- Buffet is a response problem result of pressure fluctuations exciting the structural modes
- Buffet flow consists of random, fluctuating pressures caused by separated flow - Buffet Onset = the first appearance of a significant area of separated flow
thereby causing fluctuating pressures - Buffet loads = Loads due to flow separation
2- Classify different types of buffet according to the aircraft
component affected
- Wing buffet (usually self-generated, i.e., flow separation of the same lifting surface)
o Magnitude of response depends on magnitude of unsteady flow disturbances
o May be due to… Alpha (AoA)
Transonic effects (Shock waves)
- Empennage buffet (Impingement of the flow detached from a forward lifting surface or deflected device)
o HTP affected by flow separated from the wing
o VTP affected by flow separated from the wing leading edge apex o VTP affected by flow separated from airbrakes…
- Component buffet (local flow separations) o Flaps (deployed) buffet
o Pods/Pod fin o Landing gear doors
3- Explain the differences between flutter and buffet
Flutter:
- Flutter is a STABILITY problem
- Classical Flutter is produced by the coupling of two or more normal modes.
- There is a coalescence of frequencies in the evolution of frequency of these modes with flight speed.
- The phase between the modes is such that the coupling allow extracting energy from the free flow and makes the subsequent structural motion divergent.
- Flutter may be a potentially catastrophic event
- Flutter use to be a high speed phenomena (Flutter onset usually triggered by dynamic pressure)
Buffet:
- Buffet is a RESPONSE problem
- Buffet is the structural response to flow separation. Buffet loads are loads produced by flow separation. Most often is the response of a single mode.
- Buffet use to be a fatigue issue
- Buffet onset usually triggered by angle of attack
4- Describe the Von-Karman vortex street as a classical example of
flow separation
A von Kármán vortex sheet is a repeating pattern of swirling vortices caused by the unsteady separation of flow of a fluid around blunt bodies. A vortex street will only form at a certain range of flow velocities, specified by a range of Reynolds numbers (Re).
The range of Re values will vary with the size and shape of the body from which the eddies are being shed, as well as with the kinematic viscosity of the fluid. Over a large Re range (47<Re<105 for circular cylinders) eddies are shed continuously from each side of the body, forming rows of vortices in its wake. The alternation leads to the core of a vortex in one row being opposite the point midway between two vortex cores in the other row, giving rise to the distinctive pattern shown in the picture. Ultimately, the energy of the vortices is consumed by viscosity as they move further downstream, and the regular pattern disappears.
6- Use the Strouhal number and speeds probabilities to derive
frequency design requirements in a civil structure
7- Buffet in the airworthiness regulations
CS25.251 Vibration and buffeting
- The aeroplane must be demonstrated in flight to be free from any vibration and buffeting that would prevent continued safe flight in any likely operating condition. - Each part of the aeroplane must be demonstrated in flight to be free from excessive
vibration, under any appropriate speed and power conditions up to VDF/MDF. The maximum speeds shown must be used in establishing the operating limitations of the aeroplane in accordance with CS 25.1505.
- Except as provided in sub-paragraph (d) of this paragraph, there may be no buffeting condition, in normal flight, including configuration changes during cruise, severe enough to interfere with the control of the aeroplane, to cause excessive fatigue to the crew, or to cause structural damage. Stall warning buffeting within these limits is allowable.
- There may be no perceptible buffeting condition in the cruise configuration in straight flight at any speed up to VMO/MMO, except that the stall warning buffeting is allowable.
- For an aeroplane with MD greater than 0·6 or with a maximum operating altitude greater than 7620 m (25,000 ft), the positive maneuvering load factors at which the onset of perceptible buffeting occurs must be determined with the aeroplane in the cruise configuration for the ranges of airspeed or Mach number, weight, and altitude for which the aeroplane is to be certificated. The envelopes of load factor, speed, altitude, and weight must provide a sufficient range of speeds and load factors for normal operations. Probable inadvertent excursions beyond the boundaries of the buffet onset envelopes may not result in unsafe conditions. (See AMC 25.251(e).)
8- Describe qualitatively the relevant characteristics in a wing buffet
case
9- Describe qualitatively the relevant characteristic of empennage
buffet using the F-18 case as example
Slides 15 to 19
10-
Describe qualitatively the relevant characteristics of fin buffet
using the Eurofighter fin buffet due to aero brake deployment as
example
11-
Application of wind tunnel tests in understanding the buffet
problem
12-
The solution of the Buffet Problem (both in the aerodynamic
side and in the structural side)
AERODYNAMIC SIDE: