On the peculiar structure of a helical wake vortex behind an inclined prolate spheroid

2016 ◽  
Vol 801 ◽  
pp. 1-12 ◽  
Author(s):  
Fengjian Jiang ◽  
Helge I. Andersson ◽  
José P. Gallardo ◽  
Valery L. Okulov
Keyword(s):  
Prolate Spheroid ◽  
Self Similarity ◽  
Flow Topology ◽  
Near Wake ◽  
Recirculation Bubble ◽  
Helical Vortex ◽  
Gradual Development ◽  
Coherent Flow Structure ◽  
First Time ◽  
Similarity Law

The self-similarity law for axisymmetric wakes has for the first time been examined and verified in a complex helical vortex in the far part of an asymmetric wake by means of direct numerical simulation (DNS). The helical vortex is the main coherent flow structure in the transitional non-axisymmetric wake behind an inclined 6:1 prolate spheroid at Reynolds number 3000 based on the minor axis. The gradual development of the complex helical vortex structure has been described in detail all the way from its inception at the spheroid and into the far wake. We observed a complex vortex composition in the generation stage, a rare jet-like wake pattern in the near wake and an abrupt change of helical symmetry in the vortex core without an accompanying change in flow topology, i.e. with no recirculation bubble.

scholarly journals Near-Wake Observations behind Azimuthally Perforated Disks With Varying Hole Layout and Porosity in Smooth Airstreams at High Reynolds Numbers

2018 ◽  
Vol 141 (5) ◽  
Author(s):  
Raf Theunissen ◽  
Robert Worboys
Keyword(s):  
Drag Coefficient ◽  
Experimental Studies ◽  
Reynolds Numbers ◽  
Wake Flow ◽  
Mean Flow ◽  
Flow Topology ◽  
Near Wake ◽  
Two Component ◽  
High Reynolds ◽  
Component Particle

Porous disks are commonly encountered in experimental studies dealing with flow through objects such as wind turbines, parachutes, and fluidic devices to regulate pressure and/or downstream turbulence. Perforations are typically staggered and only porosity is altered to attain the required disk drag coefficient, despite a documented influence of topology. Few works have reported, however, to which extent the spatial distribution of the circular perforations affect the mean flow pertaining freestanding disks, and for this reason, this work presents a first, more systematic study focused on the effect of azimuthally varying hole topology and porosity on disk drag and near-wake characteristics. An experimental study performed in airflows of negligible freestream turbulence at Reynolds numbers in the order of 105 is reported and related to the existing literature to ensure reliability. Complementary to drag measurements, near-wake surveys have been performed on a variety of perforation layouts using two-component laser Doppler velocimetry and two-component particle image velocimetry. It is shown that minor changes in perforations can cause drastic changes in near-wake flow topology and no perforation layout can be consistently associated with highest drag. Explicit empirical expressions for drag coefficient linked with the simplified topologies considered have been derived.

A Late Upper Aurignacian Station in North Lincolnshire

1932 ◽  
Vol 6 (4) ◽  
pp. 335-339
Author(s):  
A. Leslie Armstrong
Keyword(s):  
Upper Palaeolithic ◽  
Research Committee ◽  
Distinctive Character ◽  
Rock Shelter ◽  
English Expression ◽  
Gradual Development ◽  
The Subject ◽  
University Of Bristol ◽  
The University ◽  
First Time

The site forming the subject of this communication is an open-air station of Upper Palæolithic date, situated near the northern extremity of the Lincolnshire Cliff range, and previously unrecorded. The cultural horizon of the site closely corresponds with that of the upper levels of the rock-shelter known as ‘Mother Grundy's Parlour,’ Creswell Crags, Derbyshire, excavated by the writer in 1924, under the auspices of the British Association Research Committee for the Exploration of Caves in Derbyshire. Those excavations revealed, for the first time, the gradual development of our English phase of the Upper Aurignacian, and established the fact that this was of a distinctive character, and had been evolved practically free from Magdalenian influences. Excavations in the Mendip caves by the University of Bristol Spelæological Society, and elsewhere, have since confirmed these conclusions; and it is now recognised tlhat the culture is essentially an English expression of Upper Aurignacian, which is typical of the Upper Palæolithic in this country.

Near Wake Regime Study on Wind Turbine Blade Tip Vortex

2019 ◽  
Author(s):  
Ojing Siram ◽  
Niranjan Sahoo
Keyword(s):  
Wind Turbine ◽  
Strouhal Number ◽  
Vortex Shedding ◽  
Flow Condition ◽  
Pitch Angle ◽  
Tip Vortex ◽  
Near Wake ◽  
Helical Vortex ◽  
Blade Tip ◽  
Blade Pitch Angle

Abstract In the present research article results on wind turbine blade tip vortex have been presented, the measurements have been done behind a model scale of horizontal axis wind turbine rotor. The rotor used for flow characterization is a three-bladed having NACA0012 cross-section, the study has been performed for low range tip speed ratio of 0–2 and wind speeds range of 3–6 m/s. The investigation has been conducted specifically to near wake regime, which is often expressed as the region of regular helical vortex structures. Although this nature of regular helical vortex pattern has always been a question of debate with respect to changes in the flow condition, rotor geometry and point of measurements. A systematic experiment was done mainly on the frequency of vortex shedding through hot-wire anemometry (HWA), and the corresponding frequency is express in terms of Strouhal number. Present article work within near wake regime includes tip vortex shedding stability analysis for different blade pitch angle and flow condition. From the systematic experimental observation, the evaluated data indicate that the Strouhal number has an incremental trend when the blade pitch angle is close to 40°, and above it inconsistency in frequency response is observed.

scholarly journals An LES Investigation of the Near-Wake Flow Topology of a Simplified Heavy Vehicle

2018 ◽  
Vol 102 (2) ◽  
pp. 389-415 ◽  
Author(s):  
Anirudh N. Rao ◽  
Jie Zhang ◽  
Guglielmo Minelli ◽  
Branislav Basara ◽  
Siniša Krajnović
Keyword(s):  
Wake Flow ◽  
Heavy Vehicle ◽  
Flow Topology ◽  
Near Wake

Numerical analysis of bluff body wakes under periodic open-loop control

2013 ◽  
Vol 739 ◽  
pp. 94-123 ◽  
Author(s):  
Derwin J. Parkin ◽  
M. C. Thompson ◽  
J. Sheridan
Keyword(s):  
Strouhal Number ◽  
Cross Section ◽  
Vortex Shedding ◽  
Open Loop ◽  
Wake Flow ◽  
Dynamic Mode ◽  
Two Dimensional ◽  
Near Wake ◽  
Recirculation Bubble ◽  
Number Range

AbstractLarge eddy simulations at$Re= 23\hspace{0.167em} 000$are used to investigate the drag on a two-dimensional elongated cylinder caused by rear-edge periodic actuation, with particular focus on an optimum open-loop configuration. The 3.64 (length/thickness) aspect-ratio cylinder has a rectangular cross-section with rounded leading corners, representing the two-dimensional cross-section of the now genericAhmed-body geometry. The simulations show that the optimum drag reduction occurs in the forcing Strouhal number range of$0. 09\leq S{t}_{act} \leq 0. 135$, which is approximately half of the Strouhal number corresponding to shedding of von Kármán vortices into the wake for the natural case. This result agrees well with recent experiments of Henninget al. (Active Flow Control, vol. 95, 2007, pp. 369–390). A thorough transient wake analysis employing dynamic mode decomposition is conducted for all cases, with special attention paid to the Koopman modes of the wake flow and vortex progression downstream. Two modes are found to coexist in all cases, the superimposition of which recovers the majority of features observed in the flow. Symmetric vortex shedding in the near wake, which effectively extends the mean recirculation bubble, is shown to be the major mechanism in lowering the drag. This is associated with opposite-signed vortices reducing the influence of natural vortex shedding, resulting in an increase in the pressure in the near wake, while the characteristic wake antisymmetry returns further downstream. Lower-frequency actuation is shown to create larger near-wake symmetric vortices, which improves the effectiveness of this process.

Separated Flows Around the Rear Window of a Simplified Car Geometry

2008 ◽  
Vol 130 (2) ◽  
Author(s):  
Mathieu Rouméas ◽  
Patrick Gilliéron ◽  
Azeddine Kourta
Keyword(s):  
Blunt Body ◽  
Wake Flow ◽  
Complex Flow ◽  
Flow Topology ◽  
Rear Window ◽  
Near Wake ◽  
Longitudinal Vortices ◽  
Simulation Based ◽  
Aerodynamic Performances ◽  
Boltzmann Method

A 3D numerical simulation based on the lattice Boltzmann method is carried out on a simplified car geometry (initially proposed by Ahmed, Ramm, and Falting, 1984, SAE Technical Paper series No. 840300) to analyze and establish a method for controlling the near-wake flow topology of a generic blunt body model. The results indicate the existence of a complex flow topology consisting of transverse and longitudinal vortices emanating from flow separations that occur on the top and the lateral edges of the slanted rear window, respectively. The topology of each structure is detailed and the numerical results are compared with the experimental results in the literature. The results presented in this paper can then be used to develop and parametrize active control solutions conducive to improving the aerodynamic performances of automobile vehicles.

Critical points in a turbulent near wake

1994 ◽  
Vol 275 ◽  
pp. 59-81 ◽  
Author(s):  
Y. Zhou ◽  
R. A. Antonia
Keyword(s):  
Critical Points ◽  
Phase Plane ◽  
Reynolds Shear Stress ◽  
Saddle Points ◽  
Normal Stresses ◽  
Flow Topology ◽  
Near Wake ◽  
Spanwise Vortex ◽  
Plane Technique ◽  
Insight Into

Velocity data were obtained in the turbulent wake of a circular cylinder with an orthogonal array of sixteen X-wires, eight in the (x, y)-plane and eight in the (x, z)-plane. By applying the phase-plane technique to these data, three types of critical points (where the velocity is zero and the streamline slope is indeterminate) were identified. Of these, foci and saddle points occurred most frequently, although a significant number of nodes was also found. Flow topology and properties associated with these points were obtained in each plane. Saddle-point regions associated with spanwise vortices provide the dominant contribution to the Reynolds shear stress and larger contributions to the normal stresses than focal regions. The topology was found to be in close agreement with that obtained from other methods of detecting features of the organized motion. The inter-relationship between critical points simultaneously identified in the two planes can provide some insight into the three-dimensionality of the organized motion. Foci in the (x, z)-plane correspond, with relatively high probability and almost negligible streamwise separation, to saddle points in the (x, y)-plane and are interpreted in terms of ribs aligned with the diverging separatrix between consecutive spanwise vortex rolls. Foci in the (x, z)-plane which correspond, with relatively weak probability, to foci in the (x, y)-plane seem consistent with a distortion of the vortex rolls in the (y, z)-plane.

scholarly journals 3D Diatom–Designed and Selective Laser Melting (SLM) Manufactured Metallic Structures

2019 ◽  
Vol 9 (1) ◽  
Author(s):  
Izabela Zglobicka ◽  
Agnieszka Chmielewska ◽  
Emre Topal ◽  
Kristina Kutukova ◽  
Jürgen Gluch ◽  
...  
Keyword(s):  
Selective Laser Melting ◽  
Three Dimensional ◽  
Model Systems ◽  
Self Similarity ◽  
Laser Melting ◽  
Metallic Structures ◽  
Perfect Model ◽  
X Ray Computed ◽  
Biomimetic Approach ◽  
First Time

AbstractDiatom frustules, with their diverse three-dimensional regular silica structures and nano- to micrometer dimensions, represent perfect model systems for biomimetic fabrication of materials and devices. The structure of a frustule of the diatom Didymosphenia geminata was nondestructively visualized using nano X-ray computed tomography (XCT) and transferred into a CAD file for the first time. Subsequently, this CAD file was used as the input for an engineered object, which was manufactured by applying an additive manufacturing technique (3D Selective Laser Melting, SLM) and using titanium powder. The self-similarity of the natural and the engineered objects was verified using nano and micro XCT. The biomimetic approach described in this paper is a proof-of-concept for future developments in the scaling-up of manufacturing based on special properties of microorganisms.

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