Uniform Pressure Field Research Articles

Effects of non-uniform pore pressure field on hydraulic fracture propagation behaviors

Hydraulic fracturing is one of the key technologies to stimulate oil and gas production in low-porosity and low-permeability reservoirs. Thus, understanding the hydraulic fracture (HF) propagation behaviors is of great significance for optimizing the fracturing design. It is usually assumed that HF tends to take a straight path in a symmetric stress and uniform pore pressure field. However, much more complex stress and pore pressure distribution could be encountered during the fracturing treatment as a result of long-time fluid injection and production in the reservoir. In this paper, considering the arrangement of five-spot well pattern, the effects of unsymmetric stress and non-uniform pore pressure distribution on HF growth is investigated using the cohesive zone method (CZM) and extended finite element method (XFEM). The interesting results indicate that in-situ stress and pore pressure distribution are gradually altered due to fluid injection and production, which affects subsequent HF growth. And in the reservoir developed by five-spot well pattern, HF tends to initially propagate along a straight line with fracture growth rate gradually decreasing and then prefers to extend towards the nearby injection well with fracture growth rate gradually increasing when there exists an intersection angle (0°<θ<45°) between the direction of well arrays and minimum horizontal stress orientation. When θ equals 0° or 45°, HF will extend along a straight line without curving. The results also show that a larger water injection pressure leads to a greater fracture deviation angle, but a larger fracturing fluid injection rate and a larger water injection well distance results in a smaller fracture deviation angle. Moreover, the influence of fracturing fluid viscosity on fracture deviation angle can be ignored.

Dominant Synoptic Patterns and Their Relationships with PM2.5 Pollution in Winter over the Beijing-Tianjin-Hebei and Yangtze River Delta Regions in China

This paper concerns about the episodes of PM25 pollution that frequently occur in China in winter months. The severity of PM2.5 pollution is strongly dependent on the synoptic-scale atmospheric conditions. We combined PM2.5 concentration data and meteorological data with the Hybrid Single Particle Lagrangian Integrated Trajectory model (HYSPLIT4) to investigate the dominant synoptic patterns and their relationships with PM2.5 pollution over the Beijing-Tianjin-Hebei (BTH) and Yangtze River Delta (YRD) regions in the winters of 2014–17. The transport of PM2.5 from the BTH to YRD regions was examined by using cluster analysis and HYSPLIT4. It is found that the level of PM2.5 pollution over the BTH region was higher than that over the YRD region. The concentration of PM2.5 in the atmosphere was more closely related to meteorological factors over the BTH region. The episodes of PM2.5 pollution over the BTH region in winter were related to weather patterns such as the rear of a high-pressure system approaching the sea, a high-pressure field, a saddle pressure field, and the leading edge of a cold front. By contrast, PM2.5 pollution episodes in the YRD region in winter were mainly associated with the external transport of cold air, a high-pressure field, and a uniform pressure field. Cluster analysis shows that the trajectories of PM2.5 were significantly different under different weather patterns. PM2.5 would be transported from the BTH to the YRD within 48 h when the PM2.5 pollution episodes were associated with three different kinds of weather patterns: the rear of a high-pressure system approaching the sea, the high-pressure field, and the leading edge of a cold front over the BTH region. This suggests a possible method to predict PM2.5 pollution episodes based on synoptic-scale patterns.

Inter-annual variability in fine particulate matter pollution over China during 2013–2018: Role of meteorology

Based on the long-term observed fine particulate matter (PM2.5) data obtained from the Ministry of Environmental Protection of China, the present study focused on the analysis of annual and seasonal trends of PM2.5 over the polluted areas in China from 2013 to 2018. The MODIS-derived aerosol optical depth (AOD) data was used as an auxiliary data to supplement the analysis with surface measured PM2.5. The emission source inventory and meteorology datasets are also used and analyzed to examine the causes of changes in trends of PM2.5. The results revealed that the annually averaged values of AOD and surface PM2.5 concentrations over the high-polluted areas are decreasing year-by-year, with stable values found over the Pearl River Delta (PRD) region. The decreasing trends of PM2.5 concentration and fine particle pollution frequency over the Beijing, Tianjin, and Hebei (BTH) area are most obvious. Over the PRD region, PM2.5 concentration maintains below the annual air quality standard (35 μg/m3) with low fine particle pollution frequency. The decreasing trend of averaged PM2.5 in each region is closely related to the reduction of anthropogenic PM2.5 emissions. However, the fluctuated features in different season are mostly influenced by the meteorological conditions. Based on the weather type classifications observed from the COST733 model, the weather types under uniform pressure and eastern high pressure more likely resulted in the occurrence of fine particulate pollution over China. When the weather under the bottom with high pressure occurs, Northeast China (NEC) and BTH regions are most likely to be polluted, which is more likely to lead to heavy fine particle pollution events. The air pollution in the Yangtze River Delta (YRD) area is the most serious when the uniform pressure field weather occurs in winter.

Potential impacts of cold frontal passage on air quality over the Yangtze River Delta, China

Abstract. Cold frontal passages usually promote quick removal of atmospheric pollutants over North China (e.g. the Beijing–Tianjin–Hebei region). However, in the Yangtze River Delta (YRD), cold fronts may bring air pollutants from the polluted North China Plain (NCP), thereby deteriorating the air quality in the YRD. In this study, a cold frontal passage and a subsequent stable weather event over YRD during 21–26 January 2015 was investigated with in situ observations and Weather Research and Forecasting – Community Multiscale Air Quality Modeling System simulations. Observations showed a burst of PM2.5 pollution and an obvious southward motion of PM2.5 peaks on the afternoon of 21 January, suggesting a strong inflow of highly polluted air masses to YRD by a cold frontal passage. Model simulations revealed an existing warm and polluted air mass over YRD ahead of the frontal zone, which climbed to the free troposphere along the frontal surface as the cold front passed, increasing the PM2.5 concentration at high altitudes. Strong north-westerly frontal airflow transported particles from the highly polluted NCP to the YRD. As the frontal zone moved downstream of YRD, high pressure took control over the YRD, which resulted in a synoptic subsidence that trapped PM2.5 in the boundary layer. After the cold frontal episode, a uniform pressure field took control over the YRD. Locally emitted PM2.5 started to accumulate under the weak winds and stable atmosphere. Tagging of PM2.5 by geophysical regions showed that the PM2.5 contribution from the YRD itself was 35 % and the contribution from the NCP was 29 % during the cold frontal passage. However, under the subsequent stable weather conditions, the PM2.5 contribution from the YRD increased to 61.5 % and the contribution from the NCP decreased to 14.5 %. The results of this study indicate that cold fronts are potential carriers of atmospheric pollutants when there are strong air pollutant sources in upstream areas, which may deteriorate air quality in downstream regions.

A severe fog-haze episode in Beijing-Tianjin-Hebei region: Characteristics, sources and impacts of boundary layer structure

Beijing–Tianjin–Hebei (BTH) region is recognized as one of the most polluted regions in China. In this study, characteristics, synoptic condition, boundary layer structure, and sources of a severe fog–haze episode during the December of 2016 and January of 2017 are investigated. The results show that BTH region is controlled by the uniform pressure field and southwest wind, and the average mass concentrations of PM2.5 in Beijing (BJ), Tianjin (TJ), and Shijiazhuang (SJZ) are 263.5, 192.7, and 296.1 μg/m3, respectively. Besides, the maximum hourly concentration and growth rate of PM2.5 are 578.8 μg/m3 and 13.1 μg/(m3·h), respectively for BJ. According to the wind profiler radar and L–band radar data, the maximum concentration of PM2.5 in two stages basically coincides with the minimum ventilation index (VI) value, and the minimum VI value in stage I and stage II are 5124 m2/s and 4524 m2/s. Moreover, the two stages are accompanied by significant inversion layer in the low levels. Although the maximum temperature inversion (10.8 °C) in stage I is higher that in stage II (8.3 °C) in the boundary layer, the maximum inversion strength in stage II (0.33 °C/hPa) is stronger than that in stage I (0.20 °C/hPa). Besides, weighted potential source contribution function (WPSCF) and conditional probability function analysis(CPF) are applied to investigate the potential source regions of PM2.5, the results show that southern BJ, as well as central and southern Hebei province (HB) are the main potential sources, and CPF values from the wind direction (180°–270°) are all over 0.3.

Cause and Effect Evaluation of PM2.5 During Three Red Alerts in Beijing from 2015 to 2016

Beijing authorities issued three red alerts for heavy air pollution at 6 pm on December 7 and 7 am on December 18, 2015, and at 1 pm on December 15, 2016, respectively. To better understand the different causes of the three red alerts and assess the effectiveness of emergency measures dealing with heavy air pollution, the PM2.5 concentrations, meteorological conditions, weather conditions, and air mass transmission were analyzed during the red alerts using data from environmental and meteorological monitoring stations in Beijing. All three red alerts were affected by secondary transformation, but the high PM2.5 concentrations were mainly affected by meteorological conditions. During the first and second red alerts, the ground in Beijing was controlled by a uniform pressure field and the regional transport was mainly affected by southwestern and southern air masses. During the third red alert, the ground was under the control of a wide range of low pressure and affected by the superposition of southwestern and local air masses. During the third red alert, the PM2.5 pollution was the most serious. Its average concentration was the highest (273.6 μg·m-3), that is, 1.2 times and 1.3 times higher than that of the previous two alerts, respectively. The WRF-CMAQ model was used in combination with the emission reduction of each source to evaluate the effects of the emergency measures related to the third red alert. The results show that the average daily emission reduction of pollutants was 678.4 t, the average concentration of PM2.5 decreased by 79.1 μg·m-3, and the average reduction ratio was 26.9% after the emergency measures were implemented. The increase of coal-fires, traffic, and other sources control the emergency measures, early alert start-up time, and inter-region joint prevention and effectively mitigate the increasing PM2.5 concentration.

Characterization of individual particles and meteorological conditions during the cold season in Zhengzhou using a single particle aerosol mass spectrometer

To investigate the formation of haze during the cold season, continuous ambient air measurements were taken at an urban site in Zhengzhou from October 10, 2016 through December 31, 2016 using a single particle aerosol mass spectrometer. In total, 4,099,800 particles were analyzed and classified into eight major particle types: elemental carbon (EC, 36.7%), organic carbon (OC, 30.0%), ECOC (8.6%), K-rich (13.0%), levoglucosan (1.2%), metal (2.2%), NH4-K (2.1%), and dust (6.2%). By combining these measurements with correlation analysis and wind data, particle sources were determined to be vehicles, industrial emissions, coal combustion, biomass burning, secondary aerosols, agriculture, and dust. Additionally, analysis of mixing states indicated that particles underwent substantial aging and secondary OC particles were dominant OC species. Temporal profiles of meteorological parameters, mixing states, and particle types during a typical haze episode revealed that EC and OC particles were dominant components during haze formation, and a northeastern transport route (Anyang-Zhengzhou and Puyang-Xuchang) for OC particles was identified by potential source contribution function and concentration weighted trajectory analysis. Relatively higher humidity and lower temperature favored the formation of secondary inorganic aerosol. Wind direction and speed determined the transport, formation, and elimination of stagnant weather conditions.In sum, heavy haze during the cold season in Zhengzhou was observed due to extensive aerosol aging under adverse weather conditions (i.e., northeastern wind direction, wind speed <2 m s−1, temperature < 10 °C, relative humidity >60%, temperature inversion, and uniform pressure field).

Bianisotropic acoustic metasurfaces for wavefront manipulation

Materials with sub-wavelength asymmetry and long-range order have recently been shown to respond to acoustic waves in a manner that is analogous to electromagnetic biansiotropy [Sieck et al. Phys. Rev. B 96, 104303, (2017)]. A characteristic of these materials is the generation of dipolar scattering when subjected to a uniform time-varying pressure field. This behavior leads to a characteristic acoustic plane-wave impedance that has a preferential direction, known as a polarization, which results in direction-dependent magnitude and phase of an acoustic field scattered from bianisotropic acoustic media. These materials can therefore be used as acoustic metasurfaces to control the reflected or transmitted acoustic fields. This work presents the use of bianisotropic acoustic media for wavefront manipulation through analytical and finite element modeling and discusses potential acoustic analogues to existing bianisotropic electromagnetic metasurfaces. [This work was supported by ONR and the National Science Foundation.]

Chemical composition and sources of PM1 and PM2.5 in Beijing in autumn

Beijing, the capital of China, suffers from severe atmospheric aerosol pollution; nevertheless, a comprehensive study of the constituents and sources of PM1 is still lacking, and the differences between PM1 and PM2.5 are still unclear. In this study, an intensive observation was conducted to reveal the pollution characteristics of PM1 and PM2.5 in Beijing in autumn. Positive matrix factorization (PMF), backward trajectories and a potential source contribution function (PSCF) model were used to identify the source categories and source areas of PM1 and PM2.5. The results showed that the average concentrations of PM1 and PM2.5 reached 78.20μg/m3 and 95.47μg/m3 during the study period, respectively. PM1 contributed greatly to PM2.5. The PM1/PM2.5 value increased from 73.6% to 90.1% with PM1 concentration growing from <50μg/m3 to >150μg/m3. Higher secondary inorganic aerosol (SIA) proportions (31.3%–70.8%) were found in PM1. The higher fraction of SIA, OC, EC and typical elements in PM1 illustrated that anthropogenic components accumulated more in smaller size particles. Three typical weather patterns causing the heavy pollution in autumn were found as follows: (1) Siberian high and uniform high pressure field, (2) cold front and low-voltage system, and (3) uniform low pressure field. A PMF analysis indicated that secondary aerosols and coal combustion, vehicle, industry, biomass burning, and dust were the important sources of PM, accounting for 53.8%, 8.0%, 13.0%, 13.2% and 12.0% of PM1, respectively, and for 47.5%, 9.9%, 12.4%, 8.4% and 21.8% of PM2.5, respectively. The HYSPLIT and chemical components analysis indicated the potential contribution from biomass burning and fertilization ammonia emissions to PM1 in autumn. The source areas were similar for PM1 and PM1–2.5 under general polluted conditions, but during the heavily polluted periods, the source areas were distributed in farther regions from Beijing for PM1 than for PM1–2.5.

Impact of synoptic weather patterns on 24 h-average PM2.5 concentrations in the North China Plain during 2013–2017

North China Plain area (NCP) is one of the most densely populated and heavily polluted regions in the world. In the last five years, frequently happened fine particulate matter (PM2.5) serious pollution events were one of the top environmental concerns in China. As PM2.5 concentrations are highly influenced by synoptic flow patterns and local meteorological conditions, a two-stage hierarchical clustering method based on dynamic principal component analysis (DPCA) and standard k-means clustering algorithm was employed to classify synoptic wind fields into 6 patterns over the NCP area using the data of 5 PM2.5 seasons (Sept. 15th–Apr. 15th) from 2013 to 2017. Among the six identified synoptic patterns, pattern of uniform pressure field (U) and that of zonal high pressure (ZH) accounted for 78.21%, 65.55%, 63.56%, 57.11%, 59.13% and 58.27% studied heavy smog pollution events in Beijing, Tianjin, Tangshan, Baoding, Shijiazhuang and Xingtai city. The two particular patterns were associated with uniform pressure field and sparsely latitudinal isobar in 850 hPa level, respectively. They were also characterized by high relative humidity, low temperature, low-speed northerly wind in Tianjin and Tangshan, and southerly wind in the other cities. Under the continuous control of pattern ZH, the values of 24 h–average PM2.5 were found to increase at a rate of 31.78 μg/m3 per day. To evaluate the contribution of meteorological factors and precursors to PM2.5 levels, linear mixed-effects models (LMMs) were applied to establish relations among 24 h–average PM2.5 concentrations, concentrations of main precursors, local meteorological factors and synoptic patterns. Results show that the variations of precursors, local meteorological factors and synoptic flow patterns can explain 51.67%, 19.15% and 14.01% changes of the 24 h–average PM2.5 concentrations, respectively. This study illustrates that dense precursor emissions are still the main cause for heavy haze pollution events, although meteorological conditions play almost equal roles sometimes.

Tracking sensitive source areas of different weather pollution types using GRAPES-CUACE adjoint model

In this study, the sustained pollution processes were selected during which daily PM2.5 concentration exceeded 75 μg/m3 for three days continuously based on the hourly data of Beijing observation sites from July 2012 to December 2015. Using the China Meteorological Administration (CMA) MICAPS meteorological processing system, synoptic situation during PM2.5 pollution processes was classified into five weather types: low pressure and weak high pressure alternating control, weak high pressure, low pressure control, high rear, and uniform pressure field. Then, we chose the representative pollution cases corresponding to each type, adopted the GRAPES-CUACE adjoint model tracking the sensitive source areas of the five types, and analyzed the critical discharge periods of Beijing and neighboring provinces as well as their contribution to the PM2.5 peak concentration in Beijing. The results showed that the local source plays the main theme in the 30 h before the objective time, and prior to 72 h before the objective time contribution of local sources for the five pollution types are 37.5%, 25.0%, 39.4%, 31.2%, and 42.4%, respectively; the Hebei source contributes constantly in the 57 h ahead of the objective time with the contribution proportion ranging from 37% to 64%; the contribution period and rate of Tianjin and Shanxi sources are shorter and smaller. Based on the adjoint sensitivity analysis, we further discussed the effect of emission reduction control measures in different types, finding that the effect of local source reduction in the first 20 h of the objective time is better, and if the local source is reduced 50% within 72 h before the objective time, the decline rates of PM2.5 in the five types are 11.6%, 9.4%, 13.8%, 9.9% and 15.2% respectively. And the reduction effect of the neighboring sources is better within the 3–57 h before the objective time.

One-dimensional numerical simulations of heat transfer on the piston effect

Strong singularities in thermodynamic and transport properties appear in near-critical fluids, which give birth to the piston effect. In the present study, the piston effect is investigated numerically in a one-dimensional cavity filled with near-critical CO2, being heated at one side while the other side keeps at initial temperature. The thermodynamics model, which ignores velocity field and assumes uniform pressure field, is numerically solved by explicit finite difference method. The results are presented and discussed from two aspects. In the first part, the results show typical evolutions of temperature and density fields. The variations of temperature, density and pressure are results of competitions and eventual equilibrium between the heating piston effect and the cooling piston effect. In the second part, the results indicate influences of the critical proximity on the equilibrium. Results show that with the increase of critical proximity, piston effect gets weaker, equilibrium time gets longer, and equilibrium temperature becomes smaller.

Characteristics and classification of PM2.5 pollution episodes in Beijing from 2013 to 2015

During the period of 2013–2015, a total of 34 PM2.5 pollution episodes occurred in Beijing, each of which remained for at least 2days. Among that, 28 times occurred in winter half year with the average concentration of 243.1μg/m3 and summer half year with the average concentration of 194.1μg/m3. These episodes were mainly associated with lower wind speed and lower visibility as well as higher relative humidity, indicating that they belonged to heavy pollution under static stability. The PM2.5 pollution was classified into two categories according to the back trajectory analysis and meteorological background field. Category I, accounting for 22 times among all the pollution episodes, was due to air mass transport from Beijing's southern regions with north-south direction pressure gradient and sparse isopiestic. And category II was mainly led by northwestern air masses accompanied with a large area of uniform pressure field. Then, a typical case study was conducted for each category to recognize the sub-region contribution to Beijing's PM2.5 pollution based on WRF-CAMx modeling system, and the simulation results indicated that local emission source contribution decreased significantly during the accumulation phase for category I, but increased during that of category II, with an average contribution of 47.3% and 77.1% during the entire pollution period of each category, respectively. Two red alerts of air pollution occurred in December 2015 were also analyzed based on the episode classification. It was found that the second red alert pollution episode belonged to category II. The emission control measures in Beijing worked more obviously with the reduction effect ratio of 15.4% compared to the first red alert period (9.7%).

Source-receptor relationships for PM2.5 during typical pollution episodes in the Pearl River Delta city cluster, China

Located in the Southern China monsoon region, pollution days in Pearl River Delta (PRD) were classified into “Western type”, “Central type” or “Eastern type”, with a relative percentage of 67%, 24% and 9%, respectively. Using this classification system, three typical pollution events were selected for numerical simulations using the WRF-Chem model. The source sensitivity method for anthropogenic emissions of PM2.5 and its precursors was applied to identify the source-receptor relationships for PM2.5 among 9 cities in PRD. For “Western type” case, the PRD region was under control of a high-pressure system with easterly prevailing winds. The PM2.5 concentrations in the western PRD region were higher than those in the eastern region, with emissions from cities in the eastern PRD region having higher contributions. Within the PRD's urban cluster, PM2.5 in Huizhou, Dongguan and Shenzhen was mainly derived from local emissions, whereas the PM2.5 in the other cities was primarily derived from external transport. For “Eastern type” case, the PRD was influenced by Typhoon Soulik with westerly prevailing winds. Emissions from cities in the western PRD region had the highest impacts on the overall PM2.5 concentration. PM2.5 in Jiangmen and Foshan was primarily derived from local emissions. Regarding “Central type” case, the PRD region was under control of a uniform pressure field with low wind speed. PM2.5 concentrations of each city were primarily caused by local emissions. Overall, wind flows played a significant role in the transport and spatial distribution of PM2.5 across the PRD region. Ideally, local governments would be wise to establish joint prevention and control measures to reduce regional atmospheric pollution, especially for “Western type” pollution.

Impact of atmospheric circulations on aerosol distributions in autumn over eastern China: observational evidence

Abstract. Regional heavy pollution events in eastern China (110–122° E, 28–40° N) are causing serious environmental problems. In this study, the relationship between the degree of regional pollution and the patterns of large-scale atmospheric circulation over eastern China in October is investigated using 10-year (2001–2010) Terra/MODIS aerosol optical depth and NCEP reanalysis data by both case study and composite analysis. Eighteen polluted and 10 clean episodes are selected and categorised into six polluted types and three clean types respectively. Generally speaking, weather patterns such as a uniform surface pressure field in eastern China or a steady straight westerly in the middle troposphere, particularly when being at the rear of the anticyclone at 850 hPa, are typically responsible for heavy pollution events. Meanwhile, clean episodes occur when strong southeastward cold air advection prevails below the middle troposphere or air masses are transported from sea to land. Uniform descending motion prevails over the study region, trapping pollutants in the lower atmosphere. Therefore, the value of vertical velocity averaged from 1000 to 100 hPa and divergence of wind field in the lower troposphere are used in this study to quantify the diffusion conditions in each circulation type. The results reveal that it is often a clean episode when both the mean downward motion (larger than 2.56 × 10−2 Pa s−1) and the divergence of low-level winds (larger than 1.79 × 10−2 s−1) are strong. Otherwise, it is more likely to be a polluted episode.

Investigation on the effect of pore pressure gradient on fracture propagation in rock materials

When rock is subjected to internal hydraulic pressure and external mechanical loading, the fluid flow properties will be altered by newly induced fractures. In turn, the fluid flow driven by pore pressure gradient can influence the fracturing behaviour. To better capture the complex hydraulic fractures in rock materials, a ‘pinch-off’ breaking test is numerically conducted to illustrate the tensile failure of a rock specimen within a uniform pore pressure field. A double-notched specimen, with water pressure in one notch while keeping another one open to the atmosphere, is numerically extended to investigate how the water flow direction or the pore pressure gradient will influence the fracturing behaviour. The simulation results indicate that both pore pressure magnitude and the orientation and distribution of pore pressure gradient surrounding the fracture tip can affect fracturing process and macroscopic strength behaviour of rock materials.

Influence of pore pressure on tensile fracture growth in rocks: a new explanation based on numerical testing

The diffusion of pore fluid pressures may create both spatial and temporal effective stress gradients that influence or control the development and evolution of fractures within rock masses. To better understand the controls on fracturing behavior, numerical simulations are performed using a progressive fracture modeling approach that shares many of the same natural kinematic features in rocks, such as fracture growth, nucleation, and termination. First, the pinch-off breaking test is numerically performed to investigate the tensile failure of a rock specimen in a uniform pore pressure field. In this numerical simulation, both mechanical and hydrological properties of a suite of rocks are measured under simulated laboratory conditions. The complete tensional failure process of the rock specimen under pore pressure was reproduced. Second, a double-notched specimen is numerically extended to investigate how the water flow direction or pore pressure gradient influences the fracture growth. An exhaustive sensitivity study is conducted that examines the effects of varying both hydrological and mechanical boundary conditions. The simulation results indicate that local fluid pressure gradients strongly influence the state of stress in the solids and, thereby, fracture growth. Fracture and strength behavior is influenced not only by the pore pressure magnitude on a local scale around the fracture tip, but also by the orientation and distribution of pore pressure gradients on a global scale. Increasing the fracture growth rate increases the local model permeability and decreases the sample strength. The results of this study may provide useful information concerning the degree of hydrological and mechanical coupling action under geologic conditions.

An improved Doppler model for obtaining accurate maximum blood velocities

Maximum blood velocity estimates are frequently required in diagnostic applications, including carotid stenosis evaluation, arteriovenous fistula inspection, and maternal-fetal examinations. However, the currently used methods for ultrasound measurements are inaccurate and often rely on applying heuristic thresholds to a Doppler power spectrum. A new method that uses a mathematical model to predict the correct threshold that should be used for maximum velocity measurements has recently been introduced. Although it is a valuable and deterministic tool, this method is limited to parabolic flows insonated by uniform pressure fields. In this work, a more generalized technique that overcomes such limitations is presented. The new approach, which uses an extended Doppler spectrum model, has been implemented in an experimental set-up based on a linear array probe that transmits defocused steered waves. The improved model has been validated by Field II simulations and phantom experiments on tubes with diameters between 2mm and 8mm. Using the spectral threshold suggested by the new model significantly higher accuracy estimates of the peak velocity can be achieved than are now clinically attained, including for narrow beams and non-parabolic velocity profiles. In particular, an accuracy of +1.2±2.5cm/s has been obtained in phantom measurements for velocities ranging from 20 to 80cm/s. This result represents an improvement that can significantly affect the way maximum blood velocity is investigated today.

Nonlinear Buckling Analysis of Shallow Spherical Shell under Uniform Pressure and Temperature Field

Shallow spherical shell is an important kind of sensor elastic element. Because of the particular metal material texture, it can work under high temperature field. The nonlinear buckling problems of shallow spherical shell in load and temperature filed are very important. In this paper, nonlinear buckling of shallow spherical shell under uniform pressure and temperature field in simply supported boundary conditions is studied by the modified iteration method, and the analytic formulas for determining the critical buckling load are obtained. The effect of temperature field on critical buckling load and critical geometrical parameter are discussed.

酸化剤流旋回型ハイブリッドロケットエンジンのC*効率の評価方法について

The characteristic velocity C* is derived from the assumption of one-dimensional flow in a uniform pressure field, and is usually evaluated using a combustion chamber pressure measured on the chamber wall. In the swirling-oxidizer-flow-type hybrid rocket engines, the C* efficiency in excess of 1.0 was sometimes measured due to an increased wall pressure in the centrifugal force field of the swirling flow. It was induced that the specific impulse efficiency was useful to evaluate the C* efficiency accurately without using the combustion chamber pressure when the CF efficiency was given. This evaluation method of the C* efficiency was confirmed to be appropriate and available by the burning tests using the swirling-oxidizer-flow-type hybrid rocket engines with optimum expanding nozzles.