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Staff Researches

Ahmed Mohamed Ali Ashour

Silo outflow of soft frictionless spheres

Ahmed Mohamed Ali Ashour

Torsten Trittel, Tamás Börzsönyi, and Ralf Stannarius

PHYSICAL REVIEW FLUIDS

2017

Outflow of granular materials from silos is a remarkably complex physical phenomenon that has been extensively studied with simple objects like monodisperse hard disks in two dimensions (2D) and hard spheres in 2D and 3D. For those materials, empirical equations were found that describe the discharge characteristics. Softness adds qualitatively new features to the dynamics and to the character of the flow.We report a study of the outflow of soft, practically frictionless hydrogel spheres from a quasi-2D bin. Prominent features are intermittent clogs, peculiar flow fields in the container, and a pronounced dependence of the flow rate and clogging statistics on the container fill height. The latter is a consequence of the ineffectiveness of Janssen’s law: the pressure at the bottom of a bin containing hydrogel spheres grows linearly with the fill height.

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Linking bottleneck clogging with flow kinematics in granular materials: The role of silo width

Ahmed Mohamed Ali Ashour

D. Gella, D. Maza, I. Zuriguel, R. Arévalo and R. Stannarius

PHYSICAL REVIEW FLUIDS

2017

We demonstrate experimentally that clogging in a silo correlates with some features of the particle velocities in the outlet proximities. This finding, that links the formation of clogs with a kinematic property of the system, is obtained by looking at the effect that the position of the lateral walls of the silo has on the flow and clogging behavior. Surprisingly, the avalanche size depends nonmonotonically on the distance of the outlet from the lateral walls. Apart from evidencing the relevance of a parameter that has been traditionally overlooked in bottleneck flow, this nonmonotonicity supposes a benchmark with which to explore the correlation of clogging probability with different variables within the system. Among these, we find that the velocity of the particles above the outlet and their fluctuations seem to be behind the nonmonotonicity in the avalanche size versus wall distance curve.

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Elongated grains in a hopper

Ahmed Mohamed Ali Ashour

Tamás Börzsönyi, Ellák Somfai, Balázs Szabó1, Sandra Wegner, and Ralf Stannarius

EPJ Web of Conferences

2017

Flow and clogging of granular materials in a 3-dimensional hopper is investigated experimentally. We use X-ray tomography and optical methods to study this phenomenon for spherical and elongated particles. The X-ray tomograms provide information on the bulk of the hopper filling, and allow to determine the particle positions and orientations inside the silo, as well as spatial variations of the local packing density. We find that particles show a preferred orientation and thereby an enhanced order in the flowing zone of the silo. Similarly to simple shear flows, the average orientation of the particles is not parallel to the streamlines but encloses a certain angle with them. The clogged state is characterized by a dome, i. e. the geometry of the layer of grains blocking the outflow. The number of grains forming this blocking layer is larger for elongated grains compared to the case of spheres of the same volume.

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Flow of anisometric particles in a quasi-two-dimensional hopper

Ahmed Mohamed Ali Ashour

Balázs Szabó, Zsolt Kovács, Sandra Wegner, David Fischer, Ralf Stannarius, and Tamás Börzsönyi

PHYSICAL REVIEW E

2018

The stationary flow field in a quasi-two-dimensional hopper is investigated experimentally. The behavior of materials consisting of beads and elongated particles with different aspect ratio is compared.We show, that while the vertical velocity in the flowing region can be fitted with a Gaussian function for beads, in the case of elongated grains the flowing channel is narrower and is bordered with sharper velocity gradient. For this case, we quantify deviations from the Gaussian velocity profile. Relative velocity fluctuations are considerably larger and slower for elongated grains.

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Outflow and clogging of shape-anisotropic grains in hoppers with small apertures

Ahmed Mohamed Ali Ashour

S. Wegner, T. Trittel, T. Börzsönyi and R. Stannarius

Soft Matter

2017

Outflow of granular material through a small orifice is a fundamental process in many industrial fields, for example in silo discharge, and in everyday’s life. Most experimental studies of the dynamics have been performed so far with monodisperse disks in two-dimensional (2D) hoppers or spherical grains in 3D. We investigate this process for shape-anisotropic grains in 3D hoppers and discuss the role of size and shape parameters on avalanche statistics, clogging states, and mean flow velocities. It is shown that an increasing aspect ratio of the grains leads to lower flow rates and higher clogging probabilities compared to spherical grains. On the other hand, the number of grains forming the clog is larger for elongated grains of comparable volumes, and the long axis of these blocking grains is preferentially aligned towards the center of the orifice. We find a qualitative transition in the hopper discharge behavior for aspect ratios larger than E6. At still higher aspect ratios 48–12, the outflowing material leaves long vertical holes in the hopper that penetrate the complete granular bed. This changes the discharge characteristics qualitatively.

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Analysis of Surface Roughness Using Laser Optical Imaging Techniques

Ahmed Mohamed Ali Ashour

Hatem El-Ghandoor and Mohamed Saudy

Journal Of Materials Science and Engineering

2012

Speckle phenomena were produced by using interference of scattered laser beams from certain rough object. Digital speckle images were recorded for different rougher and smoother surfaces, using an optical imaging system in two and three dimensions with a high resolution CCD camera. The obtained speckle images were transformed to equivalent binary images. The values of surface roughness depend on the degree of agglomeration of the speckle images. The optical density was calculated and it was found that it depends on the different conditions for the optical imaging system. The back projection technique was used to reconstruct 3-dimensional surface roughness profiles from multi-directional projection data. Also, interference microscope was used for the reconstruction of surface topography for different rough surfaces.

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Birefringence dispersion in uniaxial material irradiated by gamma rays cellulose triacetate films

Ahmed Mohamed Ali Ashour

Fouad El-Diasty, Mona A Soliman and Abdel Fatah T. Elgendy

Journal of Optics A: Pure and Applied Optics

2007

Cellulose triacetate, CTA, as a uniaxial material, has many diverse scientific and technological applications. These applications depend basically on the optical properties of the material. In this paper, a simple two-beam interferometric technique is used to measure the absolute (built-in) birefringence in cellulose triacetate film plates. By a least squares fitting to the Cauchy dispersion formula, the quantum parameters of CTA are deduced. The dispersion of the induced-birefringence due to gamma irradiation, at the visible region of the spectrum, is determined. The relative error in birefringence is 5 × 10−3. It is found that the birefringence is inversely proportional to the wavelength and its rate of increase is greater at shorter wavelengths. As the gamma irradiation dose increases, the birefringence increases, which in turn enhances the anisotropic character of the CTA films. The variation in birefringence dispersion (as the retardance or relative retardation) versus the irradiation dose is investigated. We conclude that measuring the birefringence is essential in order to provide correct refractometric data to study the structure of irradiated materials.

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