The overall goal of the following experiment is to investigate the three dimensional flow separation induced by a model vocal fold polyp. This is achieved by mounting a prote hemisphere with a two to one aspect ratio into the test section of a wind tunnel to model a wall-mounted protrusion, such as a polyp or nodule. As a second step, oil flow visualization techniques are performed, which enable the visualization of skin friction lines and separation and attachment points within the surface flow.
Next unsteady surface pressure measurements are acquired around the wall-mounted protuberance. In order to quantify the wall pressure loadings surrounding the model polyp, the results in combination with particle image villas symmetry data show the presence of a horseshoe vortex shedding hairpin vortices and spatial and temporal pressure loadings surrounding the model vocal fold polyp, which are expected to contribute to irregular vocal fold dynamics observed in patients with pathological conditions such as polyps or nodules. This method can provide insight into the fluid dynamics of pathological vocal fold disorders.
It can also be applied to other systems such as blood flow and stenosis arteries, management of coastal sand dunes, secondary flow, and heat exchangers and wind energy applications. Generally, individuals new to this method can have difficulty determining the correct oil-based mixture for their specific testing conditions. To create the model vocal fold polyp, begin by building a three-dimensional computer-aided design or CAD model polyp.
As a prolate hemisphere, add a square base to the bottom of the model vocal fold polyp to be used to anchor the model to the test section floor. Export the 3D CAD model as a stereolithography or STL file. Choose a resolution of at least 600 dots per inch to ensure a smooth surface on the model polyp.
Upload the STL file into the appropriate software and print it using a high resolution 3D printer or rapid prototyper. With a built layer resolution of at least 20 micrometers, the polyp is now ready to be mounted in the wind tunnel. The wind tunnel test section has a removable bottom plate with a square hole located along the span-wise center line, and at the desired downstream location.
The hole is used for mounting the model vocal fold polyp. To prepare for the oil flow tests, cover the inside surface of the wind tunnel test section with white adhesive paper. Carefully place the adhesive paper to ensure that there are no creases or bumps due to air bubbles.
Cut a hole in the adhesive paper above the square hole in the test section floor. Then replace the removable bottom plate in the test section. Now insert the model polyp into the anchor position.
To prepare for testing using a high resolution camera mounted above the wind tunnel test section, focus the camera so it's field of view, includes the model polyp and surrounding test section area. Set the camera to acquire either images or video. Prepare the flow visualization oil mixture by combining baby oil copy toner, powder and kerosene in a seven to one to two ratio by volume.
Mix the baby oil and toner powder together in a container and stir for a few seconds until the toner is completely dissolved. Then add the kerosene and mix. Well transfer the mixture to a spray bottle for easy application to the test section surface.
Prior to applying the oil mixture, clean the test section surface with water and dry it with a paper towel. Then let it air dry for a few minutes. Use the spray bottle filled with the oil mixture to spray a thin even layer of fluid over the area surrounding the model vocal fold polyp.
A thin even oil mixture layer is important for producing proper oil. Film visualization images initiate the image or video acquisition on the camera. Camera acquisition is begun before the wind tunnel is powered on.
In order to capture the initial transient oil mixture motion, turn on the suction wind tunnel at the desired velocity. The oil mixture will begin to flow along the test section surface. Once the oil mixture stops flowing and as reached a steady state as shown by stationary patterns, or when the desired time is elapsed, stop the camera recording and power down the wind tunnel to add static pressure.
Taps to the test section floor surface. Start at the midline of the anchor position of the model polyp and drill holes into the removable plate to make a grid pattern that spans eight point 89 centimeters wide and 22 point 86 centimeters downstream. Then mount stainless steel tabulations into the holes surrounding the model polyp in the desired configuration.
The tribulations have a bulge on one end for attaching flexible tubing and are straight on the other end for mounting. When mounted properly, the tribulations are flushed with the test section floor. Next, attach short pieces of clear flexible polyvinyl chloride tubing to connect the mounted stainless steel tabulations to the scanning pressure transducer measurement ports.
The scanning pressure transducer has 16 pressure port. Begin surface pressure data acquisition by connecting the scanning pressure transducer to a computer and configuring the acquisition parameters using the scanning pressure transducer software. Set the acquisition software to acquire data at 500 hertz for the desired duration of data acquisition.
Next, set the suction wind tunnel to the desired velocity. Now begin pressure measurement acquisition. The pressure measurements may be acquired simultaneously with any desired flow diagnostic technique.
For example, PIV laser, doppler vela symmetry, and hot wire anemometry. The model polyp was tested under steady flow conditions with a Reynolds number of 9, 000. The primary upstream separation line is displayed as the dark line upstream of the polyp.
Near the wake of the polyp are two vorticity concentration nodes, which are the attachment points for two counter-rotating vortex tubes that form the legs of the downstream hairpin vortex. Shown here is an oil flow visualization image for a model polyp in cross flow with a Reynolds number of 9, 000. The dark lines extending downstream from the sides of the polyp, representing the outer limits of the wake converge until the attachment point due to the recirculation vortex behind the object.
The oil flow visualization results for the steady flow conditions confirm the formation of a horseshoe vortex system upstream of the model polyp and hairpin vortices downstream of the polyp. The upstream and downstream pressure measurements throughout a single oscillatory cycle are shown here. The red line located at position number three indicates the site of the lowest pressure in the backflow region directly downstream of the polyp.
The individual pressure transducer values were found to change throughout the cycle, and the pressure difference among the transducer locations varied as a function of the cycle location and therefore mean velocity shown. Here is a related study using driven model vocal folds and a wall mounted model vocal fold polyp. The direction of the fluid flow is out of the screen.
The colored arrows show the vector plots of the velocity magnitude as the vocal folds begin to open. A convergent channel is formed and a favorable pressure gradient is developed. The flow passes over the polyp and is directed towards the midline.
As the glottis reaches its maximum width and assumes a parallel configuration, two counter-rotating stream wise vortices are formed. As the vocal folds close, the flow is forced around the polyp and away from the anterior posterior midline Once mastered, this technique can be done in a few minutes if it is performed properly. After watching this video, you should have a good understanding of how to acquire oil flow visualization data, which can be used to identify skin friction lines high and low velocity regions, and to construct topological maps of complex three dimensional flows.
