; ^_Oh+'0HP
$TU Delft Repository search results0TU Delft Repository search results (max. 1000)TU Delft LibraryTU Delft Library@4c A@4c A՜.+,0HPX`hp
x
WorksheetFeuilles de calcul
B=%r8X"1Calibri1Calibri1Calibri1
Calibri 83ffff̙̙3f3fff3f3f33333f33333.H[TU Delft Repositoryg ;uuidrepository linktitleauthorcontributorpublication yearabstract
subject topiclanguagepublication type publisherisbnissnpatent
patent statusbibliographic noteaccess restrictionembargo datefaculty
departmentresearch group programmeprojectcoordinates)uuid:58b22f7631c7429f8120a3b585d32b01Dhttp://resolver.tudelft.nl/uuid:58b22f7631c7429f8120a3b585d32b01OTransition in a separation bubble under tonal and broadband acoustic excitationKurelek, J.W. (TU Delft Aerodynamics; University of Waterloo); Kotsonis, M. (TU Delft Aerodynamics); Yarusevych, S. (University of Waterloo)Q Transition and flow development in a separation bubble formed on an airfoil are studied experimentally. The effects of tonal and broadband acoustic excitation are considered since such acoustic emissions commonly result from airfoil selfnoise and can influence flow development via a feedback loop. This interdependence is decoupled, and the problem is studied in a controlled manner through the use of an external acoustic source. The flow field is assessed using timeresolved, twocomponent particle image velocimetry, the results of which show that, for equivalent energy input levels, tonal and broadband excitation can produce equivalent changes in the mean separation bubble topology. These changes in topology result from the influence of excitation on transition and the subsequent development of coherent structures in the bubble. Both tonal and broadband excitation lead to earlier shear layer rollup and thus reduce the bubble size and advance mean reattachment upstream, while the shed vortices tend to persist farther downstream of mean reattachment in the case of tonal excitation. Through a crossexamination of linear stability theory (LST) predictions and measured disturbance characteristics, nonlinear disturbance interactions are shown to play a crucial role in the transition process, leading to significantly different disturbance development for the tonal and broadband excited flows. Specifically, tonal excitation results in transition being dominated by the excited mode, which grows in strong accordance with linear theory and damps the growth of all other disturbances. On the other hand, disturbance amplitudes are more moderate for the natural and broadband excited flows, and so all unstable disturbances initially grow in accordance with LST. For all cases, a rapid redistribution of perturbation energy to a broad range of frequencies follows, with the phenomenon occurring earliest for the broadband excitation case. By taking nonlinear effects into consideration, important ramifications are made clear in regards to comparing LST predictions and experimental or numerical results, thus explaining the trends reported in recent investigations. These findings offer new insights into the influence of tonal and broadband noise emissions, resulting from airfoil selfnoise or otherwise, on transition and flow development within a separation bubble.Baeroacoustics; boundary layer separation; transition to turbulenceenjournal articleAerodynamics)uuid:6758b9fed20e4bfda057f7a86df7f9ffDhttp://resolver.tudelft.nl/uuid:6758b9fed20e4bfda057f7a86df7f9ffKOn the origin of spanwise vortex deformations in laminar separation bubbleszMichelis, Theodorus (TU Delft Aerodynamics); Yarusevych, S. (University of Waterloo); Kotsonis, M. (TU Delft Aerodynamics) This work investigates the threedimensional, spatioTemporal flow development in the aft portion of a laminar separation bubble. The bubble is forming on a flat plate geometry, subjected to an adverse pressure gradient, featuring maximum reverse flow of approximately 2 % of the local freestream velocity. Timeresolved velocity measurements are performed by means of planar and tomographic particle image velocimetry, in the vicinity of the reattachment region. The measurements are complemented with a numerical solution of the boundary layer equations in the upstream field. The combined numerical and measured boundary layer is used as a baseline flow for linear stability theory analys< is. The results provide insight into the dynamics of dominant coherent structures that form in the separated shear layer and deform along the span. Stability analysis shows that the flow becomes unstable upstream of separation, where both normal and oblique modes undergo amplification. While the shear layer roll up is linked to the amplification of the fundamental normal mode, the oblique modes at angles lower than approximately are also amplified substantially at the fundamental frequency. A model based on the stability analysis and experimental measurements is employed to demonstrate that the spanwise deformations of rollers are produced due to a superposition of normal and oblique instability modes initiating upstream of separation. The degree of the initial spanwise deformations is shown to depend on the relative amplitude of the dominant normal and oblique waves. This is confirmed by forcing the normal mode through a controlled impulsive perturbation introduced by a spanwise invariant dielectricbarrierdischarge plasma actuator, resulting in the formation of spanwise coherent vortices. The findings elucidate the link between important features in the bubble shedding dynamics and stability characteristics and provide further clarification on the differences in the development of coherent structures seen in recent experiments. Moreover, the results present a handle on the development of effective control strategies that can be used to either promote or suppress shedding in separation bubbles, which is of interest for system performance improvement and control of aeroacoustic emissions in relevant applications.Dboundary layer separation; boundary layer stability; boundary layers)uuid:13cb5cc7be32442eac257f91f7358514Dhttp://resolver.tudelft.nl/uuid:13cb5cc7be32442eac257f91f7358514<Response of a laminar separation bubble to impulsive forcingyMichelis, Theodorus (TU Delft Wind Energy); Yarusevych, S. (University of Waterloo); Kotsonis, M. (TU Delft Aerodynamics)6The spatial and temporal response characteristics of a laminar separation bubble to impulsive forcing are investigated by means of timeresolved particle image velocimetry and linear stability theory. A twodimensional impulsive disturbance is introduced with an alternating current dielectric barrier discharge plasma actuator, exciting pertinent instability modes and ensuring flow development under environmental disturbances. Phaseaveraged velocity measurements are employed to analyse the effect of imposed disturbances at different amplitudes on the laminar separation bubble. The impulsive disturbance develops into a wave packet that causes rapid shrinkage of the bubble in both upstream and downstream directions. This is followed by bubble bursting, during which the bubble elongates significantly, while vortex shedding in the aft part ceases. Duration of recovery of the bubble to its unforced state is independent of the forcing amplitude. Quasisteady linear stability analysis is performed at each individual phase, demonstrating reduction of growth rate and frequency of the most unstable modes with increasing forcing amplitude. Throughout the recovery, amplification rates are directly proportional to the shape factor. This indicates that bursting and flapping mechanisms are driven by altered stability characteristics due to variations in incoming disturbances. The emerging wave packet is characterised in terms of frequency, convective speed and growth rate, with remarkable agreement between linear stability theory predictions and measurements. The wave packet assumes a frequency close to the natural shedding frequency, while its convective speed remains invariant for all forcing amplitudes. The stability of the flow changes only when disturbances interact with the shear layer breakdown and reattachment processes, supporting the notion of a closed feedback loop. The results of this study shed light on the response of laminar separation bubbles to impulsive forcing, providing insight into the attendant changes of flow dynamics and the underlying stability mechanisms.</
20180115Wind Energy)uuid:94aaf935aaf945279ec1d7cd22d94d98Dhttp://resolver.tudelft.nl/uuid:94aaf935aaf945279ec1d7cd22d94d98ISpanwise flow structures within a laminar separation bubble on an airfoilMichelis, Theodorus (TU Delft Aerodynamics); Kotsonis, M. (TU Delft Aerodynamics); Yarusevych, S. (TU Delft Aerodynamics; University of Waterloo)The present study considers the development of a Laminar Separation Bubble on the suction side of a NACA0018 airfoil under natural and forced conditions. Deterministic forcing is applied by means of a twodimensional plasma actuator installed on the airfoil surface. The spatiotemporal characteristics of the bubble are measured using timeresolved, twocomponent Particle Image Velocimetry in streamwise and spanwise planes. Analysis of the results shows that while the timeaverage bubble is strongly twodimensional, the dominant coherent structures assume three dimensional organisation in the vicinity of laminarturbulent breakdown in both natural and forced conditions.conference paperTSFP)uuid:8b5f268d2eee416ba296b08838e81c43Dhttp://resolver.tudelft.nl/uuid:8b5f268d2eee416ba296b08838e81c43QEffect of Local DBD Plasma Actuation on Transition in a Laminar Separation BubbledYarusevych, S. (TU Delft Aerodynamics; University of Waterloo); Kotsonis, M. (TU Delft Aerodynamics)This work examines the effect of local active flow control on stability and transition in a laminar separation bubble. Experiments are performed in a wind tunnel facility on a NACA 0012 airfoil at a chord Reynolds number of 130 000 and an angle of attack of 2 degrees. Controlled disturbances are introduced upstream of a laminar separation bubble forming on the suction side of the airfoil using a surfacemounted Dielectric Barrier Discharge plasma actuator. Timeresolved twocomponent Particle Image Velocimetry is used to characterise the flow field. The effect of frequency and amplitude of plasma excitation on flow development is examined. The introduction of artificial harmonic disturbances leads to significant changes in separation bubble topology and the characteristics of coherent structures formed in the aft portion of the bubble. The development of the bubble demonstrates strong dependence on the actuation frequency and amplitude, revealing the dominant role of incoming disturbances in the transition scenario. Statistical, topological and linear stability theory analysis demonstrate that significant mean flow deformation produced by controlled disturbances leads to notable changes in stability characteristics compared to those in the unforced baseline case. The findings provide a new outlook on the role of controlled disturbances in separated shear layer transition and instruct the development of effective flow control strategies.Airfoil; Boundary layer separation; DBD plasma actuator; Flow control; Laminar separation bubble; Laminartoturbulent transition; Separated shear layer
*+&ffffff?'ffffff?(?)?"dXX333333?333333?U}}}}}}}}}} }
}}}
}}}}}}}}}}}}
@
!
"
#
$@
%
&
'
(
)
*@
+
&
,

.
/
0
1@
2
3
4*ʡB
5
6
7
8@
9
:
!"#$%&'()*+,./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{}~
!"#$%&'()*+,./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{}~
!"#$%&'()*+,./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{}~
!"#$%&'()*+,./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{}~
!"#$%&'()*+,./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{}~
!"#$%&'()*+,./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{}~
!"#$%&'()*+,./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{}~
!"#$%&'()*+,./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{}~>@ddyKyKhttp://resolver.tudelft.nl/uuid:58b22f7631c7429f8120a3b585d32b01yKyKhttp://resolver.tudelft.nl/uuid:6758b9fed20e4bfda057f7a86df7f9ffyKyKhttp://resolver.tudelft.nl/uuid:13cb5cc7be32442eac257f91f7358514yKyKhttp://resolver.tudelft.nl/uuid:94aaf935aaf945279ec1d7cd22d94d98yKyKhttp://resolver.tudelft.nl/uuid:8b5f268d2eee416ba296b08838e81c43gg
Root Entry F4c A4c A@SummaryInformation( F<Workbook FDocumentSummaryInformation8 F
!"#$%&'()*+,./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]