Source: Jose Roberto Moreto, Gustaaf Jacobs and Xiaofeng Liu, Department of Aerospace Engineering, San Diego State University, San Diego, California

The delta wing, shown in Figure 1D, is a popular design in high-speed airplanes due to its superb performance in transonic and supersonic flight regimes. This type of wing has a small aspect ratio and high sweep angle, which reduces drag at high subsonic, transonic and supersonic flight regimes. The aspect ratio is defined as the wing span divided by the average chord .

An important advantage of the delta wing is its high stall angle. The stall of a delta wing is delayed compared to the stall of a high aspect ratio wing. This is because the lift of a delta wing is enhanced by the leading-edge vortex over the wing.

An effective way to observe this vortex flow phenomenon and study the vortex breakdown in a delta wing is by visualizing the flow in a water tunnel. By injecting dye in the flow surrounding a model from dye ports on the leading-edge, the vortex development and breakdown can be observed and its position measured. The data can also be used to estimate the stall angle.

Figure 1. Typical wing planform shapes: A) Rectangular, with constant chord along span, B) elliptical, C) tapered, with variable chord along the span, and D) delta wing, an aft-swept wing with zero taper ratio.

1. Preparing the water tunnel

1. Obtain three 500-mL containers, and fill each container at least half-full with dye. There should be one container with blue dye, one with green dye and one with red dye. The concentration is not important because the dye flow rate will be adjusted accordingly.
2. Install the delta wing on its support in the water tunnel. Attach the C-strut support to the water tunnel with screws, keeping the yaw angle at zero. See Figure 3.
3. Fill the water tunnel

From the experiment, we can identify the vortex breakdown, as illustrated in Figure 4. The distance from the wing apex to the vortex breakdown can be measured using the scale drawn in the wing (Figure 4B). During the experiment, the angle of attack of the wing was incrementally increased, and the vortex breakdown location, l_b, with respect to the wing apex, was measured. The breakdown location, x/c, with respect to the wing trailing edge was graphed against the angle o

By using flow visualization in a water tunnel, the vortex breakdown locations for various angle of attacks in a delta wing were identified. Flow visualization in a water tunnel is performed by injecting dye into specific locations of the flow field. The dye follows the flow, which enables us to observe the flow streaklines. This method is similar to the smoke visualization technique that is used in a wind tunnel. However, the ability to use multiple different dye colors allowed easy visualization of the flow structures a