Dynamic laser light scattering studies of turbulence in fluids and its drag reduction

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DYNAMIC LASER LIGHT SCATTERJNG STUDIES OF TURBULENCE IN FLUIDS

AND ITS DRAG REDUCTION

SURESH KUMAR P. BHAT

Department of Physics

Submitted

in fulfilment ofthe requirements for the Award ofthe Degree of

DOCTOR OF PHILOSOPHY

to the

INDIAN INSTITUTE OF TECHNOLOGY, DELHI

APRIL, 1998

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CERTIFICATE

This is to certify that the thesis entitled

‘のYNAMICLASER LICHT SCATTER-

[NC STUDIES OF TURB ULENCE IN FL UIDS A ND ITS DRA C REDUCTION"

Leing submitted by Mr. Suresh Kumar P' Bhat to the Indian Institute of

脆

cli- nology, Delhi for the award of the degree of DOCTOR. OF PHILOS 01

〕

1-ly is a record of bonafide research work carried out by him. He has worked under my guidance arid supervision and has fuiffihled the requirements for the submission of the thesis.

The results contained in this thesis have not been submitted in part or full to any othei

・

University or Institute for award of any degree or diploma

_・

(O 。ノと七ん遥

9

^！へ～し

s. CHOPRA

Professor, Department of Physics Jndian Institute of Technology New Delhi-i i 0016,India.

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ACKNOWLEDGEMENTS

It is indeed a great pleasure for me to express my sincere gratitude to my super- visor, Prof. S. Chopra for his guidance and competent advice durinlg my Ph

_・

D programme. I really appreciate him for his friendly approach towards his students that gives them confidence to discuss with him any problems related to 1

_・

esca!

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ch or personal life. I would like to extend my sincere thanks to him for all his help and support during my stay in lIT.

I also wish to express my sincere gratitude to Prof. Kehar Singh, Head, De- partment of Physics for his encouragement throughout my work. My sincere thanks also to Professors P.K.C. Pillai and S.C. Abbi, Department of Physics and Prof.

H.M. Chawla, Department of Chemistry for their timely help and advice.

I really enjoyed working with my senior colle

昭

ue in Quantum Electronics Lab, Dr. Anul Razdan who was a constant source of inspiration for me. I wish to thank hiin for all his help during my experiments. I thank my juniors Karan Pal who joined QEL in the later stages for giving me good company. I acknowledge the contributions of some of my seniors from QEL who have tried their best to help me.

A long period of more than six years in IlT has given me lot of friends with- out whom life in the campus would have been very dull. It will be a di

伍

cult task for inc to mention the names of each one of them, but I would like to acknowledge the contributions of some of my friends, Reji Thomas,Anul Govindan, Dr.

Sreemoolanathan, Dr. Zachariah C. Alex and Dr. Prabhakar Rao for giving me good company and pulling me through the highs and lows of my stay in IlT.

j would like to express my heartfelt thanks to Dr. Joby Joseph and iiis wife A.ncy Joby for their unselfs1l:untiring support throughout my tenure and especially

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(luring the time when I was not at the peak of my physical

石

t fl es s

・

No words of praise can express my thanks for the many sacrifices of my parents, cS[)ecially my mother, to whom I owe everyt}iing. I really thank them for being siipportive, caring and understa,nding throughout roy life.

Suresh Kumar P. Bhat

New Delhi April,i 998

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ABSTRACT

知

rbulence is a phenomena of great importance in many flelds of science and engineering both in fundamental understanding of its physics and in applications some of which are still unresolved. Thle high degree of diLusivity associated with a turbulent flow compared to molecular di

伽

siGn has found wide variety of applications ill various

五

elds. Sorne of the characteristics features of a turbulent flow IiI

く

^{e inter-}

mittency or non-Gaussian nature of the statistics of turbulence, presence of inactive regions and coherent structures in turbulent

丑

^ow

五

eld and turbulent drag reduction by polymer additives are very interesting from a fundamental point of view and still eludes satisfactory physical expla,nation because of their complex nature.

Photon Correlation Spectroscopy (PCS) is one of the most versatile recent tech- nique to study turbulence phenomena because of its noninvasive nature and high degree of spatial and temporal resolution. Together with Laser Doppler Velocimetry (LDV) it can completely characterize a turbulent flow. PCS is very sensitive to the relative velocity fluctuations V(R, t) in a turbulent flow and hence can provide information about the spatial structures in a turbulent flow field which are very important in understanding the dynamnics of a turbulent flow. In PCS, the laser light scattered from turbulent fluctuations is detected by means of a Pliotomultiplier tube (PMT) and analyzed in the time domain by using fast digital correlators to obtain the intensity autocorrelation function g(r) of the scattered light. g(r) is very sensitive to the eddies generated in a turbulent flow and hence provides information about the relative velocities V(R, t) between a pair of points s

叩

arateci by a distance R in the turbulent fluid, In this thesis, we report our studies of a grid generated turbulent flow using Photon Correlation Spectroscopy.

Cliapter I gives a brief history of laser light scattering followed by basic aspects of Dynamic Light Scattering (DLS). In cha,pter II, sorne of Liie ba.sic definitions and characteristics of turbulence phenomena aie presented along with somlle tlieoretical

Iv

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mnodels. A theoretical expression for the intensity autocorrelation function g

_（り

for ligh1t scattered from turbulent fluctuations is also presented in the same chapter.

The details of the experimental setup used for studying grid generated turbulence is (I i S CUS

肥

d in chapter III along with the working principle of a digital photon correlatoi

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ln chapter IV, we propose a theoretical model for relative fluctuations V(R, t) in a

・

tui

・

bulent flow and experimentally verify its validity under di

飛

rent flow conditions

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We experimentally extract the probability distribution function of relative velocity flt

_に

tuations P(V(R,t )) from the measurements of intensity autocorrelation fund

_・

ion g(T)using a

・

computer program. Our experimental results show that the probability distribution function P(V(R,t)) is Reynolds number dependent and changes the functional form with increased Reynolds number. P(V(R, t)) is found to be a product of Lorentzian and Gaussian function for intermediate Reynolds number in agreement with our theoretical predictions. With increased Reynolds number, P(V(R, t))tends to a Gaussian function due to the space-fllliug nature of turbulence. Our experimental results are also in support of the presence of 'inactive' regions and small scale coherent structures in a turbulent flow fleld which have become a subject of controversy for long time arid has not been satisfactorily explained so far.

Although turbulence finds

叩

plications in various diverse fields of science and engineering, it is regarded as a nuisance in applications where one is interested in increasing the flow rates like in transport of oils/gases through pipelines and in defence applications because of the 'drag' associated with turbulent fluctuations. It was found tllat certain high molecular weight polymers of linear structure like Polyethylene Oxide, Polyacrylamide etc. reduce the turbulent drag to a considerable extent. In chapter V, we present the results of our experiment in a grid gener

航

ed turbulent in thle presence of a drag reducing polymer, Polyacrylamide. From the measurements of t

・

lie intensity autocorrelation function g

（ァ）

,probability distribution P(V(R, t)) and the decay time r of g(r), it is seen that the turbulent fluctuations are suppressed by l>olyacrylainide solution when tliey are added in minute concentrations in a turbulent

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flow and the

う・

a

什

ed small sca'e eddies and coherent structures in the bulk of the flow.

Tule thesis concludes with summary and conclusion in chapter VI,

The work included in this thesis has resulted in the following publiGations;

Research Publications

i . s'K.P. Bliat, S. Rai, A.K. R.azdan and S. Chopra, 'Relati

り

e

り

elocity

戸

?_tC tuat ions ifl turlulence", Phys. Rev. E 50, 5127 (1994).

2. S.K.P. Bi

、硫，

A.K. Razdan and S. Chopra, "Study of turtidence by tase.

，・

ligh.t.

SCCLLtCriflg

、、，

Laser News, 6(1), 6(1995).

3. S. Chopra, S.K.P. Bhat and A.K. Razdan,t

負

study of acti

り

e and mnacti

り

e

，・留

joris in tuγ

る

ulence by Photon Correlation Spectroscopy

刀，

in Coherence and Quantun Optics VII, Eds. J. Eberly, L. Mandel&E

・

Wolf, Plenum Press, New York(1996),pp. 609-611.

4. S. Chopi

・

a, S.K.P. Bhat and A.K. Razdari,

‘翫り

estigation of tui

・

b nien i

戸

ow char- acieristics by Hornodyne correlation spectroscopy"in Photon Correlation 9 Scat- terii

》タ，

Opt. Soc. Am.,14 (1996) pp. 108-110.

5. S. Chopra and S.K.P. Bhat, "Study

可

drag

，・

eduction and intermittency in tur- bulerice by lase

，・

light scattering

刀，

in Quantum Optics 9 Laser Physics, Edited by S.Y. Zllu, (Springer

－恥

nlag, Singapore

。

1998) (In press).

6. S.K.P. l3hat and S. Chopia, "Characterization ofturbulence phenomena by Dy- rtamic laser light scattering-An overview", Communicated to Current Science (1.998),

7. S.K.P. H}iat an

（」

s.Chop i

・

a,

（シ

In Experim,,ental Study of m:;iei ' ,niltcncy iii ilL?

・－

i) uic,ice

、、，

COfl1111U11 ica,tcd to Experiments in Flui ds(i g y 8).

VI

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