In the development of modern electric SUVs, noise, vibration, and harshness (NVH) performance is a critical factor influencing passenger comfort. Among various noise sources, wind noise becomes predominant at high speeds, particularly when the sunroof is open. This phenomenon, known as sunroof buffeting, generates low-frequency, high-intensity noise that can cause discomfort, such as ear pressure, even if it is not always audible. As electric SUVs lack the masking effect of traditional engine noise, wind-induced noises like buffeting are more pronounced. This article explores the mechanisms, simulation, and optimization strategies for mitigating sunroof buffeting in electric SUVs, drawing from computational fluid dynamics (CFD) simulations and wind tunnel testing. We present a comprehensive approach that includes structural enhancements and operational adjustments to improve the driving experience.
Sunroof buffeting occurs due to the interaction between external and internal airflow when the sunroof is open. At high vehicle speeds, a shear layer forms at the leading edge of the sunroof opening, where high-velocity external air meets relatively stagnant interior air. When the velocity difference exceeds a critical threshold, this shear layer becomes unstable, leading to vortex shedding. These vortices travel backward and impinge on the trailing edge of the opening, generating pressure waves that propagate. A portion of these waves feedback to the leading edge, reinforcing the vortex formation in a closed-loop cycle. This process excites the shear layer at a specific frequency, which can couple with the acoustic modes of the vehicle cabin, resulting in resonance. The buffeting frequency is typically below 20 Hz, but the sound pressure level can exceed 90 dB, causing significant discomfort. For electric SUVs, where quiet operation is expected, addressing this issue is essential during the design phase.
The fundamental frequency of sunroof buffeting can be estimated using the formula: $$f = \frac{v}{L}$$ where \( f \) is the frequency in Hz, \( v \) is the velocity of the airflow in m/s, and \( L \) is the characteristic length, such as the sunroof opening length. In practice, the actual frequency may vary due to factors like vehicle geometry and cabin acoustics. The pressure fluctuations associated with buffeting are governed by the Bernoulli equation: $$P + \frac{1}{2} \rho v^2 = \text{constant}$$ where \( P \) is the pressure, \( \rho \) is the air density, and \( v \) is the flow velocity. Disruptions in this balance lead to the oscillatory behavior characteristic of buffeting.
To analyze and mitigate sunroof buffeting in electric SUVs, we employed CFD simulations using STAR-CCM+ software. The simulation model was based on the exact CAD geometry of the electric SUV, with simplifications applied to non-critical exterior features to reduce computational cost. The mesh was refined around key areas, including the sunroof opening, roof, and interior cabin, to capture flow details accurately. The minimum grid size was set to 2 mm, resulting in a total of 116 million cells. This high-resolution mesh allowed for precise modeling of vortex dynamics and pressure distributions.
The simulation examined two scenarios: full sunroof opening and a proposed comfort position with reduced opening. In the full-open condition, the airflow over the sunroof deflector created large-scale vortices that penetrated the cabin, leading to strong buffeting. In contrast, the comfort position minimized vortex formation by altering the flow path, reducing the impact on interior noise. The following table summarizes the key simulation parameters and outcomes:
| Parameter | Full Open Condition | Comfort Position |
|---|---|---|
| Sunroof Opening Size | 60 cm | 53 cm (88% of full open) |
| Mesh Cells | 116 million | |
| Vortex Intensity | High | Low |
| Pressure Fluctuation | Significant | Reduced |
The simulation results indicated that the comfort position could effectively suppress buffeting by preventing large vortices from entering the cabin. This insight guided subsequent design modifications and experimental validation for the electric SUV.
Controlling sunroof buffeting in electric SUVs involves both hardware and software interventions. One primary method is the optimization of the sunroof deflector, which disrupts the airflow to prevent vortex formation. Initially, the electric SUV featured a mesh-type deflector, but its small size and exposed support arms generated additional noise. To address this, we redesigned the deflector to include a fully wrapped R-angle around the mesh, integrating it with the front beam structure. This change minimized flow separation and reduced unsteady pressure fluctuations. The deflector was also transitioned from a composite assembly to a single-piece plastic design for better aerodynamic performance.
Another critical aspect is defining a comfort position for the sunroof opening. Based on CFD simulations, we identified an initial comfort position at 88% of the full opening size. However, to balance customer desires for a large opening with NVH requirements, we incorporated this position into the vehicle’s control system. The electric SUV allows users to activate the comfort position via touchscreen or voice commands, ensuring that the sunroof does not open fully in conditions prone to buffeting. The control logic was embedded in the central infotainment system, enhancing usability without compromising comfort.
The effectiveness of these measures was evaluated through wind tunnel testing. Subjective assessments by NVH engineers rated the buffeting intensity on a scale of 1 to 9, with lower scores indicating greater discomfort. In the electric SUV, buffeting became noticeable at speeds of 60–70 km/h, peaking at 100 km/h. Objective measurements were conducted using a binaural head microphone to record interior noise during speed sweeps. The data confirmed that the dominant buffeting frequency was around 17 Hz, consistent with theoretical predictions.
We compared the noise levels between full open and comfort positions at 65 km/h, a common urban speed where buffeting is perceptible. The sound pressure level at 17 Hz dropped from 52 dB in the full open condition to 30 dB in the comfort position, demonstrating a significant reduction. Further tests involved gradually opening and closing the sunroof to identify the critical opening size that triggers buffeting. The results showed that an opening of 54.5 cm (91% of full open) maintained noise levels similar to the CFD-proposed comfort position, with no noticeable buffeting. The table below summarizes the experimental findings:
| Condition | Sunroof Opening | Buffeting Frequency (Hz) | Sound Pressure Level (dB) |
|---|---|---|---|
| Full Open | 60 cm | 17 | 52 |
| CFD Comfort Position | 53 cm (88%) | 17 | 30 |
| Experimental Comfort Position | 54.5 cm (91%) | 17 | 30 |
The experimental data validated the CFD predictions and led to the adoption of the 91% opening as the comfort position for the electric SUV. This adjustment ensures that passengers enjoy ample ventilation and visibility while minimizing buffeting-related discomfort.
In addition to time-domain analysis, we used waterfall plots to visualize the noise energy distribution over frequency and time. These plots confirmed that the comfort position effectively suppressed the 17 Hz peak across various speeds. The optimization process also considered the impact of the deflector redesign; post-modification tests showed a reduction in high-frequency noise components caused by the earlier exposed arms.
The success of this approach highlights the importance of integrating simulation and testing in the development of electric SUVs. The CFD model provided a cost-effective way to explore design alternatives, while wind tunnel testing offered real-world validation. The combined use of hardware improvements and software controls exemplifies a holistic strategy for NVH optimization in electric vehicles.
In conclusion, addressing sunroof buffeting in electric SUVs requires a multifaceted approach that leverages advanced simulations, structural enhancements, and user-centric features. By optimizing the sunroof deflector and implementing a comfort position, we achieved a significant reduction in buffeting noise without sacrificing the open-air experience that customers desire. This study underscores the value of early NVH considerations in electric SUV design, contributing to quieter and more comfortable vehicles. Future work could explore adaptive control systems that dynamically adjust the sunroof opening based on real-time sensor data, further refining passenger comfort in varying driving conditions.
The methodologies described here are applicable to other vehicle types, but the focus on electric SUVs is particularly relevant due to their growing market share and unique acoustic challenges. As the automotive industry shifts toward electrification, continued innovation in NVH engineering will be essential to meet consumer expectations for refinement and tranquility.