Vortex Presentation

8/2/2019 Vortex Presentation

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EXPERIMENTAL STUDY ONVORTEX FLOW

By

K DURGA DEVI

101D61003

Under the Guidance of

Dr. P. Lovaraju

Dept. of Mechanical Engineering,

LBRCE, Mylavaram


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NEEDOFVORTEX

Vortex dynamics play an important role in all the threetransport processes. Therefore, vortex manipulation is the keyfor the efficiency of any device that involves mixing.

A mixing process involving more than two streams or two

species should have an appropriate combination of large andsmall vortices for efficient mixing.

large vortex structures are efficient suction creators, but theyare highly unstable and easily get fragmented into smallvortices.

Small vortices are stable and could travel long distances in aflow field, but because of their small size, they are poorsuction creators.


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It is understood that an appropriate proportion of largeand small vortices are essential for efficient mixing,identifying such a proportion and generating the desiredsizes of large and small structures is much more

important.

The vortex size and its management are essential forapplications involving mass entrainment and mixing.

An attempt is made to visualize and measure the size of

vortices is required.


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VORTEX

Is a spinning, often turbulent , flow of fluid. Anyspiral motion with closed streamlines is vortex flow.

The motion of the fluid swirling rapidly around acenter is called a vortex.


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TYPES

Free (irrotational) vortex

Forced (rotational) vortex

The speed and rate of rotation of the fluid in a free(irrotational) vortex are greatest at the center, anddecrease progressively with distance from thecenter.

The speed of a forced (rotational) vortex is zero atthe center and increases proportional to thedistance from the center


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A vortex can be seen in the spiraling motion of airor liquid around a center of rotation. The circularcurrent of water of conflicting tides often formvortex shapes.

Turbulent flow makes many vortices.A good example of a vortex is the atmospheric

phenomenon of a whirlwind or a tornado or dustdevil .

This whirling air mass mostly takes the form of ahelix, column, or spiral.


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In this paper presents an experimental investigation onsuppression of cross-flow vibrations of two circularcylinders in tandem arrangement studied.

To suppress the vibrations of the cylinders, tripping wireswere deployed.

Five spacing ratios were examined ,i.e., L/D =0.1,0.3,0.8,2.0and3.2.

Tripping wire position measured from th eleadingstagnation lines of the cylinders was changed from 200 to600.


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In Regime A ( = 200400), the boundary layers separated fromthe tripping wires reattached on the cylinder surface behind,and then separation from the cylinder surface occurred.

In Regime B ( = 450600), the boundary layers separated fromthe cylinder surface sufficiently upstream of the tripping wires ,and the presence of the tripping wires forced the separatedboundary layers to deflect outwards.

Resulting in enhanced vortex shedding and an enlarged time-averaged drag, fluctuating drag and lift on the cylinder.

Subsequently the shear layers separated from the upstreamcylinder reattach on the downstream cylinder, without rolling upto form Karman vortices, thus producing no vibration.


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Fig shows the response characteristics of the two cylinders withtripping wires attached for the five regimes.

Data without tripping wires (plain cylinders) are also included in thefigure.

For each L/D, results with varying from 200

to 600

are also given. Anumber of observations can be made

The cylinders do not vibrate for = 200 and 300 in Regimes IIV

Both cylinders vibrate in all regimes for = 400600, specially theupstream cylinder, whose vibration becomes divergent at = 400 for

all regimes. The down stream cylinder vibration also becomes divergent at = 450

and 600 in Regime I and 600 in Regime II.

The tripping wires are in effective for the range of examined tosuppress the vibration of the cylinders in Regime V.


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There is no vortex shedding from the plain cylinders or thecylinders with tripping wires attached at = 300

Fig. shows the flow structures around the two cylinders(L/ D =0.3) with tripping wires attached at = 300 and400 ,for one

complete cycle of the upstream cylinder vibration. The shearlayers separating from the upstream cylinder do not roll up toform vortices behind the cylinders at = 300but do at = 400.As a result, a strong vibration is generated on the two cylinders.[1]


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This paper presents studies of vortex sheddingmechanisms in a single bluff body and two dual bluffbody combination.

These studies are based on video imaging of vortexshedding in open channel flow.

The vortex mechanism for a typical half cycle ispresented in six stages.

The combinations were tested at a channel width D of200 mm, a channel depth of 47.5 mm and awater massflow rate of 0.612 kg/s.

This gave a mean channel velocity of 0.064 mm, achannel Reynolds number of 12 800 and a bodyReynolds number of 1920.


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The bluff body combinations tested are shown inFigure


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Fig. (af) shows six stages in the mechanism for Sbetween the reversal and the reversal in the flow atthe rear of the bluff body.


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Flow at rear of body reverses anticlockwise to clockwise .Flow of RH boundary layers moving into the RH vortex isabout to stop as it becomes detached. Anticlockwise vortexabout to move downwards to form the left-hand (LH) vortex.

Flow at rear clockwise. Clockwise vortex starting to form onright, i.e. C flow. LH boundary layers flow into LH vortex. RHlayers starting to move left, i.e. B flow starting.


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As for (b) with clockwise vortex continuing to develop, RHlayers moving further left as the flow develops.

As for (c) with a clockwise vortex developing further, also

RH layers moving further to left . LH layers still moving intothe LH vortex.

As for (d), LH layers still moving into LH vortex but RHlayers moving left are about to cut off this flow of off.


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Referring to Fig. 8(af, Dual combination A+B) the sixstages in a half cycle are as follows.


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Examination of cycles for single S body with very longperiods shows that the above detachment process may bedelayed.

The RH layers becomes temporarily attached to thecirculation and rotate with it so that they cannot move tothe left to cutoff the flow into the LH vortex. These layerssubsequently become detached from the circulation.

Examination of cycles for (A+ B) body A clockwise gap flowis established, this gap flow is often slow and uncertain,some of it is entrained by the circulation.

This increases the size and angular momentum of the

circulation so that it is now sufficiently strong to entraininner LH layers. This does not occur in the single bluff bodyS


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Examination of cycles for (A+ I) body, The gap flow isobserved to be generally more certain and has higher

velocity than for A+B, also it grows and decays morequickly.

This is a consequence of the gap geometry offering agreater restriction to flow. Also all of the gap flow isentrained in the clockwise circulation, the gap flow layersrotate with the circulation inside the RH layers

The circulation is grow more quickly so that LH layers aremore quickly entrained .

This entrainment is far more effective than in A+B, innerLH layers rotate around the circulation moving right to therear of the combination(2)


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Twin vortex behind the plate is formed at Reynoldsnumbers which are much higher than that for a circularcylinder.

This indicate the dominate role played by the reverse flowon the vortex formation, the vortex size behind the arcplate is found to be smaller than the a comparable flatplate at identical flow conditions.

It implies that the base smoothness's reduces thestrength of the reverse flow.

The vortex formation behind the flat and arc plates isdicated by the pressure hill around the forwardstagnation location and shape of the gradual geometrybehind the edge


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For the plate, the edges are sharp with out any gradualchange in curvature.

Due to the low pressure region formed at the rear. This low pressure level reaches a certain minimum at

which pressure at the base is lower thant thatdownstream of the vortices, the flow just behind the vortexbegins to towards the plate, establishing a reverse flow.

During this process, depending on the plate length andflow Reynolds number, the vortex formation behind theplate is due to up stream flow rolling at the edges due thereverse flow.

Thus there appears to be a limiting situation at which thevortex formation is dominated by the reverse flow


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It is found that, the low pressure caused by thevortices due to roll- in of the upstream flow inducesa reverse flow to wards the plate base.

The vortex size behind the arc is found to be

smaller than that behind a comparable flat plate atidentical flow conditions.

It implies that the, smooth base reduces thestrength of the reverse flow.[3]


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In addition to the design of bodies in streamlined shapes to reducethe wake width, many ideas for controlling boundary layer separationor the structure of eddies near the bodies have been developed.

Drag reduction of a hollow circular cylinder which has rows of holesalong its axis. The mechanism of drag reduction due to these smallholes is discussed here.


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The dye was sucked into a hollow cylinder at the front

holes, and ejected to the wake region through the rearholes for attack angles 20 60 .

On the other hand, for 80 the dye initially inside thehollow cylinder was sucked out into the wake region from

upper rows and lower rows of holes alternately. For an attack angle 0, the pressure difference

between the front and rear surfaces of the cylinder isreduced because of the presence of the holes.

For medium attack angles 0 60 , pressure differencebetween the front and rear surfaces of the cylinder is alsoreduced. Thus a total reduction of 40% of CD is achieved.


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For attack angle 80 the boundary layer separation point shiftseither forward or backward across the hole, which greatlyinfluences the departure and the roll-over of eddies. This will be

the cause of very large CD fluctuation. This result seems to suggest that the flow around the cylinder

with slits with large attack angles is similar to that of a semi-circular arc or semi-circular cylinder, in which the boundarylayer separations are fixed at the sharp edges of the arc.[4]


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Flow characteristics around the square cylinder and theirinfluence on the wake properties are studied.

Vortex shedding characteristics are measured by a single-wire hot-wire anemometer.

The subcritical, supercritical, and wedge flows accordingto the prominently different features of the topological flowpatterns.

In the subcritical regime (0 15 ), the boundary

layers separate from the leading edges of the lateral facesof the square cylinder and never reattach to the surfaces.


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In-between the separated boundary layers and the lateralfaces, small dual-ring bubbles exist near the leading

edges of the lateral faces. In the super critical regime (15 45 ), ), the separated boundary layer on the lateralsurface facing wind ward reattaches to the surface andhence forms are circulation bubble.

the separated boundary layer never reattaches to thesurface; instead, a shallow and wide recirculation bubblecovering the rear part of the surface is formed.

The change of the flow patterns on the lateral surfaces atthe critical incidence angle reduce the wake width to a

minimum, which contributes to a maximum Strouhalnumber and a minimum drag coefficient.(5)


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REFERENCES

Lei Shi, Zhibin Yu, Artur J. Jaworski, Vortex shedding flow patterns and their transitions inoscillatory flows past parallel-plate thermoacoustic stacks, Experimental Thermal and FluidScience 34 (2010) 954965.

J.P. Bentley , J.W. Mudd, Vortex shedding mechanisms in single and dual bluff bodies, FlowMeasurement and Instrumentation 14 (2003) 2331.

Y. Takama, K.Suzuki, E. Rathakrishnan, Visulazation And Size Measurement Of Vortex Shed By

Flat Arc Plates In An Uniform Flow, International review of aerospaceEngineering(I.RE.AS.E),Vol.1,N.1February 2008.

Yoshio Yajima, Osamu Sano, A note on the drag reduction of a circular cylinder

due to double rows of holes,

R.F. Huanga, , B.H.Lina, S.C.Yenb , Time-averaged topological flow patterns and their influenceon vortex shedding of a square cylinder in crossflow at incidence , Journal of Fluids and Structures26 (2010) 406429 .


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