Knowledge of wave kinematics, velocity and acceleration of fluid particles is very important for the design of ocean floating platforms and structures. Measured ocean kinematic data are not easily obtained data nor readily available, and consequently, theoretical estimates of the wave kinematics are normally used for design, Transient waves (i.e. breaking waves, freak waves, etc.) exist in the ocean and these waves are severely asymmetric with respect to vertical and horizontal axes. Wave theories do not accurately predict the kinematics of these waves. In addition, most theories do not predict the kinematics above the still water level which important for design. With the introduction of sophisticated and accurate instruments, such as hot wire anemometer, laser Doppler anemometers, etc., measurements of waves kinematics in laboratory wave tanks are possible. These laboratory data can now be used for development of more accurate wave theories.
In order to conduct research in the area of wave kinematics, the Ocean Engineering Program of the Civil Engineering Department at Texas A&M University purchased a laser Doppler anemometer (LDA) from DANTEC Electronics in 1988. The LDA is a three beam Argon-Ion laser capable of measuring two velocity components simultaneously. This system consists of five parts: laser generator, transmitting and receiving optics, traverse system, signal processing equipment, and data acquisition system. A frequency tracker signal processor is used and the receiving optics of LDA are aligned for the backscatter mode. Output voltages from the frequency trackers corresponding to the two velocity components are acquired by a Hewlett Packard computer and data acquisition system. An IBM PC/XT compatible computer is also available for data acquisition. However, commercial software for the IBM PC/XT had limitations in analysis, graphical display and acquisition of data from other instruments such as wave probes, and therefore it was necessary to develop new software to meet the requirements for the two dimensional wave kinematics research.
The software described in this report, which is written in HP BASIC version 5.0 is called “TAMULDA” which means Texas A&M University LDA software. TAMULDA has four main parts: optical parameter settings, frequency tracker and frequency shifter parameter settings, data acquisition parameter settings, and data acquisition for keyboard trigger mode, time delay trigger mode, monitor display trigger mode and TTL trigger mode. Twelve color modes are used for the parameter setting screen and data acquisition screen. The green color corresponds to the green laser beam from TAMULDA, and the blue color to blue beam so that errors are minimized during the parameter set-up process.
This project provides a brief explanation of LDA theory and equipment, and it describes in detail the procedures for running TAMULDA and some example results. TAMULDA has proven to be very flexible and useful in the measurement and acquisition of wave kinematics data.
Related Publications: Kim, C.H., Randall, R.E., Krafft, M.J., and Boo, S.Y., “Experimental Study of Kinematics of Large Transient Wave in 2-D Wave Tank”, Proceedings of the 22nd OTC Conference, Houston, 1990