2P4 Thermofluid Mechanics Flashcards
What are the methods of mixing in turbulent compared to laminar flows?
Turbulent - large scale eddie mixing
Laminar - molecular diffusion at small scales
For laminar flow through a pipe what different dimensionless groups for wall friction (cf) can be formed for wall shear stress?
- Dynamic τ/0.5ρV^2 for higher Re
- Viscous τ/0.5μ(V/D)
What is a measure of roughness in a pipe?
k/D, where k is the characteristic length scale of the roughness, and D is the diameter of the pipe.
Under what conditions does the flow stay laminar in terms of roughness?
When k/D «1.
What is the specific steady flow availability function?
b = hi - T0si
where T0 is the environmental temperature
What is the maximum power that can be extracted between two flow states?
mdot (b1-b2)
where b is the specific steady flow availability function
What is exergy?
The difference between b1 and b0 where b0 is the specific steady-flow availability of the environemnet.
What is the energy flow rate unavailable for work?
=T0(S1-S0)
the energy unavailable is fixed by the entropy flow rate
How can you rewrite the availability equation in terms of work out?
m(b2-b1) = -Wx + integral(1-T0/T)dQ -T0Sirr
interesting because it contains carnot efficiency
Give three examples of irreversibility in a flow
- viscous dissipation in boundary layers
- viscous mixing of flows of different velocity
- heat transfer accross a finite ΔT
What is a limitation of the air-standard Joule cycle?
Accross combustor there is a 2-4% drop in pressure.
Combustion changes gas composition and therefore its thermodynamic properties.
Turbine and compressor have irreversibilities, so they are not isentropic, loss of available power.
What is the 2-property rule?
In the absence of external effects, the state of a pure substance is fixed by the values of 2 independent property
What is the triple line?
Where all three phases can exist together.
What is the saturated liquid line?
Where the fluid is wet saturated (left hand side)
What is the critical point.
The max temperature where liquid and vapor phases can exist. Above which, there is no distinction.
What is the heat transfer called during melting?
latent heat of fusion.
What is the heat transfer called during melting?
latent heat of vaporisation
How is dryness fraction defined?
x = mass of vapour/total mass
What is Poiseuille flow?
Flow between two stationary plates driven by a constant pressure gradient.
What assumptions can be made about viscous flow down a slope?
Laminar
Streamlines straight and parallel
Pressure, and velocity does not depend on distance down the slope
Shear stress at surface is zero.
What is Couette flow?
one plate moves in a set of parallel plates, no pressure gradient.
What happens in combined Couette and poiseuille flow?
Can get flow reversal depending on adverse of favourable pressure gradient.
How do you derive the expression for the growth of a boundary on a plate in a free stream?
use the navier stokes equation to scale pressure, intertia and viscous terms.
In x direction you can show that the way the thickness grows.
In y direction can show that the perpendicular pressure gradient is neglibible
Where can Bernoulli be used for the flow of a free stream over a fixed plate?
Bernoulli can be used outside the boundary layer where viscous effects are neglibible but not inside the boundary layer.
What happens to a boundary layer in a favourable pressure gradient?
Fuller boundary layer
What happens to a boundary layer in an adverse pressure gradient?
Depleted boundary layer, up to flow reversal. Can be derived from thinking of a Couette flow.
What happens to pressure around a ellipse placed in a free stream?
high p at stagnation point,
low p at top and bottom due to curvature of streamlines,
high p at other end of ellipse (if inviscid)
What is the consequence of adverse pressure gradient on backside of objects?
Can lead to flow reversal and therefore seperation.
What determines whether flow will reverse inside an adverse pressure gradient?
Competing effects of pressure gradient and diffusion of momentum from external flow towards the wall.
There is more diffusion of momentum when the flow is more viscous, making it more robust to seperation.
How can seperation be delayed?
By injection high-momentum fluid directly into the boundary layer, which means the boundary layer can stay attached more easily.
By using suction on the wing, to suck in the lower momentum fluid.
What happens after transition to turbulence?
Weak viscous forces, flow breaks down into small disorganised eddies
What effect does turbulence have on the growth of the boundary layer?
It quickens the growth of the boundary layer, as turbulence increases the rate of momentum transfer between surface and free stream.
This leads to a fuller profile than laminar. Higher skin friction due to steeper gradient at wall.
What effect does turbulence have on seperation?
More robust against flow reversal, delays seperation due to fuller boundary layer.
How can turbulence be created?
By ‘tripping’ the flow such as the dimples in golf balls.
How is form drag created?
By the seepration of the flow, the pressure behind the object won’t return to the high p seen in an inviscid flow.
What causes laminar boundary layers to become unstable and transition to turblunece?
Because their velocity profiles contain an inflexion point. This leads to seperation, unless the flow is very viscous
How can boundary layers re-attach?
Turbulence after seperation causes the velocity profile to become fuller, this new profile may be robust enough to overcome adverse pressure gradient, and reattach creating a seperation bubble.
