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Low-frequency wake dynamics for a square-back vehicle with side trailing edge tapers.

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Title: Low-frequency wake dynamics for a square-back vehicle with side trailing edge tapers.
Authors: Pavia, Giancarlo1 G.Pavia@lboro.ac.uk, Passmore, Martin1, Varney, Max1
Source: Journal of Wind Engineering & Industrial Aerodynamics. Jan2019, Vol. 184, p417-435. 19p.
Subjects: Vector beams, Stable isotopes, Vortex methods, Aerofoils, Vortex motion
Abstract: Abstract In this paper, the effects of side trailing edge tapering on the wake of a simplified square-back vehicle are investigated. The tapered surfaces are reported to trigger a switch from a laterally asymmetric bi-stable wake to a vertically asymmetric stable wake. The wake structure reported in the literature for lateral symmetry breaking states is seen to rotate by 90 ∘ as the angle of the tapered surfaces ϕ s is increased. A 6 % drag reduction over the simple square-back case is reported for 6 ∘ < ϕ s < 12 ∘ . This gain is found to be the result of the stretching of the circular vortex responsible for the suction zone visible in any symmetry breaking state. A downwash dominated wake is observed in these conditions. The sensitivity of such a wake to small variations of the model pitch angle (for ϕ s = 12 ∘ ) is also assessed. As the pitch angle α is reduced from 0 ∘ to − 2 ∘ , the time averaged wake is reported to switch from a downwash dominated topology to an upwash dominated topology. A strengthening of the long-time instability is observed when the symmetry in the vertical direction is recovered and is accompanied with a 4.9 % reduction in base drag over the same model tested at α = 0 ∘ . Graphical abstract Image 1 Highlights • Side edge tapers can suppress the long-time instability while also reducing drag. • The long-time instability stems from the equilibrium between all shear layers. • Small perturbations of pitch angle can turn stable wakes into multi-stable wakes. • For a square-back bluff body, a lower drag wake is not always a more stable wake. [ABSTRACT FROM AUTHOR]
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Database: Engineering Source
Description
Abstract:Abstract In this paper, the effects of side trailing edge tapering on the wake of a simplified square-back vehicle are investigated. The tapered surfaces are reported to trigger a switch from a laterally asymmetric bi-stable wake to a vertically asymmetric stable wake. The wake structure reported in the literature for lateral symmetry breaking states is seen to rotate by 90 ∘ as the angle of the tapered surfaces ϕ s is increased. A 6 % drag reduction over the simple square-back case is reported for 6 ∘ < ϕ s < 12 ∘ . This gain is found to be the result of the stretching of the circular vortex responsible for the suction zone visible in any symmetry breaking state. A downwash dominated wake is observed in these conditions. The sensitivity of such a wake to small variations of the model pitch angle (for ϕ s = 12 ∘ ) is also assessed. As the pitch angle α is reduced from 0 ∘ to − 2 ∘ , the time averaged wake is reported to switch from a downwash dominated topology to an upwash dominated topology. A strengthening of the long-time instability is observed when the symmetry in the vertical direction is recovered and is accompanied with a 4.9 % reduction in base drag over the same model tested at α = 0 ∘ . Graphical abstract Image 1 Highlights • Side edge tapers can suppress the long-time instability while also reducing drag. • The long-time instability stems from the equilibrium between all shear layers. • Small perturbations of pitch angle can turn stable wakes into multi-stable wakes. • For a square-back bluff body, a lower drag wake is not always a more stable wake. [ABSTRACT FROM AUTHOR]
ISSN:01676105
DOI:10.1016/j.jweia.2018.12.009