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Wagner, P.

Publications and source records attributed to Wagner, P..

32 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Damped response of shells by a constrained viscoelastic layer

Vibration absorbers are introduced into an asymmetric configuration of thin cylinders and tori enclosing an acoustic medium. The absorbers consist of thin axial strips bonded to the cylinder with a thin viscoelastic layer. The constrained layer dissipates the energy of relative motions between strip and cylinder. The absorber is most effective on response modes with two or more circumferential waves. The use of transfer matrices is extended to the coupled cylinder-absorber system.

El-Raheb, M.↗
NASA NTRS1985

Nonlinear effects in the coupled response of tiles bonded to a plate

The coupled response to a large impulse on tiles bonded to a finite plate is studied. The analysis includes geometric nonlinearity born from boundary restraint in the plane, which stiffens the plate transversely. It also includes material nonlinearities born from plasticity of the plate's material and from properties of a polymer bond including memory and dissipation. The equations of motion are solved by the Galerkin method using linearized eigenfunctions of the system as trial functions. A strip of plate which is one tile wide is modeled.

El-Raheb, M.↗
NASA NTRS1985

Harmonic response of cylindrical and toroidal shells to an internal acoustic field. I - Theory. II - Results

In the first part of the present work concerning the coupled elastic and acoustic response of a system of cylindrical and toroidal shells enclosing an acoustic medium, the theoretical model for the cylindrical and toroidal segments' elastodynamics incorporates an elastic simulation based on transfer matrices, while the acoustic simulation adapts Green's function and curved surface elements. The coupled response is determined by the equilibrium of the acoustic pressure and internal elastic reactions of the shell, and compatibility between acoustic and elastic accelerations at the shell-fluid interface. The second part of this work obtains and discusses numerical results based on the theory for the cases of shell configurations having a plane of symmetry, as well as asymmetric ones. The inadequacy of beam theory in modeling the response of short, thin shell configurations at frequencies above the fundamental elastic resonance is demonstrated.

El-Raheb, M.↗
NASA NTRS1985

Coupled transient response of tiles bonded elastically to a finite flexible plate

Presented are several methods to study a system of tiles bonded elastically to a finite flexible plate responding to an impulse. First, a one-dimensional (1-D) approximation to the system is constructed using Timoshenko beam theory and transfer matrices. This simulates a strip one tile wide. Modal analysis determines the transient response to an impulse of short duration. Also presented are parametric studies of the effect of bond stiffness and the effect of stiffening from inplane stresses produced by large out-of-plane deflections. A two-dimensional (2-D) model is constructed from the eigenfunctions of the 1-D model in a Galerkin method using pairwise products of the 1-D modes as trial functions. Comparison of the two models establishes a factor applied to peak impulse pressure for results to agree.

El-Raheb, M.↗
NASA NTRS1983

Acoustic loading in planar networks

The acoustic loading in a complex planar network of ducts is determined by a method in which Green function surface elements are used. The network consists of straight ducts, elbows and branched ducts. A transfer matrix technique is developed in which each duct is treated separately and the matrix of the influence coefficients is transformed to tri-diagonal form allowing efficient inversion.

El-Raheb, M.↗
NASA NTRS1982

Acoustic propagation in rigid ducts with blockage

Acoustic levitation has been suggested for moving nonmagnetic material in furnaces for heat processing in space experiments. Basically, acoustic standing waves under resonant conditions are excited in the cavity of the furnace while the material blockage is located at a pressure node and thus at a maximum gradient. The position of the blockage is controlled by displacing the node as a result of frequency change. The present investigation is concerned with the effect of blockage on the longitudinal and transverse resonances of a cylindrical cavity, taking into account the results of a one-dimensional and three-dimensional (3-D) analysis. Based on a Green's function surface element method, 3-D analysis is tested experimentally and proved to be accurate over a wide range of geometric parameters and boundary shapes. The shift in resonance depends on the change in pressure gradient and duct shortening caused by the blockage.

El-Raheb, M.↗
NASA NTRS1982

Acoustic propagation in a rigid torus

The acoustic propagation in a rigid torus is analyzed using a Green's function method. Three types of surface elements are developed; a flat quadrilateral element used in modeling polygonal cavities, a curved conical element appropriate for surfaces with one curvature, and a toroidal element developed for such doubly curved surfaces as the torus. Curved elements are necessary since the acoustic pressure is sensitive to slope discontinuities between consecutive surface elements especially near cavity resonances. The acoustic characteristics of the torus are compared to those of a bend of square cross section for a frequency range that includes the transverse acoustic resonance. Two equivalences between the different sections are tested; the first conserves curvature and cross-sectional dimension while the second matches transverse resonance and duct volume. The second equivalence accurately matches the acoustic characteristics of the torus up to the cutoff frequency corresponding to a mode with two circumferential waves.

El-Raheb, M.↗
NASA NTRS1981

Vibration of a liquid with a free surface in a spinning spherical tank

The problem of forced fluid vibrations in a partially filled spinning spherical tank is solved numerically by using the finite element method. The governing equations include Coriolis acceleration and spatially homogeneous vorticity. An exponential instability is detected in the present simulation for fill ratios below 0.5 and centrifugal acceleration to thrust ratios less than 1.7. This fictitious instability appears in the model as a result of the homogeneous vortex assumption since the free slosh equations are neutrally stable in the Liapunov sense.

El-Raheb, M.↗
NASA NTRS1981

Static and dynamic characteristics of large deployable space reflectors

A linear numerical model of the structural characteristics of deployable reflectors was developed. Due to cyclic symmetry of the reflector structure about its axis, only one of many segments is modeled using finite elements. The succeeding segments satisfy continuity of displacement and slope at the interface between consecutive segments. This process leads to (N/2+1) static or dynamic problems of smaller order and bandwidth where N is the number of segments. The solution of each reduced problem leads to motions having a distinct circumferential wave number. The dynamic coupling to the feed support structure is studied adopting modal synthesis.

El-Raheb, M.↗
NASA NTRS1981

Inconsistency of the homogeneous vortex model for liquid slosh in spinning spherical tanks

The problem of liquid slosh in spinning containers represents an important factor in an analytical assessment of the destabilizing energy dissipation in dual spin spacecraft. The solution of the governing equations in this problem is involved due to the occurrence of the Coriolis term. Pfeiffer (1974) introduced the concept of homogeneous vorticity. Although the assumption is valid for completely filled ellipsoidal cavities, it is only an approximation for a partially filled cavity. On the basis of the results of a stability analysis, it appears that the assumption of homogeneous vorticity may be true for filling volumes more than 55%, while for lesser volumes this assumption cannot possibly be true, even approximately. Great care should then be taken in using Pfeiffer's model in flow problems involving intrinsic resonances.

El-Raheb, M.↗
NASA NTRS1980

A consistent set of trial functions for conical shells

A consistent approach is developed for the determination of trial functions for a class of shells of revolution. The trial functions satisfy the in-plane and out-of-plane boundary conditions at the shell ends and include the effects of tangential inertia. The resulting expressions converge rapidly for the problem of linear free vibration

El-Raheb, M.↗
NASA NTRS1980

Acoustic propagation in rigid sharp bends and branches

The linear acoustic propagation in rigid planar sharp bends and bifurcation ducts is analyzed using a Green's-function integral technique often known as the surface element method. The acoustic characteristics of the sharp bend differ substantially from those of a circular bend with identical turning angle, width, and centerline length. The acoustic loading resulting from a duct bifurcation is highly two dimensional beyond the first cutoff frequency.

El-Raheb, M.↗
NASA NTRS1979

Homogeneous vortex model for liquid slosh in spinning spherical tanks

The problem of forced fluid sloshing in a partially filled spinning spherical tank is solved numerically using the finite element method. The governing equations include Coriolis acceleration, empirical fluid damping and spatially homogeneous vorticity first introduced by Pfeiffer. An exponential instability similar to flutter is detected in the present simulation for fill ratios below 50 percent. This instability appears in the model as a result of the homogeneous vortex assumption since the free slosh equations are neutrally stable in the Liapunov sense.

El-Raheb, M.↗