Whole field velocity measurements in three-dimensional periodic flows

To quantify flows around rotorcraft, rapid measurements of scalar and vector fields are needed over large volumes. The techniques used must be suitable for large test facilities. This thesis studies methods for acquiring and reconstructing four-dimensional, spatio-temporal measurements of flow properties in periodic flows. It involves both the theoretical studies needed for algorithm development and the solution of practical problems required to enable multi-dimensional velocity field measurement in flows typical of full-scale rotorcraft.; Resolving the four-dimensional flowfield is viewed as a problem in the tomographic reconstruction of scalar and vector fields. Theoretical formulations reconstructing n-dimensional scalar fields from (n-1)-dimensional projections are studied. This work was a precursor to the extraction of three-component, three-dimensional velocity fields from planar Spatial Cross-Correlation Velocimetry (SCV). SCV measures a planar displacement field by cross-correlating two time-separated images of the flow. A scalable system that uses inexpensive pulsed white light sources and enables large-area imaging has been integrated for use in full-scale test facilities.; The flowfield around a V22 half-model was studied using this technique. SCV discovered the existence of a transient upflow above the rotor plane, unique to compressible rotor flows, and verified other flow features. Measurements in a turbofan engine test cell validated system performance in the highly turbulent and vibrating test environment, under time limitations typical of industry testing. Studies of a two-bladed rotor in axial flight revealed basic vortex pairing and merger phenomena. These tests provided the first proof that full-scale rotor wakes at high Reynolds number and Mach number are cleanly periodic when facility interference effects are eliminated.; A method was developed to compute the 3D, three-component, periodic velocity field by integrating 2D, phase-resolved, SCV data across several measurement planes. A second order finite difference formulation is used to satisfy mass conservation for incompressible flows. The 3D technique was used to obtain the phase-resolved velocity field in an experimental rotor/fixed wing configuration. Vorticity contours showed the divergence of the tip vortex trajectories and the associated flowfield effects. The strong spanwise wall jet near the wing surface was captured. The resolution of periodic, large-scale, compressible rotorcraft flows is thus shown to be within reach.