A numerical and experimental study on tank wall influences in drag estimation

This study has been undertaken to quantify the tank wall effects on resistance estimation of ship models. Given the finite width of a tank, the flow around a ship model has been numerically modelled and the pressure and pressure related drag have been estimated.Since the model runs at speeds essentially in the laminar and transient speed range, an inviscid model has been chosen for obtaining the pressure drag component in the numerical studies. Grid dependency study has been done to optimize the mesh in the control volume for the numerical studies. An unstructured grid consisting of hexahedral cells has been used in the volume of fluid (VOF) model. The model chosen is a medium speed, ocean going barge and the residuary resistance has been obtained for different tank width conditions. The tank width has been defined using a non-dimensionalized parameter W/B (tank width W, model width B) ratio. The study shows that the residuary resistance obtained at W/B=5.0 is free from tank wall influence for the chosen model. The findings of the study have been compared by testing two geosim models under the same tank width conditions. The residuary resistance values have been compared with numerical results. The combined numerical experimental approach provides interesting results of consistency for comparison. The tank wall influences suggested by the numerical study are well quantified in the experimental study and give useful guideline for limiting wall influences.     

Adsorption of Metal Ions onto the Chemically Modified Agricultural Waste

This paper discusses about the adsorption of metal ions such as Cu(II), Cd(II), Zn(II), and Ni(II) from aqueous solution by sulfuric acid treated cashew nut shell (STCNS). The adsorption process depends on the solution pH, adsorbent dose, contact time, initial metal ions concentration, and temperature. The adsorption kinetics was relatively fast and equilibrium was reached at 30 min. The adsorption equilibrium follows Langmuir adsorption isotherm model. The maximum adsorption capacity values of the modified cashew nut shell (CNS) for metal ions were 406.6 mg/g for Cu(II), 436.7 mg/g for Cd(II), 455.7 mg/g for Zn(II), and 456.3 mg/g for Ni(II). The thermodynamic study shows the adsorption of metal ions onto the STCNS was spontaneous and exothermic in nature. The kinetics of metal ions adsorption onto the STCNS followed a pseudo‐second‐order kinetic model. The external mass transfer controlled metal ions removal at the earlier stages and intraparticle diffusion at the later stages of adsorption. A Boyd kinetic plot confirms that the external mass transfer was the slowest step involved in the adsorption of metal ions onto the STCNS. A single‐stage batch adsorber was designed using the Langmuir adsorption isotherm equation.     

Magnetic helicity in stellar dynamos: new numerical experiments

The theory of large scale dynamos is reviewed with particular emphasis on the magnetic helicity constraint in the presence of closed and open boundaries. In the presence of closed or periodic boundaries, helical dynamos respond to the helicity constraint by developing small scale separation in the kinematic regime, and by showing long time scales in the nonlinear regime where the scale separation has grown to the maximum possible value. A resistively limited evolution towards saturation is also found at intermediate scales before the largest scale of the system is reached. Larger aspect ratios can give rise to different structures of the mean field which are obtained at early times, but the final saturation field strength is still decreasing with decreasing resistivity. In the presence of shear, cyclic magnetic fields are found whose period is increasing with decreasing resistivity, but the saturation energy of the mean field is in strong super‐equipartition with the turbulent energy. It is shown that artificially induced losses of small scale field of opposite sign of magnetic helicity as the large scale field can, at least in principle, accelerate the production of large scale (poloidal) field. Based on mean field models with an outer potential field boundary condition in spherical geometry, we verify that the sign of the magnetic helicity flux from the large scale field agrees with the sign of α. For solar parameters, typical magnetic helicity fluxes lie around 10⁴⁷ Mx² per cycle.     

Intervention and assessment of earthquake knowledge at rural schools near the New Madrid seismic zone,USA

Natural Hazards - Possible recurrence of large earthquakes such as the 1811–1812 New Madrid sequence presents a significant hazard in the Central United States. The efficacy of earthquake...     

Automated Detection of Solar Radio Bursts Using a Statistical Method

Solar Physics - In early September 2017, a series of solar flares and coronal mass ejections (CMEs) erupted from the Sun. The Cor2a coronagraph, a unit of the Sun Earth Connection Coronal and...     

Angular momentum transport in quasi-Keplerian accretion disks

We reexamine arguments advanced by Hayashi & Matsuda (2001), who claim that several simple, physically motivated derivations based on mean free path theory for calculating the viscous torque in a quasi-Keplerian accretion disk yield results that are inconsistent with the generally accepted model. If correct, the ideas proposed by Hayashi & Matsuda would radically alter our understanding of the nature of the angular momentum transport in the disk, which is a central feature of accretion disk theory. However, in this paper we point out several fallacies in their arguments and show that there indeed exists a simple derivation based on mean free path theory that yields an expression for the viscous torque that is proportional to the radial derivative of the angular velocity in the accretion disk, as expected. The derivation is based on the analysis of the epicyclic motion of gas parcels in adjacent eddies in the disk.     

Primordial magnetic fields in the post-recombination era and early reionization

We explore the ways in which primordial magnetic fields influence the thermal and ionization history of the post-recombination Universe. After recombination, the Universe becomes mostly neutral, resulting also in a sharp drop in the radiative viscosity. Primordial magnetic fields can then dissipate their energy into the intergalactic medium via ambipolar diffusion and, for small enough scales, by generating decaying magnetohydrodynamics turbulence. These processes can significantly modify the thermal and ionization history of the post-recombination Universe. We show that the dissipation effects of magnetic fields, which redshifts to a present value   B ₀= 3 × 10⁻⁹ G  smoothed on the magnetic Jeans scale and below, can give rise to Thomson scattering optical depths  τ≳ 0.1  , although not in the range of redshifts needed to explain the recent Wilkinson Microwave Anisotropy Probe ( WMAP ) polarization observations. We also study the possibility that primordial fields could induce the formation of subgalactic structures for   z ≳ 15  . We show that early structure formation induced by nanoGauss magnetic fields is potentially capable of producing the early reionization implied by the WMAP data. Future cosmic microwave background observations will be very useful to probe the modified ionization histories produced by primordial magnetic field evolution and constrain their strength.     

Catastrophic debris flows triggered by the 20 August 2019 rainfall,a decade since the Wenchuan earthquake,China

Landslides - Enhanced debris flow activity observed after the 2008 Wenchuan earthquake, Sichuan Province, SW China, is still intense more than a decade since the earthquake. A heavy rainstorm on 20...     

Radio frequency self-interference from a data processing centre at a radio telescope site

Apart from externally generated Radio Frequency Interference (RFI), the occurrence of self-interference is a major concern at any modern radio telescope site. Antenna servo motor controllers, data acquisition processors, and fast computing capabilities operate very close to extremely sensitive and wideband radio astronomical receivers. In this paper, we present a set of measurements of the RFI level generated by a cluster of computers that will be installed at the site of the Sardinia Radio Telescope (SRT). The measured levels are compared to Recommendation ITU-R RA.769-2, which gives the threshold levels for interference detrimental to radio astronomy observations. Our analysis shows that, with proper shielding of the noisiest devices, it will be possible to preserve the present excellent RFI conditions of the SRT site.