The relationship between the airflow resistance of granular material and airflow velocity is usually presented in the form of equations or tables (Brooker et al. 1992). Usually, assumptions are made that airflow resistance is constant in the volume of the material and is independent of the packing structure. Numerous investigations performed recently have shown that such an assumption is not always true. In practice, local changes of the airflow resistance in various areas of grain bulk may cause serious disturbances in processes involving the flow of gases such as aeration, drying, fumigation, or cooling. According to Navarro and Noyes (2002) the values of airflow resistance calculated by means of the proposed equations or taken from tables correspond to clean, loosely packed grain and apply to vertical direction of airflow and, in consequence, are usually lower than in practical conditions. These authors pointed out that the efficiency of the aeration systems depends to a large extent on a uniform distribution of the airflow within the volume of grain.Early experiments studied the influence of the bulk density (related to porosity) on airflow resistance. Calderwood (1973) in his experiments with rice of different varieties stated that the bulk density modified the airflow resistance in an essential way. Stephens and Foster (1976) conducted their project with corn in a commercial grain silo and found that the use of a grain spreader resulted in threefold increase of airflow resistance. The same authors performed a similar project with wheat and grain sorghum (Stephens & Foster 1978) and reported that the use of a spreader resulted in an increase in airflow resistance to 110% in sorghum, while in the case of wheat airflow resistance increased to 101%. The authors explained the observed effect by the difference in the fine content that was from 1.5 to 2% in the case of sorghum and 0.2% in that of wheat. In the grain bulk containing a higher amount of fines, these filled pores and caused an increase in airflow resistance.The results of later experiments showed that airflow resistance depended also on the airflow direction. Kumar and Muir (1986) in their tests with wheat and barley stated that with the airflow velocity of 0.077 m/s, the airflow resistance in vertical direction was by as much as 60% higher than that in horizontal direction. Hood and Thorpe (1992) Department of Biosystems and Agricultural Engineering, University of Kentucky, Lexington, USAAbstract: A study was conducted to estimate the degree of variability of the airflow resistance in wheat caused by the filling method, compaction of the sample, and airflow direction. Two types of grain chambers were used: a cylindrical column 0.95 m high and 0.196 m in diameter, and a cubical box of 0.35 m side. All factors examined were found to influence considerably the airflow resistance. Gravitational axial filling of the grain column from three heights (0.0, 0.95 and 1.8 m) resulted in the pressure drops of 1.0, 1.3, and 1.5 kPa at the airflow velocity of 0....

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The horizontal to vertical pressure ratio k is one of the three most important parameters required for the calculation of stresses that granular materials exert on the wall and floor of a silo, introduced by JANSSEN (1895). The author assumed that the vertical pressure was uniform in a horizontal section of the silo and that ratio k was constant everywhere within the material.Two states of stress are commonly associated with the pressure ratio in a deep silo: the active state for filling and storage and the passive state for discharge mode (MOYSEY 1979). The pressure ratio k depends not only on stress state, but also on type of grain, moisture content, bedding structure of grain formed during the filling process, angle of internal friction (ŁUKASZUK, HORA-BIK 2002) and coefficient of friction on the wall. Experimental determination of pressure distribution in a bulk of grain may be performed in a model silo or in practical conditions of silo operation.ATEWOLOGUN and RISKOWSKI (1991) used four different experimental methods for determination of soybeans pressure. Static stresses were measured in a 0.91 m diameter and 2.74 m high model smooth galvanized steel silo. The results of this study indicated that the pressure ratio k decreased with grain depth and that the highest values of k occured between the silo center and wall.LOHNES (1993) determined pressure ratio k in the triaxial test apparatus for cereal grains and several levels of moisture content during loading and unloading. Experimental values of pressure ratios were compared to the values estimated form RANKINE (1857), HARTMANN and JAKY'S (1948) theories. The Rankine equation results were below the measured values, Hartmann were higher and Jaky's showed reasonable agreement.WILMS (1991) compared and evaluated values of loads calculated from silos codes and standards. General criteria included the most important physical parameters of bedding such as: wall friction, cohesion, stress state. Question of non-uniform pressure associated with eccentric discharge was also considered.HORABIK and RUSINEK (2002) used uniaxial compression tester for determination of pressure ratio of cereal grains for five levels of moisture contents. Experiments were performed according to the Eurocode 1 recommendations. The tester was 210 mm in diameter and 100 mm high. The specimen was loaded to the reference vertical stress of 100 kPa using a universal loading frame by the top cover plate at the constant displacement rate of 0.35 mm/min. Mean lateral to vertical pressure ratio was found dependent on procedure of the sample deposition. The pressure ratio of cereal grain generally decreased with an increase in moisture content. MATERIALS AND METHODSA model silo 0.6 m high and 0.6 m in diameter was constructed and instrumented to measure mean pressure ratio k, mean tangent stress on the wall σ t , as well as distribution of vertical pressure σ z along the radius of the silo (Fig. 1). The cylindrical silo wall consisted of two semicircular halves cut along the axis and connected wit...