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2011-05-17
Technical Paper
2011-01-1696
Nickolas Vlahopoulos, Ricardo Sbragio, Aimin Wang
Stiffened panels are encountered in many engineering systems since the stiffeners comprise the mechanism which provides support and rigidity to the panel's skin. Either a mechanical excitation or an acoustic load can be applied on a stiffened panel creating vibration that is transmitted in all panel components. Mechanical excitation tends to be localized in nature, originating from operating machinery mounted on the panel, while the acoustic excitation tends to be distributed over the entire panel, since it typically originates from an external acoustic source which creates an acoustic field impinging on the entire panel. In the Energy Finite Element Analysis (EFEA) various degrees of fidelity are possible when modeling the response of a stiffened panel. In this paper, the theoretical background and the corresponding implications associated with each alternative modeling approach are presented first.
2011-05-17
Journal Article
2011-01-1734
Nickolas Vlahopoulos, Noah Schiller, Sungmin Lee
The Energy Finite Element Analysis (EFEA) has been utilized successfully for modeling complex structural-acoustic systems with isotropic structural material properties. In this paper, a formulation for modeling structures made out of composite materials is presented. An approach based on spectral finite element analysis is utilized first for developing the equivalent material properties for the composite material. These equivalent properties are employed in the EFEA governing differential equations for representing the composite materials and deriving the element level matrices. The power transmission characteristics at connections between members made out of non-isotropic composite material are considered for deriving suitable power transmission coefficients at junctions of interconnected members. These coefficients are utilized for computing the joint matrix that is needed to assemble the global system of EFEA equations.
2005-05-16
Technical Paper
2005-01-2337
Sang Bum Hong, Nickolas Vlahopoulos, Robert Mantey, David Gorsich
ADRPM (Acoustic Detection Range Prediction Model) is a software program that models the propagation of acoustic energy through the atmosphere and evaluates detectable distance. ADRPM predicts the distance of detection for a noise source based on the acoustic signature of the source. The acoustic signature of a vehicle is computed by combining BEA and EBEA computations with nearfield measurements. The computed signature is utilized as the input to ADRPM. Once the initial detection range is predicted the main contributors to the acoustic detection are identified by ADRPM and their location on the vehicle is modified in order to assess the corresponding effect to the detectable distance of the vehicle.
2005-05-16
Technical Paper
2005-01-2373
Jiulong Sun, Nickolas Vlahopoulos, Kevin Hu
Numerical models are used for computing the shock response in many areas of engineering applications. Current analysis methods do not account for uncertainties in the model parameters. In addition, when numerical models are calibrated based on test data neither the uncertainty which is present in the test data nor the uncertainty in the model are taken into account. In this paper an approach for model update under uncertainty and error estimation for shock applications is presented. Fast running models are developed for the model update based on principal component analysis and surrogate models. Once the numerical model has been updated the fast running models are employed for performing probabilistic analyses and estimate the error in the numerical solution. The new developments are applied for computing the shock response of large scale structures, updating the numerical model based on test data, and estimating the error in the predictions.
2005-05-16
Technical Paper
2005-01-2372
Aimin Wang, Nickolas Vlahopoulos, Ralph D. Buehrle, Jacob Klos
An energy finite element analysis (EFEA) formulation is developed for predicting high frequency vibration response of cylindrical shells with periodically axial and circumferential stiffeners. In this method, the structure is modeled using EFEA method. The power transfer coefficients, employed in the joint matrices of the EFEA formulation at the location of the periodic stiffeners are computed based on Periodic Structure (PS) theory. Results from the new simulation method are compared to experimental data for the vibration response of a periodically circumferentially and axially stiffened cylinder (NASA aluminum testbed cylinder). The observed good correlation indicates that the new EFEA formulation captures properly the periodic characteristics for both the axial stringers and ring stiffeners.
2005-05-16
Technical Paper
2005-01-2421
Sang Bum Hong, Nickolas Vlahopoulos
A hybrid finite element formulation for analyzing flexible plates connected to stiff frame was developed. The excitation was considered to be applied on the stiff members. Conventional FEA models were employed for modeling the behavior of the stiff members in a system. Appropriate damping elements were introduced in the connections between stiff and flexible members in order to capture the presence of the flexible members during the analyses of the stiff ones. Once the vibration of the stiff members and the amount of power dissipated at the damping elements was identified, an EFEA analysis was performed in order to determine the amount of vibrational energy in the flexible members. The hybrid FEA is applied to a Body-In-White (BIW). The results of the hybrid FEA are compared with results from very dense conventional finite element analyses.
2009-05-19
Technical Paper
2009-01-2248
Washington J. de Lima, Nickolas Vlahopoulos, Ricardo Sbragio, Jim He
The Energy Finite Element Analysis (EFEA) has been developed for evaluating the vibro-acoustic behavior of complex systems. In the past EFEA results have been compared successfully to measured data for Naval, automotive, and aircraft systems. The main objective of this paper is to present information about the process of developing EFEA models for two configurations of a business jet, performing analysis for computing the vibration and the interior noise induced from exterior turbulent boundary layer excitation, and discussing the correlation between test data and simulation results. The structural EFEA model is generated from an existing finite element model used for stress analysis during the aircraft design process. Structural elements used in the finite element model for representing the complete complex aircraft structure become part of the EFEA structural model.
2013-05-13
Technical Paper
2013-01-1998
Nickolas Vlahopoulos, Geng Zhang, Walter Brophy, Madhan Ramaswami
The Energy Finite Element Analysis (EFEA) has been developed for computing the structural vibration and the interior noise level of complex structural-acoustic systems by solving numerically governing differential equations with energy densities as primary variables. In this paper a complete simulation process for evaluating airborne noise in an automotive vehicle is presented and validated through extensive comparison to test data. The theoretical elements associated with the important paths of the noise transfer from the exterior of the vehicle to the interior acoustic space are discussed. The steps required for developing an EFEA model for a vehicle are presented. The model is developed based on the physical construction of the vehicle system and no test measurements are utilized for adjusting the numerical model.
2013-05-13
Journal Article
2013-01-1995
Nickolas Vlahopoulos, Sungmin Lee, Paul Braunwart, Jeff Mendoza, Donald Butts
The Hybrid FEA method is based on combining conventional Finite Element Analysis (FEA) with Energy Finite Element Analysis (EFEA) for mid-frequency computations. The difficulty in using conventional FEA at higher frequencies originates from requiring a very large number of elements in order to capture the flexible wavelength of the panel members which are present in a structure. In the Hybrid FEA the conventional FEA model is modified by de-activating the bending behavior of the flexible panels in the FEA computations and introducing instead a large number of dynamic impedance elements for representing the omitted bending behavior. The excitation is considered to be applied on the conventional FEA model and the vibration analysis is conducted. The power flow through the dynamic impedance elements is computed and applied as excitation to the EFEA model of the flexible panels. The EFEA analysis computes the vibration of the flexible panels.
2009-05-19
Technical Paper
2009-01-2196
Ricardo Sbragio, Aimin Wang, Nickolas Vlahopoulos, Davide Caprioli Claudio Bertolini
The hybrid FEA method combines the conventional FEA method with the energy FEA (EFEA) for computing the structural vibration in vehicle structures when the excitation is applied on the load bearing stiff structural members. Conventional FEA models are employed for modeling the behavior of the stiff members in the vehicle. In order to account for the effect of the flexible members in the FEA analysis, appropriate damping and spring/mass elements are introduced at the connections between stiff and flexible members. Computing properly the values of these damping and spring/mass elements is important for the overall accuracy of the computations. Utilizing in these computations the analytical solutions for the driving point impedance of infinite or semi-infinite members introduces significant approximations.
2009-05-19
Technical Paper
2009-01-2198
Geng Zhang, Aimin Wang, Nickolas Vlahopoulos
In order to be effective and maximize the weight and cost savings when designing for noise and vibration attributes, the structural-acoustics design effort must be concurrent with the efforts of other engineering disciplines (i.e. durability, crashworthiness, etc.). In this manner, it will be possible to account for the effects of structural changes across disciplines and improve the NVH performance while the structure is being configured rather than attempting to improve NVH characteristics after the structural design has been completed.
2012-04-16
Technical Paper
2012-01-0725
Nickolas Vlahopoulos, Matthew Castanier, Nicholas Stowe, Eric Maes
In this paper a Multi-Level System (MLS) optimization algorithm is presented and utilized for the multi-discipline design of a ground vehicle track. The MLS can guide the decision making process for designing a complex system where many alternatives and many mutually competing objectives and disciplines need to be considered and evaluated. Mathematical relationships between the design variables and the multiple discipline performance objectives are developed adaptively as the various design considerations are evaluated and as the design is being evolved. These relationships are employed for rewarding performance improvement during the decision making process by allocating more resources to the disciplines which exhibit the higher level of improvement. The track analysis demonstrates how a multi-discipline design approach can be pursued in ground vehicle applications.
1993-04-01
Technical Paper
931188
Nickolas Vlahopoulos, Erich J. Vorenkamp, Jean-Pierre Coyette
An indirect variational boundary element formulation and two typical applications are presented in this paper. The significance of this method is that it can include openings in the model, and it considers the acoustic medium on both sides. Computationally it is superior to the direct method because the assembled fully populated boundary element matrices are symmetric. The theoretical background is presented. A typical generic interior cab noise analysis is performed. The excitation is comprised by an exterior impinging acoustic field and loads applied at the mounts. The coupled option was selected to solve this problem. A typical acoustic uncoupled radiation analysis is also performed. The noise radiated from a T-drive is computed and the solution time is compared to the direct method.
1993-09-01
Technical Paper
932433
Thomas C. Tecco, Nickolas Vlahopoulos, Erich J. Vorenkamp
The advancement of numerical methods for acoustics has enhanced the ability to make meaningful predictions of acoustic responses in vehicle passenger compartments, such as those found in automobiles, trucks, and construction equipment. A design objective of growing importance is to isolate the occupants from both structural and air-borne noise. This paper presents how an indirect boundary element formulation can be used to study the effect of holes on the transmission of air-borne sound, and how design changes effect the transmission of sound through heater and air conditioning ducts. The theoretical background of the indirect formulation is also presented. The significance of this method is that it can include openings in the model while considering the acoustic medium on both sides of the mesh. It is also computationally superior to the direct method because the assembled matrices are symmetric.
2006-04-03
Technical Paper
2006-01-0931
Jiulong Sun, Nickolas Vlahopoulos, Tara J. Stabryla, Rich Goetz, Roel Van De Velde
One of the main threats to military vehicles originates from landmine blasts. In order to improve the survivability of the occupants it is important to design a military vehicle for increased occupant safety. Simulation technology that combines modeling of the blast loads from the landmine explosion, the response of the vehicle to the blast load, and the loads developed on the members of an occupant are important factors in this effort. The ability to simulate the landmine explosion is validated first by comparing simulation results to test data collected by gages placed in the ground and above the ground. Combined simulations predicting the damage to a target structure due to a landmine explosion are also compared to test data for further validation. Principal component analysis and metamodel theory is employed for generating fast running models in order to adjust the soil parameters in the simulation models during the correlation effort.
2006-04-03
Technical Paper
2006-01-1224
Sang Bum Hong, Nickolas Vlahopoulos
A new development in the area of the hybrid Finite Element Analysis (hybrid FEA) is presented. The hybrid FEA method combines the conventional FEA method with energy FEA (EFEA) for analysis of systems that contain both flexible and stiff members, and is suitable for mid-frequency computations. A formulation for analyzing flexible plates spot-welded to stiff beams when the excitation is applied on the stiff members is developed. Conventional FEA models are employed for modeling the behavior of the stiff members in a system. Appropriate damping elements are introduced in the connections between stiff and flexible members in order to capture the presence of the flexible members during the analyses of the stiff ones.
2006-04-03
Technical Paper
2006-01-0762
Jiulong Sun, Nickolas Vlahopoulos, Tara J. Stabryla, Rich Goetz, Roel Van De Velde
One of the main threats to military vehicles originates from landmine blasts. In order to improve the survivability of the occupants it is important to design a military vehicle for increased occupant safety. Simulation technology that combines modeling of the blast loads from the landmine explosion, the response of the vehicle to the blast load, and the loads developed on the members of an occupant are important factors in this effort. Uncertainties from the soil properties can influence the blast loads and thus the occupants' safety. In this paper, principal component analysis along with metamodel theory are employed for developing fast running models for the response functions of interest. The response functions of interest are the time domain loads which are developed on an occupant's members due to the blast. The fast running models allow assessing the probability level associated with injury for an occupant.
2003-05-05
Technical Paper
2003-01-1733
Jin Wang, Nickolas Vlahopoulos, Zissimos P. Mourelatos, Omidreza Ebrat, Kumar Vaidyanathan
This paper presents the development of surrogate models (metamodels) for evaluating the bearing performance in an internal combustion engine without performing time consuming analyses. The metamodels are developed based on results from actual simulation solvers computed at a limited number of sample points, which sample the design space. A finite difference bearing solver is employed in this paper for generating information necessary to construct the metamodels. An optimal symmetric Latin hypercube algorithm is utilized for identifying the sampling points based on the number and the range of the variables that are considered to vary in the design space. The development of the metamodels is validated by comparing results from the metamodels with results from the actual bearing performance solver over a large number of evaluation points. Once the metamodels are established they are employed for performing probabilistic analyses.
2003-05-05
Technical Paper
2003-01-1724
Omidreza Ebrat, Zissimos P. Mourelatos, Kexin Hu, Nickolas Vlahopoulos, Kumar Vaidyanathan
A comprehensive formulation is presented for the dynamics of a rotating flexible crankshaft coupled with the dynamics of an engine block through a finite difference elastohydrodynamic main bearing lubrication algorithm. The coupling is based on detailed equilibrium conditions at the bearings. The component mode synthesis is employed for modeling the crankshaft and block dynamic behavior. A specialized algorithm for coupling the rigid and flexible body dynamics of the crankshaft within the framework of the component mode synthesis has been developed. A finite difference lubrication algorithm is used for computing the oil film elastohydrodynamic characteristics. A computationally accurate and efficient mapping algorithm has been developed for transferring information between a high - density computational grid for the elastohydrodynamic bearing solver and a low - density structural grid utilized in computing the crankshaft and block structural dynamic response.
2008-04-14
Technical Paper
2008-01-0781
Geng Zhang, Christopher J. Hart, Nickolas Vlahopoulos, Krishan Bishnoi, Richard Goetz, Farzad Rostam-Abadi
Utilizing simulation technology is important for designing a structure with increased survivability to a load from an explosion. The pressure wave from the blast and the fragments hitting the structure must be simulated in such an analysis. Commercial software can be utilized through the development of appropriate interfaces for performing such computations. In this paper an approach is presented for combining commercially available Eulerian and Lagrangian solvers for performing blast event simulations. A capability has been developed for automatically creating the Eulerian finite element given the finite element model for the structure. The effect of moisture in the soil properties is considered during the generation of the soil - explosive - air model used by the Eulerian solver. Tracers are defined in the Eulerian model for all structural finite elements which are on the outer part of the structure and are subjected to the load from the blast.
2008-04-14
Journal Article
2008-01-0218
Jim He, Geng Zhang, Nickolas Vlahopoulos
In this paper the development of statistical metamodels and statistical fast running models is presented first. They are utilized for propagating uncertainties in a multi-discipline design optimization process. Two main types of uncertainty can be considered in this manner: uncertainty due to variability in design variables or in random parameters; uncertainty due to the utilization of metamodels instead of the actual simulation models during the optimization process. The value of the new developments and their engagement in multi-discipline design optimization is demonstrated through a case study. An underwater vehicle is designed under four different disciplines, namely, noise radiation, self-noise due to TBL excitation, dynamic response due to propulsion impact loads, and response to an underwater detonation.
2008-04-14
Technical Paper
2008-01-0269
Nickolas Vlahopoulos, Aimin Wang
The Energy Boundary Element Analysis (EBEA) has been utilized in the past for computing the exterior acoustic field at high frequencies (above ∼400Hz) around vehicle structures and numerical results have been compared successfully to test data [1, 2 and 3]. The Energy Finite Element Analysis (EFEA) has been developed for computing the structural vibration of complex structures at high frequencies and validations have been presented in previous publications [4, 5]. In this paper the EBEA is utilized for computing the acoustic field around a vehicle structure due to external acoustic noise sources. The computed exterior acoustic field comprises the excitation for the EFEA analysis. Appropriate loading functions have been developed for representing the exterior acoustic loading in the EFEA simulations, and a formulation has been developed for considering the acoustic treatment applied on the interior side of structural panels.
2007-04-16
Technical Paper
2007-01-1744
Jiulong Sun, Jim He, Nickolas Vlahopoulos, Peter van Ast
In order to develop confidence in numerical models which are used for automotive crash simulations, results are compared with test data. Modeling assumptions are made when constructing a simulation model for a complex system, such as a vehicle. Through a thorough understanding of the modeling assumptions an appropriate set of variables can be selected and adjusted in order to improve correlation with test data. Such a process can lead to better modeling practices when constructing a simulation model. Comparisons between the time history of acceleration responses from test and simulations are the most challenging. Computing accelerations correctly is more difficult compared to computing displacements, velocities, or intrusion levels due to the second order differentiation with time. In this paper a methodology for enabling the update of a simulation model for improved correlation is presented.
2007-05-15
Technical Paper
2007-01-2324
Geng Zhang, Nickolas Vlahopoulos
This paper presents a validation case study for an Energy Finite Element Analysis (EFEA) formulation through comparison to test data. The EFEA comprises a simulation tool for computing the structural response of a complex structure and the amount of the radiated power. The EFEA formulation presented in this paper can account for periodic stiffeners, for partial fluid loading effects on the outer part of the structure, and for internal compartments filled with heavy fluid. In order to validate these modeling capabilities of the EFEA two 1/8th scale structures representing an advanced double hull design and a conventional hull design of a surface ship are analyzed. Results for the structural vibration induced on the outer bottom part of the structure are compared to available test data. The excitation is applied at two different locations of the deck structure. Good correlation is observed between the numerical results and the test data.
2007-05-15
Technical Paper
2007-01-2303
Nickolas Vlahopoulos, Sheng Li, Michel Viktorovitch, Davide Caprioli
The hybrid Finite Element Analysis (hybrid FEA) has been developed for performing structure-borne computations in automotive vehicle structures [1, 2 and 3]. The hybrid FEA method combines conventional FEA with Energy FEA (EFEA). Conventional FEA models are employed for modeling the behavior of the stiff members in a system. Appropriate damping and spring or mass elements are introduced in the connections between stiff and flexible members in order to capture the presence of the flexible members during the analyses of the stiff ones. The component mode synthesis method is combined with analytical solutions for determining the driving point conductance at joints between stiff and flexible members and for defining the properties of the concentrated elements which represent the flexible members when analyzing the stiff components.
2007-05-15
Technical Paper
2007-01-2178
Nickolas Vlahopoulos, Geng Zhang, Jim He
The Energy Boundary Element Analysis (EBEA) has been utilized in the past for computing the exterior acoustic field at high frequencies (above ∼400Hz) around vehicle structures and numerical results have been compared successfully to test data [1, 2 and 3]. The Energy Finite Element Analysis (EFEA) has been developed for computing the structural vibration of complex structures at high frequencies and validations have been presented in previous publications [4, 5]. In this paper the EBEA is utilized for computing the acoustic field around a vehicle structure due to external acoustic noise sources. The computed exterior acoustic field comprises the excitation for the EFEA analysis. Appropriate loading functions have been developed for representing the exterior acoustic loading in the EFEA simulations, and a formulation has been developed for considering the acoustic treatment applied on the interior side of structural panels.
2007-05-15
Technical Paper
2007-01-2325
Geng Zhang, Nickolas Vlahopoulos
In applications of the Energy Finite Element Analysis (EFEA) there is an increasing need for developing comprehensive models with a large number of elements which include both structural and interior fluid elements, while certain parts of the structure are considered to be exposed to an external fluid loading. In order to accommodate efficient computations when using simulation models with a large number of elements, joints, and domains, a substructuring computational capability has been developed. The new algorithm is based on dividing the EFEA model into substructures with internal and interface degrees of freedom. The system of equations for each substructure is assembled and solved separately and the information is condensed to the interface degrees of freedom. The condensed systems of equations from each substructure are assembled in a reduced global system of equations. Once the global system of equations has been solved the solution for each substructure is pursued.
2009-04-20
Technical Paper
2009-01-0564
Nickolas Vlahopoulos, Jim He
A multi-disciplinary optimization under uncertainty (MDO-U) capability has been developed in order to solve optimization problems with multiple sets of objectives and constraints originating from different design disciplines while simultaneously accounting for uncertainty during the optimization process. Uncertainties are introduced in the optimization process by considering the constraints which depend on any random variables and any random parameters as probabilistic. Satisfying the probabilistic constraints in the presence of uncertainty introduces sufficient conservatism in the solution and eliminates the need for further application of safety factors. The MDO-U capability is applied for performing design optimization for the TPS of an Apollo type vehicle. The Traj and FIAT codes of NASA Ames are employed during this design process for trajectory and for thermal analyses, respectively.
2009-04-20
Technical Paper
2009-01-0344
Jim He, Nickolas Vlahopoulos
A multi-disciplinary optimization analysis is a highly iterative process that requires a large number of function evaluations for computing the objective functions and the constraints. Metamodels (i.e. response surface methodologies) can be constructed before starting the optimization for each one of the objective functions and the constraint functions. The metamodels can be employed in the multi-discipline optimization instead of high fidelity simulations resulting in significant computational savings. A multi-discipline design optimization of an aircraft wing under aerodynamic and structural analysis considerations is performed in this manner. Design variables associated with the shape of the wing are considered in the CFD simulations, while sizing structural design variables are considered in the structural discipline. At the top system level, a cost type metric is defined for driving the overall design optimization process.
2004-03-08
Technical Paper
2004-01-1143
Jin Wang, Nickolas Vlahopoulos, Zissimos P. Mourelatos, Omidreza Ebrat, Kumar Vaidyanathan
This paper presents the development of surrogate models (metamodels) for evaluating the bearing performance in an internal combustion engine. The metamodels are employed for performing probabilistic analyses for the engine bearings. The metamodels are developed based on results from a simulation solver computed at a limited number of sample points, which sample the design space. An integrated system-level engine simulation model, consisting of a flexible crankshaft dynamics model and a flexible engine block model connected by a detailed hydrodynamic lubrication model, is employed in this paper for generating information necessary to construct the metamodels. An optimal symmetric latin hypercube algorithm is utilized for identifying the sampling points based on the number and the range of the variables that are considered to vary in the design space.
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