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Title: Numerical studies of the flow around an airfoil at low Reynolds number
Author: Jones, Lloyd Edward
ISNI:       0000 0001 3592 4965
Awarding Body: University of Southampton
Current Institution: University of Southampton
Date of Award: 2008
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A study of the °ow around airfoils at low-Reynolds numbers has been performed, by a combination of direct numerical simulation (DNS) and linear stability analy- sis. The behaviour of laminar separation bubbles formed on a NACA-0012 airfoil at Rec = 5 £ 104 and incidence 5± is investigated. Initially volume forcing is introduced in order to promote transition to turbulence. After obtaining su±- cient data from this forced case, the explicitly added disturbances are removed and the simulation run further. With no forcing the turbulence is observed to `self-sustain', with increased turbulence intensity in the reattachment region. A comparison of the forced and unforced cases shows that the forcing improves the aerodynamic performance whilst requiring little energy input. Linear stability analysis of the time-averaged °ow¯eld is performed, however no absolute insta- bility is observed that could explain the presence of self sustaining turbulence. A series of simpli¯ed DNS are presented that illustrate a three-dimensional in- stability of the two-dimensional vortex shedding that occurs naturally. The in- stability leads to exponential growth in time at ¯xed streamwise locations, and a mechanism for its growth is proposed. The fact that this transition process is independent of upstream disturbances has implications for modelling sepa- ration bubbles. A further DNS, of a laminar separation bubble formed on a NACA-0012 airfoil at incidence 7± clearly exhibits sustained transition to tur- bulence via the proposed instability mechanism, and illustrates that the e®ect of a modest increase in airfoil incidence upon separation bubble behaviour ap- pears slight in comparison to that of the addition of forcing. For all airfoil °ows the transition/reattachment region of the separation bubble was observed to be a signi¯cant contributor to airfoil self-noise. Numerical simulations of the response of the time-averaged °ow¯eld to small perturbations, intended to com- plement linear stability analysis, illustrate that for two dimensional cases in the range 5± · ® · 8:5± the time-averaged °ow¯eld is unstable due to an acoustic feedback instability, whereby hydrodynamic disturbances convecting over the trailing edge generate upstream traveling acoustic waves, which ultimately gen- erate further downstream travelling hydrodynamic disturbances. As the cycle repeats, the amplitude of both hydrodynamic instabilities and acoustic waves increases. It is suggested that an acoustic feedback loop of this type may act as a frequency selection mechanism for naturally occurring vortex shedding observed in two-dimensions.
Supervisor: Sandham, Neil Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: TA Engineering (General). Civil engineering (General)