Additive manufacturing of sharkskin-inspired surfaces for improved aerodynamic performance

<p>Innovative technologies that lead to a reduction in aircraft drag as a strategy to reduce fuel burn and its associated (CO2) emissions is of continued interest to the aerospace sector. One prospective avenue of research considers surface modification as a strategy to improve aerodynamic performance. Inspired by the drag reducing properties of denticles that cover the skin of sharks, the research presented investigates the design, additive manufacture (i.e. 3D printing) and testing of three-dimensional surface features based on the sharkskin denticle structure. Down selection of additive manufacturing technologies showed that material jetting was a feasible technique to produce the desired structures at the micron scale.</p> <p>Improvement in the aerodynamic performance induced by denticle inspired surface structures was demonstrated using wind tunnel experiments. A row of denticles was positioned on and aft of the quarter chord of a NACA0012 wing and printed as an integrated structure. In addition to a rigid wing model printed using the VeroCyan(TM) photopolymer, rigid denticles were also embedded and printed simultaneously into a region of flexible photopolymer. This was comparable to a compliant surface closely mimicking the ability of individual sharkskin denticles to bristle in nature. The wing structures incorporating rigid and flexible regions of printed denticles were compared for aerodynamic performance to a wing with no denticles (i.e. a smooth wing). The rigid denticles embedded in a flexible base aft of the quarter chord exhibited an enhancement in overall aerodynamic performance by passive flow control in the turbulent boundary layer. Lift improved by 13% and drag reduced by 22% at angle of attack (AOA) of 6° at peak L/D ratio prior to stall. This improvement was most prominent in the stall region with an overall increase to the L/D ratio, which is ideal for applications with more aggressive manoeuvres.</p> <p>In addition to the high-resolution fabrication of NACA0012 wing sections with integrated surface denticles, a previously unreported technique was developed to print directly onto pre-formed substrates. This may be required for the application of denticles onto large complex structures such as drones or aircraft. Manipulation of the Stratasys J750 printer vertical Z-offset allowed sharkskin-inspired denticle surface features to be printed initially on to polymethylmethacrylate (PMMA) substrates and then aluminium coupons coated in aerospace grade polyurethane paint. Time dependent studies showed that the overcoat window (t = 48 h) played a crucial role in ensuring adhesion was not compromised during the process. Surface analysis via X-ray photoelectron spectrophotometry (XPS), and surface free energy calculations obtained from contact angle measurements validated changes in substrate surface chemistry overtime, and hence adhesion due to the difference in the interfacial interactions between the photopolymers (VeroCyan(TM)) and the aerospace polyurethane coatings.</p>