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Research Spotlight: Trevor Berg

by Marla Lobley on 2018-07-23T08:46:00-05:00 in Physical Science | 0 Comments

Main take-away: It is necessary to validate the models, instruments or tools used in research to make sure that they produce accurate and reliable results. 

Trevor Berg is an ECU student participating in a Research Experience for Undergraduates (REU) at the University of Alabama sponsored by the National Science Foundation (NSF). Trevor provided the following pictures and description of his research project for the summer.

Trevor Berg in research lab

 

 

 

 

 

 

 

 

 

 

In Dr. Amy Lang’s water tunnel lab here at the University of Alabama, Mako shark skin is being studied to isolate and study fluid control mechanisms. These mechanisms function on a micro scale (scales are 100 micro meters tall). Research already published from the water tunnel lab has shown that these scales can be passively actuated (bristled outward) by a reversing flow, and that their presence helps to control flow separation on a surface. This effect reduces the pressure drag a surface experiences, while also increasing its Aerodynamic efficiency.

Here’s how it works. Within a boundary layer (VERY near to a surface), fluid momentum is the lowest and most susceptible to changing. Under the conditions of an adverse pressure gradient (APG), the flow nearest to the surface is reversed, which causes flow separation and leads to more pressure drag (not good). An adverse pressure gradient is the flow interaction a Mako shark experiences along the surface of its skin in nature. The presence of the scales on Mako shark skin has been shown to keep the flow attached longer, which helps keep the surface more Aerodynamically efficient.

Lab

How can these effects be studied using the water tunnel? The water tunnel can be set to a certain initial velocity of laminar flow. Tiny hollow 16 micro meter particles coated with silver or glass are seeded into the flow. They look like fine powder and are used because they are highly reflective. The particles are distributed fairly evenly after flowing through a device in the tunnel called a honey comb and some filters. A 1000 Hz laser is used to provide a high contrast plane (background). It is placed level to the cameras beneath the tunnel for images/video to be taken. This laser plane and the water tunnel are shown below.

Water tunnel and laser

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

This process which takes images of the flow and tracks their position is called Digital Particle Image Velocimetry (DPIV). The camera we use takes pictures at 1000 Frames/sec. So in order to match this high frame rate, the laser is pulsed 1000 times per second too (It’s a 1000 Hz laser). Matlab coding software is currently being used to operate all of this equipment and acquire data in the form of a video. For my case, the code then splits a 10 sec video into 10,000 images. (1000 images/sec). It is then converted by the code into a format recognizable by another software called Insight.  Insight is a DPIV processing software. The way it works is by tracking each individual tiny particle in the flow from one picture to another. It calculates the distance traveled between frames, and then uses this information to generate a velocity vector field. (Velocity = distance/time, so now the computer has all of this data.) Once a velocity vector field is generated, we can then see how the flow is behaving in various experiments.

Software screeenshot

 

 

 

 

 

 

 

 

 

 

My specific task is to study the flow behind a rotating cylinder placed in the water tunnel. Remember, the Mako shark experiences an adverse pressure gradient (APG) in nature, and the APG is the condition which begins the entire process of fluid reversal, followed by the passively actuated scales, etc. So how do we re-create this parameter in the water tunnel? A rotating cylinder is used to induce an APG near the vertical plate (surface) being studied within the tunnel. The way this works is the cylinder spins causing flow to be accelerated to a higher initial velocity on the side nearest to the plate. Now, there’s a velocity difference (high progressing to low) along the surface. By Bernoulli’s Principle there is also a difference in pressures (low progressing to high), which is an adverse pressure gradient.

When a stationary cylinder is placed into a flow, it disrupts the flow behind it. This flow disruption has a pattern, which is the shedding of vortices alternating from behind one side then another downstream of the cylinder. This is called a vortex street. Research was done at the University of Hong Kong also studying the effect a rotating cylinder flow moving past it. However, this research was done in a wind tunnel experiment. The values published in the wind tunnel experiment results were non-dimensionalized, meaning I can re-create this experiment in the water tunnel. The goal of my specific project is to use the DPIV process and show that there is no flow interaction between the cylinder wake and the flow passing over the surface that is being studied downstream of the cylinder.

The published wind tunnel results showed that as the cylinder begins to rotate (causing the APG), the vortex street also begins to deflect at an angle to one side and gradually becomes shorter in length. If the results of the water tunnel experiment match with the results published from the wind tunnel experiment, it is hypothesized there will be no flow interaction between the rotating cylinder and surface being studied. The analysis of the flow behind a rotating cylinder in the water tunnel is very important because it will increase the confidence of the research done in the water tunnel lab by knowing that the use of a rotating cylinder to induce an APG isn’t detrimental when trying to isolate specific mechanisms of the Mako shark scales. This project will complement the research already published using a wind tunnel, and further extend the known effective range of mediums in which a rotating cylinder can be used to induce an APG for fluids research.

Lab with laser

For more information, click on the following links (opens a new tab). 

Dr. Amy Lang, University of Alabama

Research Experience for Undergraduates (REU)

If you would like your research or creative project featured in a blog post, contact Marla at mlobley@ecok.edu (opens a new tab).


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