Passive control of jets with indeterminate origins

Wlezien, R. W.; Kibens, Valdis

doi:10.2514/3.9430

Cited by 65 publications

(24 citation statements)

References 5 publications

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“…The three-dimensional mixing layer at the edge of the serrations generates streamwise-oriented structures with vortex cores on the suction side, as also reported by Wygnanski et al 57 and Wlezien and Kibens. 58 The generation of these vortices can be attributed to both the velocity gradient between the suction ( y/c > 0) and the pressure sides ( y/c < 0), and the serration angle ϕ (Figure 1, right). Near the tip of the serration, the core of the vortices is located closer to the serration surface because of the mean downward motion through the serrations (i.e., flow from the suction to the pressure side as also reported by Jones and Sandberg 21 ).…”

Section: B Three-dimensional Flow Organizationmentioning

confidence: 99%

Three-dimensional flow field over a trailing-edge serration and implications on broadband noise

2016

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The three-dimensional flow field over the suction side of a NACA 0018 airfoil with trailing-edge serrations was studied by means of time-resolved tomographic particle image velocimetry. Mean flow results show that the boundary layer thickness decreases along the streamwise direction with a corresponding reduction of the size of the turbulent structures developing over the suction side of the serrations. At a positive angle of attack, streamwise-oriented and counter-rotating vortices aligned with the edge of the serrations are found to be the main features of the mean flow field. Their formation is attributed to the pressure imbalance between the two sides of the airfoil and the mixing layer at the edge. They locally modify the effective angle seen by the turbulent flow approaching the serrated edge. This effect may contribute to the serration underperformance in terms of noise reduction reported in literature. The spatial distribution of the spectra of the source term of the Poisson equation, which relates the velocity field to pressure fluctuations, suggests that the contribution of the serrations to far-field broadband noise is a function of the streamwise location. This observation is congruent with the spectra of the wall-normal and spanwise velocity fluctuations, which typically show low intensity close to the tips of the individual serrations. It follows that analytical models must take into account the local contribution to the far-field noise induced by the streamwise variation of the hydrodynamic pressure on the serration surface.

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Section: B Three-dimensional Flow Organizationmentioning

confidence: 99%

Three-dimensional flow field over a trailing-edge serration and implications on broadband noise

2016

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“…38,39 It can simply be explained by the streamwise convection effect, which is the dominant effect in the slanted edge. Due to the fact that the coherence scales with the size of the vortices, and that the filtering in frequency acts as a scale separation, 40 the edge correlation length is increased.…”

Section: B Correlation Lengthmentioning

confidence: 99%

Numerical analysis of noise reduction mechanisms of serrated trailing edges under zero lift condition

Velden

Avallone

Ragni

2017

23rd AIAA/CEAS Aeroacoustics Conference

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The relation between the far-field noise and the flow field, in presence of serrated and combed-serrated trailing edge, is studied to explain the associated noise reduction mechanisms. Both serration geometries are retrofitted to a NACA 0018 wing. Computations are carried out by solving the fully explicit, transient, compressible Lattice Boltzmann equation, while the acoustic far-field is obtained by means of the Ffowcs-Williams and Hawking integral solution. A link between the far-field noise and the relevant flow parameters that contribute to noise generation is proposed. It is confirmed that the intensity of the surface pressure fluctuations varies in the streamwise direction, and that most of the low-frequency noise is generated at the root of the serrations. It is concluded that the additional noise reduction achieved by the use of combs is due to the mitigation of the outward flow motion at the root. Furthermore, it is shown that the edge-oriented correlation length and the convection velocity of the surface pressure fluctuations are the two statistical flow parameters that influence both the intensity and the frequency range of noise reduction. It is found that a larger edge-oriented correlation length contributes to noise reduction, by generating destructive interference between the pressure waves scattered along the slanted edge.

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“…One of the earliest investigations on incompressible single-stream jets issuing from modified circular nozzles were conducted by Kibens and Wlezien [24] and Wlezien and Kibens [103]. Their investigations on air jets exhausting from simple inclined circular nozzles led to important observations that shear layer ring-vortices possess both the outlines and inclinations as the inclined nozzles, so long the incline-angle remains small-to-moderate.…”

Section: Circular Modified Nozzle Jetsmentioning

confidence: 99%

“…Experimental cross-stream results taken for jets forced at St = 0.51 showed that for each peak in the crown Fig. 3 Differences in the vortex roll-up behaviour for a jet issuing from inclined circular nozzles of different inclination (From Wlezien and Kibens [103]. Reprinted by permission of the American Institute of Aeronautics and Astronautics, Inc.) nozzle, outwards-spreading mushroom-shaped counter-rotating vortex pairs (i.e.…”

Section: Circular Modified Nozzle Jetsmentioning

confidence: 99%

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Dynamics of Jets Issuing from Trailing-Edge Modified Nozzles

New

Tsovolos

Tsioli

2015

Fluid Mechanics and Its Applications

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This chapter will elaborate upon the fundamental flow behaviour associated with vortex-rings and jets issuing from nozzles, where the nozzle trailingedges or lips are physically modified with selected geometries. This technique represents a passive but robust form of manipulating the underlying vortex-ring and jet circulation, such that improvements to their entrainment and mixing characteristics can be achieved. Implementations of simple inclined, hybrid inclined, notched, crown-shaped, chevron and stepped nozzles, as well as some of their implementations in circular and noncircular jets, single-stream or dual-stream coaxial jets, will be discussed as part of the overall understanding. In particular, recently observed influences of trailing-edge modifications upon the axis-switching behaviour of noncircular jets, as well as their relationships with coaxial jet flow parameters such as the velocity-and area-ratios will be presented. On top of key flow physics insights in terms of how the nozzle trailing-edge geometry will confer distortionary effects upon the basic vortex structures, the impact of such nozzles upon jet mixing efficacies will be discussed as well.

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Passive control of jets with indeterminate origins

Cited by 65 publications

References 5 publications

Three-dimensional flow field over a trailing-edge serration and implications on broadband noise

Three-dimensional flow field over a trailing-edge serration and implications on broadband noise

Numerical analysis of noise reduction mechanisms of serrated trailing edges under zero lift condition

Dynamics of Jets Issuing from Trailing-Edge Modified Nozzles

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