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Experimental Investigation of the Flow Structure over a Delta Wing Via Flow Visualization Methods
Chapters
Summary April 23rd, 2018
Here, we present a protocol to observe unsteady vortical flows over a delta wing using a modified smoke flow visualization technique and investigate the mechanism responsible for the oscillations of the leading-edge vortex breakdown locations.
Transcript
The overall goal of this experiment is to observe unsteady vortical flows over a delta wing using visualization techniques and investigate the mechanism responsible for the oscillations of the leading edge vortex breakdown locations. This method can help answer key questions of the unsteady flow phenomenon of a cylinder delta wing, such as the oscillation of the leading edge of vertex breakdown location. The main advantage of this technique is that large scale global leading edge vortex flow field and the small scale cellular substructures can be illustrated simultaneously.
Start with the model for the experiment. For this experiment, the model is an aluminum delta wing 280 millimeters long. Take the model to mount in the wind tunnel, which is closed-loop and low speed.
Its test section has glass walls to allow optical access for the experiment. Here, the delta wing has been properly mounted in the wind tunnel. Two lasers can illuminate the flow structures, a dual-pulse 532 nanometer laser during particle image velocimetry or a continuous-wave 532 nanometer laser during smoke flow visualization.
This schematic provides information on the optical setup with the continuous laser. A convex lens controls the laser beam size. A cylindrical lens expands the beam to a sheet.
A mirror reflects the laser sheet into the wind tunnel. Wear appropriate laser goggles and begin with the continuous laser. For safety, reduce the beam transmittance with a neutral density filter.
Begin with only the laser, filter, and mirror in place. Adjust the mirror to introduce the beam into the wind tunnel. Ensure the beam intersects the model at about 1/4 of the model's length behind its front.
Next, install the convex lens and the cylindrical lens. Then, at the model, measure the laser line that intersects it to check the laser sheet thickness. Adjust the convex lens as necessary.
Stop the adjustments when the desired thickness is obtained. Place a calibration target plate on the delta wing with its surface coincident to the laser sheet. With the laser off, move on to set up the camera.
Use a commercial digital camera for smoke flow visualization and operate it manually. Place the camera to obtain the desired field of view and focus it on the calibration target plate. When the camera is ready, take several frames for calibration purposes.
Then, remove the calibration target plate. Turn on the wind tunnel at a low speed, and inject oil particles to uniformly seat it. To visualize smoke flow, turn on the wind tunnel to the desired velocity.
Next, turn on the continuous laser without a filter. At the camera, capture five to 10 snapshots of the flow structure. Use the images to check that the laser sheet is at the longitudinal cross-section of the leading edge vortices core.
This example image shows a clear vortex core and substructures, indicating a well-positioned laser sheet. Adjust the camera settings as needed for image quality, and record images and video. When done, shut off the airflow and approach the model.
There, mark the position of the laser sheet as a reference for later measurements. Finish by turning off the laser and transferring the data to the computer. This measurement requires a different laser.
Replace the continuous laser with the pulsed laser. With the laser on, check its path, and if necessary, adjust the convex lens. Ensure the beam intersects the model along the line marking the position of the continuous laser.
Also, replace the digital camera. In its place, substitute a high-speed CCD camera. Control the camera with the synchronizer and the dual-pulse laser.
Now, go to the model and identify a region where structures can be observed. Once again, place a calibration target plate on the wing in that region. Focus on the calibration plate and take several photos.
Remove the calibration plate before continuing. Start the PIV software and set the parameters. Start the wind tunnel flow at the desired velocity.
Adjust the laser to its highest power level. Use the software to start data acquisition for 100 seconds. If the acquired images are acceptable, save them and continue by running other cases.
This is a processed image of the longitudinal cross-section from smoke flow visualization. There is a 10 millimeter scale for reference. The image reveals the primary leading edge vortex core, the rolling-up shear layer with vortical substructures, the vortex breakdown, and the turbulent wake.
This video show the oscillations of the leading edge vortex breakdown locations in the transfer's cross-sections. Analysis of the measured breakdown locations for each instant indicates a strong antisymmetric interaction. This is a time-averaged particle image velocimetry result showing the dimensionless vorticity in the longitudinal cross-section.
The primary vortex core can be identified by the inflection line of the positive and negative vorticity, marked by the black dotted line. Solid lines encircle regions with the local swirling strength lower than zero, indicating shear layer vortices. Additional substructure are encircled by white dotted lines.
This time-averaged particle image velocimetry vorticity result is at the span-wise cross-section. The flow separates from the leading edge, forms a shear layer, and rolls into the vortex core on either side of the wing. There are stationary vortical substructures inside the shear layers.
Once mastered, this technique can be done in one hour if it performed appropriate. After watching this video, you should have a good understanding of how to observe the unsteady flow phenomenon of a cylinder delta wing using the above-mentioned flow visualization techniques. Don't forget that working with lasers can be extremely hazardous, and precautions such as wearing laser protection goggles should always be taken while performing each experiment.
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