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Sources of sound
Published in John Watkinson, The Art of Sound Reproduction, 2012
Supersonic aircraft produce no audible warning of their approach. In subsonic aircraft the air moves out of the way because an area of high pressure precedes the craft. This cannot happen in a supersonic craft. Consequently the air is moved out of the way by brute force and the result is a shock wave which sweeps back from the nose like a bow wave. At the rear of the craft a vacuum is left after its passage and the air rushes to fill it causing another shock wave. The pressure with respect to the craft is relatively constant at a given place, but on the ground the pressure profile between the two shock waves sweeps past the listener. The result is known as an N-wave because of its waveform. Another way of considering the origin of the N-wave is the mechanism shown in Section 3.4 for extremely high amplitude sounds.
Speed of Sound and Mach Number
Published in Rose G. Davies, Aerodynamics Principles for Air Transport Pilots, 2020
The free-stream Mach number of an aircraft is used to classify the flights. Subsonic flights: The free-stream Mach number of aircraft is less than its critical Mach number, Mfs < Mcrit. The airflow around the subsonic aircraft is always subsonic, i.e., the local air flow speed around a subsonic aircraft is always less than the local speed of sound. The airflow can be treated as incompressible if the true airspeed of the aircraft is less than 250 kt, low subsonic. Otherwise, the airflow around the subsonic is compressible.Transonic flights: The free-stream Mach number of aircraft is greater than its critical Mach number, and less than, approximately, 1.2, Mcrit < Mfs < 1.2. The airflow around transonic aircraft can be subsonic, as well supersonic, even when the free-stream Mach number is less than 1. The air definitely is compressible, and shockwaves may be formed on aerofoils and on other parts of the aircraft body.Supersonic flights: The free-stream Mach number of aircraft is greater than 1.2, Mfs > 1.2. This is also called hypersonic if the free-stream Mach number of any traveling object is greater than 5 or 6. Airflow around the supersonic aircraft is supersonic in general, except the airflow behind a normal shockwave, and within boundary layers. The air is highly compressible, and the kinetic heating is a significant concern due to the speed change of airflow around supersonic aircraft.
Straight-level flight
Published in Mohammad H. Sadraey, Aircraft Performance, 2017
One of the reasons why airplanes fly at high altitude is the lower cost of flight. This is due to a lower fuel consumption with higher speed at high altitude, which also leads to a longer range. Table 5.3 demonstrates the maximum speed of several jet aircraft. With the current technology, the highest speed of supersonic aircraft reaches Mach 3 and in special configuration could pass Mach 5. The maximum speed of a high subsonic aircraft is about Mach 0.95.
Enhanced outer peaks in turbulent boundary layer using uniform blowing at moderate Reynolds number
Published in Journal of Turbulence, 2022
Gazi Hasanuzzaman, Sebastian Merbold, Vasyl Motuz, Christoph Egbers
FCT for TBL has a wide engineering application from modern aircraft to the high speed trains and cars. For aero and thermodynamic applications, flow control based on the varying surface conditions has been of keen interest where motivations towards the drag reduction, which in turn, can lead to significant financial benefit. Estimations show that nearly 50% of the total drag of an subsonic aircraft and 30% of automobiles are constituted from skin friction drag [1] which is directly linked to the fuel consumption. A particular example is that, in the U.S.A. all the transportation systems including automobiles consumes 25 % to 27% of their total energy to overcome their aerodynamic drag [2]. Only 1% of reduction of such skin friction drag can save up to 400,000 litres of fuel yearly based on Airbus A320 data [3]. Moreover, Airline and truck industry consumes 238.5 billion and 190 billion litres of oil per year out of which 25% and 27% fuel is spent to overcome the viscous drag [3,4].
Experimental investigation of turbulent boundary layers at high Reynolds number with uniform blowing, part I: statistics
Published in Journal of Turbulence, 2020
G. Hasanuzzaman, S. Merbold, C. Cuvier, V. Motuz, J.-M. Foucaut, Ch. Egbers
Large part of the fuselage, wing, tail wing and radar section of a subsonic aircraft has the potential to deploy drag reduction mechanisms. Due to their size and operating speeds, the majority of commercial and military aircraft in service today are dominated by flows that results from the presence of turbulent boundary layer. This generally cover the most of the aircraft's surface. It is well known that TBL significantly increases the skin friction drag penalties when compared to laminar boundary layers. Moreover, they do result in a reduced susceptibility to flow separation due to their robustness to surface imperfections. Therefore, turbulent drag reduction has a direct relationship to the eddy structures of different size and scales present in the boundary layer.