| 1. |
- Mihaescu, Mihai, 1976-, et al.
(author)
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Computational Aeroacoustics of a Separate Flow Exhaust System with Eccentric Inner Nozzle
- 2007
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In: 45th AIAA Aerospace Sciences Meeting 2007. - 9781563478901 ; , s. 28-39
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Conference paper (peer-reviewed)abstract
- A major issue related to our modern way of life is the noise caused by jet engine powered airliners. Methods to reduce the jet noise were proposed in the last decades starting with the introduction of high bypass turbofan engines. A numerical investigation of flow and acoustics of a separate flow exhaust system is performed in the present paper. The goal is to investigate how the noise generated by a separate flow jet engine exhaust is influenced if the symmetry of the jet exhaust geometry is broken by having the inner nozzle and the center body off the center line axis of the fan nozzle. A decomposition of flow variables is used that allows separation of flow and acoustic computations. Large Eddy Simulation approach is employed to compute the flow field and the acoustic sources. The inhomogeneous wave equation is used to compute the acoustic near- and far-fields. Using this method, previous numerical studies of the symmetric (baseline) configuration showed good agreement between the computed data and the experimental results. Comparisons between the near- and far-field baseline concentric case and the computations obtained using the eccentric configuration are performed showing an acoustic benefit for the offset case.
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| 2. |
- Mihaescu, Mihai, 1976-, et al.
(author)
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Computational Aeroacoustics of the Coaxial Flow Exhaust System of a Gas Turbine Engine
- 2007
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In: Volume 6: Turbo Expo 2007, Parts A and B. - : ASME. - 0791847950 ; , s. 1531-1539
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Conference paper (peer-reviewed)abstract
- Jet engine noise is an environmental problem that needs to be addressed. Several methods to reduce the jet noise have been proposed in the last decades. The main issue is to find methods that reduce noise without causing considerable loss of thrust. Experimental and computational tools are mandatory in successfully reducingjet engine noise emissions. One of the challenging issues ofcomputing the jet engine noise is the presence of verylarge scales (associated with the wave length of the acousticwave) and at the same time also small scales that are responsible for the acoustical sources. In the field of Computational Aero-Acoustics (CAA) different hybrid approaches have been introduced to handle the different scales using problem specific models and methods.Here, a decomposition of flow variables is used that allowsseparation of flow and acoustic computations. Large Eddy Simulation approachis employed to compute the flow field and the acousticsources. An inhomogeneous wave equation is used to perform acousticcomputations. The paper investigates numerically the flow and the near-fieldacoustic data from a coaxial jet case with chevrons onthe core nozzle that are compared with those obtained froma baseline coaxial jet, showing the spatial character of the acoustic benefit when chevrons are used on the core nozzle.Comparisons in terms of sound pressure levels with experimental data performed with the same geometry show a good agreement.
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| 3. |
- Mihaescu, Mihai, 1976-, et al.
(author)
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Computational Fluid Dynamics Analysis of Upper Airway Reconstructed From Magnetic Resonance Imaging Data
- 2008
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In: Annals of Otology, Rhinology and Laryngology. - : SAGE Publications. - 0003-4894 .- 1943-572X. ; 117:4, s. 303-309
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Journal article (peer-reviewed)abstract
- Objectives: We performed flow computations on an accurate upper airway model in a patient with obstructive sleep apnea and computed the velocity, static pressure, and wall shear stress distribution in the model. Methods: Cartesian coordinates for airway boundaries were determined from cross-sectional magnetic resonance images, and a 3-dimensional computational model of the upper airway was constructed. Flow simulations were then performed within a FLUENT commercial software framework. Four different flow conditions were simulated during inspiration, assuming the steady-state condition. The results were analyzed from the perspectives of velocity, static pressure, and wall shear stress distribution. Results: We observed that the highest axial velocity was at the site of minimum cross-sectional area (retropalatal pharynx) resulting in the lowest level of wall static pressure. The highest wall shear stresses were at the same location. The pressure drop was significantly larger for higher flow rates than for lower flow rates. Conclusions: Our results indicate that the presence of airway narrowing, through change in the flow characteristics that result in increased flow velocity and reduced static pressure, can itself increase airway collapsibility. Additionally, the effects of wall shear stress on airway walls may be an important factor in the progression over time of the severity of obstructive sleep apnea.
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| 4. |
- Mihaescu, Mihai, 1976-, et al.
(author)
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Computational Modeling of Upper Airway Before and After Adenotonsillectomy for Obstructive Sleep Apnea
- 2008
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In: The Laryngoscope. - 0023-852X .- 1531-4995. ; 118:2, s. 360-362
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Journal article (peer-reviewed)abstract
- Adenotonsillectomy, the first-line surgical treatment for obstructive sleep apnea (OSA) in children, is successful in only 50% of obese children. Computational fluid dynamics tools, which have been applied to differentiate OSA patients from those without OSA based on the airway flow characteristics, can be potentially used to identify patients likely to benefit from surgical intervention. We present computational modeling of the upper airway before and after adenotonsillectomy in an obese female adolescent with OSA. The subject underwent upper airway imaging on a 1.5 Tesla magnetic resonance imaging (MRI) scanner, and three-dimensional airway models were constructed using airway boundary coordinates from cross-sectional MRI scans. Our results using computational simulations indicate that, in an obese child, the resolution of OSA after adenotonsillectomy is associated with changes in flow characteristics that result in decreased pressure differentials across the airway walls and thus lower compressive forces that predispose to airway collapse. Application of such findings to an obese child seeking surgical treatment for OSA can potentially lead to selection of the surgical procedure most likely to result in OSA resolution. Effective intervention for OSA in this high-risk group will result in reduction in morbidity and the public health concerns associated with OSA.
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| 5. |
- Mihaescu, Mihai, 1976-, et al.
(author)
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Flow and Acoustics Characteristics of Chevron Nozzles in Coaxial Jets – LES and Acoustic Analogy Investigation
- 2007
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In: 13th AIAA/CEAS Aeroacoustics Conference (28th AIAA Aeroacoustics Conference). - 9781563478963 ; , s. 12-
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Conference paper (peer-reviewed)abstract
- In the last decades there has been significant interest in developing source-based jet noise suppression methods as well as developing jet noise prediction approaches that may involve modified geometry and complex flow fields. In this numerical study the acoustic field generated by flow unsteadiness in a non-isothermal coaxial turbulent jet with and without forced mixers on the core nozzle is determined using a hybrid approach. In the flow solver the Navier-Stokes equations are resolved using Large Eddy Simulation (LES) to model the turbulence. The acoustic solver is based on an inhomogeneous wave equation, where the instantaneous acoustic sources are computed from the LES data. The flow and the acoustic data from the coaxial jet case with chevrons on the core nozzle are compared with those obtained from a baseline coaxial jet, showing the spatial character of the acoustic benefit when forced mixers are used on the core nozzle. The computed results are compared with experimental data performed with the same geometry. The agreement between the two data sets in terms of sound pressure levels (SPLs) and noise directivity is very good.
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| 6. |
- Mihaescu, Mihai, 1976-, et al.
(author)
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Flow and Acoustics of a Coaxial Nozzle : a Sensitivity Study to the Inlet Boundary Conditions
- 2006
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In: 44th AIAA Aerospace Sciences Meeting 2006. - 9781563478079 ; , s. 7375-7393
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Conference paper (peer-reviewed)abstract
- The jet noise generated by the high velocity hot stream exhausting from a jet engine represents a major component of the aircraft noise. Reductions in jet noise has been achieved by using high bypass turbo-fan engines. To solve numerically this problem the flow and acoustic fields generated by two coaxial jets are considered. Large Eddy Simulation (LES) is used to handle the non-isothermal turbulent flow field, while for the acoustic field a wave equation with source terms provided by the instantaneous LES is employed. The effects of the inlet boundary conditions on the flow and acoustics are assessed in this paper. A comparison of the computed mean flow field and the SPL with experimental data show excellent agreement at distances of over a few jet diameters downstream of the the jet exit plane. In the proximal part the agreement is less good. However, the discrepancy between the simulations and the experiments is less than the corresponding variations due to the uncertainties in the experimental boundary conditions.
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| 7. |
- Mihaescu, Mihai, 1976-, et al.
(author)
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Flow and Acoustics Simulations Based on LES and an Acoustic Analogy : an Application to Laryngeal Airflow
- 2007
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In: 45th AIAA Aerospace Sciences Meeting 2007. - 9781563478901 ; , s. 11141-11153
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Conference paper (peer-reviewed)abstract
- The paper presents an efficient method for computational aeroacoustics applied to simulate the flow and the acoustics for a static laryngeal model considering the vocal folds widely open. The work analyses the whisper-like process defined as the sound generated by the turbulent glottal airflow in the absence of vocal fold vibration. A decomposition of the flow variables is used that allows separation of flow and acoustic computations. The approach consists in solving the low Mach number flow field by incompressible Large Eddy Simulation. This provides the possibility to compute the acoustic sources. The inhomogeneous wave equation derived from the fundamental equations of motion for a compressible fluid is employed to compute the acoustic field. The purpose of the study is to provide realistic numerical predictions of the flow patterns and the generated acoustic field in the glottis and the vocal tract regions.
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| 8. |
- Mihaescu, Mihai, 1976-, et al.
(author)
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Large Eddy Simulation and Reynolds-Averaged Navier–Stokes modeling of flow in a realistic pharyngeal airway model : An investigation of obstructive sleep apnea
- 2008
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In: Journal of Biomechanics. - : Elsevier BV. - 0021-9290 .- 1873-2380. ; 41:10, s. 2279-2288
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Journal article (peer-reviewed)abstract
- Computational fluid dynamics techniques employing primarily steady Reynolds-Averaged Navier–Stokes (RANS) methodology have been recently used to characterize the transitional/turbulent flow field in human airways. The use of RANS implies that flow phenomena are averaged over time, the flow dynamics not being captured. Further, RANS uses two-equation turbulence models that are not adequate for predicting anisotropic flows, flows with high streamline curvature, or flows where separation occurs. A more accurate approach for such flow situations that occur in the human airway is Large Eddy Simulation (LES). The paper considers flow modeling in a pharyngeal airway model reconstructed from cross-sectional magnetic resonance scans of a patient with obstructive sleep apnea. The airway model is characterized by a maximum narrowing at the site of retropalatal pharynx. Two flow-modeling strategies are employed: steady RANS and the LES approach. In the RANS modeling framework both k–ε and k–ω turbulence models are used. The paper discusses the differences between the airflow characteristics obtained from the RANS and LES calculations. The largest discrepancies were found in the axial velocity distributions downstream of the minimum cross-sectional area. This region is characterized by flow separation and large radial velocity gradients across the developed shear layers. The largest difference in static pressure distributions on the airway walls was found between the LES and the k–ε data at the site of maximum narrowing in the retropalatal pharynx.
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| 9. |
- Mihaescu, Mihai, 1976-, et al.
(author)
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Large Eddy Simulation for Turbulent Mixing in Elliptic Jets with Round Center-Body
- 2009
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In: th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition. - : AIAA. - 9781563479694
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Conference paper (peer-reviewed)abstract
- Separate-flow exhaust nozzle systems generate jets from round nozzles that include a conical plug. The present research shows results of unsteady simulations of turbulent hot jets issued from elliptic nozzles with a conic center-body. Three geometrical configurations of the elliptic nozzle are investigated keeping the same conic plug. All designs considered were intended to match the jet stream exit area, mass flow, and thrust of an existing round conical nozzle. The first configuration in the study (C1@3:1AR) was designed with the expectation of producing a jet which would take on a 3:1 aspect ratio (AR) elliptic cross-section downstream of the center-body. For a round conical plug nozzle the inner surface of the nozzle towards exit is axisymmetric with reference to the nozzle center-line and conical. Naturally, the elliptic plug nozzle is not axisymmetric. In the major axis plane the inner surface of the elliptic nozzle towards the exit has a slope close to zero, while in the minor axis plane the slope is steeper that it would be for a corresponding round nozzle. This forces the flow stream towards the conic plug in the minor axis plane. It was observed that the 3:1AR elliptic plug nozzle (C1@3:1AR) generates a bifurcated jet. The second and the third elliptic plug nozzle geometries (C2@3:1ARnf and C3@2:1ARnf) were intended to find how the jet behavior is influenced by not forcing the flow towards the conic plug in the minor axis plane and by changing the aspect ratio of the elliptic plug nozzle to 2:1. Large Eddy Simulation (LES) approach was used for the turbulence flow modeling. In the major axis plane the largest jet spreading was found for the 3:1AR elliptic plug nozzle (C1@3:1AR), while in the minor axis plane the jet exhausting from the 2:1AR elliptic plug nozzle (C3@2:1ARnf) spread the most.
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| 10. |
- Mihaescu, Mihai, 1976-, et al.
(author)
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Modeling Flow in a Compromised Pediatric Airway Breathing Air and Heliox
- 2008
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In: The Laryngoscope. - : Wiley, The Triological Society. - 0023-852X .- 1531-4995. ; 118:12, s. 2205-2211
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Journal article (peer-reviewed)abstract
- Objectives/Hypothesis: The aim of this study was to perform computer simulations of flow within an accurate model of a pediatric airway with subglottic stenosis. It is believed that the airflow characteristics in a stenotic airway are strongly related to the sensation of dyspnea. Methodology: Computed tomography images through the respiratory tract of an infant with subglottic stenosis, were used to construct the three-dimensional geometry of the airway. By using computational fluid dynamics (CFD) modeling to capture airway flow patterns during inspiration and expiration, we obtained information pertaining to flow velocity, static airway wall pressure, pressure drop across the stenosis, and wall shear stress. These simulations were performed with both air and heliox. Results: Unlike air, heliox maintained laminar flow through the stenosis. The calculated pressure drop over stenosis was lower for the heliox flow, in contrast to the airflow case. This lead to an approximately 40% decrease in airway resistance when using heliox, and presumably causes a decrease in the level of effort required for breathing. Conclusions: CFD simulations offer a quantitative method of evaluating airway flow dynamics in patients with airway abnormalities. CFD modeling illustrated the flow features and quantified flow parameters within a pediatric airway with subglottic stenosis. Simulations with air and heliox conditions mirrored the known clinical benefits of heliox as compared with air. We anticipate that computer simulation models will ultimately allow a better understanding of changes in flow caused by specific medical and surgical interventions in patients with conditions associated with dyspnea.
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