Recent studies on infrared energy addition to high-pressure air, motivated by the Radiatively Driven Hypersonic Wind Tunnel (RDHWT) concept, are presented. Experiments show that absorption properties of high-density air dramatically differ from those extrapolated from atmospheric-pressure data. A time-domain model is developed that describes the line interference phenomena and produces a good agreement with the absorption data. One-dimensional modeling of RDHWT with HF laser as the energy source is performed, taking into account real gas equation of state, the new absorption data, vibrational relaxation, and chemistry. Strong coupling of absorption, relaxation, chemistry, and gas dynamics is noted. The computations show the possibility to absorb and thermalize very large amounts of radiative power in high-density air, and to obtain hypersonic flows representative of high-altitude flight with Mach numbers above 10.