901 publications from this institution
NothingEconomies of scale moves unless it is driven. The environment resists the movement. The flier and the swimmer experience “drag”. The skin of the fish and the airplane fuselage experiences “friction,” and the impala and the...
This paper reports similarity solutions for vertical boundary layer natural convection near a solid wall adjacent to a fluid-saturated porous medium, in the case where the pore Reynolds number is high enough for the Darcy flow model to break down. Based on scaling arguments, it is shown that the departure from the Darcy flow model is dictated by the dimensionless number G = (v/K)(bgβΔT)− 1 2 , with G → 0 representing the high pore Reynolds number limit, and with G → ∞ representing the Darcy flow limit. Similarity solutions are reported for flow and heat transfer in the following cases: (1) the nonDarcy limit G = 0 near an isothermal wall and near a constant-heat-flux wall, and (2) the intermediate regime G = 0(1) for natural convection near an isothermal wall. It is shown that the heat transfer results differ fundamentally from those known from traditional Darcy flow studies.
This paper extends to electrical machines the thermodynamics and heat transfer optimization approach that has been developed for heat engines. Four models of thermodynamically irreversible electrical motors with heat transfer to the ambient are proposed and optimized: general reversible motor in series with its resistance, induction motor with transformer, synchronous motor with transformer and direct current motor. The conversion efficiency at maximum power is 1 2 . When, as in specific applications, the operating temperature of the windings must not exceed a specified level, the power output is lower and the efficiency higher. In the case of an electrical power generator it is shown that the generator and the heat engine that drives it can be optimized separately for maximum power.
No abstract is provided for this article.
A numerical study of two-dimensional natural convection in a horizontal water layer heated from below is reported. The density maximum associated with water at 3.98 °C and atmospheric pressure occurs inside the layer, as the top surface is maintained at 0 °C while the bottom surface temperature varies in the range 4 °C–10 °C. Three separate series of numerical simulations document the effect of Rayleigh number, bottom wall temperature and layer horizontal length on the flow pattern and on the net heat transfer rate vertically through the layer. These effects are documented numerically and graphically in the domain104<Ra<3×105, 1/6<H/L<1, where Ra is the Rayleigh number for a fluid with density maximum, and H/L is the (height/length) ratio of the water system selected for analysis. It is found that steady-state convection is present at Rayleigh numbers as low as 2.3×104 when TH =8 °C and H/L=0.5, and as low as 105 when TH =6.3 °C and H/L=0.5. These Rayleigh numbers agree very well with the corresponding critical values recommended by linear stability analysis.
This article reports a numerical study of the geometric minimization of the resistance to Darcy flow between a finite-size volume and one point. The volume is two dimensional and contains materials with several permeabilities. The optimization starts with the smallest volume subsystem, and proceeds toward larger subsystems (assemblies) until the given volume is covered. It is shown that at every scale the geometric shape of the subsystem can be optimized. This principle allows us to construct the volume-to-point flow path by using assemblies of previously optimized building blocks, hence the "constructal" name for the associated theory of shape and structure formation in natural flow systems. The optimized flow architecture is such that the regions of relatively high permeability form a tree network that is completely deterministic. Every feature of this architecture is the result of a single optimization principle: the geometric minimization of flow resistance subject to constraints.
No abstract is provided for this article.
This paper reports a series of experiments concerning the buckling of a slender fluid layer in a state of longitudinal compression. The experiments consist of floating a layer of highly viscous oil on a pool of water and, manually, compressing the layer from the side. Photographs of the buckled layer show conclusively that the buckling wavelength is largely insensitive to either the rate of compression or the viscosity of the fluid layer. The observations suggest that the buckling wavelength is actually a characteristic length scale (a property) of the fluid layer, in contrast with the buckling theory of purely viscous layers (Buckmaster, Nachman, and Ting, [7]) where the buckling wavelength remains to be determined randomly by initial disturbances.
Here, we show how the performance of a paddle that pushes a fluid can be increased by making parallel slits through it. The slit spacing is varied to see its effect on the drag force and the maximum stress in the paddle. The effect of water speed and paddle dimensions is documented. Designs with one or more slits are investigated. The drag force is maximum when the slit spacing matches the boundary layer thickness of the flow through the slit. Furthermore, the drag force is greater when the slit spacing is nonuniform: larger in the central slits than in the peripheral slits. The paddle with slits of one size performs almost as well as the best design with nonuniform spacings. The paddle design with slits achieves the same drag force and maximum stress with less material compared with a paddle without slits.
Scaling arguments and numerical analysis are used to document the transient and steady-state regimes of natural convection in a triangular porous layer cooled from above (along the sloping wall). The numerical simulations are conducted in the high Rayleigh number domain, Ra = 100, 1000, where Ra is the Darcy-modified Rayleigh number based on height, H. The scale analysis predicts the existence of distinct thermal boundary layers if Ra1/2 (H/L) > 1, where H/L is the height/length geometric ratio of the attic-shaped porous layer. The numerical simulations confirm the scaling results, as well as the prediction that the flow consists primarily of an elongated horizontal counterflow driven by the cold wall. In addition, the numerical solutions show the presence of a Be´nard-type instability at high enough Rayleigh numbers. For example, if H/L = 0.2, the instability is present when Ra > 620; this critical Rayleigh number is found to increase as H/L increases.
Heat transfer through a vertical skin surface covered with perpendicular hair strands of uniform density is investigated numerically. The heat transfer rate is the result of (1) direct heat transfer to the air that makes contact with the skin and (2) the heat conducted by each strand away from the skin. The hair strand and its surrounding air are not in local thermal equilibrium. Hair strands have the desirable effect of slowing the air that sweeps the vertical surface and the undesirable effect of acting as fins, thereby augmenting the overall heat transfer rate. Two distinct air flow models are considered: the Darcy model and the Forchheimer-Brinkman extended Darcy model. The overall heat transfer charts reported in this paper show that heat transfer rate can greatly exceed the estimate based on the traditional homogeneous porous medium model. By means of numerical examples, the Darcy model is shown to be adequate for modeling air flow through mammal hair.
The objective of the Journal of Mechanics is to provide an international forum to foster exchange of ideas among mechanics communities in different parts of world.
La loi Constructale est une loi de la physique qui presente la tendance naturelle de tout systeme d’ecoulement (anime comme inanime) a evoluer vers des configurations offrant progressivement un acces plus facile aux ecoulements dans le temps. Les domaines scientifiques couverts par la loi Constructale sont tres vastes. Ils concernent aussi bien les ecoulements de fluide que les transferts de chaleur ou de masse et font que la loi Constructale trouve parfaitement sa place dans la thermodynamique.
No abstract is provided for this article.
In this paper we determine the fundamental relation between global performance and flow configuration (constructal design) in the case of steam power generation with superheater and reheater placed in parallel in the same stream of hot gases of combustion. The superheater heats the steam for the high pressure turbine, and the reheater supplies steam to the low pressure turbine. Both turbines operate irreversibly. We consider superheaters and reheaters with balanced counter flows and unbalanced counter flows. The total heat transfer area (or the overall number of heat transfer units) is finite. We show that the heat transfer area can be allocated to the superheater and reheater such that the overall power output is maximum. The optimal area allocation ratio is reported for two scenarios: designs with and without a maximum allowable steam temperature.
List of Symbols Thermodynamics Concepts and Laws Definitions Closed Systems Open Systems The Momentum Theorem Useful Steps in Problem Solving The Temperature-Energy Interaction Diagram, and the Entropy Interaction-Energy Interaction Diagram Problems Entropy Generation and Exergy Destruction The Gouy-Stodola Theorem Systems Communicating with More than One Heat Reservoir Adiabatic Systems Exergy Analysis of Steady Flow Processes Exergy Analysis of Non-Flow Processes Characteristic Features of Irreversible Systems and Processes Problems Entropy Generation in Fluid Flow Relationship between Entropy Generation and Viscous Dissipation Laminar Flow Turbulent Flow The Transition Buckling Theory of Turbulent Flow Entropy Generation in Isothermal Turbulent Flow The Bernoulli Equation Entropy Generation in Heat Transfer The Local Rate of Entropy Generation in Convective Heat Transfer Fluid Friction vs. Heat Transfer Irreversibility Internal Flows External Flows Conduction Heat Transfer Convective Mass Transfer General Heat Exchanger Passage Heat Transfer Augmentation Techniques Problems Heat Exchangers Counterflow Heat Exchangers Heat Exchangers with Negligible Pressure Drop Irreversibility The Three-Part Structure of Heat Exchanger Irreversibility Two-Phase-Flow Heat Exchangers Other Heat Exchanger Entropy Generation Studies Distribution of Heat Exchanger Area on the Absolute Temperature Scale Distribution of Heat Transfer Area in Counterflow Heat Exchangers Problems Insulation Systems Power Plants and Refrigeration Plants as Insulation Systems The Generation of Entropy in an Insulation with Fixed Geometry Optimum Continuous Cooling Regime Counterflow Heat Exchangers as One-Dimensional Insulations Parallel Insulations Intermediate Cooling or Heating of Insulation Systems for Power and Refrigeration Plants Problems Storage Systems Sensible Heat Storage Optimum Heating and Cooling Processes Subject to Time Constraint Hot Storage vs. Cold Storage Latent Heat Storage Power Generation Model with Bypass Heat Leak and Two Finite-Size Heat Exchangers Power Plant Viewed as an Insulation Between Heat Source and Ambient Combined-Cycle Power Plant Optimal Combustion Chamber Temperature Other Power Plant Optimization Studies Why Maximum Power Means Minimum Entropy Generation Rate Maximum Power from Fluid Flow Problems Solar-Thermal Power Generation Models with Collector Heat Loss to the Ambient Collector-Ambient Heat Loss and Collector-Engine Heat Exchanger Collector-Ambient Heat Loss and Engine-Ambient Heat Exchanger Storage by Melting Extraterrestrial Solar Power Plant Nonisothermal Collectors Time-Varying Conditions Other Areas of Solar Power Conversion Study Problems Refrigeration Refrigeration Plant Model with Heat Transfer Irreversibilities Model with Heat Leak in Parallel with Reversible Compartment Model with Cold End Heat Exchanger and Room Temperature Heat Exchanger Minimization of the Heat-Leak Entropy Generation Problems Time-Dependent Operation Defrosting Refrigerators Cleaning the Heat Exchanger of a Power Plant Power Plants Driven by Heating from a Bed of Hot Dry Rock Maximum Rate of Ice Production Problems Appendices Local Entropy Generation Rate Variational Calculus Author Index Subject Index
Identification of complex clinical phenotypes among critically ill patients is a major challenge in clinical research. The overall research goal of our work is to develop automated approaches that accurately identify critical illness phenotypes to prevent the resource intensive manual abstraction approach. In this paper, we describe a text processing method that uses Natural Language Processing (NLP) and supervised text classification methods to identify patients who are positive for Acute Lung Injury (ALI) based on the information available in free-text chest x-ray reports. To increase the classification performance we enhanced the baseline unigram representation with bigram and trigram features, enriched the n-gram features with assertion analysis, and applied statistical feature selection. We used 10-fold cross validation for evaluation and our best performing classifier achieved 81.70% precision (positive predictive value), 75.59% recall (sensitivity), 78.53% f-score, 74.61% negative predictive value, 76.80% specificity in identifying patients with ALI.
