Pressure Drag

3 Key excerpts on "Pressure Drag"

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  eBook - ePub

  Advanced Aircraft Design

  Conceptual Design, Analysis and Optimization of Subsonic Civil Airplanes

  • Egbert Torenbeek, Peter Belobaba, Jonathan Cooper, Roy Langton, Allan Seabridge, Peter Belobaba, Jonathan Cooper, Roy Langton, Allan Seabridge(Authors)
  • 2013(Publication Date)
  • Wiley

    (Publisher)

  4 Aerodynamic Drag and Its Reduction

  It has been noted that transportation is fundamentally 0% efficient as it involves moving mass from rest at one point to rest at another point, so that the energy of the system is unchanged. That it does take energy to accomplish this objective is due to the presence of drag, and the reduction of drag has been the primary focus of aircraft design over the last century.

  —I.H. Kroo [72] (2001) 4.1 Basic Concepts

  The drag estimation for a new design is not generally a single exercise, but a continuous process through its life from the early project study stage through preliminary design and development. Similar to weight prediction, the fidelity of the drag prediction methodology used during advanced design (AD) varies with the accuracy required, with the degree of airplane geometry definition, and with the amount of computed or experimental data available. Initially, predictions are mostly semi-empirical with a gradual shift to quasi-analytical and numerical methods. A wide variety of handbook methods, computations and wind tunnel data are used as the design proceeds and it is essential to be consistent with the definition of drag components. Although aircraft design over the past century has evolved into a process of increasing sophistication, the prediction of aerodynamic drag still poses a formidable challenge to the AD engineer. Even elemental flow physics driving drag can be quite complex. In addition, there are myriad ways in which flow fields around airplane components can interact to produce interference drag which is very difficult to predict accurately. But it is clear that full-scale aircraft drag prediction errors of 10 to 20% that have occurred in the past in certain development programs are outwith the range needed for success [38].

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  eBook - ePub

  Sports Biomechanics

  The Basics: Optimising Human Performance

  • Prof. Anthony J. Blazevich(Author)
  • 2017(Publication Date)
  • Bloomsbury Sport

    (Publisher)

  FIG. 13.2 A drag force can be conceptualised by imagining each particle of a fluid applying a force against an object as they collide. The larger the number of collisions (i.e. greater surface area of the object, faster flow of the fluid or a greater density of the fluid) the greater the rate of collisions and therefore the greater the force exerted by the fluid.

  Whichever way you choose to model it, you can see that the movement of an object within a fluid will tend to slow the object. This is undesirable in many sports, so we have to minimise it. Form drag

  As I hinted above, one way to minimise drag is to reduce the area of the object that touches the fluid. This will reduce the amount of fluid that has its velocity changed in the collision with the object (or in a collision with other fluid molecules that have been deflected) and therefore reduce the energy lost from the object. In this sense, we need to find a body position on the bike that has the smallest possible frontal surface area, so that collisions are minimised. This is one benefit of the ‘tuck’ position, which is shown in the photograph at the start of this chapter.

  A second factor that influences drag is the shape of the object, because this affects how much the laminar flow will become turbulent. If the leading edge of an object is pointed, the direction of the fluid hitting the object will be changed more slowly than if the fluid hits the object abruptly (see Figure 13.3 (A)). Remember from Chapter 11 that when a ball collides with a bat with a larger angle of incidence (that is, more parallel to the bat) the coefficient of restitution is increased? Similarly, if the fluid hits the object at a larger angle of incidence, less energy will be lost from the object.

  FIG. 13.3 A. By shaping objects with a longer leading edge, fluid particles diverge earlier and strike the object’s surface at a larger angle of incidence. This minimises the ability of the fluid to exert a force on the object and reduces drag. B.

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  Understanding Aerodynamics

  Arguing from the Real Physics

  • Doug McLean(Author)
  • 2012(Publication Date)
  • Wiley

    (Publisher)

  propulsive effort being expended. Dissipative effects tend to contribute to drag, and propulsive effort tends to contribute to thrust, but when both kinds of effects are present in the same flow, as they often are, these connections are not clean and simple. And if we look at the drag or thrust on just part of a body, the connections are often nonexistent. This is especially true of the pressure forces, and in Section 6.1.6 we'll address some misunderstandings that can arise in this regard. In most of our discussion of the basic physics of drag, we'll limit our attention to the passive case, without propulsion. Even then, to avoid errors, we must look beyond simply resolving the forces and come to understand drag in terms of the flow mechanisms responsible for it. Then in Section 6.1.10 we'll take a brief look at the basic physics issues associated with propulsion.

  Drag, being parallel to the direction of a vehicle's motion through the local air mass, requires work to be done in the reference frame of the air mass. The energy to do this work can come from combinations of active propulsive effort, gravity, motions of the atmosphere, and the vehicle's kinetic energy. And of course when work is done, it has thermodynamic implications for the flowfield. These work and energy considerations can be tricky to deal with because they look different in different reference frames. We'll touch on this issue again in Section 6.1.3.

  Our understanding of drag has a complicated history. Most early work in theoretical fluid mechanics assumed an inviscid fluid. For bodies without vortex shedding or shocks, inviscid theory predicts zero drag, in contradiction to all experiments, a result known as d'Alembert's paradox. Fluid dynamicists knew what the source of the problem was, but devising computationally manageable theories that could predict drag was more easily said than done. For drag in external flows, analytic solutions to the Navier-Stokes (NS) equations were feasible only for simple 2D bodies (cylinders and spheres) in the limit of low Reynolds number. Attempts to treat high-Reynolds-number bluff-body drag in an inviscid framework required empirical input and were not very successful. For streamlined bodies at high Reynolds numbers, a correct understanding finally arrived with Prandtl's boundary-layer theory in 1904. Accurate calculations of the total viscous drag, including the pressure contribution, didn't come until much later. We'll discuss the theoretical issues involved in Section 6.1.6 and look at the particular case of airfoils in Section 7.4.2.

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