Dual Fourier-Bessel series arise during mixed boundary value problems in cylindrical coordinates where the radial dimension is of finite extent. Here we show a few examples.
•Example 3.3.1
Let us find16 the potential for Laplace’s equation in cylindrical coordi- nates:
∂2u
∂r2 +1 r
∂u
∂r +∂2u
∂z2 = 0, 0≤r <1, 0< z <∞, (3.3.1)
16 Originally solved by Borodachev, N. M., and F. N. Borodacheva, 1967: Considering the effect of the walls for an impact of a circular disk on liquid.Mech. Solids,2(1), 118.
subject to the boundary conditions lim
r→0|u(r, z)|<∞, ur(1, z) = 0, 0< z <∞, (3.3.2) uz(r,0) = 1, 0≤r < a,
u(r,0) = 0, a < r <1, (3.3.3) and
zlim→∞u(r, z)→0, 0≤r <1, (3.3.4) wherea <1.
Separation of variables yields the potential, namely u(r, z) =A0+
∞ n=1
Ane−knzJ0(knr), (3.3.5) where kn is the nth positive root of J0(k) = −J1(k) = 0. Equation 3.3.5 satisfies Equation 3.3.1, Equation 3.3.2, and Equation 3.3.4. Substituting Equation 3.3.5 into Equation 3.3.3, we obtain the dual series:
∞ n=1
knAnJ0(knr) =−1, 0≤r < a, (3.3.6) and
A0+ ∞ n=1
AnJ0(knr) = 0, a < r <1. (3.3.7) Srivastav17 showed that this dual Dini series has the solution
A0=−2 a
0
t h(t)dt, (3.3.8)
and
An=− 2 knJ02(kn)
a 0
h(t) sin(knt)dt, (3.3.9) where the unknown function h(t) is given by the regular Fredholm integral equation of the second kind:
h(t) + a
0
L(t, η)h(η)dη=t, 0≤t < a, (3.3.10) and
L(t, η) = 4 π2
∞
0
K1(x)
I1(x) sinh(tx) sinh(ηx)dx. (3.3.11)
17 Srivastav, R. P., 1961/1962: Dual series relations. II. Dual relations involving Dini series. Proc. R. Soc. Edinburgh, Ser. A,66, 161–172.
0 0.2
0.4 0.6
0.8 1
0 0.05 0.1 0.15 0.2
−0.4
−0.3
−0.2
−0.1 0 0.1
r z
u(r,z)
Figure 3.3.1: The solution to Laplace’s equation subject to the boundary conditions given by Equation 3.3.2, Equation 3.3.3, and Equation 3.3.4 whena= 0.5.
Figure 3.3.1 illustrates this solution whena= 0.5.
•Example 3.3.2
A similar problem18 to the previous one arises during the solution of Laplace’s equation in cylindrical coordinates:
∂2u
∂r2 +1 r
∂u
∂r+∂2u
∂z2 = 0, 0≤r <1, 0< z <∞, (3.3.12) subject to the boundary conditions
rlim→0|u(r, z)|<∞, ur(1, z) = 0, 0< z <∞, (3.3.13) u(r,0) = 1, 0≤r < a,
uz(r,0) = 0, a < r <1, (3.3.14) and
zlim→∞|uz(r, z)|<∞, 0≤r <1, (3.3.15) wherea <1.
Separation of variables gives u(r, z) =A0z+
∞ n=1
Ane−knzJ0(knr) kn
, (3.3.16)
18 See Hunter, A., and A. Williams, 1969: Heat flow across metallic joints – The con- striction alleviation factor. Int. J. Heat Mass Transfer,12, 524–526.
where kn is the nth positive root of J0(k) = −J1(k) = 0. Equation 3.3.16 satisfies Equation 3.3.12, Equation 3.3.13, and Equation 3.3.15. Substituting Equation 3.3.16 into Equation 3.3.14, we obtain the dual series:
∞ n=1
AnJ0(knr) kn
= 1, 0≤r < a, (3.3.17)
and
A0− ∞ n=1
AnJ0(knr) = 0, a < r <1. (3.3.18) Srivastav19 has given the solution to the dual Fourier-Bessel series
αa0+ ∞ n=1
an
J0(knr) kn
=f(r), 0≤r < a, (3.3.19)
and
a0+ ∞ n=1
anJ0(knr) = 0, a < r <1. (3.3.20) Then,
a0= 2 a
0
h(t)dt, (3.3.21)
and
an= 2 J02(kn)
a
0 h(t) cos(knt)dt, (3.3.22) where the functionh(t) is given by the integral equation
h(t)− a
0
K(t, τ)h(τ)dτ =x(t), 0< t < a, (3.3.23)
x(t) = 2 π
d dt
t 0
rf(r)
√t2−r2dr
, (3.3.24)
and
K(t, τ) = 4
π(1−α) + 4 π2
∞
0
K1(ξ)
ξI1(ξ)[2I1(ξ)−ξcosh(τ ξ) cosh(tξ)]dξ.
(3.3.25)
19 Ibid. See also Sneddon, op. cit., Equation 5.3.27 through Equation 5.3.35.
0 0.2 0.4 0.6 0.8 1 0
0.2 0.4 0.6 0.8 1
−1
−0.5 0 0.5 1 1.5 2
r z
u(r,z)
Figure 3.3.2: The solution to Laplace’s equation subject to the boundary conditions given by Equation 3.3.13 through Equation 3.3.15 whena=12.
Equation 3.3.19 through Equation 3.3.25 provide the answer to our problem if we setα= 0 andf(r) = 1. Figure 3.3.2 illustrates the solution whena= 12.
•Example 3.3.3
Let us solve Laplace equation20
∂2u
∂r2 +1 r
∂u
∂r +∂2u
∂z2 = 0, 0≤r < a, 0< z <∞, (3.3.26) subject to the boundary conditions
rlim→0|u(r, z)|<∞, u(a, z) = 0, 0< z <∞, (3.3.27) uz(r,0) = 1, 0≤r <1,
ur(r,0) = 0, 1< r < a, (3.3.28) and
zlim→∞u(r, z)→0, 0≤r < a, (3.3.29) wherea >1.
The method of separation of variables yields the product solution u(r, z) =
∞ n=1
AnJ0(knr)e−knz, (3.3.30)
20 See Sherwood, J. D., and H. A. Stone, 1997: Added mass of a disc accelerating within a pipe.Phys. Fluids,9, 3141–3148.
where kn is the nth root of J1(ka) = 0. Equation 3.3.30 satisfies not only Laplace’s equation, but also the boundary conditions given by Equation 3.3.27 and Equation 3.3.29. Substituting Equation 3.3.30 into Equation 3.3.28, we obtain the dual series
∞ n=1
AnknJ0(knr) =−1, 0≤r <1, (3.3.31)
and ∞
n=1
AnknJ1(knr) = 0, 1≤r < a. (3.3.32) We begin our solution of these dual equations by applying the identity21
∞ n=1
Jν+2m+1−p(ζn)Jν(ζnr)
ζn1−pJν2+1(ζna) = 0, 1< r < a, (3.3.33) where |p| ≤ 12, ν > p−1, m = 0,1,2, . . ., and ζn denotes the nth root of Jν(ζa) = 0. By direction substitution it is easily seen that Equation 3.3.32 is satisfied ifν= 1 and
k2−n pJ22(kna)An = ∞ m=0
CmJ2m+2−p(kn). (3.3.34)
Here pis still a free parameter. Substituting Equation 3.3.34 into Equation 3.3.31,
∞ n=1
∞ m=0
Cm
J2m+2−p(kn)J0(knr)
kn1−pJ22(kna) =−1, 0≤r <1. (3.3.35) Our remaining task is to computeCm. Although Equation 3.3.35 holds for anyrbetween 0 and 1, it would be better if we did not have to deal with its presence. It can be eliminated as follows: From Sneddon’s book,22
∞
0
η1−kJν+2m+k(η)Jν(rη)dη =Γ(ν+m+ 1)rν(1−r2)k−1
2k−1Γ(ν+ 1)Γ(m+k) Pm(k+ν,ν+1) r2 (3.3.36)
21 Tranter, C. J., 1959: On the analogies between some series containing Bessel functions and certain special cases of the Weber-Schafheitlin integral. Quart. J. Math., Ser. 2,10, 110–114.
22 Sneddon, op. cit., Equation 2.1.33 and Equation 2.1.34.
if 0≤r <1; this integral equals 0 if 1< r <∞. HerePm(a,b)(x) =2F1(−m, a+
m;b;x) is the Jacobi polynomial. If we view Equation 3.3.36 as a Hankel transform ofη−kJν+2m+k(η), then its inverse is
η−kJν+2m+k(η)
= 1
0
Γ(ν+m+ 1)r1+ν
1−r2k−1
2k−1Γ(ν+ 1)Γ(m+k) Jν(rη)Pm(k+ν,ν+1) r2
dr. (3.3.37) We also have from the orthogonality condition23of Jacobi polynomials that
1
0
r2ν+1
1−r2k−1Pm(k+ν,ν+1) r2
dr= Γ(ν+ 1)Γ(k)
2Γ(ν+k+ 1)δ0m, (3.3.38) whereδnm is the Kronecker delta. Multiplying both sides of Equation 3.3.35 byr
1−r2−p
Pm(1−p,1)(r2) and applying Equation 3.3.38, we obtain
−Γ(1−p) 2Γ(2−p)δ0j=
∞ n=1
∞ m=0
CmJ2m+2−p(kn)J2j+1−p(kn)Γ(j+ 1−p) 2pΓ(j+ 1)kn2−2pJ22(kna) ;
(3.3.39)
or ∞
m=0
AjmCm=Bj, j = 0,1,2, . . . , (3.3.40) where
Ajm= ∞ n=1
J2m+2−p(kn)J2j+1−p(kn)
k2−2n pJ22(kna) , (3.3.41) and
Bj=
−2p−1/Γ(2−p), j= 0,
0, otherwise. (3.3.42)
For a givenp, we can solve Equation 3.3.40 after we truncate the infinite num- ber of equations to justM. For a givenkn we solve the truncated Equation 3.3.40, which yields Cm form = 0,1,2, . . . , M. Then Equation 3.3.34 gives An. Finally, the potential u(r, z) follows from Equation 3.3.30. Figure 3.3.3 illustrates this solution whena= 2 andp= 0.5.
•Example 3.3.4
In the previous example we solved Laplace’s equation over a semi-infinite right cylinder. Here, let us solve Laplace’s equation24 when the cylinder has
23 See page 83 in Magnus, W., and F. Oberhettinger, 1954:Formulas and Theorems for the Functions of Mathematical Physics.Chelsea Publ. Co., 172 pp.
24 See Galceran, J., J. Cecilia, E. Companys, J. Salvador, and J. Puy, 2000: Analytical expressions for feedback currents at the scanning electrochemical microscope. J. Phys.
Chem., Ser. B,104, 7993-8000.
0 0.5
1 1.5
2
0 0.05 0.1 0.15 0.2
−0.8
−0.6
−0.4
−0.2 0 0.2
z r
u(r,z)
Figure 3.3.3: The solution to Equation 3.3.26 subject to the boundary conditions given by Equation 3.3.27 through Equation 3.3.29 whena= 2 andp= 0.5.
a heightb. Our problem now reads
∂2u
∂r2 +1 r
∂u
∂r+∂2u
∂z2 = 0, 0≤r < a, 0< z < b, (3.3.43) subject to the boundary conditions
lim
r→0|u(r, z)|<∞, u(a, z) = 0, 0< z < b, (3.3.44) u(r,0) = 1, 0≤r <1,
uz(r,0) = 0, 1< r < a, (3.3.45) and
zlim→∞u(r, z)→0, 0≤r < a, (3.3.46) wherea >1.
The method of separation of variables yields the product solution u(r, z) =
∞ n=1
Ancoth(knb) [cosh(knz)−tanh(knb) sinh(knz)]J0(knr) kn
, (3.3.47) where kn is the nth root of J0(ka) = 0. Equation 3.3.47 satisfies not only Laplace’s equation, but also the boundary conditions given by Equation 3.3.44 and Equation 3.3.46. Substituting Equation 3.3.47 into Equation 3.3.45, we obtain the dual series
∞ n=1
Ancoth(knb) kn
J0(knr) = 1, 0≤r <1, (3.3.48)
and ∞
n=1
AnJ0(knr) = 0, 1≤r < a. (3.3.49) Let us reexpressAn as follows:
An = 1
√knJ12(kna) ∞ m=0
BmJ2m+1
2(km). (3.3.50) Substituting Equation 3.3.50 into Equation 3.3.49, we have25that
∞ n=1
AnJ0(knr) = ∞ m=o
Bm ∞
n=1
J2m+1
2(km)J0(knr)
√knJ12(kna)
= 0 (3.3.51)
for 1 < r ≤ a. Therefore, Equation 3.3.49 is satisfied identically with this definition ofAn.
Next, we substitute Equation 3.3.50 into Equation 3.3.48, multiply both sides of the resulting equation by r F2 1
−s, s+12,1, r2 dr/√
1−r2 and in- tegrate betweenr= 0 and r= 1. We find that
∞ m=0
Cm,sBm=
√2 Γ(s+ 1) Γ
s+12 1
0
√ r
1−r22 1F
−s, s+12,1, r2
dr (3.3.52)
= 2/π, s= 0,
0, s >0, (3.3.53)
where
Cm,s= ∞ n=1
coth(knb)J2m+1
2(kn)J2s+1 2(kn)
k2nJ12(kna) (3.3.54) ands= 0,1,2, . . .. We used
J2s+1 2(kn)
√kn
=
√2 Γ(s+ 1) Γ
s+12 1
0
√ r
1−r22 1F
−s, s+12,1, r2
J0(knr)dr.
(3.3.55) Equation 3.3.55 is now solved to yieldBm. Next, we computeAnfrom Equa- tion 3.3.50. Finallyu(r, z) follows from Equation 3.3.47.
Figure 3.3.4 illustrates the solution to Equation 3.3.43 through Equa- tion 3.3.46 when a = 2 and b = 1. As suggested by Galceran et al.,26 the
25 Tranter, op. cit.
26 Galceran, Cecilia, Companys, Salvador, and Puy, op. cit.
0 0.5 1 1.5 2 0
0.2 0.4
0.6 0.8
1 0
0.2 0.4 0.6 0.8 1
z r
u(r,z)
Figure 3.3.4: The solution to Equation 3.3.43 through Equation 3.3.46 whena= 2 and b= 1.
computation ofBm is assisted by noting that Cm,s=
∞ n=1
[coth(knb)−1]J2m+1
2(kn)J2s+1 2(kn) k2nJ12(kna)
−∞
n=1
J2m+1
2(kn)J2s+1 2(kn)
kn2J12(kna) (3.3.56)
= ∞ n=1
[coth(knb)−1]J2m+1
2(kn)J2s+1 2(kn)
k2nJ12(kna) (3.3.57)
−a2 2
δms
4s+ 1 −2(−1)m+s π
∞
0
K0(t) t I0(t)I2m+1
2
t a
I2s+1
2
t a
dt
, where δms is the Kronecker delta and we used results from a paper27 by Tranter to replace the second summation in Equation 3.3.56 with an integral.
•Example 3.3.5
Let us find28 the electrostatic potential due to a parallel plate condenser that lies within a hollow cylinder of radiusa >1 that is grounded and infinitely long. SeeFigure 3.3.5. The governing equations are
∂2u
∂r2 +1 r
∂u
∂r +∂2u
∂z2 = 0, 0≤r < a, −∞< z <∞, (3.3.58) subject to the boundary conditions
rlim→0|u(r, z)|<∞, u(a, z) = 0, −∞< z <∞, (3.3.59)
27 Tranter, op. cit.
28 See Singh, B. M., 1973: On mixed boundary value problem in electrostatics. Indian J. Pure Appl. Math.,6, 166–176.
z
r z = h
z = 0 1
a
Figure 3.3.5: Schematic of a hollow cylinder containing discs atz= 0 andz=h.
u(r,0−) =u(r,0+), u(r, h−) =u(r, h+), 0≤r < a, (3.3.60) u(r,0) =G(r), u(r, h) =F(r), 0≤r <1,
uz(r,0−) =uz(r,0+), uz(r, h−) =uz(r, h+), 1< r < a, (3.3.61) and
|zlim|→∞u(r, z)→0, 0≤r < a. (3.3.62) Here, G(r) and F(r) denote the prescribed potential on the discs at z = 0 andz = h, respectively. The parameters h+ and h− denote points that are slightly above or belowh, respectively.
Separation of variables yields the potential, namely u(r, z) =
∞ n=0
Ane−kn(z−h)J0(knr) kn
, h≤z <∞, (3.3.63)
u(r, z) = ∞ n=0
An
sinh(knz) sinh(knh)+Bn
sinh[kn(h−z)]
sinh(knh)
J0(knr) kn
, 0≤z≤h, (3.3.64) and
u(r, z) = ∞ n=0
BneknzJ0(knr) kn
, −∞< z≤0, (3.3.65)
where kn is the nth positive root of J0(ka) = 0. Equation 3.3.63 through Equation 3.3.65 satisfy Equation 3.3.58, Equation 3.3.59, Equation 3.3.60, and Equation 3.3.62. Substituting Equation 3.3.63 through Equation 3.3.65 into Equation 3.3.61, we obtain the following system of simultaneous dual series equations:
∞ n=0
An
J0(knr) kn
=F(r), 0≤r <1, (3.3.66) ∞
n=0
Bn
J0(knr) kn
=G(r), 0≤r <1, (3.3.67) ∞
n=0
[1 + coth(knh)]An− Bn
sinh(knh)
J0(knr) = 0, 1< r < a, (3.3.68) and
∞ n=0
[1 + coth(knh)]Bn− An
sinh(knh)
J0(knr) = 0, 1< r < a. (3.3.69) For 0≤r <1, let us augment Equation 3.3.68 and Equation 3.3.69 with
∞ n=0
[1 + coth(knh)]An− Bn
sinh(knh)
J0(knr) =−1 r
d dr
1
r
t g(t)
√t2−r2dt
, (3.3.70) and
∞ n=0
[1 + coth(knh)]Bn− An
sinh(knh)
J0(knr) =−1 r
d dr
1
r
t h(t)
√t2−r2dt
, (3.3.71) whereg(t) andh(t) areunknownfunctions.
Taken together, Equation 3.3.68 through Equation 3.3.71 are a Fourier- Bessel series over the interval 0≤r < a. From Equation 1.4.16 and Equation 1.4.17, it follows that
[1 + coth(knh)]An− Bn
sinh(knh)
=− 2
a2J12(kna) 1
0
d dr
1
r
t g(t)
√t2−r2 dt
J0(knr)dr (3.3.72)
=− 2
a2J12(kna) 1
r
t g(t)
√t2−r2dt
J0(knr) 1
0
− 2 a2J12(kna)
1
0
knJ1(knr) 1
r
t g(t)
√t2−r2dt
dr (3.3.73)
= 2
a2J12(kna) 1
0
g(t)dt
− 2kn
a2J12(kna) 1
0
t g(t) t
0
J1(knr)
√t2−r2dr
dt. (3.3.74)
Now, t
0
J1(knr)
√t2−r2dr= 1
0
J1(kntη) 1−η2dη=π
2J21
2(knt/2) = 1−cos(knt)
knt , (3.3.75) where we used tables29to evaluate the integral. Therefore,
[1 + coth(knh)]An− Bn
sinh(knh) = 2 a2J12(kna)
1
0
g(t) cos(knt)dt. (3.3.76) In a similar manner,
[1 + coth(knh)]Bn− An
sinh(knh)= 2 a2J12(kna)
1
0
h(t) cos(knt)dt. (3.3.77) Solving forAn andBn, we find that
An = 1
a2J12(kna) 1
0
g(t) cos(knt)dt+e−knh 1
0
h(t) cos(knt)dt
, (3.3.78) and
Bn = 1
a2J12(kna) 1
0
h(t) cos(knt)dt+e−knh 1
0
g(t) cos(knt)dt
. (3.3.79) Substituting An and Bn into Equation 3.3.66 and 3.3.67 and interchanging the order of integration and summation,
1
0
g(t) ∞
n=0
J0(knr) cos(knt) a2knJ12(kna)
dt
+ 1
0
h(t) ∞
n=0
e−knhJ0(knr) cos(knt) a2knJ12(kna)
dt=F(r), (3.3.80)
and 1
0
h(t) ∞
n=0
J0(knr) cos(knt) a2knJ12(kna)
dt
+ 1
0
g(t) ∞
n=0
e−knhJ0(knr) cos(knt) a2knJ12(kna)
dt=G(r). (3.3.81)
29 Gradshteyn and Ryzhik, op. cit., Formula 6.552.4.
It is readily shown30that 2
a2 ∞ n=0
J0(knr) cos(knt) knJ12(kna) =
∞
0
J0(rη) cos(tη)dη
− 2 π
∞
0
K0(aη)
I0(aη)I0(rη) cosh(tη)dη.(3.3.82) In a similar manner,
2 a2
∞ n=0
e−knhJ0(knr) cos(knt) knJ12(kna) =
∞
0
J0(rη) cos(tη)e−hηdη (3.3.83)
−2 π
∞
0
K0(aη)
I0(aη)I0(rη) cosh(tη) cos(hη)dη.
Substituting Equation 1.4.14, Equation 3.3.82, and Equation 3.3.83 into Equa- tion 3.3.80 and Equation 3.3.81, we have that
r 0
√g(t)
r2−t2dt= 2 π
1
0
g(t) ∞
0
K0(aη)
I0(aη)I0(rη) cosh(tη)dη
dt
−2 π
1
0
h(t) ∞
0
K0(aη)
I0(aη)I0(rη) cosh(tη) cos(hη)dη
dt
− 1
0
h(t) ∞
0
J0(rη) cos(tη)e−hηdη
dt+ 2F(r), (3.3.84) and r
0
√h(t)
r2−t2dt= 2 π
1
0
h(t) ∞
0
K0(aη)
I0(aη)I0(rη) cosh(tη)dη
dt
− 2 π
1
0
g(t) ∞
0
K0(aη)
I0(aη)I0(rη) cosh(tη) cos(hη)dη
dt
− 1
0
g(t) ∞
0
J0(rη) cos(tη)e−hηdη
dt+ 2G(r). (3.3.85) Equation 3.3.84 and Equation 3.3.85 are integral equations of the Abel type.
From Equation 1.2.13 and Equation 1.2.14, we find g(t) = 4
π d dt
t 0
r F(r)
√t2−r2dr
+ 4 π2
1
0
g(ξ) ∞
0
K0(aη)
I0(aη) cosh(tη) cosh(ξη)dη
dξ + 4
π2 1
0
h(ξ) ∞
0
K0(aη)
I0(aη) cosh(tη) cosh(ξη) cos(hη)dη
dξ
− 2 π
1
0
h(ξ) ∞
0
e−hηcos(ξη) cos(tη)dη
dξ, (3.3.86)
30 See Section 2.2 in Sneddon, op. cit.
0 0.5 1 1.5 2
−2
−1 0
1 2
3 4
−1
−0.5 0 0.5 1
r z
u(r,z)
Figure 3.3.6: The electrostatic potential within an infinitely long, grounded, and hollow cylinder of radius 2 when two discs with potential−1 and 1 are placed atz= 0 andz= 2, respectively.
and
h(t) = 4 π
d dt
t 0
r G(r)
√t2−r2dr
+ 4 π2
1
0
g(ξ) ∞
0
K0(aη)
I0(aη) cosh(tη) cosh(ξη) cos(hη)dη
dξ + 4
π2 1
0
h(ξ) ∞
0
K0(aη)
I0(aη) cosh(tη) cosh(ξη)dη
dξ
− 2 π
1
0
g(ξ) ∞
0
e−hηcos(ξη) cos(tη)dη
dξ. (3.3.87)
For a given F(r) and G(r), we solve the integral equations Equation 3.3.86 and Equation 3.3.87 forg(t) and h(t). Equation 3.3.78 and Equation 3.3.79 giveAnandBn. Finally, we can use Equation 3.3.63 through Equation 3.3.65 to evaluate the potential for any givenr and z. Figure 3.3.6 illustrates the electrostatic potential whenF(r) = 1,G(r) =−1, anda=h= 2.
•Example 3.3.6: Electrostatic problem
In electrostatics the potential due to a point charge located atr= 0 and z=hin the upper half-planez >0 above a grounded planez= 0 is
u(r, z) = 1
r2+ (z−h)2 − 1
r2+ (z+h)2. (3.3.88)
Let us introduce a unit circular hole atz = 0 and attach an infinite pipe to
(0,h) z
r (1,0)
Figure 3.3.7: Schematic of the spatial domain for which we are finding the potential in Example 3.3.6.
this hole. See Figure 3.3.7. Let us find the potential31 in this case.
The potential in this new configuration is
u(r, z) = 1
r2+ (z−h)2 − 1
r2+ (z+h)2 + 1 2i
1
−1
g(t)
r2+ (z+it)2dt (3.3.89) when 0≤r <∞and 0≤z <∞and
u(r, z) = ∞ n=1
AnJ0(knr)eknz (3.3.90) when 0≤r≤1 and−∞< z≤0. The integral32in Equation 3.3.89 vanishes when z = 0 and 1 ≤ r <∞. Here g(t) is an odd real-valued function and kn denotes the nth root of J0(k) = 0. To determine g(t) and the Fourier coefficients An, the potential and its normal derivative must be continuous across the aperture z = 0 and 0 ≤r ≤1. Mathematically these conditions are
u(r,0−) =u(r,0+), 0≤r≤1, (3.3.91) and
uz(r,0−) =uz(r,0+), 0≤r≤1. (3.3.92)
31 Taken from Shail, R., and B. A. Packham, 1986: Some potential problems associated with the sedimentation of a small particle into a semi-infinite fluid-filled pore. IMA J. Appl.
Math.,37, 37–66 with permission of Oxford University Press.
32 See Section 5.10 in Green, A. E., and W. Zerna, 1992: Theoretical Elasticity. New York: Dover, 457 pp.
Equation 3.3.91 yields ∞
n=1
AnJ0(knr) =− 1
r
√g(t)
t2−r2dt, 0≤r≤1. (3.3.93) In deriving Equation 3.3.93, we used
r2+ (z+it)2=ξeiη/2,
r2+ (z−it)2=ξe−iη/2, (3.3.94) withξ2cos(η) =r2+z2−t2,ξ2sin(η) = 2zt,ξ≥0, and 0≤η ≤π. On the other hand, Equation 3.3.92 gives
1 r
∂
∂r r
0
t h(t)
√r2−t2dt
= ∞ n=1
knAnJ0(knr)− 2h
(r2+h2)3/2, 0≤r≤1.
(3.3.95) Using Equation 1.2.13 and Equation 1.2.14, we can solve forg(t) in Equation 3.3.93 and find that
g(t) = 2 π
∞ n=1
knAn t
0
rJ0(knr)
√t2−r2dr−4h π
t 0
r
(t2−r2)(r2+h2)3dr (3.3.96)
= 2 π
∞ n=1
Ansin(knt)− 4t
π(t2+h2), 0≤t≤1. (3.3.97) We used Equation 1.4.9 and tables33 to evaluate the integrals in Equation 3.3.96.
Substituting the results from Equation 3.3.97 into Equation 3.3.95, we obtain
∞ n=1
AnJ0(knr) =−2 π
∞ n=1
An
1
r
sin(knt)
√t2−r2dt+4 π
1
r
t (t2+h2)√
t2−r2dt.
(3.3.98) The left side of Equation 3.3.98 is a Fourier-Bessel expansion. Multiplying both sides of this equation byrJ0(kmr) and integrating with respect torfrom 0 to 1, we find that
12J12(km)Am=−2 π
∞ n=1
An
1
0
rJ0(kmr) 1
r
sin(knt)
√t2−r2 dt
dr +4
π 1
0
rJ0(kmr) 1
r
dt (t2+h2)√
t2−r2
dr. (3.3.99)
33 Gradshteyn and Ryzhik, op. cit., Formula 2.252, Point II.
0 0.5
1 1.5
2 −2
−1 0
1 2 0
1 2 3 4 5 6 7 8 9 10
r z
u(r,z)
Figure 3.3.8: The electrostatic potential when a unit point charge is placed atr= 0 and z=hfor the structure illustrated inFigure 3.3.7.
Interchanging the order of integration in Equation 3.3.99 and evaluating the innerrintegrals, we finally achieve
12πJ12(km)Am+ ∞ n=1
An
sin(kn−km)
kn−km −sin(kn+km) kn+km
= 4 1
0
t
t2+h2sin(kmt)dt, (3.3.100) wherem= 1,2,3, . . .. To findAm, we truncate the infinite number of equa- tions given by Equation 3.3.100 to a finite number, sayM. As M increases the Am’s with a smaller m increase in accuracy. The procedure is stopped when the leadingAm’s are sufficiently accurate for the evaluation of Equation 3.3.89, Equation 3.3.90 and Equation 3.3.97. Figure 3.3.8 illustrates this so- lution whenh= 1 and the first 300 terms have been retained whenM = 600.