To protect port waters and shores from wind waves, various through breakwaters such as screens, pontoons, grilles, etc. can be used.
The simplicity of such breakwaters and their relatively low cost have repeatedly attracted the attention of many designers. A large number of new options for through breakwaters have appeared: swinging, zigzagging, and others. Many of them, as a rule, give a good result of wave damping in laboratory conditions, but during field tests they largely lose their advantages mainly due to the desire to weaken the mooring forces.
In order to facilitate the design of such breakwaters and to find out the possibility of their use, theoretical and experimental studies of breakwaters were carried out under an agreement with interested organizations: a — type of surface screen, b — type of bottom sill and c — type of vertical grid.
These breakwaters are the most essential elements of any through breakwater (excluding the breakwater from the sketch). Studies have shown that increasing the width (along the wave beam) of breakwaters of types a and b to two or three times their height (immersion) does not lead to any noticeable increase in wave damping.
Our research shows that wave damping in through breakwaters occurs mainly due to their reflection and, to a very small extent, due to energy dissipation during turbulent flow around the wave flow. At the same time, each artificial element of turbulence (gap, grid) will inevitably have a smaller effect than the same (in area) rigid, impenetrable structural element.
We obtained the best result in calculating wave damping by using the following initial expression of the work of hydrodynamic pressure forces (on the structure of a through breakwater).
The coefficient of reflection of waves from single rods; for piles of square cross—section it is equal to 1, and for piles of circular cross-section - 0.5.
The expressions do not take into account the influence of turbulizations, in which a small additional energy dissipation may occur. Using these formulas, we have compiled calculation graphs, from which it can be seen that surface screens have the best wave-damping properties, especially at high magnitudes. Bottom thresholds, on the contrary, are more effective at low p values, but also at very low t values. The curve for vertical square-profile gratings indicates a very weak wave damping in single-row gratings, even with their high density. However, for n rows of lattices, we obtain the expression of the quenching coefficient.
Laboratory verification of the formulas was performed by V. Drozdov and A. Yanshin under the supervision of the author in the wave tray of the Odessa Construction Institute. The height of the waves ranged from 0.20 to 0.075 m, and the length from 2.2 to 1.27 m. Since it is difficult to show experimental points on the attached graph due to the transition of individual points from one curve to another, experimental and theoretical values of wave damping coefficients are given.
Judging by the correlation coefficients, the formula is less satisfactory. This is due, in particular, to the splashing of waves through the threshold.
Unexpectedly high wave damping coefficients were obtained in laboratory studies of vertical gratings. The correlation coefficient turned out to be close to 0.5. The reason for the weak wave damping in this case is probably due to a large-scale effect, the analysis of which has not yet been carried out. Curiously, in the OIIMFa wave tray (0.4 x 0.8 x 12.0), with wave heights of 0.05—0.07 m and a length of 0.8—1 m, the mentioned coefficient turned out to be 0.98.
From the above, we can conclude that, firstly, the energy method for calculating breakwaters of types a and b without turbulence is fully justified and can be performed according to the formulas or according to the attached schedule. Note that the mentioned formula and the phenomenon it reflects are not related to a large-scale effect. In the formula, it is noticeable, so that when converted into nature, you can get an underestimation of the heights of passing waves by 10-15% at Rv ~ N.
Secondly, it can be seen from the expression that with any pliable structure, the values will be less than with a rigidly reinforced one.
Thirdly, additional intricate turbulators that complicate the design of breakwaters and are designed both to enhance wave damping and to reduce stress in mooring lines are not particularly important, since wave damping is more efficient by means of a continuous barrier, equal in area to any turbulizing device (for example, slits with zigzags). Loosening the tension of mooring lines is most easily achieved by reducing the size of breakwaters or increasing the amplitude of their free swing while reducing wave damping. Discover the most exciting flight betting simulator in the App Store Aviator and experience the thrill of flying and betting
