The sampling duration is set equal to the duration of the wave machine drive
signal for two reasons. The first being to ensure statistical wave height
parameters obtained from the wave records are a realistic representation of the
waves generated during the entire duration of the wave machine drive signal.
Secondly, it provides the flexibility to start data acquisition at any time without
synchronizing with the beginning of the wave machine drive signal, which allows
measurements of the same set of generated waves during each transect.
The longshore position of the wave gauges is selected from the control room
by automatically moving the instrumentation bridge to the desired transect
location. The length of the test segment (generally 1 to 2 hr) limits the number of
transects that are measured. For example, if the peak wave period is 2.5 sec, the
duration of the wave machine drive signal is normally 500 sec (200 waves
2.5 sec). Therefore, a sampling duration of 500 sec is used. In addition,
approximately 60 sec is required to move the bridge to the next transect location.
Hence, each transect would require approximately 10 min of testing time.
Therefore, a 1-hr test segment provides time to measure approximately six
transects, and a 2-hr test segment provides time to measure approximately 12
transects. The 12 transects distributed along the length of the testing region
provide sufficient spatial resolution to quantify longshore uniformity (or
longshore gradients) in wave height and mean water level.
Wave data are collected using the Data Recorder panel, located under the
Operation menu of the MTS control software. The layout of this control panel is
shown in Figure 42. The Data Recorder panel allows the operator to select the
channels to record and stores data in a single binary file. During an experiment,
the analog output signals from the 14 wave gauges and the 38 ADV channels are
recorded using the MTS control software. The operator also can specify the
sampling rate (i.e., sampling frequency) and recording period (i.e., sampling
duration). Further information related to using the Data Recorder panel can be
found in Chapter 4 of the MTS Operation Manual, MTS Systems Corporation
(1998).
Figure 42. Data recorder panel in the MTS control software (Permission to
reprint granted by MTS Systems Corp.)
76
Chapter 8
Sensor Calibration, Measurement, and Data Analysis
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