a visual investigation of flow around objects

8/11/2019 A Visual Investigation of Flow Around Objects

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A Visual Investigation of Flow around Objects

By

Robert P. Skeehan

902990644

Aero 3130, group 1, Section C

Auburn University

September 2, 2013


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Abstract

A smoke tunnel was used to observe airflow around a cylinder, an infinite wing, a

wing tip, and a scale wing, and from these observations, conclusions about the

characteristics of the airflow were drawn. The streamlines of the airflow around the airfoil

and wingtip were recorded at an angle of attack of 0, 15, and 30. A cylinder was used

to view airflow moving around a smooth circular object. Sandpaper was then placed on the

cylinder, and a comparison was made. The sandpaper acted as a turbulator, and the

transition point was moved slightly upstream. The scale wing was used to observe the

effects of both the airfoil and the wingtip simultaneously. Wing wash, separation,

turbulence, and wake could be observed behind the trailing edge of the wing.


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Table of Contents

Abstract................................................................................................................................iiNomenclature......................................................................................................................iv

List of Figures......................................................................................................................v

Introduction..........................................................................................................................1Description of Relevant Theory...........................................................................................2Description of Test Equipment and Procedure.....................................................................3

Results and Discussion.........................................................................................................4

Conclusions..........................................................................................................................7References Page...................................................................................................................8

Figures..................................................................................................................................9


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Nomenclature

................................................................................................................... Angle of AttackV........................................................................................................Free Stream Velocity


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List of Figures

Figure 1: Infinite Laminar Cylinder....................................................................................9Figure 2: Turbulent Cylinder...............................................................................................9

Figure 3: Infinite wing, = 0...........................................................................................10

Figure 4: Infinite wing, = 15.........................................................................................10Figure 5: Infinite wing, = 30.........................................................................................11Figure 6: Wingtip, = 0...................................................................................................11

Figure 7: Wingtip, = 15.................................................................................................12

Figure 8: Wingtip, = 30.................................................................................................12Figure 9: Full Wing....................................................................................... ....................13

Figure 10: Stall...................................................................................................................13


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Introduction

The smoke tunnel is an extraordinary piece of equipment. Since the 1980s, the

smoke tunnel has assisted researchers and engineers in viewing streamlines and flow

around various objects. Now, smoke tunnels still use the same basic approach to observe

various flow phenomena. The smoke tunnel at Auburn University involves a basic wind

tunnel, a smoke generator, and an injection system. [1]

In this investigation, 4 objects will be placed in the smoke tunnel: A cylinder, an

air foil, a wingtip, and a scale wing. These objects were used to compare several different

flow characteristics: Wing wash, separation, streamlines, buffeting, and wake.


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Discussion of Relevant Theory

A streamline is a path traced out by a massless particle moving with the flow. [2]

Streamlines will be regular and parallel with one another all points in an undisturbed flow.

Laminar flow is flow where a given streamline maintains a smooth, regular path

across a given distance. This flow has a lower viscosity and more shallow velocity profile

than a turbulent flow. The laminar flow is more susceptible to separation due to an adverse

pressure gradient.

Turbulent flow is flow where fluid motion consists of eddies. These flows have a

higher skin friction than the laminar counterpart. A streamline in a turbulent flow will

diffuse and follow the individual eddies it comes in contact with.

The static pressure gradient means the derivative of pressure with respect to

distance. The streamwise static pressure gradient means the change in pressure along a

given streamline. If the pressure increases along a given streamline, then it is

encountering an adverse pressure gradient, and if the pressure decreases, then it is called

a favorable pressure gradient. [3]

Vortices are caused by the adverse pressure gradient at the edge of a given wing.

The high pressure beneath the wing moves to the low pressure above the wing.


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Description of Test Equipment and Procedure

Four models were tested and photographed in a smoke tunnel. The streamlines are

revealed by heating the oil and the smoke was pulled into the viewing window by the lower

pressure, moving air. The models were tested by mounting them in the tunnel and turning

the machine on. Results are taken by making observations about the pathway of the

streamlines as they flowed around the models.


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Results and Discussion

Initially, the infinite cylinder was placed in the smoke tunnel. The flow began to

transition to turbulent flow .25 past the 90 line as seen in Figure 1. Beyond the separation

point, an oscillating wake appeared between 4 and 8 inches behind the cylinder. As Vwas

increased, the frequency of the wake increased proportionally to V while the magnitude

of the wake decreased. (Note: the viewing window of the wind tunnel was limited and

further investigation is needed to verify if the magnitude increases further downstream.)

Additionally, the point at which the oscillating wake began to form moved further back

proportionally to the V, up to an additional 12 inches behind the cylinder itself at the

maximum V.

The cylinder was then rotated 180 in order to expose the sandpaper as seen in

figure 2. The sand paper acted as a turbulator and thus caused the boundary layer to become

turbulent. This induction of turbulence naturally resulted in the transition point being

moved forward to .25 ahead of the 90 line, indicating a difference of 15as observed in

Figure 2. Oscillations observed behind the cylinder were slightly closer to the cylinder

than the oscillations of the laminar cylinder counter-part, appearing about 3-7 inches

behind the cylinder.

The infinite airfoilat = 0 had only a very small amount of turbulence as observed

in figure 3. The streamlines ahead of the leading edge are almost identical to those of the

trailing edge. The turbulent flow can be observed from 1.5 ahead of the trailing edge to

up to 5 behind the trailing edge.

The airfoil was then rotated up 15 as shown in Figure 4. At this , one is able to

observe the effects of viscous flow around the airfoil. Beneath the leading edge, the


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streamlines begin sloping upward toward the leading edge before following the surface of

the underside to the trailing edge in a laminar fashion. On the upper surface, the streamlines

encounter an adverse pressure gradient and began to transition into a state of turbulent flow

2from the leading edge. Furthermore, flow separation was observed 2 from the trailing

edge. The area of turbulent flow between the upper surface of the airfoil and laminar flow

above is known as the bubble during the bubble point as shown in Figure 10.

At = 30, the airfoil was in full stall. As seen in figure 5, large amounts of

turbulent flow and flow separation was observed up to within an inch of the leading edge.

This is because the adverse pressure gradient is even more severe than the = 15 airfoil.

The wing tip was then placed into the smoke tunnel at = 0. As the wing tip had

a mean camber line which was different than the cord of the profile, the wingtip possessed

some amount of lift. This lift resulted in a vortex being formed, beginning 2 ahead of the

trailing edge as seen in figure 6. This vortex was thin, possessing a radius of only .5at

most.

At = 15, the vortex became more prominent with a radius of 1.25 and starting

only .5behind the 90 line as seen in figure 7. The more prominent vortex represents a

larger loss in lift as more air slips from beneath the wing.

When = 30, the vortexbegan to form only 1 behind the leading edge of the wing

tip. Furthermore, it grew until it had a diameter of 3.5 as observed in figure 8. This

represents a very large pressure difference between the upper and lower surfaces of the

wingtip. At 5 behindthe trailing edge, the vortex deteriorates into a standard turbulent

flow. The large amount of non-laminar flow would result in buffeting at the wingtip.


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Finally, the wing was placed in the smoke tunnel. There, the effects of the previous

2 models could be observed simultaneously. Furthermore, the turbulent flow appearing

about halfway down the length of the chord is known as downwash and does not provide

the most noticeable lift. At the wingtip, vortices can be observed coming off of the edge of

each side. Another observation one could make is the fact that the vortices seem to migrate

away from the centerline of the wing. The reason one can observe the downwash on this

wing, which was not observed on the infinite airfoil, is because the wing has camber to it,

which leads to a pressure difference between the top and bottom. [4] The high pressure air

beneath the wing was drawn up and around the trailing edge into the fast moving, low

pressure air. The downwash was naturally erratic in nature and would result in buffeting.


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Conclusions

Various aerodynamic phenomena can be observed via a smoke tunnel. As the air

flows around various objects, the smoke reveals the trajectory a given particle in the flow

will take. The streamlines also are able to reveal various flow phenomena such as wake,

The cylinder showed that a symmetric object in a flow would cause oscillations

downstream. The addition of sandpaper to the cylinder caused the separation and stagnation

points to change and move upstream. A higher free stream velocity would cause the

oscillations behind the cylinder to move more rapidly.

As the angle of attack was increased, the transition point of the flow was moved

nearer to the leading edge. The stall angle of the airfoil in the wind tunnel was 15 because

there was no flow attachment on the upper surface of the airfoil. More prominent vortices

were formed when a larger angle of attack was instigated on the wingtip, which reduced

lift.

As the angle of attack of the airfoil and the wingtip was increased, the transition

point was moved up along the chord of the airfoil. Vortices were also observed forming

along the trailing edge of the wingtip. Vortices along a wingtip indicate a loss of lift.

The scale wing was used to observe the effects of both the airfoil and the wingtip

simultaneously. Wing wash, turbulence, and wake could be observed behind the trailing

edge of the wing.


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References

Merzkirch, W. (1987).Flow visualization (2nd ed.). Orlando: Academic Press.

Definition of Streamlines. (n.d.).NASA. Retrieved September 5, 2013, from

http://www.grc.nasa.gov/WWW/k-12/airplane/stream.html

Ahmed, A. (2013).Aerodynamics Laboratory(2013 ed.). Auburn: Auburn University.

AL0966B LESSON 2. (n.d.). GlobalSecurity.org - Reliable Security Information.

Retrieved September 4, 2013, from

http://www.globalsecurity.org/military/library/policy/army/accp/al0966/le2.htm

Wake turbulence - Wikipedia, the free encyclopedia. (n.d.). Wake turbulence. Retrieved

September 4, 2013, from http://en.wikipedia.org/wiki/Wake_turbulence


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Figures and Tables

Figure 1: Infinite Laminar Cylinder

Figure 2: Turbulent Cylinder


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Figure 3: Infinite wing, = 0



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Figure 6: Wingtip, = 0


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Figure 9: Full Wing

Figure 10: Stall [4]

a visual investigation of flow around objects

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