Browsing by Author "Mohany, Atef"

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The aeroacoustics response and shear layer dynamics of confined cavities subject to low Mach number turbulent flow
(2023-04-01) Hanna, Marc; Mohany, Atef
Cavities exposed to low Mach number flow in various engineering applications are often liable for generating flow-excited acoustic oscillations, resulting in large acoustic amplitudes and vibrations. This compromises the safety and reliability of critical equipment due to a phenomenon attributed to interaction between the instability of the shear layer and the acoustic modes of a given system. This thesis experimentally investigates the aeroacoustics response of cylindrical cavities having aspect ratios of h/L = 0.5, 1, and 1.5, where h is the cavity depth and L is the shear layer impingement length, up to flow velocities of Mach 0.4. In view of the cavity confinement, the effects of the admission ratio w/W, where w is the cavity width and W is the duct width, on the aeroacoustics response and shear layer dynamics are also considered. The work extends the investigation to two-dimensional rectangular cavities and square cavities with similar aspect and admission ratios to the cylindrical cavities, as to establish the effect of the cavity shape on the resonance excitation frequencies and hydrodynamic modes of the system. Acoustic pressure measurements present Strouhal periodicities that agree well with values reported in literature. Cylindrical and square cavities with aspect ratio h/L = 0.5, however, exhibit unique behaviour due to the interference of the recirculation region within the cavity, ultimately modifying the symmetry of the shear layer. Particle image velocimetry (PIV) measurements present spatial characteristics of the shear layer dynamics, revealing improved flow modulation with increasing acoustic pressure, and significant asymmetry for shallow aspect ratios. The work presented in this thesis provides novel insight of the shear layer instability in confined cavities, and its effect on the flow-sound interaction mechanism.
Control of resonant excitation in piping systems
(9th International Symposium on Fluid-Structure Interactions, Flow-Sound Interactions, Flow-Induced Vibration & Noise, 2018) Lato, Thomas; Mohany, Atef
Acoustic resonance is a phenomenon which is known to have severe repercussions in a variety of industrial systems. Acoustic resonance can cause high levels of vibrations leading to damage or premature failure of critical components. Although acoustic resonance affects a broad spectrum of industrial equipment, piping systems will be of focus in this work. Both passive and active damping techniques were previously investigated. However, there is a need to investigate the practicality of such devices when implemented in industrial systems. Herschel-Quincke (HQ) tubes have been selected for experimental study throughout this work. The experimental setup consists of an open-air loop pipeline system which is capable of exciting a standing wave with a fundamental frequency of 30 Hz and a target dominant fifth mode of 150 Hz. Transmission loss measurements were performed by means of the two source-location method. Insertion loss measurements were performed with a straight pipe used as the baseline. The current work has shown that Herschel-Quincke devices have potential for practical implementation into resonant piping systems in industry.
Damping acoustic pressure pulsations in pipelines using Helmholtz resonators
(2019-05-01) Sachedina, Karim; Mohany, Atef
In industrial piping systems, centrifugal and reciprocating turbomachinery generate acoustic pressure pulsations, which propagate into the pipeline and interact with piping components, potentially causing vibrations, increased fretting wear, and even fatigue failure. In this thesis, an acoustic damping device known as the Helmholtz resonator (HR) is experimentally studied. The effects of HR cavity volume, pipeline diameter, HR location, the use of multiple HRs, and mean flow velocity are investigated to determine their effects on the acoustic attenuation achieved within a pipeline. Measurements are also performed to clarify the mechanism of attenuation and the effects of incident pressure amplitude on the transmission loss of an HR. The findings of this thesis may be used as practical guidelines for the use of HRs in industrial systems, where characterizing the acoustics is usually difficult and costly, and the available space for damping devices may be limited.
Development of a semi-autonomous directional and spectroscopic radiation detection mobile platform
(2014-03-01) Miller, Alexander Luke; Machrafi, Rachid; Mohany, Atef
This thesis presents a method for a small, inexpensive mobile robot equipped with a single high resolution scintillation detector to quickly survey an area and convey information about local sources of gamma radiation to a remote human operator. This is achieved by surrounding the detector with a lead sheath that blocks all gamma rays except those incident along the detector’s axial direction. A horizontal scan is performed by rotating the detector and a directional profile of gamma radiation is constructed. In addition a visual panorama of the local area is assembled using a camera mounted on the detector. A plot of the detector signal versus angle is then overlaid on top of the visual panorama and visible peaks clearly indicate the direction of local gamma radiation sources. Moreover, measuring the energy spectrum of gamma rays in each direction produces a 2D count frequency histogram where distinct peaks indicate the energy and direction of local gamma ray sources allowing the identification of different radio-isotopes.
Development of multi-physics capabilities in coupling computational fluid dynamics and thermodynamics for molten salt reactor applications
(2024-03-01) Scuro, Nikolas Lymberis; Mohany, Atef; Piro, Markus H.A.
Existing knowledge gaps in Molten Salt Reactors (MSR), such as understanding fuel salt chemistry and fission product retention, require rigorous investigations to support reactor safety. The objective of this work was to develop a novel computational toolset tailored for multi-physics simulations of MSR studies that could fill the aforementioned knowledge gaps. This toolset studied the intricate dependence between thermal-hydraulics and salt chemistry by coupling the computational fluid dynamics code OpenFOAM with the computational thermodynamics code Thermochimica . This coupling facilitates the simulation of scenarios relevant to MSR reactor safety, which is especially important given that MSRs have a low technical readiness level relative to other reactor technologies. Two demonstration problems exemplify the applications and outcomes of this project, which was in support of the SAMOSAFER Co-ordinated Research Project of the European Commission. The initial problem revolves around the first step of the molten salt clean-up fluorination process, which vaporizes molten fuel components (i.e., UF4) into its volatile form (i.e., UF6). Simulations revealed that the fluorination time is strongly dependent on the molten salt system, salt viscosity, and temperature. Compared with experimental results, the simulations displayed a strong correlation in vaporization rates under steady-state conditions, giving credence to the validity of the local equilibrium hypothesis. The second demonstration problem centred on the molten salt fast reactor. Here, normal operating conditions were examined, focusing on fission product retention and release, such as Cs, La, Xe in promising molten fluoride systems (e.g., LiF – ThF4 – UF4 77.5-20-2.5 mol%). Simulations demonstrated that most fission products are retained by the salt and the evaporation rates of several compounds, such as, LiF, ThF4, UF4, CsF, Cs2, Cs2F2, LaF3, F, F2 posed to be almost insignificant when compared to other known volatile/gaseous fission products, such as xenon. This investigation provided an understanding of how fast the UF4/UF3 molar ratio reaches optimal design limits, which plays a pivotal role in controlling corrosion. In conclusion, the outcomes underscore advancements in computational capabilities, promising to elucidate further the intricacies of designing and testing multifaceted scenarios in the realm of MSRs.
Development of polymer based nanocomposite using electrospinning for sound absorption and isolation
(2017-01-01) Elkasaby, Mohamed Ali; Mohany, Atef; Rizvi, Ghaus
Excessive noise has adverse effects on human health. Therefore, it is necessary to find innovative methods for decreasing the exposure to noise in the environment. Various materials are used for dampening noise. In order to obtain promising results for sound absorption, these materials should have a large volume. Nanofibers, derived from polymeric materials, offer enormous benefits, and therefore, there is an intense focus on how this technology can be implemented to improve the performance of different applications. Nanofibers have a high surface area to volume ratio as well as a high porosity. In addition, fillers like carbon nanotubes (CNTs), wollastonite (WS), graphene (GN) and fiberglass (FG) show significant results for improving the mechanical and other properties. Therefore, in this work, polymeric nanofibers with and without fillers, have been investigated as sound absorbing materials. The polymers used in this study were polyvinyl alcohol (PVA), polyvinyl chloride (PVC), and polystyrene (PS), which were processed using electrospinning to produce nanofiber mats and tested for the sound absorption properties. To optimize fiber diameters, the process parameters were studied including the solution concentration, the solution flow rate, the high voltage used for generating an electric field, the distance over which the fibers were collected, and the speed of the rotating drum on which the fibers were collected. Statistical analysis and multiple regression techniques were applied to investigate the effects of the process parameter on the fiber diameters. Solution concentration and flow rate were determined to have the most significant effects on the fiber diameters. The study resulted in the development of a predictive model, which can be used to determine the parameter values required to produce nanofibers with a specified average diameter in the range of interest. Various fillers were added to the polymer matrix in order to enhance its mechanical and sound absorption properties. The fillers used were CNTs, WS, GN, and FG. Single and multi-layered mats, with different fiber diameters, were produced to investigate the sound absorbing properties. Also, a mixture solution of two different polymers was electrospun to obtain a mat that consists of two polymers. The results show that the nanofiber mats exhibit good sound absorption in the mid and low frequency ranges. As the fiber diameter decreases, the sound absorption increases. The addition of fillers to the nanofibers increases the sound absorption and improves the mechanical properties. Multi-layer mats produced from different types of polymer show a good sound absorption. The sound absorption improved using mats with graded fiber diameters structure. Increasing the mats thicknes enhances the sound absorption as well. Finally adding nanofibers mats to conventional sound absorbing materials improve the sound absorption in the low frequency range by 32%. This study presents a promising road map for using electrospun polymer materials, with and without different types of fillers, for sound absorption in the mid and low frequency ranges. These materials will be particularly cost-effective in applications where saving space or volume is a major consideration, such as in the aerospace industry or in electronic devices.
Development of thermoplastic foams with heterogeneous cellular structures for acoustic applications
(2016-08-01) Naeem, Ibrahim Samir Saad; Mohany, Atef; Rizvi, Ghaus
Manufacture of heterogeneous thermoplastic foams is of great interest to damp low frequency noise in transportation and building industries. Most of the fabrication methods reported either use cumbersome batch processes or produce foams that are not appropriate for sound absorption. In this context, this work aimed at manufacturing of thermoplastic acoustic foams with double porosity and graded porosity by processes that can potentially be scaled up for mass production Foams containing a combination of small cells and large cells were developed by foaming of a low melt strength polymer in presence of sodium bicarbonate and wollastonite. The same results were obtained by foaming a blend of a base polymer and a low melt strength polymer. Thermoplastic foams with graded porosity were fabricated by molding the samples under a temperature gradient before expanding them in a heating oven. For all these foams, sound absorption coefficient increased by about three times.
Effect of diameter ratios on the flow-sound interaction of the straight circular finned cylinders
(2020-01-01) Islam, Md Rashidul; Mohany, Atef
Finned tubes that are widely used in heat exchangers could be susceptible to the flow-excited acoustic resonance that generates intense pressure fluctuations, which may lead to premature fatigue failure of the tubes. This thesis explores the unsteady flow development around circular finned tubes and its role in the excitation of acoustic resonance. Finned cylinders with same effective diameter (De f f ), fin pitch (p), fin thickness (t) and a range of diameter ratios between 1.25 _ Df =Dr _ 2.5 are considered. Results from the PIV measurements at Re = 20000 demonstrated that the extent of the recirculation region decreases downstream of the finned cylinders when the diameter ratio increases due to higher flow entrainment between the fins. The frequency spectra of the streamwise velocity fluctuations showed that the temporal evolution of the vortex shedding in the wake becomes more periodic when the diameter ratio of the finned cylinders (Df =Dr) increases. Moreover, the strength of the vortices increases significantly in the wake. The increase in the flow entrainment in between the fins leads to a pronounced reduction in the random velocity fluctuations in the near wake of the cylinders. The three-dimensional flow development and structural loading on the finned cylinders are studied using Large Eddy Simulation. In addition to the flow entrainment mechanism in between the fins, another entrainment mechanism in the near wake is found to be caused by the downwash flow induced by an array of edge vortices that generate at the tip of the fins. The RMS of the fluctuating lift coefficient increases significantly on the surface of the finned cylinders due to a reduction in the vortex formation length and enhanced vortex strength. Moreover, the increase in skin friction on the surface of the cylinders, as a result of the higher flow entrainment, causes a significant increase in the drag coefficient for finned cylinders with larger diameter ratios (Df =Dr ). The results from the aeroacoustic response show that the excitation of acoustic resonance in the case of finned cylinders occurs at lower flow velocities compared to that of bare cylinders. This is mainly due to the existence of more energetic vortex shedding in the wake of the finned cylinders. Moreover, the normalized acoustic pressure generated during resonance excitation increases significantly as a result of an enhanced vortex strength and a reduced vortex formation length. Phase-locked particle image velocimetry (PIV) measurements during resonance excitation reveal that the global effect of the fins on the vortex shedding affects the location of the acoustic sources and sinks in the flow field as well as the energy transfer mechanism between the flow and the sound fields.
Effect of self-excited acoustic perturbations on flow characteristics around rectangular cylinders with varied geometrical aspects
(2023-08-01) Shoukry, Ahmed; Mohany, Atef
This study explores the effects of aspect ratio, edge geometry and incidence angles on the dynamic lift force, wake behaviors, and the susceptibility of such geometrical aspects to self-excited acoustic resonance in rectangular cylinders inside a high-speed wind tunnel. Experimental findings demonstrate a notable shift in both acoustic pressure and dynamic lift force during resonance excitation for rods characterized by an aspect ratio of l/h = 0.5. For cylinders with an aspect ratio of l/h = 2, there is unexpected excitation of the third acoustic mode, imposing a considerable reduction in the dynamic lift force and alteration in shear layer dynamics, which subsequently impacts the shedding pattern. The study reveals that modifying the shape of the upstream edges to be rounded can alter the shedding pattern and decrease dynamic the loading, whereas alterations to the downstream edges amplify the sound pressure level (SPL) during resonance. Particle image velocimetry (PIV) measurements further accentuate the crucial role of incidence angles in modulating flow structures, vortex generation, and wake dynamics. The combined effect of a small angle of incidence and self-excited acoustic resonance was found to have an added streamwise length effect. This research emphasizes the significant influence of incidence angles and self-induced acoustic resonance on the ILEV/TEVS shedding pattern, underscoring the importance of rod geometry and orientation in the mechanism of flow-sound interaction. Note: Rectangular rod and cylinder are used interchangeably in this thesis.
Effects of the geometric modifications on the shear layer excitation source in co-axial side branches
(2023-08-01) Hammad, Omar S.; Mohany, Atef
This comprehensive study delves into the interaction between shear layer oscillations and the acoustic field within co-axial cavities, spotlighting the efficacy of the sound source model as a reliable semi-empirical modeling methodology. The focus is on how the downstream acoustic boundary conditions, when varied, markedly alter the acoustic field and the aeroacoustic sound source within deep co-axial cavities. Furthermore, it highlights how different upstream distances influence the aeroacoustic resonance of a coaxial side branch and the resultant peak acoustic pressure and the sound source term. Interestingly, edge geometry modification, specifically edge rounding and chamfering, plays a crucial role in the phasing of the shear layer with the acoustic field, thereby affecting the excitation and resonance behavior of the aeroacoustic system. These alterations significantly impact the aeroacoustic response, including peak pressure and lock-in range. These findings, derived from rigorous experimentation and predictive modeling, provide valuable insights into aeroacoustic modeling within coaxial cavities, contributing significantly to the design of industrial applications seeking to avoid and mitigate aeroacoustic resonance.
Experimental and numerical investigation of the dynamic seat comfort in aircrafts.
(2013-12-01) Ciloglu, Hakan; Mohany, Atef; Kishawy, Hossam
This research focuses on the dynamic seat comfort in aircrafts specifically during takeoff, landing and cruise through turbulence flight conditions. The experiments are performed using a multi axis shaker table in the Automotive Centre of Excellence (ACE) at the University of Ontario Institute of Technology subjected to sample takeoff, landing and cruise vibration recordings obtained onboard of an actual flight. The input vibrations introduced to the aircraft seats during actual flight conditions and during the experiments in the ACE are compared and it is concluded that the given flight conditions were successfully replicated for the interest of this thesis. The experiments are conducted with two different aircraft seats, economy class and business class. Furthermore, to investigate the importance of seat cushion characteristics in addition to economy and business class seat cushions, three laboratory made cushions were included in the investigation as well. Moreover, the effect of passenger weight is also discussed by conducting the experiments with 1 and 2 identical dummies. It is concluded that static seat properties play a significant role in the comfort perception level as well as flight conditions. Among the three flight condition, landing appeared to be the most uncomfortable case comparing to takeoff and cruise. In addition to experimental work, a numerical study to simulate the flight conditions is undertaken with the initial work of CAD modelling. The simulated responses of the seat is partially matching with experimental results due to unknown parameters of the cushion and the connections of the aircraft seat that cannot be created in the CAD model due to unknown manufacturing processes.
Flow characteristics and acoustic resonance excitation of finned cylinders in cross-flow
(2022-12-01) Alziadeh, Mohammed; Mohany, Atef
The flow characteristics and acoustic resonance excitation of bare and finned cylinders of different arrangements in cross-flow were experimentally investigated. In the design phase of finned tube heat exchangers, finned cylinders are treated as bare cylinders with a diameter equivalent to their flow blockage. This technique is used in conjunction with empirical data obtained from bare cylinder measurements to estimate the vibration/acoustic excitation parameters such as the Strouhal number and critical flow velocity at which resonance is expected to materialize. However, detailed particle image velocimetry (PIV) measurements revealed that the equivalent diameter approach does not consider the intrinsic changes in the flow characteristics caused by the addition of fins. Dynamic lift force and aeroacoustic response measurements revealed that these changes affected the finned cylinder’s susceptibility to acoustic resonance excitation, different than its equivalent diameter bare cylinder. These variations were amplified when finned cylinders were placed in a tandem arrangement, causing significant changes in the impinging flow mechanism and topology. This resulted in quantitative differences in the excitation parameters between the finned and bare cylinders. These findings ultimately show the need for empirical finned cylinder data in order to reliably estimate excitation parameters in the design phase of heat exchangers. Another simplification made in the design phase of heat exchangers is that the flow is assumed to approach the tube bundle at a zero angle of attack. However, this is not the case in industrial applications. Strouhal periodicities measured using tubes instrumented with pressure taps at different locations within a square tube bundle showed strong dependence on the flow approach angle. This greatly influenced the aeroacoustic response. The viability of non-uniformly distributing the fins along their span to suppress acoustic resonance in the tandem arrangement was studied. Tandem non-uniform finned cylinders reduced the vortex shedding periodicity compared to uniform-finned cylinders with the same number of fins. This led to weaker acoustic resonance associated with the vortex shedding process. However, further work is required to optimize the distribution of the non-uniform fins to control the shear layer instability in the cylinders’ gap, which is the dominant excitation source for pre-coincidence acoustic resonance.
Flow-sound interaction mechanism of a single finned cylinder in cross-flow
(2017-04-01) Arafa, Nadim; Mohany, Atef
The flow-sound interaction mechanism of a single finned cylinder in cross-flow is experimentally investigated and compared to that resulting from an equivalent bare cylinder. The flow field changes that result from the fins are captured by flow measurements and numerical simulations in order to be correlated to the observed acoustic resonance excitation. It is observed that the finned cylinders experience an earlier acoustic resonance and higher levels of acoustic pressure compared to their equivalent bare cylinders. This suggests that adding fins to the cylinder changes the flow field in a manner that makes it more susceptible to acoustic excitation. Flow measurements show that the vortex shedding process becomes stronger in the case of finned cylinders and the spanwise flow coherence is enhanced. The stronger vortex shedding process and coherent flow field promote the occurrence of the acoustic resonance and increase the observed noise levels. The acoustic resonance excitation is found to be significantly dependent on the fin spacing and thickness, which is explained by the impact of these fin parameters on the wake structures. Numerical simulations of the flow field downstream of the finned cylinders show that the fins lead to an organized flow structure, which is attained by concentrating the flow energy in the large-scale vortices and reducing the small-scale hair-pin vortices. This leads to the reduction of the vortex formation region in the near-wake of the cylinder, which subsequently increases the dynamic loading on the cylinder. The effect of the cylinder's aspect ratio on the acoustic resonance excitation is also investigated using the same analysis. It is shown that the finned cylinder provides a coherent flow field that is not affected by the aspect ratio, and hence, its acoustic resonance excitation levels increase with the increase of the cylinder's length. On the other hand, the bare cylinder exhibits a significant stretching in the vortex formation region with the increase in the aspect ratio. Therefore, the acoustic resonance excitation levels for the bare cylinder decrease with the increase of aspect ratio.
Flow-sound interaction mechanism of a single spirally finned cylinder in cross-flow
(2017-01-01) Alziadeh, Mohammed; Mohany, Atef
Over the years, some effort has been expended in the improvement of heat transfer performance in tubular heat exchangers. This can be achieved by adding different types of fins to the outer tube surface, effectively increasing convective heat transfer. However, the addition of fins may lead to the generation of severe noise, caused by the coupling between the vortex shedding frequency and one of the acoustic cross-modes of the duct housing the finned tubes. This may reduce the service life of the heat exchangers, and adversely affect the health of individuals working in the proximity of such noise. Since the flow-acoustic phenomenon of finned tubes are not well understood, it can be dangerously unpredictable. Therefore in this thesis, the flow-sound interaction mechanism of a single spirally finned cylinder in cross-flow is investigated. Moreover, a simple noise control technique is proposed to suppress the onset of acoustic resonance excitation.
Investigation of the ultrasonic based hydrogen production process: Sonohydrogen
(2021-04-01) Seifeldin, Sherif Samir Ahmed Rashwan; Dincer, Ibrahim; Mohany, Atef
The present work carries out various sets of numerical investigations to link the primary effect of the acoustic parameters with the secondary effect of developing a chemical reaction mechanism for water vapor dissociation into hydrogen and radicals. The first set of numerical modeling predicts the acoustic pressure distribution inside a typical geometry cylindrical sonoreactor. The study validates the acoustic pressure according to different geometrical and acoustical parameters. Secondly, the analysis and assessments give access to the sonication process's acoustic streaming. The second set validates the acoustic streaming result according to the velocity profile and streamlines, which gives an excellent agreement with the literature's experimental data. Analysis of variance ANOVA investigates the performance of 27 different configurations for the sake of optimization and determines the most influential factors for the design of a sonoreactor. Nevertheless, the chemical reaction module develops a chemical kinetics model and simulates the sonohydrogen process. The reaction kinetics mechanism consists of 19 reversible reactions and investigates the effect of the acoustic bubble temperature and the dissolved gases on the hydrogen production rate. The study quantifies the amount of hydrogen produced from the sonohydrogen process successfully and reveals the energy consumption to produce one μmol of hydrogen per kWh. The chemical kinetics results reveal that the higher the bubble temperature, the higher the chemical reaction rate. In the case of the H2O/O2 bubble, the energy consumption ranges between 1.05-1.63 μmol/kWh, with a maximum hydrogen yield of 4% and a maximum energy efficiency of 2% depending on the bubble’s temperature. However, in the H2O/Ar bubble, the hydrogen production shows an outstanding improvement with energy efficiency in the range 20-30 μmol/kWh with a maximum hydrogen yield of 35% and a maximum overall efficiency of 15%. The theory beyond this finding lies in the lower thermal conductivity, higher heat capacity, and lower thermal diffusivity of water vapor and carbon dioxide composition. We find this study is promising as a start for a new technique for hydrogen production.
Modeling and analysis of metal cutting process using self-propelled rotary tools
(2021-02-01) Ahmed, Waleed A. Abdelfattah; Kishawy, Hossam; Mohany, Atef
Machining difficult-to-cut materials is still one of the challenges facing different industries such as aerospace, nuclear and automotive sectors. That’s mainly because of the excessive heat generated, which affects the tool wear behavior and machinability performance. Rotary tools can be employed to solve these issues as they offer an acceptable tool life compared to traditional tools, especially under dry-environment conditions. Despite the attempts offered in the area of machining with rotary tools, there is a noticeable lack in understanding the physical aspects and mechanics of this process. Thus, the current work focuses on three main pillars to fully address this research gap. The first pillar of this study presents a novel analytical model to predict the cutting forces and tool rotational speeds during the machining process using self-propelled rotary tools with considering the tool bearing friction. The prediction capabilities of this analytical model are higher than all existing models in the open literature. The objective of the second pillar is to propose a hybrid finite element model which is able to predict the temperature distribution during cutting with self-propelled rotary tools. The proposed model addresses the limitation of other previous models as it is purely focused on significant aspects such as; heat partition factor and the contact area between the tool and the chip. The simulation results in terms of cutting forces, temperature, heat flux, and the contact area between the chip and the tool are obtained, and good agreement is observed between the numerical and experimental results. In terms of the third pillar, deep-understanding of the process mechanisms is fully discussed through conducting experimental tests on AISI 4140 hardened-steel, followed by analysis of variance, empirical modeling of the process, and process optimization. Besides, a detailed mechanism for the machining process with self-propelled rotary tools is concluded. It should be stated that this work offers a valuable comprehensive analysis for the metal cutting industry in terms of modeling, optimization, and assessment of the machining process with rotary tools.
Numerical investigation of the fluidelastic instability of two-phase flow in a parallel triangular tube array
(2021-05-01) Sadek, Omar; Mohany, Atef; Hassan, Marwan
Steam generators are always susceptible to vibrations induced by the flow in either the shell or tube sides. The fluidelastic instability phenomenon (FEI) is considered one of the most devastating flow excitations since it may cause excessive wear and structural failure to the tubes in a short time span, yet the phenomenon is not well understood. The U-bend region of the steam generator is very prone to the FEI and the flow in this region is characterized by a two-phase nature. Most studies of this phenomenon have been carried out experimentally on specific tube arrays at certain conditions, and design guidelines were developed based on them. Thus, a need emerges to provide a model to predict the onset of FEI at any flow condition or geometry. Firstly, this research focuses on developing and validating a model to predict the onset of FEI in two-phase flows. Secondly, the work attempts to address the problem of varying the flow’s angle of attack inside the U-bend, known as flow’s approach angle, and how it influences the onset of the instability. Finally, due to the curvature of the tubes inside the U-bend region, they are not tuned to a single natural frequency, a case known as frequency detuning. The presented work inspects the effect of frequency detuning and the key parameters controlling its influence. In this study, a model based on Computational Fluid Dynamics was proposed to simulate the onset of FEI. The model was validated and tested for a two-phase air-water flow in parallel triangular array against FEI in transverse and streamwise directions. Predictions obtained were in good agreement with experiments in the literature. Furthermore, the influence of flow approach angle was relatively understood and an efficient approximate semi-analytical model was successfully developed to predict the FEI dynamic forces at any flow angle. Finally, isolation of FEI mechanisms was carried out. Generally, frequency detuning was found to stabilize the tube bundle and its effect is sensitive to the mass-damping parameter. This work is a step forward towards a better understanding and an accurate prediction of the onset of FEI in the U-bend region.
Parametric investigation of flow-sound interaction mechanism of circular cylinders in cross-flow
(2016-12-01) Afifi, Omar; Mohany, Atef
Flow-excited acoustic resonance in heat exchangers has been an ongoing issue for the past century. The main challenge in this issue, is in the actual prediction of the resonance occurrence. This is due to the complexity of the flow-sound interaction mechanism that takes place between the packed cylinders. Most of the research lately has therefore shifted focus to simpler geometries that resemble the same mechanisms of flow-sound interaction found in actual heat-exchangers. The research presented hereafter summarizes an extensive experimental parametric work performed on multiple simple configurations such as single, tandem and side-by-side cylinders in cross-flow. The main objective of the research is to identify the critical parameters that should be included in the damping criteria to reliably predict the occurrence of acoustic resonance in tube bundles. Special attention is given to the geometrical characteristics of the duct (i.e. cross-sectional area) and how they affect the acoustic resonance. To achieve this; more than one hundred experiments have been performed in three different wind-tunnels of different cross-sectional areas. The research is motivated by the fact that most of the criteria developed to date, fail to predict the destructive phenomena of acoustic resonance in 30-40% of the cases.
Passive damping mechanism of Herschel-Quincke tubes for pressure pulsations in piping systems
(2018-12-01) Lato, Thomas; Mohany, Atef
The acoustic pressure pulsations in industrial piping systems can induce fluctuating loads on inline equipment which may cause fatigue failure and in severe cases, initiate a phenomenon known as acoustic resonance. Passive damping devices such as the Herschel-Quincke (HQ) Tube can be implemented into a resonant system to mitigate the pressure pulsations. Some practical considerations have been clarified in the current work which include the normalization of the transmission loss with respect to the HQ tube to pipeline diameter ratio and the change in attenuation when placing an HQ device at different locations along the standing wave formed in the pipeline. The attenuation mechanism of the HQ device was clarified for the application to resonant piping systems. It was found that the second acoustic mode of an open-open pipe is excited within the device. A Computational Aeroacoustic simulation was performed to visualize the acoustic state variables within the HQ device.
Passive methods for suppressing acoustic resonance excitation in shallow rectangular cavities.
(2014-08-01) Omer, Ahmed; Mohany, Atef
The flow-excited acoustic resonance in shallow rectangular cavities can be a source of severe noise and/or excessive vibration. This phenomenon is excited when one of the acoustic modes in the accommodating enclosure is coupled with the flow instabilities resulting from the shear layer formation at the cavity mouth. In this thesis, two passive methods for suppressing the flow-excited acoustic resonance phenomenon are addressed. The first passive method considers the edge geometry effect on the phenomenon. Several edge geometries including chamfered, round, and different configurations of spoilers are considered. The effect of the spoilers dimensions is investigated to provide criteria that help designing and optimizing spoilers. Some of the spoilers are found to be effective in suppressing the acoustic resonance excitation, while some other edges including chamfered and round edges result in shifting the resonance excitations to higher velocities with amplification in the acoustic pressure. To enrich the understanding of the suppression mechanism introduced by these passive methods, hotwire measurements are performed revealing the existence of orthogonal vortices interacting with the shear layer at the cavity mouth. The second passive method investigated is the effect of placing a high frequency vortex generator (control cylinder) in vicinity of the upstream edge of the cavity on the acoustic resonance excitation. The method is investigated experimentally and numerically. The effectiveness of the control cylinder method is studied by investigating different cylinder diameters and locations on both horizontal and vertical directions. It is found that locating the cylinder at relatively small height from the bottom wall and with a distance of 25.4 mm upstream the leading edge can significantly suppress the resonance excitation. To further understand the interaction between the cylinder vortex shedding and the shear layer at the cavity mouth and the influence on the shear layer thickness, a 2D numerical simulation using K-epsilon and Detached Eddy Simulation (DES) models has been carried out and compared to the experimental results. For both passive methods, the study included two cavities with different aspect ratios (L/D=1.0 and L/D=1.67, L: cavity length, D: cavity depth) to address the effectiveness of the methods with respect to the cavity depth. The methods are investigated in flow with Mach number up to 0.45. All different configurations investigated are compared to the base case which is the bare cavity with sharp edges installed upstream and downstream.