How is surface pressure measured in the drag reduction optimization process in automotive engineering?

In automotive engineering drag optimization, measuring surface pressure is key to understanding airflow behavior, identifying sources of drag (such as separation zones and vortices), and validating / improved designs. The main measurement methods are as follows:

1. Pressure sensor arrays (pressure holes / pressure points)

• Principle: Tiny holes (pressure holes) are drilled on the surface of the car model or real vehicle, connected via thin tubes (pressure tubes) to pressure sensors (usually located inside or outside the model). The sensors measure the local static pressure at the hole openings.
• Equipment:

❣️ Pressure holes: very small diameter (usually <1mm ), perpendicular to the surface, with smooth edges to avoid disturbing airflow.

❣️ Pressure tubes: small inner diameter, rigid tubing (such as stainless steel or nylon tubes), connecting pressure holes and sensors, minimizing delay and signal distortion.

❣️ Pressure sensors: typically use miniature electronic pressure sensors (such as piezoresistive or capacitive types). They require high accuracy, good temperature stability, and fast response (especially for dynamic measurements). Sensors are usually integrated into multi-channel scanning valve systems.

❣️ Scanning valves: devices that automatically switch multiple pressure channels to a few high-precision sensors, reducing cost and complexity.

❣️ Data acquisition system: records the voltage signals output by sensors and converts them into pressure values ( Pa or psi ).

• Advantages:

👉 Direct and accurate: directly measures local static pressure with high precision.

👉 High temporal resolution: suitable for measuring dynamic pressure fluctuations (such as A pillars, side mirrors, and turbulent wake regions).

👉 Mature and reliable: well-developed technology with wide applications.

• Disadvantages:

✅ Limited spatial resolution: only provides discrete point data. To obtain detailed distributions, dense placement in key areas (such as front bumper, windshield base, pillars, roof, rear window, spoilers, rear end) is required, which is very time-consuming and labor-intensive. A ✅ Model body modification: requires drilling holes and embedding tubes in the model or vehicle body, which may damage structure or affect appearance.

✅ Complex wiring: large amounts of tubing and sensor wiring are complex and may interfere with airflow or increase model weight / and volume. ✅ Measures only static pressure: usually only static pressure is measured; total pressure and velocity require other measurements (such as pitot tubes) or calculations.

Applications: This is the most basic and commonly used surface pressure measurement method in wind tunnel tests (both scaled models and full-size). It is especially suitable for locations requiring high-precision point measurements or dynamic analysis. Pressure-sensitive paint / Principle: A special paint layer is sprayed on the model surface. This paint contains:

💐 Luminescent molecules (luminophores): emit fluorescence or phosphorescence when illuminated by specific wavelength light (excitation light). 💐 Oxygen-sensitive molecules (quenchers): oxygen in the air "quenches" (reduces) the luminescence intensity.

• Working mechanism: The paint layer is exposed to airflow. The local pressure on the model surface determines the oxygen concentration diffusing into the paint layer. Higher pressure means higher oxygen concentration and lower luminescence intensity; lower pressure means lower oxygen concentration and higher luminescence intensity.

2.  Therefore, the spatial distribution of luminescence intensity directly reflects the surface pressure distribution.

• 💐 PSP coating: different formulas exist for different speed ranges (subsonic, transonic) and accuracy requirements.

                   💐  发光分子（发光团）：  在特定波长光（激发光）照射下会发出荧光或磷光。

                   💐  氧气敏感分子（猝灭剂）：  空气中的氧气会“猝灭”（减弱）发光强度。

• 工作机制：漆层暴露在气流中。模型表面的局部压力决定了扩散到漆层中的氧气浓度。 压力越高，氧气浓度越高，发光强度越低；压力越低，氧气浓度越低，发光强度越高。  因此，发光强度的空间分布直接反映了表面压力的分布。
• Equipment:

                   💐 PSP涂层：针对不同速度范围（亚音速、跨音速）和精度要求有不同配方。

💐 Excitation light source: high intensity LED Lamp or laser providing illumination of specific wavelength to the model.

💐 High sensitivity camera: scientific-grade equipped with specific filters CCD or CMOS Camera used to capture the light emitted by the coating. Usually requires capturing reference images (typically under known uniform pressure, such as before wind tunnel start) and data images during wind tunnel operation.

💐 Data acquisition and processing system: controls the light source and camera, and processes images. By comparing the luminescence intensity ratio of each pixel between the reference image and data image, the calibration curve is used to convert the intensity ratio into pressure values, generating a full-field pressure distribution map.

• Advantages:

✨ Full-field measurement: Provides a continuous pressure distribution map over the entire coated surface with very high spatial resolution (up to pixel level).

✨ Non-contact (model surface): No need to drill holes or arrange numerous sensor lines on the model surface (but spraying is required).

✨ Visual and intuitive: Pressure contour maps are very intuitive, making it easy to identify high-pressure zones, low-pressure zones, separation lines, reattachment lines, and other flow features.

✨ Suitable for complex surfaces: Can easily cover A complex geometric areas such as pillars, rearview mirrors, wheel wells, etc.

• Disadvantages:

🌹 Relatively low accuracy: Absolute pressure accuracy is generally lower than point sensors (especially at low wind speeds), sensitive to temperature changes, requiring precise temperature compensation and calibration.

🌹 Low time resolution: Traditional PSP response is slow (on the order of seconds), not suitable for capturing rapid dynamic pressure changes (although high-speed PSP technology is developing).

🌹 High surface treatment requirements: Spraying requires professional skills and time; the optical properties of the coating (reflectivity, uniformity) and surface finish affect the results.

🌹 Complex data processing: Requires complex image processing, calibration, and temperature correction.

🌹 High cost: System (camera, light source, coating) has a high initial cost.

• Applications: Increasingly used in modern wind tunnels, especially suitable for quickly obtaining large-scale detailed pressure distributions in early design stages and identifying key flow structures. Often used complementarily with point sensors.

SPI is a pressure measurement product brand from Italy, providing significant support for surface pressure measurement during automotive engineering drag optimization processes, SPI Other technical collaborations of the product series:

Summary:

Pressure sensor arrays provide high-precision, high temporal resolution discrete point pressure data and are the gold standard for dynamic analysis and precise measurement of key points. Pressure-sensitive paint offers high spatial resolution full-field pressure distribution maps and is a powerful tool for visualizing flow structures and quickly assessing pressure over large areas. In the practice of automotive drag reduction optimization, these two technologies are often used complementarily. Point sensors are used to verify pressure at key locations and perform dynamic analysis, while PSP the other is used to obtain a global view and identify areas that require further detailed study with point sensors. By accurately measuring and understanding surface pressure distribution, engineers can more effectively optimize vehicle body shapes and significantly reduce aerodynamic drag.