Coupling of RANS and BEM Solvers for Simulation of Propeller Behind the Hull in Full Appended Model

To design a propeller for ship power plant,the interaction between ship hull and propeller must be taken into account.The main concern is to apply the wake effect of ship stern on the propeller performance.In this paper,a coupled BEM(Boundary Element Method)/RANS(Renolds-Averaged Navier−Stokes)solver is used to simulate propeller behind the hull in the self-propulsion test.The motivation of this work is to develop a practical tool to design marine propulsion system without suffering long computational time.An unsteady boundary element method which is also known as panel method is chosen to estimate the propeller forces.Propeller wakes are treated using a time marching wake alignment method.Also,a RANS code coupled with VoF equation is developed to consider the ship motions and wake field effects in the problem.A coupling algorithm is developed to interchange ship wake field to the potential flow solver and propeller thrust to the RANS code.Based on the difference between hull resistance and the propeller thrust,a PI controller is developed to compute the propeller RPM in every time step.Verification of the solver is carried out using the towing tank test report of a 50 m oceanography research vessel.Wake factor and trust deduction coefficient are estimated numerically.Also,the wake rollup pattern of the propeller in open water is compared with the propeller in real wake field.

Validation and Coupling of the SWAN Wave Prediction Model by WRF for the Persian Gulf

Generation of waves is affected by forces that exerted constantly in the oceans. The most obvious reason for the appearance of surface-waves is a process of interaction between atmosphere and sea surface that results in wind generation. Wave predictions are usually issued for a maximum of a few days for using in different fields such as shipping, fishing, oil industry, tourism, and to increase the safety of seafarers and beach habitants, maintaining economic assets and optimal utilization of natural resources. In this study, SWAN model has been run for this research over the Oman sea and the Persian Gulf. For implementation of SWAN, another dynamic model with prediction ability of 99-hours also has been used. In this example, wind field is obtained from the outputs of the WRF model converted to the required format for SWAN model. The computational network of SWAN model has been set to spatial grid points of 6 minutes with 1-hour temporal scale. Standard validation ways, including experimental verification, Multiplicative Bias, Mean Error and Root Mean Square Error are used in this study by comparing together for evaluation of accuracy of the model outputs. The results show that the prediction of wave heights by the model for 9 to 24-hour prediction could be the most accurate.

Gorgan Bay： a microcosm for study on macrobenthos speciesenvironment relationships in the southeastern Caspian Sea

The relationship between spatial patterns of macrobenthos community characteristics and environmental conditions（salinity, temperature, dissolved oxygen, organic matter content, sand, silt and clay） was investigated throughout the Gorgan Bay in June 2010. Principal components analysis（PCA） based on environmental data separated eastern and western stations. The maximum（4 500 ind./m2） and minimum（411 ind./m2） densities were observed at Stas 1 and 6, respectively. Polychaeta was the major group and Streblospio gynobranchiata was dominant species in the bay. According to Distance Based Linear Models results, macrofaunal total density was correlated with silt percentage and salinity and these two factors explaining 64% of the variability while macrofaunal community structure just correlated with salinity（22% total variation）. In general, western part of the bay showed the highest number of species and biodiversity while, the highest density was found at Sta. 1 and in the middle part of the bay. Furthermore, relationship between diversity indices and macrobenthic species with measured factors is also discussed. Our results confirm the effect of salinity as an important factor on distribution of macrobenthic fauna in south Caspian brackish waters.

Water-Level Fluctuations of Urmia Lake: Relationship with the Long-Term Changes of Meteorological Variables (Solutions for Water-Crisis Management in Urmia Lake Basin)

Urmia Lake in northwest of Iran, through the recent years has been extremely faced with the water crisis. Climate variations and anthropogenic impacts could be two main affiliated factors in this regard. We considered the long term data series of precipitation, temperature and evaporation in monthly and yearly scales in order to compare to water-level values of Urmia Lake. The statistics approaches such as: standard deviation, trend analysis, T test, Pearson and Spearman correlations, liner regression are used to analyze all variables. The results released that the water-level of Urmia Lake along with the precipitation and temperature of the lake’s basin have experienced the periodic changes through 1961 to 2010, as there are some gradual dryness trends on the study area according to precipitation and temperature variations. Urmia Lake periodic water-level fluctuations show more significant correlation to temperature than the precipitation. Whiles, the water-level’s decreasing behavior especially through 1998 to 2010 is more harsh and different than the rate that is considered for precipitation’s decrease and temperature’s increase. Thus, there could be some anthropogenic factors in the basin which produced some supplementary causes to shrink Urmia Lake. Extracting the double precipitation over the basin through introducing and categorizing of atmospheric synoptic systems in order to cloud seeding operation could be one of urgent and innovative solutions to mitigate water crisis in the basin.

使用非定常粘性和无粘求解器研究双螺旋桨在不同船舶尾流场中的性能

In this study,the performance of a twin-screw propeller under the influence of the wake field of a fully appended ship was investigated using a coupled Reynolds-averaged Navier–Stokes(RANS)/boundary element method(BEM)code.The unsteady BEM is an efficient approach to predicting propeller performance.By applying the time-stepping method in the BEM solver,the trailing vortex sheet pattern of the propeller can be accurately captured at each time step.This is the main innovation of the coupled strategy.Furthermore,to ascertain the effect of the wake field of the ship with acceptable accuracy,a RANS solver was developed.A finite volume method was used to discretize the Navier–Stokes equations on fully unstructured grids.To simulate ship motions,the volume of the fluid method was applied to the RANS solver.The validation of each solver(BEM/RANS)was separately performed,and the results were compared with experimental data.Ultimately,the BEM and RANS solvers were coupled to estimate the performance of a twin-screw propeller,which was affected by the wake field of the fully appended hull.The proposed model was applied to a twin-screw oceanography research vessel.The results demonstrated that the presented model can estimate the thrust coefficient of a propeller with good accuracy as compared to an experimental self-propulsion test.The wake sheet pattern of the propeller in open water(uniform flow)was also compared with the propeller in a real wake field.

Spectral Wave Modeling in Very Shallow Water at Southern Coast of Caspian Sea

This study evaluates the capability of the Simulating WAves Nearshore(SWAN)wave model(version 41.01)in predicting significant wave height and spectral peak energy content for swell waves in very shallow water of surf zone during depth-induced wave breaking and dissipation.The model results were compared with field measurements at five nearshore stations.The results demonstrated that some breaker index formulations were successful for significant wave height prediction in surf zones.However,an incorrect shape of the energy spectrum and overestimated near spectral peak energy content at shallow water stations were obtained using all of the embedded depth-induced wave breaking formulations in SWAN.The dependent breaker index on relative depth(Kpd)formulation,which was successful in predicting near spectral peak energy content,resulted in an average error of 30%.Finally,this formulation was modified to enhance the model performance in reproducing the spectral peak energy content.

Seasonal variation of the double diffusion processes at the Strait of Hormuz

Profiles of salinity and temperature were measured in the strait of Hormuz（SH） during the winter of 2012, spring and summer of 2013. To investigate the double diffusion（DD） processes, Turner（TU） angle values are calculated in all the stations in the SH. Different TU angle values correspond to salt fingering（SF）, diffusive convection（DC）and stable stratification. The distributions of the two forms of DD were plotted vertically along transects in the eastern, central and western part of the SH, and corresponding DD processes were described. The results show that both SF and DC occurred in most part of the study area. Two different water masses（the Indian Ocean surface water and the Persian Gulf water） were evident at the SH, and SF and DC were evident at the interface of two water masses. Due to evaporation, SF occurred in the surface layer of most Stations throughout the year. In the eastern part of the SH, occurrences of DC were more feasible in wintertime. SF was the main phenomenon at the end of hot season. For central part, SF occurred throughout the year in water column. In the western part,water column was stable in summer and DC happened in most part of water column in winter.

Nematode community structure in relation to metals in the southern of Caspian Sea

Spatial distribution and structure of nematode assemblages in coastal sediments of the southern part of the Caspian Sea were studied in relation to environmental factors. By considering metals, organic matter, Shannon diversity index（H）, maturity index（MI） and trophic diversity（ITD）, ecological quality status of sediment was also determined. Fifteen nematode species belonging to eleven genera were identified at the sampling sites. Average density of nematode inhabiting in sediment of the studied area was 139.78±98.91（ind. per 15.20 cm^2）. According to redundancy analysis（RDA）, there was high correlation between metals and some species. Based on biological indicators, the studied area had different environmental quality. Generally, chemical and biological indices showed different results while biological indices displayed similar results in more sites.

Assessment of thermal stratification, stability and characteristics of deep water zone of the southern Caspian Sea

Variations of seawater properties and stratification of water column in the southern deep water zone(470 m depth)of the Caspian Sea were evaluated based on field measured data.Variations of physical characteristics(temperature,salinity,density and sound speed)of seawater mainly were limited to the upper 100 m layers.Below this layer,seasonal changes of seawater parameters were minor.Vertical structure of temperature in the southern Caspian Sea waters was characterized by a significant seasonal thermocline approximately between 20 and 50 m depths in summer with a vertical gradient of around 16℃.Destruction of the thermocline occurred with the general cooling of the sea surface water and deepening of the mixed layer during late of autumn and winter.Frequency of stability in water column often across the upper layers(surface mixed and thermocline)was more than deeper levels.Variations of salinity were around 12.4 PSU in the time of measurements.Structures of thermocline and pycnocline were characterized by a significant homogeneity especially in offshore stations during the year.According to high agreement between density and temperature in the Caspian Seawater,a strong seasonal pycnocline was observed in the position of the thermocline.Furthermore,structure of sound speed variations followed vertical variations of seawater temperature.Variations of chemical characteristics(dissolved oxygen and pH)of seawater mainly were continued from sea surface to near the sea bed in deep water zone.