# Velocity and velocity profile

## Presentation on theme: "Velocity and velocity profile"— Presentation transcript:

Flow Properties Tools to describe flows
Lecture 3 Flow Properties Tools to describe flows

Velocity and velocity profile
The velocity perpendicular to a surface is the flow per unit of area In a generic reference, velocity is a vector with 3 components.

Spatial variability: 1D, 2D, 3D flow
3D flow: velocity varies in all the directions of the space. 2D flow: velocity varies in two space directions. 1D flow: velocity varies in one direction of the space. Uniform flow: the velocity is uniform, independent of directions. Steady & Uniform (escoamento tampão): uniform and steady.

Velocity Profile Boundary layer type (tipo camada limite)
Uniform Parabolic Steady/permanent (estacionário/permanente) Flow: the velocity does not change in time.

Steady flow, imcompressible flow and inviscid flow
Steady flow (escoamento estacionário): flow properties do not change over time (the partial derivative of any property in order to time is null) at any point in the system. Incompressible flow (incompressivel): density of a fluid element does not change during its motion (liquids and gas if the gas velocity is less than 30% of the speed of sound of the gas). Inviscid flow (de fluido ideal): is the flow of an ideal fluid that is assumed to have no viscosity.

Streamlines Streamlines are lines tangent to the velocity vector.

Pathline and streakline
Pathline (trajetória) is the actual pathway of a fluid particle. Streakline (linha de emissão) is the locus of particles which have earlier passed through a prescribed point (e.g. smoke from a chimney seen from far away). Streamlines, pathlines and streaklines are identical in steady flow

Streamline, pathline and streakline (Linha de corrente, trajectória e linha de emissão)

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Information given by the streamlines
How much is the flow passing across a streamline? Zero, because the velocity is parallel to streamlines! How does the flow rate vary between two streamlines from one cross section to another? Remains constant. How does the velocity vary between two non-parallel streamlines? Increase when they approach and decrease when they move away (if ρ constant). How the resultant of the forces varies between two streamlines? It is against the flow when streamlines move away and is in favor when they approach. According to Newton's Law the resultant force is equal to mass times acceleration. If the pressure is the most important force, we conclude that it decrease when velocity increase. How the pressure varies from one side to the other of a streamline with curvature? Increases to the outside. When we have curvature we gain velocity (and momentum) perpendicular to the streamline, to the inside of the curve and so we have to have an applied force.

Basic principles of Fluid mechanics
Conservation of mass => Newton law:

Eulerian and lagrangian description
E se o escoamento fosse não estacionário?

Flow analysis techniques