The Experts below are selected from a list of 42 Experts worldwide ranked by ideXlab platform
Kartik Jain - One of the best experts on this subject based on the ideXlab platform.
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Transition to turbulence in an oscillatory flow through stenosis
Biomechanics and Modeling in Mechanobiology, 2020Co-Authors: Kartik JainAbstract:Onset of flow transition in a sinusoidally oscillating flow through a rigid, constant area circular pipe with a smooth sinusoidal obstruction in the center of the pipe is studied by performing direct numerical simulations, with resolutions close to the Kolmogorov Microscales. The studied pipe is stenosed in the center with a 75% reduction in area in two distinct configurations—one that is symmetric to the axis of the parent pipe and the other that is offset by 0.05 diameters to introduce an eccentricity, which disturbs the flow thereby triggering the onset of flow transition. The critical Reynolds number at which the flow transitions to turbulence for a zero-mean oscillatory flow through a stenosis is shown to be nearly tripled in comparison with studies of pulsating unidirectional flow through the same stenosis. The onset of transition is further explored with three different flow pulsation frequencies resulting in a total of 90 simulations conducted on a supercomputer. It is found that the critical Reynolds number at which the oscillatory flow transitions is not affected by the pulsation frequencies. The locations of flow breakdown and re-stabilization post-stenosis are, however, respectively shifted closer to the stenosis throat with increasing pulsation frequencies. The results show that oscillatory physiological flows, while more stable, exhibit fluctuations due to geometric complexity and have implications in studies of dispersion and solute transport in the cerebrospinal fluid flow and understanding of pathological conditions.
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Direct numerical simulation of transitional hydrodynamics of the cerebrospinal fluid in Chiari I malformation: The role of cranio‐vertebral junction
International Journal for Numerical Methods in Biomedical Engineering, 2017Co-Authors: Kartik Jain, Geir Ringstad, Per-kristian Eide, Kent-andre MardalAbstract:Obstruction to the cerebrospinal fluid (CSF) outflow caused by the herniation of cerebellar tonsils as a result of Chiari malformation type I leads to altered CSF hydrodynamics. This contribution explores the minutest characteristics of the CSF hydrodynamics in cervical subarachnoid space (SAS) of a healthy subject and 2 Chiari patients by performing highly resolved direct numerical simulation. The lattice Boltzmann method is used for the simulations because of its scalability on modern supercomputers that allow us to simulate up to approximately 109 cells while resolving the Kolmogorov Microscales. The results depict that whereas the complex CSF flow remains largely laminar in the SAS of a healthy subject, constriction of the cranio-vertebral junction in Chiari I patients causes manifold fluctuations in the hydrodynamics of the CSF. These fluctuations resemble a flow that is in a transitional regime rather than laminar or fully developed turbulence. The fluctuations confine near the cranio-vertebral junction and are triggered due to the tonsillar herniation, which perturbs the flow as a result of altered anatomy of the SAS.
A Silva P Freire - One of the best experts on this subject based on the ideXlab platform.
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on the connection between Kolmogorov Microscales and friction in pipe flows of viscoplastic fluids
Physica D: Nonlinear Phenomena, 2017Co-Authors: H R Anbarlooei, D O A Cruz, F Ramos, Cecilia Mageski Madeira Santos, A Silva P FreireAbstract:Abstract The present work extends Kolmogorov’s micro-scales to a large family of viscoplastic fluids. The new micro-scales, combined with Gioia and Chakaborty’s (2006) friction phenomenology theory, lead to a unified framework for the description of the friction coefficient in turbulent flows. A resulting Blasius-type friction equation is tested against some available experimental data and shows good agreement over a significant range of Hedstrom and Reynolds numbers. The work also comments on the role of the new expression as a possible benchmark test for the convergence of DNS simulations. The formula also provides limits for the maximum drag reduction of viscoplastic flows.
Kent-andre Mardal - One of the best experts on this subject based on the ideXlab platform.
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Direct numerical simulation of transitional hydrodynamics of the cerebrospinal fluid in Chiari I malformation: The role of cranio‐vertebral junction
International Journal for Numerical Methods in Biomedical Engineering, 2017Co-Authors: Kartik Jain, Geir Ringstad, Per-kristian Eide, Kent-andre MardalAbstract:Obstruction to the cerebrospinal fluid (CSF) outflow caused by the herniation of cerebellar tonsils as a result of Chiari malformation type I leads to altered CSF hydrodynamics. This contribution explores the minutest characteristics of the CSF hydrodynamics in cervical subarachnoid space (SAS) of a healthy subject and 2 Chiari patients by performing highly resolved direct numerical simulation. The lattice Boltzmann method is used for the simulations because of its scalability on modern supercomputers that allow us to simulate up to approximately 109 cells while resolving the Kolmogorov Microscales. The results depict that whereas the complex CSF flow remains largely laminar in the SAS of a healthy subject, constriction of the cranio-vertebral junction in Chiari I patients causes manifold fluctuations in the hydrodynamics of the CSF. These fluctuations resemble a flow that is in a transitional regime rather than laminar or fully developed turbulence. The fluctuations confine near the cranio-vertebral junction and are triggered due to the tonsillar herniation, which perturbs the flow as a result of altered anatomy of the SAS.
J. L. Balint - One of the best experts on this subject based on the ideXlab platform.
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An experimental study of helicity density in turbulent flows
Physics of Fluids, 1992Co-Authors: James M. Wallace, J. L. BalintAbstract:Databases for a turbulent boundary layer at Rθ=2685, a turbulent two‐stream mixing layer at Rθ=5800, and a turbulent grid flow at RM=23 400 have been examined for properties of the relative helicity density, h=(U⋅Ω)/‖U‖‖Ω‖. The velocity and vorticity vectors U and Ω were simultaneously measured in these flows using a miniature probe with nine hot‐wire sensors with a spatial resolution of a few Kolmogorov Microscales. The results of this analysis are in generally good agreement with a similar analysis of a direct numerical channel flow simulation of Rogers and Moin [Phys. Fluids 30, 2662 (1987)]. The results do not support the suggestion that there is a high probability for the flows locally to achieve a Beltrami‐like state with the velocity and vorticity vectors often nearly aligned. Such preferred alignment does not occur in the grid flow and only slightly occurs in regions of the shear flows where it is known that the mean velocity is somewhat aligned with coherent vortices. A joint probability analysis...
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THE VELOCITY AND VORTICITY VECTOR FIELDS OF A TURBULENT BOUNDARY LAYER. PART 1, SIMULTANEOUS MEASUREMENT BY HOT-WIRE ANEMOMETRY
Journal of Fluid Mechanics, 1991Co-Authors: Petar Vukoslavcevic, James M. Wallace, J. L. BalintAbstract:A nine-sensor hot-wire probe is described which is capable of simultaneously measuring the velocity and vorticity vectors with a spatial resolution of about six Kolmogorov Microscales just above the viscous sublayer in a thick turbulent boundary layer at a Reynolds number of R θ = 2685. Results from tests of the probe performance are presented to show that the three velocity components at each of its three arrays are measured with sufficient accuracy to allow determination of velocity gradients and from them the vorticity vector. Measurements with this probe of statistical properties of the velocity and vorticity fields of the turbulent boundary layer are given in Part 2 of this paper. When compared to the results of others, they further demonstrate the capability of this probe to measure simultaneously the velocity and vorticity vectors in turbulent flows of low to moderate Reynolds numbers.
H R Anbarlooei - One of the best experts on this subject based on the ideXlab platform.
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on the connection between Kolmogorov Microscales and friction in pipe flows of viscoplastic fluids
Physica D: Nonlinear Phenomena, 2017Co-Authors: H R Anbarlooei, D O A Cruz, F Ramos, Cecilia Mageski Madeira Santos, A Silva P FreireAbstract:Abstract The present work extends Kolmogorov’s micro-scales to a large family of viscoplastic fluids. The new micro-scales, combined with Gioia and Chakaborty’s (2006) friction phenomenology theory, lead to a unified framework for the description of the friction coefficient in turbulent flows. A resulting Blasius-type friction equation is tested against some available experimental data and shows good agreement over a significant range of Hedstrom and Reynolds numbers. The work also comments on the role of the new expression as a possible benchmark test for the convergence of DNS simulations. The formula also provides limits for the maximum drag reduction of viscoplastic flows.
