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The aim of this paper is to provide an advanced analysis of the shear stresses exerted on vessel walls by the flowing blood, when a limb or the whole body, or a vessel prosthesis, a scaffold… is placed in an external static magnetic field
B
_{0}. This type of situation could occur in several biomedical applications, such as magnetic resonance imaging (MRI), magnetic drug transport and targeting, tissue engineering, mechanotransduction studies… Since blood is a conducting fluid, its charged particles are deviated by the Hall effect, and the equations of motion include the Lorentz force. Consequently, the velocity profile is no longer axisymmetric, and the velocity gradients at the wall vary all around the vessel. To illustrate this idea, we expand the exact solution given by Gold (1962) for the stationary flow of blood in a rigid vessel with an insulating wall in the presence of an external static magnetic field: the analytical expressions for the velocity gradients are provided and evaluated near the wall. We demonstrate that the derivative of the longitudinal velocity with respect to the radial coordinate is preponderant when compared to the
θ-derivative, and that elevated values of
B
_{0} would be required to induce some noteworthy influence on the shear stresses at the vessel wall.

The aim of this paper is to provide an advanced analysis of the shear stresses exerted on vessel walls by the flowing blood, when a limb or the whole body, or a vessel prosthesis, a scaffold… is placed in an external static magnetic field B_{0}.

As explained in [

・ magnetic resonance imaging (MRI) [

・ magnetic drug transport and targeting [

・ tissue engineering [

・ mechanotransduction studies and applications for regenerative medicine strategies (for example, with stem cells) [

This analysis would also provide a risk assessment for the vessel wall (plaque rupture in case of atherosclerotic lesion [

Since blood is a conducting fluid, its charged particles are deviated by the Hall effect thus inducing electrical currents and voltages along the vessel walls and in the neighboring tissues. The equations of motion include the Lorentz force j^B, where j is the electric current density. Consequently, the velocity profile is no longer axisymmetric, even in a cylindrical vessel; and the velocity gradients at the wall vary all around the vessel.

To illustrate this idea, we chose to expand the exact solution given by Gold [

As explained by Gold [_{0}.

where u and P are the fluid velocity and pressure; μ is the magnetic permeability; ρ, η and σ are the fluid density, viscosity and conductivity and the electric current density is expressed as j = ( c u r l B ) / μ .

Gold [

velocity and magnetic field are defined in the cylindrical frame (e_{r}, e_{θ}, e_{z}) as:

The induced magnetic field, B_{I}, is parallel to the flow and guarantees div B = 0 . The continuity equation div u = 0 is also satisfied.

The longitudinal projections (along e_{z}) of Equations ((1) and (2)) in the cylindrical frame are thus:

The boundary conditions associated with this problem are:

and u z ( R , θ ) = 0 , because of the no-slip condition at the rigid wall (6b)

The equation system (4) to (6) is expressed in a non-dimensional form, using the following definitions:

U ˜ = u z u 0 , _{0} is some characteristic mean velocity).

The numerical values of the different quantities are taken from Abi-Abdallah et al. [

η = 4 × 10 − 3 Pa ⋅ s , u 0 = 0.4 m / s , ∂ P / ∂ z = − 128 Pa / m , and R = 0.01 m ; then G equals +8.

The non-dimensional solution given by Gold [

and

U ˜ ( r ˜ , θ ) = G 2 H a [ E 1 ( r ˜ , θ ) ) A 1 ( r ˜ , θ ) + E 2 ( r ˜ , θ ) A 2 ( r ˜ , θ ) ] (8)

with:

E 1 ( r ˜ , θ ) = e − H a 2 r ˜ cos θ ,

A 1 ( r ˜ , θ ) = α 0 I 0 ( H a 2 r ˜ ) + ∑ n = 1 ∞ 2 α n I n ( H a 2 r ˜ ) cos ( n θ ) ,

E 2 ( r ˜ , θ ) = e H a 2 r ˜ cos θ ,

A 2 ( r ˜ , θ ) = α 0 I 0 ( H a 2 r ˜ ) + ∑ n = 1 ∞ 2 ( − 1 ) n α n I n ( H a 2 r ˜ ) cos ( n θ ) ,

and

α 0 = I ′ 0 ( H a 2 ) I 0 ( H a 2 ) ; α n = I ′ n ( H a 2 ) I n ( H a 2 ) (9)

The Hartmann number, H_{a}, is defined as: H a = B 0 R σ η , the magnetic Reynolds number, R_{em}, as: R e m = R u 0 σ μ , and the functions I_{n} are the n^{th} order modified Bessel functions of the first kind.

In order to evaluate the derivatives of the I_{n} functions, the following identities are used:

I ′ 0 ( X ) = I 1 ( X ) and I ′ n ( X ) = I n − 1 ( X ) − n X I n ( X ) (10).

In such flow configuration, the classical definition of the dimensional shear stresses would yield:

τ r z = η ∂ u z ∂ r ; τ θ z = η r ∂ u z ∂ θ (11)

The corresponding non-dimensional expressions would be: τ ˜ r z = ∂ U ˜ ∂ r ˜ ; τ ˜ θ z = 1 r ˜ ∂ U ˜ ∂ θ , where τ ˜ r z = τ r z τ 0 ; τ ˜ θ z = τ θ z τ 0 , and τ 0 = η u 0 R ( τ 0 = 0. 16 Pa , with the numerical data of this study).

It is thus necessary to calculate the velocity gradients (from Equation (8)). This can be done as follows:

∂ E 1 ∂ r ˜ = − H a 2 cos θ e − H a 2 r ˜ cos θ and ∂ E 2 ∂ r ˜ = H a 2 cos θ e H a 2 r ˜ cos θ (12)

∂ A 1 ∂ r ˜ = H a 2 [ α 0 I 1 ( H a 2 r ˜ ) + ∑ n = 1 ∞ 2 α n cos ( n θ ) ( I n − 1 ( H a 2 r ˜ ) − 2 n H a r ˜ I n ( H a 2 r ˜ ) ) ] (13)

∂ A 2 ∂ r ˜ = H a 2 [ α 0 I 1 ( H a 2 r ˜ ) + ∑ n = 1 ∞ 2 ( − 1 ) n α n cos ( n θ ) ( I n − 1 ( H a 2 r ˜ ) − 2 n H a r ˜ I n ( H a 2 r ˜ ) ) ] (14)

∂ E 1 ∂ θ = H a 2 r ˜ sin θ e − H a 2 r ˜ cos θ and ∂ E 2 ∂ θ = − H a 2 r ˜ sin θ e H a 2 r ˜ cos θ (15)

∂ A 1 ∂ θ = − ∑ n = 1 ∞ 2 n α n I n ( H a 2 r ˜ ) sin ( n θ ) and ∂ A 2 ∂ θ = ∑ n = 1 ∞ 2 n ( − 1 ) n + 1 α n I n ( H a 2 r ˜ ) sin ( n θ ) (16)

Gathering all, one obtains:

∂ U ˜ ∂ r ˜ = G 2 H a [ ∂ E 1 ∂ r ˜ A 1 + E 1 ∂ A 1 ∂ r ˜ + ∂ E 2 ∂ r ˜ A 2 + E 2 ∂ A 2 ∂ r ˜ ] (17)

and:

∂ U ˜ ∂ θ = G 2 H a [ ∂ E 1 ∂ θ A 1 + E 1 ∂ A 1 ∂ θ + ∂ E 2 ∂ θ A 2 + E 2 ∂ A 2 ∂ θ ] (18)

The term ∂ U ˜ ∂ r ˜ ( 1 , θ ) , 0 < θ < 2 π , represents the change of velocity in the radial direction, all around the vessel wall.

The term ∂ U ˜ ∂ θ ( r ˜ , θ ) represents the change of velocity in the azimuthal direction, at a given value of r ˜ . The velocity is zero everywhere at the wall (r = R); consequently the velocity gradient ∂ U ˜ ∂ θ ( 1 , θ ) is also zero. The interesting quantities are thus ∂ U ˜ ∂ θ ( r ˜ , θ ) , for r ˜ close to 1, but lower than 1.

The dependence of the non-dimensional velocity U ˜ upon θ (for – π < θ < + π ) is presented in _{a} = 0), the situation is axisymmetric and the velocity does not depend upon θ. The flow is the classical Poiseuille flow and, as expected, U ˜ ( r ˜ = 0.5 ) = 3 / 2 . When the Hartmann number increases, the flow is furthermore reduced (this is the decelerating effect due to the Lorentz force), and the dependence upon θ (asymmetry of the flow) is more and more pronounced. The velocity is maximal in the direction θ = 0 and θ = π (or −π), according to the fact that the profile is flattened and stretched parallel to the direction of B_{0} (along Ox) [

The same type of results is shown in _{a} = 0.16 is superimposed with the curve H_{a} = 0, meaning that the influence of a magnetic field B_{0} =1.5 T (corresponding to H_{a} = 0.16) remains negligible. Moreover, when H_{a} = 0, the

value obtained for the non-dimensional velocity at r = 0.99 * R is 0.0398, which is the Poiseuille value.

The dependence of the non-dimensional velocity gradient ∂ U ˜ / ∂ r ˜ upon θ (for – π < θ < + π ), at r ˜ = 0.99 is illustrated in _{a} = 0.16) is not discernible, and the absolute values of the gradients ∂ U ˜ / ∂ r ˜ are maximum for θ = 0, and θ = π, or −π (due to the fact that the profile is stretched along Ox).

In the absence of a magnetic field (H_{a} = 0), the Poiseuille value ( ∂ U ˜ / ∂ r ˜ = − 4 , at the wall) is obtained, and no dependence upon θ is observed (axisymmetric situation). The maximum values of ∂ U ˜ / ∂ r ˜ are increased by about 25% in the case of a very strong magnetic field (H_{a} = 4.47, B_{0} = 40 T), when compared to the case H_{a} = 0.

The dependence of the non-dimensional velocity gradient ∂ U ˜ / ∂ θ upon θ (for – π < θ < + π ), at r ˜ = 0.5 and at r ˜ = 0.99 is illustrated in

In the absence of a magnetic field (H_{a} = 0), the situation is axisymmetric, and there is no dependence upon θ.

The non-dimensional shear stress, τ ˜ θ z , could be obtained dividing ∂ U ˜ / ∂ θ by the value of the corresponding r ˜ (Equation (11)). For example, if we look at the maximum value of τ ˜ θ z for r ˜ = 0.99 , we obtain | τ ˜ θ z | < 0.02 , which is negligible when compared to τ ˜ r z (

In this paper, we demonstrate that the quantities τ ˜ r z = ∂ U ˜ ∂ r ˜ and τ ˜ θ z = 1 r ˜ ∂ U ˜ ∂ θ

both depend upon θ, but that this dependence may be considered negligible for low values of B_{0} (B_{0} < 3 T). We also demonstrate that, at the vessel wall, τ ˜ θ z is several orders of magnitude smaller than τ ˜ r z , and that, in the presence of a very strong magnetic field (H_{a} = 4.47, B_{0} = 40 T), the maximum value of τ ˜ r z is only increased by 25%, when compared to its value in the absence of a magnetic field (H_{a} = 0). Consequently, in most of the situations encountered in biomedical applications, the classical calculation (η(∂u/ ∂r)) remains a good approximation to evaluate the shear stresses at the wall.

Competing interests: none.

Funding: none.

Ethical approval: not required.

Drochon, A, Beuque, M. and Abi-Abdallah Rodriguez, D. (2017) Impact of an External Magnetic Field on the Shear Stresses Exerted by Blood Flowing in a Large Vessel. Journal of Applied Mathematics and Physics, 5, 1493- 1502. https://doi.org/10.4236/jamp.2017.57122