Fluid Motion - Boundary layers and turbulence Flashcards
What is an ideal fluid?
A fluid that only experiences normal stresses is ideal (pressure forces)
What is the viscosity of ideal fluids?
Zero
What do we assume when drawing streamlines?
That the fluid is ideal as we ignore friction
What do real fluids have?
Viscosity which generates shear forces and experiences friction
What do real fluids create?
Boundary layers
What does the no slip condition state?
The no-slip boundary condition assumes that the fluid layer touching a surface moves at the same speed as the surface itself. Essentially, there is no slipping or difference in speed between them.
What is the fluid velocity at the surface of a boundary?
Zero
What is the boundary layer?
The boundary layer is a flow region where there are large differences in velocity (next to a wall)
What is viscosity?
How much resistance a fluid has to shear
How is the dynamic viscosity related to the shear stress/
π = π*ππ’/ππ§
What is shear force associated with?
A gradient in the velocity
When do you get shear?
If there are differences in velocity
What does the shear stress at the wall represent?
The shear stress at the wall represents the friction of the fluid with the wall
What does a higher dynamic viscosity mean?
Less displacement
How can you calculate kinematic viscosity?
v =π/π
When do you use kinematic viscosity?
Kinematic viscosity is used when dealing with motion
What is laminar flow?
Organized layered flow
What is turbulent flow?
Disorganised and random flow
What is flow characterised as?
Reynolds number
How do you calculate the Reynolds number?
Re = pUL/π
When is a flow laminar?
If Re Β«1: viscosity dominates
When is a flow turbulent?
If ReΒ»1: viscosity negligible
What is the Reynolds number?
- The Reynolds number is the order of magnitude
- The Reynolds number expresses the ratio between inertial and viscous forces.
Why is turbulence good?
Efficiently mixes stuff like momentum causing large friction
What is the drag force coefficient?
The drag coefficient is a dimensionless quantity that is used to quantify the drag or resistance of an object in a fluid environment, such as air or water.
What are turbulent flows proportional to?
U^2
