Speaker
Description
Axisymmetric turbulent wakes generated by bluff bodies are relevant to several applications, including aviation, road transport, and the design of wind turbine masts, among others. This experimental study focuses on the unsteady properties of turbulence dissipation resulting from the non-equilibrium energy cascade in the turbulent axisymmetric wake. Recent numerical studies (e.g., Goto and Vassilicos, Physical Review E, Volume 94, 2016, and Alves Portela et al., Physical Review Fluids, 2018) suggest two key findings: (1) the turbulence production rate may be correlated with the turbulent energy dissipation rate (denoted as ε) with a spatio-temporal lag, and (2) the relationship εₗₒw / ε = constant (where εₗₒw represents the low-pass filtered ε) may hold over a range of streamwise distances. Experimentally verifying these predictions is very challenging because it requires resolving various quantities, such as turbulence production and ε, in both time and space.
We conducted a series of experiments combining 2D2C particle image velocimetry (PIV) and hot-wire anemometry (HWA) to characterize some unsteady aspects of the energy cascade. A bluff fractal plate, suspended perpendicular to the freestream flow and with a characteristic length of 64 mm (defined as the square root of the frontal area), was tested in the LMFL Boundary Layer Wind Tunnel. This facility has a measurement section 20~m long and a cross-sectional area of 2 m by 1 m. The freestream velocity, U_∞ (measured with a pitot tube at the beginning of the test section), was maintained at a constant value of 8.5 m/s, resulting in Reynolds numbers of approximately 35,000 based on the plate's characteristic length.
Four cameras with a temporal resolution of 4 Hz were aligned longitudinally to capture PIV fields covering 1.35 m in length and 30 cm in height. These fields included the wake's centerline and lower part. By repositioning the cameras, we recorded three measurement stations spanning the ranges 24 < x/D < 40, 38 < x/D < 54, and 54 < x/D < 70, respectively. The spatial resolution was approximately 3 mm, sufficient to resolve several low-pass filtered turbulence quantities. Within the last camera's field of view, HWA measurements, sampled at 50 kHz and synchronized with the PIV data, were performed. These HWA measurements enabled the correlation of fully resolved turbulence quantities, such as ε, with low-pass and coherent fluctuation quantities (such as εₗₒw) and turbulence production rates obtained from the PIV data.
