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About 1,426 papers for “Starting vortex”
David Hasler (4971865), Dominik Obrist (4971868) · Figshare · 2018
<p>Flow field in the medium sized aortic root (M) at <i>t</i> = 0.12 s: RMS velocity fluctuations <i>u</i><sub>rms</sub> (a); instantaneous velocity magnitude field |〈<b>U</b>〉| (b); 3D maximum shear rate <i>γ</i><sub>3D</sub> (c).</p>
Cited by 0Open access
Thomas Leweke, Toshiki Sawada · 1992
Cited by 1
Jiao Jian Ai, S. C. M. Yu, Adrian Wing‐Keung Law et al. · Physics of Fluids · 2004
This paper presents the results of an experimental study on starting square jets at three Reynolds numbers (Re=U0De∕υ=2358,3528,4716) utilizing planar laser induced fluorescence. Starting circular jets under the same initial Re were also investigated as a basis for comparison. Observations showed that for both circular and square jets, the rate of penetration is almost constant within the first four diameters from the exit plane. Beyond that, the rate reaches an asymptotic behavior that is inversely proportional to the square root of time (t1∕2). It was also found that the jet front travels slower in square jets. The interaction and deformation between the spanwise and streamwise vortices in square jets lead to an enhanced entrainment. The phenomena of axis switching, vortex pinch-off, vortices leapfrogging and coalescence were all observed in the experiments and their formation mechanisms are discussed in detail. The normalized value for the pinch-off time obtained is found to be about 7 and the expansion rate of the head vortex core is proportional to t1.5.
Cited by 40
NTUR (臺灣機構典藏) · 2007
Cited by 0
D. I. Pullin, A. E. Perry · Journal of Fluid Mechanics · 1980
A simple dye in water method has been used to visualize the growth of a two-dimensional starting flow vortex formed at a wedge-like sharp edge. Several cases were tested corresponding to different wedge angles and to different values of the time exponent in the velocity–time power law describing the starting flow. Photographic sequences showing the time-wise primary vortex growth are presented from which various secondary-flow details are identified. For the larger wedge angles these include a strong secondary vortex and in some cases a small separation bubble-like flow region immediately adjacent to the wedge apex. For a thin-wedge model the formation of what might be interpreted as small rotation centres along the outer turns of the primary-vortex shear layer is observed but these are not seen as a manifestation of an instability phenomenon in the fluid. Measurements of the trajectories of the primary-vortex centre are compared with the predictions of an inviscid similarity theory of the vortex growth. Although the appropriate Reynolds number in the present experiments was relatively low, comparison between theory and experiments is regarded as reasonable with differences being attributed to viscous effects absent in the similarity theory, and also to apparatus wall effects.
Cited by 114
A. Goyal, Jovan Nedić · Physics of Fluids · 2025
This work characterizes the starting vortex and unsteady shear layer generated by an impulsively started National Advisory Committee for Aeronautics 0010 (NACA 0010) airfoil at angles of attack in the range of 3°–7°. Measurements are obtained by using time-resolved particle image velocimetry. The net circulation in the field is accurately predicted by Wagner's function. It is observed that the starting vortex detaches from the shear layer, while the airfoil is surging. Starting vortex circulation at detachment is presented for a range of surge speeds at pre-stall angles of attack for a given surge distance. Kinematic conditions resulting in starting vortex detachment are examined qualitatively through finite time Lyapunov exponents and quantitatively through a non-dimensional velocity ratio. Furthermore, the formation of secondary Kelvin–Helmholtz-type vortices in the shear layer is found to depend on the Reynolds number rather than the detachment of the starting vortex.
Cited by 0
P. Freymuth, Fathi Finaish, W. Bank · Journal of Fluid Mechanics · 1985
Vortical-pattern visualization for the starting flow behind airfoils is extended from a chordwise or two-dimensional mode to a spanwise or three-dimensional mode. We demonstrate the capabilities and limits of the extended method by means of selected photographic frames and sequences.
Cited by 12
Fei Zheng, A. V. Kuznetsov, William L. Roberts et al. · Journal of Fluids Engineering · 2011
For many propulsion devices, the thrust may be augmented considerably by adding a passive ejector, and these devices are especially attractive for unsteady propulsion systems such as pulse detonation engines and pulsejets. Starting vortices from these unsteady devices dominate the flowfield and control to a great extent the level of the thrust augmentation. Therefore, it is of fundamental interest to understand the geometric influences on the starting vortex and how these manifest themselves in augmenter/ejector performance. An unsteady Reynolds averaged Navier–Stokes calculation was used to study the physics of a starting vortex generated at the exit of a pulsed jet and its interaction with an ejector. A 50 cm long pulsejet (typical hobby scale, allowing comparison with experimental data) with a circular exit was modeled as the resonant driving source and used to suggest an optimal ejector geometry and relative position. Computed limit-cycle thrust augmentation values compared favorably to experimentally obtained values for the same ejector geometries. Results suggest that the optimal diameter of the ejector is related to its relative position, dictated by the trajectory of the vortex toroid. The effect of the length of the ejector (which determines the natural frequency of the ejector, related to the acoustic processes occurring in the ejector) on overall performance was also investigated and shown to be less important than the ejector diameter.
Cited by 18
GRASSO, Francesco, PIROZZOLI, Sergio, GATSKI T. B. · IRIS Research product catalog (Sapienza University of Rome) · 1999
Cited by 0
David M. Salazar, Tianshu Liu · European Journal of Mechanics - B/Fluids · 2024
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Hiroyuki Honji, Sadatoshi Taneda · Reports of Research Institute for Applied Mechanics. · 1972
Some years ago, an experimental investigation of starting flow past circular cylinders was made by the present authors, and the result obtained was reported in a preceding paper. 1 > Although many graphs and photographs were included in that paper, experimental raw va.lues have been omitted totally for want of space.In the course of further study of various types of unsteady flows, however, frequent use has be~n made of the concrete numerical values for comparison's sake.At present their presention in a tabular form seems to be desirable.
Cited by 0Open access
F. Grasso, Sergio Pirozzoli, T. B. Gatski · Physics of Fluids · 1999
The numerical simulation of a shock induced, turbulent compressible vortex is analyzed. The vortex is generated by the movement of an (unsteady) shock over the trailing-edge of a two-dimensional airfoil at different incidence angle. A theoretical analysis is performed for the case of a turbulent line vortex, at a low (vortex) Mach number and high Reynolds number, as a basis of comparison for the dynamic structure of the starting vortex. This theoretical analysis confirms the existence of an equilibrium structure of the isolated vortex similar to the laminar case. In the numerical computations, both an isotropic eddy viscosity two-equation turbulence model and an algebraic stress model are employed to assess the influence of turbulence model on the predictive capabilities of such a flow including the realization of the self-similar behavior found in the isolated vortex case. The results are also compared to theoretical estimates of the production-to-dissipation rate ratio for both types of turbulence models, and to recent experiments on such flows.
Cited by 6
Goyal, A., Nedić, J. · eScholarship@McGill (McGill) · 2025
Cited by 0
Mark Pinsky, Eshkol Eytan, Ehud Gavze et al. · Journal of the Atmospheric Sciences · 2023
Abstract A developing cumulus cloud (Cu) was modeled, and dynamic, thermodynamic, and microphysical properties of an ascending head bubble reproducing the upper part of a developing Cu were investigated. The data for analysis are taken from 10-m-resolution LES of trade wind Cu under BOMEX conditions. The detection of a rising bubble is carried out using wavelet filtering of the velocity fields and microphysical fields, while a low-frequency signal of the filtering is associated with the convective-scale structure of cloud. We substantiate and discuss the representation of the bubble as a vortex ring, and estimate the parameters of this vortex ring. The simplest Hill’s vortex was chosen as a model of a vortex ring inside cloud. Analytical approximations of the radial profiles of the vertical velocity and of conservative quantities (such as total water mixing ratio and liquid water potential temperature inside and outside the bubble) are obtained. The spatial structure of these quantities is investigated using analytical expressions. Analytical models for spatial distributions of liquid water content (LWC) and adiabatic fraction (AF) are also designed and analyzed. The results demonstrate the existence of a cloud core with high values of LWC and AF up to the height of 1800 m. The horizontally averaged value of the adiabatic fraction, calculated analytically using the Hill’s vortex concept, is evaluated as 0.39, which is the typical AF value in the upper parts of such Cu. The vertical profiles of different important quantities characterizing cloud structure are presented. The analysis performed in this study allows us to conclude that a rising vortex ring plays the dominating role in formation of the thermodynamic and microphysical structure of developing Cu. Significance Statement 1) Dynamic and thermodynamic fields of a developing cumulus cloud simulated by high-resolution LES with spectral bin microphysics are separated into convective and turbulent components by means of the wavelet technique. 2) The analysis of convective component of the cloud revealed the existence of a vortex ring at the developing stage of the cloud and evaluate its parameters. 3) The analysis performed in this study allows us to conclude that a rising vortex ring plays the important role in formation of the thermodynamic and microphysical structure of developing Cu. 4) The study provides a novel insight into the cloud–environment interaction. 5) The approximating equations describing the vortex ring can be usefully applied for developing new schemes of convective parameterization.
Cited by 6Open access
C. Marugán-Cruz, Javier Rodríguez‐Rodríguez, C. Martı́nez-Bazán · Journal of Fluid Mechanics · 2013
Abstract The formation of vortex rings in negatively buoyant starting jets has been studied numerically for different values of the Richardson number, $\mathit{Ri}$ , covering the range of weak to moderate buoyancy effects ( $0\leq \mathit{Ri}\leq 0. 20$ ). Two different regimes have been identified in the vortex formation and the transition between them takes place at $\mathit{Ri}\approx 0. 03$ . The vorticity distribution inside the vortex ring after pinching off from the trailing stem as well as the total amount of circulation it encloses (characterized by the formation number, $F$ ) show different behaviours with the Richardson number in the two regimes. The differences are associated with the different mechanisms by which the head vortex absorbs the circulation injected by the starting jet. While secondary vortices are engulfed by the leading vortex before separating from the trailing jet in the weak buoyancy effects regime ( $0\lt \mathit{Ri}\lt 0. 03$ ), this phenomenon is not observed in the moderate buoyancy effects regime ( $0. 03\lt \mathit{Ri}\lt 0. 2$ ). Moreover it is shown that the formation number of a negatively buoyant vortex ring can be determined by considering that its dynamics are similar to that of a neutrally buoyant vortex but propagating with velocity corresponding to the negatively buoyant one. Based on this simple idea, a phenomenological model is presented to describe quantitatively the evolution of the formation number with the Richardson number, $F(\mathit{Ri})$ , obtained numerically. In addition, the limitations of different vortex identification methods used to evaluate the vortex properties in buoyant flows are discussed.
Cited by 22
Ling Xu · 2015
Cited by 0
Huifen Guo, Lei Gao, S. C. M. Yu · Physics of Fluids · 2020
In this paper, the formation process of the leading vortex ring in positively and negatively buoyant starting jets with moderate Richardson number in the range of 0.06 &lt; |Ri| ≤ 0.321 has been investigated numerically and theoretically. Using the similarity variables |Ri| and |Ri|2/5 for the positively and negatively buoyant starting jets, respectively, fitting equations can be obtained to predict the buoyant jet penetration rate. Based on these fitting equations, a revised circulation model is proposed by incorporating the effects of both over-pressure and buoyancy. The revised model is well consistent with the numerical results for all positively buoyant starting jets. However, for the negatively buoyant starting jets, this model can predict well only during the initial period. The over-pressure has little influence on the vortex ring characteristics of the starting jets with positive buoyancy, whereas it can significantly affect the vortex ring formation of the negatively buoyant starting jets at moderate Richardson numbers. As the positive Richardson number increases, the instabilities of the trailing shear layer occur earlier. At moderately negative Richardson numbers, a “double plume” structure (an inner sinking circular forced plume and an outer rising annular plume) can be observed. The outer negative vorticity layers develop gradually due to the baroclinic effect. Consequently, the size and strength of the leading vortex progressively decrease. As the negative Richardson number decreases, the negative vorticity layers occur earlier and grow faster.
Cited by 4
Haojun Zheng, Lei Gao, S. C. M. Yu · AIAA Journal · 2023
The vortex dynamics of starting rectangular jets with aspect ratios of 1, 4, 8, and 10 is investigated numerically at Reynolds number of 1000. The 45°-axis-switching for the square vortex ring and 90°-axis-switching for the rectangular vortex ring are found to be controlled by the secondary flow induced by the ring deformation. Vortex stretching appears near both the [Formula: see text] and [Formula: see text] centerline planes, which is weakened by the increasing aspect ratio. Vortex ring pinch-off on both centerline planes can be identified by the magnitude of the vorticity flux delivered into the vortex ring. The dimensionless time for pinch-off (the formation numbers) for the square jet is at about 2.0 on both diagonal plane and midplane. For the rectangular jets, the formation number is reduced from 2.3 to 1.5 on the major axis plane by increasing aspect ratio, while it approaches 1.7 on the minor axis plane, which is independent of the aspect ratio. Our results suggest that the vortex deformation accelerates the pinch-off process on both the major and minor axis planes. In particular, the appearance of secondary vortices on the major plane and the arc-shaped vortices on the minor plane signals the onset of pinch-off.
Cited by 6
Philippa Gail Brown, S. N. Brown, F. T. Smith et al. · Mathematika · 1993
An oncoming two-dimensional laminar boundary layer that develops an unstable inflection point and becomes three-dimensional is described by the Hall-Smith (1991) vortex/wave interaction equations. These equations are now examined in the neighbourhood of the position where the critical surface starts to form. A consistent structure is established in which an inviscid core flow is matched to a viscous buffer-layer solution where the appropriate jump condition on the transverse shear stress is satisfied. The final result is a bifurcation equation for the (constant) amplitude of the wave pressure. A representative classical velocity profile is considered to illustrate solutions of this equation for a range of values of the wave-numbers.
Cited by 35
Eric Limacher, Vaishnavi Ramaswamy, Sidrah Aslam et al. · 2016
Cited by 0Open access