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This Defense Intelligence Reference Document from the Defense Intelligence Agency, dated 20 November 2010, was one of a series of advanced technology reports produced in FY 2010 under the Advanced Aerospace Weapon System Applications (AAWSA) Program. It reviews the theory of subsonic, supersonic and hypersonic flow. It then compares electromagnetic, optical, and acoustic and seismic methods for detecting and tracking hypersonic objects, and it makes four recommendations for progress over the next 30 years.
“Anderson”2 pages
UNCLASSIFIED//FIHl 8FFIIIIIIL ~81!! SHLY (6) It is possible to statistically model the motion of the molecules of oxygen and nitrogen in air, and their kinetic energy, KE, can be determined based on the mass, m, of each molecule and the temperature of the gas. I V' 3KE= - m - = - k T (7) 2 2 In this equation, k is the Boltzmann constant (1.3807 x 10-23 J/K). We can calculate the velocity of air molecules based on the temperature of the air. V = f-lkT (8) ~ -----;;;- Or, for the average velocity of a molecule: V=J8:T (9) For air near sea level (p = 101,320 Pa, T = 293 K) the average velocity of molecules in air is 463 m/s or 1,035 MPH. This value is just a little higher than the speed of sound in air (343 m/s or 767 MPH) as computed earlier. These molecules only travel a short distance before they collide with each other. This distance is defined as the "mean free path" given by the symbol A. (IO) In this expression, d is the diameter of a molecule, which is approximately 0.3 nanometers. For air at 20° C and 101,325 Pa, the mean free path (A) is approximately 100 nanometers or about 333 molecular diameters. When the supersonic projectile in Figure 4 moves through the air, molecules of nitrogen and oxygen in the air bounce off the vehicle's surface and collide with other molecules of air a short distance away. At the speed of sound, these molecules are not moving fast enough to get out of the way and a large number of molecules pile up along a straight line that emanates from the nose or leading edge of the projectile as a "shock wave." Supersonic flow in the atmosphere labeled as region 1 passes through the shock and moves parallel to the surface of the body. The flow "expands" through a Prandtl-Meyer expansion fan at the end of the airfoil and speeds back up to its original Mach number. Flow in the boundary layer separates from the end of the airfoil and forms a highly turbulent wake downstream of the airfoil. The wake is also composed of Strauhal eddies that can subsist in the air long after the airfoil has passed by. The flow density, pressure, and temperature increases dramatically across the bow shockwave and returns to the original Mach number downstream of the airfoil. For a blunt-nosed object, as shown in Figure 5, the magnitude of the impact that the shockwave has on the flow is easier to explain. For a blunt object, the shock detaches from the surface of the object into the freestream in front of the object. Since the original flow moving at M1 is traveling at 90° with respect to the shock near the nose of the object, the shock in this region is referred to as a "normal" shock. The properties across a normal shock 7 UNCLASSIFIED/ ,'F8"1 8FFU!lit.l! l!l91!! 8Hl:Y
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Report, from the dia collection. The PDF is mirrored here; the original link is above. 46 pages are in the text index: search them above, or from the library's search.