A Physical Interpretation of Stagnation Pressure and Enthalpy Changes in Unsteady Flow

This paper provides a physical interpretation of the mechanism of stagnation enthalpy and stagnation pressure changes in turbomachines due to unsteady flow, the agency for all work transfer between a turbomachine and an inviscid fluid. Examples are first given to illustrate the direct link between the time variation of static pressure seen by a given fluid particle and the rate of change of stagnation enthalpy for that particle. These include absolute stagnation temperature rises in turbine rotor tip leakage flow, wake transport through downstream blade rows, and effects of wake phasing on compressor work input. Fluid dynamic situations are then constructed to explain the effect of unsteadiness, including a physical interpretation of how stagnation pressure variations are created by temporal variations in static pressure; in this it is shown that the unsteady static pressure plays the role of a time-dependent body force potential. It is further shown that when the unsteadiness is due to a spatial nonuniformity translating at constant speed, as in a turbomachine, the unsteady pressure variation can be viewed as a local power input per unit mass from this body force to the fluid particle instantaneously at that point. © 2012 American Society of Mechanical Engineers.

Choosing to make an effort: the role of striatum in signaling physical effort of a chosen action.

The possibility that we will have to invest effort influences our future choice behavior. Indeed deciding whether an action is actually worth taking is a key element in the expression of human apathy or inertia. There is a well developed literature on brain activity related to the anticipation of effort, but how effort affects actual choice is less well understood. Furthermore, prior work is largely restricted to mental as opposed to physical effort or has confounded temporal with effortful costs. Here we investigated choice behavior and brain activity, using functional magnetic resonance imaging, in a study where healthy participants are required to make decisions between effortful gripping, where the factors of force (high and low) and reward (high and low) were varied, and a choice of merely holding a grip device for minimal monetary reward. Behaviorally, we show that force level influences the likelihood of choosing an effortful grip. We observed greater activity in the putamen when participants opt to grip an option with low effort compared with when they opt to grip an option with high effort. The results suggest that, over and above a nonspecific role in movement anticipation and salience, the putamen plays a crucial role in computations for choice that involves effort costs.

CdS/CdSe lateral heterostructure nanobelts by a two-step physical vapor transport method.

The two-dimensional heterostructure nanobelts with a central CdSe region and lateral CdS structures are synthesized by a two-step physical vapor transport method. The large growth rate difference between lateral CdS structures on both +/- (0001) sides of the CdSe region is found. The growth anisotropy is discussed in terms of the polar nature of the side +/- (0001) surfaces of CdSe. High-resolution transmission electron microscopy reveals the CdSe central region covered with non-uniform CdS layer/islands. From micro-photoluminescence measurements, a systematic blueshift of emission energy from the central CdSe region in accordance with the increase of lateral CdS growth temperature is observed. This result indicates that the intermixing rate in the CdSe region with CdS increases with the increase of lateral CdS growth temperature. In conventional CdSSe ternary nanostructures, morphology and emission wavelength were correlated parameters. However, the morphology and emission wavelength are independently controllable in the CdS/CdSe lateral heterostructure nanobelts. This structure is attractive for applications in visible optoelectronic devices.

Effects of the biodiesel fuel physical properties on the swirl stabilised spray combustion characteristics

An increasin g interest in biofuel applications in modern engines requires a better understanding of biodiesel combustion behaviour. Many numerical studies have been carried out on unsteady combustion of biodiesel in situations similar to diesel engines, but very few studies have been done on the steady combustion of biodiesel in situations similar to a gas turbine combustor environment. The study of biodiesel spray combustion in gas turbine applications is of special interest due to the possible use of biodiesel in the power generation and aviation industries. In modelling spray combustion, an accurate representation of the physical properties of the fuel is a first important step, since spray formation is largely influenced by fuel properties such as viscosity, density, surface tension and vapour pressure. In the present work, a calculated biodiesel properties database based on the measured composition of Fatty Acid Methyl Esters (FAME) has been implemented in a multi-dimensional Computational Fluid Dynamics (CFD) spray simulation code. Simulations of non-reacting and reacting atmospheric-pressure sprays of both diesel and biodiesel have been carried out using a spray burner configuration for which experimental data is available. A pre-defined droplet size probability density function (pdf) has been implemented together with droplet dynamics based on phase Doppler anemometry (PDA) measurements in the near-nozzle region. The gas phase boundary condition for the reacting spray cases is similar to that of the experiment which employs a plain air-blast atomiser and a straight-vane axial swirler for flame stabilisation. A reaction mechanism for heptane has been used to represent the chemistry for both diesel and biodiesel. Simulated flame heights, spray characteristics and gas phase velocities have been found to compare well with the experimental results. In the reacting spray cases, biodiesel shows a smaller mean droplet size compared to that of diesel at a constant fuel mass flow rate. A lack of sensitivity towards different fuel properties has been observed based on the non-reacting spray simulations, which indicates a need for improved models of secondary breakup. By comparing the results of the non-reacting and reacting spray simulations, an improvement in the complexity of the physical modelling is achieved which is necessary in the understanding of the complex physical processes involved in spray combustion simulation. Copyright © 2012 SAE International.

Boundary conditions in physical model tests-the influence of deck pinning on the response of piled bridge abutments in laterally spreading soils

Physical models are widely used in the study of geotechnical earthquake engineering phenomena, and the comparison of modelling results to observations from field reconnaissance provides a transparent means of evaluating the design of our physical models. This paper compares centrifuge tests of pile groups in laterally spreading slopes with the response of piled bridge abutments in the 2011 Christchurch earthquake. We show that the model foundation's fixity conditions strongly affect the success with which the mechanism of response of the real abutments is replicated in the tests. © 2012 American Society of Civil Engineers.

Physical properties of a hybrid and a nanohybrid dental light-cured resin composite.

This work was aimed at the study of some physical properties of two current light-cured dental resin composites, Rok (hybrid) and Ice (nanohydrid). As filler they both contain strontium aluminosilicate particles, however, with different size distribution, 40 nm-2.5 mum for Rok and 10 nm-1 mum for Ice. The resin matrix of Rok consists of UDMA, that of Ice of UDMA, Bis-EMA and TEGDMA. Degree of conversion was determined by FT-IR analysis. The flexural strength and modulus were measured using a three-point bending set-up according to the ISO-4049 specification. Sorption, solubility and volumetric change were measured after storage of composites in water or ethanol/water (75 vol%) for 1 day, 7 or 30 days. Thermogravimetric analysis was performed in air and nitrogen atmosphere from 30 to 700 degrees C. Surface roughness and morphology of the composites was studied by atomic force microscopy (AFM). The degree of conversion was found to be 56.9% for Rok and 61.0% for Ice. The flexural strength of Rok does not significantly differ from that of Ice, while the flexural modulus of Rok is higher than that of Ice. The flexural strengths of Rok and Ice did not show any significant change after immersion in water or ethanol solution for 30 days. The flexural modulus of Rok and Ice did not show any significant change either after immersion in water for 30 days, while it decreased significantly, even after 1 day immersion, in ethanol solution. Ice sorbed a higher amount of water and ethanol solution than Rok and showed a higher volume increase. Thermogravimetric analysis showed that Rok contains about 80 wt% inorganic filler and Ice about 75 wt%.

Physical connection and disconnection control based on hot melt adhesives

Physical connection and disconnection control has practical meanings for robot applications. Compared to conventional connection mechanisms, bonding involving a thermal process could provide high connection strength, high repeatability, and power-free connection maintenance, etc. In terms of disconnection, an established bond can be easily weakened with a temperature rise of the material used to form the bond. Hot melt adhesives (HMAs) are such material that can form adhesive bonds with any solid surfaces through a thermally induced solidification process. This paper proposes a novel control method for automatic connection and disconnection based on HMAs. More specifically, mathematical models are first established to describe the flowing behavior of HMAs at higher temperatures, as well as the temperature-dependent strength of an established HMA bond. These models are then validated with a specific type of HMA in a minimalistic robot setup equipped with two mechatronic devices for automated material handling. The validated models are eventually used for determining open parameters in a feedback controller for the robot to perform a pick-and-place task. Through a series of trials with different wooden and aluminum parts, we evaluate the performance of the automatic connection and disconnection methods in terms of speed, energy consumption, and robustness. © 1996-2012 IEEE.