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

2019 ◽  
Vol 86 ◽  
pp. 70-93 ◽  
Author(s):  
Nadim Arafa ◽  
Atef Mohany
Keyword(s):  
Cross Flow ◽  
Acoustic Resonance ◽  
Resonance Excitation

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

2020 ◽  
Vol 143 (2) ◽  
Author(s):  
Omar Afifi ◽  
Atef Mohany
Keyword(s):  
Heat Exchangers ◽  
Interaction Mechanism ◽  
Cross Flow ◽  
Acoustic Resonance ◽  
Industrial Applications ◽  
Resonance Excitation ◽  
Acoustic Damping ◽  
Radiation Losses ◽  
Cylinder Diameter ◽  
Tube Bundles

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.

Characteristics of Acoustic Resonance Excitation by Flow Around Inline Cylinders

2019 ◽  
Vol 141 (5) ◽  
Author(s):  
Mahmoud Shaaban ◽  
Atef Mohany
Keyword(s):  
Acoustic Pressure ◽  
Tube Bundle ◽  
Cross Flow ◽  
Acoustic Resonance ◽  
Environmental Risks ◽  
Critical Parameters ◽  
Resonance Excitation ◽  
Tube Bundles ◽  
Flow Oscillations

Excitation of acoustic resonance by flow over tube bundles in heat exchangers can cause hazardous levels of acoustic pressure that may pose operational and environmental risks. The previous studies have indicated that inline arrangements of cylinders excite acoustic resonance of a nature different from that of a single cylinder. In this work, the excitation of acoustic resonance by cross-flow around inline arrangements of cylinders is experimentally investigated to identify the role of critical parameters on resonance characteristics. Results show that flow around inline tube bundles can excite acoustic resonance due to periodic flow oscillations over the cavity formed between successive cylinders rather than periodic wake phenomena. Based on precoincidence resonance characteristics, a criterion is introduced to predict the occurrence of acoustic resonance in inline arrangements of cylinders. The proposed parametric criterion does not only identify the potential for resonance excitation for inline arrangements of cylinders experimentally investigated in this work but it also provides a method to separate resonant from nonresonant cases for inline tube bundle data from the literature.

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

2015 ◽  
Vol 137 (5) ◽  
Author(s):  
Nadim Arafa ◽  
Atef Mohany
Keyword(s):  
Aspect Ratio ◽  
Acoustic Pressure ◽  
Interaction Mechanism ◽  
Cross Flow ◽  
Acoustic Resonance ◽  
Resonance Excitation ◽  
Acoustic Excitation ◽  
Pressure Amplitude ◽  
Fin Thickness ◽  
Fin Spacing

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.

Vorticity Shedding and Acoustic Resonance Excitation of Two Tandem Spirally Finned Cylinders in Cross-Flow

2020 ◽  
Vol 143 (2) ◽  
Author(s):  
Mohammed Alziadeh ◽  
Atef Mohany
Keyword(s):  
Reynolds Number ◽  
Strouhal Number ◽  
Flow Characteristics ◽  
Cross Flow ◽  
Acoustic Resonance ◽  
Root Diameter ◽  
Circular Cylinders ◽  
Resonance Excitation ◽  
Equivalent Diameter ◽  
Spacing Ratio

Abstract The aeroacoustic response of two tandem spirally finned cylinders is experimentally investigated. Three different pairs of finned cylinders are studied with fin pitch-to-root diameter ratios (p/Dr) ranging between 0.37≤p/Dr≤0.74. The spiral fins are crimped similar to those used in industrial heat exchangers. The results of the finned cylinders are compared with bare, circular cylinders with a modified equivalent diameter (Deq). The spacing ratio (L/Deq) between the cylinders are kept constant at L/Deq=2.00. The Strouhal number (StDeq) of the tandem finned cylinders is found to be higher compared to the tandem bare cylinders, resulting in an earlier onset of coincidence resonance. Moreover, unlike the tandem bare cylinders, the Strouhal number of the finned cylinders did not depend on the Reynolds number, suggesting that the flow characteristics around the finned cylinders are unaffected by Reynolds number. Only the tandem finned cylinders with the lowest fin pitch-to-root diameter ratio (p/Dr=0.37) were capable of exciting precoincidence acoustic resonance. The precoincidence resonance mechanism is similar to that observed in in-line tube bundles.

Effect of Flow Approach Angle on Acoustic Resonance Excitation of In-Line Tube Bundles in Cross-Flow

2019 ◽  
Author(s):  
Mohammed Alziadeh ◽  
Atef Mohany ◽  
Marwan Hassan
Keyword(s):  
Tube Bundle ◽  
Flow Direction ◽  
Cross Flow ◽  
Acoustic Resonance ◽  
Resonant Mode ◽  
Acoustic Energy ◽  
Resonance Excitation ◽  
Approach Angle ◽  
Tube Bundles ◽  
Lock In

Abstract This paper presents preliminary experimental results on the influence of the flow approach angle on the acoustic resonance excitation of in-line tube bundles in cross-flow. The pitch-to-diameter ratio (P/D) of the in-line tube bundles investigated is P/D = 1.733. The flow approach angle was investigated by physically rotating the tube bundle clockwise relative to the flow direction. The tube bundles are capable of rotating with increments of 5° up to an angle of 45°. For brevity, only the results for the 0° and 30° orientation will be presented herein. For the 0° orientation, two Strouhal frequencies (St1 = 0.437 and St2 = 0.252) were observed. However, only one of these frequencies (St1 = 0.437) was capable of exciting resonance. During resonance, a peak sound pressure level (SPL) of 170 dB was achieved. The Strouhal frequencies and peak SPL agrees well with what has been presented in the literature. For the 30° orientation, only one Strouhal frequency (St1 = 0.98) was measured. At this orientation, the lock-in phenomenon occurred at a much lower flow velocity compared to the 0° orientation with a peak SPL reaching 153 dB. Jumps in the lock-in frequency were observed at the 30° orientation. This phenomenon is associated with two reasons. The first reason is a partial lock-in with an acoustic resonant mode, due to the acoustic energy not being fully trapped within the tube bundle. The second reason is related to the changes in the apparent speed of sound resulting in variations in the acoustic cross-mode frequency depending on where the excitation source is emanating from within the tube bundle. A brief summary of the results is presented in this paper.

Equivalent Diameter for Predicting Vortex Shedding of Finned Cylinders in Cross-Flow

2021 ◽  
Author(s):  
Mohammed Alziadeh ◽  
Atef Mohany
Keyword(s):  
Vortex Shedding ◽  
Tube Bundle ◽  
Cross Flow ◽  
Acoustic Resonance ◽  
Finned Tube ◽  
Resonance Excitation ◽  
Equivalent Diameter ◽  
Frequency Scaling ◽  
Vortex Shedding Frequency ◽  
Shedding Frequency

Abstract This article explores different equivalent diameter equations found in the literature for shedding frequency scaling and applying it to various types of finned cylinders in industrial heat exchangers. The focus is on three finned cylinder types: straight, twist-serrated, and crimped spirally finned cylinders. Within each finned cylinder category, at least three different finned cylinders are investigated. The results indicate that utilizing the appropriate equivalent diameter for vortex shedding frequency scaling collapses the data within the Strouhal bounds of a bare cylinder away from resonance excitation. However, the onset of flow-excited acoustic resonance and peak acoustic pressure in all the finned cylinder cases occur at a reduced flow velocity earlier than their equivalent diameter bare cylinder. This suggests that although utilizing the appropriate equivalent diameter can predict the shedding frequency away from resonance, it cannot be used in velocity scaling to predict the onset of acoustic resonance in finned tube bundle.

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

2021 ◽  
Author(s):  
Mohammed Alziadeh ◽  
Atef Mohany
Keyword(s):  
Strouhal Number ◽  
Vortex Shedding ◽  
Interaction Mechanism ◽  
Acoustic Resonance ◽  
Resonance Excitation ◽  
Equivalent Diameter ◽  
Effective Diameter ◽  
Finned Tubes ◽  
Vortex Shedding Frequency ◽  
Shedding Frequency

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 type of fins, 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.

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