## 2 Materials and Methods
### Experimental Test Bench  
The test bench used to perform the airflow resistance experiments is shown schematically in Figure 1.  
A cylindrical acrylic-glass test bin (480 mm diameter, 3400 mm height and 5 mm wall thickness) with a wall friction coefficient of 0.32 \(\pm\) 0.02 was used to accommodate approx. 500 kg of wheat kernels (\(V_{b}\) = 0.62 m\({}^{3}\)). A perforated floor (3.80 mm apertures,  
Figure 1: Test bench comprising (1) frequency inverter, (2) centrifugal fan, (3) safety valve, (4) air duct, (5) flow sensor, (6) data logger, (7) laboratory computer, (8) airflow straightener, (9) test bin, (10) wheat bulk, (11) sensor grid and (12) pressure taps. \(P\) denotes the equalized pressure of six pressure taps, whereas P\({}_{1.0}\), P\({}_{2.0}\), P\({}_{3.0}\) and P\({}_{3.4}\) represent the pressure data at bulk depths of 1.0, 2.0, 3.0 and 3.4 m, respectively.  
18.60% opening area) was installed at the bottom of the test bin to allow undisturbed upward movement of air within the pore volume of the grain bulk and to assert minimal resistance to airflow according to ASAE [43]. Air was supplied by a centrifugal fan (RD6-NRD80S/2, Elektror GmbH, Esslingen, Germany) with a maximal volumetric air capacity of 1230 m\({}^{3}\cdot\)h\({}^{-1}\), pressure of 2500 Pa at the nominal fan speed of 2890 min\({}^{-1}\) and power consumption of 0.75 kW at 380 V/50 Hz. The fan speed was adjusted to the experimental requirements by a frequency inverter (ST 8100, Sourcetronic GmbH, Bremen, Germany).  
An air duct (150 mm diameter, 2000 mm length) was employed to connect the fan with the test bin. A thermal flow sensor with an integrated transducer (TA10, Hontzsch Instrument, Waiblingen, Germany) with a measurement accuracy of \(\pm\)2.0% was used to measure the airflow velocity at a distance of 10-fold diameter of the duct [44]. The flow sensor was calibrated using a bench wind tunnel (8390, TSI Incorporated, Shoreview, MN, USA). Based on the airflow velocity in the duct, the volume flow was calculated and subsequently the superficial velocity \(v_{a}\) in the test bin. At the duct end, a 90\({}^{\circ}\) bow rubber pipe and a honeycomb-shaped polycarbonate straightener (100 mm diameter, 50 mm thickness) were installed to prevent the fan propagating vibrations and ensure uniform flow conditions [12]. All joints of the test bench were examined for air leakage and were tightly sealed.  
Pressure taps (2 mm diameter, 30 mm length) were attached in the wall of the test bin at depths of 1.0, 2.0, 3.0 and 3.4 m (P\({}_{1.0}\)-P\({}_{3.4}\)). The taps protruded 2 mm inside the bulk to reduce the possible wall effects during the pressure measurements. At each depth, six taps were evenly distributed around the circumference at a segment angle of 60\({}^{\circ}\) and connected by a loop of 4 mm diameter transparent polyethylene hose, which was also used to connect the loops with the pressure sensors (GMSD 25 MR & GHM 3151-Ex, GHM Messtechnik GmbH, Remscheid, Germany) with an accuracy of \(\pm\)0.50 Pa. Combined temperature/humidity sensors (SHT25, Sensirion AG, Zurich, Switzerland) were placed in the centerline of the test bin at the same depths as pressure sensors to measure the temperature \(T_{a}\) (\(\pm\)0.20 \({}^{\circ}\)C) and relative humidity \(\varphi_{a}\) (\(\pm\)1.80%) of intergranular air in the grain bulk. A data logger (Agilent 34901A, Agilent Technologies Inc., Santa Clara, CA, USA) was used to acquire data from all sensors and record them on a laboratory computer. Manual sampling and offline thermogravimetric analysis were also conducted to determine the grain moisture content in the bulk [39].