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A profound understanding of gas flow in porous media is of great interest for various technological and scientific fields. Its investigation by laboratory measurements, however, poses several challenges. In particular, the determination of macroscopic flow parameters from pressure and gas flow measurements is prone to various error influences, some of which are very difficult to analyze experimentally. Computer simulations are a solution in this context as they facilitate modifications of the underlying geometry and boundary conditions in a flexible way. Here we present a simulation framework for the analysis of a recent experiment for determining the Knudsen diffusion coefficient and viscous permeability of various porous granular materials. By combining the finite element method with analytical models and other numerical methods, we were able to identify previously neglected physical effects that increase the uncertainty of the measurements. In particular, the porosity increase due to finite sample dimensions, in a layer of about a grain diameter thickness near the container wall, creates a deviation of the measured pressure gradient. This deviation amounts to ca. 5% for a sample width of about 100 grains and a porosity of 0.5, and is indirectly proportional to the porosity. The second most prominent error source, the sample support sieve, causes a slight constriction of the flow volume. Simulations of this effect show an error around 4%–7%, dependent on the grain size. Based on these findings we recommend an overall sample dimension of 100 grains or larger. As an example of failures of the sample homogeneity, we elaborate how channels through the sample influence the flow properties. Respective suggestions for keeping all discussed effects negligible are discussed in detail. Our methodology demonstrates how the combination of finite element computations with analytical representations of the involved macroscopic parameters can assess the validity and accuracy of laboratory experiments.

Electrolytic tank measurements were applied often to determine the antenna properties of spaceborne monopole antennas. This technique, called rheometry in the present context, yields the effective length vectors of antennas for the quasi-static frequency range. When receivers or preamplifiers are connected to the antennas, their input impedances and the capacitances of the cables change the effective length vectors. In the evaluation of former rheometry measurements this effect has been taken into account for each antenna individually, tacitly neglecting the mutual antenna capacitances. Our analysis shows that this neglect can cause errors in the measured effective axes (directions of effective length vectors) of up to about 10°. However, in order to apply direction finding techniques with a triaxial antenna system an accuracy of about 2° is required. We therefore extend rheometry to the measurement of the complete antenna capacitance matrix. An application of the presented technique to the WAVES antenna system onboard the STEREO spacecraft verifies that the mutual capacitances are crucial in this context. With the extended rheometry technique the antenna transfer and capacitance matrices can be completely determined, thereby yielding a full representation of the reception properties of the antenna system as part of an electronic circuit.