Non-standard Configurations Research Articles

Nonstandard cages in the formation process of methane clathrate: Stability, structure, and spectroscopic implications from first-principles

Endohedral CH(4)@(H(2)O)(n) (n = 16, 18, 20, 22, 24) clusters with standard and nonstandard cage configurations containing four-, five-, six-, seven-membered rings were generated by spiral algorithm and were systematically explored using DFT-D methods. The geometries of all isomers were optimized in vacuum and aqueous solution. In vacuum, encapsulation of methane molecules can stabilize the hollow (H(2)O)(n) cage by 2.31~5.44 kcal/mol; but the endohedral CH(4)@(H(2)O)(n) cages are still less stable than the pure (H(2)O)(n) clusters. Aqueous environment could promote the stabilities of the hollow (H(2)O)(n) cages as well as the CH(4)@(H(2)O)(n) clusters, and the CH(4)@(H(2)O)(n) clusters possess larger stabilization energies with regard to the pure (H(2)O)(n) clusters except for n = 24. The lowest energy structures of the CH(4)@(H(2)O)(20) and CH(4)@(H(2)O)(24) cages are identical to the building units in the crystalline sI clathrate hydrate. All of the low-energy cages (including both regular and irregular ones) have large structural similarity and can be connected by "dimer-insertion" operation and Stone-Wales transformation. Our calculation also showed that in the range of cluster size n = 16-24, the relative energies of cage isomers tend to decrease with increasing number of the adjacent pentagons in the oxygen skeleton structures. In addition to the regular endohedral CH(4)@(H(2)O)(20) and CH(4)@(H(2)O)(24) cage structures, some nonstandard CH(4)@(H(2)O)(n) (n = 18, 20, 22, 24) cages have lower energies and might appear during nucleation process of methane hydrate. For the methane molecules in these low-energy cage isomers, we found that the C-H symmetric stretching frequencies show a red-shift trend and the (13)C NMR chemical shifts generally move toward negative values as the cavity size increases. These theoretical results are comparable to the available experimental data and might help experimental identification of the endohedral water cages during nucleation.

Global terrestrial isoprene emission models: sensitivity to variability in climate and vegetation

Abstract. Due to its effects on the atmospheric lifetime of methane, the burdens of tropospheric ozone and growth of secondary organic aerosol, isoprene is central among the biogenic compounds that need to be taken into account for assessment of anthropogenic air pollution-climate change interactions. Lack of process-understanding regarding leaf isoprene production as well as of suitable observations to constrain and evaluate regional or global simulation results add large uncertainties to past, present and future emissions estimates. Focusing on contemporary climate conditions, we compare three global isoprene models that differ in their representation of vegetation and isoprene emission algorithm. We specifically aim to investigate the between- and within model variation that is introduced by varying some of the models' main features, and to determine which spatial and/or temporal features are robust between models and different experimental set-ups. In their individual standard configurations, the models broadly agree with respect to the chief isoprene sources and emission seasonality, with maximum monthly emission rates around 20–25 Tg C, when averaged by 30-degree latitudinal bands. They also indicate relatively small (approximately 5 to 10 % around the mean) interannual variability of total global emissions. The models are sensitive to changes in one or more of their main model components and drivers (e.g., underlying vegetation fields, climate input) which can yield increases or decreases in total annual emissions of cumulatively by more than 30 %. Varying drivers also strongly alters the seasonal emission pattern. The variable response needs to be interpreted in view of the vegetation emission capacities, as well as diverging absolute and regional distribution of light, radiation and temperature, but the direction of the simulated emission changes was not as uniform as anticipated. Our results highlight the need for modellers to evaluate their implementations of isoprene emission models carefully when performing simulations that use non-standard emission model configurations.

MAC Design for WiFi Infrastructure Networks: A Game-Theoretic Approach

In WiFi networks, mobile nodes compete for accessing a shared channel by means of a random access protocol called Distributed Coordination Function (DCF). Although this protocol is in principle fair, since all the stations have the same probability to transmit on the channel, it has been shown that unfair behaviors may emerge in actual networking scenarios because of non-standard configurations of the nodes. Due to the proliferation of open source drivers and programmable cards, enabling an easy customization of the channel access policies, we propose a game-theoretic analysis of random access schemes. We show that even when stations are selfish, efficient equilibria conditions can be reached when they are interested in both uploading and downloading traffic. We explore the utilization of the Access Point as an arbitrator for improving the global network performance. Finally, we propose and evaluate some simple DCF extensions for practically implementing our theoretical findings.

In-Stent Thrombosis and Stenosis After Neck-Remodeling Device-Assisted Coil Embolization of Intracranial Aneurysms

Intrinsic thrombosis and stenosis are complications associated with the use of neck-remodeling devices in the treatment of intracranial aneurysms. To examine the technical and anatomic factors that predict short- and long-term stent patency. We undertook a retrospective review of 161 patients who underwent coil embolization of 168 ruptured and unruptured aneurysms assisted by the use of a neck-remodeling device. One hundred twenty-seven patients had catheter-based angiographic follow-up to evaluate 133 stent-coil constructs (mean, 15.4 months; median, 12.7 months). The technique of microcatheter jailing was used in a majority of patients; nonstandard stent configurations were also used. Clinical follow-up for all patients who had catheter-based angiograms demonstrated that among 133 stent constructs, a total of 9 (6.8%) had an in-stent event: 6 acute or subacute thrombosis (4.5%) and 3 delayed stenosis or occlusion (2.3%). Seven of these constructs were associated with a symptomatic event (5.3%). A significantly higher rate of in-stent events was seen with the use of constructs to treat anterior communicating artery aneurysms. When all patients are considered, including those who did not receive catheter-based follow-up imaging, 2 of 168 procedures (1.2%) resulted in the death of a patient, and procedural morbidity was 14.9%. From these results and those in the published literature, in-stent complication rates are low in carefully selected patients. The use of dual antiplatelet therapy, sensitivity assays, and glycoprotein IIb/IIIa inhibitors may decrease the rate of acute and chronic in-stent complications.

The muon high level trigger of the ATLAS experiment

The ATLAS experiment at CERN's Large Hadron Collider (LHC) has been designed and built for new discoveries in High Energy Physics as well as for precision measurements of Standard Model parameters. To satisfy the limited data acquisition and recording capability, at the LHC project luminosity, the ATLAS trigger system must select a very small rate of physically interesting events (~200 Hz) among about 40 million events per second. In the case of events containing muons, as described in this work, the first hardware-based level (Level-1) starts from coincidence of hits in the Muon Spectrometer trigger chambers to select Regions of Interest (RoI) where muons produce significant activity. Such RoIs are used as seeds for the two subsequent trigger levels (Level-2 and Event Filter), running on dedicated online farms, which constitute the High Level Trigger (HLT). This seeding strategy is crucial to drastically reduce the total processing time. Within the Muon HLT, few algorithms are implemented in different steps according to predefined sequences of Feature Extraction (FEX) and Hypothesis (HYPO) algorithms, whose goal is to validate the previously selected muon objects. The ATLAS muon trigger system, thanks to its particular design and to the peculiar structure of the Muon Spectrometer, is able to provide muon stand-alone event trigger decisions, that can be furtherly refined by exploiting the muon information coming from the other ATLAS subdetectors. Muon HLT algorithms are described here in terms of working functionality and performance both on simulated and real data, including non-standard trigger configurations (like cosmic data and LHC start-up scenarios).

Non-standard sequence subgroups in finite fields

In previous papers, see, for example, [O.J. Brison, J.E. Nogueira, Second order linear sequence subgroups in finite fields, Finite Fields Appl. 14 (2008) 277–290], the authors studied f-sequence subgroups when f ( t ) was a polynomial of degree 2. Here we study non-standard groups when f ( t ) has degree k. The non-standard configurations found when k = 2 extend to the general case, while new configurations when k > 2 are described.

Flap Gap Oscillatory Blowing on 2D and 2.5D Wing

Here we present preliminary results obtained in developing an active flow control system for high lift systems at advanced TRL level. The work is based on theoretical and experimental work performed in AVERT EU FP6 project where the oscillatory flap gap blowing system was designed and tested on a INCAS F15 2D wing model. Pressure data and global loads have been recorded for a complex evaluation of the basic flow control mechanism. In 2.5D test cases this work has been extended so that the proposed system may be selected as a mature technology in the JTI Clean Sky, Smart Fixed Wing Aircraft ITD. For this goal, new experimental setup was used and also updated electronics for the blowing system have been introduces. This was complemented by a new extension for the data acquisition system and visualization tools. Finally global correlations for basic lift increments have been compared with the reference 2D case and analysed with respect to the system efficiency. INTRODUCTION The overall objective of AVERT Project is to deliver upstream aerodynamics research that will enable breakthrough technology development and innovative aircraft configuration development leading to a step change in aircraft performance. The project objective is to be achieved through the evaluation of selected types of sensor, actuator and control systems, the assessment of these devices against baseline aircraft configurations and the evaluation of the most promising technologies in a medium/large scale wind tunnel test. Figure 1 – AVERT F15 model with end plates (basic 2D setup) The considered flow control technologies are focussed on low-speed applications, mainly investigations with AFC for standard (e.g. DLR F15) and non-standard (e.g. ONERA DND A310) high-lift configurations. Several technologies are investigated and developed INCAS BULLETIN No. 2/ 2009 122 DOI: 10.13111/2066-8201.2009.1.2.17 through both numerical simulations and experiments in wind tunnel. INCAS activities are related to separation control by flap gap oscillatory blowing on a high lift configuration. A high-lift, 2-D model geometry suitable for the application of oscillatory blowing in the flap gap has been selected from previous investigations (DLR F15 model) and manufactured from new at INCAS (Figure 1). A number of test cases with different slot and blowing parameters have been investigated numerically for wind tunnel conditions by DLR. From the numerical results, recommendations for the model design were given (blowing angle and slot widths). A new F15 model has been designed and manufactured for wind tunnel testing at INCAS subsonic wind tunnel. This model was designed so that 19 TU Berlin actuators could be integrated in a 2m span model and tested in a wide range of blowing conditions for the F15 proposed geometry (Figure 2). TU Berlin has designed and manufactured dedicated actuators suitable for flap integration and flap gap oscillatory blowing experimental investigation on a high lift configuration in INCAS facility. Figure 2 – Flap setup (external view) and actuators integration Wind tunnel tests results for the selected test cases have been compared and validated against numerical results and existing experiments (with previous existing WT results at DLR for the F15 high-lift airfoil). EXPERIMENTAL SETUP The experiments in the Subsonic Wind Tunnel at INCAS have been conducted so that we could benefit from the medium size of the facility and to enable large model validation. 1. The maximum wind tunnel test room is of 2.5m width & 2 m height; the maximum permissible span of the model is aprox. 2.0m, in order to make room for end plates and to enable popper distance to the side walls of the test section. 2. In order to achieve a high Reynolds number in the range of 3 million (for wt speed close to 90 m/s), the basic chord length was selected as 600 mm. However, the basic experiments were performed at Reynolds 2 million. INCAS BULLETIN No. 2/ 2009

Ionization chamber gradient effects in nonstandard beam configurations

For the purpose of nonstandard beam reference dosimetry, the current concept of reporting absorbed dose at a point in water located at a representative position in the chamber volume is investigated in detail. As new nonstandard beam reference dosimetry protocols are under development, an evaluation of the role played by the definition of point of measurement could lead to conceptual improvements prior to establishing measurement procedures. The present study uses the current definition of reporting absorbed dose to calculate ionization chamber perturbation factors for two cylindrical chamber models (Exradin A12 and A14) using the Monte Carlo method. The EGSnrc based user-code EGS_chamber is used to calculate chamber dose responses of 14 IMRT beams chosen to cause considerable dose gradients over the chamber volume as previously used by Bouchard and Seuntjens ["Ionization chamber-based reference dosimetry of intensity modulated radiation beams," Med. Phys. 31(9), 2454-5465 (2004)]. The study shows conclusively the relative importance of each physical effect involved in the nonstandard beam correction factors of 14 IMRT beams. Of all correction factors involved in the dosimetry of the beams studied, the gradient perturbation correction factor has the highest magnitude, on average, 11% higher compared to reference conditions for the Exradin A12 chamber and about 5% higher for the Extradin A14 chamber. Other perturbation correction factors (i.e., Pwall, Pstem, and Pcel) are, on average, less than 0.8% different from reference conditions for the chambers and beams studied. The current approach of reporting measured absorbed dose at a point in water coinciding with the location of the centroid of the chamber is the main factor responsible for large correction factors in nonstandard beam deliveries (e.g., intensity modulated radiation therapy) reported in literature. To reduce or eliminate the magnitude of perturbation correction factors in nonstandard beam reference dosimetry, two possible ways to report absorbed dose are suggested: (1) Reporting average dose to the sensitive volume of the chamber filled with water, combined with removing the reference field implicit gradient effect when measuring output factors, and (2) reporting average dose to the chamber itself during output factor verifications. The first option could be adopted if clinical beam correction factors are negligible. The second option could simplify quality assurance procedures when correction factors are not negligible and have to be calculated using Monte Carlo simulations.

WE‐A‐211A‐01: New Dosimetry Measurements — New Developments

Image‐guided intensity modulated radiation therapy deliveries are becoming increasingly complex and their verification involves dosimetry measurements in situations that are no longer covered by reference dosimetry protocols or relative dosimetry procedures with simple corrections. These beams are generated as part of intensity modulation (IMRT), stereotactic radiosurgery (SRS), cyber‐knife and gamma‐knife deliveries and involve the use of extremely small fields in order of few millimeters. In this era of joint imaging‐therapy developments, accurate dosimetry techniques are sometimes trivialized and important effects not understood or ignored.This presentation consists of three parts: the first part revisits the guiding principles of measurement dosimetry in standard and nonstandard conditions, definitions of detectors and phantoms, reference dosimetry of conventional beams and also discusses the new developments in reference dosimetry of nonstandard beams. The second part of the presentation concentrates on practical aspects of relative dose measurements with the goal to generate 3D distributions as well as integrated measurements and derived quantities. The third part concentrates on a discussion of relative dosimetry in narrow fields and deals with the choices of possible radiation detectors, the difficulties and possible solutions to measurements in disequilibrium situations created by this era's narrow photon fields.Learning Objectives1. To understand the principles of clinical measurement dosimetry and applicability to standard and nonstandard beams;2. To understand standard clinical reference dosimetry techniques and be exposed to some of the new developments in reference dosimetry in nonstandard beam configurations;3. To get an overview of practical aspects of relative dosimetry techniques for the purpose of 3D dose distributions in standard fields, small photon fields and buil‐up regions;4. To be aware of the complications of measurements and detector choices in electronic disequilibrium situations created by small photon fields.

Modelling ionization chamber response to nonstandard beam configurations

Novel technologies aiming at improving target dose coverage while minimising dose to organs at risk use delivery of radiation fields that significantly deviate from reference conditions defined in protocols such as TG-51 and TRS-398. The use of ionization chambers for patient-specific quality assurance of these new delivery procedures calibrated in reference conditions increases the uncertainties on dose delivery. The conversion of the dose to the chamber cavity to the dose to water becomes uncertain; and the geometrical details of the chamber, as well as the details of the delivery, are expected to be significant. In this study, a realistic model of the Exradin® A12 Farmer chamber is simulated. A framework is applied for the calculation of ionization chamber response to arbitrarily modulated fields as a summation of responses to pencil beams. This approach is used with the chamber model and tested against measurements in static open fields and dynamic MLC IMRT fields. As a benchmark test of the model, quality conversion factors values calculated by Monte-Carlo simulation with the chamber model are in agreement within 0.1 % and 0.4 % with those in the AAPM TG-51, for 6 MV and 18 MV photon beams, respectively. Pencil-beam kernels show a strong dependence on the geometrical details of the chamber. Kernel summations with open fields show a relative agreement within 4.0 % with experimental data; the agreement is within 2.0 % for dynamic MLC IMRT beams. Simulations show a strong sensitivity of chamber response on positioning uncertainties, sometimes leading to dose uncertainties of 15 %.

Large scale optimization strategies for zone configuration of simulated moving beds

Simulated moving bed (SMB) processes are widely used in sugar, petrochemical, and pharmaceutical industries. However, systematic optimization of SMB, especially finding the optimal zone configuration including the standard and modified non-standard configurations, is still a challenging problem. This paper proposes a simultaneous, fully discretized approach with an SMB superstructure using an interior-point solver. To find the optimal structure, two superstructures are analyzed to develop standard and non-standard configurations. In case studies of the linear and bi-Langmuir isotherms, optimal zone configurations have been successfully obtained without introducing discrete variables. Finally, the effect of the number of columns on the optimal throughput is investigated.

A Game-Theoretic Study of CSMA/CA Under a Backoff Attack

CSMA/CA, the contention mechanism of the IEEE 802.11 DCF medium access protocol, has recently been found vulnerable to selfish backoff attacks consisting in nonstandard configuration of the constituent backoff scheme. Such attacks can greatly increase a selfish station's bandwidth share at the expense of honest stations applying a standard configuration. The paper investigates the distribution of bandwidth among anonymous network stations, some of which are selfish. A station's obtained bandwidth share is regarded as a payoff in a noncooperative CSMA/CA game. Regardless of the IEEE 802.11 parameter setting, the payoff function is found similar to a multiplayer Prisoners' Dilemma; moreover, the number (though not the identities) of selfish stations can be inferred by observation of successful transmission attempts. Further, a repeated CSMA/CA game is defined, where a station can toggle between standard and nonstandard backoff configurations with a view of maximizing a long-term utility. It is argued that a desirable station strategy should yield a fair, Pareto efficient, and subgame perfect Nash equilibrium. One such strategy, called CRISP, is described and evaluated

SWAS: A shear-wave analysis system for semi-automatic measurement of shear-wave splitting above small earthquakes

The complexity of shear wave-arrivals above small earthquakes makes the polarisations and time-delays of shear-wave splitting above small earthquakes difficult to measure. We report a semi-automatic shear-wave analysis system, SWAS, that appears to combine the benefits of both visual and automatic techniques. Initially, SWAS automatically estimates shear-wave polarisations and picks shear-wave arrivals by an expert system, which provides sufficiently accurate initial measurements for visual adjustment. SWAS then allows easy comparison and adjustment of picks between screen images of original seismograms, seismograms rotated into anisotropic polarisations, and polarisation diagrams (hodograms), with immediate plotting in various standard or non-standard configurations. This speeds up visual measurements by well over an order of magnitude, and typically allows records of almost all small earthquakes ( M ≥ −1.0) to be reliably measured for shear-wave splitting polarisations and time-delays. SWAS was developed and tested for data from the SIL seismic network in Iceland.

Buckling of laser-welded sandwich panels. Part 1: Elastic buckling parallel to the webs

This is the first of three companion papers that examine the elastic buckling and collapse of laser-welded sandwich panels with an adhesively bonded core and uni-directional vertical webs. In compression parallel to the webs, the face sheet in between two webs can be treated as a long rectangular plate supported at the sides and resting on a continuous elastic foundation. Using a generalized form of the Kirchoff equation, the plate buckling load is evaluated for a Winkler and Pasternak foundation, and for a foundation model where the core is characterized by an elastic half-space. Although the elastic half-space model is most sophisticated in the way that the decay of transverse stress through the core is described, it requires that the buckling load is calculated iteratively and is therefore not suitable for engineering applications. In contrast, the Pasternak foundation model is able to demonstrate the effect of different boundary conditions at the laser weld and different core properties by the variation of a simple clamping factor and two foundation moduli. By changing the clamping factor, the model can be used to describe plate buckling as well as wrinkling, and, by varying the foundation moduli, it is possible to distinguish between symmetrical and anti-symmetrical buckling modes. Evaluation of the buckling solution shows that, for low modulus core materials, the plate buckling strength is determined only by the boundary conditions at the laser weld, while for high modulus materials anti-symmetrical wrinkling dominates. To improve the accuracy of the solution for non-standard configurations, the foundation moduli are calibrated using the elastic half-space model and finite element results as a reference. It is found that, by using a smaller Winkler foundation modulus, the accuracy and consistency of the Pasternak model are improved considerably. However, the results also show that, for current sandwich configurations, the proportional limit of the face plate material is usually reached well before elastic buckling occurs. All experimental work is therefore delegated to the third paper, which deals with extension of the Pasternak foundation model into the elasto-plastic regime and non-linear finite element modelling.

Wind Tunnel Analysis On The Influence OfCantilever Parapets On The Wind Loads OnCurved Roofs

Different methods to reduce the high suction caused by conical vortices have been reported in the literature: vertical parapets, either solids or porous, placed at the roof edges being the most analysed configuration. Another method for alleviating the high suction peaks due to conical vortices is to round the roof edges. Very recently, the use of some non-standard parapet configurations, like cantilever parapets, has been suggested. In this paper, its efficiency to reduce suction loads on curved roofs is experimentally checked by testing the pressure distribution on the curved roof of a low-rise building model in a wind tunnel. Very high suction loads have been measured on this model, the magnitude of these high suction loads being significantly decreased when cantilever parapets are used, thus the suitability of these parapets to reduce wind pressure loads on curved roofs is demonstrated.

Experimental design: Electrical resistivity data sets that provide optimum subsurface information

Although multielectrode electrical‐resistivity systems have been commercially available for more than a decade, resistivity imaging of the subsurface continues to be based on data sets recorded using one or more of the standard electrode arrays (e.g., the Wenner or conventional dipole‐dipole array). To exploit better the full capabilities of multielectrode acquisition systems, we have developed an experimental design procedure to identify suites of electrode configurations that provide subsurface information according to predefined optimization criteria. The experimental design algorithm includes a goodness function that ranks the sensitivity of every possible electrode configuration to changes in the subsurface parameters. To examine the potential and limitations of the new algorithm, comprehensive data sets that included data from all standard and nonstandard electrode configurations were (a) generated for a complex 2D resistivity model and (b) recorded across a well‐studied test site in Switzerland. Images determined from the resultant comprehensive data sets were used as benchmarks against which the images derived from the optimized data sets were assessed. Images from relatively small optimized data sets, containing 265–282 data points, provided more information than did those from standard data sets of equal size. By far the best images, comparable to those determined from the much larger comprehensive data sets, were obtained from optimized data sets with 1000–6000 data points. These images supplied reliable information over depth ranges that were three times greater than the depth ranges resolved by the standard images. The first ∼600 electrode configurations selected by the experimental design procedure were nonstandard dipole‐dipole‐type arrays, whereas the following ∼4800 electrode configurations were an approximately equal mix of nonstandard dipole‐dipole‐type arrays and nested configurations (i.e., mostly gradient and other nonstandard arrays).

A comparison of time versus frequency domain antenna patterns

This paper compares the performance of antenna arrays that use sinusoidal and ultra-short pulse waveforms. There are two parts to the paper: first, a comparison of narrowband versus ultra-wideband linear arrays using an analytic approach and second, a series of computer simulations used to extend the analytic results and to show the characteristics of nonstandard array configurations. Analysis shows that antennas using ultra-short waveforms have highly desirable pattern properties that do not appreciably degrade under conditions of high array sparsity, failed elements, or antenna element position errors. Patterns are given that show these properties directly. These results are of particular importance to the emerging technology of base-band or time-domain communications and remote sensing. It addresses the practical problem of designing extremely large aperture space-based arrays without populating those arrays densely with antenna elements and also reducing the traditional fabrication tolerances.

Complete allele information in the diagnosis of facioscapulohumeral muscular dystrophy by triple DNA analysis.

Facioscapulohumeral muscular dystrophy is caused by partial deletion of the D4Z4 repeat array on chromosome 4q35. Genetic diagnosis is based on sizing of this repeat array, which is complicated by cross-hybridization of a homologous polymorphic repeat array on chromosome 10 and by the frequent exchanges between these chromosomal regions. The restriction enzyme XapI optimizes the diagnosis of facioscapulohumeral muscular dystrophy by uniquely digesting 4-derived repeat units and leaving 10-derived repeat units undigested, thus complementing BlnI, which uniquely digests 10-derived repeat units. A triple analysis with EcoRI, EcoRI/BlnI, and XapI unequivocally allows characterization of each of the four alleles, whether homogeneous or hybrid. This is particularly useful in the case of identical sized 4-derived and 10-derived arrays, in situations of suspected facioscapulohumeral muscular dystrophy with nonstandard allele configurations, and for assignment of hybrid fragments to their original alleles.

Simplified Load Distribution for Vehicles with Nonstandard Axle Gauges

Several simplified methods have been developed to determine the live-load distribution factors for overweight vehicles on slab-on-girder bridges; however, these methods were developed for vehicles with standard axles. Many vehicles exist with nonstandard axle configurations, such as two-wheel axles that are wider than 6 ft (1.83 m) and four-wheel axles with wheels that are evenly or unevenly spaced. For these vehicles, a rigorous analysis is generally desired but is often deemed uneconomical. Therefore, a simplified method should be an asset to the bridge community and the trucking industry. A simplified method for determining live-load distribution factors for vehicles with nonstandard axle configurations is presented. Distribution factor formulas for moment and shear in interior and exterior girders are given. These formulas account for the transverse axle configurations that compose a vehicle. Several two- and four-wheel axle configurations are considered. The distribution factor formulas for slab-on-girder bridges presented in the AASHTO LRFD Bridge Design Specifications are incorporated into the proposed simplified method. The simplified method formulas were developed to approximate the results from a rigorous finite strip method. Comparisons are presented as verification of the accuracy of the simplified method. The simplified method results are usually conservative and correlate reasonably well with the rigorous results. In general, simplified methods worked better for interior girders than for exterior girders, and moment was better predicted than shear.

NEW DESIGN RECOMMENDATIONS FOR FLUIDELASTIC INSTABILITY IN HEAT EXCHANGER TUBE BUNDLES

Design equations are presented to determine the critical velocities for the occurrence of fluidelastic instability in uniform single-phase cross-flow. These equations are an essential part of a new guideline in the “VDI-Wärmeatlas” for estimating vibration excitation in real tube bundle heat exchangers. Six existing guidelines for fluidelastic instability were tested and compared with about 300 experimental data from 34 papers (eight of them being not yet considered in a guideline before). New equations for the stability factor K as a function of the pitch ratio for different tube configurations were derived with statistical methods by a variation of the reference definitions for the structural parameters and the exponents of either the dimensionless mass and the damping or the mass-damping parameter. The criterion used here was first to be on the safe side with a minimum number, and minimum deviation, of experimental data below the recommended threshold line, and second a minimum r.m.s. error for all data considered. The pitch ratio has the strongest influence for the normal triangular array and the influence becomes less for the rotated as well as for the in-line square configuration. This significant gradation of the stability constants as a function of the pitch ratio and the tube bundle configuration enables a reasonable interpolation for non-standard configurations. The exponent of the dimensionless mass-damping parameter for gas cross-flow depends on the tube configuration and is 0·5 for the 30°- and 45°-configuration, and 0·4 for the 60°- and 90°-configuration. For liquid flow, an average exponent of 0·15 has been observed.