dimensionless coefficients were calculated using the standard formulas:
$$C_D = \frac{F_D}{\frac{1}{2} \rho V^2 A}$$
$$C_L = \frac{F_L}{\frac{1}{2} \rho V^2 A}$$
where $F_D$ is the drag force, $F_L$ is the lift force, $\rho$ is the air density (1.225 kg/m³), $V$ is the freestream velocity (33.33 m/s), and $A$ is the frontal reference area of the electric car.
Results and Discussion: Aerodynamic Performance
The simulation results for the drag coefficient across the four configurations are summarized in the table below. The values clearly demonstrate the impact of the spoiler and the subsequent effect of slotting on this SUV electric car.
| Spoiler Configuration Type | Description | Drag Coefficient, $C_D$ | Change vs. Type II |
|---|---|---|---|
| I | No Spoiler | 0.306 | +11.7% |
| II | Solid Spoiler | 0.274 | 0% (Baseline) |
| III | Single Central Slot (280mm) | 0.288 | +5.1% |
| IV | Three Slots (3x140mm) | 0.313 | +14.2% |
The results indicate that adding a solid rear spoiler (Type II) to this SUV electric car significantly reduces the drag coefficient by approximately 10.5% compared to the spoiler-less baseline (Type I). This reduction is attributed to the spoiler’s ability to manage the flow separation off the rear windshield and roof, delaying the detachment of the wake and reducing its intensity. However, introducing square slots into the spoiler does not yield further drag reduction. On the contrary, it increases drag. The single-slot configuration (Type III) increases $C_D$ by 5.1% relative to the solid spoiler, while the three-slot configuration (Type IV) causes a more substantial increase of 14.2%. This trend suggests that increasing the number of slots exacerbates the drag penalty for this electric car.
The effect on the lift coefficient is even more pronounced and critical for the high-center-of-gravity stability of an SUV electric car. Excessive lift reduces the effective contact force between the tires and the road, potentially leading to rear-end float, understeer, and increased sensitivity to crosswinds at high speeds, severely compromising safety and handling for the electric car.
| Spoiler Configuration Type | Lift Coefficient, $C_L$ | Change vs. Type II |
|---|---|---|
| I (No Spoiler) | 0.237 | +415.2% |
| II (Solid Spoiler) | 0.046 | 0% (Baseline) |
| III (Single Slot) | 0.216 | +369.6% |
| IV (Three Slots) | 0.127 | +176.1% |
As shown in Table 2, the solid spoiler (Type II) dramatically reduces the lift coefficient by over 80% compared to the no-spoiler case, generating beneficial downforce and enhancing high-speed stability for the electric car. In stark contrast, both slotted configurations severely degrade this performance. The single-slot spoiler increases lift by 369.6%, nearly returning it to the high levels of the spoiler-less case. The three-slot design performs slightly better but still increases lift by 176.1% compared to the solid spoiler, offering significantly less downforce for the electric car.
Flow Field Analysis and Physical Mechanisms
The degradation in performance caused by square slots can be explained by analyzing the simulated flow fields. For the baseline electric car without a spoiler (Type I), airflow accelerating over the roof separates at the sharp curvature change at the rear windshield’s trailing edge. This separation creates a large, recirculating vortex in the wake, leading to a low-pressure region behind the vehicle that is a primary source of pressure drag.
The solid rear spoiler (Type II) effectively guides this airflow, creating a favorable pressure gradient that delays separation. It pushes the core of the recirculating vortex further downstream, away from the vehicle’s tailgate, reducing the intensity of the low-pressure region and thereby lowering both drag and lift for the electric car.
The introduction of square slots disrupts this managed flow. The sharp geometric edges of the slots act as strong flow separation triggers. As air passes over the spoiler, it encounters these sharp edges, leading to immediate local separation and the formation of unsteady, small-scale vortices shed from each slot edge. This phenomenon increases turbulent kinetic energy (TKE) dissipation in the wake. The turbulent kinetic energy, governed by the equation from the k-ω model, is enhanced in these regions:
$$\frac{\partial k}{\partial t} + U_j \frac{\partial k}{\partial x_j} = P_k – \beta^* k \omega + \frac{\partial}{\partial x_j} \left[ (\nu + \sigma_k \nu_t) \frac{\partial k}{\partial x_j} \right]$$
where $k$ is turbulent kinetic energy, $\omega$ is specific dissipation rate, $P_k$ is production term, and $\nu_t$ is turbulent viscosity. The slots increase $P_k$ locally due to the introduced shear and separation.
For the single large slot (Type III), the central opening locally weakens the spoiler’s flow-deflecting capability, creating a concentrated path for air to rush through. This jet-like flow interacts chaotically with the main wake, destabilizing it and increasing drag. For the three-slot configuration (Type IV), while the slots are smaller, they create multiple separation points across the spoiler’s span. The interference between the vortical structures shed from each slot creates a complex, highly turbulent wake closer to the vehicle body. The velocity vectors show a strong, coherent vortex forming just behind the slotted spoiler area, which sustains a low-pressure zone through vortex-induced suction, explaining the higher drag coefficient. Furthermore, these separated flows over the top of the spoiler reduce its effectiveness in creating a downward force, leading to the significant increase in lift coefficient observed for the electric car.
Conclusion
This investigation into rear spoiler configurations for an SUV electric car leads to two primary conclusions. First, the addition of a solid rear spoiler provides substantial aerodynamic benefits for the electric car, reducing the drag coefficient by approximately 10.5% and drastically reducing the lift coefficient by over 80%. This enhances energy efficiency and high-speed stability, which are critical parameters for electric car range and safety.
Second, and contrary to what might be intuitive for some aerodynamic components, incorporating square slots into the rear spoiler of this SUV electric car does not improve its aerodynamic performance. Both tested slotted configurations resulted in increased drag and significantly increased lift compared to the solid spoiler baseline. The single 280 mm central slot increased $C_D$ by 5.1% and $C_L$ by 369.6%. The three 140 mm slots performed worse in drag, increasing $C_D$ by 14.2%, while the lift increase was slightly lower at 176.1%. The fundamental physical mechanism is the flow separation induced by the sharp edges of the square slots. This separation generates unsteady vortices and increases turbulent kinetic energy dissipation in the wake, disrupting the pressure management achieved by the solid spoiler and ultimately degrading the overall aerodynamic performance of the electric car. Future work on spoiler optimization for electric cars should therefore explore alternative slot shapes (e.g., aerodynamically contoured vents) or other flow control devices that can manage boundary layers without introducing detrimental separation.