Steady drift force on vertical cylinder - viscous vs. potential

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.     

Soft-sediment deformation structures in the Late Quaternary sediments of Ladakh: Evidence for multiple phases of seismic tremors in the North western Himalayan Region

The exposed Quaternary sections of Ladakh show evidence of seismicity during the late Quaternary. Multiple levels of soft-sediment deformation structures (seismites) are recorded from the Quaternary sediments of the Spituk-Leh, along Indus Suture Zone (ISZ) and the Khalsar palaeolakes, along Shyok Suture Zone (SSZ) and Karakoraum Fault (KF). The studied sections are a part of two major tectonically formed palaeolakes at 35,000 yr BP. Nine levels of seismites from Spituk-Leh and eight levels from Khalsar sections are recorded. The deformed sediments comprise of clay, silts and sand and are restricted to a single stratigraphic layers bounded by undeformed beds suggesting synsedimentary deformation. The various deformational structures identified are simple and complex convolutes, pinch and swell bedding, microfolds and microfaults, flame-like structures, pseudonodules, clay diapirs, ball and pillow structures, pillar structures, sedimentary dykes and mud lenses. The release of stress along the ISZ, SSZ and KF, may have been responsible for inducing seismicity in the area during the late Quaternary times which may have caused liquefaction as a direct consequence of permanent deformation of ground surface due to earthquakes of large magnitudes (>5 intensity). The chronological data suggests release of stresses along ISZ, SSZ and KF in the form of earthquakes between 35,000 yr BP to approximately the Last Glacial Maxima (LGM) as evident form the available chronology.     

Empirical calculation of roll damping for ships and barges

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.     

Approximate methods in short-term wave response distribution

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.     

Dissipative accretion flows around a rotating black hole

We study the dynamical structure of a cooling dominated rotating accretion flow around a spinning black hole. We show that non-linear phenomena such as shock waves can be studied in terms of only three flow parameters, namely the specific energy     , the specific angular momentum (λ) and the accretion rate     of the flow. We present all possible accretion solutions. We find that a significant region of the parameter space in the     plane allows global accretion shock solutions. The effective area of the parameter space for which the Rankine–Hugoniot shocks are possible is maximum when the flow is dissipation-free. It decreases with the increase of cooling effects and finally disappears when the cooling is high enough. We show that shock forms further away when the black hole is rotating compared to the solution around a Schwarzschild black hole with identical flow parameters at a large distance. However, in a normalized sense, the flow parameters for which the shocks form around the rotating black holes are produced shocks closer to the black hole. The location of the shock is also dictated by the cooling efficiency in that higher the accretion rate     , the closer is the shock location. We believe that some of the high-frequency quasi-periodic oscillations may be due to the flows with higher accretion rate around the rotating black holes.     

Study of terrestrial carbon flux by eddy covariance method in revegetated manganese mine spoil dump at Gumgaon,India

The micrometeorological technique of eddy covariance is a powerful tool for characterizing the carbon (C) budget of terrestrial ecosystems. Eddy covariance method was used for estimating Net Ecosystem Exchange (NEE) of carbon dioxide between atmosphere and revegetated manganese mine spoil dump at Gumgaon, India. In this paper, we analyzed the diel CO₂ flux pattern and its response to various physical environmental conditions. The carbon balance of terrestrial ecosystems is particularly sensitive to climatic changes. Study of diel pattern of CO₂ flux showed that carbon uptake was dependent on sunlight. Effect of temperature and latent heat on the CO₂ flux showed that rate of CO₂ uptake increased proportionally, but later declined due to various factors like stomatal response, high evaporative demand, circadian rhythm and/or a combination of all three. Net ecosystem production of revegetated land was found to be 28.196 KgC/ha/day whereas average net carbon release by the ecosystem, through respiration was observed to be 5.433 KgC/ha/day. Thus, quantifying net carbon (C) storage in degraded land is a necessary step in the validation of carbon sequestration estimates and in assessing the possible role of these ecosystems in offsetting adverse impacts of fossil fuel emissions.