The Experts below are selected from a list of 264 Experts worldwide ranked by ideXlab platform

K Ramamurthi - One of the best experts on this subject based on the ideXlab platform.

  • Internal Instability of thin liquid sheets
    Physics of Fluids, 2009
    Co-Authors: K Ramamurthi
    Abstract:

    Linear stability analysis of an inviscid liquid sheet with different velocity profiles across its thickness is reported. The velocity profiles for which there is a progressive increase or decrease in velocities between the two interfaces are demonstrated to be inherently unstable even in the absence of the destabilizing aerodynamic shear at the liquid-gas interfaces. Compared to a flat velocity profile, a linear or a parabolic profile, symmetric at the center line of the sheet reduced both the maximum growth rate and the wavelength range over which the waves grow. The convective acceleration from the velocity gradient is found to stabilize longer waves while the growth of shorter waves is hampered by the combined effect of the surface tension and a decrease in the interface velocity between gas and liquid media. The wave forms are dominantly sinuous for symmetric velocity profiles; however, with larger velocity gradients the dilatational modes are observed. The inherent Instability of liquid sheets with a...

Hans Rönnqvist - One of the best experts on this subject based on the ideXlab platform.

R J Fannin - One of the best experts on this subject based on the ideXlab platform.

  • Discussion: On the distinct phenomena of suffusion and suffosion
    Geotechnique Letters, 2015
    Co-Authors: R J Fannin, P Slangen, Amirhassan Mehdizadeh, M. Disfani, Arul Arulrajah, Robert Evans
    Abstract:

    A valuable conceptual framework for the characterisation of seepage-induced Internal Instability was provided in the article under discussion, particularly for the ‘suffusion and suffosion’ phenomena. A few modifications are suggested here to clarify the definitions in the original paper.

  • Fabric and Effective Stress Distribution in Internally Unstable Soils
    Journal of Geotechnical and Geoenvironmental Engineering, 2014
    Co-Authors: Tom Shire, Catherine O'sullivan, Kevin J Hanley, R J Fannin
    Abstract:

    Internal Instability is a form of Internal erosion in broadly graded cohesionless soils in which fine particles can be eroded at lower hydraulic gradients than predicted by classical theory for piping or heave. A key mechanism enabling Internal Instability is the formation of a stress-transmitting matrix dominated by the coarse particles, which leaves the finer particles under lower effective stress. In this study, discrete element modeling is used to analyze the fabric and effective stress distribution within idealized gap-graded samples with varying potential for Internal stability. The reduction in stress within the finer fraction of the materials is directly quantified from grain-scale data. The particle-size distribution, percentage finer fraction, and relative density are found to influence the stress distribution. In particular, effective stress transfer within a critical finer fraction between 24 and 35% is shown to be highly sensitive to relative density.

  • On the distinct phenomena of suffusion and suffosion
    Geotechnique Letters, 2014
    Co-Authors: R J Fannin, P Slangen
    Abstract:

    Seepage-induced Internal Instability is a phenomenon whereby fine particles are transported from a non-plastic soil. A distinction can readily be made between a washed-out soil structure that remains intact and one in which some form of destruction or collapse of the structure accompanies the migration of fine particles. The three variables of a measured value of mass loss, a measured value of volume change and a value of change in hydraulic conductivity, deduced from measurements of hydraulic gradient and flow rate, are sufficient to quantify, and hence distinguish between, seepage-induced Internal Instability phenomena. The term ‘suffusion’ is advocated to describe the non-destructive response, which may be quantified by a mass loss, no change in volume and an increase in hydraulic conductivity. The term ‘suffosion’ is recommended to describe the Instability phenomenon whereby the transport of fine particles by seepage flow is accompanied by a collapse of the soil structure. Accordingly, this distinct i...

  • Capillary Tube Model for Internal Stability of Cohesionless Soil
    Journal of Geotechnical and Geoenvironmental Engineering, 2013
    Co-Authors: M Li, R J Fannin
    Abstract:

    AbstractKovacs’ criterion to assess the potential for Internal Instability of cohesionless soils that was proposed, but never tested experimentally, is verified with reference to a database of laboratory test results. The inherent margin of safety is described, and a threshold boundary between stable and potentially unstable gradations is established. A variation on the method is then presented, termed the simplified-Kovacs criterion.

  • a theoretical envelope for Internal Instability of cohesionless soil
    Geotechnique, 2012
    Co-Authors: M Li, R J Fannin
    Abstract:

    The nature of seepage-induced Internal stability is characterised with reference both to effective stress in the finer fraction of the grain size distribution curve, and also to hydraulic gradient within the soil. The concept of a hydromechanical envelope is proposed in stress-gradient space. The onset of Instability is triggered by a critical value of hydraulic gradient that is found consistent with the observations of Terzaghi on ‘piping' in uniform sand, and the observations of Skempton and Brogan on ‘segregation piping' in widely graded sandy gravels. The envelope takes the form of a linear relation that, for a particular soil, is governed by the proportion of effective stress in the finer fraction of soil grains.

Farshad Merrikh-bayat - One of the best experts on this subject based on the ideXlab platform.

  • Performance enhancement of non-minimum phase feedback systems by fractional-order cancellation of non-minimum phase zero on the Riemann surface: New theoretical and experimental results
    arXiv: Optimization and Control, 2016
    Co-Authors: Farshad Merrikh-bayat, Aliakbar Salimi
    Abstract:

    The non-minimum phase (NMP) zero of a linear process located in the feedback connection cannot be cancelled by the same pole of controller according to the Internal Instability problem. However, such a zero can partly be cancelled by the same fractional-order pole of a pre-compensator located in series with process without facing Internal Instability. This paper first presents new theoretical results on the properties of this method of cancellation, and provides design techniques for the pre-compensator. It is especially shown that by appropriate design of pre-compensator this method can simultaneously increase the gain and phase margin of the system under control without a considerable reduction of open-loop bandwidth, and consequently, it can make the control problem easier to solve. Then, a method for realization of such a pre-compensator is proposed and performance of the resulted closed-loop system is studied through an experimental setup.

  • Fractional-order unstable pole-zero cancellation in linear feedback systems☆
    Journal of Process Control, 2013
    Co-Authors: Farshad Merrikh-bayat
    Abstract:

    Abstract As a very well-known classical fact, non-minimum phase zeros of the process put some limitations on the performance of the feedback system. The source of these limitations is that non-minimum phase zeros cannot be cancelled by unstable poles of the controller since such a cancellation leads to Internal Instability. The aim of this paper is to propose a method for fractional-order cancellation of non-minimum phase zeros of the process and studying its properties. It is specially shown that the proposed cancellation strategy increases the phase and gain margin without leading to Internal Instability. Since the systems with higher gain and phase margin are easier to control, the proposed method can be used to arrive at more effective controls, which is also verified by the simulation results.

  • Practical and efficient method for fractional-order unstable pole-zero cancellation in linear feedback systems ⋆
    arXiv: Optimization and Control, 2012
    Co-Authors: Farshad Merrikh-bayat
    Abstract:

    As a very well-known classical fact, non-minimum phase zeros of the process put some limitations on the performance of the feedback system. The source of these limitations is that such non-minimum phase zeros cannot be cancelled by unstable poles of the controller since such a cancellation leads to Internal Instability. The aim of this paper is to propose a method for fractional-order cancellation of nonminimum phase zeros of the process and studying its properties. It is specially shown that the proposed cancellation strategy increases the phase and the gain margin without leading to Internal Instability. Since the systems with higher gain and phase margin are easier to control, the proposed method can be used to arrive at more effective controls.

Piltan Tabatabaie Shourijeh - One of the best experts on this subject based on the ideXlab platform.

  • a comparison of micromechanical assessments with Internal stability Instability criteria for soils
    Powder Technology, 2015
    Co-Authors: Mojtaba Farahnak Langroudi, Abbas Soroush, Piltan Tabatabaie Shourijeh
    Abstract:

    Suffusion is the erosion of small particles through the skeleton of coarse grains. Soils susceptible to suffusion are described as Internally unstable. In this study three dimensional discrete element modeling is employed to investigate Internal Instability in soils. The simulation is achieved by assessing contacts distributions, forces analysis and transmitted stresses between particles. Three types of gradations have been selected for the analyses: linear, concave upward and gap-graded. Observations of mechanical coordination number and contact distributions during isotropic compression show that the number of fine particles with low connectivity is comparatively higher for gap-graded and concave upward gradations. The evolution of contact force networks confirms that Internally stable soils have a relatively homogeneous network of contact forces compared to Internally unstable soil. Force distribution analyses reflect higher percent of weak contacts and low connectivity for fine particles in Internal Instability. In addition four commonly used Internal Instability assessment criteria were contrasted with micromechanical parameters, and findings revealed reasonable compliance between stability indices and micromechanical measures. Finally the stress reduction factor of the soils is calculated, confirming previous experimental and numerical studies that α is higher for Internally stable soils.

  • A comparison of micromechanical assessments with Internal stability/Instability criteria for soils
    Powder Technology, 2015
    Co-Authors: Mojtaba Farahnak Langroudi, Abbas Soroush, Piltan Tabatabaie Shourijeh
    Abstract:

    Suffusion is the erosion of small particles through the skeleton of coarse grains. Soils susceptible to suffusion are described as Internally unstable. In this study three dimensional discrete element modeling is employed to investigate Internal Instability in soils. The simulation is achieved by assessing contacts distributions, forces analysis and transmitted stresses between particles. Three types of gradations have been selected for the analyses: linear, concave upward and gap-graded. Observations of mechanical coordination number and contact distributions during isotropic compression show that the number of fine particles with low connectivity is comparatively higher for gap-graded and concave upward gradations. The evolution of contact force networks confirms that Internally stable soils have a relatively homogeneous network of contact forces compared to Internally unstable soil. Force distribution analyses reflect higher percent of weak contacts and low connectivity for fine particles in Internal Instability. In addition four commonly used Internal Instability assessment criteria were contrasted with micromechanical parameters, and findings revealed reasonable compliance between stability indices and micromechanical measures. Finally the stress reduction factor of the soils is calculated, confirming previous experimental and numerical studies that α is higher for Internally stable soils.