Dissertations / Theses on the topic 'Airflow'

Insects have evolved very elaborate sensing systems. The airflow sensors of crickets are among the most sensitive sensors in the animal world. The sensor comprises a thin hair, which protrudes from the surface of the cuticle and sits in a specialised socket. Its elliptical base is surrounded by a flexible joint membrane to allow the hair to rotate. The polarity of the base restricts the rotation of the hair to a preferred plane of movement. The results of the morphometric analysis of the airflow sensors presented in this study show that the maximal diameter of the hair is a strong predictor for the other parameters determining the sensors geometry, such as the hair length, its socket geometry, as well as the hair's elliptical base, which is otherwise hidden within the socket and out of view unless the hair and its base are removed.

This thesis investigates a mechanism for air flow limitation in children with croup. Croup is a common condition affecting many young children. Infection (usually viral) causes swelling of the mucosa in the subglottic region of the airway with consequent narrowing of the airway. Although researchers have investigated croup for the past sixty years, there is still very little information available on how croup affects air flow dynamics. The current theory assumes that the stenosis formed by croup in the subglottis of infants leads to a dynamic collapse of the extrathoracic trachea (Chernick, 1990). According to this literature, the dynamic collapse of the extrathoracic trachea will limit the inspiratory flow. It was believed that in severe cases of croup, the dynamic collapse may even temporarily block the airways. In order to investigate the mechanism for air flow limitation in croup the author used the intrathoracic pressure - flow traces from twenty patients with croup, four patients who had been intubated for croup and five normal subjects. Laryngeal X-rays from another twenty patients with croup were analysed as well as five videos, made during laryngoscopy, of the subglottic cross-sectional area of an additional five patients with croup requiring intubation. All data used in this project was collected by an experienced paediatrician from the Red Cross War Memorial Children's Hospital who is also the supervisor of this thesis. Both the video and the X-ray data showed that the dynamic collapse of the trachea contributes much less to airflow obstruction than the subglottic swelling itself. The hypothesis investigated in this thesis is that air flow becomes restricted due to wave speed limitation. According to the theory of wave speed limitation, an increase in driving pressure (the intrathoracic pressure) does not increase the flow if the speed of the air particles exceeds the wave speed. In our case the wave speed is the speed of sound within the lumen of the compliant, narrowed airway. In order to test that theory, it was necessary to obtain the flow, the driving pressure in the subglottis and the cross-sectional area of the subglottis of patients with croup. Unfortunately, the measurement of subglottal cross-sectional areas from videos made during laryngoscopies, proved to be impossible due to both ethical and practical constraints. The measurement of the subglottal cross-sectional areas from X-rays was also difficult in practice. Therefore, the cross-sectional area is calculated. The general orifice equation is modified m order to calculate the subglottal cross-sectional areas in patients with croup. Two methods are used to test the hypothesis of wave speed limitation: i) The wave speed limitation formula. The wave speed limitation formula directly calculates the maximum flow from the pressure - flow data. Hereafter the calculated maximum flow is compared with the measured flow. ii) A lumped component model. A nonlinear, lumped component model has been used to calculate the flow from the driving pressure (intrathoracic pressure). Flow is not limited in this model and an increase in driving pressure will result in a corresponding increase in flow. The flow which is calculated using this model has also been compared to the measured flow. It was found that, in children with croup, there is a good correlation (r=0.82) between calculated and measured values of maximum flow using the wave speed limitation model. The slope of the linear fit using a least square's approximation is 0.98 and this linear relationship is valid for a 0.05 level of significance for Conover's nonparametric test (Daniel and Terrell, 1989). The lumped component model was able to fit the inspiratory flow data with a small sum of square error in the case of both normal ((7.56 ± 0.86) · 10⁻⁹ (ml/s)²) and intubated patients ((3.2 ± 0.75)·10⁻⁹ (ml/s)²). However, the error rose dramatically in patients with croup ((2.04 ± 0.5) -10⁻⁸ (ml/s)²) thus indicating that the lumped component model is no longer valid in these patients. It is concluded that the measured flow velocities in patients with croup approach the calculated velocity of sound in the region of the subglottic swelling, and that the wave speed theory accurately describes the flow limitation. Further support of this is the fact that the lumped component model, which does not incorporate a flow limiting mechanism, breaks down in patients with croup.

Directional airflow has been utilized to enable targeted air conditioning in cars and airplanes for many years, where the occupants could adjust the direction of flow. In the building sector however, HVAC systems are usually equipped with stationary diffusors that can only supply the air either in the form of diffusion or with fixed direction to the room in which they have been installed. In the present thesis, the possibility of adopting directional airflow in lieu of the conventional uniform diffusors has been investigated. The potential benefits of such a modification in control capabilities of the HVAC system in terms of improvements in the overall occupant thermal comfort and energy consumption of the HVAC system have been investigated via a simulation study and an experimental study. In the simulation study, an average of 59% per cycle reduction was achieved in the energy consumption. The reduction in the required duration of airflow (proportional to energy consumption) in the experimental study was 64% per cycle. The feasibility of autonomous control of the directional airflow, has been studied in a simulation experiment by utilizing the Reinforcement Learning algorithm which is an artificial intelligence approach that facilitates autonomous control in unknown environments. In order to demonstrate the feasibility of enabling the existing HVAC systems to control the direction of airflow, a device (called active diffusor) was designed and prototyped. The active diffusor successfully replaced the existing uniform diffusor and was able to effectively target the occupant positions by accurately directing the airflow jet to the desired positions.
M.S.
The notion of adjustable direction of airflow has been used in the car industry and airplanes for decades, enabling the users to manually adjust the direction of airflow to their satisfaction. However, in the building the introduction of the incoming airflow to the environment of the room is achieved either by non-adjustable uniform diffusors, aiming to condition the air in the environment in a homogeneous manner. In the present thesis, the possibility of adopting directional airflow in place of the conventional uniform diffusors has been investigated. The potential benefits of such a modification in control capabilities of the HVAC system in terms of improvements in the overall occupant thermal comfort and energy consumption of the HVAC system have been investigated via a simulation study and an experimental study. In the simulation study, an average of 59% per cycle reduction was achieved in the energy consumption. The reduction in the required duration of airflow (proportional to energy consumption) in the experimental study was 64% per cycle on average. The feasibility of autonomous control of the directional airflow, has been studied in a simulation experiment by utilizing the Reinforcement Learning algorithm which is an artificial intelligence approach that facilitates autonomous control in unknown environments. In order to demonstrate the feasibility of enabling the existing HVAC systems to control the direction of airflow, a device (called active diffusor) was designed and prototyped. The active diffusor successfully replaced the existing uniform diffusor and was able to effectively target the occupant positions by accurately directing the airflow jet to the desired positions.

Aeolian dune interdunes have been relatively ignored when compared with the research attention on the morphodynamics of the dune bodies themselves. This neglect is in spite of the possible significance of interdune dynamics for the geomorphology of the sand dune system as a whole, especially with regard to dune spacing. This project involved the collection of geomorphologically relevant airflow data for four relatively simple transverse dune interdunes. The study locations were chosen in order to sample interdunes with different size and surface type characteristics, the dynamics of which were investigated for when incident flow was normal to the upwind crest. The findings confirm existing models of aeolian dune lee-side flow in terms of flow re-attachment length and recovery attributes. A consistent pattern of increasing near-surface velocity downwind of re-attachment provides a mechanism for interdunes as sand-free features. Where studies for comparison from other aeolian examples are limited, the field-measured turbulence shows the importance of the shear layer as a source of turbulence, and agrees with studies from subaqueous bedforms. The importance of shear stress variability and the possible contribution of turbulence structures to the maintenance of sediment transport at re-attachment where velocity and mean stress is low or negative is also emphasised. At the downwind edge of interdunes, the mean and turbulent velocity properties, and therefore morphodynamics, vary according to the interdune size. In this case, interdune length leads to greater recovery, and a balance exists in this region between the recovering flow at the surface, dissipating wake from above and the obstacle effect of the dune. The flow dynamics are characterised for the different types of interdune observed. Dynamics accordant with the flow response model are seen to characterise the interdune setting with the closest spacing. The occurrence of other “extended” aeolian interdunes with a length well over that for flow separation demanded the development of a new descriptive model to characterise the dynamics therein. In this model, the variation in near-surface flow allowed process zones to be identified through the interdune. The geomorphological significance of the processes dominating each zone are discussed and comparisons are made between the flow response case and the new interdune model from this study

This work comprises an investigation of airflow and transport in the human upper airways, which not only perform essential air conditioning physiological functions (heat and water exchange and primary filtration) but also house the olfactory receptors. The conflicting requirements posed by efficient air transit on the one hand and sampling for olfaction on the other renders the geometry of the upper airways complex. Knowledge of the geometry and flow conditions are primary requirements for understanding the physiological mechanics of the airways. This work describes the application of imaging and experimental measurement techniques to determine the variations in nasal airway geometry and the characteristics of nasal inspiratory flow. Whilst the results are relevant to a host of applications, the particular case of sinonasal ventilation well illustrates the interrelation between form, flow and function as well as motivating the development of improved techniques for clinical management. Specifically 3T MR imaging has been investigated as a means to define the anatomy in congested and decongested states. Results show very large changes in nasal airway calibre and moreover allow the variation in mucosal engorgement throughout the nasal cavity to be mapped. Highly time resolved hot wire measurements of inspiratory flow profiles revealed for the first time the rapid temporal development of inspiratory flow during normal inspiration and dramatically so during sniffing. Variations in flow profile were recorded across a cohort of subjects for conditions of normal inspiration, sniffing and smelling. Sinonasal gas exchange is of particular interest given the common occurrence of sinus pathologies. Here short half-life Krypton imaging has been used to investigate gas exchange between the maxillary sinus and the nasal cavity. It has been shown that the technique can provide quantitative assessment of volume flow rate in a model, demonstrating the rapid venting associated with an accessory ostium.

The mechanics of airflow in the large airways during inspiration affects important physiological functions such as ventilation, olfaction, heat exchange and mass transfer. The behaviour of the airflow is important not only for healthcare applications including diagnosis, intervention planning and assessment, but for inhalation toxicology. This research aims to further the understanding of human nasal physiology through computational modelling. Specifically, the effects of transient inhalation conditions on flow dynamics and transport were characterised and the changes in flow behaviour in response to certain pathologies quantified. The key findings can be summarised as follows: Firstly, the time scales for airflow in the large airways have been identified and the initial flow patterns revealed. Three phases in the temporal behaviour of the flow were identified (flow initiation, quasi-equilibrium and decay). The duration of each phase differs depending on the quantity of interest. Flow in the nose was characterised as transitional, whilst in parts of the descending airways it is turbulent, particularly in the faster moving regions around the jets which may occur in the pharynx, larynx and at the superior end of the trachea. The bulk of the flow is biased to fill only certain regions of the airways, whilst other regions carry little flow, due to features upstream. Analysis of cross-sectional images provided by medical imaging does not necessarily provide a representative view of the area available to the flow. Various scalar species were employed to represent the fate of nanoparticles and gaseous species within the airways. Only species with high diffusion rates exhibited significant absorption at the airway walls. Airway pathologies often cause changes to the geometry of the airway. One such pathology, the goitre, was found to curve the trachea and in some cases cause constriction. Both these geometric changes were found to increase the pressure loss and energy required to drive flow through the trachea. Furthermore, the flow in pathological cases was more disturbed. High resolution simulations have been used to address these topics and the scales simulated have been analysed in terms of the smallest features possible in the flow to determine their fidelity.

Though heating, insulation, wall claddings and cavity-wall construction are considered as measures for remediating moisture and condensation in buildings, ventilation of wall cavities has however become a mantra among architects and other building professionals. Holes of any size and shape are made and located on building facades based on the accepted wisdom that a little air movement will keep the wall cavities dry. Whilst ventilation has been found to be successful in the control of moisture and condensation in rooms and larger enclosures, there is however insufficient understanding of how it works in thin spaces with high aspect ratios, such as the wall cavities studied in this thesis.In order to put in place good control and management practices in the remediation of moisture and condensation in vertical wall cavities by natural ventilation, it is vital to understand the dynamics of airflow in these cavities. In this thesis therefore, different size and shape of slots were employed to numerically investigate the effects of size, spacing and number of the slots on the characteristics of the velocity fields (patterns of airflow and distributions of velocity) in different cavity models. The Reynolds-Averaged-Navier-Stokes (RANS) methodology was employed to simulate the cavity flows under different modelling conditions using FLUENT. The BS 5925 model, an empirical relation for predicting ventilation rates in rooms and other larger enclosures, was employed and modified to predict ventilation rates in these cavities. Experimentally, the mapping of the airstreams in these cavities was obtained under similar reference (inlet) wind speeds employed for the numerical investigations.The results of this study show that there exists a potential at higher wind speeds for natural ventilation in the remediation of moisture and condensation in the cavities of vertical walls. The steady state approach employed in the RANS-based computation of cavity flows in this thesis averages out the peak values of air velocities and therefore gives no information about regions of maxima or minima velocity values even at higher wind speeds. This makes the predicted air change rates insensitive to the inlet air velocities from the ventilation slots and therefore makes the results more applicable for long term control and management of moisture in these cavities. In order to therefore put in place short, medium and long term plans for remediation of moisture in these wall cavities, a time-dependent computation is required. This will also allow the efficiency of the cavity ventilation to be properly assessed. Using the modified BS 5925 model, reasonable predictions were obtained for the air change rates of the wall cavities with the different size of ventilation slots employed. Close agreements are also obtained at lower and higher wind speeds between the predicted ventilation rates from the modified BS 5925 model and the experimental results employed as benchmark for validating the results.

Schlieren imaging is a non-invasive research tool that enables real-time visualization of airflow through refraction of light. Used predominantly in aerospace and ballistics research, its suitability for observing airflow in speech was proposed nearly 40 years ago. To date, this potential has been virtually unexplored. The following proof-of-concept study investigates the visual correlates of nasal versus non-nasal airflow to provide a preliminary demonstration of the tool’s ability to visualize aerodynamic events in speech. Simultaneous schlieren and audio recordings were made of three French nasal/non-nasal minimal pairs spoken by eight native-French speakers. These stimuli were presented to 10 raters in Video-only, Audio-only and Combined Audio-Visual formats. The raters coded each stimulus as either “nasal” or “not nasal”. Accurate designation of Video-only stimuli was significantly above chance response (p < .05), indicating that the difference between airflow for nasal and non-nasal sounds can be visualized and perceived through schlieren imaging alone. Non-significant improvements were observed over time in the Video-only condition. Differences between Combined Audio-Visual and Audio-only stimuli were non-significant and likely influenced by a ceiling effect for the auditory information presented in both conditions. Further research is needed with more difficult auditory-perceptual tasks to explore potential supplementary advantages of schlieren visual feedback alongside auditory ratings of resonance. Future research may also benefit from improved training procedures for schlieren imaging. Nonetheless, schlieren imaging has promising potential for future implementation in both speech research and clinical applications, particularly for speech resonance disorders.
Medicine, Faculty of
Audiology and Speech Sciences, School of
Graduate

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2012.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 185-191).
Naturally ventilated buildings can significantly reduce the required energy for cooling and ventilating buildings by drawing in outdoor air using non-mechanical forces. Buoyancy-driven systems are common in naturally ventilated commercial buildings because of their reliable performance in multi-story buildings. Such systems rely on atria or ventilation shafts to provide a pathway for air to rise through the building. Although numerous modeling techniques are used to simulate naturally ventilated buildings, airflow network tools (AFNs) are most commonly used for annual simulations. These AFNs, however, assume minimal momentum within each zone, which is a reasonable approximation in large atria, but is inappropriate in smaller ventilation shafts. This thesis improves AFNs by accounting for momentum effects within ventilation shafts. These improvements are validated by Computation Fluid Dynamics (CFD) models that haven been validated by small scale and full scale experiments. The full scale experiment provides a detailed data set of an actual atrium that can be used in further validations and demonstrates the first use of a neutrally buoyant bubble generator for flow visualization and particle image velocimetry within a buoyancy driven naturally ventilated space. Small scale experiments and CFD simulations indicate an "ejector effect" within the shaft that uses momentum from lower floors to induce flow through upper floors. In some configurations, upper floors achieve higher flow rates than lower floors. Existing AFNs do not predict this "ejector effect" and are shown to significantly under predict flow rates through ventilation shafts by 30-40%. Momentum effects are accounted for in AFNs using empirical relationships for discharge coefficients. This approach maintains the current structure of AFNs while enhancing their ability to simulate airflow through ventilation shafts. These improvements are shown to account for the "ejector effect" and predict airflow rates that agree with CFD simulations to within 1-25%.
by Stephen Douglas Ray.
Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Architecture, 1998.
Includes bibliographical references (p. 157-165).
It is important to predict indoor environment in order to design thermally comfortable and healthy indoor spaces. Heating, Ventilating and Air Conditioning (HVAC) design engineers and architects widely use the Computational Fluid Dynamics (CFD) technique for indoor environment predictions. The CFD technique requires a turbulence model to correctly calculate indoor air distribution. However, the currently available turbulence models in the literature are either inaccurate or inefficient for the indoor environment predictions. To solve the problem, this thesis proposes two two-layer turbulence models and a zero-equation turbulence model. The two-layer models use a one-equation (k) model for the near wall region and the "standard" k -£ model in the outer region. The zero-equation model calculates turbulent viscosity based on local velocity and a length-scale. The near wall models have been developed with the aid of the data of natural and forced convection flows by Direct Numerical Simulation (DNS), while the zero-equation model has been proposed empirically. One of the two-layer turbulence models is used for predicting natural convection in rooms. The other two-layer model and the zero-equation model can be used to predict forced, natural, and mixed convection in rooms. These three new models have been applied to predict different types of indoor airflows. The corresponding DNS or experimental data were used to validate the models. This study concludes that the two-layer models can predict airflows most accurately, better than many k -E models. The computing cost is significantly lower than that of the low Reynolds number k-E models and is only slightly higher than that of the "standard" k-E models. The zero-equation model is at least ten times faster than the "standard" k-E model and it is numerically stable and can predict indoor airflow with acceptable accuracy.
by Weiran Xu.
Ph.D.

Thesis (Ph.D.)--City University of Hong Kong, 2009.
"Submitted to Department of Building and Construction in partial fulfillment of the requirements for the degree of Doctor of Philosophy." Includes bibliographical references (leaves 217-263)

Thesis: M. Eng., Massachusetts Institute of Technology, Department of Civil and Environmental Engineering, 2014.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 82-84).
Vapor intrusion is the vapor-phase migration of volatile organic compounds (VOCs) into buildings due to subsurface soil or groundwater contamination. Oxygen replenishment rates beneath a building are significant for quantifying potential contaminant degradation rates within the vadose zone. Additionally, the migration of VOC soil gas vapors into buildings is partly due to pressure differences between the building and the subsurface. This study addresses these issues through two laboratory scale experiments. The Wind Experiment quantifies oxygen replenishment rates as a function of above ground wind speed, while the Depressurization Experiment examines the flow rate of air into a model building as a function of decreased building pressure. For the Wind Experiment, tests were run for basement and slab-on-grade building configurations, as well as with and without a simulated sidewalk. Results show that increased above ground wind speed increases the oxygen replenishment rate and that the presence of a simulated sidewalk inhibits the oxygen replenishment rate. For the Depressurization Experiment, tests were again run for basement and slab-on-grade building configurations, as well as for two different foundation crack percentages. Results of the experiment indicate that increased building vacuum increases the flow rate of air into the building. In addition, basement configuration, increased foundation crack percentage, or some combination of the two results in increased airflow into the building. Additional research is needed for both experiments in order to obtain statistically significant results and resolve remaining uncertainties. Specific research needs include an improved wind source, additional monitoring locations, various sidewalk sizes and shapes, and different foundation crack configurations.
by Joseph William Corsello.
M. Eng.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2000.
Includes bibliographical references (p. 47).
Double-skin airflow facades have been in use for several years in European countries. They have yet to be used in the United States. One factor is that there is a lack of a heat transfer model which can accurately predict facade performance. A model of performance has been developed by Daniel Arons. The goal of the work presented is to verify the model with experimentation on a small-scale facade. Sunlight was simulated with a 400 W metal halide High Intensity Discharge lamp. Outdoor summer conditions were simulated with a residential space heater. Two 1/8" thick panes of uncoated window glass, separated by 7", with 1" white aluminum blinds in the center, made up the facade structure. Air was driven through the channel at velocities up to 0.7 mis. The results of experimentation validate the model when no light is being projected on the facade. More work needs to be done to refine the model for cases where there is little or no airflow and also when light is shining on the facade. More specifically, the interaction between incident radiation and blinds should be refined. The greatest discrepancy between model and experiment occurs for the surface temperature of glass on the heated side.
by Jui-Chen Chang.
S.B.

This dissertation examines the interaction between a series of progressive gravity waves and an overlying airflow. A critical review of the existing literature is made, from which it is concluded that although a large number of wind-wave interactions have been proposed, few have been verified experimentally. Furthermore, virtually no consideration has been given to the effect of the airflow on the underlying wave motion. A comprehensive experimental investigation of the interaction between wind and waves has therefore been undertaken. In respect of the airflow, a complete set of kinematics measurements, involving both horizontal and vertical velocity data, is presented. The mean, wave-induced and fluctuating velocities are presented in a wave-following frame of reference. The flow streamlines are deduced from the measured velocity data, and the structure of the airflow examined in detail. Analysis of the velocity profiles using boundary layer theory enables the effects of the flow structure identified above to be quantified and an assessment of the surfece stresses made. A corresponding set of kinematics measurements were undertaken in the underlying water flow. The wind-induced currents and wave kinematics are examined. Numerical models to predict the wave motion in combined wind-wave field are developed. The models incorporate the effects of a wind-induced current and the varying surface stresses, and allow the relative importance of each of these effects to be investigated. Furthermore, measurements of the water surface elevation are used to investigate the modification to the underlying long waves caused by the airflow, and the nature of the superposed wind-waves. The variation in the properties of the wind-generated waves with the phase of the long waves are also examined. Finally, the fundamental mechanisms of wind-wave interaction are considered, and the relative importance of the various mechanisms assessed. The implications for the oflfehore engineer, in terms of wind loading, gas ventilation and wave loading are outlined.

Energy usage in buildings has become a major topic of research in the past decade, driven by the increased cost of energy. Designing buildings to use less energy has become more important, and the ability to analyze buildings before construction can save money in design changes. Computational fluid dynamics (CFD) has been explored as a means of analyzing energy usage and thermal comfort in buildings. Existing research has been focused on simple buildings without much application to real buildings. The current study attempts to expand the research to entire buildings by modeling two existing buildings designed for energy efficient heating and cooling. The first is the Viipuri Municipal Library (Russia) and the second is the Margaret Esherick House (PA). The commercial code FLUENT is used to perform simulations to study the effect of varying atmospheric conditions and configurations of openings. Three heating simulations for the library showed only small difference in results with atmospheric condition or configuration changes. A colder atmospheric temperature led to colder temperatures in parts of the building. Moving the inlet only slightly changed the temperatures in parts of the building. The cooling simulations for the library had more drastic changes in the openings. All three cases showed the building cooled quickly, but the velocity in the building was above recommended ranges given by ASHRAE Standard 55. Two cooling simulations on the Esherick house differed only by the addition of a solar heat load. The case with the solar heat load showed slightly higher temperatures and less mixing within the house. The final simulation modeled a fire in two fireplaces in the house and showed stratified air with large temperature gradients.
Master of Science

To examine the influence of airway smooth muscle shortening and airway wall thickness on pulmonary resistance a model of the tracheobronchial tree has been developed which allows simulations of these mechanisms of airway narrowing. The model is based on both a symmetrical branching system proposed by Weibel (61) and an asymmetric branching scheme developed by Horsfield(21). Fluid mechanic equations proposed by Pedley (49) are used to calculate inspiratory resistance and to allow for changes in lung inflation the pressure area curves described by Lambert(32) are used. The model is easily implemented using a spreadsheet and personal computer which allows calculation of total and regional pulmonary resistance. At each generation or order in the model provision is made for the airway wall thickness, the maximal airway smooth muscle shortening achievable and an S shaped dose response relationship to describe the smooth muscle shortening. Measurements of airway wall thickness from 23 normal subjects whom died suddenly and 19 asthmatic subjects whom died of complications of their disease are related to internal airway perimeter using an iterative restricted maximum likelihood technique. Using the estimated relationship for airway wall thickness it is possible to partition the changes to central or peripheral airways. It is concluded that the model provides a realistic qualitative estimate of the tracheobronchial pressure drop that may provide valuable insights into the interaction of airway smooth muscle shortening and airway wall thickness as important contributors to airway hyperresponsiveness.
Graduate and Postdoctoral Studies
Graduate

Due to millions of years of evolution, sharks have evolved to become quick and efficient ocean apex predators. Shark skin is made up of millions of microscopic scales, or denticles, that are approximately 0.2 mm in size. Scales located on the shark’s body where separation control is paramount (such as behind the gills or the trailing edge of the pectoral fin) are capable of bristling. These scales are hypothesized to act as a flow control mechanism capable of being passively actuated by reversed flow. It is believed that shark scales are strategically sized to interact with the lower 5% of a boundary layer, where reversed flow occurs at the onset of boundary layer separation. Previous research has shown shark skin to be capable of controlling separation in water. This thesis aims to investigate the same passive flow control techniques in air.

To investigate this phenomenon, several sets of microflaps were designed and manufactured with a 3D printer. The microflaps were designed in both 2D (rectangular) and 3D (mirroring shark scale geometry) variants. These microflaps were placed in a low-speed wind tunnel in the lower 5% of the boundary layer. Solid fences and a flat plate diffuser with suction were placed in the tunnel to create different separated flow regions. A hot film probe was used to measure velocity magnitude in the streamwise plane of the separated regions. The results showed that low-speed airflow is capable of bristling objects in the boundary layer. When placed in a region of reverse flow, the microflaps were passively actuated. Microflaps fluctuated between bristled and flat states in reverse flow regions located close to the reattachment zone.

Data on the resistance of wood pellets to air flow are required for the design and control of ventilation and drying of bulk pellets in storage. In this study, pressure drops versus air flows were measured for several sizes of wood pellets, with diameter of 6 mm and lengths varying from 4 to 34 mm. Air flow rates ranging from 0.014 to 0.80 m s⁻¹ were used in the experiment. The maximum pressure drop measured was 2550 Pa m⁻¹. Three predictive models - Shedd, Hukill-Ives, and Ergun equations that relate pressure drop to air flow in bulk granular materials were used to analyze the data. The Ergun equation was found to provide the best fit to the data. Aeration of bulk pellets in storage requires a low airflow. The airflow range used for low permeability tests was from 0.0002 to 0.0220 m s⁻¹. The corresponding measured pressure drop ranged from 0.18 to 8.30 Pa m⁻¹ for low permeability tests. Three models were investigated and compared for the low permeability data. The increase in moisture content over a wider range of airflows (0.0042 to 0.7 148 m ⁻¹) showed a slight decrease in the resistance to airflow due to increased moisture content. Broken and fines are produced when pellets are handled. The resistance to air flow for wood pellets was measured in the presence of fine materials. Fines were defined as broken pellets passed a 4 mm sieve. The average geometric diameter of the fines was 0.75 mm. The pressure drop for pellets mixed with fines ranged from (2.0 to 191.2 Pa m⁻¹) and (7.9 to 1779.0 Pa m⁻¹) for 1% and 20% fines content (mass basis) respectively. Coefficients of Hukill and Ives’ equation for pellets were estimated as a function of percent fines content.

In recent years the drivage advance rates achieved within the UK coal industry have increased. In the 1980's average drivage rates were 35m per week compared to the 100/150m per week possible today. These extended rates of advance have resulted in an increase in the potential methane, dust and heat generation within the vicinity of the drivage face. In order to effectively disperse this additional pollutant load a controlled increase in air quantity is required. Although advance rates have changed, current auxiliary ventilation practice has not. UK mining law requires that the fresh air must be delivered to within 5m of the face. This has lead to the wide spread adoption of the use of overlap auxiliary systems within mechanised drivages, since a pure forcing system set at this distance from the face would lead to excessive airborne dust. UK mining law does not at present consider on-board mounted exhaust scrubber fans to constitute an effective overlap fan within mechanised drivages. Consequently an additional overlap exhaust fan is required to be installed within such drivages. In an attempt to determine whether working conditions could be safely and economically improved within mechanised rapid development drivages utilising an on-board mounted exhaust fan, a series of preliminary full scale gallery trials were conducted. A summary of the principal findings of these trials is presented together with an outline of a series of representative CFD simulations. This thesis examines the accuracy of CFD simulations for auxiliary ventilated headings. This is achieved by utilising Laser Doppler Anenometry (LDA) in a scale model representative of an underground heading and a detailed underground measurement programme conducted in production headings. These measured airflow values are then compared with representative CFD simulations and conclusions

This work aims to assess human nasal blockage by investigating its influence on nasal airflow dynamics, both computationally and experimentally. An in-house CFD code (Lithium) computes the steady (mean) nasal airflow for a cavity constructed from CT images of a healthy adult, for the internal cavity and for the first time for the external flow. To account for turbulence occurrence, the low Reynolds number k-ω Reynolds-Averaged-Navier-Stokes (RANS) model is used. The flow field is calculated at different breathing rates by varying the influx rate. Blockages are introduced at various locations inside the cavity to investigate common nasal blockages. The computational results are assessed against published literature and the Particle Image Velocimetry experimental (PIV) results, carried out on a 2.54:1 scale model of the computational nasal cavity. Schlieren optical technique is also used for external nasal airflow visualizations of a human subject, to comment on using an optical system for clinical application. These computations reveal a significant dependency of both, the internal and external nasal airflow fields on the nasal cavity’s geometry. Although for this model, the flow is found to be turbulent in the inspiratory phase of 200 ml/s and higher, it is suggested that the nature of flow can vary depending on the nasal cavity’s structure which is influenced by genetics. Nevertheless, some common flow features were revealed such as higher flow rate in the olfactory region and main flow passage through lower airways during inspiration. More uniform flow passage was found in expiration. The results also suggest a possible correlation between the internal geometry of the cavity and the external nasal airflow angle and thickness. This correlation can allow an application of optical systems such as Schlieren which is shown to give accurate qualitative images of the external nasal airflow for assessment of the nasal blockage.

Blowouts are erosional depressions that occur on pre-existing vegetated sand deposits. Their morphodynamic behaviour can reflect changes in anthropogenic activity, climatic conditions and animal behaviour; however patterns of deflation are poorly understood as near-surface airflow is complex. Previous research has indicated that flow is topographically manipulated as it moves through the landform, causing steering, reversal and jetting of the airflow. However, empirical data on these effects have been limited due to inadequacies in anemometer and sediment trap deployments. As a consequence knowledge of flow behaviour inside blowouts is limited to crude conceptual approaches. This study used ultrasonic three-dimensional anemometry (50 Hz) to validate a three dimensional computational fluid dynamics (CFD) model. This enabled a detailed description and explanation of near surface wind flow within a bowl blowout from a variety of wind direction and wind speeds up to hurricane force. Three-dimensional anemometry complemented by a CFD simulation was used to quantify wind conditions, whilst high resolution (25 Hz) electronic load cell traps and saltation impact responders, measured sediment transport within a trough blowout. The results demonstrate that considerable flow streamline compression, expansion, steering and reversal occur within trough and saucer blowouts. Airflow within blowouts alters with incident wind direction but does not change structurally with wind speed. Sediment transport flux and intermittency vary considerably within the landform, whilst the best correlation between wind flow and sediment transport varies between wind speed and turbulent kinetic energy (TKE). Where wind speed correlates best with TKE, the optimum averaging interval is much lower than those locations which correlate best with wind speed.

Thesis: S.M., Massachusetts Institute of Technology, Department of Architecture, 2016.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 98-105).
Bi-directional airflow in natural ventilation is an essential but not-well-understood scenario due to the complexity of airflow patterns as well as the strong coupling effect between temperature and ventilation. Neglecting bi-directional natural ventilation will result in problematic solutions and inaccuracy in estimation of ventilation performance. This work is focused on filling the knowledge gap by understanding the bi-directional airflow using computational fluid dynamics (CFD). Two important scenarios are simulated and analyzed: 1. Two-zone model with pure buoyancy forces, 2. Multi-zone model with combined wind and buoyancy forces. In the 1st model, a new concept of "local discharge coefficient" is proposed for its consistency under different boundary conditions. The influence of radiative heat transfer on simulation accuracy and ventilation performance is also investigated. In the 2nd model, the transient behaviors of airflow and the dynamics of wind and buoyant forces are analyzed and characterized. A new physical model is proposed based on simplified assumptions and nondimensionalization. This model is able to predicting the transient behavior of multi-zonal ventilation that involves bidirectional airflow patterns. The result of this study is to be integrated in CoolVent, the software designed by Building Technology Lab.
by Qin Zhang.
S.M.

In the current work the focus is on natural ventilation occurring in buildings. The first step is to discuss the potential of numerical models applied to determine building performance and air flow as part of a mixed mode building control scheme with respect to a test case. A dynamic simulation software (TRNSYS) is used to estimate the annual energy demand. An optimization program (GenOpt) changes iteratively the parameters regulating the airflow within the building model in order to minimize the whole year energy use. Elements which are considered in the analysis are outdoor climatic conditions and elements representing the building use, such as internal gains. The numerical results of this analysis have shown how a proper analysis of natural ventilation phenomena occurring in small premises can lead to energy saving while thermal comfort is not compromised. This has been tested in various Italian climates, by means of tuning different parameters handling the natural ventilation in a mixed mode office room. The natural ventilation to be effective has to be carefully designed and it may not just occur. On the other hand in large enclosures such as atria or churches, where much more complicated phenomena occur in the indoor air flow, the natural ventilation is not so straightforward to analyse. This fact pushes the application of numerical methods capable of higher resolution in order to catch the features of the streams, with the aim of achieving good levels of thermal comfort and indoor air quality. In particular, in the last years a growing attention has been paid on the analysis of building airflow, mostly due to the diffuse interest in reducing energy losses and optimizing the efficiency of heating systems. A nontrivial technical problem is the heating of churches, because nowadays they are used both for religious services and as cultural places. Such problem is still open and no solution has been so far found. In the course of the last century, with the installation of new heating systems, an increase in damage and the decay of valuable interior decoration have been noticed. Moreover in this kind of environments, because of the remarkable heights and the presence of great windows, relevant natural and mixed convection flows may show up and, on the other hand, stratification phenomena may occur, with hot air stagnation far from occupied zones. This can cause people discomfort or energy waste. Therefore it is not possible to design heating plants only to maximize energy efficiency, but the heating systems have to accomplish the best compromise between preservation of cultural property, economy, energy and comfort. For handling all these conflicting requirements at once, simplified or macroscopic models, which describe the real system only with a small number of temperature, pressure and flow rate values, are no more enough, but microclimate field flow models, based on computational fluid dynamics methods (CFD), provide a powerful and versatile tool to obtain a more reliable prediction of the air movement and temperature distribution within the built environment. The case of St. Marien’s church has been investigated to prove the utility of this kind of analysis. On the basis of experimental data collected during winter 2003-2004, a thermal model of St. Marien church has been produced and tuned. The results provided by this model have been used in this work to perform several simulations on St. Marien church with the commercial CFD software FLUENT, in order to find out a computational model which could be a good compromise between simplicity on geometrical representation, saving on computational resources, accuracy and reliability of the solution. Once the model was set up, it has been validated against some temperature values recorded during the monitoring period. Simulations have highlighted a shortcut of the flow from the inlet to the outlet, giving the reason why energy model of the church tuned on the experimental data are overestimating energy consumption. This would not have been possible with a macroscopic analysis, showing the need for large enclosure to carry on both the analyses together. This promising result pushed to test a more direct way to couple energy models and CFD models to predict energy consumptions in buildings or buildings components involving remarkable ventilation phenomena. It came out a method to estimate ventilated components performance on annual bases in a sensible amount of time. The method was applied to a ventilated roof. In this case the considered vented roof showed to increase energy saving with respect to the typical roof layout in climates with high solar radiation levels during a wide period of the year.
Nel lavoro svolto in questa tesi l'attenzione è rivolta ai fenomeni di ventilazione naturale che avvengono negli edifici. Il primo studio ha riguardato il potenziale della modellazione numerica relativamente alle prestazioni dell'edificio come parte di uno schema di controllo della ventilazione mista in riferimento a un caso test. A tal fine è stato utilizzato un software di simulazione dinamica (TRNSYS) per valutare il fabbisogno annuale di energia accoppiato a un programma di ottimizzazione (GenOpt) in grado di modificare i parametri di interesse in maniera iterativa; i risultati dell’analisi hanno permesso di individuare la combinazione di parametri per cui il consumo energetico è minimo su base annuale. Sono state considerate nell'analisi le condizioni climatiche e gli elementi rappresentativi del tipo di utilizzo dell'edificio come i carichi interni. Dai risultati numerici di questa analisi è stato possibile mostrare come un’analisi dettagliata della ventilazione naturale all’interno di ambienti di piccole dimensioni possa portare a un risparmio di energia senza compromettere il comfort termico. Questo è stato provato per diverse regioni climatiche, tarando i diversi parametri che gestiscono la ventilazione naturale nella stanza adibita a ufficio soggetta a ventilazione mista. Il lavoro ha dimostrato come la ventilazione naturale per essere efficiente debba essere pianificata a priori sia per definire le aperture sia per stabilirne la gestione. D'altro canto, in ambienti interni di grandi dimensioni come atri o chiese, dove avvengono fenomeni molto più complessi nei campi di moto dell'aria, la ventilazione naturale non è così semplice da analizzare. Questo fatto spinge all'adozione di metodi caratterizzati da una maggiore risoluzione al fine di meglio definire le caratteristiche del deflusso, quando si studiando intende studiare il comfort termico e la qualità dell'aria. In particolare, negli ultimi anni, c'è stata una crescente attenzione riguardo allo studio dei campi di moto dell'aria all’interno degli edifici, dovuto al diffuso interesse nel ridurre le perdite di energia e a incrementare l'efficienza dei sistemi di riscaldamento. Un problema tecnico non banale riguarda il riscaldamento delle chiese, dal momento che queste oggi sono sempre più utilizzate sia per funzioni religiose che come centri culturali. Tale questione è tuttora aperta, non avendo ancora trovato una soluzione definitiva. Nel corso dell'ultimo secolo, a seguito dell'installazione dei sistemi di riscaldamento, si è manifestato un aumento di danni delle opere d’arte e delle preziose decorazioni all’interno delle chiese storiche. Inoltre in ambienti di questo tipo, a causa dell’accentuato sviluppo verticale e della presenza di grandi finestrature, si possono verificare importanti fenomeni di convezione naturale o fenomeni di stratificazione in cui l'aria calda tende a ristagnare in regioni lontane dalla zona occupata. Le conseguenze possono essere discomfort termico o spreco di energia. Pertanto non è possibile progettare impianti di riscaldamento secondo metodologie standard, quanto dal momento che gli impianti di riscaldamento devono realizzare il miglior compromesso fra conservazione dei beni culturali, costi di esercizio e di manutenzione, risparmio energetico e comfort. Per gestire queste esigenze spesso contrastanti, i modelli macroscopici o ingegneristici che descrivono il sistema reale con un numero ridotto di valori di temperatura, pressione e portata di massa, non sono molto spesso adeguati, mentre la fluidodinamica numerica è uno strumento potente e versatile per ottenere una previsione più affidabile del moto dell'aria e dei campi termici che avvengono negli edifici. Dopo aver illustrato il problema del riscaldamento delle chiese storiche e i principi della CFD, si è condotta un'analisi dettagliata della chiesa di St.Marien a Wismar per dimostrare l'utilità di questi metodi per questo tipo di applicazioni. Sulla base dei dati sperimentali raccolti durante l'inverno 2003-2004, è stato realizzato e tarato un modello energetico dinamico della chiesa. I risultati forniti hanno permesso di stimare le condizioni al contorno per una serie di simulazioni della chiesa di St.Marien basate sul codice commerciale FLUENT, per identificare un modello numerico che potesse essere un buon compromesso fra semplicità del dominio spaziale di calcolo, risparmio di risorse di calcolo, accuratezza e affidabilità della soluzione. Una volta realizzato, il modello è stato validato con alcuni valori di temperatura registrati durante il periodo di monitoraggio. Le simulazioni hanno evidenziato la presenza un cortocircuito in corrispondenza a un fan-coil installato a pavimento. Questo non sarebbe stato possibile con un'analisi basata su modelli semplificati, indicando la necessità, per i grandi ambienti, di portare avanti insieme sia le analisi macroscopiche che quelle di dettaglio con metodi CFD. Questo risultato ha spinto a provare in modo più stretto di accoppiare modelli energetici e CFD al fine di predire le prestazioni energetiche degli edifici. E’ stato quindi prodotto un metodo per stimare le prestazioni di componenti ventilati dell'involucro edilizio su base annuale. Il metodo è stato testato su un tetto ventilato. Si è quindi potuto verificare il miglior comportamento energetico del tetto ventilato rispetto a una equivalente copertura tradizionale.

The original objective of this work, to improve the efficiency of a forward curved centrifugal fan, was redirected to the urgent solution of a serious impeller life problem arising under certain service conditions. Using a variety of experimental and theoretical techniques the flow pattern within the blade passage was analysed and the cause of the problem diagnosed. A new impeller has been designed and has been found to solve the service life problem while also yielding an improvement in efficiency. Because the project was carried out under the Total Technology programme the scope was widened to include consideration of the fan application in a suction roadsweeper : as a result of the wider technical and commercial investigation an opportunity has been identified for a new product offering benefits much greater than those sought within the scope of the original objective.

Thesis (M.S.) -- University of Maryland, College Park, 2004.
Thesis research directed by: Dept. of Mechanical Engineering. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

In this work a suggested method to reduce the energy consumption of the cooling system in a data center is modelled and evaluated. Introduced is different approaches to distributed airflow control, in which different amounts of airflow can be supplied in different parts of the data center (instead of an even airflow distribution). Two different kinds of distributed airflow control are compared to a traditional approach without airflow control. The difference between the two control approaches being the type of server rack used, either traditional ones or a new kind of rack with vertically placed servers. A model capable of describing the power consumption of the data center cooling system for these different approaches to airflow control was constructed. Based on the model, MATLAB simulations of three different server work load scenarios were then carried out. It was found that introducing distributed airflow control reduced the power consumption for all scenarios and that the control approach with the new kind of rack had the largest reduction. For this case the power consumption of the cooling system could be reduced to 60% - 69% of the initial consumption, depending on the workload scenario. Also examined was the effect on the data center of different parameters and process variables (parameters held fixed with the help of feedback loops), as well as optimal set point values.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2012.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 253-257).
Natural ventilation (NV) can considerably contribute to reducing the cooling energy consumption of a building and increase occupant productivity, if correctly implemented. Such energy savings depend on the number of hours that NV can maintain the indoor air temperature within comfort levels. CoolVent, currently the only simple multizone simulation tool that can adequately model the physics of NV in buildings, assumes a uniform temperature distribution in each room or zone. This temperature corresponds to the exhaust air temperature of each room using an energy balance. In reality, however, the air in a room is often thermally stratified, and the air temperature at occupant level can be significantly lower than the exhaust temperature. The final goal of this thesis was to develop a set of criteria to predict the vertical temperature profile in naturally ventilated rooms by comparing the strength of buoyant to inertial forces in the space, based on a few critical room airflow and physical parameters. Developing such criteria required conducting a thorough study of the physics of turbulent jets and plumes, their development in room-sized enclosures, and their effect on the airflow and heat transfer dynamics in a room. Additionally, it was necessary to investigate, using Computational Fluid Dynamics, the effect of certain parameters -such as radiative heat transfer, heat source distribution and room geometry, among others- on the physics of room airflow simulations. Results shed light on the complexity of modeling room airflow and thermal physics analytically, particularly when the air is thermally stratified. Thermal stratification predictions indicate that multi-zone models overestimate the air temperature at occupant height by up to 40% of the total room air temperature change. This work enhances the physical understanding of modeling critical elements of room airflow and improves the predictive accuracy of the natural ventilation potential in buildings. These contributions promote a wider use of passive cooling strategies, thereby increasing the energy efficiency of the built environment.
by Maria Alejandra Menchaca Brandan.
Ph.D.