Optimal model parameters for multi-objective large-eddy simulations

A methodology is proposed for the assessment of error dynamics in large-eddy simulations. It is demonstrated that the optimization of model parameters with respect to one flow property can be obtained at the expense of the accuracy with which other flow properties are predicted. Therefore, an approach is introduced which allows to assess the total errors based on various flow properties simultaneously. We show that parameter settings exist, for which all monitored errors are "near optimal," and refer to such regions as "multi-objective optimal parameter regions." We focus on multi-objective errors that are obtained from weighted spectra, emphasizing both large- as well small-scale errors. These multi-objective optimal parameter regions depend strongly on the simulation Reynolds number and the resolution. At too coarse resolutions, no multi-objective optimal regions might exist as not all error-components might simultaneously be sufficiently small. The identification of multi-objective optimal parameter regions can be adopted to effectively compare different subgrid models. A comparison between large-eddy simulations using the Lilly-Smagorinsky model, the dynamic Smagorinsky model and a new Re-consistent eddy-viscosity model is made, which illustrates this. Based on the new methodology for error assessment the latter model is found to be the most accurate and robust among the selected subgrid models, in combination with the finite volume discretization used in the present study

In-Situ Thermal Remediation of Contaminated Soil

Recently, a method for removing contaminants from soil (several meters under the ground) has been proposed by McMillan-McGee Corp. The process can be described as follows. Over a period of several weeks, electrical energy is introduced to the contaminated soil using a multitude of finite length cylindrical electrodes. Current is forced to flow through the soil by the voltage differentials at the electrodes. Water is also pumped into the soil via the injection well and out of the ground at the extraction well. The soil is heated up by the electrical current and the contaminated liquids and vapours are produced at the extraction well. The temperature of the contaminated soil, during the process, is believed to reach the maximum value (the boiling temperature of water). Normally, the electrodes are placed around the contaminated site and the extraction well is located in the centre of the contaminated region. The distance between the electrodes is usually seven to eight meters. The distance between the extraction well and an electrode is about four meters. The diameter of the electrodes is 0.2 meter and the extraction well is 0.1 meter in diameter. The reason for using the electrical current is that “flushing” the soil using water alone is not effective for removing the contaminants. By heating up the soil and vaporizing the contaminated liquid, it is anticipated that rate of extraction will increase as long as the recondensation is not significant. A major concern, therefore, is whether recondensation will occur. Intuitively, one might speculate that liquid phase may dominate near the injection well. Moving away from the injection site towards the extraction well, due to the combined effects of lower pressure and higher temperature (from heating), phase change occurs and a mixture of vapour and liquid may co-exist. There may also be a vapour-only region, depending on the values of temperature, pressure, and other parameters. In the two-phase zone, since vapour bubbles tend to rise due to the buoyancy force, and the temperature decreases along the vertical path of the bubbles out of the heated region, it is possible that the bubbles will recondense before reaching the extraction well. As a consequence, the probability exists that part of the contaminants stay in the soil. Obviously, to predict transition between single-phase and two-phase regions and to understand the transport phenomenon in detail, a thermal capillary two-phase flow model is needed. However, to simplify the problem, here we only consider the case when two-phases co-exist in the entire region

Soil Strength and Water Infiltration Under Reduced and Conventional Tillage in a Typic Haplustepts of Lamongan District

. The ability of upland non-irrigated soil to absorb and store water is critical to to provide sufficient moisture for crop grown in dry season. The objective of this study was to evaluate the effects of tillage, reduced (RT) and conventional tillage (CT), on infiltration rate and soil penetration resistance (soil strength) in soil with ustic moisture regime planted with corn. The experiment was conducted on a site, which had been continuously planted with corn twice a year. The predominant soil was Typic Haplusteps. Six positions, 15 meters a part, were chosen within each treatment to measure infiltration rate and soil strength. The mean infiltration rate values were higher under CT (91.87 ± 18.99 mm h-1) than under RT (64.36 ± 30.97 mm h-1). The amount of water infiltrated in CT is 1.4 times higher than in RT. The RT induced the formation of near surface compacted layer with a soil strength of 850 kPa, 2 times higher than under CT at the same depth. The compacted layer is expected to be responsible for lowering infiltration rate under RT. The highest correlation (R2 = 0.83) between qs and Ksat under RT was found at the second depth (8 to 12-cm) and third depth (16 to 20-cm) for CT (R2 = 0.73) indicating that soil layer with the highest soil strength was responsible to control water infiltration. The infiltration models tested (Parlange, the Green and Ampt, and Kostiakov) fit well with the measured data (r2 = 0.99–1.00). It is recommended to conduct deep tillage (20 – 25 cm) once a year to maintain favorable soil structure for water infiltration and root growth