Fluid Mechanics And Applications Chapter 5 Immersed BODY FLOW( Review) PDF

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Immersed Body Flow...

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2015-04-22

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Chapter 5 IMMERSED BODY FLOW

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When a viscous fluid passes a solid body the body experiences a net force ,F which can

be decomposed into 2 components which are  1) Drag Force ----It is parallel to the flow direction  

2) Lift Force ---- It is perpendicular to the flow direction

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A = Wetted Area --- Total surface area in contact with fluid

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Ap = Plan form Area --- Maximum projected area of an object such as the wing

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There are two types of Drag force which are 1) Friction Drag (Viscous Drag)--- Due to the wall friction shear stress exerted on the

surface of a solid body  2) Pressure Drag ( Form Drag)--- Due to the difference in the pressure on the front and the rear surfaces of a solid body   For an inviscid body the total drag force exerted on a solid body is solely contributed by the pressure drag  For a viscous flow the total drag force is a combination of both the friction and pressure drags    

Laminar to turbulent Transition BL over a finite flat plate Viscous drag is significant when ReL is low At a given ReL the value of CD is less for laminar BL which implies that for a given

length of plate , the drag coefficient is less, when laminar flow is maintained over the longest possible distance .  At range ReL( 1000) because the size of the wake is fixed by the geometry of the object .

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Combined Friction and pressure Drag For a viscous laminar BL flow the BL separation occurs around 82 degrees For a turbulent BL flow the BL separation occurs around 120 degrees For a Turbulent

case the low pressure wake region is much reduced. Therefore the pressure difference between the front and the rear of a sphere is greatly reduced .  Smooth Ball

Rough Ball

Drag force is not reduced . Wake

Dimples on a rough ball triggers

is thick

the onset of turbulence . Turbulent BL delays the BL separation and makes the low pressure wake region much narrower resulting in a much reduced pressure drag . With reduced drag force , a golf ball can fly over a much longer distance that a smooth ball

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Characteristics Area in Drag Coefficient Characteristics assumes one of the following types : 1) Wetted Area --- Total area that is in contact with fluid 2) Frontal Area --- Projected area as seen by the flow 3) Plan form Area ----Maximum projected area as seen from above SREAMLINING The extent of wake (separated flow region) behind an object can be reduced or

eliminated using streamlining and this can also reduces pressure drag.

2015-04-22 

2015-04-22 ...