Drag force study Drag force study
Drag force Case study
A body moving through the air experiences resistance and a stationary body in an air stream experiences wind force. The forces are due to the shearing action which is produced when the air flows over a surface and are generally known as drag forces.
Motor vehicle and aeronautical engineers are continually seeking to reduce these forces in a quest for improved efficiency. Architects and structural engineers must also take account of wind force in the design of buildings and bridges. Aerodynamics is the study of air flow and its effects, in which the wind tunnel testing of models plays a major role.
Drag force
Drag is the aerodynamic force that opposes an aircraft's motion through the air. The total drag force acting on a body in a fluid stream is made up of two components. They are known as form drag and skin friction drag. On streamlined objects, the form drag and skin friction drag are roughly of the same magnitude. Skin friction drag is the viscous resistance caused by the shearing action which takes place across the boundary layer adjacent to the surface of a body. It is dependant on the surface area and the surface texture. The magnitude of skin friction drag is extremely difficult to predict except on the most simple shapes. It can, however, be kept to a minimum by making surfaces as smooth as possible.
Form drag depends on the projected area which a body presents to the fluid stream. With an irregularly shaped or bluff body, the form drag will be high and with a streamlined body it
will be comparatively low. Figure 1 shows a bluff body in a fluid stream. Station (1) is in the free stream where the velocity is 𝓥₁and the pressure is 𝓟₁. It is assumed that the pressure behind the body is also 𝓟₁. Station (2) is on the front surface of the body where stagnation conditions are assumed to exist, i.e. where the fluid has been brought to rest. The pressure, 𝓟₂ will thus be greater than 𝓟₁. Applying Bernoulli’s equation to stations (1) and (2), we obtain
But 𝓥₂ = 0 and 𝓩₁ =𝓩₂. Eliminating these, we get
where
𝝆 is density kgm ⁻³
𝓥₁ is the velocity of the free stream ms⁻ ¹
𝓟₁ is the pressure behind the body kg m⁻²
𝓟₂ is the pressure on the front of body kg m⁻²
This is known as the dynamic pressure acting on the body as a result of the fluid being brought to rest. It is assumed to act uniformly over the whole projected area A(a) which the body presents to the fluid stream. The theoretical form drag is thus given by
Dynamic pressure is the pressure increase that results when a fluid is brought to rest on a surface in a fluid stream.
the dynamic pressure does not act uniformly over the projected area and in addition to the pressure build-up in front of the body, there is often a pressure decrease in the wake behind it. There is also skin friction drag present. Nevertheless, the above theoretical drag force is used as a standard against which the actual measured drag force can be compared. The actual drag force is obtained from exhaustive wind tunnel tests where it is measured on sensitive balances. The theoretical and measured drag forces can then be used to calculate the drag coefficient C𝘋 for the body.
where 𝓥 is the free stream velocity. (𝓥₁)
The value of C𝘋 for a flat plat is around 1.15 and for a cylindrical object it is around 0.9. For modern cars it is around 0.3 and for a fully streamlined teardrop shape it is around 0.05.
The drag coefficient gives a comparison of the measured drag force on a body and the theoretical drag force that is calculated from The dynamic pressure.
Drag force Case study
A body moving through the air experiences resistance and a stationary body in an air stream experiences wind force. The forces are due to the shearing action which is produced when the air flows over a surface and are generally known as drag forces.
Motor vehicle and aeronautical engineers are continually seeking to reduce these forces in a quest for improved efficiency. Architects and structural engineers must also take account of wind force in the design of buildings and bridges. Aerodynamics is the study of air flow and its effects, in which the wind tunnel testing of models plays a major role.
Drag force
Drag is the aerodynamic force that opposes an aircraft's motion through the air. The total drag force acting on a body in a fluid stream is made up of two components. They are known as form drag and skin friction drag. On streamlined objects, the form drag and skin friction drag are roughly of the same magnitude. Skin friction drag is the viscous resistance caused by the shearing action which takes place across the boundary layer adjacent to the surface of a body. It is dependant on the surface area and the surface texture. The magnitude of skin friction drag is extremely difficult to predict except on the most simple shapes. It can, however, be kept to a minimum by making surfaces as smooth as possible.
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| FIG 1 |
Form drag depends on the projected area which a body presents to the fluid stream. With an irregularly shaped or bluff body, the form drag will be high and with a streamlined body it
will be comparatively low. Figure 1 shows a bluff body in a fluid stream. Station (1) is in the free stream where the velocity is 𝓥₁and the pressure is 𝓟₁. It is assumed that the pressure behind the body is also 𝓟₁. Station (2) is on the front surface of the body where stagnation conditions are assumed to exist, i.e. where the fluid has been brought to rest. The pressure, 𝓟₂ will thus be greater than 𝓟₁. Applying Bernoulli’s equation to stations (1) and (2), we obtain
𝝆 is density kgm ⁻³
𝓥₁ is the velocity of the free stream ms⁻ ¹
𝓟₁ is the pressure behind the body kg m⁻²
𝓟₂ is the pressure on the front of body kg m⁻²
This is known as the dynamic pressure acting on the body as a result of the fluid being brought to rest. It is assumed to act uniformly over the whole projected area A(a) which the body presents to the fluid stream. The theoretical form drag is thus given by
theoretical drag = dynamic pressure × projected area
the dynamic pressure does not act uniformly over the projected area and in addition to the pressure build-up in front of the body, there is often a pressure decrease in the wake behind it. There is also skin friction drag present. Nevertheless, the above theoretical drag force is used as a standard against which the actual measured drag force can be compared. The actual drag force is obtained from exhaustive wind tunnel tests where it is measured on sensitive balances. The theoretical and measured drag forces can then be used to calculate the drag coefficient C𝘋 for the body.
drag coefficient = measured drag force / theoretical drag force
where 𝓥 is the free stream velocity. (𝓥₁)
The value of C𝘋 for a flat plat is around 1.15 and for a cylindrical object it is around 0.9. For modern cars it is around 0.3 and for a fully streamlined teardrop shape it is around 0.05.
The drag coefficient gives a comparison of the measured drag force on a body and the theoretical drag force that is calculated from The dynamic pressure.
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