Surface Dye Flow Visualization: A Qualitative Method to Observe Streakline Patterns in Supersonic Flow
Flow visualization around or on a body is an important tool in aerodynamics research. It provides a method to qualitatively and quantitatively study f…
- Observing streaklines in supersonic flow
- Mix the fluorescent dye powder and mineral oil in a plastic bowl. Add small amounts of mineral oil to the dye in increments, mixing continuously until a semi-viscous mixture is obtained. The mixture should not be runny.
- Mount the sting above the supersonic wind tunnel test chamber and lock it into place. A blow-down supersonic wind tunnel with a 6 in x 4 in test section and an operating Mach number range of 1.5 to 4 was used in this demonstration, as shown in Figure 1. The Mach number is varied by adjusting the block setting (changing the area ratio of the test section).
- Screw the 2D wedge model onto the sting mount and fix the direction of the wedge such that the wedge surface is facing the transparent sidewalls of the wind tunnel test section. All models are shown in Figure 2.
- Use a paintbrush to apply a sufficient amount of the dye mixture onto the model. Ensure that the dye does not drip off of the model. See Figure 3 for reference.
- Adjust the block setting for the desired free-stream Mach number.
- Close and secure the wind tunnel panels.
- Run the wind tunnel for 6 seconds.
- After the run is complete, shine a UV light onto the model to illuminate the dye. Capture the streakline image with a camera.
- Adjust the angle of attack or Mach number according to the test matrix listed in Table 1 for model and repeat steps 1.4 - 1.9.
- Repeat steps 1.3 - 1.9 for all models listed in Table 1.
- When all models have been tested, shut down the wind tunnel, and dismantle the set-up.
Figure 1. Blow-down supersonic wind tunnel.
Figure 2. Wind tunnel models (left to right) 2D wedge, 3D wedge, cone, blunt nose body, sphere, and missile.
Table 1. Test Matrix.
| Model | Angle of Attack (q) or Mach Number (M) setting |
| 2D 10° Wedge | θ = 0, 12, and -12° |
| 3D 10° Wedge | θ = 0, 12, and -12° |
| Cone | θ = 0, 13, and -13° |
| Blunt Nose Body | θ = 0, 11, and -11° |
| Missile | θ = 0 and 11° |
| Sphere | M = 2, 2.5, and 3 |
Figure 3. Representative image of fluorescent dye painted on the 2D wedge.
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.
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Q1: What is dye flow visualization and how does it work?
Dye flow visualization coats a body with a semi-viscous mixture of fluorescent dye particles and mineral oil to observe surface flow patterns. The highly viscous oil maintains flow patterns on the body surface, while fluorescent dye becomes visible under UV light. This cost-effective technique provides qualitative images of surface flow, including laminar bubbles, boundary layer transitions, and flow separation.
Q2: What is a streakline and what does it reveal about flow?
A streakline is the line joining all dyed particles passing through a point or area, visible when images are captured with prolonged exposure. In supersonic flow, streaklines identify flow separation points, shock formation, and flow movement across surfaces. Smooth streaklines indicate attached flow, while clumped, brighter dye regions reveal flow separation zones.
Q3: How does Mach number affect flow separation on a sphere?
As Mach number increases in supersonic flow, the separation point moves toward the aft of the sphere, showing decreasing flow separation. Higher velocity flows have greater momentum, helping fluid overcome adverse pressure gradients and achieve higher flow attachment. This effect demonstrates how increased speed enhances flow stability on curved surfaces.
Q4: What flow features can be identified using streakline patterns?
Streakline patterns reveal multiple flow features including attached flow regions shown as smooth streaklines, flow separation indicated by coalesced dye, shock waves marked by thin bright curves, and surface deformities causing streakline disturbance. These patterns help researchers understand complex aerodynamic behavior around bodies in supersonic conditions.
Q5: How does angle of attack influence streakline patterns on a wedge?
At zero angle of attack, wedge streaklines show uniform flow except at surface deformities. At positive angles, flow deflects upward along the surface, while negative angles produce downward deflection. These directional changes in streaklines reveal how angle of attack alters pressure distribution and flow direction on aerodynamic surfaces.
Q6: What are the differences between surface and off-the-surface flow visualization?
Surface flow visualization coats the body with dye to study surface flow, making it invasive but simple and cost-effective. Off-the-surface techniques like schlieren imaging and particle image velocimetry determine flow characteristics around the body without contact, providing quantitative data but requiring expensive, complex equipment setup.
Q7: How do shock waves appear in dye flow visualization on missile fins?
Shock waves appear as thin bright curves on the body surface, visible through coalesced dye patterns. On missile fins, a bow shock forms at the leading edge of the central fin, marked by the dye shape. The shock location and intensity change with angle of attack, revealing how supersonic flow compresses and deflects around the fin geometry.