Module 8 : AERODYNAMIC TEST FACILITIES

Lecture 1 : Aerodynamics Test Facilities - Part I

 

Overview

With the advancement of aerospace vehicles, the human's dream is to fly faster and higher. Speed of manned and unmanned flight vehicles has increased by some orders of magnitude over the last few decades. As the speed of the vehicle is increased, the aerodynamic environment becomes increasingly hostile. At speeds around the local speed of sound (Transonic) and higher (Supersonic), the aerodynamic loads increase and their distributions change. When the speed of the vehicle becomes several times higher than the speed of sound (Hypersonic), additional problem of aerodynamic heating demands the change in design geometry and materials. At still higher speeds (hypervelocity), the behavior of air begins to change significantly; both physically and chemically. There is a conventional ‘rule-of-thumb' that defines the flow regimes based on the free stream Mach number i.e.

The past four decades have seen major flights cruising from subsonic to hypersonic speeds. The most routine flights made possible when American Jet Transport started its first flight (Boeing-707) on October 26, 1958 cruising at Mach 0.7. Traveling at speeds faster than sound speed and thus breaking ‘Sound Barrier' became reality with the taste of supersonic travel from London to Bahrain in the aircraft ‘Concorde' commenced by British airways on January 21, 1976. The first ever-fastest commercial passenger aircraft ‘Concorde' with its cruising altitude 18 km at Mach 2, crossed Atlantic Ocean from London to New York little less than 3.5 hours as opposed to about eight hours for a subsonic flight. This mile stone was achieved on November 22, 1977. Now the age of ‘hypersonic flight' is about to dawn with the evolution of orbital space crafts, hypersonic airliners and re-entry vehicles. The first such major landmark has been achieved after the launch of USSR satellite SPUTNIK-II on November 3, 1957. It was the first man-made earth satellite that carried living organism (a dog named LAIKA) into the space and remained in orbit till April 13, 1958. In the bumper year of 1961, Yuri Gagarin (USSR) became the first man in the history to fly in space with an orbital space craft VOSTAK-I that entered the earth atmosphere at Mach 25 on April 12. His safe return from the space has inspired the future objectives of hypersonic flight. In the same year i.e. on June 23, U.S. air force test pilot Major R. White accomplished the concept of ‘miles per second' flight in an X-15 airplane by flying at Mach 5.3. White again extended this record with same X-15 flight at Mach 6. Since 1961, major space programs carried out by U.S. space agency NASA and Indian Space Research Organization (ISRO) have achieved milestones in the development of satellites and aero-assisted space transfer vehicles.

The aerodynamic flow fields at very high Mach numbers experience a very high pressure, temperature and density. Even, the temperature rise can be so high that the gaseous medium gets decomposed and thereby the properties (specific heat, gas constant and specific heat ratio) can change as well. So, even at same free stream Mach number, different velocities can be obtained. Thus, the high Mach number flows where the behavior of the medium begins to change significantly, are normally classified in terms their velocities: s uborbital velocities speed (4-7 km/s), super orbital speed (8-12 km/s) and escape velocities (>13 km/s). However, these flow conditions are normally achieved at different trajectories/altitudes of a flight vehicle in the earth atmosphere. So, the more realistic way is to simulate these conditions experimentally in the laboratory for entire range of operational speeds. Some of these aerodynamic test facilities are broadly summarized and discussed in this module.