Characterization of Vortex Dynamics in the Near Wake of an Oscillating Flexible Foil

An experimental study was conducted to explore the effect of surface flexibility at the leading and trailing edges on the near-wake flow dynamics of a sinusoidal heaving foil. Midspan particle image velocimetry (PIV) measurements were taken in a closed-loop wind tunnel at a Reynolds number of 25,000 and at a range of reduced frequencies (k = fc/U) from 0.09 to 0.20. Time-resolved and phase-locked measurements are used to describe the mean flow characteristics and phase-averaged vortex structures and their evolution. Large-eddy scale (LES) decomposition and swirling strength analysis are used to quantify the vortical structures. The results demonstrate that trailing edge flexibility has minimal influence on the mean flow characteristics. The mean velocity deficit for the flexible trailing edge and rigid foils remains constant for all reduced frequencies tested. However, the trailing edge flexibility increases the swirling strength of the small-scale structures, resulting in enhanced cross-stream dispersion. Flexibility at the leading edge is shown to generate a large-scale leading edge vortex (LEV) for k ≥ 0.18. This results in a reduction in the swirling strength due to vortex interactions when compared to the flexible trailing edge and rigid foils. Furthermore, it is shown that the large-scale LEV is responsible for extracting a significant portion of energy from the mean flow, reducing the mean flow momentum in the wake. The kinetic energy loss in the wake is shown to scale with the energy content of the LEV.

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Trailing edge flap influence on leading edge vortex flap aerodynamics

10.2514/6.1982-128 ◽

1982 ◽

Cited By ~ 2

Author(s):

J. MARCHMAN, III ◽

A. GRANTZ

Keyword(s):

Leading Edge ◽

Trailing Edge ◽

Leading Edge Vortex ◽

Edge Vortex ◽

Trailing Edge Flap

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Heat Transfer and Flowfield Measurements in the Leading Edge Region of a Stator Vane Endwall

Journal of Turbomachinery ◽

10.1115/1.2841351 ◽

1999 ◽

Vol 121 (3) ◽

pp. 558-568 ◽

Cited By ~ 58

Author(s):

M. B. Kang ◽

A. Kohli ◽

K. A. Thole

Keyword(s):

Heat Transfer ◽

Reynolds Number ◽

Leading Edge ◽

Reynolds Numbers ◽

Secondary Flows ◽

Edge Region ◽

Leading Edge Vortex ◽

Stator Vane ◽

Edge Vortex ◽

The Mean

The leading edge region of a first-stage stator vane experiences high heat transfer rates, especially near the endwall, making it very important to get a better understanding of the formation of the leading edge vortex. In order to improve numerical predictions of the complex endwall flow, benchmark quality experimental data are required. To this purpose, this study documents the endwall heat transfer and static pressure coefficient distribution of a modern stator vane for two different exit Reynolds numbers (Reex = 6 × 105 and 1.2 × 106). In addition, laser-Doppler velocimeter measurements of all three components of the mean and fluctuating velocities are presented for a plane in the leading edge region. Results indicate that the endwall heat transfer, pressure distribution, and flowfield characteristics change with Reynolds number. The endwall pressure distributions show that lower pressure coefficients occur at higher Reynolds numbers due to secondary flows. The stronger secondary flows cause enhanced heat transfer near the trailing edge of the vane at the higher Reynolds number. On the other hand, the mean velocity, turbulent kinetic energy, and vorticity results indicate that leading edge vortex is stronger and more turbulent at the lower Reynolds number. The Reynolds number also has an effect on the location of the separation point, which moves closer to the stator vane at lower Reynolds numbers.

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The structure of a highly decelerated axisymmetric turbulent boundary layer

Journal of Fluid Mechanics ◽

10.1017/jfm.2021.845 ◽

2021 ◽

Vol 929 ◽

Author(s):

N. Agastya Balantrapu ◽

Christopher Hickling ◽

W. Nathan Alexander ◽

William Devenport

Keyword(s):

Boundary Layer ◽

Pressure Gradient ◽

Shear Layer ◽

Layer Thickness ◽

Boundary Layer Thickness ◽

Large Scale ◽

Convection Velocity ◽

Mean Flow ◽

Mean Velocity ◽

The Mean

Experiments were performed over a body of revolution at a length-based Reynolds number of 1.9 million. While the lateral curvature parameters are moderate ( $\delta /r_s < 2, r_s^+>500$ , where $\delta$ is the boundary layer thickness and r s is the radius of curvature), the pressure gradient is increasingly adverse ( $\beta _{C} \in [5 \text {--} 18]$ where $\beta_{C}$ is Clauser’s pressure gradient parameter), representative of vehicle-relevant conditions. The mean flow in the outer regions of this fully attached boundary layer displays some properties of a free-shear layer, with the mean-velocity and turbulence intensity profiles attaining self-similarity with the ‘embedded shear layer’ scaling (Schatzman & Thomas, J. Fluid Mech., vol. 815, 2017, pp. 592–642). Spectral analysis of the streamwise turbulence revealed that, as the mean flow decelerates, the large-scale motions energize across the boundary layer, growing proportionally with the boundary layer thickness. When scaled with the shear layer parameters, the distribution of the energy in the low-frequency region is approximately self-similar, emphasizing the role of the embedded shear layer in the large-scale motions. The correlation structure of the boundary layer is discussed at length to supply information towards the development of turbulence and aeroacoustic models. One major finding is that the estimation of integral turbulence length scales from single-point measurements, via Taylor's hypothesis, requires significant corrections to the convection velocity in the inner 50 % of the boundary layer. The apparent convection velocity (estimated from the ratio of integral length scale to the time scale), is approximately 40 % greater than the local mean velocity, suggesting the turbulence is convected much faster than previously thought. Closer to the wall even higher corrections are required.

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A Turbulent Boundary Layer Flow Over an Open Shallow Cavity

10.1115/fedsm2016-7765 ◽

2016 ◽

Cited By ~ 1

Author(s):

Khaled J. Hammad

Keyword(s):

Boundary Layer ◽

Large Scale ◽

Vector Fields ◽

Turbulence Models ◽

Leading Edge ◽

Field Measurements ◽

Recirculation Region ◽

Mean Flow ◽

Trailing Edge ◽

Cavity Depth

Particle Image Velocimetry (PIV) was used to study the flow structure and turbulence, upstream, over, and downstream a shallow open cavity. Three sets of PIV measurements, corresponding to a turbulent incoming boundary layer and a cavity length-to-depth ratio of four, are reported. The cavity depth based Reynolds numbers were 21,000; 42,000; and 54,000. The selected flow configuration and well characterized inflow conditions allow for straightforward assessment of turbulence models and numerical schemes. All mean flow field measurements display a large flow recirculation region, spanning most of the cavity and a smaller, counter-rotating, secondary vortex, immediately downstream of the cavity leading edge. The Galilean decomposed instantaneous velocity vector fields, clearly demonstrate two distinct modes of interaction between the free shear and the cavity trailing edge. The first corresponds to a cascade of vortical structures emanating from the tip of the leading edge of the cavity that grow in size as they travel downstream and directly interact with the trailing edge, i.e., impinging vortices. The second represents vortices that travel above the trailing edge of the cavity, i.e., non-impinging vortices. In the case of impinging vortices, a strong, large scale region of recirculation forms inside the cavity and carries the flow disturbances, arising from the impingement of vortices on the trailing edge of the cavity, upstream in a manner that interacts with and influences the flow as it separates from the cavity leading edge.

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Estimation of Take-Off Performance for Supersonic Transport with Leading-Edge Vortex Flaps and Trailing-Edge Flaps

JOURNAL OF THE JAPAN SOCIETY FOR AERONAUTICAL AND SPACE SCIENCES ◽

10.2322/jjsass.51.327 ◽

2003 ◽

Vol 51 (593) ◽

pp. 327-329

Author(s):

Katsuhiro Miyata ◽

Kenichi Rinoie

Keyword(s):

Leading Edge ◽

Trailing Edge ◽

Leading Edge Vortex ◽

Supersonic Transport ◽

Trailing Edge Flaps ◽

Edge Vortex

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WKB theory for rapid distortion of inhomogeneous turbulence

Journal of Fluid Mechanics ◽

10.1017/s0022112099005340 ◽

1999 ◽

Vol 390 ◽

pp. 325-348 ◽

Cited By ~ 29

Author(s):

S. NAZARENKO ◽

N. K.-R. KEVLAHAN ◽

B. DUBRULLE

Keyword(s):

Large Scale ◽

Isotropic Turbulence ◽

Two Dimensional ◽

Turbulent Spot ◽

Mean Flow ◽

Mean Velocity ◽

Inhomogeneous Turbulence ◽

Large Scale Vortex ◽

The Mean ◽

Mean Strain

A WKB method is used to extend RDT (rapid distortion theory) to initially inhomogeneous turbulence and unsteady mean flows. The WKB equations describe turbulence wavepackets which are transported by the mean velocity and have wavenumbers which evolve due to the mean strain. The turbulence also modifies the mean flow and generates large-scale vorticity via the averaged Reynolds stress tensor. The theory is applied to Taylor's four-roller flow in order to explain the experimentally observed reduction in the mean strain. The strain reduction occurs due to the formation of a large-scale vortex quadrupole structure from the turbulent spot confined by the four rollers. Both turbulence inhomogeneity and three-dimensionality are shown to be important for this effect. If the initially isotropic turbulence is either homogeneous in space or two-dimensional, it has no effect on the large-scale strain. Furthermore, the turbulent kinetic energy is conserved in the two-dimensional case, which has important consequences for the theory of two-dimensional turbulence. The analytical and numerical results presented here are in good qualitative agreement with experiment.

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LDV and PIV Velocity Results in a Thermosiphon

Volume 2: Symposia, Parts A, B, and C ◽

10.1115/fedsm2003-45601 ◽

2003 ◽

Author(s):

J. Kulman ◽

D. Gray ◽

S. Sivanagere ◽

S. Guffey

Keyword(s):

Large Scale ◽

Single Phase ◽

Flow Characteristics ◽

Reynolds Numbers ◽

Flow Patterns ◽

Laser Doppler Velocimeter ◽

Mean Velocity ◽

Developed Turbulence ◽

The Mean ◽

Good Agreement

Heat transfer and flow characteristics have been determined for a single-phase rectangular loop thermosiphon. The plane of the loop was vertical, and tests were performed with in-plane tilt angles ranging from 3.6° CW to 4.2° CCW. Velocity profiles were measured in one vertical leg of the loop using both a single-component Laser Doppler Velocimeter (LDV), and a commercial Particle Image Velocimeter (PIV) system. The LDV data and PIV data were found to be in good agreement. The measured average velocities were approximately 2–2.5 cm/s at an average heating rate of 70 W, and were independent of tilt angle. Significant RMS fluctuations of 10–20% of the mean velocity were observed in the test section, in spite of the laminar or transitional Reynolds numbers (order of 700, based on the hydraulic diameter). These fluctuations have been attributed to vortex shedding from the upstream temperature probes and mitre bends, rather than to fully developed turbulence. Animations of the PIV data clearly show these large scale unsteady flow patterns. Multiple steady state flow patterns were not observed.

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Coherent structures and dominant frequencies in a turbulent three-dimensional diffuser

Journal of Fluid Mechanics ◽

10.1017/jfm.2012.107 ◽

2012 ◽

Vol 699 ◽

pp. 320-351 ◽

Cited By ~ 14

Author(s):

Johan Malm ◽

Philipp Schlatter ◽

Dan S. Henningson

Keyword(s):

Coherent Structures ◽

Large Scale ◽

Three Dimensional ◽

Flow Characteristics ◽

Low Frequency ◽

Bulk Velocity ◽

Time Signal ◽

Mean Flow ◽

The Mean ◽

Dominant Frequencies

AbstractDominant frequencies and coherent structures are investigated in a turbulent, three-dimensional and separated diffuser flow at $\mathit{Re}= 10\hspace{0.167em} 000$ (based on bulk velocity and inflow-duct height), where mean flow characteristics were first studied experimentally by Cherry, Elkins and Eaton (Intl J. Heat Fluid Flow, vol. 29, 2008, pp. 803–811) and later numerically by Ohlsson et al. (J. Fluid Mech., vol. 650, 2010, pp. 307–318). Coherent structures are educed by proper orthogonal decomposition (POD) of the flow, which together with time probes located in the flow domain are used to extract frequency information. The present study shows that the flow contains multiple phenomena, well separated in frequency space. Dominant large-scale frequencies in a narrow band $\mathit{St}\equiv fh/ {u}_{b} \in [0. 0092, 0. 014] $ (where $h$ is the inflow-duct height and ${u}_{b} $ is the bulk velocity), yielding time periods ${T}^{\ensuremath{\ast} } = T{u}_{b} / h\in [70, 110] $, are deduced from the time signal probes in the upper separated part of the diffuser. The associated structures identified by the POD are large streaks arising from a sinusoidal oscillating motion in the diffuser. Their individual contributions to the total kinetic energy, dominated by the mean flow, are, however, small. The reason for the oscillating movement in this low-frequency range is concluded to be the confinement of the flow in this particular geometric set-up in combination with the high Reynolds number and the large separated zone on the top diffuser wall. Based on this analysis, it is shown that the bulk of the streamwise root mean square (r.m.s.) value arises due to large-scale motion, which in turn can explain the appearance of two or more peaks in the streamwise r.m.s. value. The weak secondary flow present in the inflow duct is shown to survive into the diffuser, where it experiences an imbalance with respect to the upper expanding corners, thereby giving rise to the asymmetry of the mean separated region in the diffuser.

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