Flow measurements for an afterbody in a vertical wind tunnel

eries 01

Aerodynamics 02

Flow Measurèments for an Afterbody

in a Vertical Wind Tunnel

P.E. Skare

Flow Measurements for an Afterbody

in a yertical Wind Tunnel

8ibl i ot heek. TU Delft

1111

~

1111111

C 30,'1883

Series 01: Aerodynamics

02

'. '. ' . . .

Flow Measurements tor an

Afterbody in a Vertical Wind Tunnel

P.E. Skare

Delft University Pre ss / 1997

2392

342

7

Published and distributed by: Delft University Press

Mekelweg 4 2628 CD Delft The Netherlands Telephone +31 (0)152783254 Fax +31 (0)152781661 e-mail: DUP@DUP.TUDelft.NL by order of:

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Cover: Aerospace Design Studio, 66.5 x 45.5 cm, by:

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Copyright © 1997 by Faculty of Aerospace Engineering All rights reserved.

No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electron ic or

mechanical, including photocopying, recording or by any information storage and retrieval system, without written permission from the publisher: Delft University Press.

Acknowledgements

The author is grateful to Delft University of Technology for providing him the opportunity to be a Post-Doe Visitor during the period April-October 1994. The visit was financially supported by the "Extra Research Fellow" programme of that university.

Special thanks are due to dr.ir. R.A.W.M Henkes for his cooperation and enthusiasm. Thanks are also due to the scientists and technical staff of the Low-Speed Laboratory at TU-Delft.

Contents

Acknowledgements List of Figures List of Tables Nomenclature 1 Introduction 2 Experimental configuration 2.1 Wind tunnel . . . . .. . 2.1.1 Working section . 2.1.2 Suction devices 2.1.3 Afterbody . . . . 2.2 Hot-wire anemometry . . 2.2.1 Single-wire probe 2.2.2 Cross-wire probes 2.2.3 General .. 2.3 General Equipment 2.4 Filter problems 2.5 Figures .. 3 Flow quality

3.1 Working section symmetry 3.1.1 Free stream symmetry . . 3.1.2 Boundary layer symmetry 3.2 Local two-dimensionality

3.3 Final condition

3.4 Figures.. .. . . 4 Mean flow

4.1 Mean velocity profiles . 4.1.1 (x,y) plane . . . 4.1.2 (x, Y) plane . .

4.1.3 Representation of the mean velocity profile.

111 vii IX xi 1 3 :3 3 3 4 Ei .5 6 8 8 9 11 21 21 21 22 23 2:3 2.5 33 3:3 :33 34 35 III

4.1.4 Skin friction determination . 4.1.5 Boundary layer length scales . 4.2 Momentum balance . . . . 4.2.1 Oil-flow visualisation 4.3 Statie pres su re 4.4 Figures. 5 Reynolds stresses 5.1 Normal stresses 5.1.1 (x,y) plane . 5.1.2 (x, Y) plane 5.2 Shear stress .. . .

5.2.1 Check of shear stress value . 5.3 Another reduction method

5.4 Figures. . . . . . . . . . . 6 Conclusions and Recommendations

6.1 Conclusions . . .. . .. .. . 6.2 Recommendations for further research

6.2.1 Vertical wind tunnel 6.2.2 Hot wire anemometry . 6.2.3 Data acquisition . 6.2.4 Krohn-Hite filters 6.3 Further research. . A Boundary layer results

A.l Mean results. ..

A.2 Boundary layer development B Total flow results

B.1 Mean results . . B.2 Total flow development References 3.5 36 36 37 38 40 51 51 51 52 .52 -? .)-.53 .55 63 63 64 64 64 65 65 65 67 67 76 79 79 88 91

List of Figures

2.1 The vertical wind tunnel . . . . . . . . . 11

2.2 Working section of the vertical wind tunnel . 12

2.3 A close-up of the afterbody . . . 1:3

2.4 Typical spectra for u in the boundary layer at x

I

Ri

=

2.0 . 14 2.5 Typical spectra for u in the shear layer of the jet outer edge at x

I

Ri

= 2.0

14 2.6 Angle response for the X-wire probe (5.0 flm, lwld"." ~ 100) 15 2.7 Angle response for the X-wire probe (2.5 flm, lwl dw ~ 200) . . . . . . . 15 2.8 Effective wire angles found from the eosine law. . . . . . . . . . 16 2.9 Normal Reynolds stresses

ul

at xlRi

=

2.0 for Type A -X-Wire probe. 16 2.10 Normal Reynolds stresses

ul

at xlRi

=

2.0 for Type B -X-Wire probe. 17 2.11 Mean velocity at xlRi

=

1.0 - Filter test . . . . . . . . 17 2.12 Uncorrected Normal Reynolds stress UZ profiles at xl Ri

= 1.0

- Filter test 18 2.13 Spectra measured at xl Ri and y+ ~ 20 - Filter test 18 2.14 Test of three different band-pass filters . . . . . . 19 2.15 Corrected norm al Reynolds stress UZ - Filter test 19 3.1 Working section of the tunneL first stage. . . . . 2.5

3.2 Symmetry velocity profiles with Ue

=

10.0 mis. 26

3.3 Symmetry velocity profiles at Ue

=

20.0 mis 26

3.4 Symmetry velocity profiles at Ue = 30.0 mis . . 27

3.5 Symmetry velocity profiles at Ue = 40.0 mis . . 27

3.6 Symmetry velocity profiles at Ue = 20.0 mis: suction off 28

3.7 Skin friction symmetry at Ue

=

20.0 mis, suction off. . . 28 3.8 Boundary layer traverse at x

I

Ri

= -5.2, Suction devices on

29

3.9 Skin friction symmetry 200 mm before suction ring :30

3.10 Skin friction symmetry 470 mm after suction ring 30

3.11 Skin friction symmetry at xl Ri

= -0

. .5, No. 30ff . :31 3.12 Skin friction symmetry at xl Ri

= -0.5: No. 3 on

. :31 3.13 Local two-dimensionality at x

I

Ri

= 1.0 in the boundary layer

32 3.14 Local two-dimensionality at xlRi

=

1.0 in the shear layer of the jet 32 4.1 Overview of the traverses in the (x, y) and (x, Y) plane . . . 40 4.2 B.L profiles in the region -0.15

S

xlRi

S

:3.25 . . . . . . . . . 41 4.3 Boundary layer profiles in the region -0.15

S

xl R;

S

3.25 in inner sealing 41 4.4 Boundary layer profiles, U, in outer sealing in the region -0.15

S

xl Ri

S

3.25 - X-wire . . . .. . . .. . . . .. . . , 42 4.5 Boundary layer profiles in the region -0.15

S

xl Ri

S

3.25 - X-wire . . . 42 4.6 Boundary layer profiles, V, for the region -0.15

S

xl Ri

S

3.25 - X-wire 43

4.7 Change in O:xy in the region -0.15:S x/B.; :S 3.25 . . . 43

4.8 Mean veloeities in the region -0.15 :S X/Ri :S 4.52 -(x, Y) plane . 44 4.9 Mean veloeities in the region -0.15 :S X/Ri :S 4.52-(x, Y) plane 44 4.10 Result of the optimalisation procedure at X/Ri

=

1.00 . . . . 45 4.11 Result of the optimalisation procedure at x/B.;

=

3.00 . . . . 45 4.12 Distribution of the skin friction eoeffieient on the afterbody. 46 4.13 Length seale distribution on the afterbody. 46

4.14 Streamwise momentum balanee . . . . 47

4.15 Curvature - principle sketch . . . . . . 47 4.16 Effeetive radius, Tefj, of the afterbody . . 48 4.17 Statie pressure differenee t1P at X/Ri

=

2.00 48

4.18 Statie pressure differenee t1P at X/Ri

=

2.75 49 4.19 Statie pres su re differenee t1P at

x/

B.;

=

3.25 49

.5.1 Reynolds normal stress u2 _{- outer sealing .55} ·5.2 Reynolds norm al stress u2 _{- inner sealing .5.5}

·5.3 Reynolds norm al stress v2 _{- outer sealing 56}

.5.4 Reynolds norm al stress v2 _{- inner sealing 56} .J.·5 Normal Reynolds stress in the region -0.1·5 ~ x/ Ri ~ 4.·52 -(x. Y) plane .57 .5.6 Normal Reynolds stress in the region -0.1·5 :S x/ Ri :S 4.52- (x, Y) plane. S7 S.7 Reynolds shear stress -uv - outer sealing 58 ·5.8 Reynolds shear stress -uv - inner sealing . . . . . . . . . .. 58

5.9 Calculated shear stress

T

/T

w . . . . . . . . . . . . . 59 5.10 Deeomposition of the instantaneous velocity vector for the X-wire probe. 59 5.11 Different reduction methods, mean veloeities, X/Ri

=

-0.15 . 60 ·5.12 Different reduction methods, Reynolds stresses, x/Ri

=

-0.15 60

·

5.13 Different reduction methods, mean veloeities, x/B.;

=

1.00 . 61 S.14 Different reduetion methods, Reynolds stresses, X/Ri

=

1.00 61 A.1 Complete velocity field _U/Uref in the (x,y) plane

A.2 Complete stress field, u2_{/U;ef' in the (x, y) plane} B.1 Complete velocity field U _/Uref in the (x, Y) plane B.2 Complete stress field, u2_{/U;ef' in the (x. Y) plane}

77 78 89 90

List of Tables

2.1 Afterbody details .. .j

2.2 X-wire dimension 6

4.1 Boundary layer results :37

A.1 Boundary layer resul ts at x

I

El;

= -

.15 - (x, y) plane. 68

A.2 Boundary layer results at x

I

Ri

=

1.00 - (x, y) plane. 69

A.3 Boundary layer results at x

I

Ri

=

1.50 - (x, y) plane. 70

AA Boundary layer results at x

I

Ri

=

2.00 - (x, y) plane. 71

A.5 Boundary layer results at x

I

Ri

=

2.50 - (x, y) plane. 72

A.6 Boundary layer results at xlRi

=

2.75 - (x,y) plane.

n

A.7 Boundary layer results at x

I

Ri

=

3.00 - (x, y) plane. 74

A.8 Boundary layer results at x

I

Ri

=

3.25 - (x, y) plane. 7·5

B.1 Single-wire results at xlRi

=

-.15 - (x, Y) plane. 80

B.2 Single-wire results at xl Ri

=

.29 - (x, Y) plane. 81

B.3 Single-wire results at xl Ri

=

1.00 - (x, Y) plane. 82

B.4 Single-wire results at xl Ri

=

2.00 - (x, Y) plane. 83

B.5 Single-wire results at xl Ri

=

3.00 - (x, Y) plane. 84

B.6 Single-wire results at xl Ri

=

3.50 - (x, Y) plane. 85

B.7 Single-wire results at xl Ri

=

4.00 - (x, Y) plane. 86

B.8 Single-wire results at xl Ri

=

4.54 - (x, Y) plane. 87

I

Nomenclature

L Al B C, Ceff Cf d dw En !i'!o

!.,

!f h H k

Zw

P Pe. Pw r Re Ri,Ro

Ru"

s S t T u,V,W u.,.

U

e UTef U,V,W U 2,v2,W2 UV,UW x,Y y Greek symbols

angle bet ween the y and the Y axis free variabie

constant in the law-of-the-wall real and effective curvature radius skin friction coefficient, Cf == 2u;jU-;

outer diameter of a tube

nominal diameter of the hot-wire Power spectral density of u fiuctuations inner and outer functions

sampling and filter frequency height of the tripping band shape factor, i.e. H == ó' j8

turbulent kinetic energy, k == (u2

+

_v2

+

_w2_)j2 nominal length of the hot wire

static pressure

statie pressure in the free-stream and at the surface local radius of the af ter body

Reynolds number

inner and outer radius of the working section correlation

distance along the surface of the af ter body instantaneous velocity vector

time

mean temperature during a measurement

fiuctuating part of instantaneous velo city components

friction velocity, u.,. == Ue

J

Cf j2 free-stream velocity

reference velocity measured at

xj

Ri == 0 mean veloeities

normal Reynolds stresses Reynolds shear stresses axial and radial fiow direction distance normal to the surface

Ct flow angle, i.e. Ctxy == tan-I(VjU)

6.P difference in local and surface pressure (static)

6.wT ringwidth of the wind tunnel in radial direction 6.w distance between the wires in a X-wire arrangement

Ó boundary layer thickness ó' displacement thickness

ft IJ

von Karman's constant dynamic viscosity of the fiuid kinematic viscosity of the fiuid

<.p,i.pw IJ p 8 T Subscripts COTT T Te! xy, xY xz Superscripts

+

Overscores

angular position on the cylinder lafterbody probe angle and effective wire angle

wake parameter density of the fluid momentum thickness total shear stress wall shear stress

corrected

cylindrical effects are included measured at the reference position

measured with the X-wire aligned in the x,y or the x, Y plane measured with the X-wire aligned in the x, z plane

inner variables. i.e. normalized by the friction velocity, UT' and

the viscous length

111

UT root mean squared

average value instantaneous value

Chapter 1

Introduction

Most of the work on boundary layers and shear layers in adverse pressure gradients has been performed in two-dimensional rectangular geometries. Fewer reports are available for axisymmetric flows or three-dimensional flows. From a numeri cal point of view, the latter types are a great challenge for the modelling of the turbulence quantities, since most of the models for turbulence were constructed for two-dimensional flows. Hence, many of the mechanisms present in more complex types of flows are not accounted for. Axisymmetric flows including separation may be found in e.g. Driver (1991) who studied boundary layers in adverse pressure gradient developing on a cylinder. while separation behind blunt-based axisymmetric bodies are reported by e.g. Atli (1989) and Merz et

al. (1978), (1985).

The present study deals with a flow subjected to an adverse pressure gradient induced by a curved afterbody. The afterbody partly has an ellipsoid shape equal to the one used in an earlier investigation reported by Merz et al. (1985), (1986). In contrast to their model of the af ter body, which has full ellipsoid curves, the present ellipsoid is ended when the angle has reached 25° to avoid the earlier reported separation at the trailing edge of the body. The end of the present body is a straight cone with a top angle of .50°. The overall dimension of the afterbody is increased approximately 4.3 times compared to the one used by Merz et al. (1985).

This report includes the flow up to the trailing edge of the afterbody, which is the first stage in this investigation. Later on also the wake behind the afterbody is planned to be investigated in detail. The standard hot-wire technique is used (both single and X-wire probes) to unveil the mean flow distribution and the turbulent statistics. Both the attached boundary layer on the afterbody and the shear layer of the tunnel jet outer edge are investigated and reported.

The flow quality of the vertical wind tunnel is checked, and suggestions for further work are given.

Chapter 2

Experimental configuration

This chapter gives details of the set-up of the working section and the equipment used in the present work.

2.1

Wind tunnel

These measurements were carried out in the vertical boundarv laver tunnel ,11 the Low-Speed Laboratory at TU Delft during the period April-\"ovember 1994. Thp tunnel was designed by Professor Dobbinga, and after his death it also received his name. i.e. the Professor Dobbinga wind tunnel. An overview on the tota.! dimensions of thc tunnel is sketched in Figure 2.1. The overall dimension is roughly 16.8 x 8 m2

. anel it covers four

floors. The contraction ratio is about 100:1. The tunnel can be used both as a closeel or an open tunnel. The latter configuration is established by opening t he roof. which exchanges air with another inlet (not shown). For the present work. the t \1llncl was useel as a closed-circuit wind tunnel. The main aim of the experiment \\·a.5 to il1\'cstigate the turbulent flow induced by an afterbody.

2.1.1

Working section

A sketch of the working section of the vertical wind tunnel is shown in Figurc 2.2. In the working section the fluid (air) is flowing along a cylinder with radius R;

=

l:FJ mm. which acts as a forebody or an inner wall. The outer boundary of the working section consists of large tube sections which can be mounted on top of each ot her. The inner radius of these tube sections is Ro

=

:300 mmo No divergence in the area is present. so a slight favorable pressure gradient (dP/dx

<

0) is present in the working section. elue to thc elc\'clopment of two boundary layers. The location of :r/Ri = 0 is arbitrarily chosen at tbe junction of the forebody and the afterbody (see Figure 2.:3).

2.1.2

Suction devices

Three different suction positions are available for this wind tunnel. Two of them were located in the settling chamber, to remove the boundary layer on the inner cur\"ecl \Val! of the contraction. The first is made as a slit in the inner curved wal! aml is connectecl through tubes to the fan room upstairs (see Figure 2.1). Due to the lower prpssure in this room, a suction is present. This slit is called :\0. 1. At the junction of t h(' c\ll'\"ed wall

and the straight cylinder, the suction called No. 2 is located. This is also connected to

the fan room. The main suction is located on the cylindric inner wal! (named as !\o. :3).

:'-iote that the names do not correspond to the numbers used in Figure 2.1.

Suction Ko. :3 is the strongest suction, and is designed to remove thc tot.al boundary

layer before a new one develops on the inner cylinder. The maximum suction which is

available is up to approximately 2050 Pa (measured in the outer part of the suction tube).

However, the suction does not manage to remove the boundary layer as expected, even

not at maximum suction. This was verified with a microphone mounted in a Pitot tube.

It indicated that the boundary layer was still turbulent just af ter the suction. V/hen the

suction No. :3 is turned off, the step between the suction tube and cylinder works as a

large tripping device. Hence, also a turbulent boundary layer is detected when no suction

is used.

2.1.3

Afterbody

The main configuration of this set-up was the afterbody. mounted as an pnding of the

cylindric inner wall of the tunnel. A detailed sketch of the afterbody is shown in Figure 2.:3

where a.Jso the used coordinate system is sho\\·n. Thc .r-position \\"ber(' 111(" af ter body

begins is referred to as x j Ri

=

0, and positive values are in the dO\\"l1stream direction. 8

is the axis fitted to the surface of the afterbody. also \rith .~

=

0 at .1"

=

O. y represents

the axis normal to the surface, while Y is alwa.ys normal to t.hc .r-axis. i.C'. thc radial

distance from the surface. The total length, ~x. of the afterbody is 61:3 mm. Tbc

first part is designed as an ellipsoid. with a maximum angle of 2.5°. The rel11aining part

of the afterbody is a straight cone, with a top angle of .50°. This limit \ras chosen to

avoid separation on the afterbody. Similar measurements (ReR, ~ 0.96 . ](l") performed

by Merz et al. (198.5) indicated that separation most probably occurs al a \rall angle

between 27 anc! 29°. The same wall angle limit was also found on a t\\·o dimensional

curved backstep (Nice et al. (1966)). The outer form of the afterbody is gil"en by

r(:r)

= {

i}16 -

(-kr

Ri 0 4-')- - 0 466'3 . 1-·) . .

(x

Ni

-

.

'3 -')-)

.. )_

.

)

The derivative, drjdx. for the ellipsoid part is

dr x

=

if 0.0::;

-k ::;

:3 .. S2.S if :3 . .52.5 <

-k ::;

-l.Y3lJ dx 4J16-.r2 (2.1 ) (2·:n

\\"bile for the straight cone the derivative is. of course. constant with a vahw of drjdJ.'

=

-0.466:3. The angle. L. between the y and Y is then found as L

=

tan-I(dr/d.r).

The bounclary layer measurements were performecl at 8 stations. starting at .r/ Hi

=

-0.1.5. This station represents the incoming flow on the aft.erbody. The t1'<l\"(:,rs('s weIT

performed at the circumferential angle

a

= 1.50°. (dJ

=

0° is perpendicula1' t.o tlw Ol!t('r

wall of the building). This angle was chosen as it is in tbe section that had 111e smallest

persistent oscillations. The distance bet ween the stations decrcases from 1.1 'jRi at thc

start to 0.2.SR; at the end of the ellipsoicl part of the afterbocly.

All of the boundary layer measurements were done perpenclicular to t he afterbody

sm·face. In addition to these traverses, several traverses we re a.\so performed

.r/Ri .r [mm] r [mm] r(x)/R; L [0] oS [mm] -0.15 i -20.0 1:35.00 1.000 0.00 -20.0 1.00 I 1:3.5.0 1:30.i1 0.968 :3.69 1:3.5.09 1..50 20:3..5 125.1.5 0.92i _I ·S.i8 202.82 2.00 270.0 116.91 0.866 i _{8.21 270.8:3} 2.50 :3:3i .5 10,5 . .58 0.i81 , _i 11.:32 :3:39.:32 . 2.75 :3ïl.:3 98.00 I 0.726 i 1:3.:32 :3ï:3.8.S , i :3.00 : 405.0 89.29 0.661 i 1.5.8:3 408.i:3 :3.25 4:38,8 78.70 0.58:3 i 19.21 I 4·14.10

Table 2,1: Afterbody details.

Throughout this report y is the local distance from the wall. and normal to t he curved \\"all of the afterbody. The uppercase letter Y represents the Cartesian 1j-coordillale. Hence y

and Y are the same in the wake behind the afterbocly anel on tbc straighl C\·lineler.

2.2

Hot-wire anemometry

An ordinary hot-wire technique was used for measuring the mean and fiuctllat.ing values of the turbulent flow field. For further information about the soft\\"a.re, calibration procedures et cetera, see Wubben (1991). General information about the hot-wire technique can be found in e.g. Bradshaw (1971),

2.2.1

Single-wire

probe

The single-wire probe used in the work is described by \Vubben (1991). The active wire was of tungsten and was operated by a DISA ·5·5:\110 CTA standard bridge. \\'ith an overheat ratio of 0.8. The voltage signal was split into a mean anel a fillctllating part. The mean part was amplified to fill most of the voltage range on the A/D carcl. The second signal was band-pass filtered to remove the average value of the signal anel further amplified to increase the resolution of the signa!. The lower limit (high pass filter) was set to 0.1 Hz (6 dB/Octave). while the upper limit (low-pass filter) was set to 10 kHz (-24 dB/Octave). j'-;early all the single-wire measurements were performeel \vith the Krohn

-Hite (LRTH 19.1i.S) filter. All the fluctuating values from these measurements are reduceel by 6

o/c.

unless something else is reported (see Section 2.4).

For all the single-wire measurements the calibration was done just prior to the mea

-surements at the stations presented. If the elrift during a tra\'erse was large!' than 1.0 9f

the data set was rejectecl. Since the calibration always \\'as performed jusl prior 10 tlw measurements. no correct ion for the aging of the \\'ire was used for this type of probe.

The single-wire probe had a nominalor effective wire \\'ith a eliameter. d".. of .j.O ILm anel a length to diameter ratio, lu./d«, of approximately 200. Both ends of the \\'ire were covered by copper (see also Figure .5.10). Although the ratio is \\·ithin thE' recommendation of Ligrani anel Bradshaw (19Si), also the length of t he \\'ire in \'iscous units lt

=

uTl,u/ IJ is recommended to be lower than 20. They indicate thaI. length effects are negligible up to velocity correlations of fourth oreler if these recomnlPnclations are

Type du: Lu' Lwldw 6." Apex angle [0]

I

Manufact.ured Measured I I A 5.0 0.5 100 0.·5 90 ::::: 9:3 I B 2.·5 0,.5 200 0 .. 5 90 ::::: 8i _I

Table 2.2: X-wire dimensions (in mmo exeept for du', ,,"hich is in 1/171)

fulfilled. The same limits were found by Blaekwelder and Haritonidis (l08:3) for correct

detections of bursting frequeneies. For the present measurements the non-dimensional length l~ was approximately 5:3 at x

I

Ri = 1.0, while at :rl R; = :3.2.j the length l~ is redueed to approximately :37.

2.2.2

Cross-wire probes

In addition to the DISA 5.5MI0 CTA unit, whieh was used for operating t he single-wire

probe. a DISA .5.5DOI CTA bridge was used for the seeond circuit. Othen\"ise. the way of eollecting mean and fluctuating values was the same. The cross or X-wire probes used in

the work we re somewhat smaller than those described by Wubben ( 1(91). The dimensions

of the two different types are listed in Table 2.2.

The overall dimension of the probe was, however. the same. The diameter of the

hot-wire, dw , was ·5.0 ILm for type A and 2,5 ILm for type B. Henee, the ratio L",jd" was

approximately 100 and 200 for the two types, respectively.

The calibration of the X-wire probes was first performed in the S-tunne[l in the velocity

range :3 - 24 mis. The veloeity was determined by measuring the preSSl1re drop, dP, over the last contraetion of the S-tunnel. Arelation to correct this for the actual dynamic pressure at the probe position (1.jO mm downstream of the outlet) was given as

dPcorr = 0.482

+

1.00,5.589(dP)

+

0.000019(dP)2 (2.:3)

This was checked against a Pitot tube at the required distance and found to give less than

0.:3 % error for the used range. The tunnel had to be heated up for a couple of hours

before it could be used, as the motor was changing speed (rlmin) up and down when it

was cold. \Vhen the motor was heated up, this was redueed. but still some \·ariation in the veloeity was notieed during a ealibration run.

The angle sensitivity was ealibrated in the range _:3.)0 to +:3.)0 in steps of ,jO for a.ll

of the X-wire probes. A look-up table was generated (typieally on a 40x-l0 grid, \\"ith third order polynomiaJ interpolation); in this way for every veloeity the \\"hole range of yaw angles was ea.librated. Normally 9 veloeities were used. giving 1:3.5 points.

The first indiea.tion that something was wrong \\"ith the A-type wire ,,"as seen when the effeetive wire angles were extraeted from the calibration data. For every \·cloeity the angle sensitivity was assumed to follow the eosine formula

u

= Al cos(;,:;

+

;':;u') (2.4 )

where :.p is the angle bet ween the probe and the flow direction and l:.ptv 1 is the effective angle of the wire. Al is only a constant. The angle was found by a best-fit procedure

(e.g. Krogstad (1988)) in the range -1-5° < :.p < l.jO. As seen from Figures 2.6 and 2.8 the effective angle decreased quite a lot for the lowest velocities for Type A.

A traverse was performed at

xl

Ri

=

2.0 in the Y-direction to check if velocities and

stresses were in agreement with the single-wire measurements. \Vhile the mean velocity gave comparable results (within ±2%) with the single-wire probe, the tt2 measured in

the (x. Y) plane did not reach the level measured with the single-wire probe as shown in Figure 2.9. These reductions are comparable with the reeluctions founel by Ligrani anel

Bradshaw (1987) when they tested the influence of the ratio. l,u/d"., on the longitudinal

velocity component. tt2, in a zero pressure gradient bounelary layer. FOl" the present

measurements, the stress reached only 70-80

%

of the single-wire results. The much higher 112 measured in the (x,.:) plane, which is in this case by chance equaJ to the single-wire results, is discussed later in this section.

A new probe was fabricated in VI'hich the ratio. l",/d" .. was increased to approximately 200, by reducing the '.'lire diameter, duo, to 2 .. j pm. while the other dimensioll remained unchanged. The junction between the copper coating anc! the effective wire \\'a, also triec!

to be made a little smoother, by rounding the ends of the coating. First severa\ calibrations

were performed in the S-tunnel in the same manner as for the first X-wir<~ probe. As seen from Figures 2.7 and 2.8 the angle response turns out to be Inuch better when

{

,,,I

d

,,.

was increased. Although a. sJight drop may be seen fOf the lowest \'elocity. no dist inct drop in the 1 'Pw I, as was found for the .5.0 pm '.'lire was obsen·ed. This indicates that the end effects are considerably reduced for this X-wire probe. It. also indicates the outer limits of the correct measured instantaneous velocity angle. Due to the flattening of the curve

for flow angles,

1

'Pw

I,

larger than 20°, this probe with apex angle of approximately 900

should be used with care when larger angles are detected during measurement.s.

It was found to be very difficult to transport the X-wire probe from the S-tunnel to

the vertical tunnel where the measurements were carried out. Since the power hac! to

be switched off during transport. the \vire was much more fragile than if it remained constantly heated. Also other settings may have changed during the transport of the equipment. A new, simple calibration device was made for yaw calibration of thc probe

in the free-stream flow of the vertical wind tunnel. Hence. the calibration coule! be done in situ and the wires could remain heated for the entire measurements periocl.

Severa,] tra.verses were performed with the new probe at X/Ri = 2.0 in the Y-direction to check if there was some improvement. The probe was aligned both in the (:/'. Y) plane and in the (:r;,.:) plane. Again the difference in mean velocity. C. was small compared to the single-wire value. For the Reynolds normal stress. v2 . the difference lwt ween the

single-wire and the new probe configuration (measurements performed in thc (.r. Y) plane) was reduced to \\'ithin the experimental scatter. Figure 2.10 shows the results from three

repeated tra.verses, with the probe pitched at different a.ngles \\'ith the .1'-axis (i.e. :3.·5.

l.-5 ane! _:3.-50

). This shows the importance of making the probes within eert ain design

limits.

As seen from both Figures 2.9 and 2.10 a relative large deviation is fount! I)etween the

((2 measured in the (x. Y) plane and in the (x . .::) plane. i.e.

U;1'

and 11;.= when the probe

angle, ..p. is :3 .. 50

. For both type A and B the quantity u~., was approximatcly 20 o/r higher

than

U;y.

The reason for this was that the probe was just turned arouncl ilS own axis and was not aligned in the flow direction. In the (:1', Y) plane, this does not matter so

much as long as the "out-of-plane" velocity. i.e. \lF in this plane. is very small. Due to

the design of the X-wire probes and the relative large probe body. it has 1.0 be pitched with a positive angle relative to the surface when measuring close to the wal!. Hence, the

"out-of-plane" velocity, i.e. V in this plane. is no longer negligible. Some t.ests 1.0 confirm

this theory were performed by pitching the probe more and more in the flow direction. In

addition to the measurements with the probe pitched with an angle of :3 .. ,)°. the angle was

changed to - 1..50 and -:3.5°. The results are shown in Figure 2.10. It clearly confirms

that the "out-of-plane" velocity influences the results. By pitching the X-win" probe more and more in the flow direction, the difference bet ween

U;y

and

u;'.o

decreased considerably. On the other hand, the quantity w2 _{increased. This indicates that the 110rmal stress is}

transported from w2 _{to u~= when a large V velocity is present. Since the streamlines}

follow the a.fterbody close to the surface. while they are more or less in th(' .r-direction far [rom the surface, the "out-of-plane" velocity changes through the layer. This might

really be a problem for this flow. as the flow angle is constantly changing t.hrough the

layer. due 1.0 the st rong cun·ature.

2.2.3 General

For the long-time averages of the mean values. Reynolds stress<?s anel higher order terms

(third and fourth order) the sampling time was 60 seconcls.

The sampling frequency was set to .l~

=

166 Hz, due 1.0 the capacity of the PC used.

Although a much larger frequenc}' could be used. the time-consuming st.oring procedure

of the raw-c1ata on the hard-disk Iimited the frequency. The cut-off frequencies of the

low-pass pass filter \\"ere chosen after inspecting the Power Spectrum Density of the time

series from a. single hot-wire probe. Figures 2.4 and 2 . .') show typical spectra of time s<?ries sampled for 16.2 seconds at x / R; = 2.0 in the boundary layer on the afterbocly a.nd in the

shear layer from the jet. Bath measurements were taken at ['Tej

=

20 mis. These spectra

indicate that a filter cut-off frequency

Ij

of 10 kHz is suitable for these measlIrements. By

keeping in mind thaI. both v2 _{and u;2 probably have a larger contribution from the higher}

frequencies than u2 _{(see e.g. Krogstad et al. (199:3), Ska.re (1994)), this cut-of!' frequency}

was llsed for the present measurements. The figures also indicate differences Iwtween the

\.\\'0 flow types. \Vhile the turbulence in the boundary layer has a large cant rihution from

i j wide range of frequencies. a much more distinct peak is found for tbe shear la.yer at a

freqllency of approximately 90 Hz.

2.3

General Equipment

?denzor Quarts manometers ,,"ere used for a.ll tbe pressure readings presented in t.his re-port. \ormally the accura.cy of these manometers \\'as witbin ±O.,') Pa for thc range used. HO\H'\·er. ma.n." of the measurements were performed in a range \\'here t he large

la\\"-frequency \\'as present. BecausE' na automatic averaging was available. the mean \'alue

\\'as fOlll1d by manua] axeraging. \Vhen the fluctuation \\'a~ very strong t.his might be

il little dubious and the accuracy of the measurements is much less than the specified

accuracy of the Menzor. This fluctllation was also present in the pressure readings with

t.he Pitot or Preston tubes within the boundary layer. e.g. as used to make t.he Cj

and maximum values are reported, reftecting the difficulties of not applying an aut.omatic averaging. The temperature was re ad by a Menzor Platinum Thermometer.

2.4 Filter problems

During the experiments it was found that some of the filters did not behave as they we re

expected to do. Unfortunately this was discovered relatively late in the measurement

period. Problems with getting the same longitudinal Reynolds stress, 11 2 • \\·ith the single and X-wire measurements, was traeed back to the different filters used. During the single-wire traverses the Krohn-Hite filter (LRTH 19.17-5) was used as a band-pass filter. Since only the Dual Channel filter was available when the X-wire measurements started. the filter was exchanged. In comparison with two different I\:rohn-Hite filters. an advantage

of the Dual Channel filter is that the characteristics of the filters on th(-' two channels

are exactly equal. Problems with an always smaller u2 _{measured ,,·ith the X-\yirc probe}

compared to the single-wire probe, was first believed to be mainly duf' 10 the probe

configuration of the X-wire probe. However. when some single-wire traverses at .1) Ri

=

-0.15 and 1.0 we re repeated with only changing the filters in the circuit. the same u2 _level

as measured with the X-wire probe was found. The mean anel variance of (11(:' singlc-wire

rcsults at x j Ri

=

1.0 are shown in Figures 2.11 and 2.12 for \rhich thr (wo different

filter types were used. Since no filtering is used on the mean signal. the iigures are only showed to support that the mean velocity U compares weIl. The two measurements with the Krohn-Hite filter we re carried out with an interval of about -l months. Therefore it is assumed that the characteristic of this filter has remained constant through thc entire measuring period.

Some time series were collected at y+ ~ 20. where the turbulent intensity \ras near

maximum, and the spectra were calculated. Typical spectra for the Krohn-Hite and the

Dual Channel filter are shown in Figure 2.1:3. It may be seen that the spectra l"or the two filters al most coincide, except for the lowest frequency.

A test was performed with a signal generator. where four channels were simultaneously sampled. In accordance to the Krohn-Hite filter (LRTH 19.17,)) and the Dual Channel filter, also a second Krohn-Hite filter of the same model, marked with VTH 19.09:3, was checked. All filters were used as band-pass filters in the range 0.1 - 10000 Hz. The Krohn-Hite filters had a -24 dBjOctave slope for the low-pass filter. which is half the slope of the Dual filter. The high-pass slope was 6 dBjOctave (the same for all the filters). As seen

in Figure 2.14. the only filter which behaves normally is the Dual Channel filter. This filter follows the raw signalover the prescribed range, except for the lowest frequencies.

In the lower range more energy is filtered out compared to what was expectE'd. It may

be seen that the high-pass filter limit is approximately 4 Hz, and not 0.1 Hz. For the

Krohn-Hite filters the problem was completely different. These active filters genera.ted extra energy compareel to the raw signal. However this extra energy was fOlll1el to be

approximately constant for all frequencies, that is +6 o/c anel +9

%

for the ]\:rohn-Hite

filters, respecti,·ely. It may then be argued that the correct r.m.s. can be found by reducing the measurements with the same amount. All of the measured r.m.s. va.lues from the single-wire probe presented in this report. when using the Krohn-Hite filter in

the circuit, are reduced with 6.0

%

unless otherwise indicated. This is based on the

findings that the repeated single-wire measurements did not show any elifference in the

u2 _{level and that the "extra" r.m.s. is approximately constant for all frequency used.}

Figure 2.1.5 shows a comparison between the corrected u2 _{(with Krohn-Hite filter (LRTH}

19.17-5)) and the measured u2 _{(with the Dual Channel filter). Indeed the profiles agree}

I

I

I

2

0~+

1

t

t

~

'".""""~'''. t

1111111111111111111111 / \ 111111111111111111111111111111 1

1 4 - - - -

8.0 m ---~~

Figure 2.1: The vertical wind tunnel (Professor E. Dobbinga wind tunnel). 1: Measuring room, 2: Fan room, 3: Fan, 4: Settling chamber, 5,6 and 7: Suction devices. named No. 1,2 and 3, respectively, 8: Sound damper.

11

t

2Ri= 270

t

0 0 \Cl I 2Rn= 600 I

I

CL

I

-

I

-::::;.:::

::

I

I

I

x y

r-==---=r--+

y rex) 00 ~7---~---~~----~ R· _I s

Figure 2.3: A close-up of the afterbody mounted at the end of the cylindric forebody.

I

I 1,IIJllt.,. 'WiiUiiil! I I. I I

13

0.2 0.1 /

o

~--~~~~~~--~~~~~~--~--~~~~~~ 10 100 1000 104

f

(Hz)

Figure 2.4: Typical spectra for the streamwise velocity fluctuation u in the boundary layer at xlR;

=

2.0 and for ylb

=

0.01 (--),0.1 (- - -) and 0.5 ( - - - ) measured at Ure!

=

20.0

mis.

0.6 0.3 f-... , ... , ... , ... . oL-_~~~~~-~~~~~L-_~~~=--L_~ 10 100 1000 104

f

(Hz)

Figure 2.5: Typical spectra for the streamwise velocity fluctuation u in the shear layer of the jet outer edge at x I R.; = 2.0 and for YI R; = 1.97 ( ) , 2.12 ( ) and 2.22 ( -- -- ) measured at Ure!

=

20.0

mis.

1.6 Wire 1 ... . ...

~ire

2 1.4 1.2

....

:J=~~~

I

.. ·· ••

••

•••••

•

•

·

••

•

•

.

···< i~~ ---_._-_ ... ; ... . 0.8 1-... , .. '\ 0.6 0.4 0.2~~~~~~--~~--~~--~-L--~-L~--~~~ -40 -30 -20 -10 10 20 30 40

Figure 2.6: Examples of angle response for the X-wire probe (5.0 J.lm, lwldw ::::: 100). Arrows show increasing velocity in the range 3 - 24 mis.

Wire 1 Wire2 1.6 1.4 .... ~. ... . ... -... ~... .. _ ... ; .. 1.2 .. .. .. ; _. .. . _. -_.~ ... .. -. --. -... -. -.. -.; ... 0.8 0.6

:

,

:

'.

:.

,

.... 0.4 ...• 0.2 ~~~L-~~ __ ~~ __ ~~ __ ~-L __ ~-L~ __ ~~~ -40 -30 -20 -10 0 10 20 30 40 ({J [0]

Figure 2.7: Examples of angle response for the X-wire probe (2.5 J.lm, lwl dw ::::: 200).

Arrows show increasing velocity in the range 3 - 24 mis.

55 r---~--r---~--r---~--r---~--r---~--.---~--, 50

₁

...

..

...

.

...

·! ... : .: : ..

J

=

...

..

=~~~~=4~==~~

... .

tPw

[0] 45 ... . 40 35 ~--~~~~--~--~--~--~--~--~--~--~~

°

5 10 15 20 25 30 U [mis]

Figure 2.8: Effective wire angles found from the eosine law for different X-wire ealibrations. X-wire, 5.0 J-Lm, lw/dw ~ 100 (line with open symbols) and X-wire, 2.5 J-Lm, luJdw ~ 200

(line with filled symbols) .

0.8 ...

•

...

•

•

g .. g~. ,

..

,

• 0 0 0

q.~ ~~

•

...

..

.

...

..

.

...

....

...

..

.

.

.

..

..

qo.o

...

~~~

•

....

.

.

..

.

..

.

...

.

...

~

6 6

~

OOOoo~~

··········0·· ......... ~ ... f::. ··l5.··l::,············.···.···· ..... · ..... !.O.~··· ... -._-. o Q 0 0 oe

~ ~ê ~ ê® ~~

.

ê~~Z~

o

L---~--~~~~~--~--~~~~~----~~~~~-e'~-0.6 0.4 0.2 0.001 0.01 0.1 Y/Rj

Figure 2.9: Normal Reynolds stresses u; for single-wire (5.0 J-Lm, lw/dw ~ 200) and X-wire

probe (5.0 J-Lm, lw/dw ~ 100) measured at X/Ri

= 2.0

in both the (x, Y) and (x, z) plane. Single wire: u2 _{(e). X-wire:}

0.8 ...•

•

0.4 0.2 0.01 0.1 YIR_i

Figure 2.10: Normal Reynolds stresses

ur

for single-wire (.5.0 J1.m, /wldu; ~ 200) and X-wire probe (2.5 J1.m, /,,;jdw ~ 200) measured at xl R;

=

2.0 in both the (x, Y) and (x, z) plane. Single wire: u2 _{(e). X-wire:}

U 2x Y (0), u2xz (0), w2 (6) and 1)2 (0). Arrows show

trend for decreasing probe angle.

0.8 0.6 0.4 0.2 0'--~_~~~,",-_ _ ~~~...J..._~_~~...! _ _ ~_~...J4 1 JO 1000 10

Figure 2.11: Mean velocity profiles measured with a single·wire probe at xl Ri

=

l.O. Krohn-Hite (LRTH 19.17.'5) filter: (0) and (e) with a time space of 4 months, Dual Channel filter: (0).

17

0.8 ...

-O-.~~~~

... f ... : ... f ... . 1000 0 • •

...

...

.

....

.

....

.

.

.

...

~

...

.

...

~

...

!? ...

..

..

.

...

...

...

..

.

...

.

...

...

...

.

_

.

JD;~~t..-J

o

'

oo~~

•

...

--.

~

..

...

...

--.

.

. .

..

.

..

.

...

...

...

, ...

q

.

~

.

.

.

..

.

.

.

.L. ...

..

--

...

.

.

.

.

--o

0.4 0.2 --... , ... _-_ ... ;. ... .

o

L-~~~~~--~~~~~~~~~~--~as~~~ 1 10 100 1000 104

Figure 2.12: Uncorrected Normal Reynolds stress

u

2 _{profiles measured at}

xl

_R;

=

_{l.O. For} symbols see Figure 2.11.

10-2 10-3 10.2 10-4 Eii(J) 10-5 10-6 -.--- _10-3 0.1 10 -7 1 10 100 1000

f

[Hz]

Figure 2.13: Spectra at

xl

Ri and y+ ~ 20 where the signal were band-pass filtered in the region 0.1-10000 Hz with the Krohn-Hite filter (- - -) and a Dual Channel filter (

IIIAII 0.8 E'/E_s 0.6 0.4 0.2 0 0.1 10 100 1000

f

[Hz]

Figure 2.14: Sinus signal at different frequencies from a signal generator. Raw signa! (- -). Band-pass filtered with Krohn-Hite (LRTH 19.1i')) (0), Krohn-Hite (VTH

19.093) (6) and Dual Channel filter (0).

1.2 '---"'--"'-~~""""""'--'--'-""""'-'-'-~'-I, -~~~~.,.,!-~~~~..., . . . .: .. ~.HH. .H .. ...

o~o\

0.8 u2 2 - 2 xlO 0.6

U

e 0.2 HH. ... H .. ~..H.H. H~ •

o

~~

.

· ···_··~'i~ ···

...

....

...

..

...

..

.

...

.

@. ~ ... .I!~[ oL-~~~~L-~~~~L-~~~~L-~~~~ I 10 100 1000 104 y+

Figure 2.15: Corrected norm al Reynolds stress

u

2 _{profiles at}

x

l

_Ri

=

_{l.O. For symbols see} Figure 2.11.

19

Chapter 3

Flow quality

This chapter gives details of the circumferential flow quality in the working section of the vertical wind tunnel. The problems with the low-frequency disturbance in the boundary layer are also reported.

3.1

Working section symmetry

To study the flow quality of the working section, both the flow quality in the free stream and within the boundary layer which developed on the inner wall we re investigated.

3.1.1

Free stream symmetry

The original configuration was used to study the uniformity in the free stream part of the flow (see Figure 3.1 for details). To measure the mean velocity around the forebody a total-static Pitot tube was mounted on the sidewall of the wind tunnel. The upper tube section could be rotated around its own axis. The measurements were taken at

xlRi

=

-5.0 and at two different locations in the flow. One in the middle of the tunnel

(y = y = 0.5~WT) and one doser to the cylindric forebody at y = Y = O.l.6. wT , where

~WT is the radial distance bet ween the outer and inner part of the working section. The suction tube (No. 3) can be lifted up and down, positioning the suction at different heights on the cylivdrical inner wal!. For the results shown in this subsection, suction No. :3 is at its lowest position at x

I

Ri :::::: -15.6. First the measurements were carried out with all the suction devises active. Figures 3.2 to 3.5 show the mean velocity profiles with the free stream velocity varying from 10 to 40

mis.

For the two lowest velocities, i.e. Ue

=

10

and 20

mis,

the symmetry was excellent with r.m.s. values lower than 0.3 %. For the two highest velocities, i.e. Ue

=

30 and 40

mis

the r.m.s. increased marginally to 0.9 %. Ko

low-frequency fluctuations were found at y

=

0.5.0.wT, while some indications of the low-frequency fluctuations occurred at y

=

O.l.0.wT. Closest to the walL the measurements are near to the boundary layer edge, and therefore they may reflect the situation within the boundary layer.

Since the main suction device reduced the boundary layer thickness, a run was per-formed with suction No. 3 turned off. The traverses done with Ue

=

20

mis

were repeated

and are shown in Figure 3.6. No significant change in the middle position at y = 0.5.6. wT was found (r.m.s. below 0.1 %), while the track close to the forebody at y

=

O.l.6.wT shows a much larger deviation from the average velocity (r.m.s. increased to 2.3 %). The

- - -- - - -- -- - - - -- - - -- - -- - -- - - - -- - -- -- --- --- --

-saw-tooth form distribution had two maxima at <i> ~ 60 and 310°, while the minimum was located at <i> ~ 210°. In addition to these two traverses, the Pitot tube was positioned

close to the wall, acting as a Preston tube. The skin friction, C" was found from the calibration carried out by Patel (1965). Since no statie pressure taps we re available at this position it was assumed that the pressure difference bet ween the free stream and at the wal! was negligible. As seen from Figure 3.7, the averaged Cf value reflects the measurement at O.lb.WT. The r.m.s. had now increased to 6

%.

3.1.2 Boundary layer symmetry

The deviation in the average mean values and in particular the low-frequency disturbance had not been reported before for this vertical wind tunnel. Since the skin friction coef-ficient Cf tends to be the best value to reflect the circumferential quality of the layer. Preston tube measurements were used for this purpose. In addition, a single-wire probe was used to measure the velocity distribution through the boundary layer.

When the main suction was active, the fiuctuations in the dynamic pressure measured with the Preston tube at x

I

R; = -5.0 were real!y large. For the highest velocity, i.e.

Ue

=

40

mis

,

the dynamic pressure varied with

±

11 % while the variation increased to

over ± 50 % for the lowest free stream velocity, i.e. Ue

=

10

mis

.

This was the first

indication that the main suction (No. 3) did not work as it was intended to do.

To ensure a turbulent boundary layer to develop from the same x

I

Ri position, a tripping band (zig-zag band: 60° top angle and h

=

0 .. 55 mm) was put around the forebody at

xl

R;

=

-14.07. This is just af ter the main suction (No. 3). However, this did not change anything. To investigate which flow conditions were present before and af ter the suction ring, a Pitot tube with a built-in microphone was used. The sound from the microphone indicated that the boundary layer was laminar before it reached the suction ring . .lust af ter the ring (i.e. just upstream of the tripping band) the boundary layer had become turbulent. Of course this was also the case when the main suction was off, as then the ring acted as a 90° backward facing step. However, the change to a turbulent boundary layer was also found when the maximum suction was applied, showing that even then the suction ring more or less tripped the layer. Anyhow, the suction device did not manage to remove the boundary layer as it was expected to do.

To confirm whether the primary suction (1-'0. 3) initiated some fluctuations, the static pressure was measured in the outlet of the suction tube. On the inner wal], several pressure taps were available. This gave us the opportunity to measure the pressure within the suction tube. At the pressure tap located within the inlet of the suction tube, the pressure was approximately -20.50 Pa. Moreover, no fluctuations in the signal were observed, confirming that the compressor run steadily. It was also tried to reduce the suction, which was practically done by opening some inspection holes in the circuit. However, this did not give any significant improvement of the flow quality.

Some boundary layer traverses with a single-wire probe were performed at x

I

Ri

=

-.5.0,

and they confirmed the findings from the earlier measurements. A typical boundary layer traverse to il!ustrate the situation, is shown in Figure 3.8, where the measurements are averaged over 100 seconds for each position. As seen from the figure, the mean velocities have a large scatter, making the distribution unusable for determining boundary layer parameters such as e.g. 8 and Cf. This traverse was carried out at cf>

=

0°. A test on the minimum required sampling time to get a stabie average (± 1 %) indicated a sampling

time of about 5-6 minutes within the boundary layer.

The main suction tube (No. 3) was lifted to its highest level, which enabled to

deter-mine the flow condition before the suction point more closely. Using a Preston tube at an x-position 200 mm upstream of the in let of No. 3, the dynamic pressure was found to vary

with

±

50

%.

The Pitot tube containing the microphone indicated that the boundary layer was still laminar at this position. This means that the low-frequency fluctuations

are generated in the settling chamber.

Several combinations of the three suction devices were tried, but none of the

combina-tions was able to remove the fluctuations. It was therefore decided to make the boundary layer turbulent before it reached the main suction ring. This could easily be achieved

by mounting a tripping band (60°, h = 0.75 mm) on the cylinder surface just af ter the

junction between the curved inner wall and the cylinder. In this way the low-frequency

fluctuations were reduced considerably. It thus seems that high-frequency turhulence more or less dampens the low-frequency waves. The circumferential Cf was then measured 200

mm upstream of the out let of the suction tube (Figure :3.9) and 470 mm downstream of

it (Figure 3.10). As seen in the figures the symmetry is only acceptable in the region 90° < Ó < 270°. Again two maxima are found at 0

=

60 and :300°. the same positions as shown in Figure :3.7.

To check the initial condition of the flow symmetry before the flow reaches the af

ter-body, two traverses were performed at

xl

Ri

=

-0.50. One without and one with the main suction turned on, shown in Figure :3.11 and 3.12, respectively. The flow symmetry was

far from a perfect start condition for the boundary and wake investigations. However, the

best flow quality is found in the region 90° <

9

< 270° and in this region the symmetry

is reasonably good.

3.2

Local two-dimensionality

From the skin friction measurements the region close to tjJ

=

150° seems to be the best for detailed measurements. Traverses with a single-wire probe we re performed at 0

=

120, 150

and 180° in the (x, Y) plane to check the local two-dimensionality. As seen in Figures :3.1:3 and 3.14 the local symmetry around rP

=

150° is reasonably good. Within the boundary layer only small differences in the mean velocity distribution were found, with a little

higher velocity distribution at 180° than for q:,

=

120 and 1.50°. Also the stress distributions were in good agreement, with differences within the uncertainty of the measurements. In

addition to this, the shear layer at the outer edge of the jet was seen to have local

two-dimensionality.

3.3

Final condition

Although the circumferential symmetry in the wind tunnel is not optimaL and there still

are some low-frequency fluctuations present. it was decided to leave it as it was and to continue the boundary layer and the shear layer measurements at different

x

l

Ri positions. Since the flow quality was best in the reg ion from 120° up to 240°, this region was chosen

for the measurements. In particular detailed measurements were taken at 1·50°.

All mean and turbulence measurements which are presented in the next chapters in

this report were performed with the following settings:

24

1. The measurements are performed with UTe!

=

20 mis. The reference velo city is

defined as the free stream velocity at x

I

Ri

=

O.O.

2. The measurements are performed at ~

=

1500

3. All suction devices, i.e. No. 1,2 and 3, are active.

4. Device No.3 is working with maximum suction .

. ). Suction device No. 3 is lifted to its highest position.

6. The boundary layer is tripped far upstream of suction device No. :3.

7. The two doors leading to the the sounds dampers are widely opened.

8. The main door leading to the fan room is closed.

9. Wooden plat es are used to prevent the downfall of the jet, which hits the roof, from

I

I

I

dl _ _ 11 '.alll' Ij _{" I} I

I

€

I

RIJ = 300

UJ

I

Figure :3.1: Working section of the tunnel, first stage.

J i dl ij I ,I

25

1.04 '--~-~----r-~-~-"T'" I, -~--,---,---~-~---, 1.02 U/Ue 1.00 0.98 ... ... . ... ~ . .. .. . .... -_ ... -... _.;.

•

...

0.

.

·· 0 ···0

•

o ·O

•

··

. 0 .

0

...

···i· ...•... , ... ... _ 0.96 L -_ _ _ _ _ L -_ _ _ _ _ L -_ _ _ _ _ ..I...-_~_~____l

o

90 180 270 360 ~ [0)

Figure :3.2: Symmetry check for velocity profiles with U,

=

10.0

m

i

s

at y

=

0.1 (0) and

at 0.5b.WT (e) for an interval of b.~

=

30°. Suction devices turned on.

1.04 ~-~-~---~-~--""'I, ---~---'I "----~----\.02 ...•.... U/Ue 1.00 ··0· 0 ···~ ···O ···O ···· · e ···e · e · ··ee···l~

o

0.98 0.96 L-_~~_----l. _ _ _ ~_--1.... _ _ _ ~_...L-_~_~----'

o

90 180 270 360 ~ [0)

Figure 3.3: Symmetry check for velocity profiles with U.

=

20.0

mis

at y

=

0.1 (0) and at 0.5~WT (e) for an interval of ~<P

=

30°. Suction devices turned on.

1.04r---~--~--r---~--~--r---~---.---, 1.02 UWe 1.00 ·· ···~ ·· ~~··· O · ~... . .•

•

...

~ ~H1D o o

o

0.98

...

.

... ···0·

0.96~~--~--~--~--~--~--~~--~--~--~--~

o

90 270 360

Figure 3.4: Symmetry check for velocity profiles with Ue = 30.0 mIs at y = 0.1 (0) and at 0.5.6.wT (.) for an interval of .6.dJ = 30°. Suction devices turned on.

1.04 .---....--~,---;I--~--~---T I, --~---,----Ir--~---, 1.02 ... ; ...•... -U/U_e 1.00

..

.

..

.

.

.

~ . ~

...

~

..

....

.

.

.

~

.

.•

...

. .

H..

.

.

.

...

.

....

~ .~~ o o

o

o 0.98 ...

-o

0.96 ~~--~----'---~--~---'----~~---'---~---~

o

90 180 270 360 tP [0]

Figure 3.5: Symmetry check for velocity profiles with Ue

=

40.0

m

i

s

at y =:: 0.1 (0) and

at 0.5.6.wT (.) for an interval of .6.~ =:: 30°. Suction devices turned on.

27

J.05r---~~---TI.--~--~--~-I--~--~--~--~--~~ 1.00---. ---.

----

-

---

e

---

.

---

.--

-

---.---.-

----

.

-

-

-.

•

...

-

..

•

.

-

-

.-

-'-l~

o

o

... VIVe 0.95 _o

_o

---_._._._ -o

o

D

o

o

o

0.90

-.----.-

..

-9.

o

0.85~--~--~--~--~--~--~--~--~--~--~----~

o

90 180 270 360 lP [Ol

Figure 3.6: Symmetry check for velocity profiles with Ue

=

30.0

mis at

y = 0.1 (0) and at 0.5b.wT (. ) for an interval of b.cP

=

30°. Suction devices turned off.

1.25~--~--~--r---~--~--~--~--~--~--~--~~ 1.15 1.05 Cf/Cf 0.951-- -·----·---0.85 0.75 ~ __ ~~ __ ---1_~_~ _ ___L._~_~ _ __L__~_~__l

o

90 180 270 360 qJ [Ol

Figure 3.7: Check for symmetry in the skin friction coefficient with Ue

=

20.0

mis

mea-sured at xlR;

=

-5.2 with a Preston tube (d

=

3.0 mm) for an interval of t:.<h

=

30°. Suction devices turned off.

16 f-... , .... .. ,-.(C. 12 U [mis] 8 4 5 10 15 20 25 y [mm]

Figure 3.8: Boundary layer traverse at x

I

Ri

=

-.5.2 with Ue

=

20 mis. Suction devices

turned on.

29

1.6 ,...--,...-,...-,---.,...--...---...---_,__-_.__-_---, 1.4 1.2 0.8 0.6 '---_~~ _ ____l_~_~ _ ___'__~_~ _ ___l...._~_~__1

o

90 270 360

Figure 3.9: Symmetry check for the skin friction at 200 mm before the suction ring with

Ue

=

20

mis.

Minimum value: (0) and maximum value (e) for an interval of .0.$

=

30°.

1.4

1.2

0.8

0.6 '---_~~_----l _ _ _ _ ~ _ ___'__~_~ _ ___l...._~_~__1

o

90 270 360

Figure :3.10: Symmetry check for the skin friction at 470 mm af ter the suction ring with

1-1.4 1.2 . 0.8 0.6 ~ __ ~~ __ ~ __ ~ __ ~ __ ~ __ ~ __ ~ __ - L _ _ ~ _ _ ~~

o

90 270 360

Figure 3.11: Symmetry check for the skin friction at

xl

R; = -0.·5 with lire! = 20

mis.

Minimum value: (0) and maximum value (.) for an interval of

l::!.

m

=

:30°. SlIction No. :3

switched off. 1.6 r--~-~-..---~-"'--"--"'--"'--"""--"""--"""---' 1.4 1-... " .. 1.2 0.8 90 270 360

Figure 3.12: Symmetry check for the skin friction at

xl

Ri

=

-0.5 with Ure!

=

20

m

is.

Minimum value: (0) and maximum value (.) for an interval of

l::!.4>

=

30°. SlIction ~o. :3

switched on.