Landau-Squire Flow

I made a simulation of a Landau-Squire jet with javascript. Check it out! You can click on this gif to take you to the simulation page. It looks way better there.

This is the Navier-Stokes solution to a tiny submerged point jet. Imagine a thin needle injecting fluid into the middle of bath full of little tracer particles. Code below.

  var c = document.getElementById("myCanvas");
  c.addEventListener(‘click’, onClick, false)
  var ctx = c.getContext("2d");

  var ball_list = [];

  var is_paused = false;

  window.onblur = function() {
    if (!is_paused) {
      pause();
    }
  };

  function onClick() {
    if (is_paused) {
      unpause();
    } else {
      pause();
    }
  }

  function pause() {
    is_paused = true;
    ctx.globalAlpha=0.2;
    ctx.fillRect(360,140,100,300)
    ctx.fillRect(540,140,100,300)

    ctx.font = "30px Arial";
    ctx.fillText("Click to unpause",390,500);
    ctx.globalAlpha=1;
  }

  function unpause() {
    is_paused = false;
  }

  function Ball(x, y) {
    return {
      x: x,
      y: y,
      draw: function() {
        ctx.fillRect(this.x,this.y,2,2)
      },
      update: function() {
        var xp = (this.x – c.width/2 )/c.width;
        var yp = (this.y – c.height/2)/c.height;
        this.x += (1 * xp * xp – yp * yp) / (Math.pow(xp * xp + yp * yp, 1.5))
        this.y += (1.5 * xp * yp) / (Math.pow(xp * xp + yp * yp, 1.5))
      }
    }
  }

  function clearCanvas() {
    ctx.clearRect(0, 0, c.width, c.height);
    ball_list.forEach(function(ball, i) {
      if ((ball.x < 0) || (ball.x > c.width) || (ball.y < 0) || (ball.y > c.height)) {
        ball_list.splice(i, 1);
      }
      ball.draw();
    })
  }

  function update() {
    ball_list.forEach(function(ball) {
      ball.update();
    })
  }

  function draw() {
    clearCanvas();
    ball_list.forEach(function(ball) {
      ball.draw();
    })
  }

  function loop() {
    if (!is_paused) {
      update();
      draw();
    }
  }

  function addTracers() {
    if (!is_paused) {
      var n_tracer = 10;
      var step = c.height / n_tracer;

      var rand = 9;

      for (var x = step/2; x < c.width; x += step) {
        for (var y = step/2; y < c.height; y+= step) {
          ball_list.push(Ball(x + rand * (Math.random() – 0.5), y + rand * (Math.random() – 0.5)));
        }
      }
      console.log(ball_list.length)
    }
  }

  setInterval(loop, 1000/60);
  setInterval(addTracers, 1000/1);

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Landau-Squire Flow