The drag coefficient (Cd, Cx or Cw, depending on the country) is a dimensionless quantity that describes a characteristic amount of aerodynamic drag caused by fluid flow, used in the drag equation. In Dimensional analysis, a dimensionless quantity (or more precisely a quantity with the dimensions of 1) is a Quantity without any Physical units FLUID ( F ast L ight '''U'''ser '''I'''nterface D esigner is a graphical editor that is used to produce FLTK Source code In Fluid dynamics, the drag equation is a practical formula used to calculate the force of drag experienced by an object due to a Fluid that it is moving [1] Two objects of the same frontal area moving at the same speed through a fluid will experience a drag force proportional to their Cd numbers. Coefficients for rough unstreamlined objects can be 1 or more, for smooth objects much less.
explanation of terms on drag equation page. In Fluid dynamics, the drag equation is a practical formula used to calculate the force of drag experienced by an object due to a Fluid that it is moving
Usually A is the projected frontal area. For example, for a sphere A = πr2, (i. e. , not the surface area. )
The drag equation is essentially a statement that, under certain conditions, the drag force on any object is approximately proportional to the square of its velocity through the fluid. In Fluid dynamics, drag (sometimes called fluid resistance) is the force that resists the movement of a Solid object through a Fluid (a In Physics, a force is whatever can cause an object with Mass to Accelerate. In Physics, velocity is defined as the rate of change of Position. The required conditions are that the Reynolds number of the flow around the object must be high enough to create a turbulent wake (larger velocities, larger objects, and lower viscosities make for larger Reynolds numbers), and that the object does not approach the speed of sound in the fluid. In Fluid mechanics and Heat transfer, the Reynolds number \mathrm{Re} is a Dimensionless number that gives a measure of the Ratio In Fluid dynamics, turbulence or turbulent flow is a fluid regime characterized by chaotic Stochastic property changes A wake is the region of Turbulence immediately to the rear of a solid body caused by the flow of Air or Water around the body Viscosity is a measure of the resistance of a Fluid which is being deformed by either Shear stress or Extensional stress.
At a low Reynolds number, that is for small objects, low velocities, or high viscosity fluids, the flow around the object is laminar, Cd is no longer constant but depends on velocity, and Fd is proportional to v instead of v2. Laminar flow, sometimes known as streamline flow occurs when a fluid flows in parallel layers with no disruption between the layers
A Cd equal to 1 would be obtained in a case where all of the fluid approaching the object is brought to rest, building up stagnation pressure over the whole front surface. In Fluid dynamics, stagnation pressure is the Pressure at a Stagnation point in a fluid flow where the kinetic energy is converted into pressure energy The top figure shows a flat plate with the fluid coming from the right and stopping at the plate. The graph to the left of it shows equal pressure across the surface. In a real flat plate the fluid must turn around the sides, and full stagnation pressure is found only at the center, dropping off toward the edges as in the lower figure and graph. The Cd of a real flat plate would be less than 1, except that there will be a negative pressure (relative to ambient) on the back surface. The overall Cd of a real square flat plate is often given as 1. 17. Flow patterns and therefore Cd for some shapes can change with the Reynolds number and the roughness of the surfaces.
Contents |
Cd in automobiles
The drag coefficient is a common metric in automotive design, where designers strive to achieve a low coefficient. Minimizing drag is done to improve fuel efficiency at highway speeds, where aerodynamic effects represent a substantial fraction of the energy needed to keep the car moving. Fuel efficiency, in its basic sense is the same as Thermal efficiency, meaning the efficiency of a process that converts chemical potential energy contained in a carrier Indeed, aerodynamic drag increases with the square of speed. Aerodynamics are also of increasing concern to truck designers, where a lower drag coefficient translates directly into lower fuel costs. This article is about the semi-truck For the North American use of the word see Pickup truck.
About 60% of the power required to cruise at highway speeds is taken up overcoming air drag, and this increases very quickly at high speed. Therefore, a vehicle with substantially better aerodynamics will be much more fuel efficient. Additionally, because drag does increase with the square of speed, a somewhat lower speed can significantly improve fuel economy. This was the major reason for the United States adopting a nationwide 55 mile per hour speed limit during the early 1973 oil crisis as slower traffic would save scarce petroleum. The United States of America —commonly referred to as the The National Maximum Speed Law in the United States was a provision of the 1974 Emergency Highway Energy Conservation Act that prohibited speed limits higher The 1973 oil crisis began on October 17 1973 when the members of Organization of Arab Petroleum Exporting Countries (OAPEC consisting of the Arab members of Petroleum ( L petroleum, from Greek πετρέλαιον, lit
CdA
While designers pay attention to the overall shape of the automobile, they also bear in mind that reducing the frontal area of the shape helps reduce the drag. The combination of drag coefficient and area is CdA (or CxA), a multiplication of the Cd value by the area.
In aerodynamics, the product of some reference area (such as cross-sectional area, total surface area, or similar) and the drag coefficient is called drag area. In Aerodynamics, the zero-lift drag coefficient C_{D0} is a dimensionless parameter which relates an aircraft's zero-lift drag Force to its In 2003, Car and Driver adapted this metric and adopted it as a more intuitive way to compare the aerodynamic efficiency of various automobiles. Year 2003 ( MMIII) was a Common year starting on Wednesday of the Gregorian calendar. Car and Driver is an American automotive enthusiast Magazine. Average full-size passenger cars have a drag area of roughly 8. 5 ft² (. 79 m²). Reported drag area ranges from the 2000 Honda Insight at 5. The Honda Insight is a Hybrid electric vehicle manufactured by Honda in its first generation as a three-door hatchback from model years 2000 to 2006 1 ft² (. 47 m²) to the 2003 Hummer H2 at 26. The Hummer H2 is an SUV and SUT manufactured by General Motors under the Hummer brand 3 ft² (2. 44 m²).
More CdA examples
| Terminal velocity |
Mass | ||||
|---|---|---|---|---|---|
| 60 kg | 70 kg | 80 kg | 90 kg | 100 kg | |
| 45 m/s | 0. 487 | 0. 569 | 0. 650 | 0. 731 | 0. 812 |
| 50 m/s | 0. 395 | 0. 461 | 0. 526 | 0. 592 | 0. 658 |
| 55 m/s | 0. 326 | 0. 381 | 0. 435 | 0. 489 | 0. 544 |
| 60 m/s | 0. 274 | 0. 320 | 0. 365 | 0. 411 | 0. 457 |
| 65 m/s | 0. 234 | 0. 272 | 0. 311 | 0. 350 | 0. 389 |
| 70 m/s | 0. 201 | 0. 235 | 0. 269 | 0. 302 | 0. 336 |
| 75 m/s | 0. 175 | 0. 205 | 0. 234 | 0. 263 | 0. 292 |
This value is extremely useful as either the area or drag coefficient alone are not enough to be used in any equation. Sometimes it is not possible to get either value, but it might be possible to deduce it. For a skydiver example below, it is possible to deduce CdA from the mass of the diver and equipment and terminal velocity. Skydiver CdA examples are in both ft² and m² units.
Cd examples
|
|
See also
- Automobile drag coefficients
- Automotive aerodynamics
- Drag (physics)
- Drag equation
- Drag crisis
- Lift coefficient
- Pitching moment
References
- McCormick, Barnes W. Automotive aerodynamics is the study of the Aerodynamics of road vehicles In Fluid dynamics, drag (sometimes called fluid resistance) is the force that resists the movement of a Solid object through a Fluid (a In Fluid dynamics, the drag equation is a practical formula used to calculate the force of drag experienced by an object due to a Fluid that it is moving In Fluid dynamics, drag crisis is a phenomenon in which Drag coefficient drops off suddenly as Reynolds number increases The lift coefficient ( CL or CZ) is a non-dimensional coefficient that relates the lift generated by an Airfoil, the In Aerodynamics, the pitching moment on an Airfoil is the Moment produced by the Aerodynamic force on the airfoil if that aerodynamic force is (1979), Aerodynamics, Aeronautics, and Flight Mechanics, Chapter 4, John Wiley & Sons, Inc. New York ISBN 0-471-03032-5
Notes
- ^ McCormick, Barnes W. , Aerodynamics, Aeronautics, and Flight Mechanics, p. 24
External links
- Experienced Environmentalists improve Car Economy by improving Aerodynamics: Gas Mileage and Aerodynamics
- A. Filippone's Advanced Topics in Aerodynamics: Drag
- Danish Wind Industry Association: Aerodynamics of Wind Turbines: Drag
- Improving Aerodynamics to Boost Fuel Economy
- Tel Aviv University reduces drag on trucks by 10%
- Simple roll-down test for measuring Cd and Crr for cars and bikes
- Variation of drag coefficient with Reynolds number
- Drag of Blunt Bodies and Streamlined Bodies
Dapyx Software network: MP3 Explorer | Ebook Manager | Zenithic