Ultrasound Image processing of blood flow

Hi there,

I am supposed to write a code that models a blood flow between two stationery tissue,

To generate the data before I image, I was thinking to generatre scatter points in a shape of rectangle (boubdaries 1 to 4 for x-axis and -xis) which scatter points betwen y=3 and y=4 are stationary, also scatter points between y=1 and y=2 are stationary but the scatter point between y=2 and y=3 are moving (representing the blood flow),

any Idea how could I generate this data?

Thank