The velocity field and flow patterns in a fully baffled cylindrical flat-bottomed tank with a Rushton turbine impeller were investigated by the spectral analysis of the velocity time-series measured with two-dimensional laser doppler velocimetry (LDV) and by laser sheet flow visualization. The dynamical behavior in the range near the turbine revolution to the range related to trailing vortices and Kolomogorov wave numbers was shown to be induced by nonlinear coupling between at least three frequencies, the blade passage frequency f(p), the turbine revolution frequency f(p)/6 and the macro-instabilities frequency f(u) (f(MI)) due to the change in the eddies or vortices (movement of the center and the size) near the turbine impeller. The dynamic analysis using the Fourier spectrum under different conditions of off-bottom clearance supported the finding that the macro-instabilities frequency f(u) (f(MI)) is a component of velocity field dynamics.