Partech Midterms LEC 5

Motion of Particles through Fluids

external force

Movement of a particle through a stationary fluid requires an _____ acting on the particle.

gravitational and centrifugal force

What are the external forces considered in the mechanics of particle motion?

Particle Dynamics

the study of change in a solid particle’s velocity with time when different forces are acting on it

using streamlines which follow the flow path

What’s one of the best ways in representing variation in velocity?

Streamline or Laminar Flow

What do you call the slow fluid flow caused by the fall of the solid particle?

Turbulent Flow

What do you call the fast motion fluid flow, where fluid particles cross and re-cross the streamline?

TRUE

TRUE OR FALSE: Buoyant force is parallel to the external force but in an opposite direction.

Drag Force

It acts to oppose the motion and acts parallel with the direction of movement but in opposite direction (parang fricion on a surface)

• Terminal Settling Velocity (single particle)

• Hindered Settling Velocity (mixtures of particle)

What are the two types of settling velocity in particle dynamics?

• Particle is spherical

• Particle is non-porous, incompresible, as well as insoluble and inert with the fluid

• Fluid density and viscosity is constant

• Particle surface characteristics is negligible

• Particle is freely settling under gravity

• Fluid forms an infinite medium

What are the assumptions considered in particle dynamics?

Frictional Resistance

It is offered by the liquid on the particle due to the relative motion between the particle and the liquid.

Gravitational force - buoyancy force - frictional drag

How do you calculate for the net force acting on a particle?

Stagnation point

The point in the body surface at which the velocity decreases to zero.

Upstream pressure plus density times velocity² over 2

How is the pressure at stagnation point calculated?

Drag Force

the force acting on a body which is in the flow direction U

Lift Force

The force acting on a body that is in a direction normal to U

Drag

It is the sum of the pressure drag (in the direction of the flow velocity U) and friction drag whose proportions vary with the shape of the body.

Lift

The sum of the pressure lift (direction normal to the flow)

Integral of pdA cos theta

What is the formula for Pressure Drag or Form Drag ? (x-component of the force acting on the same direction as the oncoming velocity)

Integral of tau-dA sin theta

What is the formula for friction drag ? (y-component of the force acting tangential to the surface)

Integral of pdA sin theta

What is the formula for the pressured lift? (the y-component of the force acting on the same direction as the oncoming fluid velocity)

Integral of tau-dA cos theta

What is the formula for the pressured lift? (the x-component of the force acting tangential to the surface of the body)

30-90

At what reynold’s number will the eddies start to elongate and a periodic oscillation called wake is produced?

Twin Vortices

What do you call the eddies forming behind the body as the reynold’s number increases?

between 10² and 10^5

At this amount of Reynold’s number, the separation occurs near 80 degrees from the front stagnation point.

Karman vortex street

What do you call the arrangement of vortices when the separation occurs near 80 degrees?

3.8 × 10^5

What is the value for the critical Reynold’s number at which the boundary layer becomes turbulent?

130 degrees

What is the angle at which the boundary layer becomes turbulent and the separation position is moved further downstream from the stagnation point?

It gradually decelerates with the velocity gradient reaching zero

What happens to the flow on the rear half of the cylindrical body when flow is turbulent?

Separation point

What do you call the point where downstream flow get reversals and developing eddies?

This is because the location of the separation point suddenly changes as it reaches this Re

Why is it that when Re = 10³ to 2 × 10^5, the drag coefficient Cd is roughly constant at 1-1.2, but when it reaches the critical Re = 3.8 × 10^5, it suddenly decreases to 0.3?

buoyancy, drag and gravity

The relative motion of particles in a fluid is generally under the action of the forces of ___?

Single Particle Terminal Velocity

The velocity at which the acceleration is zero

Centrifuges

These are extensively used for separating fine solids from suspension in a liquid.

Stoke’s law region

In centrifuges, it is necessary to consider only the _____ ___ in calculating the drag between the particle and the liquid.

Hindered Settling

This is what occurs when particles are crowded and surrounding particles interfere with the motion of individual particles.