Understanding the effect of sheared flow on microinstabilities

The competition between the drive and stabilization of plasma microinstabilities by sheared flow is investigated, focusing on the ion temperature gradient mode. Using a twisting mode representation in sheared slab geometry, the characteristic equations have been formulated for a dissipative fluid model, developed rigorously from the gyrokinetic equation. They clearly show that perpendicular flow shear convects perturbations along the field at a speed we denote by $Mc_s$ (where $c_s$ is the sound speed), whilst parallel flow shear enters as an instability driving term analogous to the usual temperature and density gradient effects. For sufficiently strong perpendicular flow shear, $M >1$, the propagation of the system characteristics is unidirectional and no unstable eigenmodes may form. Perturbations are swept along the field, to be ultimately dissipated as they are sheared ever more strongly. Numerical studies of the equations also reveal the existence of stable regions when $M < 1$, where the driving terms conflict. However, in both cases transitory perturbations exist, which could attain substantial amplitudes before decaying. Indeed, for $M \gg 1$, they are shown to exponentiate $\sqrt{M}$ times. This may provide a subcritical route to turbulence in tokamaks.Comment: minor revisions; accepted to PPC

The effect of open-cell metal foams strut shape on convection heat transfer and pressure drop

The effect of strut shape on convection heat transfer and pressure drop in open-cell metal foams has been investigated numerically in this paper by introducing the foam shape factor, a parameter that characterizes the shape of the strut. The analysis has been carried out both on real and ideal foams. The geometry of three 40 PPI real foams, with 0.87, 0.94 and 0.96 average measured porosities, was determined with CT-tomography and a morphological analysis of the CT data was carried out. The geometry of the ideal foams, based on the Kelvin foam, with the same PPI and porosities as those of the real foams, was generated with the free-to-use software Surface Evolver, building up foams with different strut shapes. Governing equations have been solved with a finite element scheme, assuming a uniform heat flux condition at the solid/fluid boundary of the foam. Results are presented in terms of convection heat transfer coefficients and pressure distributions as a function of strut shape parameters. The comparison between predictions for ideal and real foams, for different strut shapes, confirms that, in all cases, the closer the form of the ideal strut to that of the real strut the better the agreement among predictions

Shear Flow and Kelvin-Helmholtz Instability in Superfluids

The first realization of instabilities in the shear flow between two superfluids is examined. The interface separating the A and B phases of superfluid 3He is magnetically stabilized. With uniform rotation we create a state with discontinuous tangential velocities at the interface, supported by the difference in quantized vorticity in the two phases. This state remains stable and nondissipative to high relative velocities, but finally undergoes an instability when an interfacial mode is excited and some vortices cross the phase boundary. The measured properties of the instability are consistent with the classic Kelvin-Helmholtz theory when modified for two-fluid hydrodynamics