Abstract:A theoretical solution for an oscillatory flow of an electrically conducting, viscous, incompressible visco-elastic fluid is obtained without neglecting Hall current. The magnetohydrodynamic (MHD) flow is bounded by two infinite horizontal plates filled with porous medium. The fluid is injected with constant velocity through the lower stationary plate and the upper plate is subjected to the same constant suction velocity. The effects of Hall current and Hartmann numbers on velocity profile and shear stress for different values of the visco-elastic parameter with the combination of the other flow parameters are illustrated graphically and physical aspects of the problem are discussed.
Keywords: visco-elastic, oscillatory, porous channel, porous medium, MHD, Hall current.
The study of flows through porous medium have stimulated considerable interest due its applications in the fields of agricultural engineering for irrigation processes; in petroleum technology to study petroleum transport; in chemical engineering for filtration and purification processes. Raptis  investigated the unsteady two-dimensional flow through a porous medium bounded by an infinite porous plate subjected to a constant suction and variable temperature. Further, Raptis and Perdikis  studied the problem of free convective flow through a porous medium bounded by a vertical porous plate with constant suction when the free stream velocity oscillates in time about a constant mean value. Singh et al.  investigated the problem of heat transfer in three dimensional flow through a porous medium with periodic permeability. Singh and Verma  studied further the three dimensional oscillatory flow through a porous medium where the free stream velocity oscillates in time about a non-zero constant mean. Singh et al.  studied the effect of permeability variation on the heat transfer and three dimensional flow through a highly porous medium bounded by an infinite porous plate with constant suction. Attia  studied the Hall current effect on the velocity and temperature fields of an unsteady Hartmann number. Singh and Mathew  studied the injection/suction effect on a hydromagnetic oscillatory flow in a horizontal porous channel in a rotating system. Choudhury and Das  extended the problem studied by Singh and Mathew  to the case of visco-elastic fluid. Singh and Kumar  investigated the problem of on oscillatory MHD flow through a porous medium bounded by rotating porous channel in the presence of Hall current. The aim of the present investigation is to study the effect of the Hall current on the visco-elastic fluid flow when the porous horizontal channel filled with a porous medium is rotating about an axis normal to the planes of the plates.
2. Mathematical Analysis
Consider an unsteady oscillatory flow of an electrically conducting visco-elastic, incompressible second order fluid through a porous medium bounded between two insulated infinite parallel porous plates distance d apart. The fluid is injected with constant velocitythrough the lower stationary plate and is being sucked with the same velocity through the upper plate which is oscillating in its own plane with a velocity about a non-zero constant mean velocity . A coordinate system is taken with the origin at the lower stationary plate lying in plane and -axis parallel to the direction of motion of the upper plate. The -axis taken perpendicular to the planes of the plates, is the axis of the rotation about which the entire system is rotating with constant angular velocity. A strong magnetic field of uniform strength is applied along - axis. The magnetic Reynolds number is considered to be small so that the induced magnetic field is neglected. Since the plates are infinite in extent, all the physical quantities except the pressure depend only on and for this fully developed laminar flow.
The constitutive equation for the incompressible second order fluid is of the form
where is the stress tensor, (n=1, 2) are the kinematic Rivlin-Ericksen tensors; are the material coefficients describing the viscosity, elasticity and cross-viscosity respectively. The material coefficients are taken constants with and as positive and as negative (Coleman and Markovitz ). The equation (1) was derived by Coleman and Noll  from that of the simple fluids by assuming that stress is more sensitive to the recent deformation than to the deformation that occurred in the distant past.
Denoting the velocity components in the directions, respectively, the continuity equation gives on integration and solenoidal relation for the magnetic field gives (constant) everywhere in the flow field. The physical configuration of the problem is shown in Figure 1.
Figure1: Physical configuration of the problem.
The equation of conservation of electric charge gives =constant. This constant is zero i.e. at the plates which are electrically non-conducting. Taking Hall current into account the generalized Ohm’s law (Cowling ) is of the form
where is the velocity vector, is the magnetic field, is the current density, is the electric field, is the electric conductivity, is the magnetic permeability, is the cyclotron frequency and is the electron collision time.
For the and components of Ohm’s law (2), for large magnetic field, which include Hall current, are
Since the external electric field arising due to polarization of charges is negligible.
Hence Therefore, solving for and we get
Under the above assumptions, the governing equations for the flow, in presence of Hall current, are as follows: (3)
where are the kinematic viscosity, is the time, is the density, is the pressure and is the permeability of the porous medium, is the Hall parameter.
The boundary conditions for the problem are
where is the frequency of oscillations, is the mean velocity and is a very small positive constant.
Eliminating the pressure gradient, under the usual boundary layer approximations, equations (3) and (4) reduces to
Introducing the following non-dimensional quantities
the rotation parameter, , the frequency parameter, is the injection/suction parameter, is the permeability parameter, is the Hartmann number, and into the equations (6) and (7), we get
subject to boundary conditions
Equations (8) and (9) can now be combined into a single equation, by introducing the complex function , as
subject to the boundary conditions
where , the visco-elastic parameter and
3. Solution of the problem
In order to solve equation (11) subject to the boundary conditions (12), we look for a solution of the form
Substituting (13) into the equations (11) and (12) and comparing the harmonic and non-harmonic terms, we get
where primes denote differentiation with respect to.
The corresponding transformed boundary conditions are
To solve the equations (14) to (16) under boundary conditions (17), we consider very small value of non-Newtonian parameter, and substituting
into equations (14) to (16) and boundary conditions (18) up to first order of and equating the coefficients of like powers of , we obtain the following sets of ordinary differential equations and corresponding boundary conditions:
Solving the equations (19), (21), (23) under the boundary conditions (20), (22), (24) respectively and substituting these values in (18), we get the solutions for velocity. The solutions and constants of the differential equations are obtained but not presented here for the sake of brevity.
4. Results and discussion
Now for the resultant velocities and the shear stress of the steady and unsteady flow, we write
The steady part consists of as the primary and as the secondary velocity components. The amplitude and phase difference due to these primary and secondary velocities for the steady flow are given by
Figures 2 and 3 depict the resultant velocity for the steady and unsteady part of the flow against to observe the visco-elastic effects for the various values of Hall current and Hartmann number. It is observed from figures 2 and 3 that both and increase rapidly
Figure 2: Variation of resultant velocity against with
Figure 3: Variation of resultant velocity against with
from zero near the stationary plate for Newtonian as well as non-Newtonian cases. It is evident from the Figures 2 and 3 that both and increase with the increase of Hartmann number for both Newtonian and non-Newtonian cases. But these results decrease with the increase with the increase of Hall current
The amplitude and the phase difference of the shear stress at the stationary plate for the steady flow can be obtained as,
Here are, respectively, the shear stresses at the stationary plate due to the primary and the secondary velocity components.
Table 1: Values of andfor various values of with .
Table 1 exhibits the effects of the visco-elastic parameter on the amplitude and the phase difference of the shear stress at the stationary plate for the steady part of the flow with the combination of the other flow parameters and It is observed from the table 1 that the values of increase but decrease with the increasing values of the non-Newtonian parameter This table shows that increase with the increase of Hartmann number but decrease with the increase of Hall current for both Newtonian as well as non-Newtonian cases.
The solutions of and together give the unsteady part of the flow. The unsteady primary and secondary velocity components and , respectively, for the fluctuating flow can be obtained as
The resultant velocity or amplitude and the phase difference of the unsteady flow are given by
For the unsteady part of the flow, the amplitude and phase difference of shear stresses at the stationary plate () can be obtained as
Table 2: Values of andfor various values of with .
The amplitude and phase difference of the unsteady shear stresses at the stationary plate have been listed in Table 2. It is observed from the Table 2 that the values of decrease with the increase of Hall current But increase with the increase of Hartmann number for both Newtonian as well as non-Newtonian cases. The phase difference increase with the increase of , but decrease with the increase of for both Newtonian and non-Newtonian cases.
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