Flow-sound Interaction Mechanism Research Articles

Characteristics of the flow-induced noise from rectangular rods with different aspect ratios and edge geometry

The dynamic lift force, wake characteristics and the susceptibility to self-excited acoustic resonance is experimentally investigated for rectangular rods with aspect ratios of l/h=0.5, 1, and 2, where l is the rod length in the flow direction and h is the rod length perpendicular to the flow. The effect of rounding different edges on the flow-sound interaction mechanism is also considered. Rectangular rods with an aspect ratio of l/h=0.5 showed a significant escalation of the acoustic pressure and the dynamic lift force during acoustic resonance excitation. However, when the aspect ratio of the rod increased to l/h=2, an early excitation of the third acoustic mode is observed accompanied with a significant reduction in the dynamic lift force compared to the values observed before the onset of resonance excitation. It is revealed that the wake structure around the rectangular rod with an aspect ratio of l/h=2 has significant dependence on the edge geometry and this results in different shedding modes. The first shedding mode is the typical vortex shedding with a Strouhal value of Stl≈0.16−0.18 and it occurs for rods with aspect ratios of l/h=2 when either all the edges are sharp or the two upstream edges are sharp. Although the vortex shedding is dominant for this case, a masked shear layer instability along the lateral faces of the rectangular rod is what triggers the early excitation of the third acoustic mode. When either all the edges are rounded or the two upstream edges are rounded, the shear layer instability mode becomes dominant with a Strouhal value of Stl≈0.5−0.52 and the dynamic lift force decreases substantially. This mode classically excites the first and the third acoustic modes at their frequency coincidence and no typical single-street vortex shedding is detected downstream of the rod. The work presented in this paper reveals that rounding the upstream edges of a rectangular rod with an aspect ratio of l/h=2 can switch the shedding pattern making the shear layer instability mode dominant and hence a reduction of the dynamic loading and an attenuation of the acoustic energy is obtained.

Applicability of the Equivalent Diameter Approach to Estimate Vortex Shedding Frequency and Acoustic Resonance Excitation From Different Finned Cylinders in Cross-Flow

Abstract This article explores the applicability of utilizing different equivalent diameter (Deq) equations to estimate the vortex shedding frequency and onset of self-excited acoustic resonance for various types of finned cylinders. The focus is on three finned cylinder types that are commonly used in industrial heat exchangers: straight, twist-serrated, and crimped spirally finned cylinders. Within each fin type, at least three different finned cylinders are investigated. The results indicate that at off-resonance conditions, utilizing the appropriate equivalent diameter collapses the Strouhal number data within the typical Strouhal number variations of an equivalent diameter circular, bare cylinder. However, when acoustic resonance is initiated, the onset and the peak of resonance excitation in all of the finned cylinder cases generally occurred at a reduced flow velocity earlier than that observed from their equivalent diameter bare cylinders. This suggests that although utilizing the appropriate equivalent diameter can reasonably estimate the vortex shedding frequency away from acoustic resonance excitation, it cannot be used to predict the onset of acoustic resonance in finned tubes. The findings of this study indicate that the effective diameter approach is not sufficient to capture the intrinsic changes in the flow-sound interaction mechanism as a result of adding fins to a bare cylinder. Thus, a revision of the acoustic Strouhal number charts is required for finned tubes of different types and arrangements.

Parametric Investigation of the Flow-Sound Interaction Mechanism for Single Cylinders in Cross-Flow

Abstract Flow-excited acoustic resonance is a design concern in many industrial applications. If not treated, it may lead to excessive vibrational loads, which could subsequently result in premature structural failure of critical equipment. For the case of tube bundles in heat exchangers, several acoustic damping criteria were proposed in the literature to predict the occurrence of resonance excitation. However, these criteria, in some cases, are not reliable in differentiating between the resonant and nonresonant cases. A primary reason for that is the geometrical differences between reduced scale models and full-scale tube bundles, and their effect on the flow-sound interaction mechanism. Therefore, the effect of two geometrical aspects, namely, the duct height and the cylinder diameter, on the self-excited acoustic resonance for single cylinders in cross-flow is experimentally investigated in this work. Changing the duct height changes the natural frequency of the excited acoustic modes and the duct's acoustic damping and radiation losses. Changing the cylinder diameter changes the flow velocity at frequency coincidence, the pressure drop, and Reynolds number. It is found that increasing the duct height decreases the acoustic impedance, which makes the system more susceptible to resonance excitation. This, in turn, changes the magnitude of the acoustic pressure at resonance, even for cases where the dynamic head of the flow is kept constant. The acoustic attenuation due to visco-thermal losses is quantified theoretically using Kirchhoff's acoustical damping model, which takes into account the geometrical aspects of the different ducts. Results from the experiments are compared with the acoustic damping criteria from the literature for similar cases. It is revealed that the height of the duct is an important parameter that should be included in damping criteria proposed for tube bundles of heat exchangers, as it controls the acoustic damping and radiation losses of the system, which have been over-looked in the past.

Vortex dynamics and acoustic sources in the wake of finned cylinders during resonance excitation

The flow–sound interaction mechanism and its effect on the vortex dynamics in the wake of circular finned cylinders are experimentally investigated using phase-locked particle image velocimetry at Reynolds numbers between 7 × 104 and 9.5 × 104. In addition, a hybrid experimental–numerical technique using the theory of vortex sound is employed to quantify the acoustic sources and sinks in the vicinity of finned cylinders with different fin-to-root diameter ratios, Df/Dr = 1.5, 2.0, and 2.5. The results show that changing the diameter ratio of the fins induces fundamental changes in the wake structure and the vortex shedding process downstream of the cylinder. Finned cylinders induce stronger vortex cores with a shorter formation length compared to their equivalent bare cylinders. Moreover, the flow topology over the spanwise direction shows that acoustic resonance results in uniform cylindrical vortex cores with less three-dimensional distortion, which demonstrates that the flow field becomes highly two-dimensional during resonance excitation. Quantification of the energy transfer between the flow and the sound fields reveals an enhancement in the acoustic energy production closer to the cylinder with a significant dependence on its fin-to-root diameter ratio.

Wake structures and acoustic resonance excitation of a single finned cylinder in cross-flow

This paper presents an experimental and numerical investigation to clarify the effect of circular fins on the wake structures, the dynamic loading, as well as the flow–sound interaction mechanism downstream of a single finned cylinder in cross-flow. It is revealed that adding circular fins to the cylinder enhances the flow coherence along the cylinder’s span, strengthens the intensity of the vortex shedding process downstream of the cylinder, reduces the vortex formation length, and increases the dynamic lift force acting on the cylinder. Thus, adding circular fins to the cylinder makes the flow more susceptible to acoustic resonance excitation, compared to an equivalent bare cylinder. Three-dimensional numerical simulations of the flow field around the cylinder show that the flow is entrained between the fins which results in the generation of an accelerated jet-like flow in the developing shear layer region. The jet-like flow emanating from the fins leads to a closer and stronger shear layer roll-up, and hence a shorter vortex formation region. This, in turn, increases the fluctuating lift force on the cylinder, as the strong oscillating shear layers occur closer to base of the cylinder. Additionally, with the reduction of the vortex formation length, the base pressure coefficient decreases and therefore the drag force on the cylinder increases. These intrinsic flow features explain the global effect of circular fins on the vortex shedding process and there by on the excitation mechanism of acoustic resonance.

Acoustic methods to suppress self-excited oscillations in spring-loaded valves

Spring-loaded valves have been reported to experience self-excited vibration in reciprocating compressor installations as well as in safety relief devices of pressurized systems. Prior work by the authors has provided an experimental characterization (El Bouzidi et al., 2018a) and a theoretical model (El Bouzidi et al., 2018b) of the flow–sound-interaction mechanism which causes the self-excited vibration. It was found that the vibration occurred only when the fluid–structure interaction at the valve is coupled with acoustic reflections in the associated piping system. In other words, coupling with the pipe acoustics was essential for the initiation of the self-excited valve vibration.This investigation evaluates the effectiveness of three acoustic devices in suppressing the self-excited vibrations. The objective is to dampen the sound waves in the piping system and weaken its coupling with the valve oscillations. A concentric Helmholtz-type cavity resonator, an orifice plate, and an anechoic termination are placed at the downstream side of a model valve which exhibited strong self-excited vibration in previous investigations. The orifice plate was successful in eliminating the oscillation frequency, but its placement at the acoustic pressure node was critical. The concentric cavity resonator dampened the oscillations to very small amplitudes, but a knowledge of the oscillation frequency was necessary. Finally, the anechoic termination was also successful in suppressing the oscillation, but its practicality is rather limited in industrial pipelines.

Near-Wake Characteristics and Acoustic Resonance Excitation of Crimped Spirally Finned Cylinders in Cross-Flow

This paper presents an experimental investigation of the near-wake flow characteristics for isolated crimped spirally finned cylinders in cross-flow and its influence on the generated sound pressure during flow-excited acoustic resonance. Four crimped spirally finned cylinders are investigated, which have pitch-to-root diameter ratio (p/Dr) ranging between 0.384 ≤ p/Dr ≤ 1. A new equivalent diameter equation (Dc) has been developed to better capture the vortex shedding frequency emanating from the crimped spirally finned cylinders. The addition of crimped spiral fins reduces the coherence of the vortex shedding process as compared to that of a bare cylinder. Moreover, the addition of crimped spiral fins causes an elongation in the vortex formation region, as well as induces a larger velocity deficit in the near-wake. Reduction in the pitch-to-diameter ratio (p/Dr) leads to a progressive increase in the strength and coherence of the vortex shedding process. It also results in a gradual reduction in the vortex formation length and velocity deficit. The near-wake flow characteristics of the crimped spirally finned cylinders inherently affect the sound pressure during flow-excited acoustic resonance. Furthermore, the helical fins impose an asymmetrical inclination of the acoustic particle velocity. This hinders the flow-acoustic coupling, leading to a weakened energy transfer between the flow and sound fields. The findings of this investigation provide better understanding of the complex flow-sound interaction mechanism from crimped spirally finned cylinders in heat exchanger tube bundle.

Flow-sound interaction mechanisms during the resonance of trapped diametral modes in shallow cavities

Flow-sound interaction mechanisms during the resonance of trapped diametral modes in shallow cavities

Aeroacoustic Response of a Single Cylinder With Straight Circular Fins in Cross-Flow

The phenomenon of sound generation has been investigated in some detail for the case of bare cylinders; however, the effect of adding fins to the cylinder on the flow–sound interaction mechanism is not yet fully understood. Thus, the aeroacoustic response of a cylinder with straight circular fins in cross-flow is investigated experimentally in this work. During the experiments, the acoustic modes of the duct housing the cylinder are self-excited due to the vortex shedding that emerges from the cylinder's surface. In order to determine the effect of different fin parameters on the onset and intensity of acoustic resonance, 14 different finned cylinders with fin thickness ranging from 0.35 to 1.5 mm and fin density ranging from 4 to 13.7 fin/in. are investigated. It is observed that the finned cylinders experience an earlier acoustic resonance and higher levels of acoustic pressure compared to their equivalent bare cylinders. Moreover, it is observed that, for constant fin spacing, the acoustic pressure amplitude increases and the acoustic resonance occurs at earlier velocities as the fin thickness increases. On the other hand, for constant fin thickness, as the fin spacing increases the amplitude of the acoustic pressure decreases while the onset of the resonance is delayed. Finally, the effect of the cylinder's aspect ratio on the acoustic resonance excitation is presented. It is shown that as the finned cylinders' aspect ratio increases from 4.85 to 11.3, the normalized acoustic pressure during resonance increases drastically. However, for bare cylinders the normalized acoustic pressure during resonance is not highly dependent on the cylinders' aspect ratio. These results indicate that adding fins to the cylinder alters the flow field downstream of the cylinder in a manner that makes it more susceptible to acoustic excitation.

Flow-Sound Interaction Mechanisms in the Wake of Two Side-By-Side Cylinders

The phenomenon of flow-excited acoustic resonance, where periodic flow oscillations are enhanced by a resonant sound field, is a design concern in many engineering applications such as in heat exchangers, piping systems, and cavity flows. This study experimentally examines the phenomenon of flow-excited acoustic resonance for two side-by-side cylinders in a duct with cross-flow. This geometry has been investigated for three cylinder spacing ratios, defined as the center-to-center distance between the cylinders normalized by their diameter, of T/D = 1.25, 1.46 and 2.5, and for a range of acoustic pressure amplitudes. Intermediate and small spacing ratios have been given special attention, as these cases have been found to exhibit bistable flow in the wake in the absence of acoustic resonance. Phase-locked PIV measurements reveal that the self-excited sound field produces a strong oscillatory flow pattern in the cylinder wakes, which is symmetric for large spacing ratios and high acoustic amplitudes, but remains bistable for small spacing ratios, even during very intense acoustic resonances. The aeroacoustic sources and sinks within the flow field, computed using Howe's theory of vortex sound, are compared for the range of spacing ratios and acoustic pressure amplitudes examined in this study.

Aeroacoustic sources generated by flow–sound interaction in a T-junction

This paper experimentally investigates the flow–sound interaction mechanisms in a T-junction combining the flow from its two co-axial side-branches into the central branch. The T-junction has a sudden area expansion at each side-branch entrance. Flow separation at these area expansions forms free shear layers which are shown to excite the acoustic mode(s) of the branches over several ranges of flow velocity, each of which results from the coupling of the acoustic mode with a different shear layer oscillation mode. Phase-locked particle image velocimetry is utilized to detail the unsteady flow field over the acoustic cycle for two shear layer oscillation modes. Finite element analysis is used to characterize the excited acoustic mode shape and its associated particle velocity field. In-depth analysis of the flow–sound interaction mechanism inside the T-junction is performed by means of Howe׳s acoustic analogy. It is concluded that the flow–sound interaction mechanism in the entrance region of the T-junction produces a spatially alternating pattern of acoustic energy generation and absorption. This alternating pattern of energy exchange between the flow and sound fields results in a minimal amount of net acoustic power being generated in the entrance region. However, the increasing orthogonality between the acoustic particle streamlines and the flow streamlines near the exit of the T-junction at its center results in the majority of the generated sound power which sustains the acoustic resonance.

Closure to “Discussion of ‘Flow-Excited Acoustic Resonance: Excitation Mechanism, Design Guidelines and Counter Measures,’” (Ziada, S., and Lafon, P., 2014, Appl. Mech. Rev., 66(1), p. 010802)

Closure to “Discussion of ‘Flow-Excited Acoustic Resonance: Excitation Mechanism, Design Guidelines and Counter Measures,’” (Ziada, S., and Lafon, P., 2014, Appl. Mech. Rev., 66(1), p. 010802)

Flow-Sound Interaction Mechanism of Two Side-by-Side Cylinders in Cross-Flow

A numerical simulation of the flow-excited acoustic resonance for the case of two side-by-side cylinders in cross-flow is performed. One spacing ratio between the cylinders, T/D = 2.5, is investigated, where D is the diameter of the cylinders and T is the center-tocenter distance between them. The unsteady flow field is simulated using a finite-volume method at a Reynolds number of 30000. This simulation is then coupled with a finite element simulation of the resonant sound field, by means of Howe's theory of aerodynamics sound, to reveal the details of flow-sound interaction mechanisms, including the nature and the locations of the aeroacoustic sources in the flow field. At the off resonance conditions the vortex shedding from the cylinders occurs at a single frequency with a Strouhal number of 0.217. Moreover, a symmetric wake structure downstream of the two cylinders is observed. However, when acoustic resonance is initiated the symmetric wake structure downstream of the cylinders is eliminated and the vortex shedding from both cylinders becomes antisymmetric. Moreover, two main aeroacoustic sources located just downstream of each cylinder are observed. The numerical results are compared with the experimental results presented in a previous investigation and favourable agreement is obtained.

Self-Excited Acoustic Resonance of Isolated Cylinders in Cross-Flow

Self-excited acoustic resonance is a design concern in many engineering applications such as tube bundles of heat exchangers and boilers. Since this phenomenon is not yet fully understood, it can be dangerously unpredictable. Due to the complexity of the flow-sound interaction mechanisms in tube bundles, the simplified cases of a single cylinder and two cylinders in various arrangements, tandem and staggered, are investigated in some detail. A summary of these investigations is presented in the current paper. It is found that the aeroacoustic response of two-tandem and side-by-side cylinders in cross-flow can be considerably different from that of a single cylinder under similar flow conditions. Moreover, for the case of two tandem cylinders, the acoustic resonance is excited over two different ranges of flow velocity; the pre-coincidence and the coincidence resonance ranges. The pre-coincidence acoustic resonance phenomenon is found to be similar to the acoustic resonance mechanism of in-line tube bundles.

Vortex shedding and acoustic resonance of single and tandem finned cylinders

The effect of fins on vortex shedding and acoustic resonance is investigated for isolated and two tandem cylinders exposed to cross-flow in a rectangular duct. Three spacing ratios between the tandem cylinders ( S/D e=1.5, 2 and 3) are tested for a Reynolds number range from 1.6×10 4 to 1.1×10 5. Measurements of sound pressure as well as mean and fluctuating velocities are performed for bare and finned cylinders with three different fin densities. The effect of fins on the sound pressure generated before the onset of acoustic resonance as well as during the pre-coincidence and coincidence resonance is found to be rather complex and depends on the spacing ratio between cylinders, the fin density and the nature of the flow–sound interaction mechanism. For isolated cylinders, the fins reduce the strength of vortex shedding only slightly, but strongly attenuate the radiated sound before and during the occurrence of acoustic resonance. This suggests that the influence of the fins on correlation length is stronger than on velocity fluctuations. In contrast to isolated cylinders, the fins in the tandem cylinder case enhance the vortex shedding process at off-resonant conditions, except for the large spacing case which exhibits a reversed effect at high Reynolds numbers. Regarding the acoustic resonance of the tandem cylinders, the fins promote the onset of the coincidence resonance, but increasing the fin density drastically weakens the intensity of this resonance. The fins are also found to suppress the pre-coincidence resonance for the tandem cylinders with small spacing ratios ( S/D e=1.5, 2 and 2), but for the largest spacing case ( S/D e=3), they are found to have minor effects on the sound pressure and the lock-in range of the pre-coincidence resonance.

Azimuthal behaviour of flow-excited diametral modes of internal shallow cavities

The aerodynamic excitation of ducted cavity diametral modes gives rise to complex flow–sound interaction mechanisms, in which the axisymmetric free shear layer interacts with the asymmetric acoustic modes. This results in various azimuthal patterns and behaviours depending on different flow and geometrical parameters. The azimuthal behaviour of this self-excitation mechanism is investigated experimentally. Axisymmetric shallow cavities in a duct have been tested over the range of cavity length to depth ratio from 1 to 6 and at Mach numbers up to 0.4. A set of pressure transducers flush mounted to the cavity floor is used to determine the acoustic mode amplitude and orientation. The excited acoustic modes are classified into spinning, partially spinning, and stationary diametral modes. An analytical representation based on the duct acoustics theory is used to analyse the measurements and provides a physical explanation of the observed behaviour of the diametral modes. Splitter plates are installed inside the cavity to form a geometrical preference. The acoustic response of this geometrically altered case show that pressure oscillations at different azimuthal angles along the cavity circumference can be uncorrelated, or even oscillate at different frequencies, while the diametral modes are still strongly excited. Two hot-wire probes are also used in a separate set of measurements to investigate the azimuthal behaviour of the shear layer oscillation. The results show that the shear layer oscillation has the same azimuthal distribution as that of the excited acoustic modes, indicating that the shear layer oscillation at different azimuthal angles can be not only uncorrelated but also occur at different frequencies.

Numerical Simulation of the Flow-Sound Interaction Mechanisms of a Single and Two-Tandem Cylinders in Cross-Flow

A numerical simulation of the flow-excited acoustic resonance for the case of two-tandem cylinders in cross-flow is performed. The spacing ratio between the cylinders (L/D=2.5) is inside the proximity interference region. Similar simulation is performed for the case of a single cylinder. The unsteady flow field is simulated using a finite-volume method. This simulation is then coupled with a finite-element simulation of the resonant sound field, by means of Howe’s theory of aerodynamics sound, to reveal the details of flow-sound interaction mechanisms, including the nature and the locations of the aeroacoustic sources in the flow field. For the case of a single cylinder, acoustic resonance is excited over a single range of flow velocity. The main aeroacoustic source, which causes a positive energy transfer from the flow field to the acoustic field, is found to be located just downstream of the cylinder. For the case of two-tandem cylinders, the acoustic resonance is excited over two different ranges of flow velocity: the precoincidence and the coincidence resonance ranges. For the coincidence resonance range, the main aeroacoustic source is found to be located just downstream of the downstream cylinder, and the excitation mechanism of this resonance range is found to be similar to that of a single cylinder. However, for the precoincidence resonance range, the primary acoustic source is found to be located in the gap between the cylinders. Moreover, flow visualization of the wake structure for the two-tandem cylinders during acoustic resonance shows that for the precoincidence resonance range there is a phase shift of about 90 deg between the vortex shedding from the upstream and the downstream cylinders, which is different from the coincidence resonance range, where the vortex shedding from both cylinders seems to be in-phase.