Showing papers in "AIAA Journal in 1997"
TL;DR: In this article, an interval truncation approach is proposed to limit the growth of intervals of response parameters so that realistic and accurate solutions can be obtained in the presence of large amounts of uncertainty.
Abstract: The imprecision or uncertainty present in many engineering analysis/design problems can be modeled using probabilistic, fuzzy, or interval methods. This work considers the modeling of uncertain structural systems using interval analysis. By representing each uncertain input parameter as an interval number, a static structural analysis problem can be expressed in the form of a system of linear interval equations. In addition to the direct and Gaussian elimination-based solution approaches, a combinatorial approach (based on an exhaustive combination of the extreme values of the interval numbers) and an inequality-hased method are presented for finding the solution of interval equations. The range or interval of the solution vector (response parameters) is found to increase with increasing size of the problem in all of the methods. An interval-truncation approach is proposed to limit the growth of intervals of response parameters so that realistic and accurate solutions can be obtained in the presence of large amounts of uncertainty. Numerical examples are presented to illustrate the computational aspects of the methods and also to indicate the importance of the truncation approach in practical problems. The utility of interval methods in predicting the extreme values of the response parameters of structures is discussed.
400 citations
TL;DR: In this paper, the authors simulated flow past a circular cylinder at a Reynolds number of 3.9 X 10 3 using a solver that employs an energy-conservative second-order central difference scheme for spatial discretization.
Abstract: We have simulated flow past a circular cylinder at a Reynolds number of 3.9 X 10 3 using a solver that employs an energy-conservative second-order central difference scheme for spatial discretization. Detailed comparisons of turbulence statistics and energy spectra in the downstream wake region (7.0 < x/D < 10.0) have been made with the results of Beaudan and Moin and with experiments to assess the impact of numerical diffusion on the flowfield. Based on these comparisons, conclusions are drawn on the suitability of higher-order upwind schemes for LES in complex geometries.
397 citations
TL;DR: In this paper, a simple new zonal boundary condition has been proposed based upon the addition of dissipative and convective terms to the compressible Navier Stokes equations, which is based upon a simple addition of convective and dissipative terms.
Abstract: A simple new zonal boundary condition has been proposed. It is based upon the addition of dissipative and convective terms to the compressible Navier Stokes equations
374 citations
TL;DR: The rollup of a wingtip vortex, at a Reynoldsnumber based on chord of 4.6 £ 10 6, was studied with an emphasis on SUctionside and very near-wakemeasurements (upto x/c = 0.678 downstreamofthetrailingedge) as mentioned in this paper.
Abstract: The rollup of a wingtip vortex, at a Reynoldsnumber based on chord of 4 .6 £ 10 6 , was studied with an emphasis onsuctionsideandverynear-wakemeasurements(upto x/c = 0.678 downstreamofthetrailingedge).Theresearch was conducted in a 32 £ 48 in. (0.81 £ 1.22 m), low-speed wind tunnel. The rectangular half-wing model had a semispan of 36 in. (0.91 m), a chord of 48 in. (1.22 m), and a rounded tip. Seven-hole pressure probe measurements of the velocity ® eld surrounding the wingtip showed that a large axial velocity up to 1.77 U 1 developed in the vortex core. This high a level of core axial velocity has not been measured previously. Triple-wire probes were used to measure all components of the Reynolds stress tensor. It was determined from correlation measurements that
244 citations
TL;DR: In this article, a load projection scheme based on Gaussian integration and fast interpolation algorithms for unstructured grids is proposed to solve the problems of the load transfer and interface surface tracking.
Abstract: The loose coupling of computational fluid dynamics and computational structural dynamics solvers introduces some problems related to the information transfer between the codes. Some techniques developed to solve the problems of the load transfer and interface surface tracking are presented. The main criterion is to achieve conservation of total loads and total energy. The load projection scheme is based on Gaussian integration and fast interpolation algorithms for unstructured grids. The surface tracking algorithm, also based on interpolation, is important for many applications, including aeroelastic deformation of wings due to aerodynamic loads. The methodologies not only improve present fluid-structure interaction simulations, but also increase the range of their applicability. These techniques are of general character and can be used in other multidisciplinary applications as well.
185 citations
TL;DR: Two new robust optimization extensions are developed to evaluate the utility of using automatic differentiation in robust optimization, one is a sensitivity-based procedure, and the other makes use of experimental design techniques.
Abstract: In this study the use of automatic differentiation techniques significantly reduces the number of analysis calls and the CPU time required for robust optimum design. Traditionally, robust optimum design procedures have relied on finite differencing techniques to evaluate sensitivities for use in gradient-based optimization. Robust objective functions and robust constraints are typically formulated as compound functions that invoke the original objective or constraint many times for each robust evaluation. In this research, automatic differentiation techniques are used to avoid the costly finite differencing of robust objective functions and robust constraints. Sensitivities calculated using automatic differentiation are exact and therefore enhance performance. Two new robust optimization extensions are developed to evaluate the utility of using automatic differentiation in robust optimization. One is a sensitivity-based procedure, and the other makes use of experimental design techniques. These two new robust optimization extensions are successfully implemented in application to an aircraft concept sizing test problem.
174 citations
TL;DR: In this paper, a new mixed-laminatetheory for piezoelectric laminates is presented, where a coupled governing equation for curvilinear laminations is derived for the displacements and a layerwise representation for the electric potential.
Abstract: Mechanicsfortheanalysisoflaminatedcompositeshellswithpiezoelectricactuatorsandsensorsarepresented.A newmixedlaminatetheoryforpiezoelectricshellsisdevelopedincurvilinearcoordinatesthatcombinessingle-layer assumptions for the displacements and a layerwise representation for the electric potential. The resultant coupled governing equations for curvilinear piezoelectric laminates are described. Structural mechanics are subsequently developed and an eight-node ® nite element is formulated for the static and dynamic analysis of adaptivecomposite shell structures of general laminationscontaining piezoelectriclayers. Evaluations of themethod and comparisons with reported results were performed. Numerical results for cylindrical laminated piezoelectric composite panels with continuous piezoceramic actuators and cantilever shells with continuous or discrete piezoelectric actuators and sensors illustrate the advantages of the method and quantify the effects of curvature on the electromechanical response of piezoelectric shells.
169 citations
TL;DR: In this paper, the flow fields in two-dimensional channels with discontinuous expansions are studied numerically to understand how the channel expansion ratio influences the symmetric and nonsymmetric solutions that are known to occur.
Abstract: The flowfields in two-dimensional channels with discontinuous expansions are studied numerically to understand how the channel expansion ratio influences the symmetric and nonsymmetric solutions that are known to occur. For improved confidence and understanding, two distinct numerical techniques are used. The general flowfield characteristics in both symmetric and asymmetric regimes are ascertained by a time-marching finite difference procedure. The flowfields and the bifurcation structure of the steady solutions of the Navier-Stokes equations are determined independently using the finite element technique. The two procedures are then compared both as to their predicted critical Reynolds numbers and the resulting flowfield characteristics. Following this, both numerical procedures are compared with experiments.
142 citations
TL;DR: The development, implementation, and application of approaches to modeling the interactions in multidisciplinary design is illustrated and the rich insights and results that can be generated by exercising different realistic protocols between disciplinary players in modern design processes are illustrated.
Abstract: The development, implementation, and application of approaches to modeling the interactions in multidisciplinary design is illustrated. Given that the design of complex systems involves multiple disciplinary design teams and their associated analysis and synthesis tools, the task is to model the real interactions among the designers and their tools in order to predict the resulting design. Our approach to this problem is to abstract the interactions in multidisciplinary design as a sequence of games among a set of players, which are embodied by the design teams and their computer-based tools. The developments are applied to a subsonic passenger aircraft design case study to illustrate the rich insights and results that can be generated by exercising different realistic protocols between disciplinary players in modern design processes.
139 citations
TL;DR: In this article, a self-contained automated methodology for active flow control which couples the time-dependent Navier-Stokes system with an adjoint Navier Stokes system and optimality conditions from which optimal states, i.e., unsteady flow fields and controls (e.g., actuators), may be determined, is presented.
Abstract: This paper describes a self-contained automated methodology for active flow control which couples the time-dependent Navier-Stokes system with an adjoint Navier-Stokes system and optimality conditions from which optimal states, i.e., unsteady flow fields and controls (e.g., actuators), may be determined. The problem of boundary-layer instability suppression through wave cancellation is used as the intital validation case to test the methodology. Here, the objective of control is to match the stress vector along a portion of the boundary to a steady base flow. Control is effected through the injection or suction of fluid through a single orifice on the boundary. The results demonstrate that instability suppression can be achieved without any a priori knowledge of the flow unsteadiness such as frequencies, instability type, etc. The present methodology has been extended to three dimensions and may potentially be applied to separation control, relaminarization, and turbulence control applications using one to many sensors and actuators.
129 citations
TL;DR: In this article, the effects of largedistributed roughness located on the leading edge of an airfoil has been studied to determine the effect on boundary-layer development and transition.
Abstract: An experimentalstudy of the effects of largedistributed roughness located neartheleading edgeofan airfoilhas been performed to determine the effect on boundary-layer development and transition. Boundary-layer measure- ments werecarried out on a two-dimensional NACA0012 airfoil with a 53.34-cm chord through theuse of hot-wire anemometry at Reynolds numbers of 0 .75 £ 10 6 , 1.25 £ 10 6 , and 2.25 £ 10 6 . These measurements included mean anductuating velocity, turbulence intensity, ¯ ow® eld intermittency, and associated integral parameters. The roughness used was of the type and density observed to occur during the initial glaze ice accretion process. Results have shown that the transitional boundary layer induced by large distributed roughness is markedly different from the smooth model Tollmein± Schlicting induced transition process. No fully developed turbulent boundary layers were observed to occur near the roughness location. Instead, the large distributed roughness was observed to trigger a transitional boundary layer at or very near the roughness location. This transitional boundary layer required asubstantialchordwiseextentto obtaina fully developedturbulentstate.Streamwiseturbulenceintensity levels in the roughness induced transitional region were observed to berelatively low as compared with the smooth model transitional region.
TL;DR: In this paper, a methodology for performing optimization on three-dimensional unstructured grids based on the Euler equations is presented, where the same, low-memory-cost explicit relaxation algorithm is used to resolve the discrete equations that govern the flow, linearized direct, and adjoint problems.
Abstract: A methodology for performing optimization on three-dimensional unstructured grids based on the Euler equations is presented. The same, low-memory-cost explicit relaxation algorithm is used to resolve the discrete equations that govern the flow, linearized direct, and adjoint problems. The analysis scheme is a high-resolution local-extremum-diminishing-type scheme that uses Roe decomposition for the dissipative fluxes. Mesh movement is performed in such a way that optimization of arbitrary geometries is allowed. The parallelization of the algorithm, which permits its extension to optimization of realistic, complete aircraft geometries, is presented. Two sample optimizations are performed. The flrst is the inverse design of a transonic wing/body configuration using a surface target pressure distribution found by analyzing the geometry for known design variable deflections. The second exercise is the inverse design of a business jet configuration consisting of wing, body, strut, nacelle, horizontal fin, and vertical fin. The surface target pressure distribution in this case is provided by an analysis of the configuration with no strut-nacelle.
TL;DR: In this article, the effect of thermal and chemical nonequilibrium on stability is shown to depend on the disturbance mode and the frequency and spatial ampli® cation of disturbances that may lead to boundary layertransition on cold wall and adiabaticat plates.
Abstract: Foright at high Mach numbers, thermal and chemical nonequilibrium may exist in the meanow and thus affectthestabilityoftheow.Acomputationaltoolwasdevelopedtoanalyzeahypersonicmeanowanditsstability including thermochemical nonequilibrium. The meanow analysis employs the Navier± Stokes equations with a translational/vibrational temperature model for thermal nonequilibrium and a ® ve-species reacting air model for chemicalnonequilibrium. Thestability analysisemployslinearstability theory to describethespatialampli® cation of two- and three-dimensionaldisturbances. Thecomputational tool is used to determinethe frequency and spatial ampli® cation of disturbances that may lead to boundary layertransition on cold wall and adiabaticat plates. The effect of thermal and chemical nonequilibrium on stability is shown to depend on the disturbance mode.
TL;DR: In this article, the characteristics of a typical flow control actuator design are discussed and an optimum operating Stokes parameter is computed by matching this viscous dominated solution to an ideal, inviscid result.
Abstract: The characteristics of a typical flow control actuator design are discussed. The device is based on a resonating structure that interacts with a closed volume of fluid to create a concentrated jet through an exit orifice. The resulting unsteady flow through the orifice introduces viscous effects that are characterized by the Stokes parameter based on the orifice diameter. An optimum operating Stokes parameter is then computed by matching this viscous dominated solution to an ideal, inviscid result. The actuator is modeled with a system of coupled equations that describe its fluid-structural behavior.
TL;DR: In this article, a dual approach integrating structural tailoring and adaptive materials technology is presented to control the dynamic response of cantilever beams subjected to external excitations, where the adaptive capability is achieved through the converse piezoelectric factors.
Abstract: A dual approach integrating structural tailoring and adaptive materials technology and designed to control the dynamic response of cantilever beams subjected to external excitations is presented. Whereas structural tailor- ing uses the anisotropy properties of advanced composite materials, adaptive materials technology exploits the actuating capabilities of piezoelectric materials bonded or embedded into the host structure. A control law relat- ing the piezoelectrically induced boundary bending moment with the velocity at given points of the structure is implemented and its effect on the closed-loop frequencies and dynamic response to harmonic excitations is inves- tigated. The combination of structural tailoring and control by means of adaptive materials proves very effective in damping out vibration. I. Introduction A Sthe requirements for higherexibility on high-speed aircraft increase, so do the challenges of developing innovative de- sign solutions. Whereas the increasedexibility is likely to provide enhanced aerodynamic performance, the aircraft also must be able to ful® ll a multitude of missions in complex environmental condi- tions and to feature an expanded operational envelope and longer operational life. To achieve such ambitious goals- advanced con- cepts resulting in the enhancement of static and dynamic response of the multimission- highlyexible aircraft must be developed and implemented. One way of achieving such goals consists of the in- tegration of advanced composite materials in the aircraft structure. 1 In this regard, the directionality property featured by anisotropic composite materials is capable of providing the desired elastic cou- plings through the proper selection of the ply angle. However, such a technique is passive in nature in the sense that, once the design is in place, the structure cannot respond to the variety of conditions in which it must operate. The situation can be mitigated by incorporating into the host structure adaptive materials able to respond actively to changing conditions. In a structure with adaptive capabilities, the natural fre- quencies, damping, and mode shapes can be tuned to reduce the vibration so as to avoid structural resonance andutter instability, and in general toenhance the dynamic response characteristics.The adaptivecapabilityisachievedthroughtheconversepiezoelectricef- fect,whichconsistsofthegenerationoflocalized strainsinresponse to an applied voltage. This induced strain ® eld produces, in turn, a change in the dynamic response characteristics of the structure. It is proposedheretoenhancethefreevibrationanddynamicresponseto externalexcitationsofwing structures by incorporating theadaptive capabilityreferredtoasinduced strain actuationinconjunctionwith structural tailoring. Under consideration is a cantilevered aircraft wing, modeled as a thin/thick-walled closed cross-section beam of anisotropic material. Implementation of a control law relating the applied electric ® eld to one of the mechanical quantities charac- terizing the response of the wing according to a prescribed func- tional relationship results in eigenvalue/boundary-value problems. The solution consists of closed-loop eigenvalues/dynamic response characteristics, which are functions of the applied voltage, i.e., of the feedback control gain. Investigation of static and dynamic control of aircraft wing structures via the simultaneous implementation of induced strain
TL;DR: A class of synthetic problems for testing multidisciplinary design optimization (MDO) approaches is presented and several MDO approaches were executed on two sample synthetic test problems.
Abstract: A class of synthetic problems for testing multidisciplinary design optimization (MDO) approaches is presented. These test problems are easy to reproduce because all functions are given as closed-form mathematical expressions. They are constructed in such a way that the optimal value of all variables and the objective is unity. The test problems involve three disciplines and allow the user to specify the number of design variables, state variables, coupling functions, design constraints, controlling design constraints, and the strength of coupling. Several MDO approaches were executed on two sample synthetic test problems. These approaches included single-level optimization approaches, collaborative optimization approaches, and concurrent subspace optimization approaches. Execution results are presented, and the robustness and efficiency of these approaches an evaluated for these sample problems.
TL;DR: In this paper, a numerical procedure for the aerodynamic design of transonic airfoils by means of genetic algorithms, with single-point, multipoint, and multiobjective optimization capabilities, is presented.
Abstract: Some of the advantages and drawbacks of genetic algorithms applications to aerodynamic design are demonstrated. A numerical procedure for the aerodynamic design of transonic airfoils by means of genetic algorithms, with single-point, multipoint, and multiobjective optimization capabilities, is presented. In the ® rst part, an investigation on the relative ef® ciency of different genetic operators combinations is carried out on an aerodynamic inverse design problem. It is shown how an appropriate tuning of the algorithm can provide improved performances, better adaption to design space size and topology, and variables cross correlation. In the second part, the multiobjective approach to design is introduced. The problem of the optimization of the drag rise characteristics of a transonic airfoil is addressed and dealt with using a single point, a multipoint, and a multiobjective approach. A comparison between the results obtained using the three different strategies is ® nally established, showing the advantages of multiobjective optimization.
TL;DR: In this paper, the authors computed the flow over two different multi-element airfoil configurations using linear eddy viscosity turbulence models and a nonlinear explicit algebraic stress model.
Abstract: The flow over two different multi-element airfoil configurations is computed using linear eddy viscosity turbulence models and a nonlinear explicit algebraic stress model. A subset of recently-measured transition locations using hot film on a McDonnell Douglas configuration is presented, and the effect of transition location on the computed solutions is explored. Deficiencies in wake profile computations are found to be attributable in large part to poor boundary layer prediction on the generating element, and not necessarily inadequate turbulence modeling in the wake. Using measured transition locations for the main element improves the prediction of its boundary layer thickness, skin friction, and wake profile shape. However, using measured transition locations on the slat still yields poor slat wake predictions. The computation of the slat flow field represents a key roadblock to successful predictions of multi-element flows. In general, the nonlinear explicit algebraic stress turbulence model gives very similar results to the linear eddy viscosity models.
TL;DR: In this paper, a method of prioritizing sensor locations on a flexible structure for the purpose of determining damaged structural elements from measured modal data is presented, which is useful in applications where practicality dictates only a small subset of the total structural degrees of freedom can be instrumented.
Abstract: A method of prioritizing sensor locations on a flexible structure for the purpose of determining damaged structural elements from measured modal data is presented. This method is useful in applications where practicality dictates only a small subset of the total structural degrees of freedom can be instrumented. In such cases, it is desirable to place sensors in locations yielding the most information about the damaged structure. No a priori knowledge of the damage location is assumed. The prioritization is based on an eigenvector sensitivity analysis of a finite element model of the structure. In addition, the dual problem is presented and solved, which determines the observability of change in the measured elgenstracture from the instrumented degrees of freedom. This analysis is used to determine the extent to which damage can be localized. An analytical example is presented that illustrates the relationship between the number of measured modes, the number of instrumented degrees of freedom, and the extent to which damage can be localized. Additionally, an analysis of an experimental cantilevered eight-bay truss assembly consisting of 104 elements instrumented with eight single-axis accelerometers is presented. The extent to which structural damage can be localized from the measurement data is limited by the number of measured modes.
TL;DR: In this article, a method that eliminates Mach waves from the exhaust of supersonic jets was proposed to increase takeoff thrust with minimal impact on overall fuel consumption by surrounding the jet with an annular stream.
Abstract: Experimental results are presented on a method that eliminates Mach waves from the exhaust of supersonic jets and, hence, that removes a strong component of supersonic jet noise. Elimination is achieved by surrounding the jet with an annular stream at prescribed velocity and temperature so that all turbulent motions become intrinsically subsonic. No mechanical suppressors are used. Implementation of the technique in a typical turbofan engine is estimated to increase takeoff thrust with minimal impact on overall fuel consumption.
TL;DR: In this paper, the authors used three-component laser Doppler velocimetry to obtain the distributions of velocity, vorticity, and circulation across the tip vortex at several axial locations in the flow for several angles of incidence.
Abstract: The evolving three-dimensional flowfield of the tip vortex in the near wake of a rectangular wing at incidence was studied in detail, using three-component laser Doppler velocimetry. The flow quantities measured were the three components of the instantaneous velocity. These data were used to obtain the distributions of velocity, vorticity, and circulation across the vortex at several axial locations in the flow for several angles of incidence. The data have been used to understand the process of rollup of the shear layer into the vortex in the near wake, as well as its kinematic structure. The data indicate that the rollup takes place quite quickly and the inner part of the three-dimensional vortex becomes nearly axisymmetric within a distance of about two chord lengths downstream of the trailing edge. Even though the vortex behavior in the near wake is, in general, strongly dependent on the initial conditions, the vortex trajectory appears to be described reasonably well by the overall wing lift and the freestream velocity. Also, even in the near wake, circumferentially averaged mean flow properties in the inner part of the nearly axisymmetric vortex begin to exhibit a universal structure characteristic of conceptual asymptotic trailing vortices
TL;DR: In this article, an Froude scale helicopter rotor model with embedded piezoceramic elements as actuators to suppress vibrations was used for a hover test with a one-eighth dynamically scaled rotor model.
Abstract: The experimental results are presented of hover test with a one-eighth dynamically scaled (Froude scale) helicopter rotor model with embedded piezoceramic elements as actuators to suppress vibrations. A 6-ft-dlam two-bladed bearingless rotor model was built with banks of piezoelectric torsional actuators capable of manipulating blade twist at harmonics of the rotational speed. The twist performances of several rotor blade configurations were investigated using accelerometers embedded in the blade tip. The change in oscillatory rotor lift due to plezoactuation was measured by a hub balance. Experimental results show that linear twist distributions of up to 0.6 deg, resulting in increases of up to 10% of the nominal rotor lift, are possible with existing piezoceramic technology. Although the twist amplitudes attained in this experiment were less than the target value (1-2 deg), it is expected that partial reduction of hub vibration can be achieved with the current smart rotors.
TL;DR: In this paper, the concept of deployable structures based on pantographs that are deployed and stiffened by means of cables is applied to the design of the support structure for a large mesh reector.
Abstract: The general concept of deployable structures based on pantographs that are deployed and stiffened by means of cables is applied to the design of the support structure for a large mesh reector. The two main components of this structure are a cable-stiffened pantographic ring that deploys and pretensions a cable network that, in turn, provides a series of stiff, geometrically accurate support points to which a reective wire mesh orexible membranewould beconnected.Thepantographicring isahighly redundantstructurewith an internalmechanism that permits synchronous deployment without any strain in the rods. The geometric conditions that have to be satis® ed in order for an n-sided ring to fold without any strain are investigated, including the effects of joint size. An experimental model has been designed and tested. In the folded con® guration, it has a diameter of 0.6 m and height of 1.2 m; in the deployed con® guration, it has a diameter of 3.5 m. Stiffness and deployment tests on this model have shown its behavior to be linear and the maximum shape error to be § 0.3 mm. HIS paper is the second in a series that deals with a new type of deployable structures where a foldable bar structure, which consists of pairs of straight bars connected by pivots and forming a pantograph, is deployed and stiffened by two sets of cables, known as active and passive cables. The passive cables are short cable elements connected to joints of the pantograph that get farther apart during deployment,and the length of each passive cable is such that it becomes taut when the pantograph is fully deployed. The active cables are longer elements that run over small pulleys and whose overall length is controlled by one or more electric motors. The layout of these cables is such that deployment of the pantograph can be activated by shortening the length of at least one of the active cables.Oncethepantographisdeployed,andhenceallofthepassive cables are taut, it is possible to set up an overall state of prestress in
TL;DR: In this paper, a numerical procedure for the time integration of spatially discretized finite element equations for viscoelastic structures governed by a constitutive equation involving fractional calculus operators is presented.
Abstract: Numerical procedures for the time integration of the spatially discretized finite element equations for viscoelastic structures governed by a constitutive equation involving fractional calculus operators are presented. To avoid difficulties concerning fractional-order initial conditions, a form of the fractlonal calculus model of viscoelasticity involving a convolution integral with a singular memory kernel of Mittag-Lefler type is used. The constitutive equation is generalized to three-dimensional states for isotropic materials. A simplification of the fractional derivative of the memory kernel is used, in connection with Grunwald's definition of fractional differentiation and a backward Euler rule, for the time evolution of the convolution term. A desirable feature of this process is that no actual evaluation of the memory kernel is needed. This, together with the Newmark method for time integration, enables the direct calculation of the time evolution of the nodal degrees of freedom. To illustrate the ability of the numerical procedure a few numerical examples are presented. In one example the numerically obtained solution is compared with a time series expansion of the analytical solution.
TL;DR: In this paper, a mode selection strategy based on maximum modal strain energy produces more accurate update results than a strategy that selects modes based on minimum frequency, which is a function of the measured modal parameters.
Abstract: Amethod is presented forselecting thesubsetofidenti® edstructural vibration modesto beusedin® niteelement model correlation for structural damagedetection. Themethod is based on a ranking of themodesusing measured modal strain energy and is a function of only the measured modal parameters. It is shown that a mode selection strategy based on maximum modal strain energy produces more accurate update results than a strategy based on minimum frequency. Strategies that use the strain energy stored by modes in both the undamaged and damaged structuralcon® gurationareconsidered.Itisdemonstratedthatmoreaccurateresultsareobtainedwhen themodes are selected using the maximum strain energy stored in the damaged structural con® guration. The mode selection techniques are applied to the results of a damage detection experiment on a suspended truss structure that has a large amount of localized modal behavior.
TL;DR: In this paper, an assessment of the mean and unsteady nozzle flow to define the quiet core length, and hence performance, over the operating range of the NASA Langley Mach 6 quiet wind tunnel was performed.
Abstract: The flow in the NASA Langley Mach 6 quiet wind tunnel has been investigated to quantify the effectiveness of laminar-flow control techniques used to delay transition of the nozzle-wall boundary layer. The results of this investigation include an assessment of the mean and unsteady nozzle flow to define the quiet core length, and hence performance, over the operating range of the facility. A large, uniform region of Mach 5.91 flow was documented for a variety of unit Reynolds numbers. By using a prototype constant-voltage anemometer to measure the unsteady flowfield, acoustic radiation patterns from the transitional nozzle-wall boundary layers were mapped. These disturbances originating at the Irregular edge of the transitional nozzle-wall boundary layer were shown to follow Mach lines into the test section of the nozzle, thereby limiting the length of the quiet core. With a virtual origin downstream of the nozzle throat, a Reynolds number dependency was found for the amplitudes of the acoustic radiation.
TL;DR: In this paper, the flow in the crossflow separation region of a 6:1 prolate spheroid at 10-and 20-degrees angle of attack, Re L = 4.20 x 10 6, was investigated using a novel, miniature, three-dimensional, fiber-optic laser Doppler velocimeter (LDV).
Abstract: The flow in the crossflow separation region of a 6:1 prolate spheroid at 10- and 20-deg angle of attack, Re L = 4.20 x 10 6 , was investigated using a novel, miniature, three-dimensional, fiber-optic laser Doppler velocimeter (LDV). The probe was used to measure three simultaneous, orthogonal velocity components from within the model, and these measurements were simultaneous with wall-pressure measurements made just below the LDV probe volume. The LDV measurements extend from approximately y + = 7 out to beyond the boundary-layer edge. The design and operation of this LDV probe is summarized.
TL;DR: In this paper, a geometrically nonlinear theory of anisotropic multilayered plates of general layups featuring interlayer slips is discussed, and the pertinent equations of motion and consistent boundary conditions are derived by means of the dynamic version of virtual work.
Abstract: The formulation of a geometrically nonlinear theory of anisotropic multilayered plates of general layups featuring interlayer slips is discussed. The theory rests on a displacement field, which accounts for an arbitrary distribution of the tangential displacements through the laminate thickness, fulfills a priori the static continuity conditions of tangential stresses at the layer interfaces, and allows for jumps in the tangential displacements so as to provide the possibility of incorporating effects of interfacial imperfection. For the interlayer displacement jump, a linear shear slip law is postulated. No a priori assumption is made on the type and order of the expansion in the thicknesswise direction of the tangential displacements. The pertinent equations of motion and consistent boundary conditions are derived by means of the dynamic version of the principle of virtual work. These are given in terms of force and moment stress resultants and in terms of generalized displacements. The generalization achieved by the proposed approach is shown by deriving, as particular cases, the recently proposed first-order and third-order models for laminated plates featuring interlayer slips
TL;DR: In this article, a modification and extension of a previously developed Sensitivity-Based Element-By-Element (SB-EBE) method for damage detection and finite element model up-dating is presented.
Abstract: An algorithm is presented for updating finite element models based upon a minimization of dynamic residuals. The dynamic residual of interest is the force unbalance in the homogeneous form of the equations of motion arising from errors in the model`s mass and stiffness when evaluated with the identified modal parameters. The present algorithm is a modification and extension of a previously-developed Sensitivity-Based Element-By-Element (SB-EBE) method for damage detection and finite element model up- dating. In the present algorithm, SB-EBE has been generalized to minimize a dynamic displacement residual quantity, which is shown to improve test- analysis mode correspondence. Furthermore, the algorithm has been modified to include Bayesian estimation concepts, and the underlying nonlinear optimization problem has been consistently linearized to improve the convergence properties. The resulting algorithm is demonstrated via numerical and experimental examples to be an efficient and robust method for both localizing model errors and estimating physical parameters.
TL;DR: In this paper, adaptive airfoils, which change their shape in response to changes in velocity, are examined for potential applications in this arena, and the results show that even ordinary variations in wind speed can cause unwanted changes in behavior.
Abstract: Microaerial vehicles with characteristic lengths under 15 cm, flight speeds of 32-64 km/h, and specific payload and endurance needs are potentially useful for many military and civilian applications. The combination of small dimensions and modest flight speed results in Reynolds numbers ranging between 10 4 and 10 5 . Traditional rigid airfoil shapes experience substantial degradation in performance (specifically, the lift-to-drag ratio) as the Reynolds number falls through this range. Thus, even ordinary variations in wind speed can cause unwanted changes in behavior. In this study, rudimentary adaptive airfoils, which change their shape in response to changes in velocity, are examined for potential applications in this arena.