Chapter 3 - Fluid dynamics Flashcards
When looking at a narrowing of a tube through which a fluid flows. Taking into consideration Bernoulli’s law, is the pressure higher or lower at the narrow part?
Counter intuitively, the pressure is lower. Because the kinetic energy is higher because it flows faster. More volume needs to flow through a smaller area, therefore it must flow faster
P1 + 0.5pv1^2 + pgh1 = P2 + 0.5pv2^2 + pgh2
What is inertia?
A property of matter by which it continues in its existing state of rest or uniform motion in a straight line, unless that state is changed by an external force.
Explain which ‘wall’ layer of the flow is thinner, in laminar or in turbulent flow?
In turbulent flow the ‘wall’ layer is thinner as the vortexes mix the fluid near the wall into the rest of the flow.
Is inertia more important at high or low velocities?
It becomes more important at high velocities in the transition area from laminar to turbulent. Because at low velocities, the viscous drag has the overhand.
What is the main determinant of laminar or turbulent flow?
The viscosity (mu).
As seen from the Reynolds number, it gives the ratio between the kinetic energy and the viscous energy:
–> Re = (pudc)/mu
- p = density fluid
- u = average velocity
- dc = diameter tube
- mu = viscosity fluid
Give the Reynold numbers at which a flow becomes turbulent for:
- Tube or slit
- Flow over a flat plate
- Flow around a sphere
- Flow around a cylinder
- Tube/slit: 2300 (dc = diameter tube and with of slit)
- plate: 300 000 (dc = length)
- Sphere: +- 10 (dc = diameter sphere)
- Cylinder: +- 40 (dc = diameter cylinder)
How and when do you calculate the hydraulic diameter?
Dh = 4A/S
- A: surface of cross-sectional area in m2
- S: is the wetted perimeter in m (omtrek)
This is used when analysing other geometries such as rectangular ducts, slits, etc.
What are the three requirements for using the Hagen-Poiseuille equation?
- Constant density –> incompressible fluids
- Constant viscosity
- Laminar flow
In what situations are laminar flows preferred? Give three
- Fluid flow in an extruder (e.g. meat replacing products with high viscosity)
- Flow in a filling machine, to know exactly how much product goes in each package, while not spilling it.
- Designing a bottle of ketchup that gives just the right amount on your food with low velocity.
What is the most preferred flow in the industry?
Turbulent flow, to get as much mixing as possible while also getting a good throughput.
What velocity should be used when you are analysing a flow through a restriction, where the velocity changes?
The velocity downstream.
How can you tell the cohesiveness of a powder?
- It is glassy (rigid) or crystalline
- It is non-deformable (e.g. by moisture content)
- Have a regular shape (ideally spherical)
- Not be too small
What four unwanted types of flow behaviours are possible in a factory?
1) Rat holing
2) Bridging
3) Segregation
4) Incomplete emptying
