# SAE 20 Degree Notchback Automotive Reference Model

^{}Martin A Passmore

^{✉ }Anna-Kristina Perry

^{}

**Affiliation :** Aeronautical and Automotive Engineering, Loughborough University, UK

✉ [email protected]; [email protected]

## Introduction

The aerodynamic drag of a road vehicle has a major impact on $CO_{2}$ emissions for combustion engine vehicles and range for battery electric vehicles. A significant component of drag is that associated with the large region of separated flow at the rear of the vehicle. Real world cars have complex geometric detail and it is a common practice to study simplified bodies that focus on generic flow features. Cogotti at Pininfarina studied a generic body ^{[1]} that was later adapted by the Society of Automotive Engineers (SAE) for studies of model interference in wind tunnels^{[2]}. It has a variety of rear end geometries, and the 'notchback' variant used here is representative of a saloon car with a boot (or trunk). Experimentally these reference models allow a better understanding of the fundamental flow features and how these change with geometric modifications and this data is then especially useful for the validation of computational simulation models.

The wind tunnel measurements use a 1/5 scale model and the notchback angle is at 20 degrees (measured from the horizontal). This dataset aims to capture the flow details in the notch and shows a typical notch back flow structure with asymmetry in the trailing vortex structure and in the impingement on the boot-deck. Weak separation on the slant with flow displaced above the boot-deck is highlighted and the correlation between the Particle Image Velocimetry (PIV) results and the surface pressures is demonstrated.

Measurements includes force and moment data, surface pressures for the centerline, slant, boot-deck and base and detailed PIV data for the impingement region, model centerline, A-pillar and multiple planes on the slant and boot-deck. Time averaged, statistical and instantaneous data are available. Further information on this dataset is presented in Wood, D. et al (2014)^{[3]}. The full dataset is available from the Loughborough University repository.

### Model Geometry

The geometry used in the wind tunnel was a 1/5 scale model with a length of 840 mm, a width of 320 mm and a height (excluding pins) of 240 mm. The schematic of the model and the reference coordinate system is shown in figure 1. The reference frontal area is defined as 0.076 m^{2} and the car length L and height H are used to non-dimensionalise distances. Note that in Cogotti (1988) ^{[1]} the full-scale variant has a frontal area of 1.896 m^{2}, so that a 1/5 scale should be 0.07584 m^{2}, however the paper rounds this to 0.076 m^{2} and is the reference value that should be used. The front of the model has a 30° slant and at the rear there is a 20° backlight leading to the notchback. The underside is flat until reaching a small 6° diffuser starting at the nominal location of the rear axle. The model is mounted in the wind tunnel with four pins at a ground clearance of 40 mm and zero pitch.

The CAD geometry of the model has its origin at the centre of the vehicle (shown in figure 1) projected down to the tunnel floor, with the X-direction forward (nose +420 mm, rear -420 mm), Y-direction to the right (left side -160 mm, right side +160 mm) and Z-direction downwards (bottom -40 mm, top -280 mm). The pins have a ‘wheelbase’ of 490 mm and are placed at X=-235 mm and X=+255 mm – hence the origin for pitching moment is at X=-10 mm.

### Measurement Locations and Techniques

2D PIV and stereoscopic PIV were employed to characterise the flow field. Measurements were conducted at various planes, which is illustrated in figure 2. In addition, force balance and pressure measurments were conducted.

### Experimental Facility

The experiments were carried out in the Loughborough University large wind tunnel . This is an open circuit wind tunnel, with a test section of 1.92 m X 1.32 m X 3.6 m (W X H X L ).

### Flow Conditions

Inlet velocity = 40 m/s.

Free-stream turbulence ≈ 0.2 %.

Flow uniformity = ±0.4%.

All the above reference conditions are measured at 2000 mm upstream of the origin, on the centerline near the roof.

### Wind Tunnel Corrections

No corrections have been made to the data.

### Generated Results

$V_{x}$, $V_{y}$, $V_{z}$ – Mean Velocity components in the x, y and z directions respectively.

$C_{p}$ – Mean and RMS of Coefficient of pressure.

$C_{D}$, $C_{L}$, $C_{S}$ – Coefficient of drag, lift and side force respectively.

$C_{MY}$, $C_{MP}$, $C_{MR}$ – Coefficient of moments, yawing, pitching and rotation respectively.

### CAD models

Reference model is given in three different formats, and the measurement planes are given as part file.

SAE reference model.prt | SAE reference model.igs | SAE reference model.stl |
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Measurement planes.prt |
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### Available Datasets

All the mean flow fields are averaged over 1000 images.

All the data are formatted as Tecplot compatible.

This is a simple ASCII file format with data in columns and can be easily imported into Excel via the csv route.

##### (i) Rear Notch – Mean flow field from PIV

##### (ii) Front Stagnation – Mean flow field from PIV

##### (iii) Centreline – Mean flow field from PIV

The full vehicle centreline plane (represented in purple) has been created through two cameras located side by side, with the two images stitched together to form a larger field of view.

##### (iV) Rear Notch – Mean flow field from PIV

**Cam 1**

Y = 0.dat | Y = 15.dat | Y = 30.dat | Y = 45.dat | Y = 60.dat | Y = 75.dat | Y = 90.dat |
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y = -15.dat | Y = -30.dat | Y = -45.dat | Y = -60.dat | Y = -75.dat | Y = -90.dat |

**Cam 2**

Y = 0.dat | Y = 15.dat | Y = 30.dat | Y = 45.dat | Y = 60.dat | Y = 75.dat | Y = 90.dat |
---|---|---|---|---|---|---|

Y = -15.dat | Y = -30.dat | Y = -45.dat | Y = -60.dat | Y = -75.dat | Y = -90.dat |

##### (V) A pillar – Mean flow field from PIV

**0 degree Yaw**

X = 33.dat | X = 99.dat | X = 165.dat |
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**10 degree Yaw**

X = 33.dat | X = 99.dat | X = 165.dat |
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**20 degree Yaw**

X = 33.dat | X = 99.dat | X = 165.dat |
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**30 degree Yaw**

X = 33.dat | X = 99.dat | X = 165.dat |
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##### (Vi) Force measurement

The force and moment data is reported for ±30^{o} yaw sweep

Force data.dat |
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##### (Vii) Pressure measurement

At each pressure tapping 8192 pressure samples were collected at 260Hz, representing 31 seconds of data. The location of pressure taps are shown in the below image.

RMS of Coefficient of pressure

Backlight Cp.dat | Base Cp.dat | Bootdeck Cp.dat |
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Average Coefficient of pressure

Backlight Cp.dat | Base Cp.dat | Bootdeck Cp.dat |
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Centreline Coefficient of pressure

Centreline Cp.dat |
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## Open Access

This metadata is provided under the Creative Commons Attribution-NonCommercial 4.0 International License https://creativecommons.org/licenses/by-nc/4.0/). This license allows for unrestricted use, distribution, and reproduction in any medium, provided that proper credit is given to the original author(s) and the source. Also provide a link to the license, and indicate if any changes were made. Furthermore, this license does not allow the use of this material for commercial purposes.