Underhood Buoyancy Driven Flow—An Experimental Study

2011 ◽  
Vol 133 (8) ◽  
Author(s):  
Parviz Merati ◽  
Charles Davis ◽  
K.-H. Chen ◽  
J. P. Johnson
Keyword(s):  
Surface Temperature ◽  
Ambient Air ◽  
Engine Block ◽  
Scale Model ◽  
Flow Structures ◽  
Engine Compartment ◽  
Fluid Exchange ◽  
Thermal Measurements ◽  
The Right ◽  
Over The Top

Particle image velocimetry and thermal measurements using thermocouples are used to measure the buoyant flow of a simplified full-scale model of an engine compartment. The engine block surface temperature and exhaust heaters are kept at about 100 and 600°C, respectively. Thermal measurements include enclosure surface temperature, temperature difference on the enclosure wall at midplane, engine block temperatures, and air temperatures under the hood. The highest surface temperatures were concentrated near the top of the enclosure around the exhaust heaters. This effect was due primarily to radiation from the exhaust heaters. Highest measured air temperature was about 300°C immediately above the right exhaust heater. The measured dominant flow structures are two larger counter rotating vortices over the top right side of the engine block and two counter rotating vortices over the top left side. These flow structures weaken considerably during the first 35 min of the transient cool down of the engine block and the exhaust heaters. Colder ambient air is sucked into the engine compartment at the vents near the bottom of the compartment with some exiting as hot air through the top slots. The time scale of the fluid exchange at the vents is in the order of seconds, indicating that this process is occurring very slowly.

Unsteady Aerodynamics of an Aeroengine Double Swirler Lean Premixing Prevaporizing Burner

2004 ◽  
Vol 128 (1) ◽  
pp. 29-39 ◽  
Author(s):  
Edward Canepa ◽  
Pasquale Di Martino ◽  
Piergiorgio Formosa ◽  
Marina Ubaldi ◽  
Pietro Zunino
Keyword(s):  
Flow Field ◽  
Reynolds Stress ◽  
Large Scale ◽  
Unsteady Aerodynamics ◽  
Scale Model ◽  
Flow Structures ◽  
Laser Doppler Velocimeter ◽  
Formation Mechanisms ◽  
Meridional Plane ◽  
Stress Components

Lean premixing prevaporizing (LPP) burners represent a promising solution for low-emission combustion in aeroengines. Since lean premixed combustion suffers from pressure and heat release fluctuations that can be triggered by unsteady large-scale flow structures, a deep knowledge of flow structures formation mechanisms in complex swirling flows is a necessary step in suppressing combustion instabilities. The present paper describes a detailed investigation of the unsteady aerodynamics of a large-scale model of a double swirler aeroengine LPP burner at isothermal conditions. A three-dimensional (3D) laser Doppler velocimeter and an ensemble-averaging technique have been employed to obtain a detailed time-resolved description of the periodically perturbed flow field at the mixing duct exit and associated Reynolds stress and vorticity distributions. Results show a swirling annular jet with an extended region of reverse flow near to the axis. The flow is dominated by a strong periodic perturbation, which occurs in all the three components of velocity. Radial velocity fluctuations cause important periodic displacement of the jet and the inner separated region in the meridional plane. The flow, as expected, is highly turbulent. The periodic stress components have the same order of magnitude of the Reynolds stress components. As a consequence the flow-mixing process is highly enhanced. Turbulence acts on a large spectrum of fluctuation frequencies, whereas the large-scale motion influences the whole flow field in an ordered way that can be dangerous for stability in reactive conditions.

scholarly journals Potential Reduction of Peripheral Local SAR for a Birdcage Body Coil at 3 Tesla Using a Magnetic Shield

2021 ◽  
Vol 9 ◽  
Author(s):  
C.C. van Leeuwen ◽  
B.R. Steensma ◽  
S.B. Glybovski ◽  
M.F.J. Lunenburg ◽  
C. Simovski ◽  
...  
Keyword(s):  
Eddy Currents ◽  
Periodic Structures ◽  
Ring Resonators ◽  
Scale Model ◽  
Birdcage Coil ◽  
Magnetic Shield ◽  
Body Regions ◽  
Body Coil ◽  
Local Sar ◽  
The Right

The birdcage body coil, the standard transmit coil in clinical MRI systems, is typically a shielded coil. The shield avoids interaction with other system components, but Eddy Currents induced in the shield have an opposite direction with respect to the currents in the birdcage coil. Therefore, the fields are partly counteracted by the Eddy currents, and large coil currents are required to reach the desired B1+ level in the subject. These large currents can create SAR hotspots in body regions close to the coil. Complex periodic structures known as metamaterials enable the realization of a magnetic shield with magnetic rather than electric conductivity. A magnetic shield will have Eddy currents in the same direction as the coil currents. It will allow generating the same B1+ with lower current amplitude, which is expected to reduce SAR hotspots and improve homogeneity. This work explores the feasibility of a birdcage body coil at 3 T with a magnetic shield. Initially, we investigate the feasibility by designing a scale model of a birdcage coil with an anisotropic implementation of a magnetic shield at 7 T using flattened split ring resonators. It is shown that the magnetic shield destroys the desired resonance mode because of increased coil loading. To enforce the right mode, a design is investigated where each birdcage rung is driven individually. This design is implemented in a custom built birdcage at 7 T, successfully demonstrating the feasibility of the proposed concept. Finally, we investigate the potential improvements of a 3 T birdcage body coil through simulations using an idealized magnetic shield consisting of a perfect magnetic conductor (PMC). The PMC shield is shown to eliminate the peripheral regions of high local SAR, increasing the B1+ per unit maximum local SAR by 27% in a scenario where tissue is present close to the coil. However, the magnetic shield increases the longitudinal field of view, which reduces the transmit efficiency by 25%.

scholarly journals A nonlinear multi-scale model for blood circulation in a realistic vascular system

2021 ◽  
Vol 8 (12) ◽  
Author(s):  
Ulin Nuha A. Qohar ◽  
Antonella Zanna Munthe-Kaas ◽  
Jan Martin Nordbotten ◽  
Erik Andreas Hanson
Keyword(s):  
Blood Flow ◽  
Blood Circulation ◽  
Vascular System ◽  
Scale Model ◽  
Model Framework ◽  
Fluid Exchange ◽  
Multi Scale ◽  
Proposed Model ◽  
Porous Media Model ◽  
Capillary Bed

In the last decade, numerical models have become an increasingly important tool in biological and medical science. Numerical simulations contribute to a deeper understanding of physiology and are a powerful tool for better diagnostics and treatment. In this paper, a nonlinear multi-scale model framework is developed for blood flow distribution in the full vascular system of an organ. We couple a quasi one-dimensional vascular graph model to represent blood flow in larger vessels and a porous media model to describe flow in smaller vessels and capillary bed. The vascular model is based on Poiseuille’s Law, with pressure correction by elasticity and pressure drop estimation at vessels' junctions. The porous capillary bed is modelled as a two-compartment domain (artery and venous) using Darcy’s Law. The fluid exchange between the artery and venous capillary bed compartments is defined as blood perfusion. The numerical experiments show that the proposed model for blood circulation: (i) is closely dependent on the structure and parameters of both the larger vessels and of the capillary bed, and (ii) provides a realistic blood circulation in the organ. The advantage of the proposed model is that it is complex enough to reliably capture the main underlying physiological function, yet highly flexible as it offers the possibility of incorporating various local effects. Furthermore, the numerical implementation of the model is straightforward and allows for simulations on a regular desktop computer.

scholarly journals Study on the Application Possibility of Uninsulated Duct Firestop System using Simulation

2020 ◽  
Vol 20 (2) ◽  
pp. 119-126
Author(s):  
Heewon Seo ◽  
Myungo Yoon ◽  
Dongho Choi ◽  
Daehoi Kim ◽  
Hyungdo Lee
Keyword(s):  
Surface Temperature ◽  
Ambient Air ◽  
Radiant Heat ◽  
Radiant Heat Transfer ◽  
Insulation Material ◽  
Structure Surface ◽  
Important Input ◽  
Verification Test ◽  
Radiant Temperature ◽  
Fire Spreading

A simulation of the uninsulated duct firestop system was performed to review its application possibility to the building by analyzing the radiant heat transfer and its impact to the surrounding structures in the event of a fire. The simulation targets were divided by the presence or absence of an insulation material (1 m long) as floor and wall duct firestop systems, and the surface temperature, which is an important input data, was based on the existing verification test data. Then, the ambient air temperature, surrounding structure surface temperature, and radiant temperature at some distance from the duct were analyzed through simulations in the modeled virtual spaces. As a result of simulation, it is judged that the application of the uninsulated duct firestop system wrapped by the insulation material can significantly reduce the possibility of fire spreading due to the radiant heat.

Small and Full-Scale Modeling for the Application of Wall Solar Chimneys

2016 ◽  
Author(s):  
David Park ◽  
Francine Battaglia
Keyword(s):  
Natural Ventilation ◽  
Velocity Ratio ◽  
Thermal Conditions ◽  
Ambient Air ◽  
Full Scale ◽  
Scale Model ◽  
Small Scale ◽  
Solar Chimney ◽  
Window Position ◽  
Pi Theorem

A solar chimney is a natural ventilation technique that has a potential to save energy consumption as well as to maintain the air quality in the building. However, studies of buildings are often challenging due to their large sizes. The objective of the current study was to determine relationships between small- and full-scale solar chimney system models. In the current work, computational fluid dynamics (CFD) was utilized to model different building sizes with a solar chimney system, where the computational model was validated with the experimental study of Mathur et al. The window, which controls entrainment of ambient air, was also studied to determine the effects of window position. Correlations for average velocity ratio and non-dimensional temperature were consistent regardless of window position. Buckingham pi theorem was employed to further non-dimensionalize the important variables. Regression analysis was conducted to develop a mathematical model to predict a relationship among all of the variables, where the model agreed well with simulation results with an error of 2.33%. The study demonstrated that the flow and thermal conditions in larger buildings can be predicted from the small-scale model.

Development of a Predictive Equation for Ventilation in a Wall-Solar Chimney System

2017 ◽  
Vol 139 (3) ◽  
Author(s):  
David Park ◽  
Francine Battaglia
Keyword(s):  
Mathematical Model ◽  
Relative Error ◽  
Natural Ventilation ◽  
Thermal Conditions ◽  
Ambient Air ◽  
Maximum Error ◽  
Scale Model ◽  
Small Scale ◽  
Maximum Relative Error ◽  
Solar Chimney

A solar chimney is a natural ventilation technique that has potential to save energy consumption as well as to maintain the air quality in a building. However, studies of buildings are often challenging due to their large sizes. The objective of this study was to determine the relationships between small- and full-scale solar chimney system models. Computational fluid dynamics (CFD) was employed to model different building sizes with a wall-solar chimney utilizing a validated model. The window, which controls entrainment of ambient air for ventilation, was also studied to determine the effects of window position. A set of nondimensional parameters were identified to describe the important features of the chimney configuration, window configuration, temperature changes, and solar radiation. Regression analysis was employed to develop a mathematical model to predict velocity and air changes per hour, where the model agreed well with CFD results yielding a maximum relative error of 1.2% and with experiments for a maximum error of 3.1%. Additional wall-solar chimney data were tested using the mathematical model based on random conditions (e.g., geometry, solar intensity), and the overall relative error was less than 6%. The study demonstrated that the flow and thermal conditions in larger buildings can be predicted from the small-scale model, and that the newly developed mathematical equation can be used to predict ventilation conditions for a wall-solar chimney.

Investigation of Wedge Probe Wall Proximity Effects: Part 2—Numerical and Analytical Modeling

1997 ◽  
Vol 119 (3) ◽  
pp. 605-611 ◽  
Author(s):  
P. D. Smout ◽  
P. C. Ivey
Keyword(s):  
Large Scale ◽  
Three Dimensional ◽  
Outer Wall ◽  
Aerodynamic Characteristics ◽  
Proximity Effects ◽  
Wake Flow ◽  
Scale Model ◽  
Flow Structures ◽  
Flow Features ◽  
Wall Proximity

An experimental study of wedge probe wall proximity effects is described in Part 1 of this paper. Actual size and large-scale model probes were tested to understand the mechanisms responsible for this effect, by which free-stream pressure near the outer wall of a turbomachine may be overindicated by up to 20 percent dynamic head. CFD calculations of the flow over two-dimensional wedge shapes and a three-dimensional wedge probe were made in support of the experiments, and are reported in this paper. Key flow structures in the probe wake were identified that control the pressures indicated by the probe in a given environment. It is shown that probe aerodynamic characteristics will change if the wake flow structures are modified, for example by traversing close to the wall, or by calibrating the probe in an open jet rather than in a closed section wind tunnel. A simple analytical model of the probe local flows was derived from the CFD results. It is shown by comparison with experiment that this model captures the dominant flow features.

Investigation of Wedge Probe Wall Proximity Effects: Part 2 — Numerical and Analytical Modelling

1996 ◽  
Author(s):  
Peter D. Smout ◽  
Paul C. Ivey
Keyword(s):  
Large Scale ◽  
Three Dimensional ◽  
Outer Wall ◽  
Aerodynamic Characteristics ◽  
Proximity Effects ◽  
Wake Flow ◽  
Scale Model ◽  
Flow Structures ◽  
Flow Features ◽  
Wall Proximity

An experimental study of wedge probe wall proximity effects is described in Part 1 of this paper. Actual size and large scale model probes were tested to understand the mechanisms responsible for this effect, by which free stream pressure near the outer wall of a turbomachine may be over indicated by upto 20% dynamic head. CFD calculations of the flow over two-dimensional wedge shapes and a three-dimensional wedge probe were made in support of the experiments, and are reported in this paper. Key flow structures in the probe wake were identified which control the pressures indicated by the probe in a given environment. It is shown that probe aerodynamic characteristics will change if the wake flow structures are modified, for example by traversing close to the wall, or by calibrating the probe in an open jet rather than in a closed section wind tunnel. A simple analytical model of the probe local flows was derived from the CFD results. It is shown by comparison with experiment that this model captures the dominant flow features.

scholarly journals Wind Modifications to Density-Driven Flows in Semienclosed, Rotating Basins

2010 ◽  
Vol 40 (7) ◽  
pp. 1473-1487 ◽  
Author(s):  
Cristóbal Reyes-Hernández ◽  
Arnoldo Valle-Levinson
Keyword(s):  
Pressure Gradient ◽  
Wind Stress ◽  
Flow Structures ◽  
Pressure Gradients ◽  
Coriolis Parameter ◽  
Maximum Water ◽  
The Right ◽  
Stress Variations ◽  
Wedderburn Number ◽  
Lateral Structure

Abstract An analytical two-dimensional model is used to describe wind-induced modifications to density-driven flows in a semienclosed rotating basin. Wind stress variations produce enhancement, inversion, or damping of density-driven flows by altering the barotropic and baroclinic pressure gradients and by momentum transfer from wind drag. The vertical structure of wind-induced flows depends on αH, the nondimensional surface trapping layer, where α is the inverse of the Ekman layer depth d and H is the maximum water depth. For αH > 5 wind-driven flow structures are similar to the Ekman spiral; for αH < 2 wind-driven flows are unidirectional with depth. The relative importance of density to wind forcing is evaluated with the Wedderburn number W = τ−1ρH2D, which depends on water density ρ, mean depth H, a proxy of the baroclinic pressure gradient D, and wind stress τ. Because D depends on α and therefore on the eddy viscosity of water Az, wind speed and Az both modify W. Moreover, wind direction alters W by modifying the pressure gradient through the sea surface slope. The effect of Az is also evaluated with the Ekman number E = Az/fH2, where f is the Coriolis parameter. The alterations of the density-driven flow by the wind-driven flow are explored in the E and W parameter space through examination of the lateral structure of the resulting exchange flows. Seaward winds and positive transverse winds (to the right facing up basin in the Northern Hemisphere) result in vertically sheared flow structures for most of the E versus W space. In contrast, landward winds and negative transverse winds (to the left facing up basin) result in unidirectional landward flows for most of the E versus W space. When compared to observed and numerically simulated flow structures, the results from the analytical model compare favorably in regard to the main features.

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