L. Parras

Structure of trailing vortices: Comparison between particle image velocimetry measurements and theoretical models

The velocity field of the trailing vortex behind a wing at different angles of attack has been measured through the stereo particle image velocimetry technique in a water tunnel for Reynolds numbers between 20 000 and 40 000, and for several distances to the wing tip. After filtering out the vortex meandering, the radial profiles of the axial and the azimuthal velocity components and of the radial profiles of the vorticity were compared to the theoretical models for trailing vortices by [G. K. Batchelor, J. Fluid Mech. 20, 645 (1964)] and by [D. W. Moore and P. G. Saffman, Proc. R. Soc. London, Ser. A 333, 491 (1973)], whose main features are conveniently summarized. We take into account the downstream evolution of these profiles from just a fraction of the wing chord to more than ten chords. The radial profiles of the vorticity and the azimuthal velocity are shown to fit quite well to Moore and Saffman’s trailing vortex model, while Batchelor’s model does not fit so well, especially in the tails of the p...

Dynamics of the wing-tip vortex in the near field of a NACA 0012 aerofoil

Vortex meandering (or wandering) is a typical feature of wing-tip vortices that consists in a random fluctuation of its vortex centreline. This meandering of the vortex is quite significant a few chords downstream the wing, and was originally thought to be due to free stream turbulence [1], then to instabilities of the vortex core [2]. But, independently of the controversy about its origin [3], the quantitative characterization of the vortex wandering phenomenon is a subject of current research [4]-[6]. In this work we have undertaken a systematic visualization of the trailing vortex behind a NACA0012 airfoil at several distances near the wing tip for different angles of attack and different Reynolds numbers to characterize the structure of the vortex meandering phenomenon as well as its frequency, wavelength, and amplitude. The technique is similar to that used by Roy and Leweke [5], but we characterize the downstream evolution of these vortex meandering characteristics and, therefore, the dynamics of the wing-tip vortex in the near field.

Acute intestinal anisakiasis in Spain: a fourth-stage Anisakis simplex larva

A case of acute intestinal anisakiasis has been reported; a nematode larva being found in the submucosa of the ileum of a woman in Jaén (Spain). The source of infection was the ingestion of raw Engraulis encrasicholus. On the basis of its morphology, the worm has been identified as a fourth-stage larva of Anisakis simplex. In Spain, this is the ninth report of human anisakiasis and also probably the first case of anisakiasis caused by a fourth-stage larva of A. simplex.

Spatial stability and the onset of absolute instability of Batchelor's vortex for high swirl numbers

Batchelor’s vortex has been commonly used in the past as a model for aircraft trailing vortices. Using a temporal stability analysis, new viscous unstable modes have been found for the high swirl numbers of interest in actual large-aircraft vortices. We look here for these unstable viscous modes occurring at large swirl numbers ( q> 1.5), and large Reynolds numbers (Re > 10 3 ), using a spatial stability analysis, thus characterizing the frequencies at which these modes become convectively unstable for different values of q, Re, and for different intensities of the uniform axial flow. We consider both jet-like and wake-like Batchelor’s vortices, and are able to analyse the stability for Re as high as 10 8 . We also characterize the frequencies and the swirl numbers for the onset of absolute instabilities of these unstable viscous modes for large q.

Temporal instability modes of supersonic round jets

In this study, a comprehensive inviscid temporal stability analysis of a compressible round jet is performed for Mach numbers ranging from 1 to 10. We show that in addition to the Kelvin‐Helmholtz instability modes, there exist for each azimuthal wavenumber three other types of modes (counterflow subsonic waves, subsonic waves and supersonic waves) whose characteristics are analysed in detail using a WKBJ theory in the limit of large axial wavenumber. The theory is constructed for any velocity and temperature profile. It provides the phase velocity and the spatial structure of the modes and describes qualitatively the e!ects of base-flow modifications on the mode characteristics. The theoretical predictions are compared with numerical results obtained for an hyperbolic tangent model and a good agreement is demonstrated. The results are also discussed in the context of jet noise. We show how the theory can be used to determine a priori the impact of jet modifications on the noise induced by instability.

Experimental study of rotating Hagen-Poiseuille flow discharging into a 1:8 sudden expansion

In this paper, we present experimental evidence for the five different states that result from rotating Hagen-Poiseuille flow when it discharges into a 1:8 sudden expansion, namely: stable, convectively unstable, unstable shear layer, stable and unstable vortex breakdowns. Sanmiguel-Rojas et al. [“Three-dimensional structure of confined swirling jets at moderately large Reynolds numbers,” Phys. Fluids 20, 044104 (2008)] numerically predicted four of these five states and mapped the transition from one state to another. Our main objective is to study the onset of instabilities and vortex breakdown in these states experimentally. For this purpose, we visualize the flow at the inlet of the expansion for several values of moderately large Reynolds numbers, Re, and of swirl parameters, S. We analyze the inner region of the state that corresponds to the unstable shear layer in the sudden expansion and find two different states that share the same character, although they have different non-dimensional frequenci...

Interaction of an unconfined vortex with a solid surface

The interaction of an open vortex with a solid plane perpendicular to the axis of the vortex is analyzed numerically. We solve the axisymmetric, incompressible Navier-Stokes equations with boundary conditions that far from the axis correspond to the near-inviscid far-field of Long’s similarity solution for an open vortex. Continuation techniques are used to solve the equations of motion for varying Reynolds numbers. When this parameter is large enough, a vortex breakdown phenomenon occurs, producing a small region of reversed flow at the axis. This region increases in size and migrates toward the solid plane for increasing Reynolds numbers. The subsequent intensification of the swirl near both the axis and the surface generates a bifurcation with nonuniqueness of the solution corresponding to a new, more intense, vortex breakdown. At the end, for Reynolds numbers above a critical value, the flow acquires a two-celled structure, with a region of reversed flow all along the axis surrounded by an annular upd...

Two-dimensional instability of the bottom boundary layer under a solitary wave

The objective of this paper is to establish a detailed map for the temporal instability of the bottom boundary layer (BBL) flow driven by a soliton-like wave induced pressure gradient in a U-tube-shaped tunnel, which serves as an approximation to the BBL under surface solitary waves. Both linear stability analysis and fully nonlinear two-dimensional simulations using high-order numerical methods have been carried out. The process of delineation of the stability regions as a function of boundary layer thickness-based Reynolds number of the temporally evolving base flow, Reδ, consists of two parts. In the first part, we assess the lower limit of the Reδ range within which the standard, quasi-steady, linear stability analysis is applicable when considering individual base flow profiles sampled during its transient evolution. Below this limit, transient linear stability analysis serves as a more accurate predictor of the stability properties of the base flow. In the second step, above the Reδ limit where the ...

Frequency response of Lamb-Oseen vortex

In this numerical study we present the frequency response of the Lamb-Oseen (Gaussian) vortex for two synthetic jet configurations. The first one consists of an annular axial jet that is superimposed on the Gaussian vortex. The other configuration deals with an off-axis, single-point, axial jet (SPI). We detect that the system responds to the forcing for a given axial wavenumber, k, exciting natural modes of the vortex by a resonance mechanism. We propose an explanation for the physical mechanism responsible for the maximum energy gain obtained by comparing our results with the different branches found theoretically by Fabre et al (2006 J. Fluid Mech. 551 235–74). We find high energy gains in both cases ( for the annular jet and for the SPI jet), so these types of forcing are able to produce responses of the system strong enough to reach a non-linear state. Axisymmetric modes, with azimuthal wavenumber m = 0, produce the highest energy gain while applying an annular forcing. However, other modes, such as the helical one m = 1 and also double helix modes with m = 2, contribute in the SPI configuration. We find that the best region to be tested experimentally in both cases is the region that corresponds to the L2 branch described by Fabre and his collaborators. Furthermore, and whenever using these L2 branch frequencies, the response of the system is always axisymmetric, independently of the type of excitation. Finally, we conclude that the energy gain with the SPI jet is one order of magnitude greater than for the annular jet, so that the single-point off-axis jet is a feasible candidate to design a control device.

Experimental Study of Very Low Aspect Ratio Wings in Slender Bodies

The dynamics of very low aspect ratio wings (or strakes) vortices in slender bodies are complex due to the interaction of the shed vortex sheet and the body vortex. For missiles at supersonic speeds these interactions are not easily predicted using engineering level tools. To shed some new light onto this problem, an experimental study in a water channel for moderate Reynolds number (Re=1000) was performed for a 19D body and strake configuration with strakes having a span to body diameter ratio of 1.25. Comparisons to numerical simulations in supersonic flow are also performed. Flow visualisation has been carried out to characterize the vortex dynamics at different angles of attack; these being 11 ◦ , 16 ◦ , 22 ◦ and 27 ◦ . The comparison between a slender body without strakes and the body-strake configuration has given some key indicators in relation to the vortex position