A diffusion problem in a two-medium system

A diffusion problem in a two-medium system

A DIFFUSION PROBLEM IN A TWO-MEDIUM SYSTEM* E.M. KARTASHOV, G.M. BARTENEV and I.V. RAZUMOVSKAYA Moscow (Received November 4 1966) THE two-di...

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A DIFFUSION PROBLEM IN A TWO-MEDIUM SYSTEM* E.M.

KARTASHOV,

G.M.

BARTENEV and

I.V.

RAZUMOVSKAYA

Moscow (Received

November

4

1966)

THE two-dimensional diffusion processes in a system composed of two anisotropic media separated by a moving interface are of considerable Elementary solutions are known Interest in some branches of physics. for this type of problem for the case of constant values of the initial and boundary functions. We shall here consider a more general solution with no restrictions on the functions describing the behaviour of the reoulred concentration on the Interface and at the Initial instant. Consider one of the diffusion theory of the strength of rigid surface-active medium [2 - 41.

[II

problems of this type occurring in the bodies close to destruction in a weakly

The general physical picture of this problem is of diffusion in a system of two media, separated by an interface in uniform motion. The medium to the left of the interface (z < vt, y > 0) is anisotropic, with diffusion coefficients kl and k2 In two perpendicular directions, and with initial concentration distribution fll(z, y); the medium to the right of the Interface (z > vt, y > 0) is isotropic, with diffusion coefficient kl and initial concentration distribution fgg(z, y). There is dfffuslon contact on the interface z = vt, i.e. the concentration of the diffusing substance and the diffusion flux vary continuously In the nelghbourhood

II

of the

Interface.

Given the diffusion flux at the boundary y = 0.

To find the time distribution 1, 2, of the diffusing substance *

Zh.

vychisl.

Mat.

mat.

t)

qlI(z,

Fiz.

in medium I and g~gg(z,

of the concentration in both media. 7,

6, 269

1423

-

1429,

Ci(z,

1967,

t)

y,

in medium

t),

i =

E.M. Xartashov

290

The problem amounts diffusion equations

a2ci

a=ci

at

a22

w

a-c, -=

avz

to the

-kk2-,

initial

to

z >

of two-dimensional

t > 0;

0,

Y >

and boundary

a

vt,

=Cz(Z,Y,t)IL=11I;

Y >

t > 0,

0,

conditions

I =

(6 Y, t)

i 5 1, 2;

wr (z, t),

aY C~(z,y,t)l,=,t

a set

SOlViUg

(1)

w

Cf (z, Y, t) I t-o = fff (2, Y),

al.

z < vl,

a2c2

k iF+k,-,

at

subject

msthematically

acj

-=kk,-

et

u-0

(2:

aci (z, Y, t) 1

az

acz (z, Y, t)

1 zsot=

a2

II r=vt

in diffusion across an interlace with a Such problems arize, e.g. crystalline structure on one side and amorphous structure on the other. Movement of the interface corresponds to movement of the crsstallization process is not directly confront with constant speed V; the diffusion nected with the interface movement. A closely related problem concerns the growth at a constant rate v of a crack at the interface between an Isotropic material of diffusion COefficient kl, and a surface-active medium of surface diffusion coefficient k2 c41. We introduce new function

a new variable x = z - vt ‘and replace t) according to the equation Ni(x, y, V

Cj(z,y,t)= Problem

exp

- (2)

(1)

--x-2k,

a2Nf

aNt

-5

ki

at

w

a2Nz z

+

a2Ns ki -,

x < 0, 2

>

au2 NI(x, 19 t) It-o = fr(x, Y), aY =

1,

t) by a

2.

a2N*

0,

aNi (3, I/I t)

.Ni(~,y,t)l+-o

i =

y,

now becvacs

at--ki-+k2--,a22 aN2

3 N1(5, Y, q,

4k, 5

Ci(z,

Nz(z,

= ‘pi@, t), I Y-=0

~,t)lx=-o;

0,

t > 0:

(3)

Y > 0,

t > 0:

(4)

Y >

f 5 1,2;

(5)

1-

(6)

aN1(I, ur q az

1, 2;

= Z.-O

W(x, ax

y, t) , g-0

('1

A

diffusion

problem

in

a two-medium

291

system

t) are new (known) functions. We seek Nl(r, y, t) where fit%, y). qi(r, capable of Fourier transformand Nz(z, y, t) in the class of functions ation with respect to x, and t. Assume that

y

flux on the interwhere ~i(y, t) is the unknown function of the diffusion face, which has to be determined later. We apply to (4) the twodimensional infinite Fourier cosine transformation [51 m

m cos Es dz

R(E,rl.f)=;S Using (6)

where

z(f,.q,

and (8),

s

N2 (5, Y, t)cos

we now get

t) = I(T), t) + F2(f, t),the solution

formed initial

(9)

qg dy.

0

0

condition

of which,

with the trans-

%*((5,% 0) = f=2(&u) is

si(E,q, r)exp 1-h

(E2+ q2) (t - ~11dz +

0 -t-

Application

Nz(s,v,t)=-

to this

;,,j,

of the inverse

j “(“;B’(exp[

t- WE2 +

VII.

cosine-transformation

gives

_P7e+p3)2]+

0

0

To find Nl(z,

%$,,q)exp

y,

t) we replace

the variable

x = -LA, and transform

E.M.

292

the

region

x

(8) as ki -

at

Ni(--u,

w

= fit-4

I/,

au Application

of

2nd medium,

4n )I(kikz)

h(~-_B)~+k2(~+a)~

k,(u

-

to

conjugation

(ll),

(13),

OD

(‘12)

t);

as in the

-

case

1 +

(131

0

kt(d-P)2+kz(u-a) 1 II

.___

----

+ exp

4k,kpt

bI>

dS t

ks2

+exp

-

k,(u + a)2 + hP 4k,k,(t

[

working [61 that the and (3) respectively, conditions.

condition Ni t-0,

We have

‘pi(--4

4kikzi

1

The first

I =

ki(y-fi)-‘+kziv.-aJ2

a)Z + kly2

It can be shown by direct (10) satisfy equations (11) sponding boundary and initial

(11;

=

(9)

4kik,t

k*(y + a)‘+

and

I - ‘u.tv, 4. -.u-o

t)

0

0

(61,

u-0

DD

--.T--

(5),

t > oi’

0,

Y >

ai

the transformatior gives oi)

Xi(---u,y,t)=

(3).

write

dNi (-u, Y, t) --

Y);

aNi (-9

of the

now

u > 0,

fkz--*

au2

y, t) It-o

al.

@Ni

PNi

dNi -_=

et

0 < u < + m. We can

0 into

<

Kartashov

(7) Y,

is

used

to

t) = Nz (+O, Y, t).

-z)

functions and also

find

y(y,

da. I>

(13) and the corre-

t):

A diffusion

problem

in

a tso-medium

system

293

where

ki(v -___

-

B)” + kza2 4k,k,t

To solve the integral equation second factor in the inner Integral

c

l! TfxP -

and we apply

(14) in ‘Y(y, t), thus:

a)”

(Y -

we transform

the

=

4ki(t--7)

= exp The equation

1+

-

y2 +

a2

4ki (t -

t)

ch 1

w 2ki (t -

T)

I

(i = a, 2).

now becomes

to

It

the

infinite

Fourier

cosine

trSnSfOrmatiOn

. . . cos qy dy. 0

Changing

the

order

of

integration

and evaluating

the

inner

InteiZralS

294

E.M.

c71,

we

Kartashou

et

get

OD

1

-

3t

1 - --exp[--k&(t-T)Jdt * ,‘(t - T)

za

1 -

we can write

the

last

s 0

Y(a, z)cos aq da = F(q, z),

as

expression

The Carson - Heaviside

to this

equation.

transformation

b--kWt - T)] a$=

1 ,‘(P + kir1’)

where

P s ,. . e-PJdt 0

hl

F(~, t).

can now be

We get

Ikiy (q, P) whence

Y(cz, z)cos c&r) da = P(q, t).

s

3% 8

---e=p

applied

al.

+ __I-.

I

li(P + km

I

= F(rl, P),

A diffusion

problem

a two-medium

in

1

OL

0

-$s I

To find

the

rl)

fl(--a,

original

ml,

C

p J’(P +

F(q,

-$l@+k2r12)

kzrl”)

exp

t) we write

the

I’(P + W)

P) =



1 (P + kiq2) + V(P + kz?)

+

da

i

image

ml,

295

system

T(%P)c

thus

PI+ ’ 15)

I'(P

+ kir12)

P),

-------FS(Tl>

-I’(P + kN)+

I’(P + kzq2)

where 02

E(q,P)=-

F,(q,

v

+

&(a,

1

-$(P

p)ew

fii(-u,

p)=: I/:

+ ktrt2) Ida

i

L

0

t

[-~V(p+kzt12) Ida-

p)exp

Cl

-

al

-G 1 fi (-a,

rl)P e*p

0

The originals

FzCy.

t)

and 1

Pn(& t)=

Fs(y,t)=-

l’kz 2nk,:i

-2

2k,n

s

0

dz ‘_

(t--)2

s s o t

~

&

o (t-t)’

~‘s(,Y,

t)

I

a2 + ii2

m os

+ k2q2)

aq2(a, 7)exp

7)exp

da.

are

-

acpl (-a,

$I(P i

-

-

ha2

da +

z) I

+ du 1-

4kikz (t -

-

4ki (t -

b(y

p_-._

by2 t)

-

p)” + k2a2

4k,k,t

1+

_ _k_i_!y + B)” + ha2 4k,k,t

1

I

J dB.

B.M.

296

Evaluating

the contour 1

et

ab.

Integrals

Y+im

--s 2ni

Knrtashov

1

I@ 4- kzW

y--imT

V(p+

k2q2)+

I@

- exp (pt) 4 +

kiQ"l

and

where v isa lties

of

this

constant,

,:rcater

integrand.

than the real

we find

?‘(P+ JwP)-+-

1

2ni

lip + kN) + ,‘(P + k2n2)

P”+Yos

convolution

to

of all

1

V’(P+ W?

I;----------.------c----_ V_i,~ I’& + kN)+ Y(P + k#)

x2 + ki u2(xf = kz x.‘+x Applying

parts

(1.5).

the

SlIIgUlar-

that

,

t+(x)

=

exp W) dp -

x2 + kz

k, ~.

s2+kt

the original

of y(q,p):

is found to be

m

X

(s

xW(x)q2

7 t~2+kzf(~2+klf2

exp I-- M(r) (t -

t)q2]

dz

& f

applying the Fourier cosine transformstiti3 inversion %I farther changing the order of the integrals, and theorem to this expression, evaluating the inner integral, we finally get

A diffusion

+ (

prob

Zen

a two-medium

(Y-m2

1_

K

26(x)

(t -

= 1o Fs(B’ ‘)-(

) [ exp

.c)

(Y

-’

+ (

46(x)

-

The required expressions fixed

Due to

for

the

the

the

constants, -

such

presence

y,

of

t),

are

i = I,

2) /

t)

(i

2,

(Y+ 4a2(x)

= 1,

21,

now easily after

the

exponentials are

fi(x,

and (13)

since

the

integrals

functions (10)

(7),

Ni (x,

y,

(21,

-

),,, [ -

(t -



+

I

(yf!c

1 _

Ci(t,

(1)

PI2

k2)2a(x)

]] dx }dB. (t-r)

satisfying

obtained

again

the

from the

returning

to the

system.

improper

and boundary

to

our problem

coordinate

(13),

(31

concentrations

of

3 (x” + k,) (xr;

(t -T)

28(x)

conditions

297

system

I---

Ikn(ki - kz) * dz -___ .s ____ 2n31s o (t-Z)%

X

in

y)

and ~i(“,

represent

Fourier

in the

convergent

for If

t).

of

class

these

“generalized”

transformation

kernels

a wide

and initial

functions

solutions

obviously

(10) of

of

cannot

are problem

be applied

functions.

The authors

thank

V.I.

Levin

for

his

interest

and for

discussing

the

results. Translated

by D.E.

Brown

REFERENCES 1.

LYKOV, A. V. Theory

of

Moscow, Gostekhizdat,

2.

LIKHTMAN, V.I., mechanics

Moscow,

of

Akad.

heat conduction 1952.

SHCHUKIN, E.D. metals

Nauk

(Teoriya

and REBINDER, P.A.

(Fiziko-khimicheskaya SSSR,

teploprovodnosti),

1362.

Physico-chemica

mekhanika

metallov).

I

E.M.

298

Kartashov

et

al.

3.

BARTENEV, G.M. and RAZUMOVSKAYA,I.V. Time dependence strength of brittle bodies in surface-active media, Nauk SSSR 150, 4, 784 - ‘787, 1968.

4.

BARTENEV, G.M. and ZUEV, Yu.S. Strength and destruction of highly i razrushenie vysoko-elasticheskikh elastic materials (Prochnost’ materialov), Moscow, Khimiya. 1964.

5.

SNEDDON, I.N.

6.

TIKHONOV, A.A. and SAMARSKII, A.L. Equations of mathematical (Uravneniya matematicheskoi fiziki), MOSCOW, Gostekhizdat.

Fourier

transforms,

7.

GRADSHTEIN, I.S. and RYZHIK, I.M. and products (Tablitsy integralov, Moscow, Fizmatgiz, 1963.

8.

DITKIN, V.A. and PRUDNIKOV, A.P. (Spravochnik po operatsioanomu shkola. 1965.

McGraw-Hill,

of the DokI. Akad.

New York.

lg51, physics

1958.

Tables of integrals, sums, series summ, ryadov i proizvedenii),

Guide

to

ischisleniyu),

the

operational

Moscow,

calculus

Vysshara