Fluid mechanics of turbulent ows€¦ · Lecture 5 1/25. The Richardson cascade Kolmogorov’s 1941...

The Richardson cascadeKolmogorov’s 1941 theory

Spectral view

Fluid mechanics of turbulent flowsFluidmechanik turbulenter Stromungen

Markus Uhlmann

Institut fur HydromechanikKarlsruher Institut fur Technologie

www.ifh.kit.edu

WS 2010/2011

Lecture 5

1/25

http://www.ifh.kit.edu/213_216.php


Spectral view

LECTURE 5

The scales of turbulent motion

2/25


Spectral view

Questions to be answered in the present lecture

How are energy and anisotropy distributed among scales?

Which physical processes occur on each scale?

3/25


Spectral view

The energy cascade (Richardson 1922)

Conceptual image of energy & scales

I turbulence is composed of eddies of different sizes

I consider statistically stationary flow, very large Re ≡ LU/νI characteristic size: `, velocity: u(`), timescale: τ(`) ≡ `/u(`)

I largest eddies: ` = `0 = O(L),u0 ≡ u(`0) = O(urms) = O(U)

I eddies interact, transfer energy preferentially to smaller sizes

I for some size `� `0: Re` ≡ u(`) · `/ν = O(1)

→ dissipation by molecular viscosity becomes important

4/25


Spectral view

The energy cascade (2)

Consequences of the concept

I ‘top-down’ process

I rate of energy transfer fromlarge scales: u2

0/τ0 = u30/`0

→ dissipation scales as u30/`0

⇒ dissipation determined byenergy input! (Frisch “Turbulence”, 1995)

⇒ cascade process takes care of dissipating energy at the appropriate rate

5/25


Spectral view

Local isotropySimilarity hypothesesConsequences of the theory

Kolmogorov’s theory

Quantification of the cascade

I what is the size of the smallest scales?

I how do the scales u(`) and τ` vary along the cascade?

I how does the range of scales depend on the Reynolds number?


I provides scaling laws

I provides some measurable quantities

→ can be verified in high Reynolds number experiments

I formulated in form of hypotheses

6/25


Spectral view


Kolmogorov’s hypotheses

Hypothesis of small-scale isotropy

At high Reynolds numbers, the motion of small scales`� `0 is statistically isotropic.

I directional bias & information about flow geometry

→ lost along the cascade

⇒ small-scale statistics should be universal

7/25


Spectral view


Hypothesis 1: Small-scale isotropy

Loss of anisotropy due to repeated vortex stretching

Cartoon-like explanation

I vortex “stretching”term: (ω · ∇)u

I stretching in z→ gradients in x,y

I and so on . . .

⇒ isotropization afterrepeated steps (Bradshaw 1971)

8/25


Spectral view


Hypothesis 2: Similarity of small scales

First similarity hypothesis

At high Reynolds numbers, the statistics of the small-scale motion(` < ÈI) have a universal form determined by ν and ε.

I Kolmogorov scales (from dimensional grounds):

η ≡ (ν3/ε)1/4

, τη ≡ (ν/ε)1/2 , uη ≡ (νε)1/4

⇒ recall: Reη ≡ η uη/ν = 1 → viscous effects important!

I small scales decrease with large-scale Reynolds number:η/`0 ∼ Re−3/4 , uη/u0 ∼ Re−1/4 , τη/τ0 ∼ Re−1/2

I η decreases faster than uη → gradients increase

9/25


Spectral view


Finite limit for dissipation at high Re

Experimental evidence from homogeneous-isotropic turbulence

laboratory grid turbulence

Downloaded 17 Sep 2008 to 129.13.72.153. Redistribution subject to AIP license or copyright; see http://pof.aip.org/pof/copyright.jsp

(Sreenivasan 1984)

direct numerical simulation

yond someRl . However, the numerical value ofD` is notthe same in the two groups. To compare them meaningfullywith experiments,4 the scalesL andu used there have to beredefined slightly. The redefinition leads toD`'0.73 forsquare grids of round bars, and is in rough agreement withthe D` for the upper curve in Fig. 1. It was noted in Ref. 4that D` assumes different values for grids of different con-figurations, especially for the active grids of Gad-el-Hak andCorrsin.10

Yeung and Zhou used a stochastic forcing confined tothe lowest two or three wavenumber shells, while Wanget al. and Caoet al. maintain the energy of a few lowestmodes according to thek25/3 energy spectrum. It is hearten-ing to note that the forced data of Wanget al. and of Yeungand Zhou agree with each other, but one cannot dismiss thefact that they both differ from the forced calculations ofJimenezet al. and the decay data of Wanget al. The formermaintained the energy peak essentially atk51, and intro-duced negative viscosity fork,3 in order to compensate forthe energy decay. In all the forced cases, it might be said thatthe resolution of the large-scale is a major factor: there is noperceptible gap between the large-scale and the box-size.The energy in the decay data of Wanget al. did not peak atthe lowest wavenumber but was shifted to the right, suggest-ing that the large-scale resolution might be better. Yet, thedecay data agree with one set of forced data—though itshould be said that there are only threeRl values for theformer, and that they do not totally preclude the possibilityof further decrease with increasingRl— but not with theother two. It is not clear why this is so.

Despite this lack of clarity, the principal message of Fig.1 is thatD asymptotes to a constant value, but thatD` can

perhaps be manipulated moderately—even in isotropicturbulence—by adjusting in some manner the forcingscheme or the large structure. Some preliminary calculationsof Juneja~private communication! suggest that the same de-gree of manipulation might also be possible by varying theinitial conditions. At present, we do not know enough to sayprecisely how this can be done in a controlled way. To re-solve this issue, one ought to implement systematic changesin the forcing scheme, the large-scale structure, and initialconditions. That the large structure does influence the con-stant D` is clear from experiments in homogeneouslysheared flows; in Ref. 5, it is shown thatD`5D`(S), Sbeing a non-dimensional shear parameter.

One is now left with the question as to whether the na-ture of forcing at the large scale, and the resulting differencesin the structure of the large scale, affect other aspects ofturbulence as well. We have examined various small-scalestatistics from the sources cited here. There seems to be noperceptible difference in this regard. But the scalingrange—as determined, for example, by Kolmogorov’s 4/5-ths law11—does depend on the nature of forcing: it can beextended or contracted depending on how one deals with theenergy level of the lowest few wavenumbers.

ACKNOWLEDGMENTS

Thanks are due to Dr. S. Chen, Dr. P.K. Yeung and Dr.J. Jimenez for discussions about their data, and to a refereefor some perceptive comments.

1P. G. Saffman, ‘‘Lectures on homogeneous turbulence,’’ inTopics in Non-linear Physics, edited by N. Zabusky~Springer, Berlin, 1968!, p. 487.

2J. L. Lumley ‘‘Some comments on turbulence,’’ Phys. Fluids A4, 203~1992!.

3U. Frisch,Turbulence: The Legacy of A.N. Kolmogorov~Cambridge Uni-versity Press, Cambridge, 1995!.

4K. R. Sreenivasan, ‘‘On the scaling of the energy dissipation rate,’’ Phys.Fluids 27, 867 ~1984!.

5K. R. Sreenivasan, ‘‘The energy dissipation rate in turbulent shear flows,’’in Developments in Fluid Dynamics and Aerospace Engineering, edited byS.M. Deshpande, A. Prabhu, K.R. Sreenivasan, and P.R. Viswanath~In-terline, Bangalore, India, 1995!, p. 159.

6J. Jimenez, A. A. Wray, P. G. Saffman, and R. S. Rogallo, ‘‘The structureof intense vorticity in homogeneous isotropic turbulence,’’ J. Fluid Mech.255, 65 ~1993!.

7L. P. Wang, S. Chen, J. G. Brasseur, and J. C. Wyngaard, ‘‘Examinationof hypotheses in the Kolmogorov refined turbulence theory through high-resolution simulations,’’ J. Fluid Mech.309, 113 ~1996!.

8P. K. Yeung and Y. Zhou, ‘‘On the universality of the Kolmogorov con-stant in numerical simulations of turbulence,’’ Phys. Rev. E56, 1746~1997!.

9N. Cao, S. Chen, and G. D. Doolen, ‘‘Statistics and structure of pressure inisotropic turbulence’’~submitted!.

10M. Gad-el-Hak and S. Corrsin, ‘‘Measurements of the nearly isotropicturbulence behind a uniform jet grid,’’ J. Fluid Mech.62, 115 ~1974!.

11A. N. Kolmogorov, ‘‘Dissipation of energy under locally isotropic turbu-lence,’’ Dokl. Akad. Nauk SSSR32, 16 ~1941!; English translation inProc. R. Soc. London, Ser. A434, 15 ~1991!.

FIG. 1. The variation of the quantity«&L/u3 with the Taylor microscaleReynolds number,Rl , in simulations of homogeneous and isotropic turbu-lence in periodic box. The symbols, described on the figure, correspond todifferent sources of data noted in Table I.

529Phys. Fluids, Vol. 10, No. 2, February 1998 Brief Communications


(Sreenivasan 1998)

⇒ ε`0/u30 has finite value O(1)

10/25


Spectral view


Finite limit for dissipation at high Re (2)

Simulations by Kaneda et al. (2003) at higher Re

εL

u30

Run 2048-1. Figure 4 showsP(k) at various times in Run2048-1. The range over whichP(k) is nearly constant isquite wide; it is wider than the flat range of the correspond-ing compensated-energy-spectrum~see Fig. 5!. The station-arity is also much better than that of lower resolution DNSs~figures omitted!, andP(k)/ê& is close to 1. In the study ofthe universal features of small-scale statistics of turbulence,if there are any, it is desirable to simulate or realize an iner-tial subrange exhibiting~i!–~iii ! rather than~i!–~iii !. Thepresent results suggest that a resolution at the level of Run2048-1 is required for such a simulation. Such DNSs areexpected to provide valuable data for the study of turbulence,and in particular for improving our understanding of possibleuniversality characteristics in the inertial subrange.

These considerations motivate us to revisit anothersimple but fundamental question of turbulence: ‘‘Does theenergy spectrumE(k) in the inertial subrange follow Kol-mogorov’s k25/3 power law at large Reynolds numbers?’’Figure 5 shows the compensated energy spectrum for thepresent DNSs~the data were plotted in a slightly differentmanner in our preliminary report4!. From the simulationswith up toN51024, one might think that the spectrum in therange given by

E~k!5K0e2/3k25/3 ~1!

with the Kolmogorov constantK051.6– 1.7 is in goodagreement with experiments and numerical simulations~see,for example, Refs. 1, 3, 9, and 10!. However, Fig. 5 alsoshows that the flat region, i.e., the spectrum as described by~1!, of the runs withN52048 and 4096 is not much widerthan that of the lower resolution simulations. The higherresolution spectra suggest that the compensated spectrum isnot flat, but rather tilted slightly, so that it is described by

E~k!}e2/3k25/32mk, ~2!

with mkÞ0.The detection of such a correction to the Kolmogorov

scaling, if it in fact exists, is difficult from low-resolutionDNS databases. The least square fitting of the data of the40963 resolution simulation for (d/d logk)logE(k) to(25/32mk)logk1b (b is a constant! in the range 0.008,kh,0.03 givesmk50.10. The slope withmk50.10 isshown in Fig. 5.

It may be of interest to observe the scaling of the secondorder moment of velocity, both in wavenumber and physicalspace. For this purpose, let us consider the structure function

S2~r !5ûv~x1r ,t !2v~x,t !u2&,

where S2 may, in general, be expanded in terms of thespherical harmonics as

S2~r !5 (n50

`

(m52n

n

f nm~r !Pnm~cosu!eimf.

Here,r 5ur u andu,f are the angular variables ofr in spheri-cal polar coordinates,Pn

m is the associated Legendre polyno-mial of ordern,m, and f nm(5 f n,2m* ) is a function of onlyr ,where the asterisk denotes the complex conjugate. The timeargument is omitted. ForS2 satisfying the symmetryS2(r )5S2(2r ), we havef km50 for any odd integerk. In strictlyisotropic turbulence,f nm must be zero not only for oddn,but also for anyn and m exceptn5m50. However, ourpreliminary analysis of the DNS data suggests that the an-isotropy is small but nonzero. In such cases,f nm is also smallbut nonzero, andS2 itself may not be a good approximationfor f 05 f 00. To improve the approximation forf 0 , onemight, for example, take the average ofS2 over r /r

FIG. 3. Normalized energy dissipation rateD versusRl from Ref. 5~dataup to Rl5250), Ref. 3~n,d!, and the present DNS databases~j,m!.

FIG. 4. P(k)/ê& obtained from Run 2048-1.

FIG. 5. Compensated energy spectra from DNSs with~A! 5123, 10243, and~B! 20483, 40963 grid points. Scales on the right and left are for~A! and~B!,respectively.

L23Phys. Fluids, Vol. 15, No. 2, February 2003 Energy dissipation rate and energy spectrum


Reλ

I Kolmogorov scaling confirmed

11/25


Spectral view


Hypothesis 3: Inertial similarity

Second similarity hypothesis

At high Reynolds numbers, the statistics of motions in the range`0 � ` � η have a universal form determined by ` and ε,independent of ν.

qqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqq

qqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqq

inertial subrange

η ÈI `0 L`DI

universal equilibrium range energy-containing

dissipationrange

range

I inertial subrange scales: u(`) = (ε`)1/3, τ` = (`2/ε)1/3

12/25


Spectral view


Energy flux through the cascade

I rate of energy transfer through scale ` defined as T (`)

I determined by scales around `: T (`) ∼ u(`)2/τ` = ε

⇒ transfer rate T (`) is independent of ` !

I conceptual diagram of the cascade:

qqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqq qqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqq

η ÈI `0 L`DIdissipation range inertial subrange energy-containing range

transfer T (`)

production Pdissipation ε

13/25


Spectral view


Predictions by Kolmogorov’s theory

Second order velocity structure function (definitions)

I Dij(r,x, t) ≡〈(ui(x + r, t)− ui(x, t)) (uj(x + r, t)− uj(x, t))〉

I related to: Rij(r,x, t) = 〈ui(x, t)uj(x + r, t)〉 (lecture 3)

I local isotropy: for |r| � L only components DLL, DNN

qqqqqqqqqqqqqqqqqqqqqqqqqqqqq qqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqq qqqqqqqqqqqqqqqqqqqqqqqqqqqqq

qqqqqqqqqqqqqqqqqqqqqqqq qqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqq qqqqqqqqqqqqqqqqqqqqqqqqppppppppppppppppppppp ppppppppppppppppppppp ppppppppppppppppppppp ppppppppppppppppppppp pppppppppppppppppppppppppppppppppppppppppp ppppppppppppppppppppp ppppppppppppppppppppp ppppppppppppppppppppp ppppppppppppppppppppp DNN (transverse)

DLL (longitudinal)u1u1

u2,u3 u2,u3

r

x x + e1rI in homogeneous turbulence with 〈ui〉 = 0

from continuity: DNN = DLL + 12r∂rDLL

⇒ in this case: Dij(r, t) fully determined by DLL(r, t)14/25


Spectral view


Second order velocity structure function (K41 results)

I second similarity hypothesis (inertial subrange):

statistics depend only on r, ε DLL = C2 (εr)2/3

log(D

LL

)

log(r)(grid turbulence, Gagne & Hopfinger)

DL

L/(εr

)2/3

CHAPTER 6: THE SCALES OF TURBULENT MOTION

Turbulent FlowsStephen B. Pope

Cambridge University Press, 2000

c©Stephen B. Pope 2000

1 10 10 510 410 310 2

r/g

C′ = (4/3)C2 2

C′ = (4/3)C2 2

C = 2.02

0

1

2

3

1

2

3

1

2

3

4(a)

(b)

(c)

e

r

D

(r

)–2

/3–2

/311

e

r

D

(r

)–2

/3–2

/322

e

r

D

(r)

–2/3

–2/3

33

Figure 6.5: Second-order velocity structure functions measured in a

high-Reynolds-number turbulent boundary layer. The horizontal

lines show the predictions of the Kolmogorov hypotheses in the

inertial subrange, Eqs. (6.33) and (6.34). (From Saddoughi and

Veeravalli (1994).)

1

r

(boundary layer, Saddoughi & Veeravalli 1994)

15/25


Spectral view


K41 results: the 4/5 law for isotropic turbulence

Result derived from Navier-Stokes

I derive transport equation for DLL

(involves 3rd order structure function DLLL)

I invoke: local isotropy, negligible viscosity (inertial subrange)

⇒ Kolmogorov’s 4/5-law: DLLL(r) = −45 εr

DL

LL/(εr

)

r/η

(grid turbulence, Gagne 1987)

16/25


Spectral view

Velocity spectraSpectrum balance

Spectral view of the cascade

Previous arguments were based on physical space view

Alternative – spectral space view:

I based upon Fourier transform

1. introduce spectral quantities

2. present consequences of Kolmogorov’s theory

3. discuss energy cascade in wavenumber space

17/25


Spectral view


Velocity spectrum tensor

Homogeneous turbulence

I definition: (cf. lecture 3)spectrum tensor Φij = transform of two-point correlation Rij

Φij(κ, t) =1

(2π)3

∫ ∞−∞

e−Iκ·rRij(r, t) dr

Rij(r, t) =∫ ∞−∞

e+Iκ·rΦij(κ, t) dκ

I setting r = 0: Rij(0, t) = 〈u′iu′j〉 =∫ ∞−∞

Φij(κ, t) dκ

⇒ Φij(κ) is contribution from mode κ to Reynolds stress

18/25


Spectral view


Energy spectrum function

Reduction of information contained in Φij

I sum diagonal components, integrate over directions of κ:

E(κ, t) =∫ ∞−∞

12

Φii(κ, t)δ(|κ| − κ) dκ

I E(κ) is real, non-negative, defined for κ ≥ 0

I k =∫ ∞

0E(κ) dκ contribution to TKE

I ε = 2ν∫ ∞

0κ2E(κ) dκ contribution to dissipation

19/25


Spectral view


Kolmogorov spectra

Scaling in the inertial subrange (η � `� `0)

I recall the 2/3 law: DLL = C2 (εr)2/3

I it is possible to relate DLL(r) to spectrum function E(κ)

⇒ E(κ) = Ckol ε2/3κ−5/3

I universal constant: Ckol = 1.5

(directly related to C2, value from measurements)

I confirmed in numerous experiments at high Reynolds number

20/25


Spectral view


Kolmogorov spectra - experimental confirmation

1D energy spectra – same scaling as corresponding 3D spectra

E11ε−

2/3κ

5/3

1





10–5 10–4 10–3 10–2 10–1 1000.0

0.2

0.4

0.6

0.8

(a)

C1

j 1g

e–2/3

j 15/3 E

11(j

1)

10–5 10–4 10–3 10–2 10–1 1000.0

0.2

0.4

0.6

0.8(b)

j 1g

C1′

e–2/3

j 15/3 E

22(j

1)

10–5 10–4 10–3 10–2 10–1 1000.0

0.2

0.4

0.6

0.8

(c)

j 1g

e–2/

3 j 15/3 E

33(j

1)

C1′

Figure 6.17: Compensated one-dimensional spectra measured in a tur-

bulent boundary layer at Rλ ≈ 1, 450. Solid lines, experimental

data Saddoughi and Veeravalli (1994); dashed lines, model spec-

tra from Eq. (6.246); long dashed lines, C1 and C ′1 corresponding

to Kolmogorov inertial-range spectra. (For E11, E22 and E33 the

model spectra are for Rλ = 1, 450, 690 and 910, respectively, cor-

responding to the measured values of 〈u21〉, 〈u

22〉 and 〈u2

3〉.)

9

κ1η

(boundary layer, Saddoughi & Veeravalli 1994)

E11,E

22

κ1

(high Re jet flow, Champagne 1978)

21/25


Spectral view


A model spectrum

Pope (2000) proposes model spectrum

I valid over range of scales:E(κ) = Ckol ε

2/3κ−5/3︸︷︷︸Kolmogorov

fL(κL) fη(κη)

I fL(κL) – energy-containing range

I tends to unity for large κLI for small κL: E(κ) ∼ κp0

I fη(κη) – dissipation range:I tends to unity for small κηI for large κη:E(κ) ∼ exp(−βκη)





10-5 10-4 10-3 10-2 10-1 10010-310-210-1100101102103104105

slope -5/3

E(κ)η uη

2

slope 2

κ η

Figure 6.13: Model spectrum (Eq. 2.246) for Rλ = 500 normalized by

the Kolmogorov scales.

5

given Re, p0 = 2

22/25


Spectral view


Spectral behaviour of the large scales

Energy-containing range

I non-universal behavior!

I 3D spectrum function moreinformative than 1D (aliasing)

⇒ consider grid turbulence→ approx. isotropic

I Note:∫ ∞0

E(κ)/κ dκ =4

3πkL11

k L11 is used for normalization





0 1 2 3 4 5 6 7 8 9 100.00

0.05

0.10

0.15

0.20

0.25

κL11

E(κ)kL11

Figure 6.18: Energy spectrum function in isotropic turbulence nor-

malized by k and L11. Symbols, grid-turbulence experiments

of Comte-Bellot and Corrsin (1971): ©, Rλ = 71; ¤, Rλ =

65;4, Rλ = 61. Lines, model spectrum, Eq. (6.246): solid, p0 = 2,

Rλ = 60; dashed, p0 = 2, Rλ = 1, 000; dot-dash p0 = 4, Rλ = 60.

10

◦ Comte-Bellot & Corrsin 1971, Reλ = 60 . . . 70

—— model spectrum, Reλ = 60, p0 = 2

– – – – model spectrum, Reλ = 1000, p0 = 2

— ·— model spectrum, Reλ = 60, p0 = 4

23/25


Spectral view


Spectral behaviour of the dissipation range

Dissipation range

I universal for different flows

I lin-log plot:straight = exponential decay

I peak dissipation at `/η ≈ 24H grid-turbulence Comte-Bellot & Corrsin 1971, Reλ = 60

◦ boundary layer, Saddoughi & Veeravalli 1994, Reλ = 600

—— model spectrum, Reλ = 600

24/25


Spectral view


Energy spectrum balance in homogeneous turbulence

∂tE(κ) = Pκ(κ, t)︸︷︷︸production

− ∂κTκ(κ, t)︸︷︷︸spectral transfer

−2νκ2E(κ, t)︸︷︷︸dissipation ≡D(κ)

I production limited toenergy-containing range

I κ < κEI :∂tE = Pκ − ∂κTκ

I κEI < κ < κDI :0 = −∂κTκ

I κDI < κ:0 = −∂κTκ −D

sketch for high Reynolds no. flows





0κDIκEI

Pκ

∂κ- ∂E

∂t

κ

∂κ- ∂Tκ

- ∂Tκ - D

(b)

0κDIκEI

D(κ)E(κ)

κ

(a)

0κDI

ε

κEI κ

Tκ(c)

Figure 6.28: For homogeneous turbulence at very high Reynolds num-

ber, sketches of (a) the energy and dissipation spectra (b) the

contributions to the balance equation for E(κ, t) (Eq. 6.284), and

(c) the spectral energy transfer rate.

20

(Pope “Turbulent flows’, 2000)

25/25

Conclusion Summary

Summary of the lecture

The turbulent energy cascade

I hierarchy of eddies, downward transfer of energy

I dissipation determined by large scales, performed by smallscales


I building block of turbulence research

I valuable results for small scales (e.g. −5/3 spectrum)

BUT: Problem of non-universality of large scales remains

1/6

Conclusion Summary

Outlook on next lecture: Wall turbulence

What is the general structure of wall-bounded flows?

How does the presence of a solid boundary affect the turbulentmotion?

What is the effect of wall roughness?

2/6

Conclusion Summary

Further reading

I S. Pope, Turbulent flows, 2000→ chapter 6

I U. Frisch, Turbulence, 1995→ chapters 6-8

3/6

Conclusion Summary

Appendix

4/6

Conclusion Summary

Shortcomings and refinements

Reynolds number dependence

spectral exponent in inertial subrange





102 103 1041.2

1.3

1.4

1.5

1.6

1.7

Rλ

p

Figure 6.29: Spectrum power-law exponent p (E(κ) ∼ κ−p) as a func-

tion of Reynolds number in grid turbulence: symbols, experimental

data of Mydlarski and Warhaft (1998); dashed line, p = 53; solid

line, empirical curve p = 53 − 8R

−34

λ .

21

(Mydlarski & Warhaft 1998)

I define E(κ) ∼ κ−pI exponent p approaches 5/3 only slowly with Reynolds

5/6

Conclusion Summary

Shortcomings and refinements (2)

Higher-order statistics deviate from K41 theory

I nth order structure functionDn(r) ≡ 〈(∆ru)n〉

I K41, dimensional arguments:Dn(r) ∼ (εr)ζn ζm = n/3

I n > 3: measurements deviate→ attributed to intermittency

I refinements by Kolmogorov(1962)





0 5 10 150

1

2

3

4

ζn

n

Figure 6.31: Measurements (symbols) compiled by Anselmet et

al. (1984) of the longitudinal velocity structure function exponent

ζn in the inertial subrange, Dn(r) ∼ rζn. The solid line is the

Kolmogorov (1941) prediction, ζn = 13n : the dashed line is the

prediction of the refined similarity hypothesis, Eq. (6.323) with

µ = 0.25.

23

• Anselmet et al. 1984

—— K41

– – – – refined similarity

6/6

Date post:	11-Oct-2020
Category:	Documents
Upload:	others
View:	3 times
Download:	0 times

Fluid mechanics of turbulent ows€¦ · Lecture 5 1/25. The Richardson cascade Kolmogorov’s 1941...

Documents