Deposition Of Foreign Materials Research Articles

Characteristics of large-scale and superstructure motions in a turbulent boundary layer overlying complex roughness

ABSTRACTThe structural attributes of turbulent flow over a complex roughness topography are explored using high-frame-rate stereo particle-image velocimetry measurements in the wall-normal–spanwise plane. The roughness under consideration was replicated from a turbine blade damaged by deposition of foreign materials and contains a broad range of topographical scales arranged in a highly irregular manner. Previous results from Barros and Christensen [Observations of turbulent secondary flows in a rough-wall boundary layer. J Fluid Mech. 2014;748] revealed strong spanwise heterogeneity in the flow attributed to the formation of roughness-induced turbulent secondary flows identified by spanwise-alternating low- and high-momentum flow pathways (HMP & LMP, respectively) in the mean flow marked by enhanced Reynolds stresses and turbulent kinetic energy. Frequency spectra of streamwise velocity at fixed wall-normal location presented herein also display strong dependence on spanwise position. In particular, the roughness promotes enhanced energy content of the large-scale and smaller-scale motions (as opposed to very-large-scale ones). Depending on spanwise position, pre-multiplied spectra highlight significant modification of the energy content of the very large-scale motions (superstructures) due to roughness compared to smooth-wall flow. Of note, a shift in both TKE and RSS content to shorter streamwise scales at an LMP was noted, while less of an impact was found coincident with an HMP.

Analysis of realistic rough surface for its globally dominant parameters using continuous wavelets

Continuous Morlet and Mexican hat wavelets are used to analyze a highly irregular rough surface replicated from real turbine blades which are roughened by deposition of foreign materials. The globally dominant aspect ratio, length scale, and orientation of the roughness elements are determined. These parameters extracted from this highly irregular rough surface are important for the future studies of their effects on turbulent flows over this kind of rough surfaces encountered in Washington aerospace and power generating industries.

Observations of turbulent secondary flows in a rough-wall boundary layer

AbstractThe characteristics of a turbulent boundary layer overlying a complex roughness topography were explored with stereo particle-image velocimetry measurements in the wall-normal–spanwise $(y\mbox{--}z)$ plane. The roughness under consideration was replicated from a turbine blade damaged by deposition of foreign materials containing a broad range of topographical scales arranged in a highly irregular manner. The single-point turbulence statistics displayed strong spanwise heterogeneity, in particular spanwise-alternating low- and high-momentum flow pathways in the mean flow marked by enhanced Reynolds stresses and turbulent kinetic energy. The spanwise regions between high- and low-momentum flow pathways were occupied by swirling motions, suggesting the generation and sustainment of turbulent secondary flows due to the spanwise heterogeneity of the complex roughness under consideration. Similar observations were recently reported for more ordered spanwise roughness transitions by Nugroho, Hutchins & Monty (Intl J. Heat Fluid Flow vol. 41, 2013, pp. 90–102) and Willingham et al. (Phys. Fluids vol. 26, 2014, 025111).

Spatial structure of a turbulent boundary layer with irregular surface roughness

Particle image velocimetry experiments were performed to study the impact of realistic roughness on the spatial structure of wall turbulence at moderate Reynolds number. This roughness was replicated from an actual turbine blade damaged by deposition of foreign materials and its features are quite distinct from most roughness characterizations previously considered as it is highly irregular and embodies a broad range of topographical scales. The spatial structure of flow over this rough surface near the outer edge of the roughness sublayer is contrasted with that of smooth-wall flow to identify any structural modifications due to roughness. Hairpin vortex packets are observed in the outer layer of the rough-wall flow and are found to contribute heavily to the Reynolds shear stress, consistent with smooth-wall flow. While similar qualitative consistency is observed in comparisons of smooth- and rough-wall two-point correlations, some quantitative differences are also apparent. In particular, a reduction in the streamwise extent of two-point correlations of streamwise velocity is noted which could be indicative of a roughness-induced modification of outer-layer vortex organization. Proper orthogonal decomposition analysis reveals the streamwise coherence of the larger scales to be most sensitive to roughness while the spatial characteristics of the smaller scales appear relatively insensitive to such effects.

Low-order representations of irregular surface roughness and their impact on a turbulent boundary layer

The present effort explores the relative impact of various topographical scales present within irregular surface roughness on a turbulent boundary layer under both developing- and developed-flow conditions. Low-order representations of highly irregular surface roughness replicated from a turbine-blade damaged by deposition of foreign materials were generated using singular value decomposition to decompose the complex topography into a set of topographical basis functions of decreasing importance to the original (“full”) surface character. The low-order surface models were then formed by truncating the full set of basis functions at the first 5 and 16 modes (containing approximately 71% and 95% of the full surface content, respectively) so that only the most dominant and large-scale topographical features were included in the models, while the finer-scale surface details are excluded. Physical replications of the full surface and the two low-order models were created using rapid prototyping methods to generate short and long streamwise fetches of roughness, and particle-image velocimetry was used to acquire ensembles of instantaneous velocity fields in the streamwise–wall-normal plane for developing- and developed-flow conditions at moderate Reynolds number. Comparison of both single- and multipoint statistics (mean velocity and Reynolds normal and shear stresses) as well as quadrant analysis of the instantaneous events contributing to the mean Reynolds shear stress indicates that a 16-mode model of the full surface faithfully reproduces the characteristics of flow over the full surface for both developing- and developed-flow conditions. For the latter scenario, both the 5- and 16-mode models reproduce the outer-layer characteristics for flow over the full surface in accordance with Townsend’s wall similarity hypothesis. However, neither low-order surface representation fully reproduces important details of the Reynolds-shear-stress-producing events within the roughness sublayer, particularly the contributions of the most intense ejection and sweep events.

Outer-layer similarity in the presence of a practical rough-wall topography

High-resolution particle image velocimetry measurements are made in the streamwise–wall-normal plane of a zero-pressure-gradient turbulent boundary layer over smooth and rough walls at Reθ≈13000. The roughness considered herein is replicated from a surface scan of a turbine blade damaged by deposition of foreign materials and its topography is highly irregular and contains a broad range of topographical scales. Two physical scalings of the same roughness topography are considered, yielding two different rough surfaces: RF1 with k=4.2mm and RF2 with k=2.1mm, where k is the average peak-to-valley roughness height. At Reθ≈13000, these roughness conditions yield k+≡k∕y*=207, δ∕k=28, ks+=115, and δ∕ks=48 for RF1 and k+=91, δ∕k=50, ks+=29, and δ∕ks=162 for RF2 (where δ is the boundary-layer thickness, ks is the equivalent sand-grain height, and y* is the viscous length scale). The mean velocity deficits along with the Reynolds normal and shear stress profiles for both roughness conditions collapse on the smooth-wall baseline in the outer layer when appropriately scaled by the friction velocity, uτ. Probability density functions and quadrant analysis of the instantaneous events contributing to the mean Reynolds shear stress show similar outer-layer consistency between the smooth and rough cases when scaled appropriately with uτ. In addition, one-dimensional, two-point streamwise, and wall-normal velocity autocorrelation coefficients are also found to collapse in the outer region, indicating a similarity in the spatial structure of the outer-layer turbulence. The observed collapse of the smooth- and rough-wall turbulence statistics in the outer layer supports Townsend’s wall similarity hypothesis for flow over the unique surface topography considered herein.

Ultra-Pure Processing: a Key Challenge for Ion Implantation Processing for Fabrication of ULSI Devices

AbstractA key issue in modern ion implantation processing is the requirement for dramatic improvements in the purity of the incident ion beam and reductions in the deposition of foreign materials onto the wafer surface. These deposited materials include particles as well as sputtered and vapor deposited metals and dopants. Physical mechanisms which effect the elemental purity of atoms arriving at the surface of ion implanted wafers and progress towards achieving implantation purity levels of below 100 ppm of the implanted dose for sputtered metal and dopant films are discussed.

Experimental marginal periodontitis in rats produced by coprophagy (author's transl)

The purpose of the present study was investigated to the effects of the high carbohydrate diet (Keyes No. 2000) and coprophagy on the periodontal tissues of the normal and alloxan diabetic rats. The experimental animals were divided into four groups of normal rats (A, B, C, D) and four groups of alloxan diabetic rats (a, b, c, d). A and a groups received a powder diet (a normal laboratory diet), B and b groups a high carbohydrate diet, C and c groups a powder diet+feces, and D and d groups a high carbohydrate diet+feces. The experimental animals were killed at intervals of 1, 3, 5, 7 and 9 weeks, and the periodontal tissues between mandibular first and second molars of these animals were examined macroscopically and histpathologically. The results were as follows;1. Normal rats (A, B, C and D groups).During the entire experimental period, there were no apparent changes in the periodontal tissues of animals receiving a powder diet (A) or a high carbohydrate diet (B). At 3, 5 and 7 weeks, proliferation of the gingival epithelium and deposition of foreign materials in the gingival epithelium and connective tissue were observed in the periodontal tissues of animals receiving a powder diet+feces (C) or a high carbohydrate diet+feces (D). At 9 weeks, loss of the gingival epithelium and proliferation of the functional epithelium were observed in the periodontal tissues of animals receiving a powder diet+feces (C). In addition, at 7 and 9 weeks, a periodontal pocket formation was also observed in the animals receiving a high carbohydrate diet+feces (D). During the entire experimental period, however, the changes were not found in the periodontal ligament and alveolar bone of these grups (C and D).2. Alloxan diabetic rats (a, b, c and d groups).During the entire experimental period, there were no apparent changes in the periodontal tissues of animals receiving a powder diet (a) or a high carbohydrate diet (b). At 3 and 5 weeks, proliferation of the gingival epithelium and deposition of foreign materials in the gingival epithelium and connective tissue were observed in the periodontal tissues of animals receiving a powder diet+feces (c) or a high carbohydrate diet+feces (d). At 7 and 9 weeks, the destructive changes, such as a loss of gingival epithelium, ulceration, inflammatory cell infiltration in the connective tissue and alveolar bone resorption, were observed in the periodontal tissues of animals receiving a powder diet+feces (c). At 5, 7 and 9 weeks, the similar periodontal changes were also found in the animals receiving a high carbohydrate diet+feces (d). Moreover, at 9 weeks, severe hyaline degeneration of the gingival connective tissue and disorganization of the periodontal fibers were observed in this group (d).