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An experimental investigation of disturbance growth in boundary layer flows
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis deals with the early stages of transition to turbulence in two different baseflows, namely the Falkner-Skan-Cooke boundary layer (FSC) and the asymptotic suction boundary layer (ASBL). Grid-generated turbulence is studied in order to characterise the isotropy levels, free-stream turbulence levels and characteristic length scales that will be present in the receptivity study. By varying the grids and their location it is possible to control the turbulence intensity level, Tu, and the integral length scale independently. Comparisons with other studies show that for increasing Re_M the isotropy levels and the rate of kinetic energy decay asymptotically approach the theoretical values. The FSC describes a a 3D boundary layer subjected to a pressure gradient. The FSC is stable to TS-waves, but becomes susceptible to both travelling and stationary crossflow disturbances. In the experiments the travelling modes were triggered using free-stream turbulence (FST) and the stationary modes were triggered using an array of cylindrical roughness elements. The receptivity phase to FST was linear as well as the initial growth. For high enough $Tu$ inside the boundary layer, nonlinear behaviour was observed further downstream. The stationary mode could only be triggered using tall roughness elements, with low heights resulting in no noticeable disturbances. The receptivity is found to be nonlinear for the roughness heights tested and the growth of the disturbances is exponential. For low levels of FST, Tu < 0.25%, the travelling mode as well as the stationary mode grew. The ASBL is formed when uniform suction is applied to the surface of a porous plate with a flow over it. This baseflow is very stable to TS-waves, and was used to study the transient growth. For the ASBL, stationary disturbances were triggered using a spanwise array or cylindrical roughness elements. The velocity signals were decomposed using a spatial Fourier transform to study the growth of individual modes. The fundamental mode as well as some harmonics were seen to undergo transient growth, before finally decaying exponentially. Comparisons were made to the experimental data using optimal perturbation theory. The global optimals did not describe the transient growth effects well. The calculations were redone for suboptimal times and showed agreement with the experimental data, showing that optimal perturbation theory can describe transient growth if the initial disturbance state is known.

Place, publisher, year, edition, pages
Stockholm: KTH , 2010. , viii, 41 p.
Series
Trita-MEK, ISSN 0348-467X ; 2010:04
National Category
Fluid Mechanics and Acoustics Other Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-12949ISBN: 978-91-7415-654-6 (print)OAI: oai:DiVA.org:kth-12949DiVA: diva2:319781
Public defence
2010-06-11, F3, Linstedsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC20100702Available from: 2010-05-24 Created: 2010-05-19 Last updated: 2010-07-02Bibliographically approved
List of papers
1. Grid-generated turbulence revisited
Open this publication in new window or tab >>Grid-generated turbulence revisited
2009 (English)In: Fluid Dynamics Research, ISSN 0169-5983, E-ISSN 1873-7005, Vol. 41, no 2, 021403- p.Article in journal (Refereed) Published
Abstract [en]

In this study we characterize the turbulence, by means of energy spectra, characteristic turbulence length scales, energy dissipation, kinetic energy decay rate etc., behind a set of grids with the feature of having roughly the same solidity but different mesh and bar widths. This is one way of being able to vary the turbulence characteristic length scales while keeping the same turbulence intensity, which is usually a difficult task for experimentalists. Measurements are performed by using, on the one hand, traditional hot-wire x-probes oriented in both directions giving information about all three directional velocity components and, on the other hand, small single-wire probes for faster frequency response. Independent procedures to calculate some quantities are summarized and performed in the present paper and compared with correlation functions based on homogeneous isotropic turbulence as well as semi-empirical relations. For grid-generated turbulence, which often erroneously is described as isotropic (actually a rare condition), relations derived based on isotropic turbulence are frequently used. Here, we show that dissipation rates and length scales may be inaccurate by as much as 50% or more when compared with valid anisotropic relations. The paper ends with a comparison of the turbulence characteristics between the zero pressure gradient case and a favorable pressure gradient case with a small degree of cross flow. With the pressure gradient, a reduction of the integral and Taylor length scales of about 20% and 30%, respectively, is reported for a large mesh width, whereas no change is observed for a small one.

National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-12945 (URN)10.1088/0169-5983/41/2/021403 (DOI)000270658300004 ()2-s2.0-66149084895 (Scopus ID)
Note
QC20100524Available from: 2010-05-19 Created: 2010-05-19 Last updated: 2011-03-21Bibliographically approved
2. Receptivity of crossflow instabilities to free-stream turbulence and surface roughness
Open this publication in new window or tab >>Receptivity of crossflow instabilities to free-stream turbulence and surface roughness
(English)Article in journal (Other academic) Submitted
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-13916 (URN)
Note
QS 20120328Available from: 2010-07-02 Created: 2010-07-02 Last updated: 2012-03-28Bibliographically approved
3. Transient growth in the asymptotic suction boundary layer
Open this publication in new window or tab >>Transient growth in the asymptotic suction boundary layer
2011 (English)In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 51, no 3, 771-784 p.Article in journal (Refereed) Published
Abstract [en]

In the present experimental setup, the transient disturbance growth in a spatially invariant boundary layer flow, i.e., the asymptotic suction boundary layer (ASBL), has been investigated. The choice of the ASBL brings along several advantages compared with an ordinary spatially growing boundary layer. A unique feature of the ASBL is that the Reynolds number (Re) can be varied without changing the boundary layer thickness, which in turn allows for parameter variations not possible to carry out in traditional boundary layer flows. A spanwise array of discrete surface roughness elements was mounted on the surface to trigger modes with different spanwise wavenumbers (beta). It is concluded that for each mode there exists a threshold roughness Reynolds number (Re (k) ), below which no significant transient growth is present. The experimental data suggests that this threshold Re (k) is both a function of beta and Re. An interesting result is that the energy growth curves respond differently to a change in Re (k) when caused by a change in roughness height k, implying that Re remains constant, compared with a change in the free-stream velocity U(infinity), which also affects the Re. The scaling of the energy rowth curves both in level and the downstream direction is treated and appropriate scalings are found. The result shows a complex non-linear receptivity mechanism. Optimal perturbation theory, which has failed to predict the energy evolution in growing boundary layers, is tested for the ASBL and shows that it may satisfactorily predict the evolution of all transiently growing modes that are triggered by the roughness elements.

National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-13917 (URN)10.1007/s00348-011-1095-1 (DOI)000294347800017 ()2-s2.0-80052655039 (Scopus ID)
Note
QC20100702 Uppdaterad från submitted till published 20010927Available from: 2010-07-02 Created: 2010-07-02 Last updated: 2011-09-27Bibliographically approved

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