The liquid drop impacting onto a very thin film is investigated experimentally through time-resolved shadowgraph images for both miscible and immiscible liquids. Regime maps for five impact morphologies, including deposition, crown formation, crown breakdown, crown-type splashing, and crown-breakdown splashing, are reported and compared comprehensively. Besides the thresholds of splashing/non-splashing regimes, the divisions between crown-type and crown-breakdown splashing are also figured out. It is found that the immiscibility can promote splashing, especially the crown-breakdown type, mainly due to the interfacial instability. The crown-breakdown splashing can be suppressed by increasing film viscosity in immiscible systems, while the viscosity effect is not significant in other test cases. The statistics of crown and secondary droplets are measured and analyzed. It is found that the immiscibility can enlarge the crown diameter but decrease the height, while the effect of viscosity on the crown structure is minor. When the film viscosity is similar, more but smaller secondary droplets are generated in immiscible systems. With the increase in film viscosity, larger but less secondary droplets are observed in both miscible and immiscible systems.

A comparative study of the immiscibility effect on liquid drop impacting onto very thin films

The flow driven by electromagnetic forcing of an electrolytic fluid in a concentric sphere system with differential rotation is studied experimentally and theoretically in the laminar regime. The electromagnetic force is generated by the interaction of a direct current injected radially through electrodes located at the equatorial zone of the spheres and a dipolar magnetic field produced by a permanent magnet inside the inner sphere. Even though the flows are fully three-dimensional, under some assumptions, they can be mathematically modeled with a general one-dimensional solution from the incompressible Navier–Stokes equations. Additionally, in order to deepen the flow dynamics, a full three-dimensional numerical simulation is also obtained. Experimentally, velocity profiles in the equatorial plane between spheres were obtained with particle image velocimetry . The analytical solution reproduces qualitatively the experimental profiles, whereas the numerical solution reproduces quantitatively the experimental measurements. 

Study of the spherical Couette flow with electromagnetic stirring

The flow driven by electromagnetic forcing of an electrolytic fluid in a concentric sphere system with differential rotation is studied experimentally and theoretically in the laminar regime. The electromagnetic force is generated by the interaction of a direct current injected radially through electrodes located at the equatorial zone of the spheres and a dipolar magnetic field produced by a permanent magnet inside the inner sphere. Even though the flows are fully three-dimensional, under some assumptions, they can be mathematically modeled with a general one-dimensional solution from the incompressible Navier–Stokes equations. Additionally, in order to deepen the flow dynamics, a full three-dimensional numerical simulation is also obtained. Experimentally, velocity profiles in the equatorial plane between spheres were obtained with particle image velocimetry. The analytical solution reproduces qualitatively the experimental profiles, whereas the numerical solution reproduces quantitatively the experimental measurements.

Using direct numerical simulations, we studied the effects of roughness in a turbulent plane Couette flow where one wall is rough and the other one is smooth. Two different roughness configurations are simulated, and the results are compared with the case where both walls are smooth. Upon roughening a wall, roughness would generate vortical structures near the vicinity of the roughness elements and enhance turbulence locally. In the current study, however, we report that turbulence is enhanced near both the rough and the smooth walls.

Turbulent plane Couette flow with a roughened wall

Turbulent channel flow with underlying rough and smooth surfaces present abreast is investigated using direct numerical simulations. Secondary roll cells, exhibiting a strong updraft in the smooth half of the channel, are observed. The roll cells have significantly attenuated the bulk flow and turbulence in the smooth half of the channel. 

Direct numerical simulation of coflowing rough and smooth turbulent channel flows

Onset of shear-layer instability at the interface of parallel Couette flows

A non-planar mixing layer observed between parallel Couette flows by Narasimhamurthy et al. (Phys Rev E 85:036,302, 2012) is considered. Direct numerical simulation is chosen, and simulations are run in order to determine the critical Reynolds number at which the interface between the co-flowing laminar and non-laminar flow becomes unstable exhibiting a meandering motion. The necessary conditions required to trigger the shear-layer instability were also discussed. Different combinations of Reynolds numbers are chosen keeping the Reynolds number ratio between the laminar and non-laminar flows as constant. Preliminary results indicate that the onset of instability occurs, and a meandering motion is observed at the interface when Reynolds number for the non-laminar flow corresponds to 650.

Kalluri Manohar Teja

Papers

Onset of shear-layer instability at the interface of parallel Couette flows

DNS of an Oscillating Shear Layer Between Two Parallel Couette Flows