Supersonic Flow

5 Key excerpts on "Supersonic Flow"

• 

  eBook - ePub

  Flight Theory and Aerodynamics

  A Practical Guide for Operational Safety

  • Charles E. Dole, James E. Lewis, Joseph R. Badick, Brian A. Johnson(Authors)
  • 2016(Publication Date)
  • Wiley-Interscience

    (Publisher)

  Chapter 16 High‐Speed Flight

  In this chapter we discuss the airflow as the aircraft approaches the speed of sound (high subsonic), transonic flight, and supersonic flight. Hypersonic flight is not discussed.

  In subsonic flight, the density change in the airflow is so small that it can be neglected in the flow equations without appreciable error. The airflow at these lower speeds can be compared to the flow of water and is called incompressible flow. At high speeds, however, density changes take place in the airstream that are significant. Thus, this type of airflow is called compressible flow. Transonic, supersonic, and hypersonic flight all involve compressible flow.

  Flight speeds have been arbitrarily named as follows:

  • Subsonic Aircraft speeds where the airflow around the aircraft is completely below the speed of sound (about Mach 0.7 or less)
  • Transonic Aircraft speeds where the airflow around the aircraft is partially subsonic and partially supersonic (from about Mach 0.7 to Mach 1.3)
  • Supersonic Aircraft speeds where the airflow around the aircraft is completely above the speed of sound but below hypersonic airspeed (from about Mach 1.3 to Mach 5.0)
  • Hypersonic Aircraft speeds above Mach 5.0

  THE SPEED OF SOUND

  The speed of sound is an important factor in the study of high‐speed flight. Small pressure disturbances are caused by all parts of an aircraft as it moves through the air. These disturbances move outward from their source through the air at the speed of sound. A two‐dimensional analogy is that of the ripples on a pond that result when a stone is thrown in the water.

  The speed of sound in air is a function of temperature alone:

  (16.1 )

  where

  a	=	speed of sound
  a0	=	speed of sound at sea level, standard day (661 kts)
  θ	=	temperature ratio, T/T0

  Since Eq. 16.1

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Aerodynamics for Aviators

  • Mark Dusenbury, Gary Ullrich, Shelby Balogh(Authors)
  • 2016(Publication Date)
  • Aviation Supplies & Academics, Inc.

    (Publisher)

  In reality, air is compressible and viscous. While the effects of these properties are negligible at low speeds, compressibility effects in particular become increasingly important as speed increases. Compressibility (and to a lesser extent viscosity) is of paramount importance at speeds approaching the speed of sound. In these speed ranges, compressibility causes a change in the density of the air around an aircraft. During flight, a wing produces lift by accelerating the airflow over the upper surface. This accelerated air can, and does, reach sonic speeds even though the aircraft itself may be flying subsonic. At some extreme AOAs, in some aircraft, the speed of the air over the top surface of the wing may be double the aircraft’s speed. It is therefore entirely possible to have both supersonic and subsonic airflow on an aircraft at the same time. When flow velocities reach sonic speeds at a location on an aircraft (such as the area of maximum camber on the wing), further acceleration results in the onset of compressibility effects such as shock wave formation, drag increase, buffeting, stability difficulties, and control difficulties. Subsonic flow principles are invalid at all speeds above this point.

  A factor of great importance in the study of high-speed airflow is the speed of sound. The speed of sound is the rate at which small pressure disturbances will be propagated through the air, and this propagation speed is a function of air temperature.

  The most important variable that determines the speed of sound is the temperature. Under standard temperature conditions of 15°C, the speed of sound at sea level is 661 knots. At 40,000 feet, where the temperature is –55°C, the speed of sound decreases to 574 knots. In high-speed flight and/or high-altitude flight, the measurement of speed is expressed in terms of a “Mach number”—the ratio of the true airspeed of the aircraft to the speed of sound in the same atmospheric conditions. An aircraft traveling at the speed of sound is traveling at Mach 1.0.

  As an object moves through the air mass, velocity and pressure changes occur that create pressure disturbances in the airflow surrounding the object. Of course, these pressure disturbances are propagated through the air at the speed of sound. If the object is travelling at low speed, the pressure disturbances are propagated ahead of the object and the airflow immediately ahead of the object is influenced by the pressure field on the object. Actually, these pressure disturbances are transmitted in all directions and extend indefinitely in all directions. Evidence of this “pressure warning” is seen in the typical subsonic flow pattern in Figure 8-1A, in which upwash and flow direction changes occur well ahead of the leading edge. If the object is travelling at a speed above the speed of sound, the airflow ahead of the object will not be influenced by the pressure field on the object since pressure disturbances cannot be propagated ahead of the object. Thus, as the flight speed nears the speed of sound, a compression wave will form at the leading edge and all changes in velocity and pressure will take place quite sharply and suddenly. The airflow ahead of the object is not influenced until the air particles are suddenly forced out of the way by the concentrated pressure wave set up by the object. Evidence of this phenomenon is seen in the typical Supersonic Flow

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  High Enthalpy Gas Dynamics

  • Ethirajan Rathakrishnan(Author)
  • 2014(Publication Date)
  • Wiley

    (Publisher)

  Chapter 5 Hypersonic Flows

  5.1 Introduction

  In the perfect gas dynamic theory, hypersonic flow is defined as the flow with Mach number greater than 5, where the change in flow Mach number is dictated by the change in the speed of sound. That is, in the hypersonic flow regime, the speed of sound is more dominant than the flow speed itself. But in problems such as the flow fields around blunt bodies begin to exhibit many of the characteristics of hypersonic flow when the Mach number is 4, or greater. By definition,

  5.1

  where is the freestream Mach number, is the flow speed, and is the speed of sound. That is, the Mach number is greatly larger than unity ( ) is the basic assumption for all hypersonic flow theories. Thus, the internal thermodynamic energy of the freestream fluid particles is small compared to the kinetic energy of the freestream for hypersonic flows. In flight applications, this results because the volume of the fluid particles is relatively large. The limiting case, where approaches infinity because the freestream velocity approaches infinity while the freestream thermodynamic state remains fixed, produces extremely high temperatures in the shock layer.

  The high temperatures associated with hypersonic flight are difficult to match in ground-test facilities, such as hypersonic wind tunnel and shock tunnel. Therefore, in wind tunnel applications, hypersonic Mach numbers are achieved through relatively low speeds of sound. Thus, in the wind tunnel, the test-section Mach number, , approaches infinity because the speed of sound goes to zero while the freestream velocity is held fixed. As a result, the fluid temperatures in such wind tunnels remains below the levels that would damage the wind tunnel model.

  Another assumption common to hypersonic flow is that the ratio of the freestream density to the density just behind a shock is extremely small, that is,

  5.2

  where is the freestream density and is the density behind the shock. Equation (5.2 ) is known as the small-density-ratio

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Elements of Aerodynamics

  A Concise Introduction to Physical Concepts

  • Oscar Biblarz(Author)
  • 2022(Publication Date)
  • Wiley

    (Publisher)

  10 Transonic and Hypersonic Aerodynamics 10.1 Introduction In this chapter we examine two separate flight regimes that while based on the same overall fluid dynamic principles are sufficiently different from other aerodynamics regimes to merit separate treatment. Transonic and hypersonic flows are also different from each other and need to be treated in a more advanced manner than our coverage of flow regimes in previous chapters. Their importance is nearly matched by their complexity, so we will only consider topics that can be presented without the aid of numerical tools. Our study of transonic and hypersonic flows will focus on some aspects of their unique character. Formulations with compressible‐flow boundary layers and high temperature gas effects are beyond the scope of our presentation. Supersonic flight is presently routinely achieved, but to get there the aircraft needs to pass through the transonic region (Mach 0.7–1.4), one that generates high drag forces and often significant flow unsteadiness thereby challenging thrust production from ordinary low‐speed powerplants and requiring advanced controls on the aircraft. Recall that when going through Mach 1, the aerodynamic center location, or “neutral point” on the wing (where the moment coefficient is independent of angle of attack), changes from the quarter chord to the half chord. With transonic flows we need to depart from our default thin airfoils’ assumptions because research has shown that for cruising at this regime “supercritical airfoils” need to be thick to minimize overall drag unlike more conventional wing designs

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Essentials of Supersonic Commercial Aircraft Conceptual Design

  • Egbert Torenbeek, Peter Belobaba, Jonathan Cooper, Allan Seabridge(Authors)
  • 2020(Publication Date)
  • Wiley

    (Publisher)

  Although the first turbojet‐powered aircraft had enough thrust to allow supersonic flight, they were also subject to adverse flight dynamics behavior since the aforementioned aerodynamic phenomena had a profound influence on the forces and moments acting on the plane. In the present world of large‐scale applications of high‐speed aircraft, designers involved in the development of a supersonic cruise vehicle (SCV) should have a basic understanding of aerodynamic phenomena in high‐speed flight, such as shock and expansion waves. Moreover, the prediction of aerodynamic properties such as the airplane's aerodynamic efficiency and the variation of its aerodynamic center with Mach number are essential elements during the early development of a supersonic transport aircraft configuration. An SCV must be able to cruise efficiently at supersonic as well as high‐subsonic Mach numbers similar to those of high‐subsonic airliners. Moreover, supersonic transport aircraft must have good aerodynamic properties in low‐speed flight for taking off and landing and, since the effects of compressibility have a major effect on aerodynamic phenomena observed at the complete range of operational Mach numbers, the conceptual designer should have good insight into the aerodynamic phenomena affecting the plane's behavior.

  The aerodynamic phenomena to be discussed in this book will be limited to those occurring at speeds below Mach 5.0, which implies that the air can be treated as a calorific perfect gas with constant values of the specific heat. Since the airplane's geometry is closely related to its most essential aerodynamic properties, some selected elements of classical linearized solutions to theoretical models are presented. The present chapter offers an abstract of topics treated in [8 ] on high speed flow phenomena around body shapes representative of aircraft components as well as applications of the theory. More in‐depth treatments of aerodynamic phenomena around high‐speed flight vehicles to which attention must be paid in the aerodynamic design phase can be found in textbooks such as [4 ,9 ], and [10 ].

  4.1.1 Speed of Sound and Mach Number

  An infinitesimal pressure disturbance such as a sound wave is transmitted in the atmosphere at the sonic (or acoustic) velocity. The propagation of sound is closely related to the transfer of momentum between colliding molecules, which depends on their average speed, whereas the average kinetic energy of molecules is proportional to the (local) temperature of the medium. The implication is that, according to the kinetic theory, the molecules of a gas are moving with an average velocity of , where and

  Sign up to read

  Learn more about book