Empirical calculation of roll damping for ships and barges     

Subrata Chakrabarti   《Ocean Engineering》2001,28(7):368

For a large floating structure in waves, the damping is computed by the linear diffraction/radiation theory. For most degrees of freedom, this radiation damping is adequate for an accurate prediction of the rigid body motions of the structure at the wave frequencies. This is not particularly true for the roll motion of a long floating structure. For ships, barges and similar long offshore structures, the roll damping is highly nonlinear. In these cases the radiation damping is generally quite small compared to the total damping in the system. Moreover, the dynamic amplification in roll may be large for such structures since the roll natural period generally falls within the frequency range of a typical wave energy spectrum experienced by them. Therefore, it is of utmost importance that a good estimate of the roll damping is made for such structures. The actual prediction of roll damping is a difficult analytical task. The nonlinear components of roll damping are determined from model and full scale experiments. This paper examines the roll damping components and their empirical contributions. These empirical expressions should help the designer of such floating structures. The numerical values of roll damping components of typical ships and barges in waves and current (or forward speed) are presented.  相似文献   

Approximate methods in short-term wave response distribution     

Subrata K. Chakrabarti 《Ocean Engineering》1989,16(4)

The design and performance of an offshore structure depends largely upon the response of the structure to the environmental loading such as waves. The extreme response chosen for the design of a structure should meet its lifetime response, operational response as well as the fatigue damage. The failure of the structural member may be caused by the maximum instantaneous stress experienced by the member due to a given environment. This is considered short-term as opposed to long-term or fatigue damage. The short-term response statistics are obtained on the basis of one particular seastate. Since this seastate is invariably high, nonlinearity in the excitation and response of the structure is almost invariably present. The general nonlinear problem in the extreme response prediction is largely unsolved. Response characteristics are often obtained from the perturbation methods and equivalent linearization techniques. Unlike nonlinear problems, these methods greatly simplify the analysis for extreme values. This paper reviews the available approximation techniques in the response computation and the limits of their applicability in a design situation. Results are illustrated so that a designer may evaluate the suitability of a method in a particular design condition.  相似文献   

Steady drift force on vertical cylinder - viscous vs. potential     

Subrata K. Chakrabarti 《Applied Ocean Research》1984,6(2):73-82

The contribution of the steady drift force on a floating structure may arise from waves, wind and current. The component of the wave drift force may be due to the second-order diffraction theory or potential effect and may be due to the velocity squared force or viscous effect. The presence of current in waves increases the effect of the viscous force. The expressions for these terms for a vertical cylinder are derived and their relative importance is investigated. Plots are presented showing the regions where the viscous or potential drift force predominates. Experiments were conducted with both small and large diameter cylinders. The mean drift forces obtained in these tests are compared with the theory.  相似文献   

Thermal effect on the pulses of the rotating Sun     

G. C. Das  Smriti Sarkar  B. Karmakar 《Astrophysics and Space Science》1984,106(2):265-272

In this paper, a theoretical investigation is undertaken of the group travel-time for the Sun's pulses to travel from the source of the pulse to the solar surface. For mathematical simplicity, we consider a simple ionized model of the Sun that includes the thermal effect and rotation with uniform angular velocity. The expression for the group travel time gives two terms: the term arising from the linear theory which varies inversely to (2-)^1/2 and the term introduced by the thermal effect which is inversely proportional to (2-)^7/2. The thermal effect variation has been shown and an estimation of the temperature of the medium has been made. Furthermore, the velocity distribution and the amplitude of the magnetic field of the wave, arising from the damping of ionized particles, have been calculated.  相似文献   

Studies on flow characteristics of coal–oil–water slurry system     

Subrata Kumar Majumder  Kamal ChandnaDhiren Sankar De  Gautam Kundu 《International Journal of Mineral Processing》2006

Experiments were carried out to investigate the rheological properties of coal–oil–water suspension containing solids of different sizes. Two different coal samples with mean particle sizes of 120 mesh, 175 mesh and 220 mesh were used. The coal concentration was varied from 5% to 25% by weight. Sodium silicate has been used as an additive to study the behavior of the variation of average viscosity of the suspension. A generalized correlation has been developed to predict the average viscosity of suspension in terms of particle diameter of the coal, concentration of coal, viscosity of the suspending medium and the concentration of water. Experimental investigations revealed that coal–oil–water suspensions show an increase in the viscosity with decrease in coal size but with the addition of an additive, the average viscosity tends to decrease initially up to a certain optimum dosages and thereafter it increases with further addition of additives. Two empirical correlations are proposed for average viscosity of the coal–oil–water suspension, μ_sL in terms of physical properties of the solid and viscosity of the suspending medium with and without additives.  相似文献   

RS & GIS-based landslide susceptibility mapping of the Balason River basin,Darjeeling Himalaya,using logistic regression (LR) model     

Subrata Mondal  Sujit Mandal 《Georisk: Assessment and Management of Risk for Engineered Systems and Geohazards》2018,12(1):29-44

The present study deals with the preparation of a landslide susceptibility map of the Balason River basin, Darjeeling Himalaya, using a logistic regression model based on Geographic Information System and Remote Sensing. The landslide inventory map was prepared with a total of 295 landslide locations extracted from various satellite images and intensive field survey. Topographical maps, satellite images, geological, geomorphological, soil, rainfall and seismic data were collected, processed and constructed into a spatial database in a GIS environment. The chosen landslide-conditioning factors were altitude, slope aspect, slope angle, slope curvature, geology, geomorphology, soil, land use/land cover, normalised differential vegetation index, drainage density, lineament number density, distance from lineament, distance to drainage, stream power index, topographic wetted index, rainfall and peak ground acceleration. The produced landslide susceptibility map satisfied the decision rules and ?2 Log likelihood, Cox &; Snell R-Square and Nagelkerke R-Square values proved that all the independent variables were statistically significant. The receiver operating characteristic curve showed that the prediction accuracy of the landslide probability map was 96.10%. The proposed LR method can be used in other hazard/disaster studies and decision-making.  相似文献   

Phase transitions in ilvaite,a mixed-valence iron silicate I. A 57Fe Mössbauer study of magnetic order and spin frustration     

Kan Xuemin  Subrata Ghose  B. D. Dunlap 《Physics and Chemistry of Minerals》1988,16(1):55-60

Ilvaite, Ca(Fe²⁺,Fe³⁺)Fe²⁺Si₂O₈(OH) shows two magnetic phase transitions, which have been studied by Mössbauer spectroscopy within the temperature range 120–4 K. The continued charge localization between Fe²⁺ and Fe³⁺ ions in octahedral A-sites causes the Fe²⁺-Fe³⁺ interaction to be ferromagnetic, although the overall magnetic order is antiferromagnetic. The thermal evolution of the hyperfine fields at the Fe²⁺ (A) and Fe³⁺ (A) sites indicates B _hf: 328 and 523 kOe respectively at 0 K and T _N1= 116K. The corresponding values for Fe²⁺ (B) site are: B _hf 186 kOe and T _N2=36K. An additional hyperfine field exists at the Fe²⁺(B) site within the temperature range 116–36K due to short-range order induced by the spin ordering in A sites. The considerable difference between the two magnetic transition temperatures is due to spin frustration, because the Fe²⁺ (B) site occurs on a corner common between two triangles with respect to two sets of Fe²⁺ (A) and Fe³⁺ (A) sites with opposite spin directions.  相似文献   

Abstracts of papers held at the IMA symposium physics of minerals,novosibirsk, USSR, 1978 (appendix)     

K. Langer  John K. Winter  Fujio P. Okamura  Subrata Ghose 《Physics and Chemistry of Minerals》1979,4(1):101-103

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Characteristics of cirrus clouds and its radiative properties based on lidar observation over Chung-Li, Taiwan     

Subrata Kumar Das  Chih-Wei Chiang  Jan-Bai Nee 《Atmospheric Research》2009,93(4):723-735

In this paper, characterization of cirrus clouds are made by using data from ground based polarization lidar and radiosonde measurements over Chung-Li (24.58°N, 121.10°E), Taiwan for a period of 1999–2006. During this period, the occurrence of cirrus clouds is about 37% of the total measurement nights over the lidar site. Analysis of the measurement gives the statistical characteristics about the macrophysical properties such as occurrence height, ambient temperature, and its geometrical thickness while the microphysical properties are interpreted in terms of extinction coefficient, optical depth, effective lidar ratio and depolarization ratio. The effective lidar ratio has been retrieved by using the simulation technique of backscattered lidar signals. The effect of multiple scattering has been taken into the account by a model calculation. Summer (Jun–Aug) shows the maximum appearances of cirrus due to its formation mechanism. It is shown that tropopause cirrus clouds may occur with a probability of about 24%. These clouds are usually optically thin and having laminar in structure with some cases resembling the characteristics similar to that of polar stratospheric clouds (PSCs). The radiative properties of the cirrus clouds are also discussed in detail by the empirical equations with results show a positive feedback on any climate change.  相似文献   

Mineralogy and geochemistry of banded iron formation and iron ores from eastern India with implications on their genesis   总被引：1，自引：0，他引：1  

Subrata Roy  A. S. Venkatesh 《Journal of Earth System Science》2009,118(6):619-641

The geological complexities of banded iron formation (BIF) and associated iron ores of Jilling-Langalata iron ore deposits, Singhbhum-North Orissa Craton, belonging to Iron Ore Group (IOG) eastern India have been studied in detail along with the geochemical evaluation of different iron ores. The geochemical and mineralogical characterization suggests that the massive, hard laminated, soft laminated ore and blue dust had a genetic lineage from BIFs aided with certain input from hydrothermal activity. The PAAS normalized REE pattern of Jilling BIF striking positive Eu anomaly, resembling those of modern hydrothermal solutions from mid-oceanic ridge (MOR). Major part of the iron could have been added to the bottom sea water by hydrothermal solutions derived from hydrothermally active anoxic marine environments. The ubiquitous presence of intercalated tuffaceous shales indicates the volcanic signature in BIF. Mineralogical studies reveal that magnetite was the principal iron oxide mineral, whose depositional history is preserved in BHJ, where it remains in the form of martite and the platy hematite is mainly the product of martite. The different types of iron ores are intricately related with the BHJ. Removal of silica from BIF and successive precipitation of iron by hydrothermal fluids of possible meteoric origin resulted in the formation of martite-goethite ore. The hard laminated ore has been formed in the second phase of supergene processes, where the deep burial upgrades the hydrous iron oxides to hematite. The massive ore is syngenetic in origin with BHJ. Soft laminated ores and biscuity ores were formed where further precipitation of iron was partial or absent.  相似文献