246109893 Ae341 Lab1 Flow Visualization Using Hydrogen Bubble Chamber

8/18/2019 246109893 Ae341 Lab1 Flow Visualization Using Hydrogen Bubble Chamber

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Submitted to:

AEE-METU

AEE 341-AERODYNAMICS I LAB. REPORT #1

Submitted by: Ayşenur Bıçakçı 1881952 Ece Öztürk 1882513

Mohamed Abdulaziz 1681865Sait Can Güven 2110757

Şahin Murat Karacaoğlu 1882141

Özge Sinem ÖzçakmakÖzgür Harputlu

31 Oct. 2014


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CONTENTS LISTPage

Abstract 2Contents List 3Objective 4Introduction 4Theory 4Experimental Set-up 9

Experimental Procedure 9Results & Discussion 10Conclusion 22References 22Appendix 22


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OBJECTIVE

The lab work is aimed at improving our understanding of the flow patterns andtheir different phenomena as well as how the Reynolds number and angle of attackaffects the flow pattern. Thanks to the hydrogen bubble technique, some of theobservable properties of flows are visualized by using several shaped objects such ascylinders, rectangular prism and symmetric airfoil.

INTRODUCTION

In this experiment, we observed alterations in the flow behavior around differentsamples including cylinders with two different diameters, a rectangular prism and asymmetric airfoil. Moreover, the impact of changing the angle of attack was also viewed.Thus, in terms of aerodynamics, this experiment is essential in gaining an intuitive graspof how the flow acts around these common shapes.

In the following parts of this report, the theory of this experiment and results areverbalized and discussed. Then, the techniques and equipment used in the experiment arelisted under the equipment section. Finally, references and appendix sections are located.

THEORY

CROSS FLOW OVER NON-ROTATING CYLINDER

This external flow is normal to the axis of the non-rotating cylinder. It combines anumber of phenomena such as flow separation, turbulence transition, reattachment andturbulence separation of the boundary layer. If the flow is inviscid the velocitydistribution is given by:

Where Vr is the radial velocity, Vθ is the tangential velocity, R is the radius of thecylinder, r is the radial coordinate and the angle θ is measured from the forwardstagnation point.


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It can be seen that there are two stagnation points and that they are at the points (r,θ)= (R,0), (R, π). There is a streamline which connects the two points (there is no flownormal to the surface at r=R so the surface is by default a streamline) and therefore theflow past a non-rotating cylinder can be made up of some elementary potential flows;

namely a uniform flow and a doublet. The uniform flow outside the circular streamline(on the surface) does not interact with the doublet flow within.

Applying Bernoulli’s equation for an inviscid fluid we can find another importantcharacteristic of the flow. That is the pressure distribution is symmetric around the axis.This is intuitive since the flow velocity components and stream function are alsosymmetric around the axis.

This leads to another important result; since the pressure distribution of the

inviscid cross flow over a circular cylinder is symmetric there should not be any liftingforce or drag. This is one example of the d’Alembert paradox.

http://www.thermopedia.com/content/5637/TUBES_CROSSFLOW_OVER_FIG1.gif

However the viscous effect within fluids is not negligible and the relation between both the viscous and inertial forces were found to be an important indicator of flow pattern. This ratio of viscous to inertial forces is a dimensionless parameter known as theReynolds Number (Re).

Consider now the flow past the non-rotating cylinder with viscous effects. As thefluid moves of the cross flow over a non-rotating cylinder the fluid pressure increasesfrom the freestream value to the stagnation point value. Then the high pressure causes the

fluid to separate and travel along the surface of the cylinder. However, in this case, a boundary layer starts to build up and the pressure force is counteracted with the shearforces and this prevents the fluid from travelling across the surface of the cylinder andleaving from the rear stagnation point. This forms two shear layers those have a velocitydistribution that is a function of the displacement from the surface. This difference invelocity means that the innermost part moves at a slower rate than the uppermost part and

http://www.thermopedia.com/content/5637/TUBES_CROSSFLOW_OVER_FIG1.gifhttp://www.thermopedia.com/content/5637/TUBES_CROSSFLOW_OVER_FIG1.gifhttp://www.thermopedia.com/content/5637/TUBES_CROSSFLOW_OVER_FIG1.gif


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this causes the shear layers to roll up. Vortices appear in the wake. How exactly thevortex street develops depends on the Reynolds number.

FLOW OVER A SYMMETRIC AIRFOIL

In the thin airfoil theory, the inviscid flow over a symmetric airfoil has a velocityand pressure distributions that are also symmetric. There is no net lift or drag at a 0°angle of attack. With increasing angle of attack the coefficient of lift increases linearlywith increasing angle of attack. This theory has its advantages such as allowing us todetermine the center of pressure and the aerodynamic center but it does not account forstall. This is an important criterion for the design of airfoils and planes as it can haveadverse effects on balance and control.

With positive angles of attack the stagnation points which were at leading andtrailing edge start to move. The flow meets the airfoil on the underside of the airfoil and

separates into two flows. One flow path must flow to the leading edge and around thetopside and the other flows along the underside. Due to the Kutta condition they shouldmeet at the trailing edge and form one flow (velocities must be equal and non-zero). Inshort, the flow over the top must be faster than the one at the bottom. As a result, vortexflow occurs at the trailing edge and the fast flows result in strong viscous forces that buildup near the edge. This vortex is known as the starting vortex. The fluid tends to movefrom the bottom to the trailing edge as opposed to going around to the leading edge andthis means that the vorticity of the flow behind the airfoil is positive for positive angles ofattack. However, due to Helmholtz’s theorem which states that if the circulation of a flowwas initially zero then it should still be the same regardless then there must be a vortexthat turns the airfoil. This is known as the bound vortex.

As seen the bound and shed vortices contribute the same but have opposite sense.Therefore, the total circulation is zero as the circulation of a uniform freestream velocityis zero. As the airfoil moves along the vortices at the T.E. form a vortex sheet.


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FLOW OVER A FLAT PLATE

When the viscous flow meets the flat plate the fluid sticks to the surface (no slipcondition). That is the velocity of the fluid on the surface equals the velocity of thesurface. This results in a boundary layer forming which is zero at the wall to the value ofthe freestream velocity some distance away. This displacement is known as the boundarylayer thickness and is a function of distance x from the leading edge of the flat plate.

As the flow travels along the plate the boundary layer starts to develop and this isseen as an increase in the boundary layer thickness. The extent to which it increases aswell as the existence of separation depends on the Reynolds number (which is a functionof x as well).

http://cdn.comsol.com/wordpress/2013/09/Flow-of-a-fluid-over-a-flat-plate.png

The transition range between laminar and turbulent flow occurs as 3x10 5


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both a laminar and turbulent state. Further on, in the turbulent state vortices form whichspread as the boundary layer gets larger.

At a zero angle of attack, the only friction that exists is the skin friction and this isthe only contribution to the drag. The pressure distribution does not provide any

contribution as it is symmetric and normal to the surface. With increasing angle of attack,the separated region becomes larger and moves further towards the leading edge. Also the pressure distribution starts to contribute to the drag due to the asymmetric build-up of pressure due to the appearance of steady spirals and later wake vortices and Karmanvortices. Until the friction force due to the skin has a small effect compared with the dragdue to pressure when the angle of attack reaches 90°. At this angle of attack the wake isfully turbulent behind the plate.

http://img.photobucket.com/albums/v145/johnfarley/CH10F7.jpg

http://img.photobucket.com/albums/v145/johnfarley/CH10F7.jpghttp://img.photobucket.com/albums/v145/johnfarley/CH10F7.jpghttp://img.photobucket.com/albums/v145/johnfarley/CH10F7.jpg


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EXPERIMENTAL SET-UP

From the lab manual

The names of the parts:

1. Feet 2. Electrode Holders 3.Platinum Wire4. Mounting Pillar 5. Pump Body 6.Baffle Plate7. Weir 9, 10. Rod 11.Adjustable Connectors13. Casing 14. Flash Gun 15.Flow Straightener16. Block 17. Support 18.Guide

EXPERIMENTAL PROCEDURE

The observation of the external flow behavior over submerged bodies, being the primary objective of the experiment, was realized using the Hydrogen Bubble FlowVisualization technique. Generally, when the fluid is a liquid, the flow is visualized using bubbles or dyes as they are a cheap yet powerful tool. In this case, hydrogen bubbles areused and are generated by electrolysis of the purified water conditioned with salt filled inthe flow table. The bubbles are generated at the cathode and carried with the flowsupplied by a water pump. The water is circulated during the operation and thecirculation process is sufficiently proper so as not to create unwanted vibration inducedturbulence in the flow. The bubble generation may set to be intermittent or continuous.The intermittent bubble generation is conducted by simply turning on and off theelectrical current through the electrodes. The flow is evened by means of a honeycomb


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flow staightener so as to increase the discernibility of the flow pattern by letting the bubbles swept away in an accurate manner. A build-in light and an acrylic light guide arealso contained within the system in order to accentuate the flow.

Different acrylic objects are placed in the flow by mounting them on the pin fixed

in the flow table. Initially, flow over a cylinder is studied. The change of the flow pattern,location of the separation point, wake region, the laminar to turbulence transition andKarman vortices are the phenomena to be observed. The same procedure is repeated fordifferent speeds and for a cylinder of a larger diameter, which is the characteristic lengthfor a cylinder. Thus, by changing speed and characteristic length, the changes in the flow pattern with respect to Reynolds number is studied. After the flow over the cylinder isstudied, to see the effect of the shape of the body on the flow, a flat plate is placed ontothe pin both parallel and perpendicular to the flow direction and again the separation andwake regions are observed at different flow speeds. Finally, a symmetrical airfoil is pinned into the flow to emphasis the difference between blunt and streamlined bodies.The effects of flow speed at different values of “angle of attack” (AOA) are aimed to be

studied whilst the formation of starting vortex and the stall condition for the airplanes arediscussed.

RESULTS & DISCUSSION

FLOW PAST CYLINDER

At Reynolds numbers below 1, separation does not occur. However thestreamlines shape is different from that in an inviscid fluid. From 5 ≤ Re ≤ 45, the flowseparates from the rear side of the tube and a symmetric pair of vortices is formed in the

near wake. The streamwise length of the vortices increases linearly with Reynoldsnumber as shown in the figure below.

|Streamwise length of vortices. From Taneda S. (1956) J Phys


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With increasing Reynolds number wake becomes unstable and vortex shedding begins. At first, one of the two vortices breaks away and then the second is shed becauseof the non-symmetric pressure in the wake. This oscillating shed vortices form a laminar periodic wake of staggered vortices of opposite sign. A phenomenon called the Karmanvortex street. (See picture below).

.

http://fmeabj.lecturer.eng.chula.ac.th/FMRL/public_html/Flow%20Visualization/Flow%20Images/Karman

%20Vortex%20Street/Karman%20Vortex%20Street%202.jpg

In the Reynolds number range 150 < Re < 300, unpredictable disturbances arefound in the wake. The flow is transitional and gradually becomes turbulent as theReynolds number is increased.

The Reynolds number range 300 < Re < 1.5·105 is called subcritical (the upperlimit is sometimes given as 2·105). The laminar boundary layer separates around 80degrees downstream of the front stagnation point and the vortex shedding is periodic andstrong.

With a further increase of Re, the flow enters the critical regime. The laminar boundary layer separates on the front side of the tube, forms a separation bubble andlater reattaches on the tube surface. Reattachment is followed by a turbulent boundarylayer and the separation point is moved to the rear side, to about 140 degrees downstreamthe front stagnation point. As an effect, the drag coefficient is decreased sharply.

http://fmeabj.lecturer.eng.chula.ac.th/FMRL/public_html/Flow%20Visualization/Flow%20Images/Karman%20Vortex%20Street/Karman%20Vortex%20Street%202.jpghttp://fmeabj.lecturer.eng.chula.ac.th/FMRL/public_html/Flow%20Visualization/Flow%20Images/Karman%20Vortex%20Street/Karman%20Vortex%20Street%202.jpghttp://fmeabj.lecturer.eng.chula.ac.th/FMRL/public_html/Flow%20Visualization/Flow%20Images/Karman%20Vortex%20Street/Karman%20Vortex%20Street%202.jpghttp://fmeabj.lecturer.eng.chula.ac.th/FMRL/public_html/Flow%20Visualization/Flow%20Images/Karman%20Vortex%20Street/Karman%20Vortex%20Street%202.jpghttp://fmeabj.lecturer.eng.chula.ac.th/FMRL/public_html/Flow%20Visualization/Flow%20Images/Karman%20Vortex%20Street/Karman%20Vortex%20Street%202.jpg


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Figure of pressure coefficiet vs forward stagnation pointhttp://www.thermopedia.com/content/5637/TUBES_CROSSFLOW_OVER_FIG2.gif

http://www.thermopedia.com/content/5637/TUBES_CROSSFLOW_OVER_FIG2.gifhttp://www.thermopedia.com/content/5637/TUBES_CROSSFLOW_OVER_FIG2.gifhttp://www.thermopedia.com/content/5637/TUBES_CROSSFLOW_OVER_FIG2.gif


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Reynolds number vs flow speed for 3mm and 25mm radius cylinder


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FLOW PAST SYMMETRIC AIRFOIL

In real flows there is a viscous effect and as the flow travels along the surface ofthe airfoil a boundary layer develops. This causes two shear flows that separate near therear of the airfoil. The flow pattern changes significantly with changing angles of attack.

As the angle of attack is increased the flow separates nearer the leading edge.Flow separation begins at small angles of attack while attached flow is dominant. Withincreasing angle of attack the separated regions become larger and after a critical angle ofattack the separated flow is so dominant that it causes a reduction in lift with increasingangle of attack as the separated region continues to increase.

http://upload.wikimedia.org/wikipedia/commons/thumb/8/8d/StallFormation.svg/350px-

StallFormation.svg.png

Flow past airfoil at 0° angle of attack

http://upload.wikimedia.org/wikipedia/commons/thumb/8/8d/StallFormation.svg/350px-StallFormation.svg.pnghttp://upload.wikimedia.org/wikipedia/commons/thumb/8/8d/StallFormation.svg/350px-StallFormation.svg.pnghttp://upload.wikimedia.org/wikipedia/commons/thumb/8/8d/StallFormation.svg/350px-StallFormation.svg.pnghttp://upload.wikimedia.org/wikipedia/commons/thumb/8/8d/StallFormation.svg/350px-StallFormation.svg.pnghttp://upload.wikimedia.org/wikipedia/commons/thumb/8/8d/StallFormation.svg/350px-StallFormation.svg.png


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Coefficient of lift at varying Reynolds number vs Coefficient of drag


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Aircraft polars at varying angles of attack

As you can see increasing the velocity increases the Reynolds number (at a positive angle of attack).This further increases the coefficient of lift value in an almostlinear fashion. Near 15° angle of attack there is a sudden drop in the coefficient of lift.This is called the stall angle of attack.


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In addition, the coefficient of drag is significantly reduced. This is due to themovement of separated region in the downstream direction caused by the increase inmomentum of the flow near the surface. The starting vortex is swept away and throughthe circulation theory we can see that this causes a bound vortex on the airifoil which

gives it a positive coefficient of lift.

It is important to note that the coefficient of moment is zero for all angles ofattack. This is because the center of pressure does not move and that all the forces act onone point (there is no moment arm).

Pressure Coefficient distributions vs x/c at different angles of attack


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FLOW PAST RECTANGULAR PRISM

The transition range between laminar and turbulent flow occurs as 3·105


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Flow past rectangular prism at 30° angle of attack



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Pressure coefficient distribution vs x/c at varying angles of attack


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CONCLUSION

To conclude, this experiment displays the power of the hydrogen bubbletechnique to visualize elementary flow patterns at varying Reynolds numbers and angles

of attack for a rectangular prism, cylinders of varying diameter and the symmetric airfoil.In addition, the flow patterns characteristics for the symmetric airfoil and rectangular prism at different angles of attack were observed. Departure from the ideal flow patternwas seen. This was evidenced by the observation of phenomena such as flow separationand various vortex types. It was observed that when the stream velocity was increased,the vortex gained more length. Separation of flow was seen on the upper side of thesymmetric airfoil for non-zero angles of attack. In the case of the airfoil and flat plate it isworthy of remark that the separated region increased in size with increasing angles ofattack. This had adverse effects on the coefficients of lift when the stall angle wasreached.

It must be noted that although it was possible to observe the variations of thecharacteristics of the wake for 35< Red < 4.5·103 it was not possible to reach the criticalReynolds number and therefore the transition of the laminar boundary layer to turbulentas well as the reestablishment of the turbulent vortex street is outside the scope of thisexperiment.

As a final comment, although other flow visualization techniques exist such ashelium-filled bubbles technique, colored oil applying technique, laser sheet, surface oil,schlieren, smoke and tufts etc. they can be prohibitively expensive and complex.However, the hydrogen bubble technique is cheap and sufficiently effective at analyzingelementary flows. Therefore with this scope in mind the hydrogen bubble technique is

suitable for the purpose of a qualitative analysis of simple flows.


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REFERENCES

1. http://www.consultexim.hu/katalogus/armfield/dshtml/fseries/f14.htm 2.

www.imaph.tu-bs.de/lehre/99/irro/conformi_e.html

3.

Anderson , D.J., Fundamentals of Aerodynamics, McGraw-Hill 4. AER 303 F Aerospace Laboratory I Aerodynamic Forces on an Airfoil5. Experiment 1 Lab Handout given in the course website6.

Puttkammer, P. P. (2013). Boundary layer over a flat plate. (Master's thesis,University of Twente).

7. Trinh, K. T. (2007). On the critical reynolds number for transition from laminarto turbulent flow. (Master's thesis, Massey University).

8.

Yemenici, O. (2014). An experimental study on the aerodynamics of a symmetrical airfoil with influence of reynolds number and attack angle.

9. (30, October, 2014). Retrieved fromhttp://www.consultexim.hu/katalogus/armfield/dshtml/fseries/f14.htm

10.

8.(30, october 2014). Retrieved fromwww.imaph.tubs.de/lehre/99/irro/conformi_e.html
http://www.consultexim.hu/katalogus/armfield/dshtml/fseries/f14.htmhttp://www.consultexim.hu/katalogus/armfield/dshtml/fseries/f14.htmhttp://www.imaph.tu-bs.de/lehre/99/irro/conformi_e.htmlhttp://www.imaph.tu-bs.de/lehre/99/irro/conformi_e.htmlhttp://www.consultexim.hu/katalogus/armfield/dshtml/fseries/f14.htmhttp://www.consultexim.hu/katalogus/armfield/dshtml/fseries/f14.htmhttp://www.imaph.tubs.de/lehre/99/irro/conformi_e.htmlhttp://www.imaph.tubs.de/lehre/99/irro/conformi_e.htmlhttp://www.imaph.tubs.de/lehre/99/irro/conformi_e.htmlhttp://www.consultexim.hu/katalogus/armfield/dshtml/fseries/f14.htmhttp://www.imaph.tu-bs.de/lehre/99/irro/conformi_e.htmlhttp://www.consultexim.hu/katalogus/armfield/dshtml/fseries/f14.htm

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246109893 Ae341 Lab1 Flow Visualization Using Hydrogen Bubble Chamber

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