Model Validation Experiments Research Articles

Advanced materials modelling – E.U. perspectives

Modelling of advanced materials has become a main tool in materials development. This is particularly the case for present developments in advanced nuclear materials. This paper reviews the current status of modelling fuels and structural materials in the EU and discusses future programs. At the microscale, fundamental physics issues such as magnetism are being included to build a robust modelling scheme and to understand microstructural stability. At the meso-scale, dislocation interactions with defect clusters, void/bubbles and dispersoids are shown to be the main cause of hardening. How such issues, as well as model validation experiments are emerging to verify these modelling schemes is presented.

Inversion solutions of the elliptic cone model for disk frontside full halo coronal mass ejections

A new algorithm is developed for inverting six unknown elliptic cone model parameters from five observed CME halo parameters. It is shown that the halo parameter α includes the information on the coronal mass ejection (CME) propagation direction denoted by two model parameters. On the basis of the given halo parameter α, two approaches are presented to find out the CME propagation direction. The two‐point approach uses two values of α observed simultaneously by COR1 and COR2 on board STEREO A and B. The one‐point approach combines the value of α with such simultaneous observation as the location of CME‐associated flare, which includes the information associated with CME propagation direction. Model validation experiments show that the CME propagation direction can be accurately determined using the two‐point approach, and the other four model parameters can also be well inverted, especially when the projection angle is greater than 60°. The propagation direction and other four model parameters obtained using the one‐point approach for six disk frontside full halo CMEs appear to be acceptable, though the final conclusion on its validation should be made after STEREO data are available.

Seismic Velocity Structure and Seismotectonics of the Eastern San Francisco Bay Region, California

The Hayward Fault System is considered the most likely fault system in the San Francisco Bay Area, California, to produce a major earthquake in the next 30 years. To better understand this fault system, we use microseismicity to study its structure and kinematics. We present a new 3D seismic-velocity model for the eastern San Francisco Bay region, using microseismicity and controlled sources, which reveals a ∼10% velocity contrast across the Hayward fault in the upper 10 km, with higher velocity in the Franciscan Complex to the west relative to the Great Valley Sequence to the east. This contrast is imaged more sharply in our localized model than in previous regional-scale models. Thick Cenozoic sedimentary basins, such as the Livermore basin, which may experience particularly strong shaking during an earthquake, are imaged in the model. The accurate earthquake locations and focal mechanisms obtained by using the 3D model allow us to study fault complexity and its implications for seismic hazard. The relocated hypocenters along the Hayward Fault in general are consistent with a near-vertical or steeply east-dipping fault zone. The southern Hayward fault merges smoothly with the Calaveras fault at depth, suggesting that large earthquakes may rupture across both faults. The use of the 3D velocity model reveals that most earthquakes along the Hayward fault have near-vertical strike-slip focal mechanisms, consistent with the large-scale orientation and sense of slip of the fault, with no evidence for zones of complex fracturing acting as barriers to earthquake rupture. Online material: Velocity model validation experiments and additional seismicity plots.

Laser diagnostics and their interplay with computations to understand turbulent combustion

Laser diagnostics for fundamental investigation of turbulent combustion are discussed in the context of collaborative research that has been conducted over the past decade to contribute toward the development and experimental validation of predictive science-based models for turbulent flames. The emphasis is on simultaneous application of multiple laser techniques in flames having relatively simple fuels and flow geometries, as well as separate application of complementary diagnostics in the same flames. Data needs and design considerations for turbulent combustion model-validation experiments are outlined. Examples are given of ways in which the interplay of experiments and computations on “standard” turbulent flames has led to better understanding of these flames and also a better understanding of the capabilities of laser diagnostics and models to accurately capture the effects of turbulence–chemistry interactions. Issues of spatial resolution, differential diffusion, and LES validation are discussed, and perspectives on current research challenges are offered.

Plant canopy effects on soil thermal and hydrological properties and soil respiration

A soil–plant–air continuum multilayer model was developed to simulate soil respiration. The model can be used to investigate the effects of three canopy characteristics on soil thermal and hydrological properties and soil respiration: leaf area index, the maximum rate of carboxylation, and the vertical profile of leaf area density. The model did not consider physiological effects of canopy characteristics, such as the effect of photosynthesis on root respiration. Rather, it examined how changes in temperature and moisture in the soil layers, caused by changes in the plant canopy, affect soil respiration. Model validation and numerical experiments were performed using above-canopy hydrometeorological variables, including seasonal changes related to a dry season with strong evaporative demand (high solar radiation and high temperature) and a rainy season with weaker evaporative demand. The results suggest that the model can successfully reproduce seasonal changes in soil profiles of moisture, temperature, CO 2 gas concentration, and respiration, and that canopy characteristics can limit soil respiration because of thermal and hydrological effects. An increase in leaf area decreased soil respiration in two ways: by decreasing soil temperature through a reduction in net radiation to the soil surface and by decreasing soil moisture through an increase in canopy transpiration. An increase in the maximum rate of carboxylation decreased soil respiration because it decreased soil moisture by increasing canopy transpiration; there was little thermal effect on soil respiration. A canopy with a denser leaf area in the upper layers created a faster wind velocity in the lower portion and on the ground than did a canopy with the same leaf area index, but with a denser leaf area in the lower layers. The faster wind velocity significantly decreased soil moisture at the soil surface and decreased soil temperature through an increase in soil evaporation; these effects decreased soil respiration. The hydrological effects of the three parameters were most evident in the dry season, while the thermal effects were evident year-round.

A generalized neural network model of ball-end milling force system

The focus of this paper is to develop a reliable method to predict 3D cutting forces during ball-end milling process. This paper uses the artificial neural networks (ANNs) approach to evolve an generalized model for prediction of cutting forces, based on a set of input cutting conditions. A set of ten input milling parameters that have a major impact on the cutting forces was chosen to represent the machining conditions. The training of the networks is performed with experimental machining data. This approach greatly reduces the time-consuming mathematical work normally required for obtaining the cutting force expressions. The estimation performance of the network is evaluated through a detailed simulation study. The accuracy of an analytical model, which is a feasible alternative to the network, is compared to that of the network. With similar system parameter estimates for both methods, the network is found to be considerably more accurate than the analytical model. The results of model validation experiments on machining Ck45 are also reported. Experimental results demonstrate that this method can accurately estimate feed cutting force within an error of 4%. The results also indicate that when the combination of sigmoidal and gaussian transfer function were applied, the prediction accuracy of neural network is as high as 98%.

Fragmentation of Materials In Expanding Tube Experiments

The phenomena of dynamic fracture and fragmentation are important in a variety of engineering applications. Computational modeling of these events presents unique challenges, not the least of which is the experimental validation of models. In this report, we describe recent developments on a technique for the expansion and fragmentation of tubes using a two-stage light gas gun. Because of the controlled environment of the experiment, a variety of diagnostic techniques can be employed. The available instrumentation, along with the repeatability of the technique, makes it well suited for model validation experiments. We present results of experiments on two materials, which provide insight into the physics of the process. Results of a limited number of simulations performed with CTH are also presented.

A respiration–diffusion model for ‘Conference’ pears

Abstract A respiration–diffusion model for ‘Conference’ pears was used to simulate three-dimensional internal O 2 and CO 2 concentration profiles for intact pears as a function of the storage conditions. Although the skin was the major barrier to gas transport, the internal gas gradients in the fruit could not be neglected. Especially at high temperatures, when the respiration rate is high, the internal gradients increased considerably. A sensitivity analysis was carried out to assess the effect of biological variability present on the respiration and diffusion parameters, and on the gas concentration profiles. It was found that at room temperature, the O 2 concentration underneath the fruit skin is mainly determined by the O 2 transfer coefficient and consumption rate. However, for typical storage temperatures this dependency is less pronounced. A comparison of simulated gas concentration contours of pears stored with delayed controlled atmosphere and those immediately stored under controlled atmosphere (CA), illustrated the importance of an appropriate storage management right after harvest. When CA storage was applied immediately after harvest, the O 2 and CO 2 concentrations reached very low and very high values, respectively, possibly causing irreversible membrane damage and inducing core breakdown. These values were not found for delayed controlled atmosphere. A model validation experiment on affected pears indicated that the diffusion and respiration parameters for affected tissue were different from those for healthy tissue.

Sculpture surface machining: a generalized model of ball-end milling force system

A new mechanistic model is presented for the prediction of a cutting force system in ball-end milling of sculpture surfaces. The model has the ability to calculate the workpiece/cutter intersection domain automatically for a given cutter location (CL) file, cutter and workpiece geometries. Furthermore, an analytical approach is used to determine the instantaneous chip load (with and without runout) and cutting forces. In addition to predicting the cutting forces, the model also employs a Boolean approach for a given cutter, workpiece geometries, and CL file to determine the surface topography and scallop height variations along the workpiece surface which can be visualized in 3-D. The results of model validation experiments on machining Ti-6A1-4V are also reported. Comparisons of the predicted and measured forces as well as surface topography show good agreement.

A three-dimensional spatial model for plant competition in an heterogeneous soil environment

The model described in this paper, the three-dimensional model for the interaction of plants and soil (3DMIPS), was developed to explore how the growth and competitive ability of plants is affected by the interactions between the size and scaling properties of their root systems and the spatial properties of the resources they consume. The main characteristics of the model are as follows. (1) The soil model simulates the spatiotemporal dynamics of temperature, water and solutes (nutrients) in a heterogeneous, 3-dimensional (3-D) soil profile. (2) The plant model has a 3-D simulation of root architecture and plasticity (the ability of certain plants to redirect root growth to areas with high soil nutrient concentration). Plant root architecture is a key component in the simulation of plant nutrient uptake and transpiration. (3) The model simulates the spatiotemporal dynamics of live root, dead root, live shoot, standing dead, and litter biomass and nutrients of individual plants. It simulates the translocation of biomass and nutrients between roots and shoots as a function of plant growth, nutrient uptake, defoliation, regrowth, senescence, and changes in environmental condition. The author presents two set of simulation results. The first set consist of model validation experiments where the author compares model results with data from experiments designed to test the model's ability to predict plant growth and competition in a heterogeneous soil nutrient environment. Model and experimental results were statistically similar in 297 of the 408 variables analyzed. Experimental and modeled variables that were statistically different, were nevertheless highly correlated, with the difference between model and experimental results averaging ±28%. The second set of simulations was designed to assess how plant growth can be affected by an interaction between root lateral spread and the supply rate and spatial distribution of soil nutrients. Results showed that the competitive ability, distribution, and performance of plants could be linked in part to interactions among the size, uptake rates, and scaling properties of their root systems and the supply rate and spatial distribution of the nutrients they utilize. Comparisons are made with the SPUR, PHOENIX, CENTURY, and GEM grassland models. Computer codes and instructions for the implementation of 3DMIPS are available from the author.

Progress in bidirectional reflectance modeling and applications for surface particulate media: Snow and soils

Surface particulate media such as soil and snow have significant impacts on global systems because of their contributions to the radiation balance of the Earth system, their roles in the hydrologic cycle, and their extensive spatial distributions. At regional and global scales, characteristics of soils and snow cover can best be mapped and analyzed using satellite‐based sensors that acquire both spectral and angular reflectance information. Advances in surface reflectance models over the past decade now provide improved interpretation of remote sensing imagery over snow. However, advances have been slower in remote sensing of soil properties and a number of key issues remain. This article begins with an overview of the similarities and differences between particulate media modeling approaches for soil and snow. We review common modeling methods: radiative transfer approximations, numerical solutions, and geometrical optics. Model inversion and validation experiments are summarized and research priorities ...

A High Performance Pneumatic Force Actuator System: Part I—Nonlinear Mathematical Model

In this paper, we developed a detailed mathematical model of dual action pneumatic actuators controlled with proportional spool valves. Effects of nonlinear flow through the valve, air compressibility in cylinder chambers, leakage between chambers, end of stroke inactive volume, and time delay and attenuation in the pneumatic lines were carefully considered. We performed system identification, numerical simulation, and model validation experiments for two types of air cylinders and different connecting tubes length. The mathematical model of the present article is used in a sequel article to develop high performance nonlinear pneumatic force controllers. [S0022-0434(00)00503-7]

Development and testing of diesel engine CFD models

The development and validation of Computational Fluid Dynamic (CFD) models for diesel engine combustion and emissions is described. The complexity of diesel combustion requires simulations with many complex, interacting submodels in order to be successful. The review focuses on the current status of work at the University of Wisconsin Engine Research Center. The research program, which has been ongoing for over five years, has now reached the point where significant predictive capability is in place. A modified version of the KIVA code is used for the computations, with improved submodels for liquid breakup, drop distortion and drag, spray-wall impingement with rebounding, sliding and breaking-up drops, wall heat transfer with unsteadiness and compressibility, multistep kinetics ignition and laminar-turbulent characteristic time combustion models, Zeldovich NO x formation, and soot formation with Nagle-Strickland-Constable oxidation. The code also considers piston-cylinder-liner crevice flows and allows computations of the intake flow process in the realistic engine geometry with two moving intake valves. A multicomponent fuel vaporization model and a flamelet combustion model have also been implemented. Significant progress has been made using a modified RNG k-ε turbulence model. This turbulence model is capable of predicting the large-scale structures that are produced by the squish flows and generated by the spray. These flow structures have an important impact on the prediction of NO x formation since it is very sensitive to the local temperatures in the combustion chamber. Model validation experiments have been performed using a single-cylinder version of a heavy duty truck engine that features state-of-the-art high-pressure electronic fuel injection and emissions instrumentation. In addition to cylinder pressure, heat release, and emissions measurements, combustion visualization experiments have been performed using an endoscope system that takes the place of one of the exhaust valves. In-cylinder gas velocity (PIV) and gas temperature measurements have also been made in the motored engine using optical techniques. Modifications to the engine geometry for optical access were minimal, thus ensuring that the results represent the actual engine. Experiments have also been conducted to study the effect of injection characteristics, including injection pressure and rate, nozzle inlet condition and multiple injections on engine performance and emissions. The results show that multiple pulsed injections can be used to significantly reduce both soot and NO x simultaneously in the engine. In addition, when combined with exhaust gas recirculation to further lower NO x, pulsed injections are found to be still very effective at reducing soot. The intake flow CFD modeling results show that the details of the intake flow process influence the engine performance. Comparisons with the measured engine cylinder pressure, heat release, soot and NO x emission data, and the combustion visualization flame images show that the CFD model results are generally in good agreement with the experiments. In particular, the model is able to correctly predict the soot—NO x trade-off trend as a function of injection timing. However, further work is needed to improve the accuracy of predictions of combustion with late injection, and to assess the effect of intake flows on emissions.

Aerobic deterioration of grass silage: the need to couple models and experiments

AbstractA mathematical model of the aerobic stability of grass silage is presented. The model is shown to predict as well as more complex models previously published. Sensitivity analysis performed on model parameters suggests that current understanding of the temperature dependence of yeast growth needs to be advanced in order to produce more accurate models of deterioration. Inhibition of yeast growth by organic acids is identified as a critical process worthy of further investigation. We discuss how model validation experiments must identify different yeast species and track their growth separately. Such experiments should also attempt to minimize or measure heat losses.

Statistical comparison of spatial fields in model validation, perturbation, and predictability experiments

The comparison of spatial fields of meteorological variables is an essential component of model validation studies and is central in assessing the significance of any change between a perturbed and control run of a general circulation model. Comparisons may be made of statistics which define the time‐mean state, the temporal variability about this state, and/or spatial variability. Comparisons may also be made of the two time‐mean spatial patterns, or of the temporal evolutions of spatial patterns. We consider here a suite of univariate and multivariate statistics which may be used to make these comparisons. Some of these statistics have been used previously, while others are either new or have not previously been used in the present context. The use of these statistics, their differences and similarities, and their relative performances are illustrated by considering mean sea level pressure changes between the decades 1951–1960 and 1971–1980 over an area covering North America, the North Atlantic Ocean, and Europe. Significance levels are assessed using the pool‐permutation procedure of Preisendorfer and Barnett (1983) (henceforth P+B). This overcomes problems arising from nonideal behavior of the data (particularly spatial autocorrelation), unknown sampling distributions, and multiplicity in the case of univariate statistics. A subset of statistics is identified as most useful. For tests of differences in means these are the grid point by grid point t‐test, a test comparing the overall means, and P+B's SITES statistic. For tests of differences in temporal variability they are the grid point by grid point F‐test, and SPRET1 (the ratio of the spatial means of the time variances). SPRET1 is a modification of P+B's SPRED statistic designed to identify the direction of any variance difference. As a test of spatial variability differences, we identify SPREX1 (the ratio of the time means of the spatial variances), and for comparing spatial patterns the best statistic is the (spatial) correlation coefficient between the time‐mean fields. For comparing the temporal evolution of spatial patterns, we recommend using the time‐mean anomaly field correlation which is a more easily interpreted equivalent to P+B's SHAPE statistic.

Operational Experience with the DATS Sampler during the Canadian Tritium Modeling Experiment

An international tritium model validation experiment was held at Chalk River, Canada, during June 1987. The Princeton Plasma Physics Laboratory (PPPL) Differential Atmospheric Tritium Sampler (DATS) was one of the many types of tritium samplers used for this experiment. Besides the modeling data that were produced from this experiment, the authors learned how well our tritium samplers performed when a known tritium quantity was released. The DATS were set up at 50, 100, 200, and 400 meters downwind from the release point. Data were collected during the release period and for the next 24 hours. While the units worked very well in the field, valued operational experience was gained in the recovery of the tritium from the silica gel. Because of delays in the analysis of the collected samples, it became difficult to recover the HTO fraction quantitatively. Indications are that molecular sieves are more suitable for samples which are not going to be processed immediately. This paper reports on the field set up, the measurement results, and operational experience.

Nocturnal boundary-layer height: Observations by acoustic sounders and predictions in terms of surface-layer parameters

Acoustic sounder measurements of the stable boundary-layer height taken during the EPRI Plume Model Validation and Development Project experiment are examined. Comparison of simultaneous measurements by two sodars located 15 km apart shows good agreement. Several widely used diagnostic formulas for estimation of the boundary-layer height, based on wind speed and surface-layer parameters, such as friction velocity and Monin-Obukhov length, are tested against the sodar data. Of these, best performance is found using a simple linear relationship with friction velocity or, alternatively, wind speed at 10 m height. No evidence is found to support the more often used Zilitinkevich (1972) formula. Tests using selected data from the Cabauw site in the Netherlands confirm the results found on the basis of EPRI data.