Hiromitsu Hamakawa

Effect of Fins on Vortex Shedding Noise Generated From a Circular Cylinder in Cross Flow

In the present paper, the effect of twist-serrated fins around a bare tube on the Aeolian tone was experimentally investigated. These fins were mounted spirally around a bare tube and had the same geometry as those actually used in boiler tubes. We measured the intensity of velocity fluctuation, spectrum of velocity fluctuation, coherence of Karman vortex in the spanwise direction, dynamic lift force, and sound pressure level of the aerodynamic noise generated from finned tubes with various fin pitches. An Aeolian tone induced by Karman vortex shedding was observed in the case of a finned tube, although the complicated fin was mounted around a bare tube. A decrease in the pitch of the fin effectively caused an increase in the equivalent diameter, which acted as the characteristic length of a cylinder with fins. The equivalent diameter depended on the Reynolds number. We modified a relation to calculate the characteristic diameter of the finned tube, which in turn was used to calculate the Strouhal number. The coherent scales in the spanwise direction for the cases with various fin pitches were slightly larger than that of a simple circular cylinder. It is known that the sound pressure level of the Aeolian tone depends on the coherent scale of the Karman vortex in the spanwise direction. However, when the pitch of the fins decreased, the peak level of the sound pressure spectrum decreased. A correlation analysis between the flow field and Aeolian tone was carried out.Copyright

Experimental Study on Acoustic Energy Absorbers to Suppress Acoustic Resonance in Tube Bundles of Boiler Plants

The present paper deals with acoustic resonance in tube bundles which occurred in actual boiler plants. A practical method to suppress the resonant noise is proposed based on scale model tests. In those experiments, besides the transverse modes which are relatively well-known, longitudinal modes which are known to be difficult to suppress, were realized. The relation of both kinds of acoustic modes to vortex shedding frequency was clarified. In the following step, a design method of the acoustic energy absorbers is proposed based on the feedback system stability theory. According to this theory, acoustic energy absorbers were equipped in the duct of the test model, and remarkable increase of the critical flow velocity was confirmed for both types of resonant mode. Based on these results, this method was applied to the actual boiler plants and it was confirmed that the proposed method is the effective countermeasure for both types of acoustic resonant modes.Copyright

Effect of Flow Separation on Acoustic Resonance in In-line Tube Banks

In the present paper, the attention was focused on the effect of flow separation, which occurred at upstream of tube banks, on acoustic resonance in In-line tube banks. The flow separation was generated by using the orifice at the upstream of tube banks. We measured the acoustic pressure on the surface of side walls, spectrum, correlation and phase delay of acoustic pressure in the spanwise direction. The height of passage to diameter ratio at the orifice were 0, 2.8, 5.7, 8.6, 11.6, 14.4. When the acoustic resonance of first, second, and third mode occurred, the peak SPLs increased. As the gap velocity increased overall, the acoustic mode number and peak SPLs increased. As height of passage decreased, the peak SPLs decreased and the onset velocity of each mode of acoustic resonance increased. We have also discussed the prediction method to estimate the onset velocity of acoustic resonance with flow separation at upstream of tube banks.

Dynamics of an aspherical bubble oscillating near a rigid sphere

Behavior of a non-spherical bubble oscillating near a rigid sphere was investigated in the framework of the Lagrangian formalism and multipole expansion of the bubble boundary. In this study, shape oscillations of the bubble are taken into account up to the third oscillation mode (octupole mode) to illustrate the liquid jet formation on the bubble surface. To account for interaction between the bubble and the rigid sphere, corrections of the velocity potential in a liquid containing the bubble and the sphere will be considered up to terms of fifth order in the inverse separation distance. Derived equations describes typical bubble behavior such as volume oscillations, translation, and shape oscillations. This paper presents the motion of the bubble in the vicinity of the rigid sphere by using numerical computations of the equations. In particular, it is discussed that the dependencies of bubble behavior on the density and radius of the sphere.

Characteristics of Aerodynamic Sound Radiated From Two Finned Cylinders

In the present paper the attention is focused on the characteristics of aerodynamic sound radiated from two finned cylinders with tandem and staggered arrangement exposed to cross-flow. We measured the spectrum of SPL and flow velocity for the cylinder spacing ratios ranged from 0 to 1.05 in the transverse direction and the ratios from 1.24 to 6.8 in the flow direction at Reynolds number of 1.0×105−1.9×105. As a result, we found that the peak SPL and Strouhal number of vortex shedding for two finned cylinders depend on the cylinder spacing ratios as well as those for bare cylinders. The peak SPL of the spectrum varied complexly with the tube spacing ratio. The peak levels of SPL for tandem finned cylinders were approximately 8 dB lower than that for the tandem bare cylinders. At the cylinder spacing ratio of 1.24 in the flow direction, the peak SPL for two finned cylinders at the cylinder spacing ratio of 0.72 in the transverse direction was about 8 dB larger than that for tandem finned cylinders. The peak SPL depended on the spanwise correlation length of the Karman vortex formed in the near wake of the downstream of two finned cylinders.Copyright

Study on Stability Analysis on Acoustic Resonance in Heat Exchanger Tube Bundles

Acoustic resonance may occur in heat exchangers such as gas heaters or boilers which contain tube bundles. This resonance is classified in self-excited oscillation, and feedback effect between vortex shedding and sound field plays important role. The final goal of our study is to develop a method by which to predict the resonance attack critical gas flow velocity and maximum resonance amplitude at the design stage. In order to reach this goal, it is essential to formulate the feedback effect between vortex shedding and a resonance mode concerned, and to execute a stability analysis of the resonance mode. There are two mechanisms in the feedback process: as the acoustic resonance grows, vortex strength is increased and vortex shedding synchronization grows.This paper is concerned with the proposal of phenomenological model suitable to explain this mechanism and the formulation of these kinds of feedback mechanism with the use of this model. The model adopts vortex shedding wake oscillator model which is effective for tube vibration problems. Tube vibration movement is replaced by acoustic particle movement. Another improvement of our study is the introduction of statistical modeling of the wake oscillator to express vortex shedding synchronization effect. Here, the randomness of vortex shedding is explicitly modeled by a probability density function of the phase of the oscillator, and this function depends on the level of acoustic resonance. Based on these ideas to express the vortex/acoustics interaction, the formulas of stability analysis were derived.Copyright

Synchronization Characteristics of Vortex Shedding From Tube Banks on Acoustic Resonance

In the present paper the attention is focused on vortex shedding synchronization on acoustic resonance in in-line tube banks which occurred in the two-dimensional model of boiler. And we have examined the verification of proposed modeling method. We measured the characteristics of acoustic resonance, acoustic damping, the pressure fluctuation on the surface of tubes at the nodes of acoustic pressure and the acoustic pressure fluctuation on the side wall of the duct. As the acoustic mode number increased, the acoustic damping ratio decreased. As the tube pitch ratio in the flow direction decreased, the acoustic damping increased for all acoustic modes and the vortex shedding frequency became broad-band. The multiple resonance modes of lower acoustic damping were generated within the broad-band vortex shedding frequency. If the acoustic resonance occurred, the peak level of spectrum of surface pressure fluctuation and the coherence between vortex shedding and wall acoustic pressure in the tube banks also increased. The features of experimental results agree well with those obtained by using the proposed modeling method. We have discussed the characteristics of vortex shedding synchronization by using proposed the modeling method.Copyright

Effect of Helical Strakes Around a Finned Tube on Aeolian Tone

In the present paper, the characteristics of vortex shedding and Aeolian tone radiated from a finned tube with helical strakes were experimentally investigated. The helical strakes are mounted spirally around a serrated finned tube surface. We measured the turbulence intensity in the wake, velocity fluctuation spectrum, the coherence of velocity fluctuations in the spanwise direction and SPL spectrum. The Aeolian tone radiated from the finned tube with helical strakes at high Reynolds number was smaller than the case of no helical strakes. The helical stakes were effective to reduce the Aeolian tone radiated from the finned tube. And the existence of helical strakes of large diameter caused the decrease of the periodicity and the turbulence intensity in the wake of the finned tube. The coherent scale of Karman vortex in the spanwise direction is smaller than that of a finned tube without helical strakes. However, the effect of helical strakes of small diameter on vortex shedding depended on the Reynolds number. The Karman vortex was clearly formed even in the case of helical strakes of small diameter at low Reynolds number. The existence of helical strakes of small diameter around a finned tube caused the increase of the periodicity of vortex shedding from a finned tube. The coherent scale of Karman vortex in the spanwise direction was larger than that of a finned tube without helical strakes.Copyright

Study on Modeling Method of Vortex Shedding Synchronization in Heat Exchanger Tube Bundles II: Experimental Verification

Acoustic resonance may occur in heat exchangers such as gas heaters or boilers which contain tube bundles. This resonance is classified in self-excited oscillation, and feedback effect in vortex shedding and sound field plays important role. The purpose of this study is to develop a modeling method of the resonance level dependence of vortex shedding synchronization because this is the most essential part of critical flow velocity prediction. The level of synchronization is expressed by a coherence function between vortex shedding in any two locations in the tube bundle. Here, we introduce the wake oscillator model of vortex shedding, and based on this model, a simple method to estimate the resonance level dependence of the coherence function is proposed. In this method, the relationship of vortex shedding and the sound field in an arbitrary tube is expressed by a statistical model where the effect of resonance on the wake-oscillator is expressed by the width of the fluctuation range of phase between wake-oscillator and acoustic particle velocity. From this model, the resonance level dependence of the coherence function is derived in simple form. This method gives the result that when the resonance level increases, the synchronization level in the tube bundles also increases, which seems to be a reasonable conclusion. The results of experimental verification showed the validity of the proposed modeling method.Copyright

Study on Modeling Method of Vortex Shedding Synchronization in Heat Exchanger Tube Bundles

Acoustic resonance may occur in heat exchangers such as gas heaters or boilers which contain tube bundles. The purpose of this study is to develop modeling method of vortex shedding synchronization because this is the most essential part of critical flow velocity prediction. Here, acoustic resonance level dependence of spatial correlation of vortex shedding is expressed by coherence function between wake-oscillator behaviors in any two locations in the cavity. The feedback effect in synchronization of vortex shedding is represented by resonant level dependence of the wake-oscillator phase fluctuation range. This method gives the result that when resonance level increases, synchronization level in the tube bundles also increases, which seems to be a reasonable conclusion. Experimental method to identify the undefined parameters in the proposed method is also mentioned.Copyright