CORRELATION SPECTRUM ANALYSIS IN THE NEAR WAKE BEHIND A DISK

A large eddy simulation (LES) is conducted in the near wake behind a circular disk at Re=22 000 with the diameter-thickness ratio D/h=5. Through fast Fourier transformation (FFT) of the fluctuating velocities at x/D=1, 2 and 8, three characteristic frequencies are identified, with corresponding Strouhal numbers being 0.035, 0.123 and 1.3～1.7. It is shown that St₂=0.123, is the natural vortex shedding frequency, as in agreement with previous research results; St₁=0.035, the smaller one, may be related to the combined action of the recirculation zone's "pump" motion and the azimuthal rotation of the shedding location near the shear layer; St₃=1.3～1.7, might result from the instability of the turbulent shear layer. The correlation coefficients, the coherent spectrum and the phase spectrum are calculated between two points with different separation angles in the same axial section of r/D=2.8. Due to the vortex shedding, the correlation coefficients fluctuate between positive and negative values in the intervals of 30 or 45 degrees. This indicates that the position of the vortex shedding is random in the azimuthal direction.