orientation of the ADVs was accurately aligned with the coordinate system used
in the facility.
The average velocities obtained from the eight 3-D down-looking probes
(ADVs 3 through 10) seemed to indicate that the performance specifications
listed above were reasonable. However, the two 2-D side-looking probes (ADVs
1 and 2) measured significantly lower values on the y-axis than on the x-axis.
For the 2-D side-looking ADVs, the positive x-axis is defined from the acoustic
transmitter to Acoustic Receiver No. 1 (painted red). The positive y-axis is
defined along the axis perpendicular to the acoustic transmitter from the sensor
towards the sampling volume. After much investigation, it was determined that
the acoustic transmitter and receivers on the side-looking sensors cause
significant flow disturbance when water is flowing parallel to the y-axis of the
ADV. Because quantifying the longshore current in the facility is a higher
priority than quantifying the cross-shore current, the decision was made to
permanently rotate ADVs 1 and 2 by 90 deg in the clockwise direction.
Therefore, the positive x-axes of ADVs 1 and 2 are aligned to be parallel with the
negative y-axis of the coordinate system used in the facility (i.e., downstream).
In addition, the analog voltage cables for ADVs 1 and 2 were reversed on the
back of the MTS data acquisition system so that the orientation of the ADV axis
remains consistent with the overall coordinate system used in the facility. In
addition, the voltage signal signs were transposed so that the positive and
negative directions were correct. This was accomplished by changing the
appropriate signs on the x- and y-axes of ADVs 1 and 2, using the Existing
Sensors panel showed in Figure 44. Additional information in regard to the
operation of the ADV software and hardware can be found in the Operation and
Reference Manual, SonTek (1997).
Analysis of currents
A GEDAP batch file was developed to expedite the procedure of post-
processing several transects of current data. The general procedure consists of
the following steps. First, the program GLITCH_FIX_4A is used to remove
spikes in the velocity time series caused by air bubbles entrained in the water
column. This program was developed specifically for the type of spikes
observed in the output time series from the ADVs used in the LSTF. Input
parameters required to operate the program are: (a) the time scale length (TSL) of
the sliding window that moves along the time series; and (b) the number of
standard deviations, Alpha, used to define the upper and lower limits of the valid
data with respect to the mean value within the sliding window. Initially, the
program searches the input record to find the TSL segment having the smallest
standard deviation. The purpose of this search is to provide an initial estimate of
the mean and standard deviation of the true signal without noise glitches. A TSL
value of 5.0 sec worked well for experiments conducted with a peak wave period
of 2.5 sec.
Input parameter Alpha is usually set to a value between 3.0 and 4.0.
Theoretically, a value of Alpha = 3.5 should have no measureable influence on
the true signal, assuming that the data have a Gaussian distribution. Based on
experience, Alpha is typically selected as 3.5, which sets the initial data limits to
82
Chapter 8
Sensor Calibration, Measurement, and Data Analysis
Previous Page
