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The efficacy of underwater laser detection is considerably impacted by the intense attenuation of light resulting from the scattering and absorption effects of water. In this study, we present the simulation and design of the underwater Lidar system that integrates the paraxial multi-channel detection strategy to enhance the dynamic range in subsea environments. To evaluate the performance of the system with multiple detection channels, we introduce a multi-channel underwater Lidar simulation (MULS) method based on the radiative transfer Lidar equations. Experimental validations were conducted under varied water conditions to assess the performance of the prototype and validate the simulation results. The measured range accuracy of each channel in the prototype is better than 0.1085 m, and the simulated and measured waveforms exhibit strong correlations, verifying the reliability and validity of the simulation method. The effects of transceiver configuration and the maximum detectable range of different detection methods were also discussed. Preliminary results indicate that the paraxial multi-channel design effectively suppresses near-field backscattering and substantially enhances the maximum detectable range. The findings presented in this study may provide valuable insights for the design and optimization of future underwater laser detection systems. [ABSTRACT FROM AUTHOR]

As a new modulated degree of freedom, orbital angular momentum (OAM) could greatly improve the spectrum utilization of wireless communication.However, the uniform circular array (UCA)-based OAM communication system requires the transceiver array to be static and perfectly aligned, which makes it difficult for OAM to be practically applied.A beam management method based on OAM was proposed, which determined the best beam matching by scanning the beam of receiver and receiver, and the matched beam was used to communicate.Simulation results show that the proposed scheme could effectively reduce the system performance degradation caused by non-coaxial of receiver and transmitter arrays.

The efficacy of underwater laser detection is considerably impacted by the intense attenuation of light resulting from the scattering and absorption effects of water. In this study, we present the simulation and design of the underwater Lidar system that integrates the paraxial multi-channel detection strategy to enhance the dynamic range in subsea environments. To evaluate the performance of the system with multiple detection channels, we introduce a multi-channel underwater Lidar simulation (MULS) method based on the radiative transfer Lidar equations. Experimental validations were conducted under varied water conditions to assess the performance of the prototype and validate the simulation results. The measured range accuracy of each channel in the prototype is better than 0.1085 m, and the simulated and measured waveforms exhibit strong correlations, verifying the reliability and validity of the simulation method. The effects of transceiver configuration and the maximum detectable range of different detection methods were also discussed. Preliminary results indicate that the paraxial multi-channel design effectively suppresses near-field backscattering and substantially enhances the maximum detectable range. The findings presented in this study may provide valuable insights for the design and optimization of future underwater laser detection systems.

In this paper, a modified vertex theory is proposed, and then a non-coaxial plastic model of sand is constructed by introducing the proposed theory into a state dependent dilatancy model. The distinctive feature of this model is that non-coaxial plasticity is only given under the principal stress axis rotation condition. The capability of the model to capture the non-coaxial phenomena is verified by the simple shear test. Furthermore, in order to use the established model to analyze the bearing capacity of foundation, the formulas corresponding to the semi-implicit integration algorithm are derived, and then the applicability of the algorithm to the non-coaxial model is evaluated by simulating the simple shear test of a single element. Finally, the effects of non-coaxiality on the bearing capacity of foundation are analyzed and the results show that the introduction of non-coaxial plasticity reduces the overall stiffness of the foundation model, and neglecting the non-coaxial influences may lead to unsafe designs. [ABSTRACT FROM AUTHOR]

As a new modulated degree of freedom, orbital angular momentum (OAM) could greatly improve the spectrum utilization of wireless communication. However, the uniform circular array (UCA)-based OAM communication system requires the transceiver array to be static and perfectly aligned, which makes it difficult for OAM to be practically applied. A beam management method based on OAM was proposed, which determined the best beam matching by scanning the beam of receiver and receiver, and the matched beam was used to communicate. Simulation results show that the proposed scheme could effectively reduce the system performance degradation caused by non-coaxial of receiver and transmitter arrays. [ABSTRACT FROM AUTHOR]

The unsteady flow of a viscous incompressible fluid due to non-coaxial rotations of a porous disk and a fluid at infinity subjected to a periodic suction through a porous medium has been studied. The velocity field, shear stresses are obtained in closed form. The variations of primary and secondary velocities for different values of non dimensional parameters are depicted in figures.

Adjacent non-coaxial buildings have been widely built owing to complementary functions, while the non-coaxial layout could cause a great torsion effect and seismic-induced damage. This study proposes a negative stiffness-assisted hybrid isolation system and a corresponding earthquake-resilient design methodology to upgrade the multi-performances of the adjacent non-coaxial buildings and linking corridors. The hybrid isolation system includes synthetic employment of various isolators and negative-stiffness amplification systems comprised of nonlinear viscous dampers for adjacent structures, as well as tailored friction bearing for linking corridors. The mechanical property and model of the adjacent non-coaxial buildings are established, based on which a typical case is introduced. Next, the conception of the earthquake-resilient design method and procedure is elaborated, following which the functionality of each component in the hybrid system is investigated. The advantages of the developed hybrid isolation system and design methodology are illustrated by a systematic parametric analysis and time history analysis for the typical case. The results show that the torsional effect can be a dominant problem in non-coaxial adjacent buildings, which can be solved by the flexibly tailored negative stiffness effect produced by the negative stiffness-assisted isolation system. The collaboration within the proposed hybrid isolation system can produce improved isolating and enhanced nonlinear energy dissipation effects on adjacent buildings, simultaneously limiting the isolation displacement without weakening the isolating effect. The optimized friction sliding bearings effectively reduce the relative torsion concentrated on the linking corridors with a maintained function. The hybrid isolation system and earthquake-resilient design can be adopted as a high-efficiency solution to the non-coaxial linked historic buildings with insufficient seismic resistance. • Hybrid isolation system for adjacent non-coaxial buildings (ANBs) with corridors. • Earthquake-resilient design methodology to upgrade multiple performances of ANBs. • Dominant torsional effect solved by the developed hybrid isolation system. • High-level torsion control effect satisfied by optimized bearings with low stiffness. • Tailored friction sliding bearings for corridors with a maintained function. [ABSTRACT FROM AUTHOR]

A novel superconducting tilting-pad bearing was proposed for the advanced research of reusable liquid hydrogen turbopump in liquid rocket. The bearing is a combination of superconducting magnetic bearing and hydrodynamic fluid-film bearing. Since the viscosity of cryogenic fuel to activate superconducting state and form hydrodynamic fluid-film is very low, bearing clearance will be very small. This study focuses on the investigation of superconducting levitation force in this kind of small clearance superconductor-magnet system. Based on Bean critical state model and three-dimensional finite element method, an analysis method is presented to obtain the levitation force under such situation. Since the complicated operational conditions and structural arrangement for application in liquid rocket, center lines of bulk superconductor and magnet rotor will usually be in non-coaxial state. Superconducting levitation forces in axial direction and radial direction under non-coaxial situation are also analyzed by the presented method. [ABSTRACT FROM AUTHOR]

It is normally accepted that materials inside the shear band undergo severe rotation of the principal stress direction, which causes non-coaxiality between the principal stress and principal plastic strain rate. However, classical plasticity flow theory implicitly assumes that the principal stress and the principal plastic strain rate are coaxial; thus, it may not correctly predict the onset of the shear band. In addition, classical continuum does not contain any internal length scales; as a result, it cannot provide a reasonable shear band thickness. In this study, the original vertex non-coaxial plastic model based on the classical continuum is extended to the Cosserat continuum. The corresponding codes are implemented via the interface of the user defined element subroutine in ABAQUS. Through a simple shear test, the effectiveness of the user's codes is verified. Through a uniaxial compression test, the influence of non-coaxiality on the onset, the orientation, and the thickness of the shear band is investigated. Results show that the onset of the shear localization is delayed, and the thickness of the shear band is widened when the non-coaxial degree increases, while the orientation of the shear band is little affected by the non-coaxial degree. In addition, it is found that the non-coaxiality can weaken the micro-polar effect to some extent; nonetheless, the Cosserat non-coaxial model still has its advantage over the classical non-coaxial model in capturing the pre-bifurcation as well as the post-bifurcation behaviors of strain localization. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Within a micromechanical framework, a constitutive model for sand capable of predicting noncoaxiality between stress and strain increments is presented. Anisotropy of elastic and plastic material properties is considered by introducing fabric-like second-order coefficient tensors that arise from a description of the interparticle contacts of sand grains. Strength anisotropy is further accounted for by introducing an interlocking parameter that describes the relative ease of sliding depending on the orientation of potential failure planes. The model is calibrated to and validated against two sets of hollow cylinder triaxial compression experiments consisting of various orientations of major principle stress directions, including tests conducted with the principle stress axes noncoaxial with the material axes. Model predictions of noncoaxiality between stress and strain increments are examined and compared with laboratory measurements. The results show that by considering the arrangement of interparticle contacts, the model is capable of predicting such noncoaxiality without the need to specify a priori any assumptions about the noncoaxial behavior. [ABSTRACT FROM AUTHOR]

• A GMF non-coaxial torsion sensor with sandwich structure is proposed. • The sensor based on intermodal interference is sensitive to torsion. • The sandwich structure is formed by cascading SMF and GMF. • The torsion sensing characteristics of the sensor are analyzed. • The maximum torsion sensitivity is 0.02315 dB/° in 280°∼360°. A sandwich structure on Grapefruit microstructured fiber(GMF) for measurement of non-coaxial torsion is proposed and fabricated. The GMF and the single-mode fiber (SMF) are fused to make the SMF-GMF-SMF structure. The non-coaxial system is used to twist the GMF sensor. Because the two ends of the GMF are not at the same height, a tensile strain is introduced into the fiber. At the same time, the photoelastic effect and the Poisson effect also cause the phase difference to change, resulting in a transmission spectrum shift. The experimental results show that in the range of 0°∼360°, with the increase of the twist angle, the intensity of the transmission mode gradually increases, and the coupling efficiency of the higher-order mode increases. The torsion sensitivity can reaches 0.02315 dB/°. The sensor is simple in fabrication, high in sensitivity, and has a good application prospect in torsion measurement of various devices. [ABSTRACT FROM AUTHOR]

The main conclusion of this study is that non-coaxial strain acting parallel to a flat-lying D 1 spaced cleavage was responsible for the formation of the D 2 spaced crenulation (shear band) cleavage in Dalradian rocks of Neoproterozoic-Lower Ordovician age in the SW Highlands, Scotland. The cm-dm-scale D 2 microlithons are asymmetric; have a geometrically distinctive nose and tail; and show a thickened central portion resulting from back-rotation of the constituent D 1 microlithons. The current terminology used to describe crenulation cleavages is reviewed and updated. Aided by exceptional 3D exposures, it is shown how embryonic D 2 flexural-slip folds developed into a spaced cleavage comprising fold-pair domains wrapped by anastomosing cleavage seams. The bulk strain was partitioned into low-strain domains separated by zones of high non-coaxial strain. This new model provides a template for determining the sense of shear in both low-strain situations and in ductile, higher strain zones where other indicators, such as shear folds, give ambiguous results. Analogous structures include tectonic lozenges in shear zones, and flexural-slip duplexes. Disputes over the sense and direction of shear during emplacement of the Tay Nappe, and the apparently intractable conflict between minor fold asymmetry and shear sense, appear to be resolved. [ABSTRACT FROM AUTHOR]

Rotational shear is the type of loading path where samples are subjected to cyclic rotation of principal stress directions while the magnitudes of principal stresses are maintained constant. This paper presents results from an experimental investigation on the drained deformation behaviour of saturated sand in rotational shear conducted in a hollow cylinder apparatus. Two types of granular materials, Leighton Buzzard sand and glass beads are tested. A range of influential factors are investigated including the material density, the deviatoric stress level, and the intermediate principal stress. It is observed that the volumetric strain during rotational shear is mainly contractive and most of strains are generated during the first 20 cycles. The mechanical behaviour of sand under rotational shear is generally non-coaxial, i.e., there is no coincidence between the principal axes of stress and incremental strain, and the variation of the non-coaxiality shows a periodic trend during the tests. The stress ratio has a significant effect on soil response in rotational shear. The larger the stress ratio, the more contractive behaviour and the lower degree of non-coaxiality are induced. The test also demonstrates that the effect of the intermediate principal stress, material density and particle shape on the results is pronounced. [ABSTRACT FROM PUBLISHER]

Purpose: To assess the scope and determining risk factors related to the development of pneumothorax throughout CT-guided biopsy of pulmonary lesions in coaxial and non-coaxial techniques and the outcome of its management.Materials and Methods: The study included CT-guided percutaneous lung biopsies in 650 consecutive patients (407 males, 243 females; mean age 54.6 years, SD 5.2) from November 2008 to June 2013 in a retrospective design. Patients were classified according to lung biopsy technique into coaxial group (318 lesions) and non-coaxial group (332 lesions). Exclusion criteria for biopsy were lesions <5 mm in diameter, uncorrectable coagulopathy, positive-pressure ventilation, severe respiratory compromise, pulmonary arterial hypertension, or refusal of the procedure. Risk factors related to the occurrence of pneumothorax were classified into: (a) Technical risk factors, (b) patient-related risk factors, and (c) lesion-associated risk factors. Radiological assessments were performed by two radiologists in consensus. Mann-Whitney U test and Fisher's exact tests were used for statistical analysis. p values <0.05 were considered statistically significant.Results: The incidence of pneumothorax complicating CT-guided lung biopsy was less in the non-coaxial group (23.2 %, 77 out of 332) than the coaxial group (27 %, 86 out of 318). However, the difference in incidence between both groups was statistically insignificant (p = 0.14). Significant risk factors for the development of pneumothorax in both groups were emphysema (p < 0.001 in both groups), traversing a fissure with the biopsy needle (p value 0.005 in non-coaxial group and 0.001 in coaxial group), small lesion, less than 2 cm in diameter (p value of 0.02 in both groups), location of the lesion in the basal or mid sections of the lung (p = 0.003 and <0.001 in non-coaxial and coaxial groups, respectively), and increased needle track path within the lung tissue of more than 2.5 cm (p = 0.01 in both groups). The incidence of pneumothorax in the non-coaxial group was significantly correlated to the number of specimens obtained (p = 0.006). This factor was statistically insignificant in the coaxial group (p = 0.45). The biopsy yield was more diagnostic and conclusive in the coaxial group in comparison to the non-coaxial group (p = 0.008). Simultaneous incidence of pneumothorax and pulmonary hemorrhage was 27.3 % (21/77) in non-coaxial group and in 30.2 % (26/86) in coaxial group. Conservative management was sufficient for treatment of 91 out of 101 patients of pneumothorax in both groups (90.1 %). Manual evacuation of pneumothorax was efficient in 44/51 patients (86.3 %) in both groups and intercostal chest tube was applied after failure of manual evacuation (7 patients: 13.7 %), from which one patient developed a persistent air leakage necessitating pleurodesis.Conclusion: Pneumothorax complicating CT-guided core biopsy of pulmonary lesions, showed the insignificant difference between coaxial and non-coaxial techniques. However, both techniques have the same significant risk factors including small and basal lesions, increased lesion's depth from pleural surface, and increased length of aerated lung parenchyma crossed by biopsy needle and passing through pulmonary fissures in the needle tract. [ABSTRACT FROM AUTHOR]

Objective: To compare bone marrow biopsy needle (BMBN) with bone biopsy needle (BBN) in CT guided core biopsies considering size of core, diagnostic yield and cost of needles. Methodology: Retrospective study was done on 50 out of 68 patients referred to Interventional Radiology department for CT guided bone biopsy from 2008 till 2013. Grading of specimen length was done as small, medium and large. Diagnostic yield was determined on basis of clinical, imaging and histopathology data. Local cost of both needle types was determined. Comparison was made and statistical analysis was performed. Results: Biopsy was performed by BBN in 16 patients and by BMBN in 34 patients. Overall diagnostic yield of bone biopsies was 82 % (41 of 50 biopsies). Diagnostic yield with BMBN was 85 % and with BBN was 75 %. Overall longer specimen length was obtained using BMBN. Statistically, there was no significant difference between them in diagnostic yield and specimen length; however, BMBN costs almost one half than BBN. Conclusion: Diagnostic yield and sample length using bone marrow biopsy needle proved to be equivalent to bone biopsy needle. Bone marrow biopsy needle has an added benefit of being low in cost. Despite being a non-coaxial technique these needles can achieve diagnostic yield comparable to that of coaxial techniques as reported in past literature. [ABSTRACT FROM AUTHOR]

Using interaction forces between two magnetic sleeves, the contactless magnetic driver changes the rotating motion of a motor into reciprocating motion by a link mechanism. In an ideal state, the two sleeves of the driver are coaxial, but due to manufacturing and/or assembly errors, the real working state is non-coaxial. The air gap and consequently the magnetic field and interaction forces are altered. Therefore, it is very important to predict the changing of interaction force for some application. Assuming that the magnetizations are rigid and uniform, a mathematical model of the interaction force between two axial magnetized sleeves is developed based on the theory of equivalent magnetic charges. The model considers two magnetic sleeves at any relative position and attitude, and is verified experimentally for the two sleeves being coaxial. A numerical solution is implemented to compare interaction forces at the coaxial and non-coaxial states. Through numerical prediction, eccentricity between the sleeves of 0.5 mm generates a radial force approximately 70 N. The modeling method is also applicable to cuboidal magnets. [ABSTRACT FROM AUTHOR]

Purpose To evaluate the risk factors involved in the development of pulmonary hemorrhage complicating CT-guided biopsy of pulmonary lesions in coaxial and non-coaxial techniques. Materials and methods Retrospective study included CT-guided percutaneous lung biopsies in 650 consecutive patients (407 males, 243 females; mean age 54.6 years, SD: 5.2) from November 2008 to June 2013. Patients were classified according to lung biopsy technique in coaxial group (318 lesions) and non-coaxial group (332 lesions). Exclusion criteria for biopsy were: lesions <5 mm in diameter, uncorrectable coagulopathy, positive-pressure ventilation, severe respiratory compromise, pulmonary arterial hypertension or refusal of the procedure. Risk factors for pulmonary hemorrhage complicating lung biopsy were classified into: (a) patient's related risk factors, (b) lesion's related risk factors and (d) technical risk factors. Radiological assessments were performed by two radiologists in consensus. Mann–Whitney U test and Fisher's exact tests for statistical analysis. p values <0.05 were considered statistically significant. Results Incidence of pulmonary hemorrhage was 19.6% (65/332) in non-coaxial group and 22.3% (71/318) in coaxial group. The difference in incidence between both groups was statistically insignificant ( p = 0.27). Hemoptysis developed in 5.4% (18/332) and in 6.3% (20/318) in the non-coaxial and coaxial groups respectively. Traversing pulmonary vessels in the needle biopsy track was a significant risk factor of the development pulmonary hemorrhage (incidence: 55.4% (36/65, p = 0.0003) in the non-coaxial group and 57.7% (41/71, p = 0.0013) in coaxial group). Other significant risk factors included: lesions of less than 2 cm ( p value of 0.01 and 0.02 in non-coaxial and coaxial groups respectively), basal and middle zonal lesions in comparison to upper zonal lung lesions ( p = 0.002 and 0.03 in non-coaxial and coaxial groups respectively), increased lesion's depth from the pleural surface ( p = 0.021 and 0.018 in non-coaxial and coaxial groups respectively), increased distance of traversed lung in the needle track of more than 2.5 cm ( p = 0.001 in both groups). Insignificant risk factors were patient's age, gender or emphysema in both groups ( p value >0.1 in both groups). Concomitant incidence of pneumothorax was 32.3% (21/65) in non-coaxial group and 36.6% (26/71) in coaxial group. Pulmonary hemorrhage in the majority of cases was treated conservatively. Conclusion Pulmonary hemorrhage complicating CT-guided core biopsy of pulmonary lesions, showed insignificant difference between coaxial and non-coaxial techniques. Significant risk factors of pulmonary hemorrhage included small and basal lesions, increased lesion's depth from pleural surface, increased length of aerated lung parenchyma crossed by biopsy needle and passing through vessels within the lung during puncture. [ABSTRACT FROM AUTHOR]

When the suction bucket is subjected to vertical load, it is very easy to induce the integral shear failure of the foundation surrounding the bucket. However, it is not reasonable to ignore the non-coaxial phenomenon, which is caused by the principal stress rotation. Based on the UMAT of FEM software ABAQUS, the non-coaxial model is adopted into the numerical analysis of vertial suction bucket foundation' s bearing capacity problem. The results show that the non-coaxial model has little influence on the integral shear failure mechanism of the foundation, but it not only has a visible softening effect on the bearing capacity-settlement curve, but also decreases the vertical ultimate bearing capacity of the foundation. It's safe to take the non-coaxial model into the assessment of the foundation's bearing capacity, which is often determined by the vertical displacement. [ABSTRACT FROM AUTHOR]

We propose a double slip non-coaxial plastic model within the framework of a Cosserat continuum theory. In a Cosserat continuum, a material point possesses the degrees of freedom of an infinitesimal rigid body: two translations and one rotation in 2D. We formulate the plastic model into viscous-plastic constitutive relationships and illustrate the viscous-plastic behaviour of the model by means of numerical solution of a simple shear problem. [ABSTRACT FROM AUTHOR]

class of non-coaxial plasticity models is revisited in which the plastic deformation is carried by one (single slip) or two (double slip) slip systems that are inclined at $${\pm (\pi/4+\nu/2), 0\le \nu \le \phi}$$ to the less compressive principal stress axis; ν is the angle of non-coaxiality and $${\phi}$$ is the internal angle of friction of the material. The non-coaxial model is combined with a Newtonian viscous rheology to form a viscous-plastic model. The model is applied to model strain localisation in extension and compression on a time scale typical for deformation processes in the lithosphere. We would like to dedicate this article to the memory of Ioannis Vardoulakis who always took a keen interest in bifurcation theory and shear banding for granular materials. [ABSTRACT FROM AUTHOR]

Abstract: This paper ascertains the reasons for the numerical problems when the yield vertex non-coaxial model is implemented in the finite element analysis to predict the behaviour of complicated geotechnical engineering problems. The numerical problem, reflected in the failure of convergence in the non-linear solutions in ABAQUS, is likely to happen when a smaller non-coaxial plastic modulus is used. It is found that a large non-coaxial influence at the start of elastoplastic loading, which causes the predominance of non-coaxial plastic strain rate over the coaxial plastic strain rate in stress–strain responses, is the reason for the numerical problem. The original yield vertex non-coaxial model is modified to overcome the numerical problem. Instead of a constant non-coaxial plastic modulus in the original yield vertex non-coaxial model, the non-coaxial modulus is made a function of cumulative deviatoric plastic strain. It shows that the modified non-coaxial model retains the functions of the original non-coaxial model. Meanwhile, it overcomes the non-convergence problem when a smaller non-coaxial plastic modulus, representing a larger non-coaxial influence, is used. [Copyright &y& Elsevier]

A well-developed sand plasticity model is further revised and validated based on bi-directional simple shear test data on medium to high relative density sands, which includes circular, oval and figure-8 type stress paths at different cyclic stress ratio (CSR) levels. The sand properties revealed from non-proportional loading tests, such as extended contractive behavior for medium to dense sands, flow rule dependency on plastic strain increment direction, stress increment direction dependency of plastic moduli, as well as the so-called non-coaxial property are incorporated into the new model development. Specifically, the recently proposed strain increment direction dependent flow rule is formulated and validated using bi-directional simple shear test results; and the relatively simple plastic moduli revised for the non-proportional loading are validated. After the model parameters are calibrated using a circular stress path, predictions are made for oval and figure-8 type stress paths at two cyclic stress levels. The modeling results show that the computed deformations up to a large strain state (i.e., 6% double amplitude strain) match fairly well with the test results; and the predicted pore-water pressure generation (or the effective stress change) followed the trend of test measurements including the repeating contractive and dilative cycles near the failure surface. A fabric dilatancy concept-based stiffness reduction approach is adopted that is active for the simulations of oval and figure-8 type stress paths. By comparing with test results, the model predictions demonstrate the capability of the present model in simulating the complex deformation magnitudes and patterns for sands under bi-directional loadings. • A plasticity model for sand is revised based on the behavior of sand demonstrated in a comprehensive package of bi-directional simple shear test data. •The model simulations are compared with circular, oval and figure -8 type stress paths at two cyclic stress levels. •The computed deformations and effective stress changes match fairly well with the test results; and the strain increment dependent flow rule is formulated and validated using circular stress test results. •Specifically, the sand properties revealed from non-proportional loading tests, such as extended contractive behavior for medium to dense sands, flow rule dependency on stress or strain increment direction, stress increment direction dependency of plastic moduli, as well as the so-called non-coaxial property are incorporated in the new model's development. [ABSTRACT FROM AUTHOR]

Abstract: An exact solution of the unsteady hydromagnetic flow due to non-coaxial rotations of a porous disk and a fluid at infinity is obtained on taking Hall currents into account. An analytical solution of the problem is obtained for small and large times after the start by the Laplace transform method. It is found that for small values of time there is no inertial oscillations while for large time the steady state is reached through inertial oscillations. The frequency of these oscillations first increases, reaches a maximum and then decreases with increase in Hall parameter. [Copyright &y& Elsevier]