Uncertain Modelling Research Articles

Impurity density derivation from bandpass soft x-ray tomography: applicability, perspectives and limitations

Deriving impurity density from local soft x-ray (SXR) emissivity (reconstructed by background-subtracted tomography during trace injections) by line-emission modelling leads to uncertainties due to the unknown ionization balance, especially for medium-Z impurities. This paper proposes a paradigm shift in the way SXR tomography is used, from one that maximizes signal at the expense of uncertain modelling to one that relies on sharper spectral resolution used to better understand what exactly is being measured. It is indeed shown that the measured SXR emissivity of an impurity may be robust with respect to changes to its unknown ionization balance (i.e. with respect to impurity transport) under two conditions. First, the electron temperature must be above a certain threshold (typically 4–5 keV or higher for metals like Ni or Fe). Second, the spectral response of SXR detectors must have a tuneable band-pass and should focus on a specific spectral region for each considered impurity. Both these conditions aim at maximizing the Bremsstrahlung contribution, which has the weakest dependence on ion charge. Prospective applications for several impurities are discussed as well as practical limitations. Since this method offers diagnostics-designing potential, possible technological solutions are also discussed.

Influential fluid–structure interaction modelling parameters on the response of bridges vulnerable to coastal storms

Analysis of coastal bridges under hurricane-induced wave and surge loads is essential for safety and performance assessment of water crossing bridge inventories. A reliable numerical model that can be employed to study the behaviour of bridges in hurricane events is beneficial because it reduces the cost and effort required for experimental models. Furthermore, it is important to identify modelling parameters that have a significant effect on the simulated response in order to guide uncertainty treatment for future bridge reliability studies. To address these needs, a high fidelity numerical model for simulation of coastal bridges is utilised that takes into account the fluid–structure interaction and includes contact surfaces that permit deck shifting and unseating during surge and wave passage. After validation of the model with experimental test data, it is implemented to examine the response of a typical water crossing bridge in the Houston area, revealing the values of wave and surge loads and also the potential of deck unseating under extreme wave and surge conditions. A sensitivity study is conducted to determine the uncertain structural modelling parameters that significantly affect the bridge response when subjected to surge and wave. Concrete strength and density, coefficient of friction between super- and substructure and soil shear strength are found to influence the bridge response and should be considered in probabilistic analyses and reliability assessments of coastal bridges.

The sensitivity of seismic response parameters to the uncertain modelling variables of masonry-infilled reinforced concrete frames

The sensitivity of the seismic response parameters to the uncertain modelling variables of the infills and frame of four infilled reinforced concrete frames was investigated using a simplified nonlinear method for the seismic performance assessment of such buildings. This method involves pushover analysis of the structural model and inelastic spectra that are appropriate for infilled reinforced concrete frames. Structural response was simulated by using nonlinear structural models that employ one-component lumped plasticity elements for the beams and columns, and compressive diagonal struts to represent the masonry infills. The results indicated that uncertainty in the characteristics of the masonry infills has the greatest impact on the response parameters corresponding to the limit states of damage limitation and significant damage, whereas the structural response at the near-collapse limit state is most sensitive to the ultimate rotation of the columns or to the cracking strength of the masonry infills. Based on the adopted methodology for the seismic performance assessment of infilled reinforced concrete frames, it is also shown, that masonry infills with reduced strength may have a beneficial effect on the near-collapse capacity, expressed in terms of the peak ground acceleration.

Sources of uncertainty in future changes in local precipitation

This study considers the large uncertainty in projected changes in local precipitation. It aims to map, and begin to understand, the relative roles of uncertain modelling and natural variability, using 20-year mean data from four perturbed physics or multi-model ensembles. The largest—280-member—ensemble illustrates a rich pattern in the varying contribution of modelling uncertainty, with similar features found using a CMIP3 ensemble (despite its limited sample size, which restricts it value in this context). The contribution of modelling uncertainty to the total uncertainty in local precipitation change is found to be highest in the deep tropics, particularly over South America, Africa, the east and central Pacific, and the Atlantic. In the moist maritime tropics, the highly uncertain modelling of sea-surface temperature changes is transmitted to a large uncertain modelling of local rainfall changes. Over tropical land and summer mid-latitude continents (and to a lesser extent, the tropical oceans), uncertain modelling of atmospheric processes, land surface processes and the terrestrial carbon cycle all appear to play an additional substantial role in driving the uncertainty of local rainfall changes. In polar regions, inter-model variability of anomalous sea ice drives an uncertain precipitation response, particularly in winter. In all these regions, there is therefore the potential to reduce the uncertainty of local precipitation changes through targeted model improvements and observational constraints. In contrast, over much of the arid subtropical and mid-latitude oceans, over Australia, and over the Sahara in winter, internal atmospheric variability dominates the uncertainty in projected precipitation changes. Here, model improvements and observational constraints will have little impact on the uncertainty of time means shorter than at least 20 years. Last, a supplementary application of the metric developed here is that it can be interpreted as a measure of the agreement amongst models of their projected local precipitation change. Results differ from, but are complementary to, those of the more usual approach.

Earthquake analysis of reinorced concrete minarets using ambient vibration test results

AbstractThis paper describes a Turkish style reinforced concrete minaret, its finite element model, modal testing, finite element model updating and earthquake behaviour, before and after model updating. The minaret of a mosque located in Trabzon, Turkey is selected as an application. A three‐dimensional (3D) model of the minaret and its modal analysis is performed to obtain analytical frequencies and mode shapes using ANSYS finite element program. The ambient vibration tests are conducted on the minaret under natural excitations such as wind effects and human movement. The output‐only modal parameter identification is carried out by Enhanced Frequency Domain Decomposition and Stochastic Subspace Identification methods in Operational Modal Analysis software and in doing so, dynamic characteristics (natural frequencies, mode shapes and damping ratios) are determined. A 3D finite element model of the minaret is updated to minimize the differences between analytical and experimental modal properties by changing some uncertain modelling parameters such as material properties and boundary conditions. The earthquake behaviour of the minaret is investigated using 1992 Erzincan earthquake before and after finite element model updating. Maximum differences in the natural frequencies are reduced from 21% to 8%, and good agreement is found between analytical and experimental natural frequencies. In addition to this, it is realized that finite element model updating is effective on the earthquake behaviour of the minaret. Copyright © 2008 John Wiley & Sons, Ltd.

Modal testing and finite element model calibration of an arch type steel footbridge

In recent decades there has been a trend towards improved mechanical characteristics of materials used in footbridge construction. It has enabled engineers to design lighter, slender and more aesthetic structures. As a result of these construction trends, many footbridges have become more susceptible to vibrations when subjected to dynamic loads. In addition to this, some inherit modelling uncertainties related to a lack of information on the as-built structure, such as boundary conditions, material properties, and the effects of non-structural elements make difficult to evaluate modal properties of footbridges, analytically. For these purposes, modal testing of footbridges is used to rectify these problems after construction. This paper describes an arch type steel footbridge, its analytical modelling, modal testing and finite element model calibration. A modern steel footbridge which has arch type structural system and located on the Karadeniz coast road in Trabzon, Turkey is selected as an application. An analytical modal analysis is performed on the developed 3D finite element model of footbridge to provide the analytical frequencies and mode shapes. The field ambient vibration tests on the footbridge deck under natural excitation such as human walking and traffic loads are conducted. The output-only modal parameter identification is carried out by using the peak picking of the average normalized power spectral densities in the frequency domain and stochastic subspace identification in the time domain, and dynamic characteristics such as natural frequencies mode shapes and damping ratios are determined. The finite element model of footbridge is calibrated to minimize the differences between analytically and experimentally estimated modal properties by changing some uncertain modelling parameters such as material properties. At the end of the study, maximum differences in the natural frequencies are reduced from 22% to only %5 and good agreement is found between analytical and experimental dynamic characteristics such as natural frequencies, mode shapes by model calibration.

Finite element model updating taking into account the uncertainty on the modal parameters estimates

Model updating is a common method to improve the correlation between finite element (F.E.) models and measured data. F.E. model updating is a technique that is used to identify and correct uncertain modelling parameters that leads to better predictions of the dynamic behavior of a target structure. A number of approaches to the problem exist, based on the type of parameters that are updated and the measured data that is used. This paper concentrates on an updating method based on measured modal data. The conventional F.E. updating techniques are extended and adapted in order to take into account the uncertainty on the estimated modal parameters to be updated. The effectiveness of the suggested procedure is tested on a case study.

A comparison of CFD-simulations and measurements of the temperature stratification in a mixing box of an air-handling unit

This paper presents a comparison between CFD-simulations and measurements of the temperature stratification in a mixing box of an air-handling unit. We have used data from field measurements during a period of over a year for different outside temperatures. We performed two-dimensional CFD-simulations for four different outside temperatures with commercially available software. The measurements as well as the simulations show that the temperature difference between the upper part and the lower part of the duct downstream of the mixing box is considerable. It increases, as the outside temperature decreases. However, the discrepancies between the measurements and the simulations are large. The reasons for this are uncertain boundary conditions and modelling errors leading to an inaccurate simulation result. The stratification downstream of the mixing box implies large sensor errors and the use of the mixed air temperature for control and fault detection must therefore be questioned. Averaging sensors, which take a mean value over the duct section, can be used but do not consider differences in velocities and are therefore not accurate either. In order to, for example, use CFD as a tool to decide the optimal sensor location a more accurate model and more information regarding the boundary conditions is needed. Copyright © 2001 John Wiley & Sons, Ltd.

Long term distribution of non-linear wave induced vertical bending moments on a containership

A method is proposed for the long-term formulation of non-linear wave induced vertical bending moments. The non-linearity of the response is represented by an uncertain modelling factor that is deduced from calculations of transfer functions that account for non-linear effects in the prediction of wave induced vertical bending moments. Long term predictions are obtained for a containership hull showing that the response is clearly non-linear and is reproduced in the long-term predictions.

Dynamical Models in Geology: Sensitivity Analysis and Scientific Risk

Dynamical models are used routinely throughout various branches of geology and decisions are often based on the results of such models. Awareness of some fundamental limitations of any model used is therefore vital in order to avoid decision making based on highly uncertain results. Computer models are based on mathematical models which describe essential features of processes or system behaviour and a systematic approach to investigate and understand model limitations can therefore be applied. Sensitivity analysis provides a feel for the system response to uncertainties in both assumptions and observations. However, sensitivity analyses do not provide a feel for the level of confidence one should assign modelling results. A probabilistic approach to evaluation of uncertainties and modelling results is therefore demonstrated. Two cases are used as examples for the method and it is demonstrated how the combined sensitivity analyses and probabilistic evaluation greatly improve the use of even uncertain modelling results.

Long term distribution of non-linear wave induced vertical bending moments

A method is proposed for the long-term formulation of wave induced vertical bending moments in ship structures. The non-linearity of the response is represented by an uncertain modelling factor that is calibrated by experimental values. Long term predictions are obtained for a tanker and a container ship hull showing that only in the latter case is the response clearly non-linear and reproduced in the long-term predictions.