Modeling the interplay between the shear layer and leading edge suction during dynamic stall

Abstract In this paper, an unsteady tip leakage flow phenomenon is identified and investigated in a centrifugal compressor with a vaneless diffuser at near-stall conditions. This phenomenon is associated with the inception of a rotating instability in the compressor. The study is based on numerical simulations that are supported by experimental measurements. The study confirms that the unstable flow is governed by a Kelvin–Helmholtz type instability of the shear layer formed between the main-stream flow and the tip leakage flow. The shear layer instability induces large-scale vortex roll-up and forms vortex tubes, which propagate circumferentially, resulting in measured pressure fluctuations with short wavelength and high amplitude which rotate at about half of the blade speed. The 3D vortex tube is also found to interact with the main blade leading edge, causing the reduction of the blade loading identified in the experiment. The paper also reveals that the downstream volute imposes a once-per-rev circumferential nonuniform back pressure at the impeller exit, inducing circumferential loading variation at the impeller inducer, and causing circumferential variation in the unsteady tip leakage flow.

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On the structure of the self-sustaining cycle in separating and reattaching flows

Journal of Fluid Mechanics ◽

10.1017/jfm.2018.772 ◽

2018 ◽

Vol 857 ◽

pp. 907-936 ◽

Cited By ~ 8

Author(s):

A. Cimarelli ◽

A. Leonforte ◽

D. Angeli

Keyword(s):

Shear Layer ◽

Rectangular Plate ◽

Large Scale ◽

Reverse Flow ◽

Leading Edge ◽

Streamwise Velocity ◽

The Self ◽

Small Scale ◽

Spectral Evolution ◽

Mean Velocity

The separating and reattaching flows and the wake of a finite rectangular plate are studied by means of direct numerical simulation data. The large amount of information provided by the numerical approach is exploited here to address the multi-scale features of the flow and to assess the self-sustaining mechanisms that form the basis of the main unsteadinesses of the flows. We first analyse the statistically dominant flow structures by means of three-dimensional spatial correlation functions. The developed flow is found to be statistically dominated by quasi-streamwise vortices and streamwise velocity streaks as a result of flow motions induced by hairpin-like structures. On the other hand, the reverse flow within the separated region is found to be characterized by spanwise vortices. We then study the spectral properties of the flow. Given the strongly inhomogeneous nature of the flow, the spectral analysis has been conducted along two selected streamtraces of the mean velocity field. This approach allows us to study the spectral evolution of the flow along its paths. Two well-separated characteristic scales are identified in the near-wall reverse flow and in the leading-edge shear layer. The first is recognized to represent trains of small-scale structures triggering the leading-edge shear layer, whereas the second is found to be related to a very large-scale phenomenon that embraces the entire flow field. A picture of the self-sustaining mechanisms of the flow is then derived. It is shown that very-large-scale fluctuations of the pressure field alternate between promoting and suppressing the reverse flow within the separation region. Driven by these large-scale dynamics, packages of small-scale motions trigger the leading-edge shear layers, which in turn created them, alternating in the top and bottom sides of the rectangular plate with a relatively long period of inversion, thus closing the self-sustaining cycle.

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Particle image velocimetry measurements of induced separation at the leading edge of a plate

Journal of Fluid Mechanics ◽

10.1017/jfm.2016.532 ◽

2016 ◽

Vol 804 ◽

pp. 278-297 ◽

Cited By ~ 3

Author(s):

J. P. J. Stevenson ◽

K. P. Nolan ◽

E. J. Walsh

Keyword(s):

Particle Image Velocimetry ◽

Shear Layer ◽

Particle Image ◽

Reverse Flow ◽

Leading Edge ◽

Quadrant Analysis ◽

Bypass Transition ◽

Reynolds Stresses ◽

Free Stream Turbulence ◽

Image Velocimetry

The free shear layer that separates from the leading edge of a round-nosed plate has been studied under conditions of low (background) and elevated (grid-generated) free stream turbulence (FST) using high-fidelity particle image velocimetry. Transition occurs after separation in each case, followed by reattachment to the flat surface of the plate downstream. A bubble of reverse flow is thereby formed. First, we find that, under elevated (7 %) FST, the time-mean bubble is almost threefold shorter due to an accelerated transition of the shear layer. Quadrant analysis of the Reynolds stresses reveals the presence of slender, highly coherent fluctuations amid the laminar part of the shear layer that are reminiscent of the boundary-layer streaks seen in bypass transition. Instability and the roll-up of vortices then follow near the crest of the shear layer. These vortices are also present under low FST and in both cases are found to make significant contributions to the production of Reynolds stress over the rear of the bubble. But their role in reattachment, whilst important, is not yet fully clear. Instantaneous flow fields from the low-FST case reveal that the bubble of reverse flow often breaks up into two or more parts, thereby complicating the overall reattachment process. We therefore suggest that the downstream end of the ‘separation isoline’ (the locus of zero absolute streamwise velocity that extends unbroken from the leading edge) be used to define the instantaneous reattachment point. A histogram of this point is found to be bimodal: the upstream peak coincides with the location of roll-up, whereas the downstream mode may suggest a ‘flapping’ motion.

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Large Eddy Simulation of Wake-Shear Layer Interactions Over a Multi-Element Aerofoil

Volume 2E: Turbomachinery ◽

10.1115/gt2020-14115 ◽

2020 ◽

Author(s):

Souvik Naskar ◽

S. Sarkar

Keyword(s):

Large Eddy Simulation ◽

Shear Layer ◽

Separation Bubble ◽

Turbine Blades ◽

Leading Edge ◽

Aerodynamic Characteristics ◽

Suction Surface ◽

Free Stream Turbulence ◽

Eddy Simulation ◽

Large Eddy

Abstract Modern commercial airliners use multi-element aerofoils to enhance take-off and landing performance. Further, multielement aerofoil configurations have been shown to improve the aerodynamic characteristics of wind turbines. In the present study, high resolution Large Eddy Simulation (LES) is used to explore the low Reynolds Number (Re = 0.832 × 104) aerodynamics of a 30P30N multi-element aerofoil at an angle of attack, α = 4°. In the present simulation, wake shed from a leading edge element or slat is found to interact with the separated shear layer developing over the suction surface of the main wing. High receptivity of shear layer via amplification of free-stream turbulence leads to rollup and breakdown, forming a large separation bubble. A transient growth of fluctuations is observed in the first half of the separation bubble, where levels of turbulence becomes maximum near the reattachment and then decay depicting saturation of turbulence. Results of the present LES are found to be in close agreement with the experiment depicting high vortical activity in the outer layer. Some features of the flow field here are similar to those occur due to interactions of passing wake and the separated boundary layer on the suction surface of high lift low pressure turbine blades.

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The aerodynamics of revolving wings I. Model hawkmoth wings

Journal of Experimental Biology ◽

10.1242/jeb.205.11.1547 ◽

2002 ◽

Vol 205 (11) ◽

pp. 1547-1564 ◽

Cited By ~ 45

Author(s):

James R. Usherwood ◽

Charles P. Ellington

Keyword(s):

Insect Flight ◽

Leading Edge ◽

Dynamic Stall ◽

Leading Edge Vortex ◽

Force Coefficients ◽

Attached Flow ◽

Edge Vortex ◽

Drag And Lift ◽

Geometric Relationship ◽

Spanwise Flow

SUMMARYRecent work on flapping hawkmoth models has demonstrated the importance of a spiral `leading-edge vortex' created by dynamic stall, and maintained by some aspect of spanwise flow, for creating the lift required during flight. This study uses propeller models to investigate further the forces acting on model hawkmoth wings in `propeller-like' rotation (`revolution'). Steadily revolving model hawkmoth wings produce high vertical (≈ lift) and horizontal (≈ profile drag) force coefficients because of the presence of a leading-edge vortex. Both horizontal and vertical forces, at relevant angles of attack, are dominated by the pressure difference between the upper and lower surfaces; separation at the leading edge prevents `leading-edge suction'. This allows a simple geometric relationship between vertical and horizontal forces and the geometric angle of attack to be derived for thin, flat wings. Force coefficients are remarkably unaffected by considerable variations in leading-edge detail, twist and camber. Traditional accounts of the adaptive functions of twist and camber are based on conventional attached-flow aerodynamics and are not supported. Attempts to derive conventional profile drag and lift coefficients from `steady' propeller coefficients are relatively successful for angles of incidence up to 50° and, hence, for the angles normally applicable to insect flight.

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Force and flowfield measurements to understand unsteady aerodynamics of cycloidal rotors in hover at ultra-low Reynolds numbers

International Journal of Micro Air Vehicles ◽

10.1177/1756829319833677 ◽

2019 ◽

Vol 11 ◽

pp. 175682931983367

Author(s):

Carolyn M Reed ◽

David A Coleman ◽

Moble Benedict

Keyword(s):

Separation Bubble ◽

Fluid Dynamic ◽

Unsteady Aerodynamics ◽

Leading Edge ◽

Reynolds Numbers ◽

Dynamic Stall ◽

Static Case ◽

Low Reynolds Numbers ◽

Curvature Effects ◽

Low Reynolds

This paper provides a fundamental understanding of the unsteady fluid-dynamic phenomena on a cycloidal rotor blade operating at ultra-low Reynolds numbers (Re ∼ 18,000) by utilizing a combination of instantaneous blade force and flowfield measurements. The dynamic blade force coefficients were almost double the static ones, indicating the role of dynamic stall. For the dynamic case, the blade lift monotonically increased up to ±45° pitch amplitude; however, for the static case, the flow separated from the leading edge after around 15° with a large laminar separation bubble. There was significant asymmetry in the lift and drag coefficients between the upper and lower halves of the trajectory due to the flow curvature effects (virtual camber). The particle image velocimetry measured flowfield showed the dynamic stall process during the upper half to be significantly different from the lower half because of the reversal of dynamic virtual camber. Even at such low Reynolds numbers, the pressure forces, as opposed to viscous forces, were found to be dominant on the cyclorotor blade. The power required for rotation (rather than pitching power) dominated the total blade power.

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Inflatable Leading Edge-Based Dynamic Stall Control considering Fluid-Structure Interaction

International Journal of Aerospace Engineering ◽

10.1155/2020/9046542 ◽

2020 ◽

Vol 2020 ◽

pp. 1-28

Author(s):

Shi-Long Xing ◽

He-Yong Xu ◽

Ming-Sheng Ma ◽

Zheng-Yin Ye

Keyword(s):

Lift Coefficient ◽

Fluid Structure Interaction ◽

Leading Edge ◽

Dynamic Stall ◽

Elastic Membrane ◽

Radius Of Curvature ◽

Structural Dynamic ◽

Fluid Structure ◽

Structure Interaction ◽

Stall Control

The inflatable leading edge (ILE) is explored as a dynamic stall control concept. A fluid-structure interaction (FSI) numerical method for the elastic membrane structure is constructed based on unsteady Reynolds-averaged Navier-Stokes (URANS) and a mass-spring-damper (MSD) structural dynamic model. Radial basis function- (RBF-) based mesh deformation algorithm and Laplacian and optimization-based mesh smoothing algorithm are adopted in flowfield simulations to achieve the pitching oscillation of the airfoil and to ensure the mesh quality. An airfoil is considered at a freestream Mach number of 0.3 and chord-based Reynolds number of 3.92×106. The airfoil is pitched about its quarter-chord axis at a sinusoidal motion. The numerical results indicate that the ILE can change the radius of curvature of the airfoil leading edge, which could reduce the streamwise adverse pressure gradient and suppress the formation of dynamic stall vortex (DSV). Although the maximum lift coefficient of the airfoil is slightly reduced during the control process, the maximum drag and pitching moment coefficients of the airfoil are greatly reduced by up to 66% and 75.2%, respectively. The relative position of the ILE has a significant influence on its control effect. The control laws of inflation and deflation also affect the control ability of the ILE.

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Sting-free measurements on a magnetically supported right circular cylinder aligned with the free stream

Journal of Fluid Mechanics ◽

10.1017/s0022112007008956 ◽

2008 ◽

Vol 596 ◽

pp. 49-72 ◽

Cited By ~ 24

Author(s):

HIROSHI HIGUCHI ◽

HIDEO SAWADA ◽

HIROYUKI KATO

Keyword(s):

Circular Cylinder ◽

Shear Layer ◽

Free Stream ◽

Aerodynamic Force ◽

Cross Flow ◽

Leading Edge ◽

Magnetic Suspension ◽

Recirculation Region ◽

Mean Velocity ◽

Wide Range

The flow over cylinders of varying fineness ratio (length to diameter) aligned with the free stream was examined using a magnetic suspension and balance system in order to avoid model support interference. The drag coefficient variation of a right circular cylinder was obtained for a wide range of fineness ratios. Particle image velocimetry (PIV) was used to examine the flow field, particularly the behaviour of the leading-edge separation shear layer and its effect on the wake. Reynolds numbers based on the cylinder diameter ranged from 5×104 to 1.1×105, while the major portion of the experiment was conducted at ReD=1.0×105. For moderately large fineness ratio, the shear layer reattaches with subsequent growth of the boundary layer, whereas over shorter cylinders, the shear layer remains detached. Differences in the wake recirculation region and the immediate wake patterns are clarified in terms of both the mean velocity and turbulent flow fields, including longitudinal vortical structures in the cross-flow plane of the wake. The minimum drag corresponded to the fineness ratio for which the separated shear layer reattached at the trailing edge of the cylinder. The base pressure was obtained with a telemetry technique. Pressure fields and aerodynamic force fluctuations are also discussed.

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