W. David Pointer

DOE's Effort to Reduce Truck Aerodynamic Drag Through Joint Experiments and Computations

Class 8 tractor-trailers are responsible for 11-12% of the total US consumption of petroleum. Overcoming aero drag represents 65% of energy expenditure at highway speeds. Most of the drag results from pressure differences and reducing highway speeds is very effective. The goal is to reduce aerodynamic drag by 25% which would translate to 12% improved fuel economy or 4,200 million gal/year. Objectives are: (1) In support of DOEs mission, provide guidance to industry in the reduction of aerodynamic drag; (2) To shorten and improve design process, establish a database of experimental, computational, and conceptual design information; (3) Demonstrate new drag-reduction techniques; and (4) Get devices on the road. Some accomplishments are: (1) Concepts developed/tested that exceeded 25% drag reduction goal; (2) Insight and guidelines for drag reduction provided to industry through computations and experiments; (3) Joined with industry in getting devices on the road and providing design concepts through virtual modeling and testing; and (4) International recognition achieved through open documentation and database.

Effects of mesh density and flow conditioning in simulating 7-pin wire wrapped fuel pins.

In response to the goals outlined by the U.S. Department of Energy’s Global Nuclear Energy Partnership program, Argonne National Laboratory has initiated an effort to create an integrated multi-physics multi-resolution thermal hydraulic simulation tool package for the evaluation of nuclear power plant design and safety. As part of this effort, the applicability of a variety of thermal hydraulic analysis methods for the prediction of heat transfer and fluid dynamics in the wire-wrapped fuel-rod bundles found in a fast reactor core is being evaluated. The work described herein provides an initial assessment of the capabilities of the general purpose commercial computational fluid dynamics code Star-CD for the prediction of fluid dynamic characteristics in a wire wrapped fast reactor fuel assembly. A 7-pin wire wrapped fuel rod assembly based on the dimensions of fuel elements in the concept Advanced Burner Test Reactor [1] was simulated for different mesh densities and domain configurations. A model considering a single axial span of the wire wrapped fuel assembly was initially used to assess mesh resolution effects. The influence of the inflow/outflow boundary conditions on the predicted flow fields in the single-span model were then investigated through comparisons with the central span region of models which included 3 and 5 spans. The change in grid refinement had minimal impact on the inter-channel exchange within the assembly resulting in roughly a 5 percent maximum difference. The central span of the 3-span and 5-span cases exhibits much higher velocities than the single span case,, with the largest deviation (15 to 20 percent) occurring furthest away from the wire spacer grids in the higher velocity regions. However, the differences between predicted flow fields in the 3-span and 5-span models are minimal.Copyright

A POD-based solver for the advection-diffusion equation

We present a methodology based on proper orthogonal decomposition (POD). We have implemented the POD-based solver in the large eddy simulation code Nek5000 and used it to solve the advection-diffusion equation for temperature in cases where buoyancy is not present. POD allows for the identification of the most energetic modes of turbulence when applied to a sufficient set of snapshots generated through Nek5000. The Navier-Stokes equations are then reduced to a set of ordinary differential equations by Galerkin projection. The flow field is reconstructed and used to advect the temperature on longer time scales and potentially coarser grids. The methodology is validated and tested on two problems: two-dimensional flow past a cylinder and three-dimensional flow in T-junctions. For the latter case, the benchmark chosen corresponds to the experiments of Hirota et al., who performed particle image velocimetry on the flow in a counterflow T-junction. In both test problems the dynamics of the reduced-order model reproduce well the history of the projected modes when a sufficient number of equations are considered. The dynamics of flow evolution and the interaction of different modes are also studied in detail for the T-junction.© 2011 ASME

Prediction of Boiling Water Reactor Assembly Void Distributions Using a Two-Phase Computational Fluid Dynamics Model

This paper presents recent results obtained as part of the on-going integral validation of an advanced Eulerian-Eulerian two-phase (E2P) computational fluid dynamics based boiling model that allows the detailed analysis of the two-phase flow and heat transfer phenomena in a Boiling Water Reactor (BWR) fuel assembly. The code is being developed as a customized module built on the foundation of the commercial CFD-code STAR-CD which provides general two-phase flow modeling capabilities. Simulations of a prototypic BWR fuel assembly experiment have been completed as an initial assessment of the applicability of the E2P model to realistic BWR geometries and conditions. Initial validation has focused on comparison with measured sub-channel averaged data to enable the benchmarking of the accuracy of the E2P against the current predictive capabilities of the sub-channel methods. The paper will discuss the effects of modeling assumptions, assumed coefficient values and the computational mesh structure used to describe the fuel assembly geometry on the accuracy of the sub-channel averaged void fraction.© 2008 ASME

Commercial CFD Code Validation for Heavy-Vehicle External Aerodynamics Simulation

The issue of energy economy in transportation has grown beyond traditional concerns over environment, safety and health to include new concerns over national security and energy self-sufficiency. As part of the U.S. Department of Energy Office of FreedomCAR and Vehicle Technologies’ Working Group on Aerodynamic Drag of Heavy Vehicles, Argonne National Laboratory is independently investigating the accuracy of aerodynamic drag predictions generated by commercial Computational Fluid Dynamics (CFD) Software. In this validation study, computational predictions from two commercial CFD codes, Star-CD [1] and PowerFLOW [2], will be compared with detailed velocity, pressure and force balance data from experiments completed in the 7 ft. by 10 ft. wind tunnel at NASA Ames [3,4] using a Generic Conventional Model (GCM) that is representative of typical current-generation tractor-trailer geometries. This paper highlights results from evaluations of drag coefficient predictions using standard two-equation steady RANS turbulence models and logarithmic wall functions that were completed as part of the first phase of these studies.

Optimal Disturbances in Three-Dimensional Natural Convection Flows

Buoyancy-driven systems are subject to several types of flow instabilities. To evaluate the performance of such systems it is becoming increasingly crucial to be able to predict the stability of a given base flow configuration. Traditional Modal Linear stability Analysis requires the solution of very large eigenvalue systems for three-dimensional flows, which make this problem difficult to tackle.An alternative to modal Linear stability Analysis is the use of adjoint solvers [1] in combination with a power iteration [2]. Such methodology allows for the identification of an optimal disturbance or forcing and has been recently used to evaluate the stability of several isothermal flow systems [2]. In this paper we examine the extension of the methodology to non-isothermal flows driven by buoyancy. The contribution of buoyancy in the momentum equation is modeled through the Boussinesq approximation.The method is implemented in the spectral element code Nek5000. The test case is the flow is a two-dimensional cavity with differential heating and conductive walls and the natural circulation flow in a toroidal thermosiphon.Copyright

Development and Validation of a Computational Fluid Dynamics Model for the Simulation of Two-Phase Flow Phenomena in a Boiling Water Reactor Fuel Assembly

This paper presents the current status in the development and validation of an advanced Computational Fluid Dynamics (CFD) model, CFD-BWR, which allows the detailed analysis of the two-phase flow and heat transfer phenomena in Boiling Water Reactor (BWR) fuel assemblies under various operating conditions. The CFD-BWR model uses an Eulerian Two-Phase (E2P) approach, and is also referred to as the E2P modeling framework. It is being developed as a customized module built on the foundation of the commercial CFD-code STAR-CD which provides general two-phase flow modeling capabilities. The integral validation efforts have focused on the analysis of the NUPEC Full-Size Boiling Water Reactor Test (BFBT) within the framework of the OECD/NRC benchmark exercise. The paper reviews the two-phase models implemented in the CFD-BWR code, and emphasizes recently implemented models of inter-phase and coolant-cladding momentum and energy exchanges. Results of recent BFBT experiment simulations using these models are presented and the effects of the new models on the calculated void distribution are discussed. The paper concludes with a discussion of future model development and validation plans.Copyright

Proposed experiment for validation of CFD methods for advanced SFR design: Upper plenum thermal striping and stratification

In response to the goals outlined by the U.S. Department of Energy’s Advanced Fuel Cycle Initiative, an effort is underway to develop an integrated multi-physics, multi-resolution thermal-hydraulic simulation tool package for the evaluation of nuclear power plant design and safety. As part of this effort, initial guidance has been proposed for the development of experiments to supply validation data sets for the CFD-based thermo-fluid simulation capability. To demonstrate that the proposed data requirements can be achieved using current generation measurement methods and to refine correlation and data comparison methods suitable for very large data sets, an initial experiment focused on turbulent mixing in the upper plenum of an advanced sodium fast reactor has been proposed. Prior validation efforts to support the use of one-dimensional lumped parameter models in the analysis of reactor safety performance relied primarily on data from carefully scaled integral system experiments to validate and tune correlations used to represent the physics associated with a particular transient in a particular reactor design. Unlike the correlation-based lumped parameter codes, computational fluid dynamics simulations reduce the reliance on experimentally derived correlations to the prediction of local turbulence effects rather the prediction of integral quantities like pressure drop and heat transfer coefficients. As a consequence, simpler separate effects experiments, which capture the turbulence effects but not necessarily the integral effects within a specific component of a system, can be utilized as the primary validation basis for the CFD codes. However, while the need for large carefully scaled integral experiments is reduced, the high spatial and temporal resolution of these codes requires that experimental data be collected at fine spatial and temporal resolutions. An initial series of simulations has been completed to support the development of the proposed experimental facility using air as a surrogate for the sodium coolant. Design options considered in RANS simulations using the commercial CFD code Star-CCM+ include mixing facility dimensions, the number of inlet jets to be included and outlet position. The use of RANS simulations is supported by an initial benchmarking comparison with predictions from the spectral element large eddy simulation code Nek5000 for the nominal experimental geometry.© 2009 ASME

On the Interaction of Boundary Layer and Mixing Layer in Stratified Pipe Flow

Stratified pipe flow in a pipe has been the subject of several investigations over the years. In fact it is relevant to the operation of thermal energy systems involving significant temperature gradients and low flow conditions. Stratification affects mixing, and is one the key phenomena that need to be addressed in the design of any mixing system of devices involving significant density difference. In order for thermal stratification in a pipe to be correctly modeled the underlying hydrodynamic behavior related to the mixing of two streams in a pipe needs to be fully understood.The present paper deals with the numerical simulation of the flow in a pipe where the bottom half has a lower velocity compared to the upper half. A turbulent mixing layer develops in the streamwise direction at the interface between low flow region and higher flow region. Since the Reynolds number is low, the boundary layer and the mixing layer are about the same size. This translates in non trivial interactions between the structures in the boundary layer and the mixing layer. The failure of RANS models in accounting for the mixing points in this direction.An LES of the flow in this geometry has been performed with the spectral code Nek5000. The averaged statistics have been compared with available experimental results. Proper Orthogonal Decomposition has been applied to clarify outstanding issues in RANS modeling.Copyright

Commercial CFD Code Validation for Simulation of Heavy-Vehicle External Aerodynamics

The issue of energy economy in transportation has grown beyond traditional concerns over environment, safety and health to include new concerns over national and international security. In collaboration with the U.S. Department of Energy Office of FreedomCAR and Vehicle Technologies’ Working Group on Aerodynamic Drag of Heavy Vehicles, Argonne National Laboratory is investigating the accuracy of aerodynamic drag predictions from commercial Computational Fluid Dynamics (CFD) Software. In this validation study, computational predictions from two commercial CFD codes, Star-CD [1] and PowerFLOW [2] , will be compared with detailed velocity, pressure and force balance data from experiments completed in the 7 ft. by 10 ft. wind tunnel at NASA Ames [3, 4] using a Generic Conventional Model (GCM) that is representative of typical current-generation tractor-trailer geometries.Copyright