Θέση σε λειτουργία και μετρήσεις με το νέο ηχοβολιστικό σύστημα κυματομετρήσεων του εργαστηρίου λιμενικών έργων

Abstract

Το αντικείμενο της παρούσας έρευνας είναι η θέση σε λειτουργία του νέου εργαστηριακού ηχοβολιστικού συστήματος κυματομετρήσεων και η πρώτη του εφαρμογή με την συλλογή και επεξεργασία μετρήσεων στο φυσικό προσομοίωμα της επέκτασης του επιβατικού λιμένα Πειραιά. Οι μετρήσεις έλαβαν χώρα στο Εργαστήριο Λιμενικών Έργων (Ε.Λ.Ε.) της Σχολής των Πολιτικών Μηχανικών του Εθνικού Μετσόβιου Πολυτεχνείου. Τα πειράματα διεξήχθησαν στην τρισδιάστατη Δεξαμενή Δοκιμών 2 του εργαστηρίου. Συγκεκριμένα πραγματοποιήθηκαν μετρήσεις κυμάτων στις νέες νηοδόχους και λιμενολεκάνες μετά την κατασκευή έργων επέκτασης του επιβατικού λιμένα Πειραιά για τέσσερις διαφορετικές κυματικές συνθήκες από νοτιοδυτική διεύθυνση πρόσπτωσης κυματισμών. Ελέγχθηκαν δύο διατάξεις έργων, μία με διάταξη απορροφητικού και μία με διάταξη κατακόρυφου μετώπου. Οι μετρήσεις επεξεργάσθηκαν με χρήση κατάλληλων λογισμικών και προέκυψαν υδροδυναμικά μεγέθη των κυμάτων στις θέσεις μέτρησης. Επίσης, υπολογίσθηκε ο συντελεστής μετάδοσης ως ο λόγος του μεταδιδόμενου προς τον προσπίπτοντα κυματισμό. Αξιοποιώντας τους συντελεστές μετάδοσης με χρήση κατάλληλου λογισμικού παρήχθησαν ισοπαραμετρικές καμπύλες σε δισδιάστατη μορφή. Τέλος, πραγματοποιήθηκε η καταγραφή και παρουσίαση των συμπερασμάτων της παρούσας έρευνας.

Introduction The purpose of the present study is to activate the new sonar wave measuring system and to take measurements at the physical model of the extension of Piraeus harbour. The measurements took place at the Laboratory of Harbor Works (LHW) of the Civil Engineer Department, in the National Technical University of Athens. The experiments took place in the 3D wave basin no 2 of the laboratory. More specifically, wave measurements were taken at the port installations which were created after the construction of expansion works in Piraeus harbour. Four different wave conditions from the southwest direction were tested. Two different layouts were examined , one with absorbent quay wall and one with vertical quay wall.

Remarks on the operation of the sonar wave measuring system • The sonar wave measuring system has the ability to measure from 3 to 25 cm “eye”-water level distance (optimum distance of approximately 15 cm). Resistance meters can not be calibrated in so small depths (~ 2cm). • The high-precision sound measurement sensor, which fits the amplifier should be linked closely to the sensors and be safe from possible interference, to properly record the speed of the sound and the sensors to be correctly calibrated. • In some cases, the measurement was not adequately taken either due to the fact that some of the eight meters in one location could not take measurement, or there was an interruption during an experiment. Therefore, an insulating material was placed around of the meters’ “eyes” and a rather large removal between each other was sought. Generally, meters have proved sensitive to interference. • The recording of the measurements established that in some of the measurements appeared “noise” and hence the need to limit the time of the experiment. • The record frequency for the taken measurements was 50Hz. For high wave speed or higher measurement resolution the record frequency is often 100Hz.

Experimental setup The basic experimental installation was the 3D wave basin no 2 of the Laboratory of Harbor Works at National Technical University of Athens, which has internal dimensions 35,20 x 27, 75m and depth to 1,0 m (Figure 2.1).

Figure 1: wave basin of the Laboratory of Harbor Works

The output random waves system is installed in the tank. Dampening walls of ballast were built in order to absorb the incident waves in the walls of the tank. The output random waves system moves hydraulic and consists of the following parts: 1. Hydraulic machinery-engine 2. Hydraulic pistons 3. Three wave paddles 4. Control system 5. Appropriate software The commands for the operation of the hydraulic system are provided through a computer using the appropriate software on the control system. The software used for the production of the wave packet is “WAVEGEN SD” of the English Company ‘’H.R Wallingford’’, which has the capability to produce monochromatic (sinusoidal) ripples and spectral disruptions of various forms – random waves. Similarity scales For the choice of the appropriate similarity scale we have taken into account the following parameters:  The dimensions of the installation in relation to the dimensions of the area, which was simulated, and  The ability to satisfactorily simulate the ripples in the location of the structures. The punctual simulation of mechanisms (reflection, refraction, diffraction) requires the model to have the same similarity scale to the three dimensions. The scale, according to which the linear dimensions of the model are simulated, is known as geometric simulation scale (λ). Having defined the geometric simulation scale, the scales for the hydrodynamic parameters can be calculated according to the laws of similarity. In this model we have λ = 125. The similarity ratio values of the various parameters are shown in the following table: Parameter Scale Value Length λ = λ 125 Time λρ = λ1/2 11,18 Velocity λΤ= λ1/2 11,18 Force λΔ = λ3 1.953.125,00 Volume λο = λ3 1.953.125,00 Mass λμ= λ3 x (Μp/Mm) 2.031.250,00 Acceleration 1 1

Table 1 : Similarity ratio values of the various parameters The physical model was constructed with lightweight concrete and the slope of the gradients was chosen so that the reflection coefficient is very small. Provisions layout Two constructed layouts were checked, one with absorbent and one with vertical quay wall.

Figure 2 : Layout with vertical quay wall (simulated with smooth iron plate )

Figure 3 : Layout with absorbent quay wall (simulated with perforated iron plate)

The extension of the Piraeus port is carried out in three stages: • In the first stage facilities for cruise ships are being constructed. • In the second stage facilities of servicing coastal ships are being constructed. • In the third stage facilities of servicing tourist ships are being constructed.

Measurements processing The sonar wave sensors measure the distance from the location of sensor’s “eye” up to the free surface of the water on all the selected locations within the physical model. These measurements with the use of the software HF108TOWVD are transformed in a change of the middle level, and in such a form that they can undergo further process. Thereafter, the measurements are edited, with the help of the program “H.R.Wavedata”, the spectrum of waves and a number of parameters is calculated. After we have defined all parameters for the processing, we take the output files, which contain tabular attributes of the following sizes: • Characteristic wave height Hs (mm) • Maximum wave height Hmax (mm) • Average of 10% of the highest wavesH 1/10 (mm) • Average of 33.3% of the highest wavesH 1/3 (mm) • Maximum spectrum period Tp (sec) • Average spectrum period Tm (sec)

Based on the above, tables were created, such as the one shown in the picture below, with the counters information separated, per counter and per number of test. Test number 8 Hs (mm) Hmax (mm) H10 (mm) H3 (mm) Tp (sec) Tm (sec) probe1 13.64 24.33 17.10 13.28 0.64 0.55 probe2 11.99 21.26 14.94 11.72 0.54 0.56 probe3 11.43 25.27 14.44 11.10 0.60 0.57 probe4 10.46 20.49 13.37 10.23 0.60 0.54 probe5 11.14 22.36 13.83 10.91 0.57 0.54 probe8 9.83 20.31 12.38 9.69 0.57 0.55 Test number 6 Hs (mm) Hmax (mm) H10 (mm) H3 (mm) Tp (sec) Tm (sec) probe1 36.10 59.40 44.30 35.50 0.64 0.56 probe2 26.70 51.00 33.10 26.20 0.64 0.58 probe3 23.60 43.90 29.50 23.20 0.64 0.56

Table 2 : Test results per counter . Due to the fact the measurements were not simultaneous, we calculated the transmission coefficient, as the ratio of the size (e.g. wave height) which occurs at a specific location within the port of Piraeus, to the size (e.g. wave height) on a specific checkpoint.

The following illustration shows a table that contains transmission coefficients. Kt (Hs) Kt (Hmax) Kt (H10) Kt (H3) 0.51 0.48 0.52 0.51 0.45 0.42 0.45 0.45 0.43 0.50 0.44 0.42 0.39 0.40 0.40 0.39 0.42 0.44 0.42 0.42 0.37 0.40 0.37 0.37 Table 3 : Transmission coefficients for the hydrodynamic sizes

In the context of processing, display diagrams of all featured sizes were created for the four wave situations, for both absorbent quay wall layout and vertical quay wall layout.

Isometric curves The “gridding” method which was used to create the grid, is the “Kriging” method which consists of a linear variogram. The “Kriging” method’, uses map points to export data for areas that don't have data. This leads to the minimum and maximum values of Z in the grid that is beyond the values in the data file. Indicatively, an isometric curves map is given by the following figure:

Figure 4: Isometric curves for the transmission coefficient Hs Measurement results This chapter refers to the presentation of the results based on the tables of transmission coefficients per measuring subarea and per wave disturbance in relation to both the absorbent and vertical quay wall layout. These results are confirmed by the isometric curves maps, created by Surfer software. Conclusions In this chapter, the conclusions reached by this study are being presented, in the context of experimental measurements using the new sonar wave measuring system.

 From the counters’ operation the following conclusions are reached: • The new sonar wave measuring system makes possible the wave measurement even at a very low water depth (~ 2cm). • The sonar counters take very accurate experimental measurements. • For relatively low wave speed the 50Hz frequency is recommended in a measurement, while for high wave speed or higher measurements’ resolution a 100Hz frequency is recommended. • The high-precision sound measurement sensor should be placed near the counters and be safe from possible interference, in order to correctly record the speed of the sound. • During the measurements’ recording “noise” appeared in certain measurements and hence it was needed to limit the time of the experiment. • The counters have proved to be very sensitive and reliable measuring waves instruments.

 By measurements processing, the following conclusions were concluded: • By careful examination of the transmission coefficients’ tables, it was concluded that the differences found between vertical and absorbent quay wall is important. In particular: o In the southern part of the constructions’ expansion of the Piraeus port, (cruise ships area, south channel and south coastal platforms) it is noticed that there is an encumbrance of the order of 8% of the vertical quay wall. o In the central part of the port (leeward dock side and coasters) there is is an encumbrance ~ 7% • The isometric curves’ maps, show satisfactorily the wave disturbance within the new constructions. In particular: o In the subsistent port , the incoming wave disturbance is small. o In the cruise ships area the western dock is more biased. o The area in front of coastal platforms is very wave biased. o The north coastal constructions’ dock is also significantly biased. o The wave disturbance in the marina area is high. • The influence of the vertical quay wall is obvious in the maps, from the appearance of higher rated transmission curves, particularly in the southern and central part of the port. • Largest measurements’ number using the sonar counter meters would lead to a more precise result.