Kenneth S. Ball

Dynamical eigenfunction decomposition of turbulent pipe flow

The results of an analysis of turbulent pipe flow based on a Karhunen–Loeve decomposition are presented. The turbulent flow is generated by a direct numerical simulation of the Navier–Stokes equations using a spectral element algorithm at a Reynolds number Reτ = 150. This simulation yields a set of basis functions that captures 90% of the energy after 2763 modes. The eigenfunctions are categorized into two classes and six subclasses based on their wavenumber and coherent vorticity structure. Of the total energy, 81% is in the propagating class, characterized by constant phase speeds; the remaining energy is found in the non-propagating subclasses, the shear and roll modes. The four subclasses of the propagating modes are the wall, lift, asymmetric and ring modes. The wall modes display coherent vorticity structures near the wall, the lift modes display coherent vorticity structures that lift away from the wall, the asymmetric modes break the symmetry about the axis, and the ring modes display rings of coh...

Structure and dynamics of low Reynolds number turbulent pipe flow

Using large-scale numerical calculations, we explore the proper orthogonal decomposition of low Reynolds number turbulent pipe flow, using both the translational invariant (Fourier) method and the method of snapshots. Each method has benefits and drawbacks, making the ‘best’ choice dependent on the purpose of the analysis. Owing to its construction, the Fourier method includes all the flow fields that are translational invariants of the simulated flow fields. Thus, the Fourier method converges to an estimate of the dimension of the chaotic attractor in less total simulation time than the method of snapshots. The converse is that for a given simulation, the method of snapshots yields a basis set that is more optimal because it does not include all of the translational invariants that were not a part of the simulation. Using the Fourier method yields smooth structures with definable subclasses based upon Fourier wavenumber pairs, and results in a new dynamical systems insight into turbulent pipe flow. These subclasses include a set of modes that propagate with a nearly constant phase speed, act together as a wave packet and transfer energy from streamwise rolls. It is these interactions that are responsible for bursting events and Reynolds stress generation. These structures and dynamics are similar to those found in turbulent channel flow. A comparison of structures and dynamics in turbulent pipe and channel flows is reported to emphasize the similarities and differences.

The effect of spanwise wall oscillation on turbulent pipe flow structures resulting in drag reduction

The results of a comparative analysis based upon a Karhunen–Loeve expansion of turbulent pipe flow and drag reduced turbulent pipe flow by spanwise wall oscillation are presented. The turbulent flow is generated by a direct numerical simulation at a Reynolds number Reτ=150. The spanwise wall oscillation is imposed as a velocity boundary condition with an amplitude of A+=20 and a period of T+=50. The wall oscillation results in a 27% mean velocity increase when the flow is driven by a constant pressure gradient. The peaks of the Reynolds stress and root-mean-squared velocities shift away from the wall and the Karhunen–Loeve dimension of the turbulent attractor is reduced from 2763 to 1080. The coherent vorticity structures are pushed away from the wall into higher speed flow, causing an increase of their advection speed of 34% as determined by a normal speed locus. This increase in advection speed gives the propagating waves less time to interact with the roll modes. This leads to less energy transfer and ...

Modeling and experimental results for an RTP light-pipe radiation thermometer calibration testbed

A thermometry testbed designed for the testing, analysis, and calibration of light pipe thermometers and thermocouple-instrumented silicon wafers used in RTP tools has been constructed, and comparison of measured wafer temperature distributions on the instrumented wafers with light-pipe radiation thermometer measurements have been carried out. The test chamber has been modeled using detailed Monte Carlo simulation including measured specular/diffuse surface properties, and predictions of the model have been compared with measured results and are presented. The chamber is presently being modified to test advanced temperature measurement techniques, which are also described

Drawdown-Effect of Lightpipes in Silicon Wafer Surface Temperature Measurements

Lightpipe radiation thermometers (LPRTs) have been widely used for temperature measurement in the semiconductor industries. According to the International Technology Roadmap for Semiconductors 2004 (ITRS), temperatures for semiconductor wafer processing should be measurable to within an uncertainty of ± 1.5°C at 1,000 °C with temperature calibration traceable to ITS (international temperature standard)-90. To achieve this uncertainty, there are several issues associated with LPRTs to be resolved. The “draw-down effect” is the one that will be examined in this paper. We discuss this effect both experimentally and numerically in the temperature range of 500°C to 900°C.Copyright

Numerical Investigation of Transient Marangoni Effects on Weld Pool Dynamics in Gas Tungsten Arc Welding of Stainless Steel

In this study, the fluid flow, heat transfer, and phase change occurring during the Gas Tungsten Arc (GTA) welding of stainless steel are numerically analyzed. Of all the driving forces involved during GTA welding, the Marangoni convection due to surface tension gradient is the most dominant. The amount of surfactant present in the metal strongly influences the strength, location, and direction of the Marangoni convection. Limited work has been done in the study of transient Marangoni convection behavior in the regime where a significant amount of surfactant is present, and depending on the thermal distribution in the molten metal pool, the surface tension gradient undergoes inflection, resulting in a flow reversal. The present investigation aims to address the complex relationship among the flow reversal, surfactant activity, and thermal pattern in the weld pool.Copyright

Initial Results From an Interdisciplinary Review of Trust Research

In the days of modern engineering, a complex system can be designed and built using numerous sources of information, knowledge, hardware, and software. A factor that impacts the success of a complex system is trust. In designing a framework that allows for a unified trust model or trusting picture and defining a reliable metric for measuring trustworthiness, we are examining definitions and methodologies from social sciences and engineering. This paper uses a combination of publication analysis of research literature including psychological, sociological, economic, automation, and cyberspace perspectives of trust and technical dialogues with the subject matter experts at the Air Force Research Laboratory, to illuminate the interdisciplinary approach undertaken in hardware centric design with human interface. We review past work to highlight trustworthiness characteristics and trust measurements that conceptually could apply across fields under examination. We expect to create a more rigorous definition of trust and trustworthiness that leads to finding the appropriate metrics to measure trust and trustworthiness dynamically.Copyright

Turbulent Boundary Layer Control by Wall Oscillations

The objective of this paper is to examine the effect and the effectiveness of wall oscillation as a control scheme of drag reduction in order to better understand the mechanism of drag reduction. Two flow configurations were considered: constant flow rate and constant mean pressure gradient. The Navier-Stokes equations were solved using Fourier-Chebyshev spectral methods and the oscillation in sinusoidal form was enforced on the walls through boundary conditions for the spanwise velocity component. Results to be shown included the effects of oscillation frequency, amplitude, oscillation orientation, and peak wall speed at Reynolds number of 180 based on wall-shear velocity and channel half-width as well as the Reynolds number dependency s in both flow configurations. Comparison of effectiveness made between these two flow configurations has showed similarities as well as differences. Drag reduction as a function of peak wall speed was compared with both experimental and numerical data and the agreement was good in the trend and in the quantity. Comparison between these two flow configurations in the transient response to the sudden start of wall oscillation, turbulence statistics, and instantaneous flow fields was detailed and differences were clearly shown. Analysis and comparison are allowed to shed some light on the way that oscillations interact with wall turbulence.Copyright

Coupled Field Analysis of a Gas Tungsten Arc Welded Butt Joint—Part I: Improved Modeling

Thermally induced residual stresses due to welding can significantly impair the performance and reliability of welded structures. From a structural integrity perspective of welded structures, it is necessary to have an accurate spatial and temporal thermal distribution in the welded structure before stress analysis is performed. Existing research has ignored the effect of fluid flow in the weld pool on the temperature field of the welded joint. Previous research has established that the weld pool depth/width (D/W) ratio and heat affected zone (HAZ) are significantly altered by the weld pool dynamics. Hence, for a more accurate estimation of the thermally induced stresses it is desired to incorporate the weld pool dynamics into the analysis. Moreover, the effects of microstructure evolution in the HAZ on the mechanical behavior of the structure need to be included in the analysis for better mechanical response prediction. In this study, a three-dimensional numerical model for the thermomechanical analysis of gas tungsten arc (GTA) welding of thin stainless steel butt-joint plates has been developed. The model incorporates the effects of thermal energy redistribution through weld pool dynamics into the structural behavior calculations. Through material modeling the effects of microstructure change/phase transformation are indirectly included in the model. The developed weld pool dynamics model includes the effects of current, arc length, and electrode angle on the heat flux and current density distributions. All the major weld pool driving forces are included, namely surface tension gradient induced convection, plasma induced drag force, electromagnetic force, and buoyancy. The weld D/W predictions are validated with experimental results. They agree well. The workpiece deformation and stress distributions are also highlighted. The mathematical framework developed here serves as a robust tool for better quantification of thermally induced stress evolution and distribution in a welded structure by coupling the different fields in a welding process.

A Comprehensive Study of Residual Stresses in a Gas Tungsten Arc Welded Butt Joint

Thermally induced residual stresses due to welding can significantly impair the performance and reliability of welded structures. Existing research has ignored the effect of fluid flow in the weld pool on the temperature field of the welded joint. Hence, for a more accurate estimation of the thermally induced residual stresses it is desired to incorporate the weld pool dynamics into the analysis. Various welding parameters (like, welding speed, current, arc length, surfactant activity, plasma drag etc.) influence the weld pool dynamics, which in turn affect the thermal history of the workpiece. Such integration would help in better quantification of thermal stress evolution and residual stress distribution in the welded joint. In this study, a three-dimensional numerical model for the thermo-mechanical analysis of Gas Tungsten Arc (GTA) welding of a butt joint of thin stainless steel plates has been developed. The effects of welding parameters on the residual stress distribution are documented.Copyright