Experiments utilizing two-dimensional fixed dune profiles and varying flow depth (dune regime flows) highlight the equilibrium (self-similar) nature of the near-bed boundary layer over developing dunes with flow separation in the dune lee. The negligible variation in roughness layer (comprising the interfacial and form-induced layers) flow structure for developing dunes was confirmed in terms of spatial fields of time-averaged velocities and stresses; and vertical distributions of: (a) double-averaged (in time and space) longitudinal velocity, (b) double-averaged normal stresses, and (c) the components of the momentum balance for the flow. The finding of an equilibrium nature for the near-bed flow over developing dunes is significant in its centrality to understanding the feedback loop between flow, bed morphology, and sediment transport that controls erodible-bed development. Further research is required into the form of the distribution of double-averaged velocity in the form-induced layer above roughness tops, and also to complete generalization for varying dune steepness of the universal expression for double-averaged longitudinal velocity (varying linearly with elevation) determined herein for the interfacial layer (below roughness tops). Work is presently focusing on the additional effects on flow structure due to sediment transport and three-dimensional flow and bed morphology, although it is expected that the equilibrium boundary layer flow structure patterns identified herein will still be evident for these more complex systems.

A rigorous framework involving flow decomposition and averaging is presented, within which the mechanics of rough-(e.g., rippled-) bed oscillatory flows can be better interpreted and understood. Spatiallyaveraged equations for conservation of fluid mass and momentum are developed for analyses of rapidly-changing bed conditions, e.g., for growing ripples. Where repeated observations of the changing bed conditions are available, the ensemble and spatially-averaged versions of these equations can be used for more detailed analyses of the flow dynamics. The double-averaged (in space and phase or time) equations of mass and momentum conservation are shown to be appropriate for analyses of flows over fixed rough beds and equilibrium ripples. The value of the present framework is highlighted herein by its application to PIVmeasured oscillatory-flow velocities, stresses and vorticities over growing and equilibrium wave-induced intermediate-depth orbital-vortex ripples. In particular, discussions are provided regarding the mechanisms by which gravity-induced and pressure-gradient-induced momentum is transferred to the bed, with the analysis framework naturally and explicitly including the combination of the full range of fluid stresses and boundary form and skin friction drag that is important in defining the flow mechanics.

In this paper, a new method of calibrating an acoustic particle velocity sensor using laser Doppler anemometry (LDA) is discussed. The results were compared and were in good agreement with the results obtained by conventional methods, where the sensitivity of the microflown is obtained with the use of a reference microphone and a standing wave tube. The LDA signal generated by the acoustic particle motion was analysed using the photon-correlation method, where the signal is considered to consist of a series of discrete photon events. The photon-correlation system is used to measure particle velocity amplitude next to the microflown particle velocity sensor in a standing wave. Measurements are performed for frequencies between 250 Hz and 4 kHz and velocities between 5 mm s−1 and 25 mm s−1 (root-mean-square (rms) values) which are equivalent to sound fields of 100 and 114 dB SPL in free field. From the output voltage of the probe microflown and the LDA-derived particle velocity in a standing wave, the sensitivity of the microflown is obtained. The two different calibration methods are in good agreement showing a discrepancy of 1 dB for the frequency range of 250 Hz–4 kHz.

No abstract

No abstract