Browsing by Subject "surface roughness"
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Item Evaluation of Ti-6Al-4V Parts Produced with Rapid Prototyping Technology: Electron Beam Melting Machine(2014-09-29) Abdeen, DanaThe present study measured the corrosion properties of Ti-6Al-4V parts produced with Electron Beam Melting machine (EBM). Potentiodynamic and potentiostatic tests were applied on EBM Ti-6Al-4V in 3.5% mass NaCl solution, to determine the pitting potential and critical pitting temperature (CPT). A relation between pitting potential and temperature was established for EBM Ti-6Al-4V alloy by conducting potentiodynamic test under different temperatures from room temperature until 83?C. Critical pitting temperature was also measured in 3.5% mass NaCl solution of different pH of 2.0, 5.7 and 10.0, to examine the effect of aggressive conditions on the pitting corrosion of EBM alloy. Moreover, the same tests were performed on wrought Ti-6Al-4V for comparison purposes. The results showed that EBM alloy maintained its corrosion resistance as it had the same pitting potential of wrought alloy even at higher temperatures. EBM alloy did not pit when performing potentiostatic test at 800 mV vs. Saturated Calomel Electrode (SCE) even under different pH of the solution. Due to the manufacturing technique and conditions of the EBM machine, the resulted object has irregularities on the exterior surface and voids that are formed within the part which affects samples? properties like surface roughness, CPT, density and porosity. In this study, a statistically designed experiment was employed to manufacture Ti-6Al-4V samples in EBM machine under different process parameters of beam current, beam speed and offset focus. Surface roughness was measured for as-built samples. Then, a potentiostatic test was conducted under 2.40 V vs. SCE to determine the critical pitting temperature in 3.5 % mass NaCl solution for the same samples of different processing parameters. Moreover, density was measured and porosity was calculated from density values. Finally, a model equation was established in order to relate EBM?s process parameters to measured properties of surface roughness, CPT, density and porosity. It was noted that offset focus had the main influence on surface roughness more than beam current and beam speed. Changing processing parameters did not affect corrosion behavior of EBM Ti-6Al-4V as CPT did not vary widely, with a slight effect obtained from beam current and beam speed. Density and porosity were greatly affected with offset focus more than the other parameters. It can be concluded that uniform and precise roughness and porosity are not achievable through this machine, only a range of these properties can be attained.Item Hypersonic Measurements of Roughness-Induced Transient Growth(2014-04-17) Sharp, Nicole SusanneThe effects of surface roughness on boundary-layer disturbance growth and laminar-to-turbulent transition are not well understood, especially in hypersonic boundary layers. The transient growth mechanism that produces algebraic growth of stream wise streaks may play a key role in roughness-induced transition but has not previously been deliberately observed in hypersonic flow. To make such measurements, the present work studies the boundary layer of a 5? half-angle smooth cone paired with a slightly blunted nose tip and a ring of 18 periodically-spaced cube-like discrete roughness elements 1-mm tall by 1.78-mm wide by 1.78-mm long. The roughness element height is approximately equal to the boundary-layer thickness. Measurements are made in the low-disturbance Texas A&M Mach 6 Quiet Tunnel. No transition to turbulence is observed for freestream unit Reynolds numbers between 7.5 ? 10^(6) m^(-1) and 9.8 ? 10^(6) m^(-1). Pitot measurements reveal azimuthally-alternating high- and low-speed streaks growing downstream of the roughness. Large unsteadiness is measured in the roughness wake but decays downstream. The stream wise evolution of the steady and unsteady disturbance energy is consistent with low-speed observations of transient growth in the mid-wake region behind periodically-spaced cylindrical roughness elements. This experiment contains the first quantitative measurements of roughness-induced transient growth in a high-speed boundary layer.Item On the Microphysical Properties of Ice Clouds as Inferred from the Polarization of Electromagnetic Waves(2012-10-19) Cole, BenjaminUncertainties associated with the microphysical and radiative properties of ice clouds remain an active research area because of the importance these clouds have in atmospheric radiative transfer problems and the energy balance of the Earth. In this study, an adding/doubling model is used to simulate the top of atmosphere (TOA) radiance and full Stokes vector from an ice cloud at the wavelength lambda = 865 nm with many different combinations of assumed ice habits (shapes) and different degrees of ice surface roughness, and the polarized radiance at a wide range of scattering angles is derived. Simulated results are compared with polarized radiance data from the POLDER (POLarization and Directionality of the Earth's Reflectances) instrument on board the PARASOL (Polarization and Anisotropy of Reflectances for Atmospheric Sciences coupled with Observations from a Lidar) satellite. Bulk ice scattering properties are obtained by using five different size distributions collected during field campaigns ranging in effective diameter from 10 micrometers to 90 micrometers. Bulk scattering properties for the MODIS Collection 5 ice cloud product are used in this study, along with properties for two mid-latitude ice cloud models, a polar/mid-latitude ice model, and a model built for ice clouds over deep convection. Solid columns and hollow columns are used as well. The polarized radiance simulation results for the moderate surface roughness level best fit the satellite measurements for all ice models, though severely roughened ice crystals do fare well in a few cases. Hollow columns are the best fit to the satellite polarization measurements, but of the ensemble ice models, the polar/mid-latitude model at an effective diameter of 90 micrometers best fits the polarized radiance measurements for the one day of PARASOL data considered. This model should be the best to simulate ice cloud properties on a global scale.Item The effect of ice crystal surface roughness on the retrieval of ice cloud microphysical and optical properties(Texas A&M University, 2007-09-17) Xie, YuThe effect of the surface roughness of ice crystals is not routinely accounted for in current cloud retrieval algorithms that are based on pre-computed lookup libraries. In this study, we investigate the effect of ice crystal surface roughness on the retrieval of ice cloud effective particle size, optical thickness and cloud-top temperature. Three particle surface conditions, smooth, moderately rough and deeply rough, are considered in the visible and near-infrared channels (0.65 and 3.75 ????m). The discrete ordinates radiative transfer (DISORT) model is used to compute the radiances for a set of optical thicknesses, particle effective sizes, viewing and illumination angles, and cloud temperatures. A parameterization of cloud bi-directional reflectances and effective emittances is then developed from a variety of particle surface conditions. This parameterization is applied in a 3-channel retrieval method for Moderate Resolution Imaging Spectroradiometer (MODIS) data at 0.65, 3.75, and 10.8 ????m. Cloud optical properties are derived iteratively for each pixel that contains ice clouds. The impact of ice crystal surface roughness on the cloud parameter retrievals is examined by comparing the results for particles with smooth surfaces and rough surfaces. Retrieval results from two granules of MODIS data indicate that the retrieved cloud optical thickness is significantly reduced if the parameterization for roughened particles is used, as compared with the case of smooth particles. For the retrieval of cloud effective particle size, the inclusion of the effect of surface roughness tends to decrease the retrieved effective particle size if ice crystals are small. The reversed result is noticed for large ice crystals. It is also found that surface roughness has a very minor effect on the retrieval of cloud-top temperatures.