Axisymmetric Excitation Research Articles

Large-eddy simulations of excitation effects on a VTOL upwash fountain

The responses of an upwash fountain to various azimuthal and axisymmetric excitations, applied at the exits of its neighboring jets, are investigated. Kinematic arguments are used to determine the interactions of large-scale structures in the fountain and the effects of these interactions on the fountain’s characteristics. Numerical simulations of a row of impinging jets, which contain the essential features of twin jets impinging on the ground, are used to simulate the hovering configuration of a vertical takeoff and landing (VTOL) aircraft. The flow is assumed to be governed by the time-dependent, incompressible Navier–Stokes equations. The large-eddy simulation approach is followed. Distinct fountain characteristics are shared among the cases in which azimuthal perturbations are applied at both jet exits in the same direction. These include a high fountain spreading rate, a strong lateral interaction with the neighboring jets, and a weak upload at the aircraft’s undersurface. A strong similarity in fountain characteristics also exists for the cases of axisymmetric forcing and azimuthal forcing in opposite directions at the two jet exits. The similar characteristics here include a low fountain spreading rate and a strong upload at the aircraft’s undersurface.

An infinite, elastic, cylindrical shell with a finite number of ring constraints

The axisymmetric excitation of an infinite, elastic, cylindrical shell with a finite number of ring constraints is discussed. Exact solutions are presented for the cases of one and two constraints. These are then examined in an asymptotic limit that corresponds to light fluid-loading and small curvature.

Axisymmetric vibrations of an elastic fluid‐loaded cylinder

The response of an infinite elastic cylindrical shell immersed in an acoustic medium is examined analytically. The relatively low frequency range, (ka) < 3–4, is investigated. The cylinder is subject to a localized axisymmetric excitation, a ring drive. Starting from the known integral expression for the Green's function, approximation techniques were applied to extract the dominant behavior of the surface field. Expressions for the response are given for distances greater than an acoustic wavelength from the source location.

Large-eddy simulations of axisymmetric excitation effects on a row of impinging jets

Numerical simulations of a row of impinging jets are performed. Both the impinging jets and the fountains caused by the collision of the wall jets are modeled in the simulation. The problem considered contains the essential features of twin jets impinging on the ground, simulating the hovering configuration of a vertical takeoff and landing (VTOL) aircraft. The flow is assumed to be governed by the time-dependent, incompressible Navier–Stokes equations. The large-eddy simulation approach is followed. The present study focuses on the motion and dynamics of large-scale structures that have been experimentally observed in jet flows. The behavior of the jets and the fountain caused by the introduction of axisymmetric disturbances at the jet exits are investigated.

Analysis of dynamic structure/media interaction : Avanessian, V; Dong, S B; Muki, R Proc 3rd Conference on Dynamic Response of Structures, Los Angeles, March 31–April 2, 1986P745–756. Publ New York: ASCE, 1986

A version of the Global-Local Finite Element Method (GLFEM) is applied to the analysis of soil-structure interaction under steady-state excitation. The soil medium is considered to be homogeneous, isotropic and of semi-infinite extent. The structure is axisymmetric and may lie on the surface of the half-space or be partially or fully buried. Both axisymmetric and asymmetric excitations are discussed. The solution procedure enforces both traction and displacement continuity at the interface between the finite element mesh and the outer region, and the traction free surface conditions on the half-space surface beyond the finite element mesh. The advantages of this approach are demonstrated by a series of examples.

Transient vibratory response of fluid-loaded shells with axisymmetric excitations

A general time domain method is presented to evaluate the transient velocity field of fluid-loaded cylinders of finite length which are excited by axisymmetric, broadband excitations acting in the radial direction. A mechanical pressure which is independent of the acoustic field is used to excite the cylinder. The method is based on the use of time-dependent in vacuo modal expansions of the velocity of the cylinder and the mechanical pressure. As a result of fluid coupling, the time-dependent velocity coefficients are described by a set of coupled, linear, convolution integral equations which can be solved by marching forward in time. Typical numerical results showing the impulse responses of the fluid-loaded modal admittances of the cylinder, the time-dependent coupling coefficients, and the velocity of the cylinder are presented for the case of a cylinder which is simply supported at both ends.

Relationship between near and farfield effects in second harmonic generation in the piston beam

An earlier analysis [J. H. Ginsberg, J. Acoust. Soc. Am. 76, 1201–1214 (1984)] of the infinite baffle problem for an axisymmetric harmonic excitation derived a nonlinear King integral describing signal distortion in sound beams. That analysis, which considered only the contribution to the cumulative distortion process associated with secular source terms, agreed with experiments at high frequencies at ranges beyond approximately one quarter the Rayleigh length. However, there was significant discrepancy with measurements very close the transducer face. Such differences will be shown to result from neglecting nonsecular second‐order terms that do not carry into the farfield. In the present work, the nonsecular contribution to the second harmonic is evaluated as a numerical inversion of a multiple integral transform for the second‐order potential. The composite effects of secular distortion, which is calculated using the renormalized King integral, and the nonsecular contribution are calculated. The results obtained from the analytical model compare favorably with previoius nearfield measurements. [Work supported by ONR, Code 425‐UA.]

An improved King integral algorithm for finite amplitude sound beams

This paper supercedes the previous analysis [J.H. Ginsberg, Acoust. Soc. Am. Suppl. 1 71, S30 (1982)] of the infinite baffle problem for an axisymmetric harmonic excitation, which derived a nonlinear King integral describing the distortion associated with finite acoustic Mach numbers. That analysis was shown [M. B. Moffett and J. H. Ginsberg, J. Acoust. Soc. Am. Suppl. 1 72, S40 (1982)] to exhibit excessive nearfield distortion in comparison to experiment. Using the linear King integral in its conventional complex function form, as opposed to the real function analysis employed previously, leads to formulation of the second order potential in terms of complex functions. Asymptotic integration of this potential function reveals that there may be significant contribution from the evanescent spectrum (small transverse wavenumbers), as well as from the propagating spectrum. A coordinate straining transformation describing the full spectrum is duduced. From it, the previous analysis is shown to be only asymptotically correct. The new analysis reveals that the distortion is governed by a transformation that involves Fresnel integrals for the propagating spectrum and the error function for the evanescent spectrum. Some comparisons of the analytical prediction and the results of Moffett and Ginsberg are presented [Work supported by ONR, Code 420.]

Acoustic propagation in finite length elastic cylinders. Part I: Axisymmetric excitation

The acoustic propagation in a perfect finite length thin elastic cylinder is studied from an exact solution to the coupled elasto-acoustic equations of motion. Eigenfunction expansions are obtained for Koiter’s consistent shell equations and the Helmholtz equation governing the acoustic field. The acoustic pressure is expressed as the sum of modal acoustic pressures each factored by a normalized influence coefficient related to the corresponding generalized coordinate in the elastic cylinder eigenfunction. A set of nonlinear homogeneous algebraic equations in the generalized coordinates are obtained when satisfying the coupled equations and the compatibility condition at the fluid–cylinder interface. The method when applied to a clamped cylinder filled with water and excited by a plane wave demonstrates the existence of narrow nonresonant peaks in maximum cylinder displacement and a frequency range where the average response rises considerably.

Axisymmetric excitation of spherical helical antenna with conical screen

Axisymmetric excitation of spherical helical antenna with conical screen

Radiofrequency power absorption by a strongly magnetized, hot, bounded ion-electron plasma

We give a fluid description of a uniform, bounded, hot ion-electron plasma which includes collision effects and a full electromagnetic treatment. The absorbed r.f. power is calculated in the magneto-ion domain for a plasma column excited by a cylindrical TE wave. Axisymmetric and multipolar azimuthal excitation are considered. A resonance spectrum is obtained which is strongly dependent on the geometry and on the hot character of the plasma. These results are compared with those of an earlier collisionless quasi-static model, and the influence of the various parameters is systematically investigated. The r.f. power absorbed at some well-chosen resonances can be quite considerable when the intervening parameters are optimized.

Axisymmetric excitation of a cylinder having a varying surface impedance

Axisymmetric excitation of a cylinder having a varying surface impedance