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

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.