Large-area velocity measurement in low-speed flows

Spatial cross-correlations are used to measure velocity fields over large areas from time-separated images of instantaneous patterns in flow sections. The location of the cross-correlation peak between corresponding sub-images determines the 2-D displacement vector. Since particle image features are not sought, white light and large areas can be used as well as lasers. A single video camera is adequate for low relative displacements. A phase-shifted dual camera extends the speed range and resolution. Five example applications are presented. Quasi-static wind tunnel testing of a canard-wing model demonstrates application to configuration development. Robustness to out-of-plane motion is shown in the wake of a 2-bladed rotor in forward flight. Phase-averaging in periodic flows is discussed. A single camera is used to map the mean and r.m.s. velocities in a large rotor test chamber. A halogen tight sheet is used to validate SCV results against point probe data at the exit of a wind tunnel. Finally, extraction of the out-of-plane component in incompressible flows is presented. The swirling viscous flow above a flat plate is used to analytically represent a 3-D flow. The differential mass conservation equation is used to solve for the third component using slices of 2-D vector fields. A second-order differencing procedure is examined with added noise.