Low Mach number turbulent flows over an open cavity were studied to investigate the quantitative characteristics of large-scale vortical structures responsible for self-sustained oscillations. Wind tunnel experiments with particle image velocimetry (PIV) were conducted in the range of the ratio of cavity length (L) to depth (D), I <L/D<4, when the incoming boundary layer is turbulent at Re(0) = 830 and 1810. Self-sustained oscillation modes were classified by varying the conditions of L/D and Re(0). The oscillation modes were consistent with the number of vortical structures existing between the leading and trailing edges of the cavity. Proper orthogonal decomposition (POD) was employed to the spatial distributions of vertical velocity correlations on the lip line of cavity geometry. By examining the conditionally averaged distributions of the correlation coefficients of POD, the spatial characteristics of large-scale vortical structures for self-sustained oscillations were examined. (C) 2009 Elsevier Ltd. All rights reserved.