Proper orthogonal decomposition of an axisymmetric turbulent wake behind a disk

A proper orthogonal decomposition (POD) study of the axisymmetric turbulent wake behind a disk has been performed using multipoint hot-wire data. The Reynolds number based on the free stream velocity and disk diameter was kept constant at 28 000. The investigated region spanned from 10 to 50 disk diameters downstream. The hot-wire data were obtained using two rakes: a seven wire fixed array and a six wire array azimuthally traversable to span the cross section of the flow in increments of 15°. The instantaneous streamwise velocity component data were Fourier transformed in time and decomposed in Fourier series in the azimuthal direction to form the kernel for the POD. For all downstream positions, two distinct peaks were found in the first eigenspectrum: one at azimuthal mode 2 at near zero frequency, and another at azimuthal mode 1 at a fixed Strouhal number (fd/U∞) of 0.126. Both peaks decrease in magnitude as the flow evolves downstream, but the peak at the Strouhal number 0.126 decreases more rapidly than the one at near-zero frequency, leaving the latter to eventually dominate. Because of this evolution, the eigenvalues integrated over frequency show an azimuthal mode-1 dominance at x/D=10 and a mode-2 dominance by x/D=50. The results are compared to those recently obtained in the axisymmetric far jet, and the results of previous wake investigations.