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Experimental study on the three-dimensional wake structure of the DrivAer standard model

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Abstract

An automatic particle imaging velocimetry (Auto-PIV) measuring technique was developed based on the external triggering and automatic control technology. Measurements of the DrivAer model wake were performed using Auto-PIV in the spanwise and vertical direction. Three-dimensional time-averaged velocity distribution was reconstructed and the spatial distribution of the large coherent structures was described. The results indicate that regions with high velocity fluctuations mainly locate at the shear layer of the upper and lower edge of the recirculation bubble. Further proper orthogonal decomposition results show that the velocity fluctuations account for a very little proportion of kinetic energy compared with the time-averaged velocity, supporting that the DrivAer model has steady aerodynamic characteristics. The smooth outlines of the DrivAer notchback model can delay the flow separation on the top and the C-pillar of the model, which inhibits the formation of the back recirculation bubble at the rear window and reduces the size of the recirculation bubble behind the baggage compartment. These promote the pressure recovery at the rear of the DrivAer notchback model, and thus reduces the pressure drag. At the rear of the model, the airflow on both sides converges towards the center due to the pressure difference, producing the streamwise vorticity by shearing the baggage compartment, then separates at the rear of the baggage compartment and generates a pair of secondary vortices. Overall, the aerodynamic behavior of the DrivAer model is robust.

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Abbreviations

\(A\)::

Longitudinal projection area of the DrivAer model.

\(\sqrt A\)::

Reference length.

\(D\)::

Width of the vehicle model.

\(d\)::

Space between the adjacent measurement planes.

\(E\)::

The energy ratios of the higher order POD modes.

\(H\)::

Height of the vehicle model.

\(L\)::

Length of the vehicle model.

\(N\)::

Total number of images taken at each measuring position.

\(n\)::

Current captured image number.

O : :

Origin of the right-handed Cartesian coordinate system.

\(Q\) :

Second invariant.

\({\varvec{r}}\)::

The eigenvector of the real eigenvalue of the velocity gradient tensor.

\({\varvec{R}}\)::

Rortex vector.

\({\varvec{R}}^{*}\)::

Nondimensionalized Rortex vector.

\(R_{x}^{*}\)::

Nondimensionalized streamwise component of the Rortex vector.

\(R_{y}^{*}\)::

Nondimensionalized spanwise component of the Rortex vector.

\(R_{z}^{*}\)::

Nondimensionalized vertical component of the Rortex vector.

\(Re\)::

Reynolds number based on the reference length.

\(S\)::

Total number of measuring positions.

\(s\)::

Current measuring position number.

\(U\)::

Time-averaged streamwise velocity component.

\(u\)::

Streamwise velocity fluctuation.

\(U_{0}\)::

Free-stream velocity.

\(U_{s}\)::

Time-averaged streamwise velocity component measured in the (x, z) plane.

\(U_{t}\)::

Time-averaged streamwise velocity component measured in the (x, y) plane.

\(V\)::

Time-averaged spanwise velocity component.

\(v\)::

Spanwise velocity fluctuation.

\(V_{t}\)::

Time-average spanwise velocity component measured in the (x, y) plane.

\(W\)::

Time-averaged vertical velocity component.

\(w\)::

Vertical velocity fluctuation.

\(W_{s}\)::

Time-averaged vertical velocity component measured in the (x, z) plane.

\(x\)::

Streamwise direction.

\(x^{*}\)::

Streamwise direction nondimensionalized by the reference length.

\(y\)::

Spanwise direction.

\(y^{*}\)::

Spanwise direction nondimensionalized by the reference length.

\(z\)::

Vertical direction.

\(z^{*}\)::

Vertical direction nondimensionalized by the reference length.

\(\overline{{uu}}_{n}\)::

Reynolds normal stress based on the first n samples.

\(\overline{{uv}}_{n}\)::

Reynolds shear stress based on the first n samples.

\(\overline{{uw}}_{n}\)::

Reynolds shear stress based on the first n samples.

\(\overline{uu}\)::

Reynolds normal stress based on N = 500 samples.

\(\overline{uv}\)::

Reynolds shear stress based on N = 500 samples.

\(\overline{uw}\)::

Reynolds shear stress based on N = 500 samples.

\(\eta\)::

Relative error of the Reynolds stresses.

\(\lambda {}_{2}\) :

Second eigenvalue of the pressure Hessian.

\(\lambda_{ci}\)::

The imaginary part of the complex eigenvalues of the velocity gradient tensor.

\(\mu\)::

Dynamic viscosity coefficient of air.

\(\rho\)::

Density of air.

\(\varphi\)::

Slant angle of the Ahmed body.

\(\omega\)::

Vorticity.

\(\omega_{x}^{*}\)::

Nondimensionalized vorticity in the streamwise direction.

\(\omega_{y}^{*}\)::

Nondimensionalized vorticity in the spanwise direction.

\(\omega_{z}^{*}\)::

Nondimensionalized vorticity in the vertical direction.

\(\Phi_{U}\)::

Dynamic modes in streamwise component.

\(\left\| {\Phi_{U} } \right\|\)::

Energy norm of dynamic mode in streamwise component

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Acknowledgement

This work is supported by the funds of China Automotive Engineering Research Institute (MS-03-03) and the National Natural Science Foundation of China (Grants No. 11772173). The authors are grateful to Prof. Zhaofeng Tian from the University of Adelaide for the discussion of the results.

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Authors and Affiliations

  1. AML, School of Aerospace Engineering, Tsinghua University, Zhongguancun north street, Haidian District, Beijing, 100084, China

    Jinsheng Liu, Zhihao Zhang & Shengjin Xu

  2. College of Environmental Science and Engineering, Beijing Forestry University, Beijing, China

    Yumeng Wang

  3. Department of Mechanical Engineering, University of Melbourne, Victoria, 3010, Australia

    Mogeng Li

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  1. Jinsheng Liu
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Liu, J., Zhang, Z., Wang, Y. et al. Experimental study on the three-dimensional wake structure of the DrivAer standard model. J Vis 24, 443–460 (2021). https://doi.org/10.1007/s12650-020-00714-2

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