Low High Mode Research Articles

The Spatial Pattern and Influencing Factors of Traffic Dominance in Xi’an Metropolitan Area

Metropolitan areas shoulder the crucial task of regional and even national economic development. Analyzing the spatial patterns and influencing factors of metropolitan traffic dominance can provide a scientific basis for the optimization of its transportation and industrial layout, which is conducive to the development of its economy. Previous studies on traffic dominance paid little attention to metropolitan areas and even less so to study these areas from the perspective of town units, even though these are the basic units that narrow the gap between urban and rural areas and thus achieve regional economic integration. The traffic dominance model can comprehensively and wholly reflect regional traffic conditions, due to its multidimensional characteristics (including traffic network density, traffic arterial influence and accessibility). Consequently, taking the Xi’an metropolitan area as an exemplar and the town as the basic unit, this paper employs this model and other methods to study the spatial pattern and influencing factors of its traffic dominance. The results show the following: (1) the traffic network density, traffic arterial influence and accessibility had different distribution patterns; however, they were the same in that their superiorities were relatively high in the main urban area of Xi’an city or along and on both sides of the high-speed railways (HSRs), whereas relatively low in the peripheral areas; (2) the integrated traffic dominance consistently displayed a “point-axis” pattern, with greater superiority in the east–west axis areas within 30 km of Xi’an city, especially in the main urban area of Xi’an city; (3) the integrated traffic dominance between towns had stable agglomeration correlation in the global areas and formed three major modes in the local areas: high–high, low–low and low–high aggregation. High–high mode was concentrated in the main urban areas of Xi’an and Xianyang city, low–low mode was mainly distributed in the Weibei hilly and gully areas and the Qinling mountain areas and low–high mode always nested on the edge of high–high areas; and (4) the location, GDP and elevation had a greater impact, whereas construction intensity, population and slope had a relatively small influence on the town’s traffic dominance, and their influence ability had a decreasing trend from 2010 to 2022. Finally, this paper discusses the theoretical implications, practical values and some prospects based on the research results.

Physical simulation of waterflooding development in large-scale fractured-vuggy reservoir considering filling characteristics

The Fractured-vuggy reservoir is one of the world's most important reservoir types and has great exploration and development potential. However, due to the unique geological characteristics of the fractured-vuggy reservoir and limited understanding of its waterflooding development law, it is more difficult to develop. To strengthen the study of the oil-water flow mechanism and the waterflooding development law of fractured-vuggy reservoirs, we built a large-scale 3-D physical model that can reflect the geological morphology and structure of typical well groups. On the basis of the similarity theory, we determined the parameters of the model and its physical simulation. The physical model was applied to the experimental study of the waterflooding development process and the analysis of relevant influencing factors. The results show that the low-high mode can effectively touch the reserves in high parts, and the development effect is better than that of the high-low mode. There is an optimal water injection rate in waterflooding development to maximize the recovery. Weak bottom water energy at the bottom of the reservoir plays a positive role in waterflooding development, whereas strong bottom water energy increases the risk of premature water breakthrough, which is not conducive to waterflooding development. The substantial difference in oil and water viscosities makes viscous fingering more obvious, which reduces the stability of the displacement process and reduces the efficiency of waterflooding development. This study may provide strong guidance to further understand the oil-water flow law and improve waterflooding development efficiency for fractured-vuggy reservoirs.

Peering at the outflow mechanisms in the transitional pulsar PSR J1023+0038: simultaneous VLT, XMM-Newton, and Swift high-time resolution observations

We report on a simultaneous near-infrared, optical, and X-ray campaign performed in 2017 with the XMM-Newton and Swift satellites and the HAWK-I instrument mounted on the Very Large Telescope (VLT) on the transitional millisecond pulsar PSR J1023+0038. Near-infrared observations were performed in fast-photometric mode (0.5 s exposure time) in order to detect any fast variation of the flux and correlate this with the optical and X-ray light curves. The optical light curve shows the typical sinusoidal modulation at the system orbital period (4.75 h). No significant flaring or flickering is found in the optical, nor any signs of transitions between active and passive states. On the contrary, the near-infrared light curve displays a bimodal behaviour, showing strong flares in the first part of the curve, and an almost flat trend in the rest. The X-ray light curves instead show a few low-high mode transitions, but no flaring activity is detected. Interestingly, one of the low-high mode transitions occurs at the same time as the emission of an infrared flare. This can be interpreted in terms of the emission of an outflow or a jet: the infrared flare could be due to the evolving spectrum of the jet, which possesses a break frequency that moves from higher (near-infrared) to lower (radio) frequencies after the launching, which has to occur at the low-high mode transition. We also present the cross-correlation function between the optical and near-infrared curves. The near.infrared curve is bimodal, therefore we divided it into two parts (flaring and quiet). While the cross-correlation function of the quiet part is found to be flat, the function that refers to the flaring part shows a narrow peak at ∼10 s, which indicates a delay of the near-infrared emission with respect to the optical. This lag can be interpreted as reprocessing of the optical emission at the light cylinder radius with a stream of matter spiraling around the system due to a phase of radio ejection. This strongly supports a different origin of the infrared flares that are observed for PSR J1023+0038 with respect to the optical and X-ray flaring activity that has been reported in other works on the same source.

Experimental study on waterflood development in large–scale karst structures

On the basis of the seismic data volume of a certain well group unit of the Tahe fracture–vuggy reservoirs, the similarity theory is adopted, and a numerical model with geometric, kinematic, dynamic, and fracture–cavity characteristic parameter similarities is designed. Moreover, a large–scale three–dimensional physical karst cave model is built with the carving technology. The model is used in the experimental study of two–phase flow of oil/water during waterflood development and in the analysis of relevant influencing factors. Results show that the displacement efficiency of low–high mode is obviously superior to the case of high–low mode. A high crude oil viscosity is found to equate to a low ultimate recovery. High–velocity water flooding improves ultimate recovery but results earlier water breakthrough. Well shutdown improves recovery factor and well shutdown during middle water cut stage achieves optimum effects. Bottom water energy benefits oil reservoir exploitation to some degree.

On transport barriers and low–high mode transitions

Transport barriers and transitions between modes of low and high confinement in tokamak plasmas are often attributed to suppression of turbulence by a shear flow related to a plasma gradient, e.g., of density. However, such shear flow is also affected by the second derivative of density. When this is introduced there is no unique relation between flux and gradient—it depends on the source distribution within the plasma and on conditions at the plasma edge (e.g., imposed by the scrape-off layer). This edge gradient must lie within prescribed limits if a stationary plasma profile (which may include an improved confinement zone) is to exist.