Fundamentals Of Fundamentals Of Boundary Layer TheoryBoundary Layer Theory

What is boundary layer?What is boundary layer?The boundary layer of a flowing The boundary layer of a flowing fluid is a thin layer close to the fluid is a thin layer close to the wall.wall.It is the fluid layer in the It is the fluid layer in the neighborhood of the solid neighborhood of the solid boundary where the effects of fluid boundary where the effects of fluid friction or viscous effects are friction or viscous effects are predominant.predominant.The main flow velocity with in this The main flow velocity with in this layer tends to zero while layer tends to zero while approaching the wall.approaching the wall.Where ever flowing fluid comes in Where ever flowing fluid comes in contact with solid boundary layercontact with solid boundary layer1. element energy 1. element energy 2.internal energy 2.internal energy 3.kinetic energy3.kinetic energy

Moving fluid elementMoving fluid elementdxdydzvedtdA )2/(/ 2

Actual energy balance A=B+CActual energy balance A=B+C

Significance of boundary layerSignificance of boundary layer

This boundary layer occurs in case of flow This boundary layer occurs in case of flow past blunt body cylinders, spheres , airfoils past blunt body cylinders, spheres , airfoils and associated with the formation of and associated with the formation of vortices resulting in large energy lossesvortices resulting in large energy losses

Downstream of such bodies exists a Downstream of such bodies exists a region of highly decelerationregion of highly deceleration

This region is called wake zone or wake This region is called wake zone or wake characterized by pressure distributioncharacterized by pressure distribution

NOMINAL THICKNESS NOMINAL THICKNESS

Boundary layer is arbitrary as transition Boundary layer is arbitrary as transition starts from zero velocity to free stream starts from zero velocity to free stream velocity velocity

Boundary layer thickness is defines as the Boundary layer thickness is defines as the velocity at a small distance from the velocity at a small distance from the boundary measured in y-direction attains a boundary measured in y-direction attains a value which is close to 0.99 of free stream value which is close to 0.99 of free stream velocityvelocity

DISPLACEMENT THICKNESSDISPLACEMENT THICKNESSBoundary layer formation and resulting velocity Boundary layer formation and resulting velocity distribution displaces the flow rate slightly distribution displaces the flow rate slightly outward form solid boundary as compared with outward form solid boundary as compared with potential flow in boundary layerpotential flow in boundary layer