Unsteady PIV for regular head waves Available to Purchase

Phase-averaged results from a towing tank experiment of regular head waves are documented. Uniform flow, free surface elevation, and headwave flowfields are measured with a towed particle image velocimetry system in the Iowa Institute of Hydraulic Research towing tank of a two-dimensional, deep-water, long­wavelength (A.=4.572 m), low wave steepness (Ak=0.025) progressive head wave. The towing speed is 1.53 m/s which corresponds to Fr=0.28 for a 3.048 m surface combatant which will be the focus of a future study for measuring the unsteady boundary layer and wake with the present measurement system. Uniform-flow measurements are conducted first to establish the resolution of the mean variables, levels of turbulence in three normal stresses and two Shear stress components, and uncertainties which are roughly 0.65% for the axial flow (U) and 0.5% for the transverse (V) and vertical components (W). Then, tests are conducted in headwaves at five elevations to map the orbital wave patterns. The data is phase-averaged and Fourier analyzed to determine the harmonic content in the measured headwave patterns. Results are compared with deep-water progressive wave theory to evaluate the qualities of the harmonic amplitudes and phases of the data. The 0th harmonic results for U, V, W=l, 0, 0 to within the resolution of the measurement system. The 1st harmonic of U,W are generally within 1- 2% of the theoretical harmonic amplitudes and demonstrate same exponential decay with increasing distance from the free surface as theory. Differences between experiment and theory of the 1st harmonic phase of U, W are observed in the results that are not fully understood but described with possible explanations. The data will be archived at the IIHR towing tank website at http://www.iihr.uiowa.edu/-towtank.