Dispersed-phase structure in sheared MHD turbulence

Abstract

Direct numerical simulations are performed to examine particle dispersion in homogeneous turbulence subjected to mean shear and an externally applied magnetic field. We employ the Brucker et al. algorithm for simulating mean shear without the remeshing required in the traditional Rogallo scheme. We demonstrate that the Rogallo approach requires higher grid resolution to match the Brucker et al. approach at high shear rates. Both the shear and the magnetic field are uniform. The applied magnetic field is aligned with the direction normal to the plane of the mean shear. Results are presented for three values of the ratio, M, of the mean shear time scale to the Joule time scale τ _m . The dispersed particles all have response times near unity when scaled on the Kolmogorov time scale at the instant of magnetic field application. We find that the dispersed-phase structural anisotropy deviates noticeably from the anisotropy of the velocity field structure. The dispersed-phase anisotropy is determined by M and by the magnetic Reynolds number Re_m . When M is small, the mean shear dominates and the particle distribution appears isotropic even though the turbulent eddies align preferentially along the streamwise direction. When is much larger than τ _m , sheets of particles align along both streamwise and spanwise directions. Stronger particle clustering in the streamwise direction is observed when Re_m =20, indicating that the magnetic effect on dispersed-phase structure is less significant at moderately higher magnetic Reynolds numbers. When is comparable to τ _m , the particles disperse broadly across the flow at Re_m =1 because the mean shear and the magnetic field are equally effective in inducing dispersed-phase anisotropy. Particle clustering along the mean streamwise direction is obtained when Re_m =20 due to weaker influence of the magnetic field.

Keywords:

• direct numerical simulation
• sheared MHD turbulence
• dispersed-phase anisotropy

Acknowledgments

The first author acknowledges supported from the Office of Naval Research (contract N00014-08-1-0086) via the Research Foundation of the City University of New York. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC04-94AL85000. SCK and HR gratefully acknowledge partial support under the contract of association ERB 5005 CT 99 0100 between the European Atomic Energy Community and the Hellenic Republic.