Intakes, Nozzles, Shockwaves Flashcards
What are the 3 operational styles of Pitot tubes
Lower speed than design - stream tube area is larger than lip, air is sucked into tube
Design speed - stream tube area is same as lip
Faster speed than design - stream tube area is smaller than lip, air spills
Why can BL cause problems on supersonic inlets
Interactions with shock waves lead to airflow distortions which can cause engine stall
How do supersonic inlets deal with the BL problem
Redirect BL before it reaches engine and place inlet away from BL in the freestream
Diverters provide gap between fuselage and inlet equal to BL thickness
Bleed systems redirect unwanted flow through holes in compression ramp
What is a diverterless supersonic inlet
Has a bump that functions as compression surface and creates pressure difference that prevents majority of air from entering inlet at speeds up to Mach 2
What other considerations do designers have to accommodate for during inlet design
Configuration features e.g. nose landing gear, weapon bays, access panels
What technique do most modern intakes employ to improve pressure recovery
Combination of shockwaves such as oblique shocks from nose cone or ramp, shock reflections and normal shock before compressor inlet
What is the role of a propelling nozzle and what are the main functions
To ensure exhaust gas energy is maximised in converting to kinetic energy for maximum thrust
Thrust production, thrust reversal, noise suppression
Why do afterburning engines need variable diameter and shaped nozzles
They generate a wide range of mass flow rates during operation
How do engines generate thrust quickly from idle
Variable area propelling nozzle in open configuration keeps thrust minimum while engine rpm increases
What is thrust augmentation and how can it be done
Short duration of increased thrust needed
Liquid injection - often at takeoff - spray water into compressor causing evaporation, leads to extraction of heat and so reduced compressor inlet temperature, increased pressure ratio and mass flow due to effective increase in rotational speed
Afterburning - inject fuel into jet pipe to burn with excess oxygen, increased maximum permitted temperature higher means more thrust
Why does afterburning require a variable nozzle
Increased volumetric flow, density decreases as temperature increases from combustion - to maintain turbine pressure ratio nozzle area is increased
How does a noise suppressor work
Increased mixing between exhaust gas and ambient air reduces turbulence and so noise
What does a choked nozzle mean
Sonic speed at the throat and so Mach number cannot increase above Mach 1 even if total pressure continues to rise
Critical pressure ratio with ambient pressure is fixed for a given gas and nozzle efficiency
Where do shock waves occur
At supersonic speed in decelerating flow
e.g. at throat of nozzle (Mach 1 at critical point) normal shock occurs to slow speed to subsonic
What does Rayleigh flow mean
Heat transfer causes subsonic flow to accelerate to Mach 1
Heat transfer causes supersonic flow to decelerate to Mach 1
What are the properties of normal shock
Adiabatic
Non-isentropic
M2 always subsonic
Density, P, T increase
U, Po decrease
To constant
How are oblique shocks analysed
Split into normal and tangential components
Tangential velocity is constant
Normal velocity uses normal shock relations
Where do oblique shocks occur
At concave corners
Where do expansion waves occur
At convex corners
How do detached waves occur
When the deflection angle is greater than the maximum oblique shock deflection
What is the purpose of swept wings
Reduce wave drag if trailing edge of wing is inside Mach cone
However, increases wave drag if outside Mach cone as flow is supersonic
