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Flow control of boundary lagers and wakes
KTH, Superseded Departments, Mechanics.ORCID iD: 0000-0002-3251-8328
2003 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Both experimental and theoretical studies have beenconsidered on flat plate boundary layers as well as on wakesbehind porous cylinders. The main thread in this work iscontrol, which is applied passively and actively on boundarylayers in order to inhibit or postpone transition toturbulence; and actively through the cylinder surface in orderto effect the wakecharacteristics.

An experimental set-up for the generation of the asymptoticsuction boundary layer (ASBL) has been constructed. This studyis the first, ever, that report a boundary layer flow ofconstant boundary layer thickness over a distance of 2 metres.Experimental measurements in the evolution region, from theBlasius boundary layer (BBL) to the ASBL, as well as in theASBL are in excellent agreement with boundary layer analysis.The stability of the ASBL has experimentally been tested, bothto Tollmien-Schlichting waves as well as to free streamturbulence (FST), for relatively low Reynolds numbers (Re). For the former disturbances good agreement is foundfor the streamwise amplitude profiles and the phase velocitywhen compared with linear spatial stability theory. However,the energy decay factor predicted by theory is slightlyoverestimated compared to the experimental findings. The latterdisturbances are known to engender streamwise elongated regionsof high and low speeds of fluid, denoted streaks, in a BBL.This type of spanwise structures have been shown to appear inthe ASBL as well, with the same spanwise wavelength as in theBBL, despite the fact that the boundary layer thickness issubstantially reduced in the ASBL case. The spanwise wavenumberof the optimal perturbation in the ASBL has been calculated andis β = 0.53, when normalized with the displacementthickness. The spanwise scale of the streaks decreases withincreasing turbulence intensity (Tu) and approaches the scale given by optimalperturbation theory. This has been shown for the BBL case aswell.

The initial energy growth of FST induced disturbances hasexperimentally been found to grow linearly as Tu2Rexin the BBL, the transitional Reynolds numberto vary as Tu-2, and the intermittency function to have a relativelywell-defined distribution, valid for all Tu.

The wake behind a porous cylinder subject to continuoussuction or blowing has been studied, where amongst other thingsthe Strouhal number (St) has been shown to increase strongly with suction,namely, up to 50% for a suction rate of 2.5% of the free streamvelocity. In contrast, blowing shows a decrease ofStof around 25% for a blowing rate of 5% of the freestream velocity in the considered Reynolds number range.

Keywords:Laminar-turbulent transition, asymptoticsuction boundary layer, free stream turbulence,Tollmien-Schlichting wave, stability, flow control, cylinderwake.

Place, publisher, year, edition, pages
Stockholm: KTH , 2003. , viii, 60 p.
Series
Trita-MEK, ISSN 0348-467X ; 2003:18
Keyword [en]
Laminar-turbulent transition, asymptotic suction boundary layer, free stream turbulence, Tollmien-Schlichting wave, stability, flow control, cylinder wake
National Category
Mechanical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-3664OAI: oai:DiVA.org:kth-3664DiVA: diva2:9499
Public defence
2003-12-12, 00:00
Note
QC 20100607Available from: 2003-12-05 Created: 2003-12-05 Last updated: 2010-06-07Bibliographically approved
List of papers
1. On the disturbance growth in an asymptotic suction boundary layer
Open this publication in new window or tab >>On the disturbance growth in an asymptotic suction boundary layer
2003 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 482, 51-90 p.Article in journal (Refereed) Published
Abstract [en]

An experimental and theoretical study on the effect of boundary layer suction on the laminar-turbulent transition process has been carried out. In the study an asymptotic suction boundary layer was established in a wind tunnel with a free-stream velocity of 5.0 m s(-1). Wall-normal suction (suction velocity 1.44 cm s(-1)) was applied over a large area and the boundary layer was nearly constant over a length of 1800 mm. Measurements were made both with and without suction so comparisons between the two cases could easily be made. Measurements of the development of the mean velocity distribution showed good agreement with theory. The Reynolds number based on the displacement thickness for the suction boundary layer was 347. Experiments on both the development of forced Tollmien-Schlichting (TS) waves and boundary layer disturbances introduced by free-stream turbulence were carried out. Spatial linear stability calculations for TS-waves, where the wall-normal velocity component is accounted for, were carried out for comparison with the experiments. This comparison shows satisfactory agreement even though the stability of the asymptotic suction profile is somewhat overpredicted by the theory. Free-stream turbulence (FST) was generated by three different grids, giving turbulence intensities at the leading edge of the plate between 1.4% and 4.0%. The FST induces disturbances in the boundary layer and it was shown that for the present suction rate the disturbance level inside the boundary layer is constant and becomes proportional to the FST intensity. In all cases transition was prevented when suction was applied whereas without suction the two highest levels of grid turbulence gave rise to transition. Despite a twofold reduction in the boundary layer thickness in the suction case compared to the no suction case the spanwise scale of the streaky structures was almost constant.

Keyword
BYPASS TRANSITION, CHANNEL FLOW, SHEAR FLOWS, INSTABILITY, STABILITY, SURFACE
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-13236 (URN)10.1017/S0022112003003926 (DOI)000183514200003 ()
Note
QC 20100607Available from: 2010-06-07 Created: 2010-06-07 Last updated: 2017-12-12Bibliographically approved
2. Optimal linear growth in the asymptotic suction boundary layer
Open this publication in new window or tab >>Optimal linear growth in the asymptotic suction boundary layer
2003 (English)In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 22, no 3, 259-270 p.Article in journal (Refereed) Published
Abstract [en]

A variational technique in the temporal framework is used to study initial configurations of disturbance velocity which maximize perturbation kinetic energy in the asymptotic suction boundary layer (ASBL). These optimal perturbations (OP) excite significant and remarkably persistent transient growth, on the order of that which occurs in the Blasius boundary layer (BBL). In contrast, classical modal analysis of the ASBL predicts a critical Reynolds number two orders of magnitude larger than that for the BBL. As in other two-dimensional boundary layer flows, disturbances undergoing maximum amplification are infinitely elongated in the direction of the flow and take the form of streamwise-oriented vortices which induce strong variations in the streamwise perturbation velocity (streaks). The Reynolds number dependence of the maximum growth, and the best choice of scaling for the spanwise wavenumber of the perturbation causing it, are elucidated. There is good agreement between the streak resulting from OP and disturbances measured in experiments in which the asymptotic suction boundary layer is subject to free stream turbulence (FST). This agreement is shown to improve as the level of FST increases.

Keyword
transient growth, boundary layer, asymptotic suction, optimal perturbations, streaks, DISTURBANCE GROWTH, BYPASS TRANSITION, ENERGY GROWTH, FLOW-CONTROL, INSTABILITY, TURBULENCE, STABILITY
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-13237 (URN)10.1016/S0997-7546(03)00033-5 (DOI)000184383400004 ()
Note
QC 20100607Available from: 2010-06-07 Created: 2010-06-07 Last updated: 2017-12-12Bibliographically approved
3. On the hydrodynamic stability of channel flow with cross flow
Open this publication in new window or tab >>On the hydrodynamic stability of channel flow with cross flow
2003 (English)In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 15, no 2, 436-441 p.Article in journal (Refereed) Published
Abstract [en]

We study plane channel flow, with a homogeneous cross flow through porous walls, mainly with respect to the stability to two-dimensional wave disturbances. Since the stability of a shear flow depends both on the velocity distribution and the Reynolds number we partly investigated this flow under the conditions that the flow Reynolds number was constant. The flow exhibits some interesting and unexpected stability characteristics. The effect of the cross flow was for certain parameter regions stabilizing and for others destabilizing. The latter result is in contrast to previous studies.

National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-13238 (URN)10.1063/1.1533076 (DOI)000180316100017 ()
Note
QC 20100607Available from: 2010-06-07 Created: 2010-06-07 Last updated: 2017-12-12Bibliographically approved
4. Free stream turbulence induced disturbances in boundary layers with wall suction
Open this publication in new window or tab >>Free stream turbulence induced disturbances in boundary layers with wall suction
2004 (English)In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 16, no 10, 3530-3539 p.Article in journal (Refereed) Published
Abstract [en]

An experimental investigation of free stream turbulence (FST) induced disturbances in asymptotic suction boundary layers (ASBL) has been performed. In the present study four different suction rates are used and the highest is 0.40% of the free stream velocity, together with three different FST levels (Tu=1.6, 2.0, and 2.3%). A turbulence generating grid of the active type is used and offers the possibility to vary the Tu-level while the scales of the turbulence remain almost constant. It is known that FST induces elongated disturbances consisting of high and low velocity regions, usually denoted streaky structures, into the boundary layer. The experiments show that wall suction suppresses the disturbance growth and may significantly delay or inhibit the break-down to turbulence. Two-point correlation measurements in the spanwise direction show that the averaged streak spacing decreases with increasing FST-level, whereas the spanwise scale in the ASBL is more or less constant if scaled with the free stream velocity and viscosity. This is in contrast to what is observed in a Blasius boundary layer where streaks develop and adapt their spanwise scale close to the boundary layer thickness.

Keyword
BYPASS TRANSITION, GROWTH, RECEPTIVITY, FLOW
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-13239 (URN)10.1063/1.1775222 (DOI)000223822300003 ()2-s2.0-7244260423 (Scopus ID)
Note
QC 20100607 QC 20110922Available from: 2010-06-07 Created: 2010-06-07 Last updated: 2017-12-12Bibliographically approved
5. Transition induced by free-stream turbulence
Open this publication in new window or tab >>Transition induced by free-stream turbulence
2005 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 527, 1-25 p.Article in journal (Refereed) Published
Abstract [en]

Free-stream turbulence (FST) is perhaps the most important source inducing by-pass transition in boundary layer flows. The present study describes the initial energy growth of streamwise-oriented disturbances in the boundary layer originating from the presence of FST with intensities between 1.4% and 6.7%, but the study is mainly focused on the modelling of the transition zone. For this study three passive and one active turbulence-gene rating grids were used. The active grid was used in order to vary the turbulence intensity (Tu(2)) without changing the setup in the test section. It is shown that the initial disturbance energy in the boundary layer is proportional to Tu(2). The experiments also show that the energy grows in linear proportion to the Reynolds number based on the downstream distance. Furthermore the transitional Reynolds number is shown to be inversely proportional to Tu(2) for the whole range of Tu studied. The intermittency in the transitional zone was determined and it was shown that the intermittency function has a universal shape if the downstream distance is scaled with the length of the transition zone. The Reynolds number based on this transition zone length was found to increase linearly with the transition Reynolds number; however it was also noted that this non-dimensional length has a minimum value. With these results we were able to formulate an expression for the spot production rate which has a better physical base than previous models.

Keyword
BOUNDARY-LAYER, DISTURBANCE GROWTH, BYPASS TRANSITION, FLOW, LAMINAR, SUCTION
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-13240 (URN)10.1017/S0022112004002770 (DOI)000228618000001 ()2-s2.0-17044423373 (Scopus ID)
Note
QC 20100607Available from: 2010-06-07 Created: 2010-06-07 Last updated: 2017-12-12Bibliographically approved
6. On streamwise streaks generated by roughness elements in the boundary layeron a flat plate.
Open this publication in new window or tab >>On streamwise streaks generated by roughness elements in the boundary layeron a flat plate.
2003 (English)In: 17th AIMETA Congress of Theoreticaland Applied Mechanics, 2003Conference paper, Published paper (Other academic)
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-13241 (URN)
Conference
17th AIMETA Congress of Theoretical and Applied Mechanics
Note
QC 20100607Available from: 2010-06-07 Created: 2010-06-07 Last updated: 2010-06-07Bibliographically approved
7. Flow around a porous cylinder subject to continuous suction or blowing
Open this publication in new window or tab >>Flow around a porous cylinder subject to continuous suction or blowing
2004 (English)In: Journal of Fluids and Structures, ISSN 0889-9746, E-ISSN 1095-8622, Vol. 19, no 8, 1031-1048 p.Article in journal (Refereed) Published
Abstract [en]

In the present experimental investigation the surface pressure distribution, vortex shedding frequency, and the wake flow behind a porous circular cylinder are studied when continuous suction or blowing is applied through the cylinder walls. It is found that even moderate levels of suction/blowing (less than or similar to 5% of the oncoming streamwise velocity) have a large impact on the flow around the cylinder. Suction delays separation contributing to a narrower wake width, and a corresponding reduction of drag, whereas blowing shows the opposite behaviour. Both uniform suction and blowing display unexpected flow features which are analysed in detail. Suction shows a decrease of the turbulence intensity throughout the whole wake when compared with the natural case, whilst blowing only shows an effect up to five diameters downstream of the cylinder. The drag on the cylinder is shown to increase linearly with the blowing rate, whereas for suction there is a drastic decrease at a specific suction rate. This is shown to be an effect of the separation point moving towards the rear part of the cylinder, similar to what happens when transition to turbulence occurs in the boundary layer on a solid cylinder. The suction/blowing rate can empirically be represented by an effective Reynolds number for the solid cylinder, and an analytical expression for this Reynolds number representation is proposed and verified. Flow visualizations expose the complexity of the flow field in the near wake of the cylinder, and image averaging enables the retrieval of quantitative information, such as the vortex formation length.

Keyword
LOW REYNOLDS-NUMBERS, CIRCULAR-CYLINDER, ROTARY OSCILLATION, FEEDBACK-CONTROL, ASPECT RATIO, WAKE, DYNAMICS, DRAG
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-13242 (URN)10.1016/j.jfluidstructs.2004.06.005 (DOI)000225512600001 ()2-s2.0-8744259512 (Scopus ID)
Note
QC 20100607 QC 20110915Available from: 2010-06-07 Created: 2010-06-07 Last updated: 2017-12-12Bibliographically approved
8. PIV–measurements in the wake of a cylinder subject to continuous suction or blowing.
Open this publication in new window or tab >>PIV–measurements in the wake of a cylinder subject to continuous suction or blowing.
(English)Manuscript (preprint) (Other academic)
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-13243 (URN)
Note
QC 20100607Available from: 2010-06-07 Created: 2010-06-07 Last updated: 2010-06-07Bibliographically approved
9. Errors in hot-wire X-probe measurements induced by unsteady velocity gradients
Open this publication in new window or tab >>Errors in hot-wire X-probe measurements induced by unsteady velocity gradients
2002 (English)In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 32, no 3, 413-415 p.Article in journal (Refereed) Published
Abstract [en]

Errors in hot-wire X-probe measurements due to unsteady velocity gradients are investigated by a comparison of hot-wire and laser Doppler velocimetry (LDV) measurements. The studied flow case is a laminar boundary layer subjected to high levels of free-stream turbulence, and the hot-wire data shows a local maximum in the wall-normal fluctuation velocity inside the boundary layer. The observed maximum is in agreement with existing hot-wire data, but in conflict with the present LDV measurements as well as existing results from numerical simulations. An explanation for the measurement error is suggested in the paper.

National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-13231 (URN)10.1007/s003480100360 (DOI)000174620800013 ()
Note
QC 20100607Available from: 2010-06-07 Created: 2010-06-07 Last updated: 2017-12-12Bibliographically approved
10. Leading edge design process using a commercial flow solver
Open this publication in new window or tab >>Leading edge design process using a commercial flow solver
2004 (English)In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 37, no 6, 929-932 p.Article in journal (Refereed) Published
Abstract [en]

A design process of an asymmetric leading edge, for laminar stability measurements in a flat plate boundary layer, is reported. The purpose is to minimize the leading edge pressure gradient region, which affects the stability characteristics of the flow. Finally, the design success is verified by wind tunnel testing.

Keyword
FREE-STREAM TURBULENCE, BOUNDARY-LAYERS, DISTURBANCE GROWTH, RECEPTIVITY, SUCTION
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-13245 (URN)10.1007/s00348-004-0858-3 (DOI)000225522800015 ()2-s2.0-10844287299 (Scopus ID)
Note
QC 20100607Available from: 2010-06-07 Created: 2010-06-07 Last updated: 2017-12-12Bibliographically approved

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