Effect Of Fluid Loading Research Articles

Indomethacin in the Treatment of Ureteral Colic: Is Fluid Restriction Necessary?

The effect of fluid load on pain was estimated in 60 patients with acute ureteral colic treated with 50mg. intravenous indomethacin. One group received 3 1. fluid intravenously, while in another group all fluids were withheld for 6 hours. Pain was assessed with a visual analogue scale. No intergroup difference in regard to pain was found after 1, 3 and 6 hours of observation.

Dynamic Behavior of a Hydraulically Actuated Mechanism. Part 1: Small Perturbations

This paper considers the dynamic behavior of a mechanism actuated by an oscillating hydraulic cylinder. The analysis is generalized by introducing nondimensional parameters. In this first portion of the paper, linearization is applied to explore the basic nature of the system transient response. A wide range of parameter values is considered to illustrate the effects of fluid capacitance, fluid inertance, load, leakage and friction on the system behavior and quantitative design guidelines are indicated. Part 2 of the paper deals with nonlinear aspects of the actuator-mechanism system.

Vibrational characteristics of an infinite flat plate with varying properties

A theory to study the vibrational characteristics of a structure with properties varying along its length is developed. The analysis consists of assuming that the plate thickness and plate stiffness are functions of the axial distance x. The resulting thin plate equation becomes a fourth‐order differential equation with variable coefficients. The equation is solved by Fourier transform techniques and a solution in the transformed plane is obtained. The analysis is then applied to a constant thickness plate with sinusoidally varying stiffness. For simplicity, the effect of fluid loading is neglected and the forcing function is assumed to be given by a distributed load. Results in the form of plate displacement disribution are presented.

The effect of hydrostatic pressure fields on the structural and acoustic response of cylindrical shells

The effects of external hydrostatic pressure fields and fluid loading on the structural and acoustic response of a point-driven infinitely long circular cylindrical shell were examined over a range of frequencies. The external pressure field was modeled using static prestress terms in the shell equations of motion, and the structural response was characterized by the driving point admittance and the circumferential resonant frequencies. The acoustic response was quantified through calculation of the radiated sound power, both in an overall sense and on an individual modal basis. The analysis was performed for the in-air case as well as for the in-water case. The structural response was found to be strongly affected by fluid and pressure, resulting in significant resonant frequency shifts. However, the overall acoustic response was shown to be nearly independent of the external pressure field, both in air and in water. In addition, it was shown that relatively fewer modes contribute significantly to the sound radiation for the submerged shell as compared to the shell in air. In both cases, the sound generation was controlled by the low-order nonresonant modes.

On the acoustic power radiated by line forces on elastic beams

The importance of the spatial extent of applied forces in the topic of structural radiation is studied through formulation of the sound power radiated by a line force acting on an infinite elastic beam. The expression for sound power is integrated numerically and the results examined as a function of kL, the acoustic length of the force, and the wavenumber ratio. Below coincidence the power was found to be generally proportional to (kL)−2. Near and at coincidence, sufficiently large force scales have a significant effect on the power produced. In particular, when the length of the force is an integral multiple of the critical wavelength, the coincidence peak is completely suppressed. Above coincidence, minima in the second power occur when the force length is a multiple of the wavelength of the free bending wave. These minimal values of power are shown to be nearly equal to the power radiated far below coincidence. These results are discussed in terms of the coupled structural/acoustic wavenumber response function and the wavenumber spectrum of the applied force. In addition the relative effects of light fluid loading on the radiation at coincidence are presented.

Effects of fluid loading on air‐backed thin shells

Previously, we presented [L. H. Green, R. H. Hackman, D. H. Trivett, and L. Flax, J. Acoust. Soc. Am. Suppl. 1 76, S65 (1984)] an analysis of the low‐frequency resonances of thin spherical shells. The analysis demonstrated that the proper background to subtract (in order to identify the elastic and fluid responses) is that of a rigid sphere. Some questions were raised about the validity of this approach to very thin shells. In this paper the analysis is performed on an aluminum spherical shell with an inner to outer radius ratio of 0.99 and is extended to include thin cylindrical shells. In both cases, the effects of fluid loading on the in‐vacuum modes are discussed.

The power radiation from line forced fluid‐loaded plates

In the past, most studies of the effects of fluid loading on the response of time harmonic line forced plates have involved either “ad hoc” simplifications whose adoption was justified on physical grounds alone or numerical techniques and restricted frequency ranges below the “coincidence” frequency. We use the method of Matched Asymptotic Expansions (MAE) to obtain a series of simple analytic expressions which cover overlapping regions of the whole frequency range. A composite expression is easily formed for the radiated power. This gives a simple single expression, uniformly valid for all frequency ranges, and yields predictions for the radiated power in good agreement with the corrected curves of Nayak [J. Acoust. Soc. Am. 49, (1979)]. [Work supported by ONR Code 425.]

Acoustic radiation from doubly periodic, ring‐supported, fluid‐loaded, circular cylindrical shells for different types of mechanical drives

The farfield acoustic radiation from a doubly periodic, ring‐supported, fluid‐loaded, circular cylindrical shell is computed for moment and longitudinal mechanical drives. The shell model used by C. B. Burroughs [J. Acoust. Soc. Am. 75, 715–722 (1984)] to compute the acoustic radiation for a radial point drive is employed in this effort. The rings interact with the shell only through normal forces. The model includes the effect of fluid loading and interaction between ring supports. For a large cylinder radius, predictions are compared to published results for a point‐driven doubly periodic line supported plate and a moment‐driven unsupported plate. Predictions of the farfield acoustic radiation from the ring‐supported cylinder for moment and longitudinal drives are compared to predictions for the radial‐driven cylinder. Characteristics of the acoustic radiation are examined by example and some physical interpretation of significant features are offered.

Transient response of fluid‐loaded planar vibrators with coupled modal coefficients

An approach is presented to evaluate the transient pressure field and vibratory response of a finite fluid‐loaded rectangular vibrator when excited by a wideband force and mounted in a planar baffle. The approach utilizes an in‐vacuo eigenfunction expansion with time dependent coefficients to describe the velocity of the vibrator in the fluid. In contrast to the in‐vacuo problem, the time dependent modal coefficients are coupled for the in‐fluid problem. The coefficients are shown to be the solution of a coupled set of convolution integral equations which can be solved by marching forward in time. Near‐ and farfield pressures are expressed as a sum of convolution integrals involving the mode dependent velocity coefficients and spatially dependent impulse responses. Numerical results for several problems will be presented to illustrate the effects of fluid loading and modal coupling on the response of several vibrators. [Work supported by ONR.]

Underwater‐nearfield acoustical holography using a robotic controlled scanner

A computerized, experimental facility has been developed to implement nearfield acoustical holography in a large underwater tank. The facility contains a robotic controlled hydrophone arm which allows the precise positioning and motion of a hydrophone in the tank completely under computer control. Automated scanning is done close to,the surface of a radiating/scattering object and the resulting pressure data are stored digitally. The frequency range of operation is dc to above 30 kHz. Using the principles of nearfield acoustical holography [Williams and Maynard, Phys. Rev. Lett. 45, 554 (1980)] this data are forward and backprojected to study the radiated/scattered acoustic fields. Forward projection provides directivity patterns, and intensity and vector‐velocity field mappings. Backward projection provides surface velocity and pressure (fluid loading)—reconstructing the field in the extreme nearfield. Experimental results are shown providing a complete description of the field radiated underwater from a point‐driven rectangular plate. Shown are its surface velocity, intensity vectors in the extreme near field, fluid loading effects. radiation efficiency, and directivity patterns.

Low‐frequency vibrational modes of fluid‐loaded thin spherical shells. II.L.H

Previously, we presented [L. H. Green et al., J. Acoust. Soc. Am. Suppl. 1 75, S77 (1984)] an analysis of the low‐frequency high‐Q resonances of thin spherical shells. The previous investigation centered on providing a physical explanation for the strong coupling of the lowest frequency resonances to the fluid medium. We extend the analysis to more completely explore the effects of fluid loading on the high‐Q resonances. In particular, as the phase velocity of the low speed (first mode) wave on a thin spherical shell approaches the speed of sound in the fluid, the dispersion curve for the elastic wave appears to merge with that of the “creeping wave.” A physical explanation of this effect will be provided along with numerical results based on the linear theory of elasticity.

Acoustic radiation from fluid-loaded infinite circular cylinders with doubly periodic ring supports

Analytic expressions are derived for the farfield acoustic radiation from a point-driven, fluid-loaded circular cylindrical shell reinforced with doubly periodic ring supports. The ring supports interact with the cylindrical shell only through normal forces. The model includes the effects of fluid loading and interactions between supports. Predictions are compared to measured farfield acoustic radiation data taken on a finite circular cylindrical shell with doubly periodic ring supports. Characteristics of the acoustic radiation are examined by example and some physical interpretation of significant features are offered.

Response of an edge-supported circular membrane electret earphone. Part II—Experimental results

This paper presents experimental measurements of the static and dynamic behavior of an edge-supported circular membrane electret earphone. It is shown that the measured value of static capacitance and sound pressure level generated in a cavity agree well with theoretical predictions. The experimental results dramatize the nonlinear dependence of the earphone sensitivity on many of the transducer parameters. The effects of fluid loading of the foil electret and damping are considered. It is shown that the air surrounding the foil electret can significantly increase its apparent mass, and that damping effects may be ignored below the system fundamental resonance.

The combined effects of fluid loading and compliant coating on the acoustic and structural fields of an infinite plane elastic surface

This paper deals with the combined effects of fluid loading and compliant coating on an elastic surface, taken to be a membrane, under uniform line force excitation. The parameters defining a coating may be chosen either to decouple the fluid from the surface vibration and thereby reduce the far field radiation, or to cancel fluid loading effects and restore the far field to the level and directivity corresponding to the vibration of a surface without fluid loading. In both cases there may be attendant changes in the structural vibration levels, either close to the excitation or far from it. These changes are examined in a comprehensive fashion, the drive and transfer admittances and the acoustic far field being calculated for excitation at prescribed force amplitude, at prescribed velocity amplitude, and for prescribed input power.

Low frequency acoustic radiation and vibration response of locally excited fluid-loaded structures

The fluid loading of a thin elastic plate by static fluid can be characterized by the values of two parameters, one a ratio Ω of frequency to coincidence frequency (or equivalently, a Mach number M = Ω 1 2 ), the other a “fluid loading at coincidence” parameter ε = ϱc 0/ mω g . Fluid loading effects on the structural response are of crucial importance when Ω is close to 1 and when Ω is small—specifically when Ω = O(ε 3) or smaller, the parameter ε being small in all applications. This paper deals with the latter low frequency, heavy fluid loading limit, giving asymptotic solutions for a range of problems involving infinite, semi-infinite and finite thin elastic plates and membranes. The paper begins with a study of the infinite thin elastic plate under point force, line force and line moment excitation and gives a description of the surface and acoustic wave fields on the structure at all ranges from the excitation. This section provides some new results, and previous results for the drive admittances of heavily fluid-loaded plates are recovered. There follows an examination of the effects of ribs on an infinite plane structure (taken, in section 4, as a membrane for simplicity). The question of principal interest, in respect to later sections dealing with finite structures, is whether or not the phenomenon of resonance, associated with the finite spacing between ribs, can arise in the low frequency limit. It is in fact shown that in that limit there is generally substantial transmission of structural energy across a rib, regardless of the mechanical impedance of the rib, and then that there is no possibility of near-resonant response. However it is established that near-resonant response can indeed occur under certain specified circumstances involving finite rib impedance. Next the interaction of incident structural waves with the edge of a semi-infinite plate is discussed. A variety of baffled and unbaffled configurations, and of edge conditions, is of interest in this kind of problem. Here one specific case—that of an unbaffled plate with a free edge—is considered; this case is regarded as typical, as will be seen from future work in which many other cases will be briefly examined. It is shown that, except within a small distance of the edge, the vibration field consists of the incident wave plus a wave reflected with no change of amplitude, but with a phase change dependent upon the edge constraints. Fluid loading effects are completely specified by this phase shift, or equivalently by an “end-correction” giving the virtual origin from which the wave appears to be reflected. The acoustic field scattered from the edge is also calculated and shown to be very weak. This then suggests an approximate method for dealing with finite elastic plates in the low frequency heavy fluid loading limit; one merely calculates the vacuum dynamics of the structure, allowing for fluid loading effects in (i) the equivalent free wavenumber of the plate waves, (ii) the description of the response in the neighbourhood of concentrated excitation, where infinite plate results can be used for the forced motion, and (iii) the imposition of boundary conditions with phase shifts incorporated from the semi-infinite interaction analysis. The method is applied to a strip plate or membrane, giving quickly results which are identical with those derived elsewhere [1] from a thorough analysis of the finite fluid-loaded plate problem. In particular, it is shown that the strip plate has resonances at which the amplitude is limited only by small radiation losses and by mechanical losses, and that the plate has modes with the same shapes (except near the edges) as those in the absence of fluid loading. It is further shown here that the drive admittance of the finite heavily fluid-loaded line driven plate is purely imaginary (implying structural energy conservation), and that for particular choices of the plate length the admittance may be infinite (resonance), zero (antiresonance), or the same as the imaginary part of the admittance of the infinite plate (transparency). The paper ends with a brief discussion of further extensions of this approximate method which will be pursued in a subsequent study.

Influence of volume loading on intraoperative hemodynamics and perioperative fluid retention in patients with valvular regurgitation undergoing prosthetic replacement

Preload and afterload were controlled intraoperatively in 2 groups of patients with valvular regurgitation and congestive heart failure in order to determine the effect of fluid loading on weight gain and subsequent recovery. As a part of their anesthetic management before cardiopulmonary bypass, 16 patients were given 1 liter of crystalloid with nitroprusside infused at 1 μg/kg/min and 16 received 4 liters of crystalloid with nitroprusside at 4 μg/kg/min. Arterial and filling pressures were kept the same in both groups. Heart rate and vascular resistance became significantly lower and cardiac index and stroke volume significantly higher in the fluid-augmented group. The transition to cardiopulmonary bypass was smoother for the fluid-augmented patients. All patients survived, and 2 in each group were ventilated beyond 24 hours. The mean weight gain at surgery was the same in the 2 groups (3 kg). For the first 2 postoperative days, body weight remained unchanged and thereafter, both groups lost weight at 0.6 kg/day. Preoperative weight was reached on day 7 and discharge occurred on day 15, 2 kg below control. Thus, a modest operative weight gain in these patients is inevitable, benign, and independent of the volume of fluid infused before bypass.

Effects of isotonic fluid load on plasma water and extracellular fluid volumes in the rat.

An isotonic fluid load was given to rats by infusing 12 ml saline i.v. in 60 min. The plasma water and extracellular fluid volumes of the whole animal and selected tissues were subsequently studied with 125I human serum albumin and 51Cr EDTA. The fluid infused was equivalent to 130% of the plasma water volume. The total extracellular fluid volume increased by 17%, while the total plasma water measured with RIHSA remained unchanged. The regional extracellular fluid volumes increased in the lung (14%), the gastric fundus (15%), large intestine (21%) and skin (28%). The results illustrate the selective distribution of an isotonic fluid overload, those tissues being effected having high compliances.

Acoustic radiation from point‐driven, doubly periodic ring‐supported, fluid‐loaded circular cylinders

Closed form analytic expressions are derived for the farfield acoustic radiation from a point‐driven, fluid‐loaded circular cylindrical shell reinforced with doubly periodic ring supports. The ring supports interact with the cylindrical shell only through normal forces; moments for the supports are not included. The analytic expressions include the effects of fluid loading and interactions between supports. Characteristics of the acoustic radiation are examined by example and some physical interpretation of significant features are offered.

Vibrations of and radiation from rib-stiffened water-loaded plates driven by line loads

The vibrations of and radiated pressure from rib-stiffened water-loaded plates of infinite extent were investigated. By restricting the excitation to sinusoidally varying forces acting along lines parallel to the ribs, the problem is mathematically reduced to an infinite beam with impedance discontinuities representing the ribs. The reaction forces and moments these ribs apply to the plate were calculated and put in terms of force and moment transmissibilities similar to those defined by Snowdon in his work on beams. The cases investigated included two symmetrically spaced ribs excited by a central line force or moment. Both clamped and pinned ribs were evaluated. The effects of inherent plate damping, rib spacing and fluid loading on these transmissibility ratios and resultant acoustic radiation below the critical frequency were calculated.

An analytical theory for the response of finite elastic plates at low frequencies under heavy fluid loading

The low-frequency response to concentrated excitation of a finite thin elastic plate under heavy fluid loading is examined using a generalized Wiener-Hopf approach. The plate size is assumed large in an appropriate sense, and it is shown how the solution can be derived from the exact solution for the structure-acoustic problem at the edge of a semi-infinite plate. In particular, it is shown that the reflection, with no more than a phase change dependent on the edge conditions, of a low-frequency structural wave at the edge leads to the phenomena of resonance and antiresonance very much as in the absence of fluid loading. It is then argued that exact results for infinite and semi-infinite geometries may be used to solve, approximately, a wide variety of problems in the heavy fluid loading limit which are not otherwise susceptible to analytical treatment, and also that this method in principle permits heavy fluid loading effects to be incorporated in large-scale finite element calculations for realistic structures. [Work supported by ONR, Code 222.]