Light Versus Sound

The permanently increasing pace of today’s life and business globalization, which require high-speed transportation, stimulate research aimed at the development of civil supersonic airplanes. The development of supersonic passenger aircraft of the second generation has been pursued in Russia and abroad for the last four decades. Specialists unanimously believe that the main obstacle on this way is the environmental restriction on the sonic boom (SB) level.

When an aircraft flies in the atmosphere with a velocity greater than the sound speed, the disturbed flow region is bounded by the bow shock wave (SW) emanating from the nose part of the aircraft and the tail SW formed in the rear part of the aircraft. The area in the vicinity of the aircraft (the so-called near zone) contains intermediate shock waves and also expansion and compression waves generated by individual elements of the aircraft structure. As the disturbances generated by each point of the aircraft surface propagate with a velocity close to the sound speed (which is smaller than the aircraft velocity), the SW shape is close to conical.

The pressure, temperature, and density of air behind the bow SW increase in a jumplike manner owing to superposition of disturbances. The flow at a greater distance from the aircraft (the so-called far zone) is transformed by nonlinear effects (dependence of the propagation velocity of disturbances on their amplitude) in such a way that distribution of the excess pressure (with respect to the atmospheric value) acquires an N-shaped signature. A person on the Earth’s surface observes this N-shaped wave as one or two (depending on the aircraft size and flight altitude) remote explosions. This phenomenon induced by drastic drops of pressure on the shock wave is called the sonic boom.

The admissible excess pressure on the SW had to be limited because of the adverse effect (both psychological and physiological) of the sonic boom on human beings and animals and its destructive influence on buildings. A specific value of this restriction was periodically revised, as new information on the SB effect on the environment was gained. In view of the prediction for 2012 (15 Pa), we can state that in the last 40 years environmental requirements have become more severe almost by an order of magnitude.

According to Whitham’s classical theory used for design of supersonic aircraft, the pressure drop on the bow SW is determined by the distribution of the volume and lift force along the aircraft. This pressure drop decreases with increasing flight altitude and aircraft length and with decreasing aircraft weight. With a fixed payload and flight range, conditions that assist in SB reduction do not allow the same aerodynamic efficiency and, hence, cost efficiency of the aircraft to be ensured. For flying vehicles with a weight over 100 tons, it seems problematic to satisfy the currently imposed environmental requirements even with reduced cost efficiency.

Taking into account the limited capabilities of conventional methods, the scientists at the Institute of Theoretical and Applied Mechanics (ITAM) of the Siberian Branch of the Russian Academy of Sciences initiated the study of SB reduction by means of an active impact on the disturbed flow: with the help of mass or energy supply near the aircraft surface or, vice versa, with the help of energy removal. In particular, a unique method of controlling the parameters of the intermediate SW by coolant injection into the region of SW formation was developed in 2007. This made it possible to reduce the pressure drop on the bow SW by 40 % almost without increasing the aircraft drag.

The current research at ITAM involves the possibility of SB attenuation due to interaction between the disturbed flow and the layer of heated air. Preliminary numerical simulations predict that the shock wave passing through a low-density gas layer with a high sound speed (provided by heating) can be substantially attenuated at a certain ratio of temperatures in the layer and in the flow.

Experiments on the effect of the thermal layer on the SW generated by model objects were performed in a small-scale aerodynamic facility, which ensured a supersonic air flow with a transverse size of 100 mm. The thermal layer was formed by radiation of a gas СО₂-laser developed at ITAM. The mean power of this laser reached 4.5 kW. In view of the fast development of laser and microwave engineering, these devices can be considered as the most promising onboard sources of energy. The radiation energy can be fed to a supersonic air flow in the region of the optical breakdown. This term is usually used to indicate the transition of the gaseous substance into the plasma state under the action of an electromagnetic field with optical frequency. For this effect to be realized, the laser radiation intensity should be of the order of 10⁹—10¹⁰W/cm² during the time interval of 0.3—1 µs. Such extreme values of parameters can be ensured at the moment only in the regime of repeated (with a frequency of 80—100 kHz) pulses of focused radiation. In this case, plasma blobs (plasmoids) with a temperature of 20,000—30,000 degrees arise in the air flow. A thermal wake is formed behind the plasmoid.

In the first experiments, the researchers’ attention was focused on studying the specific features of the flow structure formed owing to interaction of the plasmoid with the air flow whose velocity was twice higher than the sound speed.

The next milestone in activities was the analysis of interaction between the optical breakdown (discharge) region and the disturbed flow generated by a conical model. The instants when the SW disappeared after passing through the thermal layer were registered in the interaction region on the background of an unsteady flow. The observations were performed with a shadowgraph and a CCD camera.

The clearly observed changes in the flow structure evidence attenuation of the bow SW generated by the model. Note, in addition, that the shock wave from the plasmoid is estimated to be much less intense than the shock wave from the model even at small distances.

Now the primary effort is aimed at quantifying the effect of the thermal wake generated by the plasmoid on the SW parameters. For this purpose, direct measurements of the pressure distribution in the air flow are planned.