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On stability and receptivity of boundary-layer flows
KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
2016 (engelsk)Doktoravhandling, med artikler (Annet vitenskapelig)
Abstract [en]

This work is concerned with stability and receptivity analysis as well as studies on control of the laminar-turbulent transition in boundary-layer flows through direct numerical simulations. Various flow configurations are considered to address flow around straight and swept wings. The aim of this study is to contribute to a better understanding of stability characteristics and different means of transition control of such flows which are of great interest in aeronautical applications.

Acoustic receptivity of flow over a finite-thickness flat plate with elliptic leading edge is considered. The objective is to compute receptivity coefficient defined as the relative amplitude of acoustic disturbances and TS wave. The existing results in the literature for this flow case plot a scattered image and are inconclusive. We have approached this problem in both compressible and incompressible frameworks and used high-order numerical methods. Our results have shown that the generally-accepted level of acoustic receptivity coefficient for this flow case is one order of magnitude too high.

The continuous increase of computational power has enabled us to perform global stability analysis of three-dimensional boundary layers. A swept flat plate of FSC type boundary layer with surface roughness is considered. The aim is to determine the critical roughness height for which the flow becomes turbulent. Global stability characteristics of this flow have been addressed and sensitivity of such analysis to domain size and numerical parameters have been discussed.

The last flow configuration studied here is infinite swept-wing flow. Two numerical set ups are considered which conform to wind-tunnel experiments where passive control of crossflow instabilities is investigated. Robustness of distributed roughness elements in the presence of acoustic waves have been studied. Moreover, ring-type plasma actuators are employed as virtual roughness elements to delay laminar-turbulent transition.

sted, utgiver, år, opplag, sider
Stockholm: KTH Royal Institute of Technology, 2016. , s. 49
Serie
TRITA-MEK, ISSN 0348-467X ; 2016:17
Emneord [en]
boundary layer receptivity, acoustic receptivity, swept-wing flow, crossflow vortices, roughness element, global stability analysis, direct numerical simulation, plasma actuator
HSV kategori
Forskningsprogram
Teknisk mekanik
Identifikatorer
URN: urn:nbn:se:kth:diva-196878ISBN: 978-91-7729-184-8 (tryckt)OAI: oai:DiVA.org:kth-196878DiVA, id: diva2:1049492
Disputas
2016-12-09, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:15 (engelsk)
Opponent
Veileder
Merknad

QC 20161124

Tilgjengelig fra: 2016-11-24 Laget: 2016-11-24 Sist oppdatert: 2016-11-25bibliografisk kontrollert
Delarbeid
1. Acoustic receptivity simulations of flow past a flat plate with elliptic leading edge
Åpne denne publikasjonen i ny fane eller vindu >>Acoustic receptivity simulations of flow past a flat plate with elliptic leading edge
2016 (engelsk)Inngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 800, artikkel-id R2Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

We present results of numerical simulations of leading-edge acoustic receptivity for acoustic waves impinging on the leading edge of a finite-thickness flat plate. We use both compressible and incompressible flow solvers fitted with high-order high-accuracy numerical methods and independent methods of estimating the receptivity coefficient. The results show that the level of acoustic receptivity in the existing literature appears to be one order of magnitude too high. Our review of previous numerical simulations and experiments clearly identifies some contradictory trends. In the limit of an infinitely thin flat plate, our results are consistent with asymptotic theory and numerical simulations.

sted, utgiver, år, opplag, sider
Cambridge University Press, 2016
Emneord
boundary layers, boundary layer receptivity, boundary layer stability
HSV kategori
Identifikatorer
urn:nbn:se:kth:diva-190550 (URN)10.1017/jfm.2016.433 (DOI)000379846600002 ()2-s2.0-84978035402 (Scopus ID)
Merknad

QC 20160817

Tilgjengelig fra: 2016-08-17 Laget: 2016-08-12 Sist oppdatert: 2017-11-28bibliografisk kontrollert
2. Application of biorthogonal eigenfunction system for extraction of Tollmien-Schlichting waves in acoustic receptivity simulations
Åpne denne publikasjonen i ny fane eller vindu >>Application of biorthogonal eigenfunction system for extraction of Tollmien-Schlichting waves in acoustic receptivity simulations
2016 (engelsk)Rapport (Annet vitenskapelig)
Abstract [en]

Acoustic receptivity of a two-dimensional boundary layer on a flat plate with elliptic leading edge is studied through direct numerical simulation (DNS). Sound waves are modelled by a uniform oscillation of freestream boundaries in time which results to an infinite-wavelength acoustic wave. Acoustic disturbances interact with strong streamwise gradients at the leading edge or surface non- homogeneities and create Tollmien-Schlichting (TS) waves inside the boundary layer. Measuring amplitude of TS waves created by sound waves is challenging due to presence of Stokes wave (acoustic boundary layer) with the same temporal frequency of TS waves. In this study biorthogonal eigenfunction system of local linear stability equations has been utilised to extract TS wave amplitudes. This method is based on the concept of using adjoint mode as a projector where the TS amplitude is obtained by projecting the DNS solution onto adjoint TS modes. However, the computed TS wave amplitude employing this method found to be modulated. It is shown that the modulation is due to existence of a small amplitude wave in the DNS data that is not expandable onto the basis of local linear stability equations. 

sted, utgiver, år, opplag, sider
KTH Royal Institute of Technology, 2016
HSV kategori
Identifikatorer
urn:nbn:se:kth:diva-196874 (URN)
Merknad

QC 20161125

Tilgjengelig fra: 2016-11-24 Laget: 2016-11-24 Sist oppdatert: 2016-11-25bibliografisk kontrollert
3. Stability and sensitivity of a crossflow-dominated Falkner–Skan–Cooke boundary layer with discrete surface roughness
Åpne denne publikasjonen i ny fane eller vindu >>Stability and sensitivity of a crossflow-dominated Falkner–Skan–Cooke boundary layer with discrete surface roughness
Vise andre…
2016 (engelsk)Inngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645Artikkel i tidsskrift (Fagfellevurdert) Submitted
Abstract [en]

With the motivation of determining the critical roughness size, a global stability and sensitivity analysis of a three-dimensional Falkner–Skan–Cooke (FSC) boundary layer with a cylindrical surface roughness is performed. The roughness size is chosen such that breakdown to turbulence is initiated by a global version of traditional secondary instabilities of the crossflow (CF) vortices, instead of an immediate flow tripping at the roughness. The resulting global eigenvalue spectra of the systems are found to be very sensitive to numerical parameters and domain size. This sensitivity to numerical parameters is quantified using the "-pseudospectrum, and the dependency on the domain is analysed through an impulse response and an energy budget. It is shown that the growth rates increase with domain size, which originates from the inclusion of stronger CF vortices in the baseflow. This is reflected in a change in the rate of advective energy transport by the baseflow. It is concluded that the onset of global instability in a FSC boundary layer as the roughness height is increased does not correspond to an immediate flow tripping behind the roughness, but occurs for lower roughness heights if su ciently long domains are considered. However, the great sensitivity results in an inability to accurately pinpoint the exact parameter values for the bifurcation, and the large spatial growth of the disturbances in the long domains eventually becomes larger than what can be resolved using finite precision arithmetics. 

sted, utgiver, år, opplag, sider
Cambridge University Press, 2016
HSV kategori
Identifikatorer
urn:nbn:se:kth:diva-196877 (URN)
Merknad

QC 20161125

Tilgjengelig fra: 2016-11-24 Laget: 2016-11-24 Sist oppdatert: 2017-11-29bibliografisk kontrollert
4. Interaction of acoustic waves and micron-sized surface roughness elements in a swept-wing boundary layer
Åpne denne publikasjonen i ny fane eller vindu >>Interaction of acoustic waves and micron-sized surface roughness elements in a swept-wing boundary layer
2016 (engelsk)Rapport (Annet vitenskapelig)
Abstract [en]

E↵ect of acoustic waves on the control performance of distributed micron-sized roughness elements in a swept-wing boundary layer is investigated through direct numerical simulations. The flow configuration conforms to experiments by Kachanov et al. (2015) who observed either no significant influence of acoustic waves on the transition location or small stabilisation e↵ect. In this work, a base set up for natural transition scenario is first established by introducing unsteady background noise in the boundary layer. The natural transition is then delayed using control roughness elements. Introduction of acoustic waves to the controlled flow promotes the transition location. In all these flow cases, stationary primary crossflow vortices dominate the disturbance environment and unsteady disturbances experience an explosive growth prior to transition. The spatial distribution of the energy production associated with z-type modes shows an increase in the local transfer of energy from the modified mean flow to perturbations. Simulation of flow with control roughness elements and acoustic waves as the only source of unsteady disturbances shows no influence of acoustic wave in transition to turbulence. 

sted, utgiver, år, opplag, sider
KTH Royal Institute of Technology, 2016
HSV kategori
Identifikatorer
urn:nbn:se:kth:diva-196875 (URN)
Merknad

QC 20161125

Tilgjengelig fra: 2016-11-24 Laget: 2016-11-24 Sist oppdatert: 2016-11-25bibliografisk kontrollert
5. Passive control of a swept-wing boundary layer using ring-type plasma actuators
Åpne denne publikasjonen i ny fane eller vindu >>Passive control of a swept-wing boundary layer using ring-type plasma actuators
(engelsk)Inngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645Artikkel i tidsskrift (Fagfellevurdert) Submitted
Abstract [en]

Application of the ring-type plasma actuators for passive control of laminar- turbulent transition in a swept-wing boundary layer is investigated thorough direct numerical simulations. These actuators induce a wall-normal jet in the boundary layer and can act as virtual roughness elements. The flow configuration resembles experiments of Kim et al. (2016). The actuators are modelled by the volume forces computed from the experimentally measured induced velocity filed at the quiescent air condition. The natural surface roughness and unsteady perturbations are also included in the simulations. The interaction of generated vortices by the actuators with these perturbations is investigated in details. It is found that for a successful transition control the power of the actuator should be increased to generate a jet velocity one order of magnitude higher than that in the considered experiments. 

sted, utgiver, år, opplag, sider
Cambridge University Press
Emneord
boundary layer control, boundary layer receptivity, ring-type plasma actuators, crossflow vortices
HSV kategori
Identifikatorer
urn:nbn:se:kth:diva-196876 (URN)
Merknad

QC 20161125

Tilgjengelig fra: 2016-11-24 Laget: 2016-11-24 Sist oppdatert: 2017-11-29bibliografisk kontrollert

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