Visualizing flow around an autobody is critical to understanding and quantifying flow structure as well as for theorizing fluid flow behavior. One type of flow visualization is called surface flow visualization which uses a dyed fluid to observe the path traced by fluid flow around an object.
Dye flow visualization involves coating the body of interest with a dye to observe flow patterns along the body surface. The dye is a semi-viscous mixture of fluorescent dye particles and oil. The highly viscous nature of the oil helps maintain the flow patterns on the body surface. While the fluorescent dye lets us visualize those patterns under a UV light.
If the image is taken with prolonged exposure, the dye can be used to track the path taken by a single fluid particle as it moves in the flow. As dye marked fluid particles pass through a point or area, we can observe the line joining all of the dyed particles. This is called the streakline.
In supersonic flow, these streaklines can be used to identify the point of flow separation, shock formation and movement of flow across the surface.
Now let's take a closer look at flow over the sphere. Attached flow appears as smooth streaklines and the direction of the streaklines tells us the direction of flow on the surface. Flow separation can be identified as the region where the dye clumps up and appears brighter. This is because dye beyond the point of flow separation is undisturbed.
In supersonic flow, we can also observe the formation of shock waves on the surface of the body like on the fins of a missile shown by a thin bright curve. We can also use this technique to identify deformities on a surface as evidenced by regions where the streaklines are disturbed.
In this lab, we will demonstrate the dye flow visualization technique using several different bodies exposed to supersonic flow.
For this experiment, we'll use a blow down supersonic wind tunnel with an operating Mach number range of 1. 5 to 4. This wind tunnel has a 6 in x 4 in test section. The Mach number is varied by adjusting the block section. In other words, by changing the area ratio of the test section. We will test and observe the streaklines around several different models: a 2D wedge, a 3D wedge, a cone, a blunt nose body, a sphere and a missile.
To begin the experiment, mix fluorescent dye powder and mineral oil in a plastic bowl. Add small amounts of mineral oil to the dye in increments mixing continuously until the mixture is semi-viscous and not thin and runny.
Now, mount the sting above the wind tunnel test chamber and lock it into place. Then, screw the 2D wedge model onto the sting mount. Fix the direction of the wedge so that the wedge surface is facing the transparent sidewalls of the test section.
Use a paint brush to apply a thick layer of dye to the surface of the model ensuring that there is not so much that it drips off. Then adjust the block setting to reach the desired free stream mach number. Adjust the angle of attack alpha to 0° using a digital level.
Now, close and secure the test section door and run the wind tunnel for 6 s. Shine a UV light on the model during the run to illuminate the dye. This allows us to observe the evolution of the streakline patterns.
Once the run is complete, capture an image of the final flow patterns. Next, adjust the angle of attack to 12°. Paint the model with dye as before and run the wind tunnel for 6 s. Illuminate the streaklines with the UV light and capture the image with a camera.
Repeat these steps for the 2D wedge model at -12°. Execute the test and capture streakline images for all of the models according to the test matrix shown here. When all of the tests have been completed on each model, shut down the wind tunnel and disassemble the setup.
Now let's take a look at the results starting with the streaklines over the 2D wedge. At 0°, the streakline pattern shows uniform flow throughout the body except in the region where there is a surface deformity in the center causing flow to separate. When the wedge is angled to 12°, the flow along the surface is deflected upwards while the flow is deflected downward at the -12° setting.
Looking at the 3D wedge, we can see that the flow pattern at the center of the model is similar to that observed for the 2D wedge at all angle settings. However, the flow pattern at the top and bottom edges show deflection and the tip vortex effect is observed along their length.
Streakline patterns for the cone show that for all angles of attack, the flow curves around the body. We can also observe that flow separation occurs at the end of the cone as indicated by the region where the dye clumps up.
For the blunt nose model, we observe attached flow throughout the body at an angle of attack of 0°. At 11 and -11°, the flow curves around the body following the surface contour and separates along the line where the dye coalesces.
While flow patterns in the front of the missile model are similar to that of the blunt nose body, the streaklines on the fins show varied features. At 0°, the streaklines on the top and bottom fins show attached flow at the front of the fin with gradual separation occurring in a cross pattern. We also observe that flow detaches a lot earlier at the root of the fins as compared to the tips.
If we look at the coalesced dye at the leading edge of the central fin, we can see that the streakline patterns indicate a bow shock with the shape of the shock marked by the dye. At an 11° angle of attack, we observe fully attached flow on the bottom fin but separated flow close to the root of the top fin. Similar to the 0° case, the presence of the central fin causes a bow shock at the fin's leading edge.
Finally, for the sphere, we varied mach number as opposed to angle of attack as the flow patterns remain the same regardless of deflection angle. We can see that as the mach number increases, the point of separation moves toward the aft of the body showing decreasing flow separation. This is due to the fact that higher velocity flows have more momentum which helps the flow overcome the adverse pressure gradient over the sphere. This leads to a higher degree of flow attachment with increased mach number.
In summary, we learned how streaklines can be used to identify the point of flow separation, shock formation and movement of flow across a surface. We then exposed several bodies to supersonic flow in a wind tunnel and observed the streaklines that formed on each surface at varying angles of attack.