Singularities of Hartmann layers

Abstract

Abstract This paper is concerned with the parallel flow of conducting fluid along an insulating pipe of uniform cross-section perpendicular to which a uniform magnetic field, B0, is applied. The cross-section is supposed to have tangents parallel to B0 only at isolated points of its peri­meter. The density, kinematic viscosity and electrical conductivity of the fluid are denoted by ρ, v and σ, respectively. It is known (Shercliff 1962) that, in the limit B0 → ∞, the flow may be divided into three parts: (i) a Hartmann boundary layer, thickness ~ (ρv/σ)½(B0 cosθ)-1, at every point of the wall except those a t which cos θ = 0, where θ is the angle between B0 and the normal to the wall at the point concerned, (ii) a ‘mainstream’, far from the walls, which is controlled by the Hartmann layers and in which a quasi-hydrostatic balance subsists between the Lorentz force and the applied pressure gradient driving the motion, and (iii) a complicated boundary-layer singularity, at each point of the wall at which cos θ = 0, which is controlled by the flow in regions (i) and (ii). The solutions for regions (i) and (ii) can be obtained easily by Shercliff’s methods. It is the purpose of this paper to elucidate region (iii). Here the boundary-layer thickness is O(M2/3) and extends round the periphery of the wall for a distance which is where O(M-½ is a Hartmann number, B0L(σ/ρv)½, based on a typical dimension, L, of the pipe. The corresponding contribution to U, the mean flow down the duct, is of order M-2/3. In fact, for a circular duct of radius a( = L), the main case discussed in this paper, it contributes the final term to the following expression: U = 64/3π U0[1/M - 3π/2M2 + 3.273/M7η] Here U0 is the mean flow in the absence of field.