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  • PIV. particle image velocimetry using fiji app

    Discussion in 'Suggest a forum topic' started by MSHOfficial, Jan 6, 2019.

    1. MSHOfficial

      MSHOfficial Well-Known Member EngineeringClicks Expert

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      So if we have a still image of a jet spray, we can use an app called fiji to be able to find some flow characteristics. For example velocity, average diameter of droplets, etc.

      What are Important steps in droplet detection in any PIV software?

      · Segmentation – ability to distinguish an object from its background.

      Thresholding – we choose the range and separate into black and white. Converts image to binary

      Can be Automatic or manual. The computer chooses the objects that fit your definition of pixel size and labels it as a droplet.

      · Water shed separation – ultimate eroded points (equidistant points from each edge) are dilated until they touch another black pixel. This is point where a watershed line is drawn. Its done so that droplets out of focus do not get into the counts.

      · Analyze particles – recognizing droplets and separating unwanted background pixels.

      Important parameters in droplet detection?

      · Threshold setting – this is the intensity of a pixel that the computer recognizes as a part of the droplet.

      · Pixel area – is a range that that allows computer to understand if the lump of neighboring pixels are a droplet or not.

      · Circularity – is important because, not all areas of droplets are circular. Some are oval, elliptic etc.

      What is SMD (sauter mean diameter)?

      It is an average of particle size. It is defined as the diameter of a sphere that has same volume/surface area ratio as a particle of interest.

      · If surface diameter and volume diameter are known then SD =dv^3 / ds^2

      · If actual areas and volumes are known, then its Vp/Ap = (4/3 pi (dv/2 )^3)/(4pi (ds/2)^2)=d_32 / 6 then we should take the mean of many particles

      What is weber number and when does it increase?

      · It is a Non dimensional number

      · It appears When there is Interface between 2 different fluids, ex. Gasoline sprayed in air.

      · We = rho*v^2*d / surface tension

      · With increasing Weber number, the overall diameter of the droplet (at the atomization point) increases, while the inner diameter of the torus decreases.

      · Extremely High weber numbers end up in atomization.

      What is the difference between primary and secondary break up?

      [​IMG]

      What droplet break up regimes we have and how do they depend on weber number?

      [​IMG]
       
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