On the Application of Contour Bumps for Transonic Drag Reduction (Invited)

Abstract

The effect of discrete contour bumps on reducing the transonic drag at off-design conditions on an airfoil have been examined. The research focused on fully-turbulent flow conditions, at a realistic flight chord Reynolds number of 30 million. State-of-the-art computational fluid dynamics methods were used to design a new baseline airfoil, and a family of fixed contour bumps. The new configurations were experimentally evaluated in the 0.3-m Transonic Cryogenic Tunnel at the NASA Langley Research center, which utilizes an adaptive wall test section to minimize wall interference. The computational study showed that transonic drag reduction, on the order of 12% -15%, was possible using a surface contour bump to spread a normal shock wave. The computational study also indicated that the divergence drag Mach number was increased for the contour bump applications. Preliminary analysis of the experimental data showed a similar contour bump effect, but this data needed to be further analyzed for residual wall interference corrections.

Review of research into shock control bumps

“…5b) yields a 'cleaner' two-step pressure rise which is less detrimental to the boundary layer [26]. Figure 6 presents results from two numerical studies [10,13], both of which illustrate this beneficial impact of a 2-D SCB on the streamwise pressure distribution over the upper surface of a supercritical aerofoil.…”

“…viscous penalty or reduced lift). It is now well-documented that (2-D and 3-D) SCBs can yield a net positive benefit for the efficiency (usually expressed in terms of L/D) of transonic aerofoils and wings [13,29]. Two examples illustrating the impact of SCBs on lift-drag polars for different transonic wings are presented in Fig.…”

“…9c). If such a SCB was installed on a real wing with length of the order of 20-30 % wing chord-a range commonly suggested in literature [10,13]-such variations in [7]; solid light grey lines, SCB using set-up of [8] shock position would correspond to excursions of just 2-4 % of overall wing chord. These numbers suggest that the impact of shock position on SCB performance is a primary concern.…”

“…For a modern turbulent supercritical wing, this only occurs at high values of C L and/or Mach number that are in excess of typical cruise values. For this reason, many argue that the true potential of SCBs for drag reduction can only be realised in the long term with an integrated approach where next-generation wings are designed specifically to take advantage of the beneficial control effects offered by SCBs [13,20,33].…”

“…However, non-optimal shock locations yield significantly increased levels of viscous drag and can Fig. 6 Results from: a Milholen and Owens [13]; b Eastwood and Jarrett [10], reproduced with permission from Jeremy Eastwood also promote boundary layer separation [26]. Despite this, some studies have argued that 2-D SCBs can still have a positive impact on off-design performance in one of the two ways: (1) Delaying the onset of transonic buffet by breaking up large regions of separated flow and inhibiting communication between an oscillating shock and an aerofoil's trailing edge [2,4,18]; and, (2) In the case of a straight ramp-type design, improving shock stability by effectively anchoring the front shock leg [17].…”