Peter Vorobieff

Effective slip on textured superhydrophobic surfaces

We study fluid flow in the vicinity of textured and superhydrophobically coated surfaces with characteristic texture sizes on the order of 10μm. Both for droplets moving down an inclined surface and for an external flow near the surface (hydrofoil), there is evidence of appreciable drag reduction in the presence of surface texture combined with superhydrophobic coating. On textured inclined surfaces, the drops roll faster than on a coated untextured surface at the same angle. The highest drop velocities are achieved on surfaces with irregular textures with characteristic feature size ∼8μm. Application of the same texture and coating to the surface of a hydrofoil in a water tunnel results in drag reduction on the order of 10% or higher. This behavior is explained by the reduction of the contact area between the surface and the fluid, which can be interpreted in terms of changing the macroscopic boundary condition to allow nonzero slip velocity.

Braiding patterns on an inclined plane

A jet of fluid flowing down a partially wetting, inclined plane usually meanders but — by maintaining a constant flow rate — meandering can be suppressed, leading to the emergence of a beautiful braided structure. Here we show that this flow pattern can be explained by the interplay between surface tension, which tends to narrow the jet, and fluid inertia, which drives the jet to widen. These observations dispel misconceptions about the relationship between braiding and meandering that have persisted for over 20 years.

Knots and Random Walks in Vibrated Granular Chains

We study experimentally statistical properties of the opening times of knots in vertically vibrated granular chains. Our measurements are in good qualitative and quantitative agreement with a theoretical model involving three random walks interacting via hard-core exclusion in one spatial dimension. In particular, the knot survival probability follows a universal scaling function which is independent of the chain length, with a corresponding diffusive characteristic time scale. Both the large-exit-time and the small-exit-time tails of the distribution are suppressed exponentially, and the corresponding decay coefficients are in excellent agreement with theoretical values.

Turbulent rotating convection: an experimental study

We present experimental measurements of velocity and temperature fields in horizontal planes crossing a cylindrical Rayleigh–Benard convection cell in steady rotation about its vertical axis. The range of dimensionless rotation rates Ω is from zero to 5×10 4 for a Rayleigh number R = 3.2×10 8 . The corresponding range of convective Rossby numbers is ∞ > Ro > 0.06. The patterns of velocity and temperature and the flow statistics characterize three basic flow regimes. For Ro [Gt ] 1, the flow is dominated by vortex sheets (plumes) typical of turbulent convection without rotation. The flow patterns for Ro ∼ 1 are cyclone-dominated, with anticyclonic vortices rare. As the Rossby number continues to decrease, the number of anticyclonic vortex structures begins to grow but the vorticity PDF in the vicinity of the top boundary layer still shows skewness favouring cyclonic vorticity. Velocity-averaging near the top of the cell suggests the existence of a global circulation pattern for Ro [Gt ] 1.

Stretching of material lines in shock-accelerated gaseous flows

A Mach 1.2 planar shock wave impulsively accelerates one of five different configurations of heavy-gas (SF6) cylinders surrounded by lighter gas (air), producing one or more pairs of interacting vortex columns. The interaction of the columns is investigated with planar laser-induced fluorescence in the plane normal to the axes of the cylinders. For the first time, we experimentally measure the early time stretching rate (in the first 220μs after shock interaction before the development of secondary instabilities) of material lines in shock-accelerated gaseous flows resulting from the Richtmyer-Meshkov instability at Reynolds number ∼25000 and Schmidt number ∼1. The early time specific stretching rate exponent associated with the stretching of material lines is measured in these five configurations and compared with the numerical computations of Yang et al. [AIAA J. 31, 854 (1993)] in some similar configurations and time range. The stretching rate is found to depend on the configuration and orientation of ...

Prediction of Transverse Injection of a Sonic Jet in Supersonic Crossflow

Navier-Stokes predictions of mixing by a sonic jet injected into a Mach 1.98 crosso w are presented. The injection is transverse, typical of a jet in crosso w (JICF) problem. Both air and helium injection into crosso w air are studied. A two-equation turbulence model by Wilcox (1998) is used. The steady predictions resolve the complicated shock and vortical structures in the o w, including the barrel shock and Mach disk and the counter-rotating vortex pair that dominates mixing in the wake. The results are in good agreement with Rayleigh scattering measurements by Gruber et al. (1996).

Fluid instabilities and wakes in a soap-film tunnel

We present a compact, low-budget two-dimensional hydrodynamic flow visualization system based on a tilted, gravity-driven soap film tunnel. This system is suitable for demonstrations and studies of a variety of fluid mechanics problems, including turbulent wakes past bluff bodies and lifting surfaces, Kelvin–Helmholtz instability, and grid turbulence.

Experimental and numerical analysis of irreversibilities among particles suspended in a Couette device

An experimental and numerical study is performed of irreversibilities among particles in a wide-gap Couette device. Five spheres are placed in the Couette in a tightly packed arrangement. The inner cylinder is rotated five revolutions counterclockwise and subsequently five revolutions clockwise. Irreversibilities in the system are characterized by the radial spread of the particles measured by a radial moment. The experimental results are compared to a set of numerical simulations performed using a lubrication-corrected completed double layer boundary element method with a particle roughness model. In both the experiments and simulations, there is a critical value of the initial radial moment above which there is little irreversibility in the systems and below which irreversibilities accumulate rapidly. Particle roughness in the simulations can be used as a parameter to achieve the best agreement with experiment. It is noteworthy that the roughness value producing the best agreement is different than the ...

Observation of the Development of Secondary Features in a Richtmyer–Meshkov Instability Driven Flow

Richtmyer–Meshkov instability (RMI) has long been the subject of interest for analytical, numerical, and experimental studies. In comparing results of experiment with numerics, it is important to understand the limitations of experimental techniques inherent in the chosen method(s) of data acquisition. We discuss results of an experiment where a laminar, gravity-driven column of heavy gas is injected into surrounding light gas and accelerated by a planar shock. A popular and well-studied method of flow visualization (using glycol droplet tracers) does not produce a flow pattern that matches the numerical model of the same conditions, while revealing the primary feature of the flow developing after shock acceleration: the pair of counter-rotating vortex columns. However, visualization using fluorescent gaseous tracer confirms the presence of features suggested by the numerics; in particular, a central spike formed due to shock focusing in the heavy-gas column. Furthermore, the streamwise growth rate of the spike appears to exhibit the same scaling with Mach number as that of the counter-rotating vortex pair (CRVP).

PLIF Flow Visualization of a Supersonic Injection COIL Nozzle

This paper describes Planar Laser-Induced Fluorescence (PLIF) flow visualization of a supersonic nozzle with supersonic injection. The nozzle simulates Chemical Oxygen Iodine Laser (COIL) flow conditions with non-reacting, cold flows, where the injected flow was seeded with iodine. A laser sheet near 565nm excited the iodine, and the fluorescence was imaged with a gated, CCD camera. Spanwise and streamwise images were taken, where the relative concentration of the injected to primary flow, turbulent structures, and penetration distance of the injected flow were identified. These images qualitatively revealed a lack of mixing of the secondary (injected) and primary flows at the centerline of the nozzle, even far downstream of the throat. Quantitative data of the penetration of the secondary flow, with varying primary to secondary flow rate ratios, helped identify the shallow angle of the injectors as an inhibiter of secondary penetration even at relatively low primary flow rates. From the PLIF results, this nozzle is characterized as a poor mixer and would not be recommended as a nozzle that produces a well-mixed medium, as required with chemical lasers. This work precedes a project that will use PLIF results to design a well-mixed supersonic nozzle with supersonic injection. The results will be compared to and enable validation of computational fluid dynamics (CFD) predictions of the designed nozzle.