INTRODUCTION
The Unsteady Magnetohydrodynamic flow between two parallel porous plates is a classical problem whose solution has many applications in magnetohydrodynamic (MHD) power generators, cooling system, aerodynamics heating, polymer technology, petroleum industry, centrifugal separation of matter from fluid, purification of crude oil and fluid droplets sprays.
Hassanien and Mansour (1990) discussed the Unsteady magnetic flow through a porous medium between two infinite parallel plates.
Bagchi (1996) studied the problem of Unsteady flow of viscoelastic Maxwell fluid with transient pressure gradient through a rectangular channel.
Attia and Kotb (1996) studied the Steady, fully developed MHD flow and heat transfer between two parallel plates with temperature dependant viscosity.
Attia (1999) extended the problem to the transient state.
Ezzat et al. (1999) studied the problem of micropolar magnetohydrodynamic boundary layer flow.
AboulHassan and Attia (2002) discussed the flow of a conducting Visco elastic fluid between two horizontal porous plates in the presence of a transversemagnetic field.
Nabil et al. (2003) studied the MHD flow of NonNewtonian viscoelastic fluid through a porous medium near an accelerated plate.
Attia (2004) has considered the Unsteady Hartmann flow with heat transfer of a viscoelastic fluid considering the Hall effect.
Hayat et al. (2004) studied the Hall effects on the Unsteady hydromagnetic oscillatory flow of a secondgrade fluid.
Krishnambal and Ganesh (2004) discussed the Unsteady stokes flow of viscous fluid between two parallel porous plates.
Attia (2005a) studied the Unsteady laminar flow of an incompressible viscous fluid and heat transfer between two parallel plates in the presence of a uniform suction and injection considering variable properties.
Attia (2005b) studied the Unsteady flow of a dusty conducting fluid between parallel porous plates with temperature dependent viscosity.
The objective of this study is to analyse the Unsteady Magnetohydrodynamic Stokes flow of viscous fluid between two parallel porous plates when the fluid is being withdrawn through both the walls of the channel at the same rate. The problem is reduced to a third order nonlinear differential equation which depends on a Suction. Reynolds number R and a Hartmann number M for which an exact solution is obtained.
MATHEMATICAL FORMULATION
The unsteady laminar flow of an incompressible viscous fluid between two parallel
porous plates is considered in the presence of a transverse magnetic field Ho
applied perpendicular to the walls. The origin is taken at the centre of the
channel and let x and y be the coordinate axes parallel and perpendicular to
the channel walls.
The length of the channel is assumed to be L and 2 h is the distance between the two plates. Let u and v be the velocity components in the x and y directions, respectively.
The equation of continuity is
Equations of momentum are
Where σ is the electrical conductivity and B_{0 } = μ_{e}H_{0, }μ_{e} being the magnetic permeability.
The boundary conditions are u (x , h) = 0, u ( x, h ) = 0, v (x, h) = v_{0 }and v ( x, h) =  v_{0 } where v_{0} is the velocity of suction at the walls of the channel.
Let η = y/h, u = u(x,y) e^{iωt}, v = v(x,y) e^{iωt}, p = p(x,y)e^{iωt} and the Eq. 13 become
Let v = μ/ρ = Kinematic Viscosity, ρ the density of the fluid, μ the coefficient of viscosity and p the pressure.
The boundary conditions are converted into
and
Let Ψ be the stream function such that
The equation of continuity can be satisfied by a stream function of the form
Where u(0) is the average entrance velocity at x = 0 . From Eq.
11, the velocity components (9) and (10) are given by
where the prime denotes the differentiation with respect to the dimensionless variable η = y/h. Since the fluid is being withdrawn at constant rate from both the walls, v_{0} is independent of x.
Using (12) and (13) in (5) and (6), the equations of momentum reduce to
Now differentiating (15) w.r.t.’x’, we get
Differentiating (14) w.r.t. ‘η’ , we get
From (16), Eq. 17 can be written as
which is true for all x.
Let R = Suction Reynolds number = hv_{0}/γ
M = Hartmann number =
Integrating (18) w.r.t. η and substituting the above expressions we get
Where
and K is an arbitrary constant.
Boundary conditions on f(η) are
Hence the solution of the equations of motion and continuity is given by a
non linear third order differential Eq. (19) subject to the
boundary conditions (20).
∴ Equation (19) can be rewritten as
where a_{1}R = M^{2 } and
D = d/dη and D^{2} = d^{2}/dη^{2}
Solving the above equation, we get 
Applying the boundary conditions on f(η) and solving the values of the arbitrary constants, we get
Substituting the values of the arbitrary constants in f(η), we get
Hence the expressions for the velocity components are
Pressure distribution: From Eq (14), we have
and since (from
19)
we have
Now, from Eq. (15) we have
Integrating (31), we get
∴ The pressure drop is given by
DISCUSSION
The graphs of the axial velocity and radial velocity profiles have been drawn for different values of M.
Figures 14 represents the axial velocity
profiles at different cross sections of the channel namely at x = 0, x = 3,
x = 4 and x = 5 when the average entrance velocity is u_{o} = 0.5 and
h = 1.0 . The magnitude of the axial velocity increases as x increases from
x = 0 to x = 5 for different values of ωt, namely ωt = 0, π/4,
π/2, 3π/4 and π, respectively.

Fig. 1: 
Axial velocity profiles when u0 = 0.5, v0 = 0.5, h = 1.0,
M = 1.0, α = 1.0, x = 0 and for different values of ωt 

Fig. 2: 
Axial velocity profiles when u0 = 0.5, v0 = 0.5, h = 1.0,
M = 1.0, α = 1.0, x = 3 and for different values of ωt 

Fig. 3: 
Axial velocity profiles when u0 = 0.5, v0 = 0.5, h = 1.0,
M = 1.0, α = 1.0, x = 4 and for different values of ωt 

Fig. 4: 
Axial velocity profiles when u0 = 0.5, v0 = 0.5, h = 1.0,
M = 1.0, α = 1.0, x = 5 and for different values of ωt 

Fig. 5: 
Axial velocity profiles when u0 = 1.0, v0 = 0.5, h = 1.0,
M = 1.0, α = 1.0, x = 0 and for different values of ωt 

Fig. 6: 
Axial velocity profiles when u0 = 0.5, v0 = 0.5, h = 1.0,
M = 2.0, α = 1.0, x = 0 and for different values of ωt 

Fig. 7: 
Axial velocity profiles when u0 = 0.5, v0 = 0.5, h = 1.0,
M = 5.0, α = 1.0, x = 0 and for different values of ωt 

Fig. 8: 
Radial velocity profiles when v0 = 0.5, h = 1.0, M = 1.0,
α = 1.0 and for different values of ωt 

Fig. 9: 
Radial velocity profiles when v0 = 0.5, h = 1.0, M = 5.0,
α = 1.0 and for different values of ωt 
The Fig. 5 represents the axial velocity profiles of u at x = 0, h = 1.0 when the inlet velocity is increased to u_{0 }= 1.0 from u_{0} = 0.5. It is clearly seen that the magnitudes of the axial velocity u are more when the values of x are increased and also the magnitudes of the axial velocity u are more when the inlet velocity is increased.
When the Hartmann number M is increased from M = 1 to M = 2 or M = 5, we see that the magnitude of the axial velocity profiles decreases as seen in Fig. 6 and 7.
The Fig. 8 and 9 represent the radial velocity
profiles of v at vo = 0.5, h = 1.0, α = 1.0 and for different values of
M. As M increases from M = 1.0 to M = 5.0 we see that there is a marginal increase
in the magnitude of the radial velocity. It is also seen from the Fig.
8 and 9 that the radial velocity vanishes for ωt
= π/2 and the radial velocity profiles are non linear for the other values
of ωt.
Special case: If the distance between the two plates is assumed to be h, we get the solution as,
where the velocity components are
and
The above result reduces to the result of (Krishnambal and Ganesh, 2004) when
the Hartmann number is zero (i.e., when M = 0) and a = 2.
where a = 1  v_{1}/v_{2}, 0<v_{1}<v_{2}
Here a = 1  v_{0}/v_{0}, = 2.
CONCLUSIONS
In the above analysis a class of solutions of unsteady magnetohydrodynamics stokes flow of viscous fluid between two parallel porous plates is presented, in the presence of a transverse magnetic field when the fluid is being withdrawn through both the walls of the channel at the same rate.