flow visualization

9
4/5/2011 1 ه آ ودﻨﺎ ﯿﮏ آزﻣﺎ ه آ ودﻨﺎ ﯿﮏ آزﻣﺎ) 2 ( ﯾﺎن آ ﮑﺎرﺳﺎزی ﯾﺎن آ ﮑﺎرﺳﺎزی١ We consider two broad categories of flow visualization. The first is surface flow visualization and the second is flow field visualization. Strictly speaking, surface flow is also a flow field, but we will use these terms to distinguish between on- body and off-body fields. ٢ FLOW VISUALIZATION Categories of flow visualization The methods considered for direct-surface flow visualization include tufts, oil flow, ink dot, china clay, and liquid crystals. The methods considered for off-body visualization include smoke injected in several ways, helium bubbles, and streamers. ٣ Methods of Recording Direct Visualizations There are basically four methods of recording direct visualizations. The first, historically most important but least permanent, method is for the engineer to observe with his or her eyes. Because of depth perception, one can see three- dimensional patterns and one always sees an evolution of the phenomena ٤ Other common methods of recording the results of flow visualizations are by film, either still or movie, by video recording using a standard VCR format, and by digital recording of digitized video, either still or movie. ٥ These methods produce a two-dimensional projection of a three-dimensional phenomenon. In principle, two or more cameras can be used to obtain multiple two-dimensional projections that contain sufficient information to reconstruct the three-dimensional image. ٦

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Page 1: Flow Visualization

4/5/2011

1

یک نا ود ه آ یک آزما نا ود ه آ ))22((آزما

یان کارسازی یانآ کارسازی آ

١

• We consider two broad categories of flowvisualization.

• The first is surface flow visualization and thesecond is flow field visualization. Strictlyspeaking, surface flow is also a flow field, but wewill use these terms to distinguish between on-body and off-body fields.

٢

FLOW VISUALIZATION

Categories of flow visualization

• The methods considered for direct-surface flowvisualization include tufts, oil flow, ink dot,china clay, and liquid crystals.

• The methods considered for off-bodyvisualization include smoke injected in severalways, helium bubbles, and streamers.

٣

Methods of Recording Direct Visualizations

• There are basically four methods of recordingdirect visualizations. The first, historically mostimportant but least permanent, method is forthe engineer to observe with his or her eyes.

• Because of depth perception, one can see three-dimensional patterns and one always sees anevolution of the phenomena

٤

• Other common methods of recording the resultsof flow visualizations are

• by film, either still or movie,• by video recording using a standard VCR format,

and• by digital recording of digitized video, either still

or movie.

٥

• These methods produce a two-dimensionalprojection of a three-dimensional phenomenon.

• In principle, two or more cameras can be used toobtain multiple two-dimensional projectionsthat contain sufficient information toreconstruct the three-dimensional image.

٦

Page 2: Flow Visualization

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SURFACE FLOW VISUALIZATION

• Information about the flow on the surface of anobject being studied is usually most critical.Many times, the flow off the body is of interestprimarily in order to understand the flowfeatures on the surface.

٧

• Key aspects of surface flows that may beinvestigated using visualization techniquesinclude stagnation point location, separationlines, location of boundary layer transition,characteristic unsteadiness, extent of separationzones, and types of critical points and theirlocations.

٨

Tufts• The simplest and most frequently used method

for surface flow visualization is to attach tufts tothe surface of interest.

• The tufts must be of light, flexible material thatwill align itself with the local surface flow as aresult of direct aerodynamic force.

٩

• The most commonly used material is light yarnwith weights and lengths chosen according tomodel size and test speeds. Very smallmonofilament has also been used. There are alsopolyester and cotton sewing threads.

١٠

• The thread is a multiple-strand material andtends to unravel with time.

• Tufts do affect the aerodynamic forces to someextent as we will show, but there are manysituations in which the method is so easy andeconomical that it is the first choice.

١١

Attaching Tufts

• Two basic methods of attaching tufts to a surfaceare by scotch tape or by glue. When tape is used,the tufts are usually made on a "tuft board." Thetuft material is strung back and forth aroundpins, then the tape is applied to the tufts and thetuft material is cut at the edge of the tape.

• The model surface is cleaned with naphtha orother solvents to remove oil so that the tape willhold under the adverse conditions of high-speedflow.

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Page 3: Flow Visualization

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“Tuft Board"

١٣

• Tufts readily show where flow is steady andwhere it is unsteady. Regions of completeseparation and buffeting flow are readilyidentified. The resolution of the determination isof the order of the spacing of the tufts.

• The possibility of significant influence of thetufts themselves on the flow is very high andmust always be kept in mind. This can beinvestigated by removing tufts upstream ofindicated flow separation.

١٤

١٥

Tufts Glued to Wing

١٦

١٧

Flow Visualization Using Yarn Tufts

• To make the flow pattern downstream of the test article visible, a tufted wire grid is useful, as shown in Figure 5.17.

١٨

Page 4: Flow Visualization

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Flow Field Description Using a Tuft Grid

١٩

Minitufts

• The glue technique is used for minitufts. Thesetufts have the least effect on the aerodynamicdata and thus are often left on the model. Thetuft material is monofilament nylon that hasbeen treated with a fluorescent dye.

٢٠

Near surface flow visualization using fluorescent minitufts applied on a rotating surface.

٢١

Fluorescent Minitufts

٢٢

• The minitufts are viewed and photographed inultraviolet light as this is the way to make thesmall monofilament most visible.

• Threads can be similarly treated and can bephotographed in either ultraviolet or white light.Minitufts provide the same type of informationas larger tufts. They can provide greaterresolution and have less influence on the flow.

٢٣

Stream lines in the Flow Field Shown by a Single Tuft Wand

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Page 5: Flow Visualization

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Oil Flow Visualization• Oils and other viscous fluids are used to show

the surface flow. The selected material is usuallyspread on the areas of interest with a paintbrush. It will then flow under the influence ofshear stress from the air stream and gravity.Since inclined surfaces are almost always ofinterest, the mixture needs to have viscositysufficient so that it will not flow rapidly underthe influence of gravity.

٢٥

• The flow speed of the air must then be sufficientto impress shear stress large enough to cause theoil to flow and reveal the surface patterns withinan acceptable time.

• This is typically of the order of 10's of secondsafter the tunnel is brought to speed. It is difficultto use oil flow on vertical surfaces.

٢٦

Oil Flow

٢٧

Oil Material• The most common material for oil flow ispetroleum lubricating oils. These materials aremessy to clean up afterward, both on the modeland more so in the tunnel.

• The color of the oil needs to contrast with thecolor of the model surface. A widely usedmethod is to add a fluorescent dye to the oil andilluminate it with ultraviolet lights, as is done forthe fluorescent minitufts.

٢٨

• A light blue works very well with a dye thatprovides fluorescence in the yellow region of thespectrum. This is the most commonly usedcombination.

• Oil can be made white by mixing titaniumdioxide into it. This can be applied to a blackmodel and ordinary light used for viewing andphotography. This is sometimes preferable to theinstallation of black lights and the subsequentrequirements on light management.

٢٩

Oil Flow

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Page 6: Flow Visualization

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٣١

Surface Oil Flow with Titanium Dioxide Over Single Delta Wing.

٣٢

Surface Oil Flow with Titanium Dioxide Over Double Delta Wing.

٣٣

Laser Light Sheet Flow Visualization Over Single Delta Wing.

٣٤

Laser Light Sheet Flow Visualization Over Double Delta Wing.

Surface Flow Visualization Obtained by Applying a Continous Oil Film Over a Body Surface

٣٥

China Clay

• China clay is a suspension of kaolin in kerosene.The fluid is applied with a paint brush, usuallywith the model set at the desired attitude.

• The tunnel is started as quickly as possible afterthe model is painted. When the mixture hasdried, photographs can be taken after the tunnelis shut down because the pattern does notchange rapidly with time.

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Page 7: Flow Visualization

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China Clay

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Effects of Tufts, Minitufts, China Clay, and Oil• As previously mentioned tufts can affect the

aerodynamic loads on a model. In Figure bellowa lift curve near stall shows the effects of varioustufts on the data. The glued minitufts and No. 60thread tufts consisted of about 900 tufts. Thetwo taped tufts consisted of about 300 tufts. Thedata are an average of five runs for each set oftufts.

٣٨

٣٩

• The minitufts and the glued No. 60 thread havethe minimum effect on lift. The effect of the tapecan be seen by comparing the two sets of No. 60thread tufts.

• The six-strand floss tufts are similar to the tuftsmade out of yarn.

• The three different tuft types were shown at a =27.3° in previous figures.

٤٠

Boundary Layers and Surface Shear Stress• Often the most important information being

sought by flow visualization methods is adefinition of the locations of transition fromlaminar to turbulent boundary layers and thelocations of any separation regions. In theprevious section, we have shown several resultsthat illustrate separated flows.

٤١

• The location of transition between laminar andturbulent flow cannot be determined by tuftsand is difficult with china clay.

• However, oil flow, sublimation methods,infrared images, and liquid crystals can be usedto locate transition.

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Page 8: Flow Visualization

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Oil Flow Detection of Transition• The basis for detecting boundary layer

transition by viewing oil flow patterns is theincrease in wall shear stress when a boundarylayer transitions from laminar to turbulent. Theresult is that the oil is swept away faster in theregion where the boundary layer is turbulent.Transition indicated by oil flow visualization isshown in the following figure.

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٤٤

٤٥

Sublimation• In one sublimation technique a mixture of

naphthalene and a carrier such as fluorine,acetone, or methyl ethyl ketone is sprayed on themodel using a standard air spray gun.

• Note that the last two can remove many paints.The operator must wear a respirator mask whendoing this.

٤٦

• The mixture will leave the model surface whiteand therefore works best on a black or darksurface.

• The turbulent boundary layer will scrub themixture off.

٤٧

Liquid Crystals• Liquid crystals that undergo changes in

reflective properties as they are exposed to shearstress can be used as detectors of transition.

• This method has recently been developed as aquantitative method as well as a method ofvisualization.

٤٨

Page 9: Flow Visualization

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Infrared Thermography

• The basis for this technique is that a surface at atemperature different from the tunnel streamwill have faster heat transfer from the regionunder a turbulent boundary layer than from theregion under a laminar layer. The resulting smalltemperature differences can be observed usingcommercially available infrared cameras.

٤٩

• The results will vary with model structure andheat transfer characteristics so it is advisable tocheck results for a particular model type byobserving artificially tripped layers.

• It should be kept in mind that it is the relativetemperature, not the specific value oftemperature, that gives the indication oftransition.

٥٠

٥١

Comparison of velocity change in different wind tunnels.

٥٢