Flow Visualisation using Shadowgraphy Technique
Shadowgraphy is one of the optical techniques for observing a flow in a transparent medium. The basic apparatus consists of a light source and a recording plane onto which the shadow of the varying density field is projected. The technique is based on the variation in refractive index of the transparent media caused by density variation in the flow field. The experiment is carried out in the laboratory and density gradient is created by generating temperature gradient or any other means (diffusion of perfume). Though, shadowgraphy is not a suitable method for quantitative measurement of the fluid density but, is a convenient method of obtaining a quick survey of a flow in which the density changes in the described way. The relative changes in intensity of light in the plane of observation i.e. the shades of grey in shadowgraph is related to refractive index field which can be correlated to the required density field. This experiment aims to understand the basic principles of shadowgraphy and carry out simple data processing to get an insight into the shadowgraphy technique.
Keywords: Shadowgraphy, MATLAB, Data analysis, Cameras, Interferometry, Lenses, Schlieren, Caustics
PACS: 42.35. ± p, 52.35.Tc, 42.15.Gy, 42.65. ± k, 42.30.Va
Introduction
Shadowgraphy - an optical measurement technique is a field measurement method (image forming method) based on variation of refractive index in the flow field. The density of a fluid varies with temperature, salinity, and pressure. And, the index of refraction changes with fluid density. If a screen is placed opposite the light source, these effects create shadows on the screen creating an image called a Shadowgraph. The image ca...
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...mage to get the intensity field
I6= rgb2gray(I4);
l= 0.3; % distance of source from screen
D= 0.1; % width of test section
I7 = I6-I5;
I8= I7./I5;
I9 = I8/(l*D);% right hand side of the poisson's equation
figure,imshow(I5),figure,imshow(I6);
figure,imshow(I7),figure,imshow(I9);
gd= [3;4;0;400;400;0;0;0;800;800]; % generating geometry description matrix by giving the coordinates of the vertices of the test volume
dl= decsg(gd); % generating geometry decomposition matrix
[p e t]= initmesh(dl); % generating initial PDE triangular mesh
u = poisolv(0,p,e,t,I9); % using poisolv to solve the Poisson's equation and
giving boundary conditions , [p e t] matrix and I9 as input
n= exp(u); %obtaining refractive index field
K= 0.23; % constant for air (in cm^3/gm)
rho = (n-ones(size(n)))/K; % obtaining the density field by using Gladstone-Dale equation
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