Soil-blade dynamics in reverse-rotational rotary tillage

Soil cutting and the clod crack formation process during reverse-rotational rotary tillage in a heavy clay soil were investigated. Of particular interest was the relationship between clod crack formation and tillage resistance during sequential rotations of the tillage blade. Investigation of the crack formation process is helpful to develop and to design more effective and high performance tillage methods. This paper describes two new discoveries. The first is that the tillage resistance showed a higher cross-correlation between sequential rotations within a certain distance of tilling, while there was little or no cross-correlation between different tillage plots that were separated more than 0.4 m. The forward distance of untilled soil that was disturbed by the tillage blade was estimated to be 36.4 mm. This is the distance of two tillage pitches. The second discovery involved the blade frequencies during tilling. Fluctuation in tillage resistance frequencies of a single blade was nearly equal to the predicted occurrence of crack intervals on the tilled clod's surface. This frequency was 120 Hz. When these frequencies were translated into the distance along the trochoid trajectory of the blade cutting edge, they were the same as the length of the clods tilled by the reverse-rotational rotary tiller. These minute vibrations in the tillage resistance were considered the important indexes for recognizing the tilled soil conditions and the tilled clod failure on the reverse-rotational rotary tiller. The analytical results of this paper will be utilized for the active occurrence of the cracks regarding with natural frequency of the blade and the operation condition of the reverse-rotational rotary tiller.     

Fuzzy prediction of soil strength based on water content and composition

Sixteen samples representing various paddy field soils of the main rice-planting regions in South China, with their characteristic values, are listed, by which means the soil strength of any other paddy field soil can be fuzzily predicted when its water content, sand content and clay content are given. Illustrative examples show that the result of prediction seems to be acceptable in engineering practice.     

Reliability-based design for soil tillage machines

Using classical design methods for tillage machines does not completely guarantee a safety and satisfactory performance, due in part to the randomness of tillage forces. This randomness is derived from the variability in soil engineering properties and the variations in tool design parameters and operational conditions. In this paper, a reliability-based design approach was developed, for the first time, by integrating the randomness of tillage forces into the design analysis of tillage machines, aiming at achieving reliable machines. The proposed approach was based on the uncertainty analysis of basic random variables and the failure probability of tillage machines. The failure probability was estimated according to two performance criteria related to the structural design requirement and the quality of tillage operation. Two reliability methods, namely the Monte Carlo simulation technique and the first-order reliability methods were used for this purpose. This approach was implemented for the design of a chisel plough shank.The results showed that there were many values of the shank dimensions that guaranteed the required reliability level. However, in order to achieve the best design solution from an economic point of view, minimizing the volume of the shank structure was integrated into the reliability-based design approach. This led to the reduction of the initial volume of the shank structure by 6.86%. It was concluded that integrating the economical constraint into the reliability-based design approach can lead to optimal designs of tillage tools that ensures the required reliability level at low cost.     

Effects of tillage systems on compaction and crop yield of Albic Luvisol in Croatia

This four-year experiment was conducted in north-west Slavonia (agricultural area of Croatia) to evaluate the effects of different tillage systems on compaction of silty loam soil (Albic Luvisol). The compared tillage systems were: (1) conventional tillage (CT), (2) conservation tillage (CM), (3) no-tillage system (NT), and the crop rotation was corn (Zea mays L.) – winter wheat (Triticum aestivum L.) – corn – winter wheat. For detecting the soil compaction, bulk density and penetration resistance were measured during the growing seasons. In all seasons and tillage systems, the bulk density and penetration resistance increased with depth and the greatest increase from surface to the deepest layer in average was observed at CT system. The bulk density and penetration resistance increased at all tillage systems during the experiment, but the greatest increase was also observed at CT system. The greatest bulk density (1.66 Mg m⁻³) and the greatest increase of 6.4% were observed at CT system in the layer 30–35 cm. In the first season, the bulk density was the greatest at NT system, but during the experiment the lowest average increase of 1.9% was observed at this system. The greatest penetration resistance of all measurements (5.9 MPa) was observed in the last season at CT system in depth of 40 cm. The lowest average increase of penetration resistance 11.4% was also observed at NT system. The highest yield of corn in the first season was achieved with CT system while in other seasons the highest yield of winter wheat and corn was achieved with CM system.     

Theoretical analysis of the spatial variability in tillage forces for fatigue analysis of tillage machines

This paper presents a new theoretical model to describe the spatial variability in tillage forces for the purpose of fatigue analysis of tillage machines. The proposed model took into account both the variability in tillage system parameters (soil engineering properties, tool design parameters and operational conditions) and the cyclic effects of mechanical behavior of the soil during failure ahead of tillage tools on the spatial variability in tillage forces. The stress-based fatigue life approach was used to determine the life time of tillage machines, based on the fact that the applied stress on tillage machines is primarily within the elastic range of the material. Stress cycles with their mean values and amplitudes were determined by the rainflow algorithm. The damage friction caused by each cycle of stress was computed according to the Soderberg criterion and the total damage was calculated by the Miner’s law. The proposed model was applied to determine the spatial variability in tillage forces on the shank of a chisel plough. The equivalent stress history resulted from these forces were calculated by means of a finite element model and the Von misses criterion. The histograms of mean stress and stress amplitude obtained by the rainflow algorithm showed significant dispersions. Although the equivalent stress is smaller than the yield stress of the material, the failure by fatigue will occur after a certain travel distance. The expected distance to failure was found to be d_f = 0.825 × 10⁶ km. It is concluded that the spatial variability in tillage forces has significant effect on the life time of tillage machines and should be considered in the design analysis of tillage machines to predict the life time. Further investigations are required to correlate the results achieved by the proposed model with field tests and to validate the proposed assumptions to model the spatial variability in tillage forces.     

A semi-collocated ocean model based on the SOMS approach

In application to the Gulf of Mexico (GOM), a new DieCAST ocean model, which uses a modified Arakawa ‘a’ grid, and the SOMS model, which uses an Arakawa ‘c’ grid, give remarkably similar results. The new model avoids ‘null space’ problems of the standard ‘a’ grid approach by first using fourth-order interpolations to a ‘c’ grid advection velocity, then applying incompressibility to the result. Accuracy is further improved by using fourth-order pressure gradient approximations at ‘a’ grid locations. Incompressibility with general geometry is satisfied efficiently by a fast-converging iteration with a regular gteometry elliptic solver. Results are compared with satellite-measured r.m.s. sea surface elevation anomaly and detailed front structures, climatological mean thermocline and empirical orthogonal functions and other observations.     

Evaluation of factors and current approaches related to computerized design of tillage tools: a review

The objectives of this paper are to evaluate the factors that are involved in the tillage process, and to explore the potential approaches for the computer-aided design of tillage tools. An overview related to the dynamic effect on the performance of tillage operations has been conducted. Compared with the analytical methods, the finite element method (FEM) has some advantages for the computerized design of tillage tools. The artificial neural networks (ANN) may be useful for the integrated evaluation of tillage performance with multi-objectives. ANN can be employed for simulation of a dynamic constitutive model and identification of soil conditions for agricultural soils. The integral approach of ANN analysis with FEM is found to be promising for optimizing design of tillage tools.     

Complex turbulent compressible flow computation using a two-layer approach

A two-layer approach is proposed to compute complex flows including separations. For high- and low-Reynolds-number regions we use a two-equation k-? model and a one-equation k-L model respectively. A robust algorithm is proposed for the treatment of the convective part of the turbulence equations. Several complex configurations including separations are computed.     

Development and laboratory evaluation of a rheometer for soil visco-plastic parameters

A motorized rheometer was developed for determining soil visco-plastic parameters that works on the principle of torsional shear applied to a standard vane with controlled strain rate. Rheological measurements were carried at different soil moisture contents (10%, 13%, 17% and 20% dry basis (gravimetric)) and soil compaction levels (100, 150, 200, 300 and 400 kPa) to find their effects on soil viscosity and yield strength. The values of viscosity of the clay loam soil (29% clay, 24% silt and 47% sand) were found to spread in the range of 53–283 kPa s, and yield stress variation had a span of 4–28 kPa. Increase in soil compaction was accompanied by a sharp increase in soil viscosity, while moisture content affected soil viscosity negatively. Effect of both these parameters was statistically significant (95% confidence interval). Yield stress was positively related to soil compaction and the effect was statistically significant. However, it was negatively related to moisture content and the effect was not statistically significant for the levels of moisture content tested.     

土结构性本构模型研究现状综述

土本构模型的建立是一个重要而又复杂的问题,到目前为止,国内外学者们已提出数以百计的土本构模型,诸多文献也对这些模型进行了评述和归纳。然而这些土本构模型多是在扰动土或砂土的基础上发展和建立起来的,它们难以描述由于土结构性引起的各种非线性行为,其计算结果与实际情况相差甚远。天然土体一般都具有一定的结构性,所以有必要建立考虑土结构性影响的土本构模型。针对这个现实,目前有些学者已基于各种理论和方法,提出了一些可以考虑土结构性影响的土本构模型,并得了较好的应用。但在目前的文献中还很少有对土的结构性本构模型研究进行归纳,基于此,本文简要介绍了一下目前土的结构性本构模型研究现状,并提出了这些本构模型在应用中所存在的问题。     

缺陷对脆性材料碎裂过程的影响

采用特征线方法模拟脆性材料中应力波的传播过程,采用内聚力模型模拟断裂点的断裂过程,运用C++语言开发了一个模拟脆性圆环发生一维膨胀碎裂过程的实用工具ExpRing,简要给出了该程序的理论基础和使用说明。采用此程序模拟了具有初始缺陷的脆性圆环在均匀膨胀作用下的碎裂过程,探讨了不同应变率下,缺陷分布特征对碎裂过程和平均碎片尺寸的影响。计算结果表明:(1)在一定的应变率范围内,等间距分布的点缺陷会控制断裂点的位置及碎片个数,在碎片尺寸 应变率曲线上形成一个缺陷控制碎裂平台;(2)点缺陷的间距和弱化程度将影响缺陷控制碎裂平台的宽度和位置;(3)具有缺陷的脆性材料的表观强度呈现应变率硬化特征;(4)在一定的应变率范围内,正弦分布型缺陷同样导致缺陷控制碎裂的现象。     

土体剖面温度物理模型试验研究

利用自主开发的土体温度物理模型试验系统,研究了土体剖面温度随时间的变化规律,通过改变土体表面的覆盖层属性,对比分析了裸土和混凝土板覆盖下土体剖面的热传递特点。结果表明:在恒定热源作用下,土体剖面温度迅速上升到一定值之后逐渐趋于稳定,初始升温速率随深度的增加而呈指数递减,最终平衡温度随深度的增加而显著衰减; 温度在土体剖面上的传递存在明显的滞后效应; 混凝土板覆盖下土体的初始升温速率和最终平衡温度较裸土高; 土体剖面热通量反映了土体中热量的传递特征,其变化规律与上下土层间的温度差变化规律一致。     

土的应力-应变关系的一种描述模式

通常将由土的剪切试验测得的应力-应变关系曲线(q～ε1曲线)分为应变硬化型和应变软化型,文中提出了一种能同时描述应变硬化型q～ε1曲线和应变软化型q～ε1曲线的新模式,并导出了统一的切线模量表达式,进一步探讨了该应力-应变关系曲线描述模式在拟合应变硬化型曲线时的简化形式,及简化形式中参数取值方法和参数与围压的关系等方面的问题。通过与邓肯-张模型和应变软化模型的对比,结果表明该应力-应变关系曲线描述模式能更好地与试验数据吻合,且用其简化形式拟合应变硬化型曲线时,可以通过调整其中一个参数值来表达出不同的曲线形式,从而体现出土体应力-应变关系的多样性。该应力-应变关系描述模式为发展更一般的非线性弹性模型提供了一定的理论基础。     

Computation of backward-facing step flows by a second-order Reynolds stress closure model

This paper scrutinizes the predictive ability of the differential stress equation model in complex shear flows. Two backward-facing step flows with different expansion ratios are solved by the LRR turbulence model with an anisotropic dissipation model and the near-wall regions of the separated side resolved by a near-wall model. The computer code developed for solving the transport equations is based on the finite-volume-finite-difference method. In the numerical solution of the time-averaged momenum equations the Reynolds stresses are treated partially as a diffusion term and partially as a source term to avoid numerical instability. Computational results are compared with experimental data. It is found that the near-wall region of the separated side resolved by the near-wall model, the LRR model with a simple modification of an anisotropic dissipation model can predict backward step flows well.     

Y态TU1环的动态脆性碎裂特性

针对膨胀环的碎裂问题,以Mott碎裂理论和Grady能量平衡理论为基础,利用裂纹张开位移碎裂模型（crack opening displacement model,COD）描述脆性无氧铜环的碎裂特性,并推导了碎片尺寸和数目表达式。利用电磁膨胀环加载技术,完成了不同应变率的脆性无氧铜（TU1）环碎裂实验,实验结果与理论计算结果符合较好,可为脆性金属环膨胀碎裂特性描述提供参考。     

An empirical model to predict soil bulk density profiles in field conditions using penetration resistance, moisture content and soil depth

Cylindrical soil probes measuring 300 mm in diameter by 300 mm in height were prepared in the laboratory using samples extracted from a well drained loamy soil (FAO classification: Vertic Luvisol). These probes were compacted at different moisture contents [3, 6, 9, 12, 15 and 18 (% w/w)] and using different compaction energies (9.81, 49.05, 98.1 and 981 J). The soil penetration resistance was determined by means of the ASAE 129 mm² base area cone and seven other different cones with base sizes of 175, 144, 124, 98, 74, 39 and 26 mm². The variability of the penetration resistance measurements increased as the size of the cone decreased. Nevertheless, the penetration resistance values proved to be independent of the cone used, as long as the size of the latter was equal to or greater than 98 mm². This confirms the possibility of using cones with areas smaller than the ASAE standard when measurements are to be carried out in dry soils with high levels of mechanical resistance. The experimental data were used to develop an empirical model, a linear additive model on a log–log plane, capable of estimating soil bulk density depending on soil penetration resistance, soil moisture content and depth. This model has provided good results under field conditions and has allowed soil bulk density profiles and accumulated water profiles to be accurately estimated.     

Computerized instrumentation system for monitoring the tractor performance in the field

A high precision computerized instrumentation package was developed and mounted on a 50 kW tractor to monitor and measure various performance parameters of a tractor and implement system. The system was intended to be used for the compilation of a database of draft requirements of tillage implements. The system designed to measure: three-point linkage forces, ground speed, tillage depth, fuel consumption, forward speed, slip, engine speed, hydraulic pressure and fluid temperatures. The data acquisition unit was based on a high speed multi processors Campbell Scientific CR3000 data logger linked to a microcomputer using suitable transducer. The average calibration constants for the rear wheel speed, fuel consumption and three point linkage transducers were 0.0364 m/pulse, 0.000142857 l/pulse and 0.66 mV/kN respectively. The data acquisition system was capable of scanning rate up to 100 K sample/s. Data acquisition system was developed to measure draft of primary tillage implements in vertisol.     

不同倾角充填节理对岩石爆破块度的影响

为了理解节理与爆炸应力波之间的相互作用,并优化节理岩体的爆破参数,通过试验与数值模拟相结合的方式研究不同节理倾角对爆破块度的影响：采用一组含有不同角度节理的混凝土模型试样开展爆破试验,在试样竖直孔中装填雷管并爆破,使用高速摄像机记录试样爆破破碎过程,观测起爆后不同时刻节理面的动态响应;利用图像处理方法进行爆破块度提取,分析节理倾角对爆破块度的影响;采用LS-DYNA有限元数值模拟获得应力波的传播过程以及应变场的演变过程。试验与数值计算结果表明：节理对爆破块度分布及应力波传播有显著影响,该影响主要源于爆炸应力波在节理处的反射,这与节理的变形特性有关;随着节理倾角增大,爆破块度先减小后增大,节理中的有效应力和峰值质点振动速度透射总体呈下降趋势,但在45°至60°之间回升,其中45°左右为爆破最有利条件。数值裂纹网络重建和图像处理结果表明,随着节理倾角的增加,试样中产生的垂直裂纹增加,水平裂纹有所减少。