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206 Pages·2000·7.75 MB·English

Investigations of disturbances developing in a laminar
separation bubble flow
by
Carl Ha¨ggmark
March 2000
Technical Reports from
RoyalInstitute of Technology
Department of Mechanics
S-100 44 Stockholm, Sweden
Akademisk avhandling som med tillst˚and av Kungliga Tekniska Ho¨gskolan i
Stockholm framl¨agges till offentlig granskning fo¨r avla¨ggande av teknologie
doktorsexamen fredagen den 31:e mars 2000kl 13.00iKollegiesalen, Adminis-
trationsbyggnaden, KTH, Valhallava¨gen79,Stockholm.
(cid:1)cCarl Ha¨ggmark 2000
H¨ogskoletryckeriet, Stockholm 2000
Till Jessica

C. H¨aggmark 2000 Investigations of disturbances developing in a laminarsep-
aration bubble flow
Department of Mechanics, Royal Institute of Technology
S-100 44 Stockholm,Sweden
Abstract
A separation bubble flow is considered in wind tunnel experiments. An exper-
imental set-up is utilized in which steady two-dimensional laminar boundary-
layer separation occurs on a flate plate due to an adverse pressure gradient.
The pressure gradient is produced by a curved ceiling in the wind tunnel test
section.
In a low disturbance level environment, high frequency instability waves
growintheseparatedshearlayer,whichreattachesandtransitiontoturbulence
occurs. Detailed investigationsof the disturbance development are carried out
using controlledforcingoflow-amplitudeinstabilitywaves. Their development
andgrowthintheseparationbubbleflowisinvestigatedwithhot-wireanemom-
etry, flow visualization technique and particle image velocimetry (PIV). Con-
ventionalhot-wiremeasurementshavebeenperformedinthelaminarboundary
layer upstream ofseparation, in the separated shear layer and in the reattach-
ment zone. A region with exponential disturbance growth is observed in the
separated shear layer associated with a highly two-dimensional flow. A local
maximumin the disturbance amplitude develops at the inflection point in the
mean velocity profile, indicating an inviscid type of instability. Further down-
stream,inthereattachmentregion,acomplexthree-dimensionalflowstructure
develops including reverse flow near the wall. Details of the flow field in this
region have been obtained.
A hot-wire technique, sensitive to the flow direction, has been developed
for measurements of reverse flow near the wall. The operation of the direction
sensitive probedonotdiffersignificantlyfromconventionalhot-wiretechnique.
Measurements have been carried out with the probe inside the laminar sepa-
ration bubble, providing informationon the reverse flow region.
Finally,measurements ofthe meanflowanddisturbance growthhavebeen
compared with direct numerical simulations.
Descriptors: Laminarseparation bubbles, instabilitywaves, adverse pressure
gradient,reverse flow,laminar-turbulenttransition,boundary-layerseparation,
hot-wire anemometry, PIV, reattachment.

Preface
This thesis considers a laminarseparation bubble flowand disturbances devel-
oping therein. It is based on the followingpapers:
I Ha¨ggmark, C. P., Bakchinov, A. A. & Alfredsson, P. H. 2000
Experiments on a two-dimensional laminar separation bubble. Phil.
Trans. R. Soc. Lond. A 358, (in press).
II Ha¨ggmark, C. P., Bakchinov, A. A. & Alfredsson, P. H. 2000
Measurements with a flow direction boundary-layer probe in a two-
dimensional laminarseparation bubble. Exp. in Fluids 28, (in press).
III Ha¨ggmark, C. P. 2000 An experimental study of instabilitywaves in
a laminarseparation bubble.
IV Hildings, C., Ha¨ggmark, C. P. & Henningson, D. S. 2000 A
numerical and experimental study of a transitional separation bubble.
(submitted to Aerosp. Sci. Technol.).
vii
viii
Contents
Preface vii
Chapter 1. Introduction 1
1.1. Aim and method 3
Chapter 2. Aspects of laminar separation bubble flows 4
2.1. Laminar separation bubbles in engineering flows 4
2.2. Classifications of separated flows 5
2.3. Experimental considerations 9
2.4. Bubble bursting 10
Chapter 3. Stability considerations 13
3.1. Basic flow equations 13
3.2. Inviscid linear stability theory - the Rayleighequation 15
3.3. Pressure gradients and separation 15
3.4. Secondary instabilityof wave disturbances 16
Chapter 4. Studies on laminar separation bubbles 18
4.1. Early work 18
4.2. Instability and transition 19
4.3. Development of three-dimensional disturbances 23
Chapter 5. Separation control 26
Chapter 6. Summary of Papers 31
Paper I 31
Paper II 31
Paper III 32
Paper IV 32
ix
x CONTENTS
Acknowledgments 34
References 35
I. Experiments on a two-dimensional laminar separation bubble 41
II. Measurements with a Flow Direction Boundary-Layer Probe
in a Two-Dimensional Laminar Separation Bubble 61
III. An experimental study of instability waves in a laminar
separation bubble 81
IV.Anumericalandexperimentalstudyofatransitionalseparation
bubble 173