Now showing 1 - 10 of 24
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    Proper Orthogonal Decomposition analysis of mode switching in supersonic jets impinging on flat and corrugated plates
    (01-01-2023)
    Sarangi, Debivarati
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    Karthik, Ramanujam
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    Understanding the occurrence of various feedback mechanisms of an under-expanded impinging supersonic jet is a crucial task in research. The presence of several jet modes is examined in this study for the flat and corrugated impinging plate geometries. The behavior of impinging plate configurations during mode switching is investigated by varying the flow state, such as the jet Mach number. The staging behavior at various jet Mach numbers is observed using acoustic spectral plots and schlieren flow visualization. To explore the presence of various types of modes during the jet impingement due to the modification of jet Mach number, ensemble averaging and Proper Orthogonal Decomposition of schlieren images are carried out. In the majority of situations, the corrugated design shows a reduction in tonal noise and overall sound pressure level. In exceptional cases, for the corrugated plates, the enhanced overall sound pressure level is caused by the existence of axisymmetric instability (A1, A2).
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    Aeroacoustic modal analysis of underexpanded pipe jets with and without an upstream cavity
    (01-01-2021)
    Baskaran, Kabilan
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    The investigation of the aeroacoustics of an underexpanded pipe-cavity jet is carried out experimentally. Two different aspect ratios of the cavity are tested for a wide range of nozzle pressure ratios. Both internal and externally radiated pipe-cavity acoustics are studied. Linear and higher-order spectral analyses are implemented on the unsteady cavity pressure to comprehend the nature of the cavity acoustics and nonlinear interactions of different acoustic modes of the pipe-cavity system. Results show that an increase in depth leads to an enhancement in the nonlinear interactions. Furthermore, the power spectral and overall sound pressure level analyses of pipe and pipe-cavity jet noise radiation are carried out. High-speed schlieren imaging techniques are used to understand jet dynamics. Highly unsteady motion of the jet initial shear layer is observed due to an upstream disturbance of the cavity. In addition, proper orthogonal and dynamic mode decomposition methods are used to extract the spatial and dynamic modes of the jet structure. These methods are used to segregate the cavity associated jet dynamics and screech dynamics.
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    Effect of Atomization Quality on Lean Blow-Out Limits and Acoustic Oscillations in a Swirl Stabilized Burner
    (02-06-2020)
    Muthuselvan, Govindaraj
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    Suryanarayana Rao, Muralidhara
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    Iyengar, Venkat S.
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    Pulumathi, Manjunath
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    Thirumalachari, Sundararajan
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    The present experimental work highlights the influence of atomization quality on lean blow-out (LBO) limits and acoustic oscillations in a swirl stabilized burner with simplex atomizer. With decrease in the initial spray droplet diameter, the LBO limit shifts toward lower equivalence ratios. Reduction in droplet size also strongly influences the mode of LBO from diffusion flame to premixed lifted flame. Correlations have been developed for the LBO limit, involving mainly the time scales for evaporation, reaction and residence times for the fuel drops, as well as the gas flow. Delay in evaporation causes vapor accumulation before combustion and hence it influences both acoustic oscillations and LBO limit. The frequency of acoustic oscillations locks-in with the quarter wave frequency of the combustor duct for all initial droplet diameters considered. The amplitude of acoustic oscillations decreases with decrease in the initial droplet size.
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    Performance Analysis of SinglePhase Space Thermal Radiators and Optimization Through Taguchi-Neuro-Genetic Approach
    (01-06-2022)
    Chiranjeevi, P. B.
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    Ashok, V.
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    Sundararajan, T.
    In the thermal management of spacecraft, space thermal radiators play a vital role as heat sinks. A serial radiator with proven advantages in ground applications is proposed and analyzed for space applications. From the performance analysis, specific heat rejection (SHR) of serial radiator is found to be higher than parallel radiator by 80% for maximum diameter of the tube, 47% for maximum thickness of the fin, and 75% for maximum pitch of the tubes under consideration. Also, serial radiator requires four times higher pumping power than parallel radiator with geometric parameters and a maximum mass flowrate under consideration. In serial radiators, the cross conduction between the fins has a significant effect on its thermal performance. Thus, conjugate heat transfer simulations and optimization operations are to be performed iteratively to optimize the serial radiator, which is computationally costly. To reduce the computational time, artificial neural network (ANN) is trained using conjugate heat transfer simulations data and combined with the genetic algorithm (GA) to perform optimization. Taguchi's orthogonal arrays provided the partial fraction of conjugate heat transfer simulations set to train the ANN. Taguchi-Neuro-Genetic approach, a process that combines the features of three powerful techniques in different optimization phases, is used to optimize both parallel and serial radiators. The optimization aims to obtain a configuration that provides the lowest mass and lowest pumping power requirement for given heat rejection. Optimization results show that the conventional parallel radiator is about 20% heavier and requires about 35% more pumping power than the proposed serial radiator.
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    Investigation of acoustic spectral variations in the pool boiling regimes of water on wire heater
    (25-05-2023)
    Barathula, Sreeram
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    Alapati, Jaswanth K.K.
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    This paper presents the experimental investigation of acoustic spectral variations in saturated pool boiling regimes of water on a heated wire. In the current study, two different wires of standard wire gauge (SWG) 36 and 42 are considered to investigate the acoustic spectral variations through the pool boiling curve. The amplitude and frequency changes are evaluated for each regime of pool boiling. In a single regime, amplitude rise is observed with respect to the heat flux without any significant change in dominant frequencies. On the other hand, frequency shifts are observed in regime transitions. A change in the diameter of the heater wire has no significant effect on the boiling acoustic spectra. However, the number of high-frequency components increased for the SWG – 42 than the SWG – 36 wire. A frequency peak near 2000 Hz is found to be crucial for boiling regime identification. The sound pressure level (SPL) for SWG – 36 is higher than the SWG – 42, and it is further noted that SPL follows an ‘N’ shaped pattern for both wires owing to the frequency shifts and variation of mean bubble departure diameter at that heat flux.
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    Review on research progress in boiling acoustics
    (01-12-2022)
    Barathula, Sreeram
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    Ever-growing miniaturization of electronic devices and space-conserving endeavours of heat transfer systems pose a challenging task to the current cooling strategies. Boiling acoustics is one of the most potent and efficacious methodologies to reliably predict the boiling regime inside the cooling systems to assay the safety and address the emergency conditions. Boiling acoustics is gripping attention out of the few foreseeable technologies for the future cooling requirements. Though the potentiality of boiling acoustics was unravelled in the late 1990s, the research data present in this arena is however lacking. This paper presents a comprehensive review of the literature reported from the 1970s to the present date. Furthermore, this paper also details the evolution of boiling acoustics from the initial application of boiling incipience detection to regime identification. Much focus is given on salient features of boiling acoustic characterization dealing with the regime detection. Effects of various parameters such as thermos-physical properties of the heater surface and the boiling liquid that directly or indirectly influence the acoustic spectra are also presented. The prediction of the boiling regime constitutes the first necessary step in producing autonomous cooling systems. Hence, the detection and characterization of boiling noise under various conditions such as pressure, heat flux, and flow rate is essential.
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    Passive control of pipe-jet noise using trailing-edge castellations
    (15-12-2020)
    Anureka, R.
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    In the current study, geometric modifications in the form of trailing edge castellations are explored for the reduction of pipe-jet noise. The number of castellations determines the interaction between the streamwise vortices, which alters the sound produced. The noise levels in pipe-jets with and without castellations are compared by measuring the polar and azimuthal directivities. The parameters, namely the number of castellations and the nozzle pressure ratio, are varied in this study. The castellations work best in the underexpanded flow regime of nozzle pressure ratio 3–5. The spectral analysis of the sound revealed that the screech tones are eliminated for all castellated pipes except the one with two castellations. The broadband shock associated noise is found to increase with the number of castellations, and screech tones reappear at a higher nozzle pressure ratio. The maximum noise mitigating and amplifying cases are identified by acoustic power analysis for all castellated pipes with equidistant teeth and gap. As the number of castellations increases, the modified pipes attain the noise characteristics of the base model, which can be considered as an infinitely castellated pipe. These results are supported by the flow visualization using schlieren imaging, which showed the differential expansion in teeth and gap plane, leading to asymmetry in flow. Proper Orthogonal Decomposition of the schlieren images indicated the presence of symmetric and anti-symmetric mode of oscillation in gap and teeth plane, respectively, of the model with two castellations.
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    Machine learning aided near-field acoustic holography based on equivalent source method
    (01-02-2023)
    Chaitanya, S. K.
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    Sriraman, Siddharth
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    Srinivasan, Srinath
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    In recent times, equivalent source method-based near-field acoustic holography methods have been extensively applied in sound source localization and characterization. The most commonly used equivalent sources are spherical harmonics. In a non-reverberant environment with no reflections, these equivalent sources could be the best choice since spherical harmonics are derived for the Sommerfeld boundary condition. However, these methods are not the best fit for reverberating environments. In such cases, a new relationship can be calculated between the field weights and the measured pressure with enough training examples. The proposed machine learning models include linear regression (LR) with adaptive moment estimation (Adam), LR with limited-memory Broyden-Fletcher-Goldfarb-Shanno (L-BFGS), and multi-layer perceptron with one and two hidden layers. These methods are tested for multiple monopoles and vibrating plate simulations in a room with different wall absorption coefficients. The data-driven methods are also studied on loudspeakers numerically and experimentally in a free field environment. The results from these methods are compared with the results of one norm convex optimization (L1CVX). LR with L-BFGS performed the best among all the methods studied and performed better than L1CVX for less absorption coefficient for geometrically separable sources. LR with L-BFGS also has much faster inference times.
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    Transition from Symmetric to Asymmetric Flame Anchoring in Two-Dimensional Channels with Sudden Expansion
    (01-02-2020)
    Mithun, Jyothi
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    Sundararajan, T.
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    Symmetric to asymmetric transitions of laminar-premixed methane-air flames anchoring in symmetric sudden expansion channels are explored. Numerical and experimental studies on flame anchoring phenomena are carried out. Simulations predict attachment of cold flow to one of the side walls (Coanda effect), above a critical Reynolds number. However, with symmetric ignition near the sudden expansion, initially asymmetric cold flow becomes symmetric and a symmetric flame anchors at the sudden expansion. Below a critical value of Reynolds number (ReC,R), both steady flow and flame are symmetric. When fuel-air mixture is ignited asymmetrically close to the sudden expansion for Re > ReC,R, both the flow and the flame attach to one of the side walls asymmetrically if the transverse distance of ignition is greater than a critical value. The flow attachment length is always smaller than flame attachment length. Experimentally visualized flame anchoring phenomena for ignition near the exit are in conformity with corresponding numerical predictions.
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    Experimental investigation of the influence of Reynolds number and buoyancy on the flow development of a plane jet in the transitional regime
    (01-01-2021)
    Suresh, P. R.
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    Sundararajan, T.
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    Heated horizontal plane jets find wide applications in engineering appliances such as air curtains and discharge of industrial effluents. In the present study, experimental investigations are conducted on a heated horizontal plane jet with the Reynolds numbers in the transitional regime, using a hotwire anemometer. In the far to very far-field (20 < x/d < 100) centreline velocity decay and jet spread increases faster with the decrease of Reynolds number. This is because, with the increase of Reynolds number, the turbulent kinetic energy is distributed on a broadband of scales. As a result, larger scales, which are responsible for increased entrainment, get weaker. The shifting of the centre plane generally occurs in the far region for low Reynolds number jets. A comparison with the result of an isothermal jet at similar Reynolds numbers from the literature at identical conditions shows that the turbulence intensity is decreased due to heating. Centreline velocity decays slowly and half-width increases marginally for a heated jet when compared with an isothermal jet. The effect of heating is prominent for low Re jets. Spectral development shows a delayed transition due to heating. Probability density function plots reveal lack of equilibrium and presence of large-scale eddies in the flow field.