Force on immersed bodies and fluid mechanics Tutorialpoint #1

Fluid Mechanics
Fluid Mechanics

👉 What do you understand by immersed bodies ? What forces an immersed body experiences ?

The bodies which remains completely submerged in a flowing fluid (liquid or air) are called immersed bodies. The vehicles such as automobiles, trains and aeroplanes moves completely immersed in atmospheric air, while submarine which moves completely immersed in water are examples of flow over immersed bodies. Building, bridges are also examples of immersed bodies.
When immersed bodies moves with a constant velocity in a large stationary mass of fluid, it experiences a resistance force which tends to oppose its motion. This resistance force remains same whether the body moves through the fluid or the fluid flows around the body so long as the relative motion remains the same. This force exerted by fluid on the moving body isgenerally acts at an angle to the direction of motion. The component of this force in the direction of motion is called drag force (FD), and the component perpendicular to the direction of motion is called lift (FL). However, for a symmetrical body e.g. sphere, cylinder, etc. facing the flow symmetrically, there is no lift and total force exerted by the fluid is drag force only.

👉Explain the various types of drag forces.

Drag can be classified into the following types –

(i) Surface or Friction Drag -When a real fluid flows past a Submerged bedy, held stationary, a layer of fluid which comes in contact with the surface of body, adheres to it. This layer of fluid, which cannot slip away from the body, undergoes retardation. This retarded layer of fluid further causes the retardation of adjacent fluid layers. In this way, there forms a small region in the immediate vicinity of solid surface, where velocity varies from zero to maximum flow velocity. Due to this large velocity variation in a small region there exists a fairly large velocity gradient. This region is known a boundary layer. This boundary layer zone because of large velocity gradient produces considerable shear stress even for alow viscous fluid. The tangential force exerted by this ‘shear stress is called surface or friction or shear drag,

(ii) Pressure or Form Drag – The another effect of viscosity is exhibited in the separation of flow. When fluid flows past a bluff body whose surface 13 no where parallel to main stream, the fluid starts to leave the body surface, due to growth of boundary layer. Due to this flow separation there forms a region of low pressure (known as wake) on downstream side of the body, while pressure at the upstream side of the body is still high. This pressure difference produces a drag on bedy, known as pressure drug. As pressure dragtdepends upon the shape/form of body, it is also known as form drag.

(iii) Deformation Drag– The third effect of viscosity is that it causes deformation of fluid particles. The force required to produce deformation offers additional resistance to motion. Component of these forces in the direction of motion is called deformation drag. However, this drag force exists Only when Re < 0.10, i.e. when viscous forces are much more predominant than inertial force. The deformation drag is usually negligible at higher Reynold’s number.

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