Experimental-laboratory study of the flow around mussel shocks
Abstract
The productivity in an organized mussel culture area is closely related to the hydrodynamics in the area where the mussel units are located. The interaction between the hydrodynamics and mussel farming in Chalastra (NW Thessalonikigulf) has been investigated during last decades. In the framework of the study of optimizing the quality of mussels production in mussel farming areas, a laboratory channel was designed, where the flow around and possibly through the mussel shocks would be studied in physical scale. The experiments were designed in physical/natural scale and the relevant variables were determined. Moreover, the specific positions for the measurements, the depth of the flow and the velocity currents were also determined. The following three mean velocity values of entrance water velocity U were used in the experiment; 5 cm/sec, 7 cm/sec and 9cm/sec. A basic research parameter used in the experiment was the distance between the mussel shocks. Four cases were taken into account: 300mm, 500mm, 700mm and 900mm. The final goal was the determination of the velocity field in the areas around the shocks. The velocity field was studied with the modern Particle Image Velocimetry technique. According to the above presented experiments, for distances between the shocks greater than 500 mm the velocity field is almost restored. Furthermore the case of larger distance between the shocks (i.e. 90cm) present the largest percentage of the velocity class 5-10cm/sec(occurring for entrance current velocities 7 and 9 cm/s)which seem to be the best range for mussel’s growth.
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Introduction
It is well known that the productivity in a mussel farm is closely related to the hydrodynamics in the area. That significant factor, i.e. the hydrodynamics in a mussel farm has been extensively investigated during last decades in the coastal area of Chalastra (W-NW gulf of Thessaloniki, North Greece). Thus, such relevant studies for the specific area started to take place, in a more insistent and organized way, at the beginning of the 21st century. More specifically, [1] realized field measurements in the area which showed that currents are quite weak in the area of the mussel farms. Galinou-Mitsoudi et al. [2] later worked on a mathematical simulation showing that field measurements, mentioned before, were very well approximated by the results of a coarse hydrodynamic model. [3], [4] and [5] conducted field and numerical experiments while [6]and [7]studied mussel cultures and hydrodynamics with the help of mathematical simulations and the development of general management tools.These afore mentioned research works finally led to the findings listed below: (a) the largest proportions of current speeds (>40%) were recorded between 0 and 5 cm/s, which corresponded to very weak currents, suitable only for low density farming, (b) the longest treatment configuration (from the four configurations corresponding to distance between shocks 30 or 50 or 70 or 90 cm)i.e. shock distance 90 cm, led to the larger values of current speeds (90cm treatment had the largest percentage for the class 5-10cm/sec which is the most suitable current speed for the mussel farming activity according to [8] (c) there was a great variability of the current direction at the long line level and (d) it was observed that sometimes the current, moving towards the mussel unit deviates its route and is not entering inside the farm, which may lead to less food availability inside the farm.[9] in the framework of his doctoral research conducted both field and laboratory experiments however the latter experiments were based on the use of cylinders. The flow over two in-line cylinders in laminar and turbulent flows was also studied with the help of numerical simulations by [10].Furthermore, [11] studied the turbulent-flow characteristics and the mechanism of vortex shedding behind one and two square obstacles centered inside a 2-D channel. The study was based on the use of large eddy simulation and finite-element technique.
Conclusion
The following conclusions have been raised from the experimental procedure and the analysis of the data collected.
a) For the distance between the mussel shocks 500 and 700 mm the velocity profiles (fig.6και10) seem to have the same values in the central area of the shock for the different entrance (initial) current velocities with small divergences concerning the velocity profiles for initial current velocity 9 cm/sec
b) For distance between the shocks 900 mm, the distinction (difference) of velocity profiles is more clear (intense) for the different entrance (initial) current velocities. Moreover the case of larger distance between the shocks (i.e. 90cm) seems to present the largest percentage of the velocity class 5-10cm/sec that seems to occur for initial entrance current velocities 7 and 9 cm/s, which is in line with previous research findings in the field.
c) For shock distances between the shocks greater than 500 mm the velocity field is almost restored. This is obvious from the pattern of the velocity fields and the velocity profiles.
d) The comparison of the diagrams with the velocity profiles show that the increase of distance between the shocks leads to increase of the velocity values. For values of the initial current velocity 5 cm/sec the velocity profiles for distances 700 and 900 mm nearly coincide
e) From the velocity fields it is clear that the vorticities seem to be intense due to spottiness of the external surface of the mussel shocks. It is this fact that causes reverse water flow.
f) In large values of the mean water velocities, large returning velocities are observed while the resultant velocities (mean values in time and space) are decreasing significantly close to the mussel shocks. Those mean values close to the shocks are of the order of 1 cm/sec or below this value.
g) Intense formation of eddies in front of each mussel shock is observed.