Effect of Trailing Edge Suction on Coherent Structures in Near Wake

1998 ◽  
Vol 120 (2) ◽  
pp. 378-384
Author(s):  
S. D. Sharma ◽  
R. K. Sahoo
Keyword(s):  
Coherent Structures ◽  
Large Scale ◽  
Marked Change ◽  
Sampling Technique ◽  
Hot Wire ◽  
Near Wake ◽  
Trailing Edge ◽  
Trailing Edges ◽  
Base Drag ◽  
Vorticity Balance

Experimental results, obtained from hot-wire measurements using a conditional sampling technique, demonstrate feasibility of controlling large-scale spanwise vortices (coherent structures) in the near wake region behind a rectangular base by means of suction through a slit at just one of the trailing edges. The suction thus employed, is found to influence the near wake topology with strong asymmetry and disturb the net vorticity balance. Moreover, a significant reduction in the base drag is achieved as a consequence of the trailing edge suction. The mechanism of the drag reduction is understood to lie in a marked change in the wake dynamics including attenuation in the size and strength of the coherent structures.

Wake Measurements and Loss Evaluation in a Controlled Diffusion Compressor Cascade

1990 ◽  
Author(s):  
R. P. Shreeve ◽  
Y. Elazar ◽  
J. W. Dreon ◽  
A. Baydar
Keyword(s):  
Large Scale ◽  
Velocity Profiles ◽  
Laser Doppler Velocimeter ◽  
Pressure Probe ◽  
Compressor Cascade ◽  
Near Wake ◽  
Trailing Edge ◽  
Controlled Diffusion ◽  
Edge Surface ◽  
Probe Measurements

The results of two component laser-Doppler velocimeter (LDV) surveys made in the near wake (to one fifth chord) of a controlled diffusion (CD) compressor blade in a large scale cascade wind tunnel, are reported. The measurements were made at three positive incidence angles from near-design to angles thought to approach stall. Comparisons were made with calibrated pressure probe and hot-wire wake measurements and good agreement was found. The flow was found to be fully attached at the trailing edge at all incidence angles and the wake profiles were found to be highly skewed. Despite the precision obtained in the wake velocity profiles, the blade loss could not be evaluated accurately without measurements of the pressure field. The blade trailing edge surface pressures and velocity profiles were found to be consistent with downstream pressure probe measurements of loss, allowing conclusions to be drawn concerning the design of the trailing edge.

Wake Measurements and Loss Evaluation in a Controlled Diffusion Compressor Cascade

1991 ◽  
Vol 113 (4) ◽  
pp. 591-599 ◽  
Author(s):  
R. P. Shreeve ◽  
Y. Elazar ◽  
J. W. Dreon ◽  
A. Baydar
Keyword(s):  
Large Scale ◽  
Velocity Profiles ◽  
Laser Doppler Velocimeter ◽  
Pressure Probe ◽  
Compressor Cascade ◽  
Near Wake ◽  
Trailing Edge ◽  
Controlled Diffusion ◽  
Edge Surface ◽  
Probe Measurements

The results of two component laser-Doppler velocimeter (LDV) surveys made in the near wake (to one fifth chord) of a controlled diffusion (CD) compressor blade in a large-scale cascade wind tunnel are reported. The measurements were made at three positive incidence angles from near design to angles thought to approach stall. Comparisons were made with calibrated pressure probe and hot-wire wake measurements and good agreement was found. The flow was found to be fully attached at the trailing edge at all incidence angles and the wake profiles were found to be highly skewed. Despite the precision obtained in the wake velocity profiles, the blade loss could not be evaluated accurately without measurements of the pressure field. The blade trailing edge surface pressures and velocity profiles were found to be consistent with downstream pressure probe measurements of loss, allowing conclusions to be drawn concerning the design of the trailing edge.

Coherent Structures in a Three-Dimensional Wall Jet

1990 ◽  
Vol 112 (4) ◽  
pp. 462-467 ◽  
Author(s):  
Hisashi Matsuda ◽  
Sei-ichi Iida ◽  
Michio Hayakawa
Keyword(s):  
Coherent Structures ◽  
Large Scale ◽  
Near Field ◽  
Three Dimensional ◽  
Sampling Technique ◽  
Wall Jet ◽  
Structure Model ◽  
Streamwise Vortices ◽  
Lateral Velocity ◽  
Circular Nozzle

The formation mechanism of streamwise vortices in the near field of the three-dimensional wall jet discharging from a circular nozzle along a flat plate is studied experimentally using a conditional sampling technique. Ensemble-averages of the lateral velocity component indicate the presence of large-scale horseshoe-like structures, whose legs are inclined and stretched to form the streamwise vortices in the mixing region of the jet. Based on the present result, a coherent structure model for the near field of the wall jet is proposed.

Topological description of near-wall flows around a surface-mounted square cylinder at high Reynolds numbers

2021 ◽  
Vol 933 ◽  
Author(s):  
Yong Cao ◽  
Tetsuro Tamura ◽  
Dai Zhou ◽  
Yan Bao ◽  
Zhaolong Han
Keyword(s):  
Large Scale ◽  
Side Wall ◽  
Wake Vortex ◽  
Junction Region ◽  
Near Wake ◽  
Trailing Edge ◽  
Tip Vortices ◽  
Wall Flows ◽  
High Reynolds ◽  
Near Wall

This study topologically describes near-wall flows around a surface-mounted cylinder at a high Reynolds number ( $Re$ ) of $5\times 10^4$ and in a very thick boundary layer, which were partially measured or technically approximated from the literature. For complete and rational flow construction, we use high-resolution simulations and critical-point theory. The large-scale near-wake vortex is composed of two connected segments rolled up from the sides of the cylinder and from the free end. Another large-scale side vortex clearly roots on two notable foci on the lower side wall. In the junction region, the side vortex moves upwards with a curved trajectory, which induces the formation of nodes on the ground surface. In the free-end region, the side vortex is compressed, which results in a smaller trailing-edge vortex and its downstream movement. Only tip vortices are observed in the far wake. The origin of the tip vortices and their distinction from the near-wake vortex are discussed. Further analyses suggest that $Re$ independence should be treated with high caution when $Re$ increases from 500 to ${O}(10^4)$ . The occurrence of upwash flow behind the cylinder strongly depends on the increase in $Re$ , the mechanism of which is also provided. The separation–reattachment process in the junction region and the trailing-edge vortices are discovered only at a high $Re$ . The former should significantly affect the strength of the side vortex in the junction region and the latter should cause a sharp drop in pressure near the trailing edge.

Coherent Structures in the Near Wake of a Blunt Trailing Edge Profiled Body

2010 ◽  
Author(s):  
Lakshmana Sampat Doddipatla ◽  
Arash Naghib-Lahouthi ◽  
Horia Hangan ◽  
Kamran Siddiqui
Keyword(s):  
Coherent Structures ◽  
Three Dimensional ◽  
Leading Edge ◽  
Near Wake ◽  
Trailing Edge ◽  
Blunt Trailing Edge ◽  
Instability Modes ◽  
Planar Laser ◽  
Flow Transitions ◽  
Mode A

Wake flows behind two dimensional bodies are mainly dominated by two coherent structures, namely the Karman-Benard vortices and the streamwise vortices, also referred to as rolls and ribs respectively. The three dimensional wake instabilities lead to distinct instability modes (mode-A, mode-B and mode-C or mode-S) depending on the flow Reynolds number and geometric shape. The present investigation explores the mechanism in which the flow transitions to three dimensionality in the near wake of a profiled leading edge and blunt trailing edge body. A combination of Planar Laser Induced Fluorescence visualizations and Particle Image Velcoimetry measurements are conducted in the Reynolds numbers ranging from 250 to 550. The results indicate that three instability modes (mode-A, mode-B and mode-C) appear in the wake transition to three dimensionality, and their order of appearance does not occur through the traditional route as observed in circular cylinder flows. It is found that mode-C instability with a spanwise spacing of 2.4D dominates the near wake development.

Detached-Eddy Simulation of High-Reynolds-Number Beveled-Trailing-Edge Boundary Layers and Wakes

2005 ◽  
Vol 127 (5) ◽  
pp. 897-906 ◽  
Author(s):  
Eric G. Paterson ◽  
Leonard J. Peltier
Keyword(s):  
Qualitative Description ◽  
Navier Stokes ◽  
Detached Eddy Simulation ◽  
Near Wake ◽  
Trailing Edge ◽  
Eddy Simulation ◽  
Instability Modes ◽  
Trailing Edges ◽  
Horseshoe Vortices ◽  
High Reynolds

Flow over three different trailing-edge geometries is studied using incompressible detached-eddy simulation and unsteady Reynolds-averaged Navier Stokes CFD methods. Of interest is the ability of DES, coupled, with localized overset-grid refinement, to resolve the proper physics of separated flows from trailing edges—trailing-edge turbulence and vortex shedding, in particular. The DES model is shown to provide a good qualitative description of the trailing-edge flow. However, the modeled separations are overly energetic due to premature separation related to artificially low turbulence levels from upstream. The transition from RANS to DES is isolated as an issue. The simulated physics of the wake are shown to be in agreement with other LES studies: the model produces the “rib/roller” structures representing the first instability modes, horseshoe vortices are observed, and in regions of high resolution, small scales are formed, as expected. The turbulence statistics are qualitatively similar to benchmark data near the trailing edge and in the near wake, however, quantitative comparisons of urms show an over prediction in magnitude of 50%–100%. Despite this, the results are promising, and future modeling efforts have been motivated and identified.

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

2016 ◽  
Vol 138 (10) ◽  
Author(s):  
Firas F. Siala ◽  
Alexander D. Totpal ◽  
James A. Liburdy
Keyword(s):  
Large Scale ◽  
Flow Characteristics ◽  
Leading Edge ◽  
Wake Flow ◽  
Small Scale ◽  
Mean Flow ◽  
Near Wake ◽  
Trailing Edge ◽  
Swirling Strength ◽  
The Mean

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.

Detached-Eddy Simulation of High Reynolds Number Beveled-Trailing-Edge Flows and Wakes

2004 ◽  
Author(s):  
Eric G. Paterson ◽  
Leonard J. Peltier
Keyword(s):  
Qualitative Description ◽  
Navier Stokes ◽  
Detached Eddy Simulation ◽  
Near Wake ◽  
Trailing Edge ◽  
Eddy Simulation ◽  
Instability Modes ◽  
Trailing Edges ◽  
Horseshoe Vortices ◽  
High Reynolds

Flow over three different trailing-edge geometries is studied using incompressible detached-eddy simulation and unsteady Reynolds-averaged Navier Stokes CFD methods. Of interest is the ability of DES, coupled with localized overset–grid refinement, to resolve the proper physics of separated flows from trailing edges—trailing-edge turbulence and vortex shedding, in particular. The DES model is shown to provide a good qualitative description of the trailing-edge flow. However, the modeled separations are overly energetic due to premature separation related to artificially low turbulence levels from upstream. The transition from RANS to DES is isolated as an issue. The simulated physics of the wake are shown to be in agreement with other LES studies: the model produces the “rib/roller” structures representing the first instability modes, horseshoe vortices are observed, and in regions of high resolution, small scales are formed, as expected. The turbulence statistics are qualitatively similar to benchmark data near the trailing edge and in the near wake, however, quantitative comparisons of urms show an over prediction in magnitude of 50–100%. Despite this, the results are promising, and future modeling efforts have been motivated and identified.

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

2016 ◽  
Author(s):  
Firas F. Siala ◽  
Alexander D. Totpal ◽  
James A. Liburdy
Keyword(s):  
Large Scale ◽  
Flow Characteristics ◽  
Leading Edge ◽  
Mean Flow ◽  
Near Wake ◽  
Trailing Edge ◽  
Mean Velocity ◽  
Leading Edge Vortex ◽  
Edge Vortex ◽  
The Mean

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. Mid-span particle image velocimetry 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 0.09–0.20. Time resolved and phase locked measurements were used to describe the mean flow characteristics and phase averaged vortex structures and their evolution throughout the oscillation cycle. Large eddy scale decomposition and swirl strength analysis were used to quantify the effect of flexibility on the vortical structures. The results demonstrate that flexibility at the trailing edge has a minimal influence on the mean flow characteristics when compared to the purely rigid foil. The mean velocity deficit for the flexible trailing edge and rigid foils is shown to remain constant for all reduced frequencies tested. However, the trailing edge flexibility increases the swirl strength of the small scale structures, which results in enhanced cross stream dispersion of the mean velocity profile. Flexibility at the leading edge is shown to generate a large scale leading edge vortex for k ≥ 0.18. This results in a reduction in the swirl strength due to the complex vortex interactions when compared to the flexible trailing edge and rigid foils. Furthermore, it is shown that the large scale leading edge vortex is responsible for extracting a significant portion of the energy from the mean flow, resulting in a substantial reduction of mean flow momentum in the wake. The kinetic energy loss in the wake is shown to scale well with the energy content of the leading edge vortex.

Entrainment vortices and interfacial intermittent turbulent bulges in a plane turbulent wake

2002 ◽  
Vol 469 ◽  
pp. 49-70 ◽  
Author(s):  
GREGORY A. KOPP ◽  
FRANCESC GIRALT ◽  
JAMES F. KEFFER
Keyword(s):  
Phenomenological Models ◽  
Coherent Structures ◽  
Large Scale ◽  
Horseshoe Vortex ◽  
Turbulent Wake ◽  
Wake Region ◽  
Hot Wire ◽  
Helmholtz Instability ◽  
Far Wake ◽  
Plane Turbulent Wake

Hot-wire measurements were made simultaneously in two homogeneous ‘horizontal’ planes in the far-wake region of a cylinder. A technique developed using hot-wire data to identify the spatial characteristics of the large-scale bulges at the interface between the internal turbulent motions and the external irrotational flow was used to unambiguously relate these outer intermittent bulges to the inner coherent structures. It was found that a turbulent bulge is made up of a combination of a horseshoe vortex (whose legs form one double-roller eddy) and the straining region present just upstream of this structure. The approach also allowed the evaluation of the two most prominent phenomenological models for the entrainment mechanism in the far-wake region: the Kelvin–Helmholtz instability and Townsend's growth–decay cycle. It was found that the decaying and re-forming of the bulges and entrainment structures is not likely to occur. Rather, the evidence is that the large-scale bulges remain coherent for long streamwise distances in equilibrium with the overall similarity of the flow.

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