Unit 3: AEROENGINE AND PROPELLER NOISE - Practice Quiz

1 What is the main cause of propeller loading noise?

2 Propeller thickness noise is primarily caused by:

3 Which propeller noise component is commonly associated with turbulent inflow?

4 What does a frequency-domain propeller noise prediction primarily show?

5 What does a time-domain propeller noise prediction provide?

6 A propeller has blades and rotates at revolutions per minute. What is its blade-passing frequency?

7 Which mathematical operation commonly converts a time-domain pressure signal into a frequency spectrum?

8 Which group contains only common aeroengine noise sources?

9 Which aeroengine component produces aerodynamic noise through rapidly rotating blades near the engine inlet?

10 What commonly generates rotor-stator interaction noise in a fan or compressor?

11 Rotor-stator interaction noise is most strongly associated with which type of sound?

12 In a fan stage, what is a stator?

13 Shock waves are likely to form in a fan or compressor when the blade-relative flow becomes:

14 What name is commonly given to the multiple tones produced by unequal fan-blade shock waves?

15 What is the primary physical cause of direct combustion noise?

16 Where does combustion noise originate in an aeroengine?

17 Indirect combustion noise can be produced when entropy fluctuations pass through a:

18 What is the main source of noise in a subsonic turbulent exhaust jet?

19 What type of spectrum is commonly associated with turbulent jet mixing noise?

20 In general, what happens to jet noise when exhaust velocity increases significantly?

21 A five-bladed propeller rotates at . What is its fundamental blade-passing frequency?

22 Propeller geometry and rotational speed remain fixed, but blade thrust is increased. Which acoustic component is most directly increased?

23 Why does propeller noise often rise sharply as the blade-tip Mach number approaches unity?

24 Which prediction approach is most convenient for computing discrete harmonics from a propeller operating periodically at constant speed?

25 In a time-domain propeller-noise calculation, the retarded source time satisfies which relation?

26 A propeller tone has source frequency . The aircraft moves directly toward a stationary observer with line-of-sight Mach number . Neglecting atmospheric motion, what frequency is observed?

27 An aeroengine spectrum contains strong narrow peaks at the fan blade-passing frequency and its harmonics. Which source is the most likely cause?

28 At high thrust, an engine's dominant aft-radiated broadband noise increases strongly with exhaust velocity. Which source is most consistent with this observation?

29 For a rotor with 20 blades interacting with a stator containing 30 vanes, what circumferential mode order follows from when and ?

30 What is the primary mechanism producing a rotor-stator interaction tone in a fan stage?

31 Which design change generally reduces rotor-wake interaction tones, assuming other operating parameters remain comparable?

32 Under which condition is fan shockwave noise most likely to become important?

33 What causes the multiple shaft-order tones commonly called fan buzz-saw noise?

34 If all fan blades and their shock structures were perfectly identical and evenly spaced, how would the shock-associated spectrum change relative to a fan with blade-to-blade variations?

35 Which statement correctly describes direct combustion noise?

36 Hot entropy fluctuations leave a combustor and accelerate through a turbine or nozzle. What type of sound can this process generate?

37 A combustor develops a narrow low-frequency peak accompanied by in-phase oscillations of pressure and heat release. What phenomenon is most likely occurring?

38 Using the approximate low-Mach-number scaling , by what factor does jet acoustic power change if exit velocity increases by 10%?

39 What is a typical acoustic trade-off when chevrons are added to a turbofan exhaust nozzle?

40 Two geometrically similar subsonic jets have the same exit velocity. If the second nozzle diameter is twice the first, how does the characteristic peak frequency change when the Strouhal number remains constant?

41 An ideal propeller has five identical, equally spaced blades and rotates at in steady uniform inflow. Neglecting broadband turbulence noise, which is the lowest nonzero frequency expected from a stationary periodic observer signal?

42 A propeller is operated at constant rotational speed and blade geometry while the axial flight Mach number increases. Which mechanism most directly explains the strong nonlinear growth of its high-frequency radiation as the advancing-blade helical tip Mach number approaches unity?

43 Two geometrically similar propellers operate with the same tip Mach number and observer geometry. The second propeller has twice the diameter, while Reynolds-number effects are neglected. How do corresponding nondimensional tonal frequencies compare?

44 In a frequency-domain model, identical blade contributions to azimuthal shaft harmonic contain the phase sum . For a perfectly symmetric rotor, when is nonzero?

45 A time-domain prediction must resolve through the 12th blade-passing harmonic of a four-bladed propeller rotating at . What is the largest sampling interval permitted by the Nyquist criterion alone?

46 A time-domain Ffowcs Williams–Hawkings calculation evaluates all source quantities at the observer time rather than at retarded time. Which error is most fundamental for a moving propeller source?

47 A turbofan spectrum measured forward of the inlet contains strong blade-passing tones, while an aft sideline spectrum is dominated by broadband low-frequency energy that rises sharply with exhaust velocity. Which source assignment is most consistent?

48 An engine test shows a narrow spectral line that tracks shaft speed exactly, persists under nearly steady combustion, and changes strongly when fan outlet-guide-vane count is modified. Which source is most strongly implicated?

49 Using the Tyler–Sofrin convention , a rotor with blades interacts with a stator having vanes. What is the lowest-magnitude circumferential mode generated at the second blade-passing harmonic?

50 For a hard-walled circular duct with no mean flow, a mode has axial wavenumber satisfying . What happens when ?

51 For the , interaction, the stator vane count is changed from to . Considering the mode, what is the principal acoustic consequence?

52 The axial gap between a fan rotor and its downstream stator is increased while blade count, vane count, and shaft speed remain fixed. Which first-order change is expected for wake-interaction tones?

53 A transonic fan produces multiple pure tones below the nominal blade-passing frequency. Which physical imperfection most directly prevents cancellation into only blade-passing harmonics?

54 A fan has a subsonic axial inflow but a supersonic relative Mach number near the blade tips. Why can strong inlet shock noise still occur?

55 A transonic fan is redesigned so that all blades have nearly identical shock systems and extremely accurate angular spacing. What spectral change is expected, assuming mean operating conditions remain unchanged?

56 A compact combustor generates unsteady heat release, and the resulting entropy fluctuations pass through a strongly accelerating turbine-nozzle region. Which pair correctly identifies the two principal combustion-noise paths?

57 A combustor's unsteady heat-release amplitude remains fixed, but the phase delay between direct acoustic waves and nozzle-generated indirect waves is varied. Why can the measured downstream sound vary non-monotonically?

58 In the low-Mach-number range where Lighthill's approximate acoustic-power law applies, exhaust velocity increases by . Approximately how much does the acoustic power level increase?

59 An imperfectly expanded supersonic jet exhibits a narrowband tone whose frequency changes when the nozzle-to-reflecting-edge distance is altered. Which mechanism best explains this behavior?

60 A high-speed jet has large turbulent structures convecting supersonically relative to the ambient sound speed. Which radiation signature is most directly associated with this condition?