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ARCHE

Lab. V.

Schepsbocd

Tedinische Hoscc

DET KONGELIGE NORSKJ VIDENSKABERS SELSKAB

FORHANDLINGER Bind 26 1953 Nr 17

Dft

532.517.4

On the Velocity Distribution of Turbulent Flow behind a

System of Thin Cylindrical Rods

B

(2)

DET KONGELIGE NORSKE VIDENSKABERS SELSKABS FORHANDLINGER Bind 26 1953 Nr 17

U

532.517.4

On the Velocity Distribution of Turbulent Flow behind a

System of Thin Cylindrkal Rods

Bx

R. GRAN OLSSON

(Frcmlagt I Fellesmøtet l2te oktober 1953)

Introduction. In an earlier paper the author has treated the problem

of the velocity distribution of turbulent flOw behilid a system of rods spaced at equal distances from each other [1]. For the shear strcss of turbulent flow the expression in terms of themixing length due to L. PRANDTL was

assumed, where the mixing length at a certain distance from the rods may be put equal to a constant 10 [2]. The velocity distribution was given by a non-linear differential equation of the second order, the solution of which can be expressed by quadrature. The purpose of this note is to shw, how a solution of the differential equation can also be expressed in terms of the elliptic functions of WEIERSTEASS [3].

The differential equation. We consider a system of cylindrical

equidistant rods of infinite length in a plane perpendicular to the direction

of flow in a stream of originally constant velocity. The problem is to calcu-late the distribution of velocity in the region behind the rods by means of the

expression of L. PRANDTL for the shear stress in turbulent flow. The

mixing length will also be constant in a plane parallel to the plane of

the r.ods because of their

4od

A

1

uniform spacing.

The coordinate system may be chosen as follows: The u-axis parallel to the plane

of the rods and perpendicular to their axes, the x-axis through the axis of one of the rods and parallel to the direction of the undisturbed flow (Fig. 1). The components of the velocity may be noted by u and v,

(3)

length is

(4) /

(2h)

The differential equation (2a) will then be transformed into

0u1 Ou Ozi

Ox 'Oy Oj2

where k =212/U is a constant parameter.

3. The solution of the differential equation. If we make the substitu-tion

(5) u==A0x'F1(y)

with the condition F1(0) = 1, A0 may be interpreted as the amplitude of the velocity distribution, and we get the following differential equation

(2c) F1(y)=A0kF'1(y)F"1 (y)

Assume now g = cl/, where c is. a constant of the dimension length, we

obtain the following equation,

R. GRAN OLSSON: On the VelocitU Distribution elc; 71

Behind the rod system at a sufficiently large distance the components of

velocity are

u=U -U1,

v=v1

where a1 and v1 are assmed to be sinai! compared with U. In establishing

the equations of motion all terms of higher order of magnitude are dropped;

furthermore the pressure term is neglected. This assumption can be justi-fied by means of the solution obtained.

The equation of motion in the x-direction is

Ou Oii

lOr

ii- +v=--

Ox Oy Og

introducing a = U - a1, v = v1 ,where a1 <<U, v1<<U, we obtain as a first

approximation the equation

(2a) Ox

c011

Because of the periodicity of the rod system, it seems convenient to assume the following expression for u

u1=A[(x) cos ay,

where a has to be chosen in such a manner that the period of u1 agrees:

with the ditanse ) of the rods, i.e. a = 2/A.

The expression for shear stress according to the theory of mixing

Ou Ou Oy Oil

(4)

(2h)

II,

F1'2 =

-the independent variable , is given by

0

I) =(G)3 f

vii-1i

which is an elliptic integral of Weierstrass tyje. To get a more convenient

expression for , and F, we introduce the parameter w by means of the ellip-tic integral

(C

ii) =

.1 (4:g

Yo)

(/Z

where the inverse function

z =p(iv)

is the elliptic p-function of Weierstrass. By comparison of the integrals (7) and (8), we see that in our case

= 0,

= -- I

From (8) and (9)

one obtains

(7a) = (6)3 J p (ui) div = (6)2/3 [ (w) - (w0)]

72 D. K. N. V. S. FORHANDLINGER BD 26 1953. NH 17

- A0k

l't(ci)= F1 (c'i) I (ca)

where on the right side the differentiations are made with respect to the

variable

,.

As c is arbitrary, we choose it so that

A0 k/c2

and obtain the differential equation

F1=F1'.F1"

which, multiplied by. F1', results in

F F1'= F,' F,' and once integrated

(?g)

where C is a constant of integration, which can be determined by the

condi-tion that for g = 0 ( = 0) F1 = 1, and F1' = 0 or C = 1/2.

With this value of C we get

From the substitution

(6) = 4: + 1,

(5)

by L. M. MILNE-THOMS0N [4]. The p-function of Weierstrass can be expres sed by the Jacobian elliptic function in the following, manner

I + cn (iivH1') p(w) = '2 +

1 en (2w111)'

310srz (2ivH) dii (2ul I1/2)

p (iv) = 4H I

-{ 1 - en (w1I I_)

}

R. GRAN QLSSON: On the VeloCitg DistHbütion etc. 73 where w0 corresponds to z = 0 , w to an arbitrary value of z and W) to the

elliptic e-function of Weierstrass defined by the integral i,(w)dtv

The function F1() of velocity distribution is given by Eqs. (6) and (9)

((iii) Ij(iv)={ 4p(w) ± i

On the other hand the derivative of the Weierstrass p-function is given by (10) p' (iv) = {4p1 (iv) - g.2p (iv)-- YB

so that the velocity distribution is given by

(6b) F1(rv)=p'(zv)

Thus we have a representation of the variables F1 and by means of the

common variable w The result of the numerical calculation is shown in Fig. 2 and is obtained by using the tables of the Jacobian elliptic functior

(6)

(7b)

74 D. K. N. V. S. FORHANDLINGER BD 26 1953, NR 17

where sn, en and dii are the Jacobian elliptic functions,

and e1 , e0, e.,1 are the roots of the equation

4w3 - g3w -- = 0

e2. being the real root, whereas e1 and e3 are complex. In our case e ==

(4)=O.63OO.

The independent variable , is expressed by the integral of the

p-func-tion, i.e.

(2;3

f [

e2 + H1 + cit (2tzH)I '

1cit (2ivH) J

'U

The result is in close agreement with the results obtained by simple quadr

-ture of an equation corresponding to (7).

The velocity distribution in the direction parallel to the plane of the

rods, but perpendicular to their axes is given by the equation of continuity

(12)

OxOy

where according to Eq. (5)

= 2AX2 F (y)

and therefore

(12a) v, =cAaf2 JF1 (c') d'i

We now use the expressions of F1(c,) from Eq. (6b), d,7 from Eq; (7) and z p (w) from Eq. (9), obtaining

= 623Ac± 2

J p

(iv) p' (iv) div 9 113

- [p2 (tO) p2 (w0fl

Expressing the p-function by the Jacobian elliptic functions we get a velo-city distribution in close agreement with that found by simple quadrature (Fig. 3).

4. Summary The velocity distribution behind a system of. cylindrical equidistant rods is found by using the expression of L. PRANDTL for the shear stress in turbulent motion (mixing tength theory) The solution of

(7)

4/6

.S 10 12 14

4/4

20 2.2

swJ.

the differential equation is given in terms of the elliptic functiOns of

\Veier-strass, which fort the numeriëal calculaton are transformed to the e1liptic

functions of Jacobi. The results are in close agreement with those'tobtained

earlier by simple quadrature.

This study was supported by the Office of Naval Research (U.S.N.) under Contract No. N 7 onr - 35801, Project Number NR-041-032. The present paper is also published as a Report All-94/10, Graduate Division

of Applied Mathematics, Brown University, Providence, R.I.

R. GRAN OLssoN: Zeitschr. 1. añgew. Math. u. Mech. vol. 16, (1936), p. 257-24, see also S. GOLDSTEIN, Modern Developments in Fluid Dynamics, Oxford University Prss,

London, 1938, vol. II, p. 586 and p. 667.

L. PaNDm: Essentials of Fluid Dynamics, Blackie & Son Ltd., London and Glasgow 1952, P. 118.

E. H. NEVILLE: Jacobian Elliptic Functions, Oxford University Press, London, 2 ad.

1951, p. 26-49.

-L-. M M1LNE-THMSON: Jacobian Elliptic Function Tables, Dover Publicalions Inc. New York 1950 p. 23-24.

Trykt 18th desember 1953

I koinniiajon ho F. Brunis Bokhandel

ALktletrykkeret i Thsndhjem

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