Flow due to waves and currents H 840 February 1994
delft hydraulics
Executive's summary
Changes in the mean horizontal-velocity profile in combined wave-current motion, outside the bottom boundary-layer, are essential for the correct prediction of cross-shore sediment transport outside the surf zone and the transport of dissolved matter, e.g. Klopman (1992). To be able to verify mathematical and numerical models (see Klopman, 1992) experimental data is needed. This is especially the case with the vertical structure of the wave and turbulence Reynolds stresses where data is lacking. For this reason, a laboratory experiment was conducted to study the flow kinematics under combined wave-current motion, for waves propagating in the current direction and for waves opposing the current.
The wave-current facility in which the tests have been carried out is equipped with two computer-controlled wave boards, one generating waves and the other absorbing waves. Both wave boards have active wave-absorption systems which eliminate spurious waves. A constant discharge was provided by a flow-circulation circuit. Special care was taken in the design of the inflow and outflow structures, in order to introduce the current smoothly into the channel and to minimize unwanted reflections of the waves at the inflow and outflow. Flow velocities were measured in one vertical cross-section of the channel, with two laser-Doppler velocimetry (LDV) systems, mounted at a fixed distance above each other. Water surface elevations were measured with six resistance-type wave-height meters, and the discharge was measured with an electro-magnetic flow (EMF) meter.
Tests were performed with mono-chromatic, bi-chromatic and random waves without current, following the current and opposing the current. Also a test series was performed for a steady current without waves.
A first data analysis was performed during the tests. The main results are:
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The measured data seems to be qualified for the study of mean velocities, wave orbital velocities, as well as wave and turbulence ReYJ;lolds-stresses under combined wave-current motion.
The mean horizontal-velocity profile under combined wave-current motion is strongly affected by the presence of waves not only inside but also outside the wave bottom boundary-layer, which is in accordance within previous experiments. In the upper half of the water column the velocity shear is reduced and may even change sign in the case of waves following the current. Waves opposing the current increase the velocity shear in the upper half of the water column, see the figure alongside.
D waves following current • waves opposing current
+ current without waves
.50 , - - - . . - - - . , - - - . . - - - - , - - - , .40 .30 .20 10
