Velocity spectra and space-time correlations have been measured downstream of a low-speed linear compressor cascade with tip gap. The objective of this work is an improved understanding of the coherent turbulence structures present in fan-tip wakes and of the turbulent source terms responsible for broadband stator noise. The two-point measurements show no correlation between turbulent motions in the wakes or leakage vortices shed by adjacent blades and that the leakage vortex is not subject to low-frequency wandering motions. They also show that the tip leakage vortex turbulence is highly anisotropic and characterized by elongated eddies inclined at about 30 deg to the vortex axis. The presence of such structures, which produce no clearly identifiable footprint in velocity spectra, appears consistent with the helical structures seen in direct numerical simulations of a line vortex with unstably large streamwise velocity defect. This suggests the same mechanism is behind the generation of turbulence in this vortex. Examination of the correlation data from the point of view of a hypothetical stator blade suggests that, because of the anisotropy in turbulence structure, velocity correlations seen by a stator in a real engine are likely to be a strong function of engine geometry and operating point. IntroductionT HIS paper is our third in a series dealing with the flow downstream of a linear compressor cascade. This cascade is designed to reproduce, in an idealized setting, some of the important conditions experienced in the subsonic fan of a large-bypass-ratio aircraft engine, specifically the mechanisms leading to the generation and evolution of the tip leakage vortices that dominate the casing endwall flow, which impinges on the bypass stator. Our first two papers 1,2 focused on single-point velocity measurements of the mean flow and Reynolds-stress fields produced by the blade wakes, the leakage vortices, and the associated endwall flow, as functions of streamwise position, tip gap, and relative motion between the blade tips and endwall. Such measurements provide data and understanding to aid in the development and testing of computational-fluid-dynamics (CFD) prediction tools for calculation of the unsteady aerodynamics and tone noise produced by the stator interaction. In this paper we focus on two-point turbulence measurements. This type of information not only provides insight into the scale and form of the coherent structures that populate the turbulent regions, but is also critical to understanding the generation of broadband noise by the stator row.To predict the broadband rotor/stator interaction noise, it is, in principle, necessary to have a complete description of the rotor wake turbulence in terms of its two-point space-time correlation function and a response model for the blade row. Complete two-point information is rarely available even for relatively simple flows and cannot be provided by performing Reynolds-averaged CFD calculations. Drastic assumptions about the form of the two-point cor-

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