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Velocity gradient

About: Velocity gradient is a research topic. Over the lifetime, 3013 publications have been published within this topic receiving 77120 citations.


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TL;DR: In this paper, the absolute/convective character of axisymmetric jets is investigated for a wide range of parallel velocity and density profiles, and an adjoint-based sensitivity analysis is carried out in order to maximize the absolute growth rate of jet profiles with and without density variations.
Abstract: The absolute/convective character of the linear instability of axisymmetric jets is investigated for a wide range of parallel velocity and density profiles. An adjoint-based sensitivity analysis is carried out in order to maximize the absolute growth rate of jet profiles with and without density variations. It is demonstrated that jets without counterflow may display absolute instability at density ratios well above the previously assumed threshold ρ jet /ρ ∞ = 0.72, and even in homogeneous settings. Absolute instability is promoted by a strong velocity gradient in the low-velocity region of the shear layer, as well as by a step-like density variation near the location of maximum shear. A new efficient algorithm for the computation of the absolute instability mode is presented.

43 citations

Journal ArticleDOI
TL;DR: In this article, the authors used laser-based velocity measurement techniques to study the flow of dilute suspensions in a rotating filter separator consisting of a rigid, porous, polyethylene cylindrical filter rotating within an outer cylinrical shell, showing that the velocity field of the particles is very similar to that of the fluid indicating that the vortical structure readily entrains particles.

42 citations

Journal ArticleDOI
TL;DR: In this paper, the Biot-Savart law is used to determine the secondary flow field by integration for the downwash of the primary flow along the dividing streamline of a sphere.
Abstract: In an earlier paper the uniform shear flow past a sphere was studied, by investigating how vortex lines are deformed by the ‘primary flow’ (flow in the absence of shear), and deducing the ‘secondary’ vorticity field (first approximation for small shear). In another paper the image system associated with each element of secondary vorticity was found, whence the Biot-Savart law can be used to determine the secondary flow field by integration. The integration is here carried out for the ‘downwash’ (secondary flow component perpendicular to the undisturbed flow, down the velocity gradient) on the dividing streamline. Difficulties due to the infinite domain of integration and singularities of the integrand are overcome by selecting variables of integration carefully and using known analytical properties of the secondary vorticity. From the computation of downwash is inferred the first approximation (for small shear A) to the ‘displacement’ δ (displacement of the dividing streamline, up the velocity gradient, far upstream of the sphere). If U is the upstream flow velocity and a the radius of the sphere, the computed value of lim (Uδ/Aa2) is 0·9. Details of the calculation show that the secondary trailing vorticity is not an important contributor to the displacement, The downwash is due almost entirely to vorticity upstream of the sphere (Hall's earlier simplified theory gave good results, e.g., 1.24 instead of 0.9, because it concentrated on the effect of local vorticity in producing downwash); further, this produces displacement principally through its image vorticity.The relation between theories for a sphere and experimental results on Pitot tubes (beginning with Young & Maas 1936) is discussed. Theoretical evidence on tertiary- and quartary-flow effects is used here in the light of recent work which renders the successive-approximation sequence uniformly valid at infinity. The conclusion is that the theories, taken together, are not inconsistent with the experimental evidence that (i) at values of the ‘shear parameter’ Aa/U at which the displacement is measurable the ratio δ/a seems to have asymptoted to an approximately constant value, and (ii) displacement is greatly reduced in supersonic flow (Johannesen & Mair 1952) or when ‘sharp-lipped’ tubes are used (Livesey 1956).

42 citations

Journal ArticleDOI
TL;DR: The velocity flow fields above the vocal folds in both the midcoronal and midsagittal planes were studied to deduce the mechanisms that cause the anterior-posterior gradient and to determine whether the vortical structures are highly 3-dimensional.
Abstract: Objectives To quantify the anterior-posterior velocity gradient, we studied the velocity flow fields above the vocal folds in both the midcoronal and midsagittal planes. It was also our purpose to use these fields to deduce the mechanisms that cause the anterior-posterior gradient and to determine whether the vortical structures are highly 3-dimensional.

42 citations

Journal ArticleDOI
01 May 1961-Tellus B
TL;DR: In this paper, a model of steady airflow past a three-dimensional mountain is considered, where the fluid is inviscid and incompressible, and the primary and secondary velocities are found for large Richardson number (proportional to gd?/? dz 0 ).
Abstract: A model of steady airflow past a three-dimensional mountain is considered. The fluid is inviscid and incompressible. The flow is under the influence of gravity, but for most of the work the earth's rotation is neglected. Far upstream the flow is parallel and horizontal, the velocity U ( z ) and the density ?( z ) varying with height z . By use of conservation of density and head along streamlines, and thence of the dependence of density and head on one variable z 0 only, the equations of motion are simplified, z 0 (r) being the height far upstream of the streamline through the point with position vector r. The flow is considered first with small and then with large density variations far upstream. In the case when these variations are small, results of the known method of simple-shear secondary flows are rederived and extended. The chief consideration of this paper is the buoyancy effects of large variations of density in the oncoming stream. For simplicity, the shear far upstream is neglected. The primary and secondary velocities are found for large Richardson number (proportional to gd ?/? dz 0 ). The method is time-symmetric and analogous to taking the first two terms of the Janzen-Rayleigh expansion, the inverse of the Richardson number being the analogue of the square of the Mach number. The primary velocity, for infinite buoyancy forces, is horizontal, the flow in each horizontal plane being the same as the potential flow about the section of the obstacle in that plane. In general there is shear between horizontal planes, with horizontal primary vorticity. The secondary flow is found explicitly for a circular cylinder with vertical axis and for a hemisphere resting in a horizontal plane. The secondary approximation is not uniformly valid near the horizontal plane at the height of the top (if any) of the obstacle. Near that plane there is an inviscid shear layer, and the velocity gradient cannot be neglected, however large the Richardson number. DOI: 10.1111/j.2153-3490.1961.tb00081.x

42 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
202318
202233
2021127
2020116
2019134
201892