Principles of Operation:
4. Waves

Wave Directional Spectra  

Aquadopp Current Meters, Aquadopp Profilers and Vector Velocimeters have been seeing increasing service as wave sensors. Scientists and engineers are choosing these instruments to measure waves because they are small, inexpensive and easy to use, and because they appear to be among the best sensors in their class for obtaining high quality wave data.

This section describes how to compute wave directional spectra using the well-established PUV method, illustrates typical results and describes how you can evaluate your results and predict performance.

Computing Wave Directional Spectra  

1. WaveExtract software is the easiest and fastest method for computing wave directional spectra using the Nortek Vector, Aquadopp Current Meter or Aquadopp Profiler.
2. The Matlab WDS Toolkit, below, includes source code for simple subroutines to compute wave directional spectra using standard PUV methods.

Wave Directional Spectra Primer  

1. The PUV Method (below). A bit of background (below on this page).
2. Wave Diretional Spectrum Measurement. Principles of how the PUV method obtains wave directional spectra, illustrated with examples of results with a Vector Velocimeter.
3. Wave Directional Spectrum Data Evaluation. Explanation of limitations of the PUV method including techniques for handling some of the limitations; a model for predicted uncertainties in direction and directional spreading estimates; procedures for getting the most out of your data.

Additional Resources

1. Waves2001 paper: Gordon and Lohrmann, Nearshore Doppler Current Meter Wave Directional Spectra, compares wave directional spectra from an Aquadopp Profiler and a Vector, located next to each other.
2. Sample wave data and Matlab tools (below) for computing and evaluating wave directional spectra.

PUV Method  

The study of waves began in earnest around the time of World War II. As people began to develop interest in wave direction, two general methods emerged: the triplet method and a variety of array methods. Array methods have the capability of providing better ability to resolve wave information, but they tend to be difficult to install, and the analysis is complex. Triplet methods are more common because the necessary measurements can be combined into a single package. It is less expensive to build, easier to install, and its analysis is simpler. The two most common triplet measurements include PUV sensors, which are mounted beneath the surface, and a wide variety of directional wave buoys.

Most early PUV sensors combined electromagnetic velocity sensors and pressure sensors. A wide variety of electromagnetic velocity sensors have been workhorse wave sensors for decades. More recently, acoustic sensors are replacing electromagnetic sensors. There are many reasons for this, but two key reasons include:

1. Acoustic sensors can make their measurements remotely, away from the disturbance of the sensor itself.
2. Acoustic sensors are capable of measuring velocity with high accuracy and precision.

Aquadopps and Vectors are among this new generation of acoustic sensors, and they are proving particularly capable of making high quality wave measurements.

See also

1. Wave Directional Spectrum Measurement
2. Wave Directional Spectrum Data Evaluation

Matlab Tools and Sample Data  

Click below to download sample velocity and pressure data and Matlab m-files that can process the data to compute and display wave directional spectra. If you do not use Matlab, you can download a text file with a 2048-point sample of the Vector data.

Matlab WDS Toolkit  

The Matlab tools contained in WDS.ZIP enable you to compute wave directional spectra from arrays of sequential velocity and pressure time series. For example, demo.m uses these tools to process data from an array of 24 2048-point time series. All of the routines have been vectorized, so they are reasonably fast.

These Matlab tools are described below. If you use these tools in a report or publication, we request that you acknowledge the source in your acknowledgements or bibiography.