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Separating flows around swept and unswept wings with laminar and turbulent free stream conditions

Suardi, C. A. (2020). Separating flows around swept and unswept wings with laminar and turbulent free stream conditions. (Unpublished Doctoral thesis, City, University of London)

Abstract

The comparison of the three-dimensional (3D), subsonic flows past an infinite wing equipped with aNACA*4412 profile is presented considering an unswept and a 30o swept-back wing at incidence values that induce flow separation. The Reynolds number based on the aerofoil chord C and the free stream velocity in the chord plane QØ is fixed at a common value Rec = CQ ∞∕v = 50 x 103. The investigation is carried out using highly-resolved-LES (Large Eddy Simulations) of the incompressible Navier-Stokes equations. The comparison between swept and unswept wings is undertaken considering both laminar and turbulent free stream conditions.

One of the two central objectives of the research has concerned the assessment of the Simple Sweep Theory when flow separation takes place. The Simple Sweep Theory is a commonly used tool that is frequently deployed in designing swept wings by a simple extension of the baseline flow around the corresponding straight wing. Another objective is the accurate and detailed characterisation of both the laminar and the turbulent flow separation behaviours on the swept and unswept wings at incidence. Especially in the turbulent boundary layer scenario, separation is still an open research topic that lately is receiving particular attention.

In the laminar inlet condition, only one incidence namely α= 5o has been considered for both the wing configurations. The wings suction sides will be shown to experience laminar boundary layer separation forming a typical laminar separation bubble (LSB). The separating shear layer bounding the separated region breakdowns to turbulence, without any reattachment taking place downstream on the wings.

When comparing the flows between the unswept and the swept wing configurations, it is found that they satisfy the Simple Sweep Theory prediction along the wing, until the boundary layer starts to detach from the wings surface. When flow separation occurs, the emerging large-scale flow structures participate in the breakdown to turbulence presenting different features in the two wing configurations, thus leading to a violation of the Simple Sweep Theory violation. The laminar separation mechanism for both wing configurations has been statistically characterised, allowing to shed some additional light to this process. The laminar separation is found to be a 3D process right from its detachment location.

Within the turbulent free stream framework, achieved by the introduction of free stream turbulence (FST) in the incoming flow, two wing incidences are considered, namely α= 5o and α= 10o. For both the wing configurations, the FST triggers a very early transition towards turbulent boundary layer thus inhibiting the formation of an LSB, which is replaced by a fully 3D and time dependent boundary layer separation. The turbulent separation of the boundary layer is found to be a stochastic process that builds up moving downstream along the chord. The early formation of localised reversed flow spots in the upstream portion of the wing merge downstream to form the so-called stall cells, regions of reversing flow with a size comparable to the chord. The location of the mean turbulent separation can be defined only when considering the time averaged flow and it does not manifest in the instantaneous flow realisations.

As already seen for laminar regimes, the Simple Sweep Theory is observed when attached mean boundary layer takes place. In regions that present a statistically mean separation, it is violated. The regions of mean separation are characterised by large-scale fluctuating flow structures, which present some analogies to those found in the laminar separation.

It is therefore conjectured that a similar mechanism leads to the formation of the separating flow structures in the laminar and in the turbulent separation processes. It is also observed that in both boundary layer scenarios, the breakdown mechanism is similarly modified by the crosswind caused by the sweep. This observation suggests a common reason for the Simple Sweep Theory violation that occurs only when a global mean separation is established for both the boundary layer regimes. It also explains the fact that the theory holds for those wing portions affected by localised reversed spots that lie before the mean separation line. A formal correction of the Simple Sweep Theory when mechanisms of flow separation take place is also proposed in this thesis.

As a finale note, it is observed that the structure of the outer flow and of wall turbulence is always modified by the introduction of a constant crosswind, particularly inside the recirculating areas.

Key words: swept wing, simple sweep theory, laminar separation, turbulent separation, NACA-4412, highly-resolved LES.

Publication Type: Thesis (Doctoral)
Subjects: T Technology > TJ Mechanical engineering and machinery
T Technology > TL Motor vehicles. Aeronautics. Astronautics
Departments: Doctoral Theses
Doctoral Theses > School of Mathematics, Computer Science and Engineering Doctoral Theses
School of Mathematics, Computer Science & Engineering > Engineering > Mechanical Engineering & Aeronautics
Date Deposited: 03 Nov 2020 10:45
URI: https://openaccess.city.ac.uk/id/eprint/25187
[img] Text - Accepted Version
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