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Measurement of unsteady flow structures in a low-speed wind tunnel using continuous wave laser-based TR-PIV: near wake behind a circular cylinder

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Abstract

The unsteady measurement of spatiotemporally varying flow structures in a low-speed wind tunnel using a continuous wave (CW) laser-based time-resolved particle image velocimetry (TR-PIV) setup was extensively evaluated in the near wake behind a circular cylinder. A CW laser with a maximum power of 25 W in combination with a high-speed camera operating at 7 kHz was used to determine the wake flows at two different free-stream flow speeds: U 0 = 5 and 10 m/s. Three different camera exposure times were selected for comparison: τ = 20, 50, and 80 μs. In the experiments, the low-repetition conventional PIV setup using the high-power pulsed laser (τ = 8 ns, 135 mJ/pulse) was used to determine the time-mean and statistical flow quantities, which served as the reference for determining the deviation in the TR-PIV measurements. At the lower flow speed of U 0 = 5 m/s, the time-mean-separated flow patterns and the streamwise velocity profiles in all of the TR-PIV systems showed satisfactory agreement with the conventional PIV measurements, along with accurate capture of the large-scale Karman vortex and its harmonic behaviors. At the higher flow speed of U 0 = 10 m/s, the measurement at τ = 50 μs gave a relatively accurate representation of the statistical flow quantities. At the longest exposure time of τ = 80 μs, considerable deviations in the time-mean streamwise fluctuation intensity and the TKE (turbulence kinetic energy) were observed due to the streaky particle image. The strong swirling motion of the large-scale vortical structures increased the deviation in the TR-PIV measurements, which increased with the increasing camera exposure time. Further POD analysis demonstrated that the leading energetic modes in the system with τ = 50 μs accurately determined the spatial features of the Karman-like vortex and its harmonic events. However, inaccurate vector representation of the second harmonic events was observed in the system with τ = 80 μs. Finally, for both flow speeds, the lower-order reconstructed phase-dependent representations of the Karman-like vortex and its harmonic behaviors were composed of the time-series velocity vector fields determined using the system with τ = 50 μs, thus providing a straightforward quantitative view of the coupled unsteady events.

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Abbreviations

D :

Diameter (m)

Re :

Reynolds number

u :

Instantaneous streamwise velocity (m/s)

\(U_{0}\) :

Free-stream velocity (m/s)

\(\overline{u}\) :

Time-averaged streamwise velocity (m/s)

\(u^{\prime }\) :

Fluctuating portion of the streamwise velocity (m/s)

\(u_{\text{rms}}^{\prime }\) :

Root-mean square of the streamwise velocity fluctuations (m/s)

x :

Streamwise coordinate (m)

y :

Longitudinal coordinate (m)

St :

Strouhal number

a(t):

POD mode coefficient

n :

Eigenmode number

\(\tau\) :

Exposure time (s)

\(\lambda_{n}\) :

POD eigenvalue

\(\phi\) :

POD eigenfunction

CMOS:

Complementary metal oxide semiconductor

CW:

Continuous wave

PIV:

Particle image velocimetry

POD:

Proper orthogonal decomposition

TKE:

Turbulence kinetic energy

TR-PIV:

Time-resolved particle image velocimetry

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Acknowledgements

The authors gratefully acknowledge the financial support for this study from the National Natural Science Foundation of China (11372189), and the support from the “Shuguang Program” of the Shanghai Education Development Foundation and Shanghai Municipal Education Commission (Grant no. 12SG16).

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Authors and Affiliations

  1. Key Lab of the Education Ministry for Power Machinery and Engineering, School of Mechanical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai, 200240, China

    Shaofei Wang, Yujia Chen & Ying Zheng Liu

  2. Gas Turbine Research Institute, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai, 200240, China

    Shaofei Wang, Yujia Chen & Ying Zheng Liu

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  1. Shaofei Wang
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  2. Yujia Chen
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Correspondence to Ying Zheng Liu.

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Wang, S., Chen, Y. & Liu, Y.Z. Measurement of unsteady flow structures in a low-speed wind tunnel using continuous wave laser-based TR-PIV: near wake behind a circular cylinder. J Vis 21, 73–93 (2018). https://doi.org/10.1007/s12650-017-0445-3

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  • DOI: https://doi.org/10.1007/s12650-017-0445-3

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