1,720,981 research outputs found
Impact of structures on the water surface: An upper bound analysis of the induced stress
No full textThe analysis of stresses induced in panels slamming on the water surface is considered. The analysis requires the development of two completely different techniques to approach the fluid dynamic and the structural responses. The hydrodynamic load is recovered by using the results of the acoustic approximation developed in [1] and in [2]. A fourth order flexural equation is used to determine the structural stress response. Although a general formulation for the coupled problem is stated, an analytical result is determined for the maximum stress induced in the impacting plate by means of an upper bound analysis of the modal response. The results obtained by the determined prediction method are finally compared with numerical simulation and experimental tests
Prediction of the compressible stage slamming force on rigid and elastic system impacting over the water surface
Full text linkIn this paper some models focusing on hydrodynamic and elasticforces arising during the impact of rigid and elastic systems on thewater surface are investigated. In particular, the supersoniccompressible stage of the impact is considered by modelling the slammingphenomenon through the Skalk–Feit acoustic approximation. The dynamicequations of the dropping system are coupled to those of the fluid and anonlinear fluid-solid interaction problem is stated. Generalrelationships between the body''s shape, slamming force and body motionare determined. These equations are applied to the wedge water entrycases, and a closed-form expression for the maximum hydrodynamic forceis found. Moreover the theoretical correlation between the hydrodynamicforce and the body geometry allows us to control the inverse problem andthe shape associated to a constant slamming force is determined.
Due to some simplifications allowed in the supersonic compressibleimpact, the results of the hydrodynamic analysis hold in closed form.This permits us to focus on the basic result of the paper addressed to asystematic correlation between hydrodynamic and elastic maximum forcesin terms of some characteristic dimensionless quantities involved influid-solid interaction.
In particular, critical conditionscorresponding to those hydorelastic parameters combinations areinvestigated, leading to severe elastic response of the impactingsystem
Hydrodynamic shock of elastic structures impacting on the water: theory and experiments
Full text linkThe aim of this paper is to provide a theoretical and experimental analysis of the response of an elastic system carried on board a wedge-shaped body impacting the water surface. The wedge entering the water has a sudden deceleration with shock characteristics, resulting in a short-time duration and a sharp peak value. On the other hand, the carried system undergoes an oscillatory motion induced by the inertial load generated by the impact. The study of this problem reveals the occurrence of special conditions in the response of the on-board oscillator, depending on the parameters associated with the water entry problem, that lead to large elastic forces. The experiments show that when varying the characteristic natural frequency of the on-board oscillator, critical impact conditions occur characterized by very large amplitudes of the structural response that confirm the theoretical predictions. The analysis, based on the water shock spectral response developed by the authors in some previous papers, is generalized here and validated experimentally. (C) 2003 Elsevier Ltd. All rights reserved
Optimum design of a ship for the reduction of structural response induced by water impact
No full textSlamming indicates the impact of a solid body entering a liquid surface at high speed. Since a long time this problems attracted the interest of the technical and scientific community in the field of aeronautics and ship design. In this paper an approach based on a simple but effective hydrodynamic force analysis coupled with two elastic models describing the impact of different structures of practical interest is proposed. The analysis, based on the Parseval equality, indicates a way to select the best mechanical parameters of the impacting system mitigating the effects on the structural response
Numerical approximations on the predictive responses of plates under stochastic and convective loads
No full textThe stochastic response of a linear system through equivalent deterministic forces.
No full textThe response of a stochastic system in a discrete coordinate set, as in the finite element approach, can be
associated to a high computational cost even when using a full modal representation. A stochastic representation of the load is needed when, for instance, the case of wall pressure fluctuations induced by turbulent boundary layer (TBL) is considered. A linear 1D system, under the stochastic dynamic load simulating TBL excitation, has been studied here aiming at enhancing a numerical procedure that, under some approximations, allows a reduction of the computational cost. The proposed numerical procedure is founded on the pseudo-excitation method (PEM) that can be considered an exact representation since it is based on a modal decomposition of the cross-spectral density matrix of the excitation. Although effective for some applications, PEM requires the extraction of the eigensolutions of the load matrix at each frequency step. To overcome this computational bottleneck, a new method called frequency Modulated Pseudo Equivalent Deterministic Excitation (PEDEM), is here proposed. The method introduces some equivalent deterministic forces on the basis of the analysis of the eigensolutions of the dynamic load matrix for increasing excitation frequency. Specifically, it is possible to identify different regions for the response defined by introducing a suitable dimensionless excitation frequency. Characteristic values for region bounds and the structural response are in this work investigated and compared with a reference solution obtained with the full stochastic response. Even if the method is here applied over a simple chain of rods, the results seem to be very promising for future applications to more realistic problems
A simplified method for the analysis of the stochastic response in discrete coordinates
No full textNowadays, the problem of the computational cost versus the accuracy of the results is an important field of research in engineering, particularly for the analysis of the response of a stochastic system. By using a set of discrete coordinates, this response can become computationally challenging, especially when using a modal representation. Many dynamic load cases, significant for engineering applications, have random and convective properties such as wall pressure fluctuations due to the turbulent boundary layer (TBL). In this work, a new method for the evaluation of the dynamic response of a linear system excited by a TBL like load is proposed: it is named as frequency Modulated Pseudo Equivalent Deterministic Excitation, (PEDEM), and it is based on the Pseudo Excitation Method, (PEM). The latter is an exact representation since it uses the modal decomposition of the cross-spectral density matrix of the load. Unfortunately, the extraction of the eigensolutions of the load matrix is required at each frequency step and thus can become computationally unacceptable when a large number of eigensolutions is required. PEDEM tries to overcome this computational bottleneck by introducing some approximations, which are
based on the analysis of the eigensolutions of the dynamic load matrix versus frequency. Three different approximations are proposed and thus modulated for the three frequency ranges wherein the dynamic matrix of the considered random and convective load has different characteristics.
A criterion to identify the above mentioned frequency ranges is also proposed by introducing a dimensionless representation of the frequency. The results show that the proposed approximations combine a good accuracy in representing the stochastic system response with a 40% of reduction of the computational costs if compared to a full stochastic response. The method is successfully applied to a simple 1D conguration, a chain of oscillators, which still presents the
main features of a generic system. The extension to 2D systems is also analysed considering a preliminary test case with a flexural plate
PEDE_M: A new method for the analysis of the stochastic response
No full textThe analysis of the response of a stochastic system, through a discrete coordinate representation, can become computationally challenging, even by using a full modal representation; in fact, many dynamic load cases have stochastic behaviour as the wall pressure fluctuations due to the turbulent boundary layer. In the present work, a new method is presented and discussed and it is named frequency modulated pseudo equivalent deterministic excitation (PEDEM). PEDEM is based on the pseudo excitation method (PEM), but it tries to overcome the computational neck-bottles of this latter by introducing some approximations which are based on the analysis of the eigensolutions of the dynamic load matrix versus frequency. The solution approach uses three different approximations for the load matrix with reference to three frequency ranges, named low, mid and high; these approximations derive from the eigenanalysis of the load matrix. A criterion to identify the three frequency ranges is proposed, too and it is expressed in terms of a reduced dimensionless frequency. PEDEM is thus applied to a plate response; this test case that contains the most relevant parameters of a structural problem. The results herein discussed show that the correlation area can play a fundamental role in discriminating the quality of the approximation. A good level of accuracy and representation of the stochastic system together with a significant reduction of the computational costs are obtained if compared to a full stochastic response (FSR) or PEM solution
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