Unit 5: SOUND GENERATION, PROPAGATION, AND RADIATION IN/FROM AN AEROENGINE NACELLE - Practice Quiz

1 Which acoustic mode normally propagates in a uniform duct at frequencies below the first higher-order-mode cut-off frequency?

2 What happens to a duct mode when its frequency is below its cut-off frequency?

3 For a perfectly rigid duct wall, which acoustic boundary condition is applied at the wall?

4 Which quantities commonly define the acoustic state in a one-dimensional duct model?

5 What is the main acoustic purpose of a liner inside an aeroengine nacelle?

6 Through which main openings can fan noise radiate from a nacelle?

7 How can mean airflow affect sound propagation inside a nacelle?

8 What occurs when sound reaches an open end of an aeroengine nacelle?

9 What does a transfer element relate in a duct-acoustics model?

10 How are several consecutive duct elements usually combined in the transfer element method?

11 A transfer matrix for a basic two-port duct element commonly connects how many ends?

12 What is a primary benefit of the transfer element method?

13 How is a smoothly varying duct commonly approximated when constructing transfer elements?

14 Which geometric property changes along a varying cross-section duct?

15 At an ideal junction between two duct sections, which acoustic quantities are commonly matched?

16 What generally improves the geometric approximation of a smoothly varying duct?

17 Which two broad components are commonly included in fan noise models?

18 If a fan has blades and rotates at revolutions per second, what is its blade-passing frequency?

19 Which fan-noise component is strongly associated with periodic blade motion?

20 What may represent a fan source in a simplified duct-acoustics model?

21 For a duct mode with acoustic wavenumber and cutoff wavenumber , what is its axial wavenumber in a stationary medium?

22 A duct mode is excited at a frequency below its cutoff frequency. How does its amplitude vary sufficiently far from the source?

23 For plane-wave propagation in a rigid duct of area , which expression gives the characteristic impedance defined as pressure divided by volume velocity?

24 A rigid, uniform duct carries sound of wavelength . If a perfect reflection creates a standing wave, what is the distance between adjacent pressure nodes?

25 A plane acoustic wave travels downstream through a nacelle with uniform mean-flow speed . Neglecting losses, which axial wavenumber corresponds to angular frequency ?

26 A nacelle liner changes a mode's axial wavenumber from a real value to under the convention , where . What is the axial amplitude factor?

27 Which modification most directly increases the range of frequencies over which a passive nacelle liner can attenuate duct noise?

28 Two nacelle modes have the same frequency, but only one has a real axial wavenumber at the inlet plane. Which mode can efficiently carry acoustic energy toward the far field?

29 In a transfer element model, the acoustic state is . If element 1 is followed by element 2 and , , what is the total transfer matrix?

30 Why are pressure and volume velocity commonly selected as transfer-matrix state variables for one-dimensional duct systems?

31 For a lossless, uniform duct element represented using pressure and volume velocity, which property is expected for its transfer matrix?

32 A duct transfer matrix is known, and a load impedance is specified at the outlet. What additional result can be calculated at the inlet?

33 A smoothly varying duct is approximated by short uniform sections. How is the overall transfer matrix constructed?

34 At an ideal abrupt area change with no mean flow or local loss, which pair of quantities should be continuous when joining two transfer elements?

35 Two short uniform sections contain the same fluid, but section 2 has twice the area of section 1. How do their plane-wave characteristic impedances compare?

36 A numerical transfer-element model of a rapidly varying duct gives results that change significantly when the number of sections is doubled. What is the best interpretation?

37 A fan has 18 blades and rotates at . What is its blade-passing frequency?

38 For a rotor-stator interaction, the circumferential mode order is modelled by . If , , and , what is ?

39 Which source representation is most directly associated with unsteady aerodynamic loading on fan blades?

40 A measured fan spectrum contains narrow peaks at the blade-passing frequency and its harmonics, superimposed on a continuous spectrum. Which modelling approach is most appropriate?

41 A rigid circular duct of radius contains a quiescent fluid with sound speed . The transverse eigenvalues satisfy . Which condition guarantees that only the plane mode can propagate?

42 For a duct mode with transverse wavenumber in a lossless uniform duct, the acoustic pressure varies as . If , what is the pressure-amplitude ratio between planes separated by distance in the decaying direction?

43 A uniform subsonic mean flow of Mach number passes through a constant-area duct. A mode has transverse wavenumber , and its dispersion relation is . What is its laboratory-frame cutoff angular frequency?

44 A plane wave in a uniform duct has characteristic impedance when volume velocity is used. If the duct terminates in impedance , what is the pressure reflection coefficient at the termination?

45 An axisymmetric nacelle duct contains a finite, axisymmetric lined section between rigid sections. At each liner discontinuity, which modal-coupling statement is correct?

46 Neglecting mean flow, a cut-on nacelle mode has axial and transverse wavenumbers and . At what approximate polar angle from the duct axis does its principal radiation occur after an ideal unflanged opening?

47 Why can choosing a liner impedance equal to fail to maximize attenuation of a nacelle mode under grazing mean flow?

48 For an upstream-travelling plane acoustic wave in a uniform subsonic inlet flow of speed , what is its wavelength at fixed laboratory angular frequency ?

49 Using the state vector and time convention , which matrix maps the state at to the state at for a lossless uniform duct?

50 Three consecutive elements satisfy , , and . What is the assembled transfer matrix from plane 3 to plane 0?

51 For a reciprocal, lossless two-port duct element described using consistently directed pressure and volume velocity, which property is expected of its transfer matrix ?

52 A long nacelle model contains strongly evanescent modes. Direct multiplication of transfer matrices becomes ill-conditioned because exponentially growing and decaying terms coexist. Which reformulation is generally most stable?

53 Under the Webster horn approximation for a slowly varying duct area , which equation governs the pressure amplitude in a quiescent fluid?

54 An exponential horn has . Substituting into Webster's equation produces which equation for ?

55 A plane wave is incident from a duct of area onto an abrupt expansion to area , with equal fluid properties and no mean flow. What is the pressure reflection coefficient?

56 Two independent pressure solutions and are known for a varying-area element. With , define and . Which matrix maps to ?

57 A rotor with blades interacts with a stator containing vanes. Under the Tyler–Sofrin model, which circumferential mode orders can occur at the th blade-passing harmonic?

58 Broadband fan noise is expanded into duct modes with random complex amplitudes . Which quantity is required to retain both modal power and intermodal coherence?

59 In a low-Mach-number acoustic analogy for a fan, how are blade thickness noise and unsteady loading noise most naturally classified?

60 A source distribution on a circular fan plane is proportional to . In a perfectly axisymmetric duct, which circumferential acoustic modes can it directly excite?