Armfield Engineering Education
APPLIED HYDRAULICS AND HYDROLOGY
S645 Random Wave Maker - Provisional
This machine utilises the base hinge weir in the discharge
tank of the Armfield S6 Flume as the paddle. Additional hardware comprises
a control box, a filter box and a 50 way ribbon connector. There are two
cards for insertion into a PC (not included in our supply).
WAVE GENERATION SOFTWARE
> Regular waves .Irregular waves - filtered white noise
> Irregular waves - Fourier series
> User defined wave generation techniques
This is an advanced software package designed to simulate long crested sea conditions. It can generate Regular, Irregular and Solitary waves.
It is capable of running on any IBM compatible-PC fitted with the two interface cards supplied with the software.
Installation is by means of an installation program which checks and modifies the configuration of the PC and loads the necessary files and directories. Following installation it allows all the relevant dimensional details of the paddle to be entered and saved in a file.
The experimental transfer function can be entered and saved for a range of water depths.
Wave height and period requirements are entered at full (prototype) scale together with the model scale being used.
Progression through the program is by highlighting options displayed in a series of menus and tables.
When it is running, the parameters of the generated sea state and a real time graphical display of the paddle movement are shown on screen.
TOPGenerated Sea States
Regular waves
This method of signal generation enables regular monofrequency waves to
be produced with a specific height and period.
Irregular waves - filtered white noise method
This method of signal generation is based on the principle of digitally
filtered white noise and is generated in real time. Three sources of white
noise are available based on either a single shift register, multiple shift
registers or random numbers with a Gaussian distribution. The white noise
source is generated and passed through a digital filter to produce the required
position demanded signal for the paddle. The characteristics of this filter
are obtained by performing a Fast Fourier transformation based on the spectral
shape that has been selected. The advantage of this method is that it creates
sequences which can be varied from a few seconds to several years whilst
only requiring enough memory to hold the sea state parameters. This ability
to create long non-repeating sequences is of great importance when testing
models which have a non-linear response. Extensive tests have been carried
out which demonstrate that the waves produced using this system are realistic
and that they do fit the statistical theories for ocean waves.
The data inputs required are:
> model scale
> spectral shape
> sequence length
> sequence start condition
> run time
The sequence start condition is a very useful feature because it allows exact/y the same random sequence to be generated each time as this is essential for comparative tests.
Besides allowing the user to completely define the spectral shape, many standard shapes are available including:
> Pierson-Moskowitz
> JONSWAP
> ITTC
> 1SSC
> Darbyshire Coastal
> Darbyshire Ocean
> Neuman
Irregular waves - Fourier series method
This method is equivalent to the summation of sine waves of different frequencies
and amplitudes to reproduce the specified energy spectrum. In fact the output
time series is produced by carrying out an inverse Fourier Transform of
the spectral amplitudes after applying the appropriate Transfer Function.
Two options are provided. In the first the frequency components are produced with random phase. In the second the components have random phase and amplitude.
The technique is not suitable for producing signals in real time so the required values are computed and stored in a file. This is read by a 'playback' program when the waves are to be generated. Once created these files may be kept and used repeatedly.
A single FFT operation cannot produce a time series of infinite length and the limit in this system has been set to 2048 points. Longer sequencies are available however, made by carrying out a number of FFTs and merging the individual sequences together to create one of the length which the user has requested.
This method provides all the spectral types which are available from the filtered white noise method.
TOPSpecifications
General
Output signal ± 10 volts
Standard sea states available:
> Pierson-Moskowitz
> JONSWAP
> ITTC
> ISSC
> Darbyshire Coastal
> Darbyshire Ocean
> Numan
Sea states may be defined by:
> Windspeed
> Wind speed and Fetch
> Wave frequency and height
> Frequency
> 16 Ordinates of spectral density
Random waves may be based on:
> White noise with single or multiple shift registers or random numbers,
all with variable test length > Fourier series with variable or fixed
amplitude, an with variable test length
> User defined techniques
Transfer Functions:
> Automatic recompilation of the paddle demand signal with changing Transfer Function
> Interpolation of Transfer Function between defined frequencies
The software will run on any IBM compatible 386 PC or better. The use of a maths co-processor is advisable and the PC must have two spare expansion slots.
TOP
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