Fluid Dynamic Drag
Fluid dynamic drag refers to the force exerted on an object as it moves through a fluid, such as air or water. It is caused by the friction and pressure of the fluid against the surface of the object. Understanding and minimizing fluid dynamic drag is crucial in designing efficient vehicles and structures, as it directly impacts their speed, fuel consumption, and performance.
5 Key excerpts on "Fluid Dynamic Drag"
eBook - ePubUnderstanding Aerodynamics
Arguing from the Real Physics
- Doug McLean(Author)
- 2012(Publication Date)
- Wiley(Publisher)
...Even for well-designed streamlined bodies, the pressure contribution is rarely less than about 10% of the total drag. For airfoils, the pressure contribution to drag is generally larger than this, as we'll see in Section 7.4.2.The pressure contribution to the force can be clearly interpreted as lift (Equation 5.4.1) or drag ("Equation 6.1.1) only after it has been integrated over the entire surface of a closed body. If the pressure contribution is integrated over only a portion of a body, interpreting the result as “lift” or “drag” is problematic. We'll see the reasons for this when we consider the issue in detail as it applies to drag in Section 6.1.3.Newton's third law requires that the force exerted by the flow on the body is reciprocated by an equal-and-opposite force exerted by the body on the flow. And the equations of motion require that this force must have manifestations in the flowfield. Generating lift requires setting some of the fluid in motion, as we'll discuss in great detail in Chapters 7 and 8. Drag requires either generating lift in 3D or the presence of the dissipative processes associated with viscosity, as we'll see in Chapter 6....
eBook - ePubSolid/ Liquid Separation
Principles of Industrial Filtration
- Stephen Tarleton, Richard Wakeman(Authors)
- 2005(Publication Date)
- Elsevier Science(Publisher)
...A particle immersed in a flowing fluid is acted upon by both pressure and viscous forces from the fluid. The sum of the forces which acts normal to the free stream direction is the lift, and the sum which acts parallel to the free stream direction is the drag. Buoyant or weight forces may also act on the particle, but are differentiated from lift and drag forces as the latter are limited by definition to those forces produced by the dynamic action of the flowing fluid. The particle Reynolds number is used to describe the flow of a particle through a fluid, and is defined by: Re = ρ u x μ (2.1) where ρ and µ are the density and viscosity of the fluid, u is the particle-fluid relative velocity, and x is the particle size. When Re < 0.2, flow around a spherical particle is completely laminar (creeping flow) and is amenable to theoretical analysis. Stokes (1851) solved the hydrodynamic equations of motion, the Navier-Stokes equations, to give F D = 3 π μ u x (2.2) which is known as Stokes' law. F D in this equation is made up from skin friction (2 πµux) and form drag (πµux). Deviations from this equation become greater at higher Reynolds numbers when the skin friction becomes proportionately less than the form drag; at Re greater than about 20, flow separation occurs with the formation of vortices in the wake of the particle. The sizes of the vortices increases with Re until, at values of between 100 and 200, instabilities in the flow cause vortex shedding. When a particle moves relative to a fluid in which it is suspended there exists a force opposing motion, known as the drag force...
eBook - ePub- Mark Dusenbury, Gary Ullrich, Shelby Balogh(Authors)
- 2016(Publication Date)
- Aviation Supplies & Academics, Inc.(Publisher)
...Drag Key Terms aspect ratio bound vortex boundary layer compressibility cooling drag effective lift fairings form drag induced angle of attack (AOA) induced drag induced drag coefficient interference drag leakage drag parasite drag Reynolds number skin friction drag span efficiency factor total drag Symbols & Abbreviations AR aspect ratio b wing span c wing chord C D coefficient of drag C Di induced drag coefficient C DP coefficient of parasite drag C L coefficient of lift D i induced drag Dp parasite drag f equivalent parasite area L lift M Mach number q dynamic pressure Re Reynolds number S surface area v velocity w downwash. vector W weight α i induced angle of attack ε span efficiency factor μ viscosity coefficient ρ air density Introduction The force that retards the forward motion of an aircraft through the air is referred to as drag. Drag acts parallel and opposite to the flightpath. Since drag tends to retard motion and increase fuel consumption, performance objectives such as range, endurance, and maximum velocity are all affected by drag. To minimize drag and increase performance, the aircraft may be operated with the landing gear and flaps retracted to reduce drag as much as possible. However, during specific maneuvers or phases of flight, increased drag becomes desirable or even necessary and the pilot must understand how to augment this force in order to obtain the required performance. There are two types of drag produced at subsonic speeds: parasite drag and induced drag. The first is called parasite because it does not aid flight. Induced drag is a result of an airfoil developing lift. Parasite Drag Parasite drag is composed of all the forces that work to slow an aircraft’s movement. As the term parasite implies, this type of drag is not associated with the production of lift. It includes the displacement of the air by the aircraft, turbulence generated in the airstream, or a hindrance of air moving over the surface of the aircraft and airfoil...
eBook - ePub- Rose G Davies(Author)
- 2020(Publication Date)
- CRC Press(Publisher)
...Drag is a force, so it is a vector. Its direction is the same as the relative air velocity of the aircraft. There are two types of the total drag the aircraft experiences in flight: induced drag, which is related to lift produced by the aerofoils, and parasite drag, which exists over the aircraft body due to the airflow around the aircraft. Understand the origin, or the cause of different parts of the drag can lead pilots controlling the aircraft more effectively. Induced Drag Airflow around an aircraft wing is three-dimensional (3-D), because all of real wings used in an aircraft are with finite spans as shown in Figure 4.4. The air pressure over an aerofoil is lower than the pressure in free-stream, in particular, at the wing root. The pressure difference will drive the air over the wing to flows inwards: the spanwise component of the air velocity heads toward the fuselage. On the other hand, the air pressure below the aerofoil is likely greater than the pressure in free-stream, and this pressure difference will make the air flow outward: the spanwise component of the air velocity heads away from the fuselage, as shown in Figure 4.14 (b). The two airflow streams above the wing and below the wing meet at the trailing edge of the wing. FIGURE 4.14 (a) Difference in pressure between over the upper surface and under lower surface of a wing; (b) Trailing edge vortices and wingtip vortex. As air is a viscous fluid, there is a tendency between the air particles to drive the air particles to reduce the difference in their movements to move together. So the air particles encounter and interact, causing rotating motion at the trailing edge, with the result being that trailing edge vortices are formed. At the wingtip, apart from airflow going in different directions between upper and lower surfaces of the wing, there is a relative strong air pressure gradient...
eBook - ePubIntroduction to Engineering Mechanics
A Continuum Approach, Second Edition
- Jenn Stroud Rossmann, Clive L. Dym, Lori Bassman(Authors)
- 2015(Publication Date)
- CRC Press(Publisher)
...The basic idea is that pressure is lower on the upper surface of a wing, so a net upward force keeps the wing aloft. We could show this using the Bernoulli equation: the flow over the smooth upper surface is much faster (therefore exerts lower pressure) than that past the lower surface. Knowing that drag D and lift L are the x and y resultants of the pressure and viscous stress forces, we could obtain expressions for D and L by integrating these pressure and viscous forces over the body’s surface: D = ∫ d F x = ∫ p cos θ d A + ∫ τ w sin θ d A, (20.19) L = ∫ d F y = − ∫ p sin θ d A + ∫ τ w cos θ d A, (20.20) where θ is the degree of inclination (with respect to horizontal) of the outward normal at any point along the body surface. To carry out this integration, we must know the body shape, including θ as a function of position along the body, and the distribution of τ w and p. This is quite difficult to do for realistic geometries. As we have seen when finding the resultant pressure forces on submerged curved surfaces, there are sometimes ways to get around messy integrations involving changing orientations. This is also the case for lift and drag. FIGURE 20.3 Forces on two-dimensional airfoil: (a) pressure force, (b) viscous force, and (c) resultant lift and drag forces. In the simplest force analysis of an airplane, the four important forces are lift, drag, thrust (forward propulsion provided by engines), and weight of the plane. Lift must exceed weight, and thrust must exceed drag, in order for flight to be possible...
