*2.3. Evaluations, Comparisons and Conclusions*

First, let us consider the results of Arima et al. [24,43], see Figure 4 for details. There are some important remarks on their evaluation method:


Figure 4 demonstrates that kinetic theory can model the behavior of the gas in the rarefied state.

**Figure 4.** Calculations of Arima et al. [24]. The solid red line shows the prediction, the squares and triangles are referring to different experimental data; here, the triangles represent the data from Rhodes [50]. The dashed line shows the behavior of the Navier-Stokes-Fourier equations.

Here, using the NET-IV continuum model, only the experiment related to *T* = 296.8 K is considered for demonstrational reasons. It is not intended to evaluate the complete series of measurements. In Figure 5, two horizontal scales are used that intend to indicate the one-to-one correspondence between the *ω*/*p* and *ρ*. It is always possible if the frequency and the temperature are known. Although the fitting procedure is conducted by hand, it is clear that the NET-IV model is also applicable to these problems. However, it is more difficult to do due to more degrees of freedom. Tables 1 and 2 show the corresponding values of each parameter. For simplicity, in the fitting procedure the ratio of relaxation times was constrained to be the same as for RET, i.e., *τq*/*τ<sup>d</sup>* = 1.46, *τs*/*τ<sup>d</sup>* = 144.

**Table 1.** Fitted relaxation time coefficients for continuum model.


**Table 2.** Fitted coupling coefficients for continuum model.


**Figure 5.** Evaluation using NET-IV (thick black line). The pressure starts at 1 atm and decreases to 2000 Pa, *ω* = 1 MHz. Error bars are placed for each measurement point to indicate the uncertainty of digitalizing data, its magnitude is ±2.5 m/s. The red dashed line shows the results of Arima et al. [24].
