fluid dynamics Flashcards
(22 cards)
What is the edge facing the direction of flow on a flat plate called?
The leading edge.
What is the rear edge of a flat plate in the direction of flow called?
The trailing edge.
Near the leading edge of a flat plate, what type of boundary layer is formed?
The boundary layer is wholly laminar.
For a laminar boundary layer, what is the velocity distribution?
The velocity distribution is parabolic.
How does the thickness of the boundary layer (δ) change with distance from the leading edge x?
δ increases with distance from the leading edge x.
What happens to the boundary layer as δ increases with x?
More fluid is slowed down by the viscous boundary, becomes unstable, and breaks into a turbulent boundary layer over a transition region.
In a turbulent boundary layer on a smooth boundary, what remains laminar?
A very thin layer called the laminar sublayer.
What are the velocity distributions in the turbulent boundary layer?
Log law or Prandtl’s one-seventh power law.
How does the boundary layer thickness δ change as the free stream velocity U increases?
δ decreases as U increases.
How does δ change as kinematic viscosity (ν) increases?
δ increases as kinematic viscosity (ν) increases.
What is the relationship between τ₀ and δ?
τ₀ ≈ µ (U / δ).
How does τ₀ change as x increases in a laminar boundary layer?
τ₀ decreases as x increases.
When the boundary layer becomes turbulent, how does τ₀ behave with increasing x?
It shows a sudden increase and then decreases with increasing x.
What happens to boundary layer growth when U increases in the downward direction?
Boundary layer growth is reduced.
What occurs when U decreases in the downward direction?
Flow near the boundary is further retarded, boundary layer growth is faster and is susceptible to separation.
What govern the various characteristics of the boundary layer on a flat plate?
Inertial and viscous forces.
The characteristics of the boundary layer are functions of which non-dimensional numbers?
Either Ux/ν or UL/ν.
If 5 ~ 10 Ux/ν < 2000 what is the state of the boundary layer?
The boundary layer is laminar (velocity distribution is parabolic).
If 5 ~ 10 Ux/ν > 4000 what is the state of the boundary layer?
The boundary layer is turbulent on that portion (velocity distribution follows Log law or a power law).
What factors does the critical value of Ux/ν, at which the boundary layer changes from laminar to turbulent, depend on?
turbulence in ambient flow,
surface roughness,
pressure gradient,
plate curvature, and
temperature difference between fluid and boundary
How can the velocity distribution in the laminar sub-layer zone be approximated?
The velocity distribution can be approximated as linear, assuming the velocity gradient is constant, due to the very small thickness of the laminar sub-layer zone
