**4.2 Operating conditions**

One of the major advantage of the facility is its almost unlimited running time, particularly suitable to obtain a good stabilization of the flow conditions prior to experiments.

The purpose of this experimental study is to investigate the influence of rarefaction effects on the flight performance of waveriders with isobar and isomach operating conditions. Among the available nozzles we have selected four to carry out this first part of the study in supersonic conditions. The Isobar conditions are obtained with a static pressure of 8 Pa with the nozzle N1 operating at Mach 2 and nozzle N3 operating at Mach 4. Isomach conditions are performed at Mach 4 with the nozzle N2 operating with a static pressure of 2.66 Pa, the nozzle N3 with a static pressure of 8 Pa and the nozzle N4 operating with a static pressure of 71 Pa. The detailed operating conditions for these nozzles are presented in **Tables 1**–**4** where *p* represents the pressure, *T* the temperature, *ρ* the volumetric mass density, *U* the flow velocity, *M* the Mach number, *λ* the mean free path and *Re* the Reynolds number based on the length L = 100 mm and calculated with the relation *Re* ¼ *U*1*L=μ*1, where *μ*<sup>1</sup> is the kinematic viscosity.

Various parameters have been proposed to quantify the rarefaction effects that make the Navier–Stokes equations invalid. The Knudsen number, defined as the ratio of the mean free path to a characteristic length, is a measure of the gas density


#### **Table 1.**

*N1 (Mach 2–8 Pa) operating conditions.*


**Table 2.** *N2 (Mach 4–2.66 Pa) operating conditions.*
