Accuracy considerations in approximate reanalysis of structures

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76782/1/AIAA-2000-4751-508.pd

Theorems of structural and geometric variation for linear and nonlinear finite element analysis

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On the use of the proximity force approximation for deriving limits to short-range gravitational-like interactions from sphere-plane Casimir force experiments

We discuss the role of the proximity force approximation in deriving limits to the existence of Yukawian forces - predicted in the submillimeter range by many unification models - from Casimir force experiments using the sphere-plane geometry. Two forms of this approximation are discussed, the first used in most analyses of the residuals from the Casimir force experiments performed so far, and the second recently discussed in this context in R. Decca et al. [Phys. Rev. D 79, 124021 (2009)]. We show that the former form of the proximity force approximation overestimates the expected Yukawa force and that the relative deviation from the exact Yukawa force is of the same order of magnitude, in the realistic experimental settings, as the relative deviation expected between the exact Casimir force and the Casimir force evaluated in the proximity force approximation. This implies both a systematic shift making the actual limits to the Yukawa force weaker than claimed so far, and a degree of uncertainty in the alpha-lambda plane related to the handling of the various approximations used in the theory for both the Casimir and the Yukawa forces. We further argue that the recently discussed form for the proximity force approximation is equivalent, for a geometry made of a generic object interacting with an infinite planar slab, to the usual exact integration of any additive two-body interaction, without any need to invoke approximation schemes. If the planar slab is of finite size, an additional source of systematic error arises due to the breaking of the planar translational invariance of the system, and we finally discuss to what extent this may affect limits obtained on power-law and Yukawa forces.Comment: 11 page, 5 figure

Improving Precipitation Estimation Using Convolutional Neural Network

Precipitation process is generally considered to be poorly represented in numerical weather/climate models. Statistical downscaling (SD) methods, which relate precipitation with model resolved dynamics, often provide more accurate precipitation estimates compared to model's raw precipitation products. We introduce the convolutional neural network model to foster this aspect of SD for daily precipitation prediction. Specifically, we restrict the predictors to the variables that are directly resolved by discretizing the atmospheric dynamics equations. In this sense, our model works as an alternative to the existing precipitation-related parameterization schemes for numerical precipitation estimation. We train the model to learn precipitation-related dynamical features from the surrounding dynamical fields by optimizing a hierarchical set of spatial convolution kernels. We test the model at 14 geogrid points across the contiguous United States. Results show that provided with enough data, precipitation estimates from the convolutional neural network model outperform the reanalysis precipitation products, as well as SD products using linear regression, nearest neighbor, random forest, or fully connected deep neural network. Evaluation for the test set suggests that the improvements can be seamlessly transferred to numerical weather modeling for improving precipitation prediction. Based on the default network, we examine the impact of the network architectures on model performance. Also, we offer simple visualization and analyzing approaches to interpret the models and their results. Our study contributes to the following two aspects: First, we offer a novel approach to enhance numerical precipitation estimation; second, the proposed model provides important implications for improving precipitation-related parameterization schemes using a data-driven approach

Diagnosis of dynamic behavior of structures using the distribution of kinetic and potential energy

U većini objekata vibracije su nepoželjne. To je zato što vibracije stvaraju dinamičke sile i udarce koji mogu izazvati zamor i otkaz strukture. Odgovor strukture na njenu pobudu zavisi od načina primene i lokacije pobudne sile, kao i dinamičke karakteristike strukture kao što su prirodne frekvencije i nivo prigušenja. Strukturalni odgovor se može poboljšati promenom raspodele masa ili krutosti strukture, pomeranjem izvora pobude na drugu lokaciju , ili povećanjem prigušenja u strukturi. Strukturalna dinamička modifikacija (SDM) je veoma efikasna i pouzdna tehnika koja se intenzivno koristi za poboljšanje dinamičkih karakteristika strukture kao što su prirodne frekvencije, glavnih oblika i funkcija frekventnih odziva ( FRFs ) . Dinamičko ponašanje konstrukcije može se poboljšati čineći modifikacije delova kao što su kruta mesta, masa, prigušenja itd. Mnogo puta se desi da struktura ne ispunjava potrebne ograničenja dizajna i da dizajn mora da bude modifikovan nekoliko puta pre nego što ona ispuni sve uslove projektovanja. Suština poboljšanja dinamičko ponašanje objekta jeste povećanje prirodnih frekvencija i povećanje intervala između susednih prirodnih frekvencija. Ovaj zahtev se može postići promenom dizajna parametara strukture. Procedura koje se koriste u ovom radu jesu analize distribucije potencijalne i kinetičke energije i razlike između njih u elementima strukture. Studija distribucije potencijalne i kinetičke energije na glavnim oblicima oscilacija strukture daje očigledno predviđanje koje elemente i kako treba izmeniti da se postigne najbolje dinamičko ponašanje. Cilj predloženog razvijenog metoda reanalise i dijagnostike ponašanja struktura je da se utvrdi stvarno ponašanje konstrukcije u eksploataciji. Tehnika reanalise strukture se izvodi primenom metode konačnih elemenata (MKE ) . Informacije o strukturi kao materijal, geometrija i granični uslovi treba da budu spremni pre nego što generiše model...In most structures vibration is undesirable. This is because vibration creates dynamic stresses and strains which can cause fatigue and failure of the structure. The response of the structure to excitation depends upon the method of application and the location of the exciting force or motion, and the dynamic characteristics of the structure such as its natural frequencies and inherent damping level. The structural response can be improved by changing the mass or stiffness of the structure, by moving the source of excitation to another location, or by increasing the damping in the structure. Structural Dynamics Modification (SDM) is a very effective and reliable technique which is extensively used to improve structure's dynamic characteristics such as natural frequency, mode shape and frequency response functions (FRFs). The dynamic behavior of the structure can be improved by predicting the modified behavior making some modifications parts like rigid links, beams, lumped masses, dampers etc. Many times it happens that the structure does not meet the required design constraints and the design has to be modified numerous times before it meets all the design constraints. This repeated analysis for each such modification becomes very expensive and time consuming, especially if there are lots of degrees of freedoms. The main point of improving dynamic behavior of a structure is increasing its natural frequencies and maximizing the interval between adjacent natural frequencies. This request can be achieved by changing the design parameters of the structure. The procedures used in this thesis are concerned with the analysis of the distribution of potential and kinetic energy and the differences between them in elements of the structure. Study of distribution of potential and kinetic energy in main oscillation modes of structure gives obvious prediction which elements need some modifications to achieve the best dynamic characteristics. The aim of developed the proposed method of reanalysis and diagnostic of structure behavior is to determine real behavior of the construction in exploitation..

Minimum mass sizing of a large low-aspect ratio airframe for flutter-free performance

A procedure for sizing an airframe for flutter-free performance is demonstrated on a large, flexible supersonic transport aircraft. The procedure is based on using a two level reduced basis or modal technique for reducing the computational cost of performing the repetitive flutter analyses. The supersonic transport aircraft exhibits complex dynamic behavior, has a well-known flutter problem and requires a large finite element model to predict the vibratory and flutter response. Flutter-free designs were produced with small mass increases relative to the wing structural weight and aircraft payload

Eigenvalue sensitivity analysis in structural dynamics

Modifikacija dinamičkih karakteristika konstrukcija se definiše kao skup metoda kojima se može poboljšati dinamičko ponašanje konstrukcije u eksploataciji. Modifikacija dinamičkih karakteristika ili reanaliza se posebno odnosi na skup metoda i tehnika koje svoje korene i osnove imaju u primeni analize senzitivnosti i metode konačnih elemenata. Analiza senzitivnosti se zasniva na selekciji konstrukcionih parametara u početnom konačnoelementnom modelu čijom modifikacijom bi došlo do popravljanja dinamičkog ponašanja posmatrane konstrukcije. Ovaj rad se bavi analizom dinamičkog ponašanja vratila elektromotora, kao i analizom uticaja oblika konzolnog nosača na vrednosti osnovnih frekvencija. U osnovi ove analize je distribucija potencijalne i kinetičke energije na glavnim oblicima oscilovanja u svim elementima posmatrane konstrukcije. Na osnovu senzitivnosti pojedinih elemenata biraju se segmenti za modifikaciju. Na osnovu ovog istraživanja pokazuje se važnost reanalize u dinamici konstrukcija.Structural dynamic modification implies the incorporation, into an existing model, of new information gained either from experimental testing or some other source, which questions or improves the accuracy of the model. The sensitivity approach is based on the prior selection of updating parameters (design variables) in the initial FE model. This paper deals with analysis of the dynamic behavior of shaft of electromotor. Two cases are done. The second example problem is dynamic analysis of 12-node cantilever beam. Distribution of potential and kinetic energy in every finite element is used for analysis. In this study it is shown that structural dynamic modification is important in structural reanalysis

Eigenvalue sensitivity analysis in structural dynamics

Modifikacija dinamičkih karakteristika konstrukcija se definiše kao skup metoda kojima se može poboljšati dinamičko ponašanje konstrukcije u eksploataciji. Modifikacija dinamičkih karakteristika ili reanaliza se posebno odnosi na skup metoda i tehnika koje svoje korene i osnove imaju u primeni analize senzitivnosti i metode konačnih elemenata. Analiza senzitivnosti se zasniva na selekciji konstrukcionih parametara u početnom konačnoelementnom modelu čijom modifikacijom bi došlo do popravljanja dinamičkog ponašanja posmatrane konstrukcije. Ovaj rad se bavi analizom dinamičkog ponašanja vratila elektromotora, kao i analizom uticaja oblika konzolnog nosača na vrednosti osnovnih frekvencija. U osnovi ove analize je distribucija potencijalne i kinetičke energije na glavnim oblicima oscilovanja u svim elementima posmatrane konstrukcije. Na osnovu senzitivnosti pojedinih elemenata biraju se segmenti za modifikaciju. Na osnovu ovog istraživanja pokazuje se važnost reanalize u dinamici konstrukcija.Structural dynamic modification implies the incorporation, into an existing model, of new information gained either from experimental testing or some other source, which questions or improves the accuracy of the model. The sensitivity approach is based on the prior selection of updating parameters (design variables) in the initial FE model. This paper deals with analysis of the dynamic behavior of shaft of electromotor. Two cases are done. The second example problem is dynamic analysis of 12-node cantilever beam. Distribution of potential and kinetic energy in every finite element is used for analysis. In this study it is shown that structural dynamic modification is important in structural reanalysis

Toward a Comprehensive Model of Snow Crystal Growth: 4. Measurements of Diffusion-limited Growth at -15 C

We present measurements of the diffusion-limited growth of ice crystals from water vapor at different supersaturation levels in air at a temperature of -15 C. Starting with thin, c-axis ice needle crystals, the subsequent growth morphologies ranged from blocky structures on the needle tips (at low supersaturation) to thin faceted plates on the needle tips (at high supersaturation). We successfully modeled the experimental data, reproducing both growth rates and growth morphologies, using a cellular-automata method that yields faceted crystalline structures in diffusion-limited growth. From this quantitative analysis of well-controlled experimental measurements, we were able to extract information about the attachment coefficients governing ice growth under different circumstances. The results strongly support previous work indicating that the attachment coefficient on the prism surface is a function of the width of the prism facet. Including this behavior, we created a comprehensive model at -15 C that explains all the experimental data. To our knowledge, this is the first demonstration of a kinetic model that reproduces a range of diffusion-limited ice growth behaviors as a function of supersaturation