Steven A. Lottes

Development of a Computational Approach to Detect Instability and Incipient Motion of Large Riprap Rocks

Computational fluid dynamics (CFD) has progressed to the point where flow problems can be solved in domains containing solid objects with complex, irregular geometry in relative motion along arbitrary paths through the fluid domain. The solvers incorporate moving mesh and mesh morphing techniques. With this new CFD capability the detailed stress distribution created by flow over the surface of a moving solid and the capability of computational structural mechanics (CSM) software to solve for both small and large displacements of solids from applied loads, it is now possible to solve a wide variety of fluid structure interaction (FSI) problems by coupling the two types of software. This paper presents development of procedures to couple STAR-CCM+® CFD software to LS-DYNA® CSM software to solve FSI problems. An initial application of the coupled software to FSI analysis of incipient motion of large riprap rocks is described. Two cases were used to test the coupling. The first has a rock layer in a channel with no bridge structures, and the second has an abutment corner that contracts the flow. Three representative rocks were included in the coupling and the approximate inlet flow velocity required to lift a rock and move it downstream was determined.

A Computational Approach to the Study of the Stability of Pier Riprap at the Middle Fork Feather River

This paper discusses the use of various technologies and advanced computational modeling techniques that were combined for monitoring the performance of pier riprap on the basis of a field case study – Pier 3 of a bridge over the Middle Fork Feather River – in northern California, USA. The first phase involved capturing the field condition of the bridge site using sonar instrumentation technology in order to obtain high resolution bathymetry data. The second phase entailed enhancement and transformation of the scanned bathymetric data into a 3D CAD model to be used as the initial geometry for numerical modeling. A Fluid Structure Interaction (FSI) numerical approach was applied to simulate the rock incipient motion i.e. shear failure by coupling Computational Fluid Dynamics (CFD) software STAR-CCM+ and a Computational Structural Mechanics (CSM) software LS-DYNA. Several coupled simulations have been performed with varying flow conditions to identify shear failure conditions for the riprap apron.

Computer Modeling and Analysis of Truck Generated Salt-Spray Transport Near Bridges

Bridge painting is very expensive. Over the lifetime of a bridge, painting to protect the steel from the elements can cost a large fraction of the original construction cost. In addition to monetary costs, maintenance workers face health risks if strict procedures are not in place to avoid exposure to hazardous dust when old paint is removed and the fumes of hazardous chemicals when new paint is applied. Use of weathering steel in bridge construction avoids the large cost of periodic painting and the hazards of paint removal and repainting because this type of steel is never painted. When exposed to weather, the initial corrosion of weathering steel forms a natural corrosion resistant layer that prevents further corrosion under a wide variety of conditions. Given the advantages of using weathering steel, the choice might appear to be obvious. However, the large potential savings in bridge maintenance costs in using weathering steel are not always realized because the protective patina layer fails to adhere when the steel is exposed to excessively salt-laden and moisture-laden environments. A multi-year study is underway by the FHWA to better quantify the conditions that lead to excessive corrosion in weathering steel bridges. This report documents work using computational fluid dynamics (CFD) analysis to study the conditions and mechanisms that lead from the salt spray thrown from truck tires to salt water droplets reaching bridge girders. Computer simulations use the advanced motion modeling capability called a “sliding mesh” in the CFD software to move one or more large trucks under bridges at 60 mph. The computations include multiphase spray droplet tracking of salt-laden water droplets coming off the tires. The objectives of the study were to build a model with a truck moving relative to a bridge in which the flow field could be solved by existing CFD software and available computational resources. Although this requirement entailed one of the major challenges of the study, it was accomplished using the sliding mesh capabilities of STAR-CCM+. The study included the goals of modeling three things that are related to the physics of droplet transport that were considered to be potentially significantly related to the amount of salt laden droplet spray that could reach the girder level of bridges when thrown from approaching truck tires when the trucks passed underneath the bridge. These three conditions were geometry effects, the effects of traffic, and the effect of the wind. Given the potential complexity of the multiphase flow field and interacting physics, a major objective of the study was to use the visualization capabilities of analysis software to show what is happening visually in the test cases that are significantly different in the amount of salt spray transported to bridge girder level in addition to the usual data processing that yields graphs to show relations between parameters. This report documents the results of the test cases that could be completed within the budget and time available for this study and also to document the model for future use. The goals for the test cases were to identify the primary mechanisms that contribute to de-icing salt transport from road surfaces up to weathering steel bridge girders that span over roads, and to test to the extent possible the relative importance of the mechanisms.

THREE-DIMENSIONAL SCOURING ANALYSIS FOR OPEN CHANNEL PRESSURE FLOW SCOUR UNDER FLOODED BRIDGE DECKS

A three-dimensional stream bed scour modeling methodology was developed using well-benchmarked commercial Computational Fluid Dynamics (CFD) software to compute the bed shear stress distribution used to calculate bed displacements and to re-mesh the computational domain as the bed is displaced. This study extends a previously developed two-dimensional iterative scouring procedure to predict the final shape and size of the scour-hole under pressure-scour flow conditions for flooded bridge decks using commercial CFD software. The current approach uses single phase flow models with an assumed flat water surface using a symmetric slip top boundary to simulate a free-surface flow condition, quasi-steady simulation to obtain the bed shear, and a moving boundary formulation based on an empirical correlation for critical shear stress to iteratively deform the bed under supercritical shear conditions until an equilibrium scour condition is obtained. The model solves the flow field using Reynolds Averaged NavierStokes (RANS) equations and the high Reynolds number k– epsilon turbulence model using the commercial CFD software STAR-CD. A Bash script was developed to use a Python script to compute bed displacements from the computed shear stress distribution and generate a STAR-CD processor command file to displace the bed followed by a step using the STAR-CCM+ software to remesh the domain as the bed is displaced and bed shear distribution is recomputed in an iterative procedure until the equilibrium bed contour is reached. Simulations were performed for different inundation ratios and for mean sand diameters of 1 mm and 2 mm. The model agrees reasonably well with limited experimental data for equilibrium scour shape and size with fully submerged cases compared to the cases where the bridge deck is partially submerged. This developed three-dimensional CFD scour computation procedure provides a basis for testing of additional scour related physical models while also providing an evaluation tool that can be used immediately by engineers engaged in scour risk analysis and assessment. NOMENCLATURE

CaBr2 hydrolysis for HBr production using a direct sparging contactor

We investigated a novel, continuous hybrid cycle for hydrogen production employing both heat and electricity. Calcium bromide (CaBr2) hydrolysis, which is endothermic, generates hydrogen bromide (HBr), and this is electrolysed to produce hydrogen. CaBr2 hydrolysis at 1 050 K is endothermic with a 181.5 KJ/mol heat of reaction and the free energy change is positive at 99.6 kJ/mol. What makes this hydrolysis reaction attractive is both its rate and the fact that well over half the thermodynamic requirements for water-splitting free energy of AGT = 285.8 KJ/mol are supplied at this stage using heat rather than electricity.

Development of an Analysis Methodology for Pressure Flow Scour Under Flooded Bridge Decks Using Commercial CFD Software

Bridges are a significant component of the ground transportation infrastructure in the United States. With about sixty percent of bridge failures due to hydraulic causes, primarily scour, application of computational fluid dynamics (CFD) analysis techniques to the assessment of risk of bridge failure under flood conditions can provide increased accuracy in scour risk assessment at a relatively low cost. The analysis can be used to make optimum use of limited federal and state funds available to maintain and replace bridges and ensure public safety while traveling on the nation’s roads and highways during and after floods. Scour is the erosion of riverbed material during high flow conditions, such as floods. When scouring of the supporting soil around the piers and abutments of bridges takes place, risk of bridge failure increases. A simulation methodology to conservatively predict equilibrium shape and size of the scour hole under pressure flow conditions for flooded bridge decks using commercial CFD software was developed. The computational methodology has been developed using C++ to compute changes in the bed contour outside of the CFD software and generate a re-meshing script to change the bed boundary contour. STAR-CD was used to run the hydrodynamic analysis to obtain bed shear stress, and a BASH script was developed to automate cycling between computing bed shear stress with the CFD software and computing changes in the bed contour due to scour predicted using the computed shear stress for the current bed contour. A single-phase moving boundary formulation has been developed to compute the equilibrium scour hole contour that proceeds through a series of quasi-steady CFD computations. It is based on CFD analysis of the flow fields around the flooded bridge deck and shear stress computed at the bed modeled as a rough wall. A high Reynolds number k-e turbulence model with standard wall functions, based on a Reynolds-Averaged Navier-Stokes (RANS) turbulence model, was used to compute bed shear stress. The scour sites on the bed were identified as those sites where the computed shear stress exceeded the critical shear stress computed from a published correlation for flat bed conditions. Comparison with experimental data obtained from the Turner-Fairbank Highway Research Center (TFHRC), McLean, VA, USA, revealed larger discrepancies than anticipated between the bridge inundation ratio and the scour hole depth. Although scour hole slopes were small for the cases tested, a correction to critical shear stress to account for bed slope was also tested. It did not significantly improve the correlation between CFD prediction and experimental observations. These results may be a consequence of using only excess shear stress above critical as a criteria for scour when other physical mechanisms also contribute to the initiation of scour. Prediction of scour depth using federal guidelines over predicts scour depth by as much as an order of magnitude in some cases. Over prediction is acceptable for purposes of ensuring bridge safety. CFD methods for scour prediction can be a significant improvement of current methods as long as under prediction of scour depth is avoided. Conservative scour prediction using CFD methods can be achieved by using conservative values of parameters such as critical shear stress and effective bed roughness.Copyright

Simulation of Open Channel Turbulent Flow Over Bridge Decks and Formation of Scour Hole Beneath the Bridge Under Flooding Conditions

This study is focused on the simulation of open channel turbulent flow over flooded laboratory scale bridge decks and formation of scour holes under various flooding conditions. Solutions for turbulent flow field are based on Reynolds Averaged Navier-Stokes (RANS) equations and turbulence closure models using the STAR-CD commercial computational fluid dynamics (CFD) software. An iterative computational methodology is developed for predicting equilibrium scour profiles using the single-phase flow model with a moving boundary formulation. The methodology relies on an empirical correlation for critical bed shear stress that is used to characterize the condition for onset of sediment motion and an effective bed roughness that is a function of sediment particle size. The computational model and iterative methodology were stable and converged to an equilibrium scour hole shape and size that compares reasonably well with experiment using a constant critical shear stress value.Copyright

Computational Modeling of Industrial Scale Reactor System Components Using Parallel Processing on a Networked Cluster of Computers

Industrial scale reactor system components are frequently characterized by a complex geometry that requires a large number of nodes in a computational grid to resolve significant flow features and by reaction and additional complex physics, such as spray vaporization, that greatly increase the number of partial differential equations (PDEs) to solve. On a fast serial workstation, simulations of these systems can take several days, making use of the model for extensive parametric and optimization studies impractical. A parallel multiphase reacting flow code was developed from a family of related serial codes using the industry standard Message-Passing Interface (MPI) and domain decomposition in the primary flow direction. The parallel computational fluid dynamics (CFD) code can now be run on a cluster of networked computers. This capability makes detailed simulation of many industrial reactors feasible without incurring the large increase in cost of moving to simulation on massively parallel computing platforms. The required computers and network are often already present in organizations and idle at night. The parallel CFD code has been applied to studies of transient heat transfer to monolith catalyst substrates and to the design of inlet chambers that evenly distribute a high speed hot air inlet stream over the monolith inlet plane. These studies are part of a program to investigate the feasibility of rapid start fuel reformers. The use of fuel reformers in light duty fuel cell powered vehicles requires a reformer start up time of less than 60 seconds. Parallelization issues and parallel performance are presented with examples of reformer component flow and comparison with experimental data from monolith heat up studies.Copyright