Nadim M. Aziz

Efficient slicing for layered manufacturing

An adaptive slicing algorithm that can vary the layer thickness in relation to local geometry is presented. The algorithm is based on three fundamental concepts: choice of criterion for accommodating complexities of surfaces, recognition of key characteristics and features of the object, and development of a grouping methodology for facets used to represent the object. Four criteria, cusp height, maximum deviation, chord length and volumetric error per unit length, are identified and the layer thickness is adjusted such that one of the four is met. Next, key characteristics of the object, such as horizontal and vertical surfaces, pointed edges and ends, are identified based on the local changes in surface complexity, and slice based feature recognition is introduced to identify the nature of a feature, protrusion or depression, by studying the slice data. Note that the present approach uses information only from the tessellated model, and thus is different from current implementations. Finally, the concept of grouping of the facets based on their vertex coordinates is developed to minimize the number of searches for possible intersection of the facets with a slice plane. The slicing algorithm is interfaced with adaptive laminated machining and the stereolithography process through a CNC post processor and a hatching algorithm respectively. A comparison of the estimated surface quality and build time indicates that adaptive slicing produces superior parts in a shorter build time. The implementation of this work is protected under US Patent laws (Patent # 5,596,504, January 1997).

Experiments on sediment transport in shallow flows in high gradient channels

Abstract The main objective of this paper is to present the results of an experimental study relating to the transport of sediment under certain conditions not typical of river sedimentation studies. The area of sediment transport lacks information on the transport down high gradient channels that are typical of streets, construction sites and agricultural land, in addition to mountain torrrents. The study of transport of non-cohesive sediment in high gradient channels includes determination of the transport capacity of shallow flows in high gradient channels with simulated bed roughness and various grain sizes. Regression analysis was performed to relate the dependent and independent variables in functional relationships and revealed good correlation.

A model for interfacing geometric modeling data with rapid prototyping systems

Rapid prototyping processes are becoming viable techniques for integrating product design and manufacturing cycles. These processes have the potential to achieve maximum designer-user interaction harmonizing the initial specifications with final operational requirements of a product. Rapid prototyping processes often begin with the creation of a model using a CAD modeler. This model is then sliced to generate data about successive layers which are then constructed one layer at a time by various processes depending on the rapid prototyping method. Currently, slicing is performed on a tessellation of the CAD model which is an approximation of the model itself. There are few problems that are at the heart of this technique, including the numerical inaccuracies resulting from round-off errors due to manipulating non-integer data, as well as the dependence on proprietary algorithms for generating the slice data. These numerical problems are reflected in the quality of the surface finish of the final product. This paper presents a technique for interfacing the CAD model with the rapid prototyping process which bypasses the surface modeling by tessellation and the approximations to the actual geometry that are inherent in this process and in the subsequent slicing of the model. The technique presented is based on slicing the model directly using its complete geometric representation. This technique represents an advancement over the currently used slicing methods and provides the means for generating smoother surface finish.

Bezier surface/surface intersection

The computational requirements and accuracy of two methods for finding the intersection of Bezier surfaces are examined. In both methods, the existence of an intersection curve is confirmed by using the convex hull property of such surfaces. The first method evaluates the intersection by recursive subdivision of two patches with overlapping hulls. The second method detects a point on the intersection curve and then incrementally traces the intersection in the parametric spaces of the two surfaces. With both methods, the intersection of a pair of first-order planar patches must be solved analytically. The intersection is approximated by first-order Bezier patches in the first case and by planar triangles in the second. Overall, the method of incremental tracing is shown to give more accurate results than the method of recursive subdivision.<<ETX>>

An Experimental Investigation of Cross-Flow Turbine Efficiency

An experimental investigation was conducted to study the effect of some geometric parameters on the efficiency of the cross-flow turbine. Turbine models were constructed with three different numbers of blades, three different angles of water entry to the runner, and three different inner-to-outer diameter ratios. Nozzles were also constructed for the experiments to match the three different angles of water entry to the runner. A total of 27 runners were tested with the three nozzles. The results of the experiments clearly indicated that efficiency increased with increase in the number of blades. Moreover, it was determined that an increase in the angle of attack beyond 24 deg does not improve the maximum turbine efficiency. In addition, as a result of these experiments, it was determine that for a 24 deg angle of attack 0.68 was the most efficient inner-to-outer diameter ratio, whereas for higher angles of attack the maximum efficiency decreases with an increase in the diameter ratio from 0.60 to 0.75.

Suspended sediment concentration profiles using conservation laws

The suspended sediment transport problem in open channels is studied from the conservation laws point-ofview. Beginning with equations describing the conservation of mass and momentum of both sediment and water, equations for suspended sediment concentrations in free surface flow are derived. Sediment shear and normal stresses are modeled in a manner similar to that of fluid turbulent stresses. Ordinary differential equations for concentration profiles in the viscous sublayer and in the logarithmic velocity region of a steady uniform flow are developed. Although analytical solutions of these equations were attempted, no closed form solution was found. Therefore, a numerical technique such as the fourth order Runge-Kutta method, is used to solve the concentration profile equations. For lack of data in the viscous sublayer, only the concentration profile for flow with the logarithmic velocity distribution is solved and compared to data.

RECOVERY: A Contaminated Sediment-Water Interaction Model

This paper describes the U.S. Army Corps of Engineers screening-level water quality model (RECOVERY version 3.0) for assessing long-term impacts of contaminated bottom sediments on surface waters. The model couples contaminant interaction between the water column and the bottom sediment, as well as between contaminated and clean bottom sediments. The analysis is intended primarily for organic contaminants with the assumption that the overlying water column is well mixed vertically. The contaminant is assumed to follow linear, reversible, equilibrium sorption and first-order decay kinetics. The system is physically represented as a well-mixed water column (i.e., zero-dimensional) underlain by a vertically-stratified sediment column (i.e., one-dimensional). The sediment is well-mixed horizontally but segmented vertically into a well-mixed surface (active) layer and deep sediment. The deep sediment is segmented into variably contaminated and clean sediment regions. Processes incorporated in the model are sorption, decay, volatilization, burial, resuspension, settling, bioturbation, and pore-water diffusion. The solution couples contaminant mass balance in the water column and in the mixed sediment layer along with diffusion in the deep sediment layers. The model was verified against laboratory and field data, as well as against an analytical solution for the water and mixed sediment layers. These comparisons indicate that the model can be used as an assessment tool for evaluating remediation alternatives for contaminated bottom sediments.

Evaluation of Abutment Scour Prediction Equations with Field Data

The U.S. Geological Survey, in cooperation with FHWA, compared predicted abutment scour depths, computed with selected predictive equations, with field observations collected at 144 bridges in South Carolina and at eight bridges from the National Bridge Scour Database. Predictive equations published in the 4th edition of Evaluating Scour at Bridges (Hydraulic Engineering Circular 18) were used in this comparison, including the original Froehlich, the modified Froehlich, the Sturm, the Maryland, and the HIRE equations. The comparisons showed that most equations tended to provide conservative estimates of scour that at times were excessive (as large as 158 ft). Equations also produced under-predictions of scour, but with less frequency. Although the equations provide an important resource for evaluating abutment scour at bridges, the results of this investigation show the importance of using engineering judgment in conjunction with these equations.

A computer-aided box stacking model for truck transport and pallets

Abstract This paper presents a practical engineering application of the use of spatial data structures to model an otherwise complicated problem. The paper explains the techniques used for the geometrical modeling of patterns composed of rectangular boxes. These patterns are used in the actual stacking of boxes into trucks. The paper discusses the special method of application of the spatial occupancy technique of solid modeling for stacking the given boxes in the desired arrangements. The stacking patterns are represented as quasi-disjoint solid models made up of individual solids (boxes). The paper also explains the developed algorithms which include a packing algorithm, an algorithm for database generation, a solid modeling algorithm and a graphics display algorithm. All programs are implemented on a microcomputer.

Impact of New Rainfall Patterns on the Design of Hydraulic Structures

The current SCS dimensionless rainfall distribution patterns (Type I, IA, II, and III) are thought to be over conservative and in need of replacement. This study develops new dimensionless design rainfall patterns for the state of South Carolina for various durations. The rainfall data from various sources dating back to 1971 was collected and analyzed. Different storm separation methods were reviewed and a modification of an existing method was adopted for the study. The rainfall events were combined into 21 different groups ranging from 2 to 51 hours. The top fifty events (by volume) within each group were selected to determine the dimensionless rainfall pattern. Similar dimensionless rainfall patterns for various groups were combined to create representative time-duration patterns for the state of South Carolina. The results showed that two distinct dimensioless rainfall patterns could be established. The new rainfall patterns were compared to the SCS rainfall curves and rainfall patterns developed by other states. The impact of these new dimensionless rainfall patterns on design practice was assessed. The new rainfall patterns were used to evaluate the response of hydraulic structures to rainfall events. In particular, the basin rainfall-runoff volume and detention pond performance were evaluated and compared to the results from the SCS rainfall patterns. For the basin test, the runoff volume, peak discharge, and time to peak were compared. For the pond test, the stage versus time and the ratio of storage volume to runoff volume were compared. The impact of varying the outflow culvert size on pond storage requirement was also investigated. The comparison showed that new rainfall patterns resulted in lower peak flow and different peak time for the runoff hydrograph. The new rainfall patterns resulted in lower peak stage and detention pond storage volume.