Samir Ziada

Flow-Induced Vibrations in Power and Process Plant Components—Progress and Prospects

This paper provides a brief overview of progress in our understanding of flow-induced vibration in power and process plant components. The flow excitation mechanisms considered are turbulence, vorticity shedding, fluidelastic instability, axial flows, and two-phase flows. Numerous references are provided along with suggestions for future research on unresolved issues.

Vortex–leading-edge interaction

Visualization of successive vortices impinging upon the leading edge of a wedge reveals patterns of deformation of each incident vortex; for certain offsets of the edge with respect to the incident vortex there is pronounced vortex shedding from the leading edge, whereby the shed vortex has a vorticity orientation opposite to that of the incident vortex. Simultaneous consideration of this visualization interaction and the force induced on the wedge gives the relation between the nature of the interaction mechanism and the relative magnitude and phase of the force exerted on the wedge. The amplitude of the induced force is found to be a strong function of the transverse offset of the leading edge with respect to the incident vortex and the degree of vorticity shedding from the leading edge. Application of Stuarts vortex model to the incident vortices provides a means for approximating the phase and relative amplitude of the induced force as a function of the transverse offset of the leading edge.

Self-excited resonances of two side-branches in close proximity

Complex piping systems with multiple side-branches in close proximity are very liable to flow-induced acoustic resonances. This is demonstrated by means of model tests of two piping systems, one with a single side-branch the second containing two side-branches. The general acoustic response of these two systems is studied as the flow velocity in the main pipe is increased. The influence of the distance between the branches, the branch length, the static pressure, the upstream turbulence level and the angle between the branches is also investigated.

Oscillations of an unstable mixing layer impinging upon an edge

The central features of lihear and nonlinear disturbance growth in the unstable shear layer, mechanisms of impingement of the resultant vortices on the edge, induced force on the wedge, and upstream influence in the form of induced velocity fluctuations at separation are examined by simultaneous visualization, velocity, and force-measurement techniques. The nature of the vortex–wedge interaction, and the associated force on the wedge, are directly related to the induced velocity at the upstream separation edge, thereby providing the essential ‘feedback’ for the self-sustained oscillation. Velocity fluctuations at the upper and lower sides of the separation edge tend to be π out of phase, a condition that is maintained along the outer boundaries of the downstream shear layer. Moreover, the phase between velocity fluctuations at separation and impingement satisfies the relation 2 n π, where n is an integer. The shear layer downstream of the separation edge initially forms an asymmetric wake, which evolves into large-scale vortices, all of which have a circulation appropriate to the high-speed side. The disturbance amplification associated with the high-speed side dominates from the separation edge onwards, precluding development of instabilities associated with the low-speed side. Regardless of the initial amplitude of the disturbance induced at the separation edge, the same saturation amplitude is attained in the downstream (nonlinear) region of the shear layer, underscoring the fact that variations in force amplitude at the wedge are dominated by the type of vortex–edge interaction mechanism. The sensitivity of this interaction to small offsets between the vortex centre and the leading edge entails that jumps in frequency of oscillation are also associated with jumps in the force amplitude.

Flow excited resonance of a confined shallow cavity in low Mach number flow and its control

Abstract Shallow cavities exposed to unbounded, low Mach number flow are generally weak aeroacoustic sources because their acoustic modes are heavily damped. This paper focuses on a cavity mounted on the wall of a duct to investigate the effect of “confinement”, i.e., solid boundaries close to the cavity, on the aeroacoustic response of shallow cavities in low Mach number flow (M

Pullout strength of pedicle screws augmented with particulate calcium phosphate: An experimental study

BACKGROUND CONTEXT Pressure-injected and in situ curing bone cements have been studied as alternatives in augmenting lumbar pedicle screw fixation but are frequently found to leak outside the confines of the target vertebra. PURPOSE The objective is set to determine the mechanical efficacy of a porous granular/particulate calcium phosphate (CP) bone augmentation product (Skelite) applied manually without pressurized injection in this application. STUDY DESIGN/SETTING The biomechanical analysis compared the axial pullout strength and insertional torque of augmented and nonaugmented pedicle screws in cellular polyurethane foams. METHODS The insertion torque and pullout strength of 6.5-mm pedicle screws inserted (via 3.5-mm pilot holes) into polyurethane blocks mimicking the porosity of cancellous bone were measured. New pilot holes were then packed with granular particles of Skelite and retested. Last, those blocks initially tested to failure without augmentation were packed with Skelite and retested. Measurements were performed for polyurethane densities of 0.16 and 0.32 g/cc (corresponding to the porosity of osteoporotic and normal bone) and strain rates of 0.5 and 5mm/min. RESULTS Peak pullout force averaged 2132.5+/-119.3 N and 1840.1+/-216.7 N in high density samples without and with augmentation and 688.2+/-91.4 N and 861.6+/-74.5 N in low density samples without and with augmentation. After failure, approximately 50% and 77% of the peak pullout force of original high and low density samples was regained by augmentation. Statistical analysis revealed significant (p<.0001) correlation between the addition of CP, peak pullout resistance, and insertion torque. CONCLUSION Granular CP augmentation improved the pullout strength in both failed (pulledout) samples and low density (porosity of osteoporotic cancellous bone) polyurethane blocks.

Vorticity shedding and acoustic resonance in an in-line tube bundle part I: Vorticity shedding

Abstract An in-line tube array with intermediate tube spacings was tested in a wind tunnel and a water channel. Extensive flow visualization and correlation measurements were carried out to reveal the nature of the vorticity-shedding excitation. The instability of the jet issuing between the tubes is found to be the source of the vorticity-shedding excitation. The jet instability occurs at its symmetric mode, whereupon large-scale vortices are formed symmetrically on both sides of each flow lane. Since the jet instability in each flow lane is 180° out of phase with that in the neighbouring lanes, vortices are formed antisymmetrically in the tube wakes.

Aeroacoustics of pipe systems with closed branches

Flow induced pulsations in resonant pipe networks with closed branches are considered in this review paper. These pulsations, observed in many technical applications, have been identified as self-sustained aeroacoustic oscillations driven by the instability of the flow along the closed branches. The fundamental aspects of the flow induced pulsations are discussed, with particular attention to the description of the sound sources. A single mode model for the prediction of the self-sustained oscillations is presented, the “energy balance”. This model consists of the evaluation of the amplitude of each acoustic mode of the system by means of a balance between the acoustic source power and the acoustic power losses. The main components of this prediction method are discussed; these are the evaluation of the acoustic behavior of a pipe network and the modeling of the sound sources and the acoustic losses. Several field and scale model examples of pipe systems displaying self-sustained oscillations are presented, in order to discuss the parameters influencing the aeroacoustic behavior of pipe networks. Finally some counter-measures for the prevention of self-sustained oscillations are reviewed and perspectives for future work are considered.

On acoustical resonance in tube arrays part I: Experiments

A staggered and an in-line array of closely packed rigid tubes have been tested, in a wind tunnel of varying height, to investigate the conditions under which acoustical resonances do or do not materialize. Among the aspects which have been investigated are: the Strouhal numbers at which flow periodicities occur; the relation between these Strouhal numbers and those at which acoustical resonances occur; the effect of the Reynolds number on the occurrence of acoustical resonances; and the effect of the array depth on the effective speed of sound. A flow visualization study of an in-line array in water flow has also been carried out to reveal the nature of the flow instability which dominates at off-resonance conditions (i.e. away from resonance effects). The findings of the flow visualization have facilitated a better understanding of the complex behaviour of the in-line array tested in air.

Flow impingement as an excitation source in control valves

The impingement of high speed separated flows, often encountered in control valves, provides a feedback mechanism which enhances the pressure fluctuations at selected frequencies. The resulting dynamic loading and noise generation can be sufficiently high to cause structural damage or noise problems. This is demonstrated by considering two case-histories, one of a by-pass control valve and the other of a turbine control valve. In the first case, the behaviour of the frequency of oscillation has been found to be strikingly similar to that associated with self-excited oscillations of cavity flows. The elimination of impingement or the addition of spoilers at the separation location drastically reduces the intensity of the pressure fluctuations. p In the second case, the above mentioned excitation mechanism is coupled to an acoustical resonance mode within the valve chest. At the onset of resonance, the Strouhal number based on the jet thickness and that based on the impingement length are found to be in full agreement with those associated with impinging shear flows. In both cases, the excitation mechanism and means of alleviation have been investigated with the aid of model tests supported by measurements on full size valves in power plants.