Name: experimental investigation of the flow structure over a delta wing via flow visualization methods
Uploaded: 2018-04-23T15:00:00
Description: Watch this Scientific Journal Video about Experimental Investigation of the Flow Structure over a Delta Wing Via Flow Visualization Methods at JoVE.com

The authors would like to thank Hong Kong Research Grants Council (no. GRF526913), Hong Kong Innovation and Technology Commission (no. ITS/334/15FP), and the US Office of Naval Research Global (no. N00014-16-1-2161) for financial support.

1. Wind Tunnel Setup
<ol>
	<li>Delta wing model
	<ol>
		<li>Construct a delta wing model from aluminum, with a sweep angle &#966; of 75&#176;, a chord length c of 280 mm, a root span b of 150 mm, and a thickness of 5 mm. Have both leading edges beveled at 35&#176; to fix the separation point17 (see Figure 1a).</li>
	</ol>
	</li>
	<li>Wind tunnel facility
	<ol>
		<li>Conduct experiments in a closed-loop low...

<ol>
	<li>Smits, A. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Flow visualization: Techniques and examples. , (2012).</li><li>Barlow, J. B., Rae, W. H., Pope, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Low-speed wind tunnel testing. , (1999).</li><li>Merzkirch, W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Flow visualization. , (1987).</li><li>Raffel, M., Willert, C. E., Wereley, S., Kompenhans, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Particle image velocimetry: A practical guide. , (2007).</li><li>Westerweel, J., Elsinga, G. E., Adrian, R. 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M., Ng, T., Nelson, R. C., Schiff, L. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Visualization+and+wake+surveys+of+vortical+flow+over+a+delta+wing.">Visualization and wake surveys of vortical flow over a delta wing.</a> AIAA J. 26 (2), 137-143 (1988).</li><li>Lowson, M. V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+three+dimensional+vortex+sheet+structure+on+delta+wings.">The three dimensional vortex sheet structure on delta wings.</a> Fluid Dynamics of Three-Dimensional Turbulent Shear Flows and Transition. , 11.11-11.16 (1989).</li><li>Riley, A. J., Lowson, M. V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Development+of+a+three-dimensional+free+shear+layer.">Development of a three-dimensional free shear layer.</a> J Fluid Mech. 369, 49-89 (1998).</li><li>Menke, M., Gursul, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Unsteady+nature+of+leading+edge+vortices.">Unsteady nature of leading edge vortices.</a> Phys Fluids. 9 (10), 2960 (1997).</li><li>Yayla, S., Canpolat, C., Sahin, B., Akilli, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Yaw+angle+effect+on+flow+structure+over+the+nonslender+diamond+wing.">Yaw angle effect on flow structure over the nonslender diamond wing.</a> AIAA J. 48 (10), 2457-2461 (2010).</li><li>Menke, M., Gursul, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nonlinear+response+of+vortex+breakdown+over+a+pitching+delta+Wing.">Nonlinear response of vortex breakdown over a pitching delta Wing.</a> J Aircraft. 36 (3), 496-500 (1999).</li><li>Sahin, B., Yayla, S., Canpolat, C., Akilli, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Flow+structure+over+the+yawed+nonslender+diamond+wing.">Flow structure over the yawed nonslender diamond wing.</a> Aerosp Sci Technol. 23 (1), 108-119 (2012).</li><li>Kohlman, D. L., Wentz, J. W. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Vortex+breakdown+on+slender+sharp-edged+wings.">Vortex breakdown on slender sharp-edged wings.</a> J Aircraft. 8 (3), 156-161 (1971).</li><li>Lu, L., Sick, V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High-speed+Particle+Image+Velocimetry+Near+Surfaces.">High-speed Particle Image Velocimetry Near Surfaces.</a> J Vis Exp. (76), e50559 (2013).</li><li>Mitchell, A. M., Barberis, D., Molton, P., D&#233;lery, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Oscillation+of+Vortex+Breakdown+Location+and+Blowing+Control+of+Time-Averaged+Location.">Oscillation of Vortex Breakdown Location and Blowing Control of Time-Averaged Location.</a> AIAA J. 38 (5), 793-803 (2000).</li><li>Shen, L., Wen, C. -. y., Chen, H. -. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Asymmetric+Flow+Control+on+a+Delta+Wing+with+Dielectric+Barrier+Discharge+Actuators.">Asymmetric Flow Control on a Delta Wing with Dielectric Barrier Discharge Actuators.</a> AIAA J. 54 (2), 652-658 (2016).</li><li>Leibovich, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+Structure+of+Vortex+Breakdown.">The Structure of Vortex Breakdown.</a> Annu Rev Fluid Mech. 10 (1), 221-246 (1978).</li><li>Mitchell, A. M., Molton, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Vortical+Substructures+in+the+Shear+Layers+Forming+Leading-Edge+Vortices.">Vortical Substructures in the Shear Layers Forming Leading-Edge Vortices.</a> AIAA J. 40 (8), 1689-1692 (2002).</li><li>Gad-El-Hak, M., Blackwelder, R. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+discrete+vortices+from+a+delta+wing.">The discrete vortices from a delta wing.</a> AIAA J. 23 (6), 961-962 (1985).</li><li>Zhou, J., Adrian, R. J., Balachandar, S., Kendall, T. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mechanisms+for+generating+coherent+packets+of+hairpin+vortices+in+channel+flow.">Mechanisms for generating coherent packets of hairpin vortices in channel flow.</a> J. Fluid Mech. 387, 353-396 (1999).</li><li>Adrian, R. J., Christensen, K. T., Liu, Z. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Analysis+and+interpretation+of+instantaneous+turbulent+velocity+fields.">Analysis and interpretation of instantaneous turbulent velocity fields.</a> Exp Fluids. 29 (3), 275-290 (2000).</li><li>Yoda, M., Hesselink, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+three-dimensional+visualization+technique+applied+to+flow+around+a+delta+wing.">A three-dimensional visualization technique applied to flow around a delta wing.</a> Exp. Fluids. 10 (2-3), (1990).</li><li>Greenwell, D. I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> RTO AVT Symposium. , (2001).</li><li>Furman, A., Breitsamter, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Turbulent+and+unsteady+flow+characteristics+of+delta+wing+vortex+systems.">Turbulent and unsteady flow characteristics of delta wing vortex systems.</a> Aerosp Sci Technol. 24 (1), 32-44 (2013).</li><li>Wang, C., Gao, Q., Wei, R., Li, T., Wang, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=3D+flow+visualization+and+tomographic+particle+image+velocimetry+for+vortex+breakdown+over+a+non-slender+delta+wing.">3D flow visualization and tomographic particle image velocimetry for vortex breakdown over a non-slender delta wing.</a> Exp Fluids. 57 (6), (2016).</li></ol>

This article presents the two flow visualization methods, improved smoke flow visualization and PIV measurement, to investigate flow structure over the delta wing qualitatively and quantitatively. The general procedures of the experiment are described step by step. The setups of these two methods are almost the same, while the devices involved are different. The basic principle of these two flow visualization methods is to illuminate the particles in the flow via the laser sheet. The improved smoke flow visualization can...

Flow field measurement via visualization techniques is a basic methodology in fluid engineering. Among the different visualization techniques, smoke wire flow visualization in wind tunnel experiments and dye visualization in water tunnel experiments are the most widely used to illustrate flow structures qualitatively. PIV and laser Doppler anemometry (LDA) are two typical quantitative techniques1.
In smoke wire flow visualization, smoke streaklines are generated from oil droplets on a heating wire or injected from the outer smoke generator/container during the experiments. High-power lights or laser sheets are used to illuminate the smoke streaklines. Images are then recorded for further analysis. This is a simple but very useful flow visualization method2. However, the effectiveness of this method may be limited by various factors, such as the short duration of smoke wires, the complex three-dimensional flow field, the relatively high velocity of the flow, and the efficiency of smoke generation3.
In PIV measurements, a cross-section of a flow field with entrained particles is illuminated by a laser sheet, and instant positions of the particles in this cross-section are captured by a high-speed camera. Within an extremely small time interval, a pair of images is recorded. By dividing the images into a grid of interrogation areas and calculating the average motion of particles in interrogation areas through cross-correlation functions, the instantaneous velocity vector map in this observed cross-section can be obtained. However, it is also known that compromises must be reached for factors including the size of the observation window, the resolution of the velocity map, the velocity magnitude in the plane, the time interval between the pair of images, the orthogonal velocity magnitude, and the particle density4. Therefore, many exploratory experiments may be needed to optimize the experimental settings. It would be expensive and time-consuming to investigate an unknown and complex flow field with PIV measurement alone5,6. Considering the above concerns, a strategy to combine smoke flow visualization and PIV measurement is proposed and demonstrated here to study the complex flow over a slender delta wing.
Numerous studies of LEV flows over delta wings have been conducted7,8, with flow visualization techniques used as the primary tools. Many interesting flow phenomena have been observed: spiral type and bubble type vortex breakdowns9,10, an unsteady shear layer substructure11,12, oscillations of LEV breakdown locations13, and effects of pitching and yaw angles14,15,16 on the flow structures. However, the underlying mechanisms of some unsteady phenomena in the delta wing flows remain unclear7. In this work, the smoke flow visualization is improved using the same seeding particles used in PIV measurement, instead of a smoke wire. This improvement greatly simplifies the operation of the visualization and increases the quality of the images. Based on the results from the improved smoke flow visualization, PIV measurement focuses on those flow fields of interest to acquire the quantitative information.
Here, a detailed description is provided to explain how to conduct a flow visualization experiment in a wind tunnel and to investigate unsteady flow phenomena over a delta wing. Two visualization methods, the improved smoke flow visualization and PIV measurement, are used together in this experiment. The procedure includes step-by-step guidance for device setup and parameter adjustment. Typical results are demonstrated to show the advantage of combining these two methods for measuring the complex flow field spatially and temporally.

<table>
	<tbody>
		<tr>
			<td>532 nm Nd:YAG laser</td>
			<td>Quantel Laser</td>
			<td>Evergreen 600mJ</td>
			<td></td>
		</tr>
		<tr>
			<td>High speed camera</td>
			<td>Dantec Dynamic</td>
			<td>HiSense 4M</td>
			<td></td>
		</tr>
		<tr>
			<td>camera lens</td>
			<td>Tamron</td>
			<td>SP AF180mm F/3.5 Di</td>
			<td></td>
		</tr>
		<tr>
			<td>PIV recording and processing software</td>
			<td>Dantec Dynamic</td>
			<td>DynamicStudio</td>
			<td></td>
		</tr>
		<tr>
			<td>cylindrical lens</td>
			<td>Newport</td>
			<td></td>
			<td>&#934;=12 mm</td>
		</tr>
		<tr>
			<td>convex lens</td>
			<td>Newport</td>
			<td></td>
			<td>f=700 mm</td>
		</tr>
		<tr>
			<td>neutral density filter</td>
			<td>Newport</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Calibration target</td>
			<td></td>
			<td></td>
			<td>custom made</td>
		</tr>
		<tr>
			<td>aerosol generator</td>
			<td>TSI</td>
			<td>TSI 9307-6</td>
			<td></td>
		</tr>
		<tr>
			<td>PULSE GENERATOR</td>
			<td>Berkeley Nucleonics Corp</td>
			<td>BNC 575</td>
			<td></td>
		</tr>
		<tr>
			<td>continuous laser</td>
			<td></td>
			<td>APGL-FN-532-1W</td>
			<td></td>
		</tr>
		<tr>
			<td>Digital camera</td>
			<td>Nikon</td>
			<td>Nikon D5200</td>
			<td></td>
		</tr>
		<tr>
			<td>Image processing</td>
			<td>Matlab</td>
			<td></td>
			<td>custom code</td>
		</tr>
		<tr>
			<td>wind tunnel support</td>
			<td></td>
			<td></td>
			<td>custom made</td>
		</tr>
		<tr>
			<td>laser level</td>
			<td>BOSCH</td>
			<td>GLL3-15X</td>
			<td></td>
		</tr>
		<tr>
			<td>angle meter</td>
			<td>BOSCH</td>
			<td>GAM220</td>
			<td></td>
		</tr>
	</tbody>
</table>

experimental investigation of the flow structure over a delta wing via flow visualization methods

It is well known that the flow field over a delta wing is dominated by a pair of counter rotating leading edge vortices (LEV). However, their mechanism is not well understood. The flow visualization technique is a promising non-intrusive method to illustrate the complex flow field spatially and temporally. A basic flow visualization setup consists of a high-powered laser and optic lenses to generate the laser sheet, a camera, a tracer particle generator, and a data processor. The wind tunnel setup, the specifications of devices involved, and the corresponding parameter settings are dependent on the flow features to be obtained.
Normal smoke wire flow visualization uses a smoke wire to demonstrate the flow streaklines. However, the performance of this method is limited by poor spatial resolution when it is conducted in a complex flow field. Therefore, an improved smoke flow visualization technique has been developed. This technique illustrates the large-scale global LEV flow field and the small-scale shear layer flow structure at the same time, providing a valuable reference for later detailed particle image velocimetry (PIV) measurement.
In this paper, the application of the improved smoke flow visualization and PIV measurement to study the unsteady flow phenomena over a delta wing is demonstrated. The procedure and cautions for conducting the experiment are listed, including wind tunnel setup, data acquisition, and data processing. The representative results show that these two flow visualization methods are effective techniques for investigating the three-dimensional flow field qualitatively and quantitatively.

Figure 2d shows the time histories of the LEV breakdown locations. The black curve indicates the portside LEV and the red curve indicates the starboard LEV. The time scale is nondimensionalized by the free stream velocity and chord length. The correlation coefficient between these two time histories is r = &#8722;0.53, indicating a strong anti-symmetric interaction of the LEV breakdown location oscillations. This result agrees well with the work of o...

Watch this Scientific Journal Video about Experimental Investigation of the Flow Structure over a Delta Wing Via Flow Visualization Methods at JoVE.com

Experimental Investigation of the Flow Structure over a Delta Wing Via Flow Visualization Methods

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.

It is well known that the flow field over a delta wing is dominated by a pair of counter rotating leading edge vortices (LEV). However, their mechanism is ...

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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