Resistance of Fluids


Resistance of Fluids
Fluid resistances are the forces which act on a solid object in the direction of fluid flow velocity.
Fluid resistances are also refers to as drag.

Fd= cd1/2 ρ v2A

where,
Fd= drag force (N)
cd= drag coefficient
ρ= density of fluid
v= flow velocity
A - cross-sectional area, and

Drag is divided into two components: frictional drag and pressure drag. Frictional drag comes from friction between the fluid and the surfaces over which it is flowing. This friction is associated with the development of boundary layers. Pressure drag comes from the eddying motions that are set up in the fluid by the passage of the body. Formally, both types of drag are due to viscosity, but the distinction is useful because the two types of drag are due to different flow phenomena. Frictional drag is important for attached flows (that is, there is no separation), and it is related to the surface area exposed to the flow. Pressure drag is important for separated flows, and it is related to the cross-sectional area of the body.

Types Of Drag

Skin Friction Drag
That part of the drag which is caused by shear stress at all points on the body concerned where it is touched by the fluid in which it is immersed; it acts tangentially to the surface. It is due entirely to the viscosity of the fluid in which the body is immersed.

Form Drag
It depends on the cross sectional area of the body presented to fluid..

Profile Drag
Profile Drag is the sum of Form drag and Skin Friction drag. It is the drag to which the Drag Coefficient Cd refers, remember that the coefficients in text books are for two dimensional airfoil sections and those wings and rotor blades are three dimensional.

Induced Drag
Induced drag is dependent upon lift production and is not usually associated with viscous effects. In fixed wing terms the induced drag is caused by part of the lift vector being inclined rearward due to the downwash created by the formation of vortices at the trailing edge which causes the relative wind to be inclined downwards, reducing the angle of attack of the aero foil. Remember lift is defined as that part of the force which acts at 90 degrees to the relative airflow.


Terminal Velocity
Terminal velocity is the velocity at which drag force from the air equals the force from the weight of an object or buoyancy. Thus the object no longer accelerates and its velocity remains constant. The object has zero acceleration since the net force applied on it is also zero.
Vt =√(2mg/ ρA Cd)

where,
Vt= terminal velocity,
m = mass of the falling object,
g = acceleration due to gravity,
Cd= drag co-efficient,
ρ= densityof the fluid through which the object is falling, and
A= projected area of the object.

Applications
The creeping flow results can be applied in order to study the settling of sediment particles near the ocean bottom and the fall of moisture drops in the atmosphere. The principle is also applied in the falling sphere viscometer, an experimental device used to measure the viscosity of high viscous fluids.


Q and A

1) What is resistance of a fluid?
A. Fluid resistances are the forces which act on a solid object in the direction of fluid flow velocity.
2) What are different types of drag?
A. a) Skin friction drag b) Form drag c) Profile drag d) Induced drag
3). what is terminal velocity?
A. Terminal velocity is the velocity at which drag force from the air equals the force from the weight of an object or buoyancy.

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