J. R. Saylor

Computational Analysis of the Interfacial Bonding Between Feed-Powder Particles and the Substrate in the Cold-Gas Dynamic-Spray Process

The cold-gas dynamic-spray process is analyzed by numerical modeling of the impact between a single spherical feed-powder particle and a semi-infinite substrate. The numerical modeling approach is applied to the copper‐aluminum system to help explain experimentally observed higher deposition efficiencies of the copper deposition on aluminum than the ones associated with the aluminum deposition on copper. To properly account for the high strain, high strain-rate deformation behavior of the two materials, the appropriate linear-elastic rate-dependent, temperature-dependent, strain-hardening materials constitutive models are used. The results obtained indicate that the two main factors contributing to the observed higher deposition efficiency in the case of copper deposition on aluminum are larger particle/substrate interfacial area and higher contact pressures. Both of these are the result of a larger kinetic energy associated with a heavier copper feed-powder particle. The character of the dominant particle/substrate bonding mechanism is also discussed in the present paper. It is argued that an interfacial instability which can lead to the formation of interfacial roll-ups and vortices can play a significant role in attaining the high strength of interfacial bonding. # 2003 Elsevier Science B.V. All rights reserved.

Transport of a passive scalar at a shear-free boundary in fully developed turbulent open channel flow

Direct numerical simulations of fully developed turbulence in an open channel geometry were performed in which a passive scalar was introduced. The simulations were intended to explore transport at free surfaces in two cases for which (1) the free surface was maintained at constant temperature and (2) the interfacial flux was fixed. These cases can be considered models for mass and evaporative heat transport where buoyancy and surface deformation effects are negligible. Significant differences were found in the thermal fields in these two cases. The turbulent statistics reveal that the surface flux in the constant temperature case was significantly more intermittent compared to the surface temperature field in the constant flux case. The surface temperature field in the latter case formed large patches of warm fluid, reminiscent of the so-called fish scale patterns revealed in recent infrared imagery of the air–water interface. The wake-like structure of the patches was evident despite the absence of surf...

Conditional scalar dissipation rates in turbulent wakes, jets, and boundary layers

The expected value Eχ≡E(χ,θ=θ0) of the dissipation rate χ of a passive scalar θ conditioned on the scalar value θ=θ0 has been measured in three varieties of turbulent shear flows: heated wakes, dyed liquid jets, and the atmospheric surface layer. The quantity Eχ depends fairly strongly on θ0 and on the flow. For the wake, Eχ exhibits two peaks—one on the low‐temperature end and the other on the high‐temperature end—and the peaks are separated by an approximately flat region. The relative strength of the two peaks varies with the spatial position. Measured in the turbulent part alone, Eχ tends to have only one peak on the hot side, but is still nonuniform. The related quantity, Eθ‘≡(∇2θ,θ=θ0), which is the expected value of the Laplacian of the scalar conditioned on the scalar concentration, has also been measured on the wake centerline and shows a simpler dependence on θ0 than Eχ. For jets, Eχ has a single peak on the high‐concentration side. This feature appears to be essentially independent of the use o...

The effect of a surfactant monolayer on the temperature field of a water surface undergoing evaporation

The surface temperature field of a body of water undergoing evaporation was measured using infrared imaging techniques, demonstrating for the first time the effect of surfactant monolayers on the spatial structure of this field. Measurements were obtained from a water surface which was covered with a monolayer of the surfactant oleyl alcohol, and also from a surface which was free of surfactants. The oleyl alcohol and surfactant-free experiments were compared at equivalent heat fluxes. The presence of surfactants increased the characteristic length scale of the surface temperature field. This conclusion is supported by both visual observation of the infrared imagery and spatial Fourier transforms of the temperature fields. The presence of the surfactant monolayer had a small effect on the root mean square of the temperature field but significantly affected the skewness, creating a more positively skewed probability density function for the surfactant covered field. These observations were found to hold when comparison between the clean and surfactant case was made at heat fluxes varying by a factor of ∼11.

Gas transport across an air/water interface populated with capillary waves

An experimental study of gas transport across an air/water interface, populated by a field of standing capillary waves is presented. The experiments were conducted in a small tank containing distilled water, enriched with carbon dioxide. The capillary waves were of the Faraday type, generated by providing a small vertical vibration to the water tank. The frequency of excitation was varied from 200 to 400 Hz, giving wavelengths from 3.62 to 2.26 mm (linear estimate). The gas transport rate across the interface increased by almost two orders of magnitude as the wave slope was increased from zero to slightly above 0.2 m/m. A unique aspect of these experiments is that capillary waves were isolated from the obfuscating effects of turbulence,aerosol generation, and other phenomena typically present in wind/wave tunnel experiments. Consequently the large enhancement in gas transfer was due to the effects of capillary waves alone, demonstrating their importance in gas exchange processes. The maximum mass transfer coefficients obtained in these experiments are not achieved in typical wind/wave tunnel experiments below wind speeds of 10 m/s.

Near-surface turbulence for evaporative convection at an air/water interface

Turbulence measurements are reported for the flow beneath an air/water interface undergoing evaporative convection. Measurements were obtained using a two component laser Doppler velocimeter system. Two hydrodynamic boundary conditions were considered for the free surface: a shear free surface, which is the case when surfactants are absent, and a constant elasticity surface, created by depositing a monolayer of oleyl alcohol. The shear free boundary condition case results in significantly higher levels of near surface turbulence than the constant elasticity case. This difference between the two cases decreases with distance from the free surface. Profiles of the turbulent fluctuations were obtained for the horizontal and vertical velocity components and are compared with the somewhat analogous case of a heated solid wall.

An experimental investigation of the surface temperature field during evaporative convection

Measurements of the surface temperature field are presented for a water surface undergoing evaporation. These temperature fields were measured using an infrared camera for a range of heat fluxes q″=30–500 W/m2. Experiments were conducted for water surfaces with and without a surfactant monolayer. A statistical analysis of the data is presented which shows the effect of heat flux and surfactants on the root mean square and skewness of the field. The data reveals a linear increase in the rms with increasing heat flux, which is similar for clean and surfactant conditions. In contrast, the skewness is markedly different for the clean and surfactant-covered cases. For clean surface conditions, the skewness attains large, negative values, becoming increasingly negative as q″ increases. When the surface is covered with a surfactant monolayer, however, the skewness exhibits small, negative values which approach zero as the heat flux increases. This behavior is reflected in the pdf which is clearly asymmetric in t...

DIFFERENTIAL DIFFUSION IN LOW REYNOLDS NUMBER WATER JETS

Experimental data on differential diffusion between two species with large and quite disparate Schmidt numbers were obtained in a turbulent water jet by optically measuring the two species concentrations simultaneously. Experimental conditions were chosen so that the species were dilute and did not affect the water density thereby avoiding inertial effects. Differential diffusion was found to be significant in magnitude, even in the absence of these effects. Schmidt number ratios of 4 and 18 were considered. Differential diffusion was found to be statistically significant and to manifest at scales larger than the computed Batchelor scale. In some instances the concentration signal for the species with larger diffusivity was simply a blurred version of the other, while in other instances structures present in one signal were completely absent from the other. This second observation, presumably a more complex effect due to diffusion across velocity gradients, is discussed.

Photobleaching of disodium fluorescein in water

Photobleaching of disodium fluorescein dissolved in water is experimentally investigated using laser induced fluorescence (LIF). It is demonstrated that significant photobleaching occurs on the millisecond time scale, resulting in a large decrease in the fluorescence signal emanating from a constant concentration sample. The importance of avoiding photobleaching when using LIF with disodium fluorescein for concentration measurements in water flow experiments is demonstrated. A half-life for photobleaching is introduced and measured for disodium fluorescein and is shown to be a more appropriate measure than the traditional ‘bleaching quantum efficiency’. It is demonstrated that the photobleaching of disodium fluorescein is at least partially reversible.

Infrared imaging of the surface temperature field of water during film spreading

Deposition of a spontaneously-spreading film on a clean water surface creates a front which propagates radially outward from the point of deposition. This rapidly spreading film was used as a tool to quickly change the boundary condition of a water surface from one which is shear-free, to a boundary condition which supports shear. Infrared images of a water surface experiencing evaporative convection were recorded as this film spread. These images were converted to surface temperature fields. The amount of turbulent structure present in these fields changes dramatically across the front. Ahead of the front, significant variations at large and small spatial scales are evident, while behind the front the small scale structures are eliminated. The time scale at which this damping occurs is short and has not been reported on heretofore. In addition to being relevant to free surface turbulence, these results demonstrate the utility of infrared imaging in the study of spreading films.