Slender delta wings (DW) have often been employed in high-speed flight. The DW flowfield is characterized by two counter rotating streamwise leading-edge vortices (LEVs) which energize the flow and provide nonlinear vortex lift, rendering a high static-stall angle. The augmented lift comes, however, at the expense of high drag. Moreover, at a sufficiently high angle of attack, the LEVs can undergo a sudden expansion know as vortex breakdown, leading to lift deterioration and an increased pitch-up moment of the delta wings. The unsteady nature of the flowfield downstream of the LEV breakdown may also affect the stability of the aircraft and cause buffeting. Excellent reviews of the vortex flow structure and characteristics over slender delta wings are given by Nelson and Pelletier [1] and Gursul et al. [2].
It has also been observed that a reverse delta wing (RDW), which also resembles forward-swept-wing aircraft, can produce certain favorable aerodynamic characteristics. Most recently, Altaf et al. [3] investigated the vortex structure and characteristics of a Λ=75° slender reverse delta wing using particle image velocimetry (PIV) and force balance measurements at selected α for x/c=1.359 and 3.418 with Re = 3.82 × 10^4. They found that the peak tangential velocities at α>5° showed a trend similar to the regular delta wing. The reverse delta wing at a particular α, however, exhibited a lower magnitude of tangential velocity, circulation and vorticity than the regular delta wing. The reverse delta wing was also found to have a lower lift and drag coefficient than a DW, however, the lowered drag coefficient resulted in a higher lift-to-drag ratio compared to the delta wing. Nevertheless, detailed variation of the vortex flow structure and characteristics of the reverse delta wing along x/c and at various α were not provided.
The objective of this study was to investigate the vortex flow structure and characteristics of a Λ=65° reverse delta wing in a water tunnel using PIV for 0.1≤x/c≤1.5 at α=4° to 30° with Re=11,000. The leading-edge vortex developed on a delta wing at the same flow condition was also investigated to serve as a comparison. The PIV data was supplemented by smoke-wire and dye-injection flow visualizations, and force balance (FB) measurements will be conducted as well. Special emphases were placed on the variation of the vortex flow parameters of the RDW vortex with x/c and α. The total lift coefficient CL determination based on the PIV vw-crossflow measurements will also be considered in the study. The PIV experiment was conducted in a 20 cm × 27.5 cm × 75 cm water tunnel at McGill University. A 65° delta wing model, constructed from flat-plate aluminum with a chord of c=10.8 cm, a span of b=10 cm and a thickness-to-chord (t/c) of 1.5% was used as the test model. A Rolling Hill Research Corporation one-component internal strain-gage force balance was also used to measure the wing normal force. Figures 1a-b show the schematic diagram of the experimental PIV setup as well as the RDW configuration.
Figures 2a-g show the evolution of the normalized iso-vorticity (ζc/u∞) contours of the RDW for 0.1≤x/c≤1.5. The roll-up of the shear layer, originating from the pressure-side surface of the RDW, and the formation of the unique “arm-and-fist” RDW vortex pattern along the side edge of the RDW can be clearly seen. The extent of the “arm” grows in length as the RDW vortex progresses downstream, where, eventually, at x/c=1.5, a single RDW vortex exhibits. The iso-vorticity contours of the leading-edge vortex (LEV), or DW vortex, developed on the DW at the same α are also included in Figures 2h-l for a direct comparison. Figures 2a-g further show that the size of the RDW vortex grew as it progressed downstream. The peak vorticity, however reaches a local maximum at around x/c=0.2 and begins to drop monotonically.
Figures 2h-l present the iso-vorticity contours of the DW vortex for 0.3≤x/c≤1.5 at α=10°. The peak vorticity of the DW vortex was found to increase with x/c, reaching a local maximum at around x/c=0.7, and began to drop dramatically as it progresses further downstream. Figure 3a is a three-dimensional representation of the iso-vorticity contours of the DW at α=14° and 24°, for 0.3≤x/c≤1.5. For α=14°, the LEV stays concentrated over the DW (for x/c<1.01) and no vortex breakdown is observed. For x/c≥1.01, the peak vorticity drops suddenly and the vortex eventually rolls into a single DW vortex at x/c=1.5. However, for α=24°, vortex breakdown was observed at x/c=0.53, characterized by a sudden drop in peak vorticity as well as a sudden expansion of the vortex core size (see figure 3a insets).
Figure 3b shows the three-dimensional iso-vorticity contours of the RDW at α=14° and 20° for 0.3≤x/c≤1.5. Once again, the unique “arm-and-fist” vortex pattern is observed. For both 14° and 20°, the vortex trajectory was above and outboard of the RDW. However, for α=20° the RDW vortex was found to have a lower peak vorticity than for α=14°. Furthermore, the α=20° had a less circular, more diffused vortex for x/c≥0.6, as compared to the α=14° case (see figure 3b insets for x/c=0.6 and 0.9). As the vortex travels further downstream, it becomes a weak, circulation-like flow for α=20°.
In summary the structure and characteristics of the vortex generated by a reverse delta wing (RDW) were investigated using particle image velocimetry (PIV) at Re = 1.1 × 104. The DW vortex had a smaller core radius and a higher circulation and peak tangential velocity compared to the RDW vortex. Finally, the RDW vortex was found to remain concentrated for α=14° while it resembles a weak circulation-like flow for α=20°. The data will also be supplemented with force balance measurements and dye and smoke-wire flow visualizations. The total lift coefficient derived based on the total circulation and effective span via the Kutta-Joukowski theorem will also be calculated.
References
[1] Nelson, R.C., Pelletier, A., 2003. The unsteady aerodynamics of slender wings and aircraft undergoing large amplitude manoeuvers. Progress in Aerospace Sciences, 39, 185-248.
[2] Gursul, I., Wang, Z., Vardaki, E., 2007. Review of flow control mechanisms of leading-edge vortices. Progress in Aerospace Sciences, 43, 246-270.
[3] Altaf, A., Omar, A.A., Asrar, W., Jamaluddin, H.B.L., 2011. Study of the reverse delta wing. Journal of Aircraft, 48(1), 277-286.
Topics: Aerodynamics of airfoils, wings, wing/fuselage interactions, nacelles, etc., inclu , Topics: Aerodynamic design of fixed and rotary wing aircraft, propellers, future aircraft , Topics: Unsteady aerodynamics, vortical flows, aircraft wakevortex dynamics including DES, , Topics: Experimental aerodynamics methods and test facilities
