A CodePen by Adam. Fluid Simulation - Javascript fluid simulation on canvas.

index.html

<canvas id= "c"> </canvas>
<button id="reset">new colours</button>
<div id="info">
  <div id="top">
  <a target="_blank" id="site" href="http://lonely-pixel.com">my website</a>
  <a id="close" href="">close</a>
  </div>
  <p>
    <br>
      - Alter particles/groups with the GROUPS variable.<br><br>
    - Customize colours with GROUP_COLOURS.<br><br>
    - Use the mouse to move the fluid.<br><br>
    - Click anywhere to make water.<br><br>
    - Use one group for a more water-like effect.<br><br>
  </p>
</div>

script.js

/*
Copyright (c) 2013 Stuffit at codepen.io (http://codepen.io/stuffit)

View this and others at http://lonely-pixel.com

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
*/

document.getElementById('close').onmousedown = function(e) {
  e.preventDefault();
  document.getElementById('info').style.display = 'none';
  return false;
};

/* Settings */

var MOUSE_INFLUENCE = 5,
    GRAVITY_X     = 0,
    GRAVITY_Y     = 0,
    MOUSE_REPEL   = false,
    GROUPS        = [50,50,50],
    GROUP_COLOURS = ['rgba(97,160,232'];

window.requestAnimFrame =
window.requestAnimationFrame       || 
window.webkitRequestAnimationFrame || 
window.mozRequestAnimationFrame    || 
window.oRequestAnimationFrame      || 
window.msRequestAnimationFrame     ||
function( callback ){
    window.setTimeout(callback, 1000 / 60);
};

var fluid = function() {
    
    var ctx, width, height, num_x, num_y, particles, grid, meta_ctx, threshold = 220, play = false, spacing = 45, radius = 30, limit = radius * 0.66, textures, num_particles;

    var mouse = {
        down: false,
        x: 0,
        y: 0
    };

    var process_image = function() {
        var imageData = meta_ctx.getImageData(0, 0, width, height),
            pix = imageData.data;

        for (var i = 0, n = pix.length; i < n; i += 4) {
            (pix[i + 3] < threshold) && (pix[i + 3] /= 6);
        }

        ctx.putImageData(imageData, 0, 0);
    };

    var run = function () {

        //var time = new Date().getTime();
        meta_ctx.clearRect(0, 0, width, height);

        for (var i = 0, l = num_x * num_y; i < l; i++) grid[i].length = 0;
        

        var i = num_particles;
        while(i--) particles[i].first_process();
        i = num_particles;
        while(i--) particles[i].second_process();

        process_image();

        if(mouse.down) {

            ctx.canvas.style.cursor = 'none';

            ctx.fillStyle = 'rgba(97, 160, 232, 0.05)';
            ctx.beginPath();
            ctx.arc(
                mouse.x,
                mouse.y,
                radius * MOUSE_INFLUENCE,
                0,
                Math.PI * 2
                );
            ctx.closePath();
            ctx.fill();

            ctx.fillStyle = 'rgba(97, 160, 232, 0.05)';
            ctx.beginPath();
            ctx.arc(
                mouse.x,
                mouse.y,
                (radius * MOUSE_INFLUENCE)/3,
                0,
                Math.PI * 2
                );
            ctx.closePath();
            ctx.fill();
        } else ctx.canvas.style.cursor = 'default';

        //console.log(new Date().getTime() - time);

        if(play)
        requestAnimFrame(run);
    };
    
    var Particle = function (type, x, y) {
        this.type = type;
        this.x = x;
        this.y = y;
        this.px = x;
        this.py = y;
        this.vx = 0;
        this.vy = 0;
    };
    
    Particle.prototype.first_process = function () {
        
        var g = grid[Math.round(this.y / spacing) * num_x + Math.round(this.x / spacing)];

        if (g) g.close[g.length++] = this;

        this.vx = this.x - this.px;
        this.vy = this.y - this.py;

        if (mouse.down) {
            var dist_x = this.x - mouse.x;
            var dist_y = this.y - mouse.y;
            var dist = Math.sqrt(dist_x * dist_x + dist_y * dist_y);
            if (dist < radius * MOUSE_INFLUENCE) {
                var cos = dist_x / dist;
                var sin = dist_y / dist;
                this.vx += (MOUSE_REPEL) ? cos : -cos;
                this.vy += (MOUSE_REPEL) ? sin : -sin;
            }
        }

        this.vx += GRAVITY_X;
        this.vy += GRAVITY_Y;
        this.px = this.x;
        this.py = this.y;
        this.x += this.vx;
        this.y += this.vy;
    };
        
    Particle.prototype.second_process = function () {

        var force = 0,
            force_b = 0,
            cell_x = Math.round(this.x / spacing),
            cell_y = Math.round(this.y / spacing),
            close = [];

        for (var x_off = -1; x_off < 2; x_off++) {
            for (var y_off = -1; y_off < 2; y_off++) {
                var cell = grid[(cell_y + y_off) * num_x + (cell_x + x_off)];
                if (cell && cell.length) {
                    for (var a = 0, l = cell.length; a < l; a++) {
                        var particle = cell.close[a];
                        if (particle != this) {
                            var dfx = particle.x - this.x;
                            var dfy = particle.y - this.y;
                            var distance = Math.sqrt(dfx * dfx + dfy * dfy);
                            if (distance < spacing) {
                                var m = 1 - (distance / spacing);
                                force += Math.pow(m, 2);
                                force_b += Math.pow(m, 3) / 2;
                                particle.m = m;
                                particle.dfx = (dfx / distance) * m;
                                particle.dfy = (dfy / distance) * m;
                                close.push(particle);
                            }
                        }
                    }
                }
            }
        }

        force = (force - 3) * 0.5;

        for (var i = 0, l = close.length; i < l; i++) {

            var neighbor = close[i];

            var press = force + force_b * neighbor.m;
            if (this.type != neighbor.type) press *= 0.35;

            var dx = neighbor.dfx * press * 0.5;
            var dy = neighbor.dfy * press * 0.5;

            neighbor.x += dx;
            neighbor.y += dy;
            this.x -= dx;
            this.y -= dy;
        }

        if (this.x < limit) this.x = limit;
        else if (this.x > width - limit) this.x = width - limit;

        if (this.y < limit) this.y = limit;
        else if (this.y > height - limit) this.y = height - limit;

        this.draw();
    };
            
    Particle.prototype.draw = function () {

        var size = radius * 2;

        meta_ctx.drawImage(
        textures[this.type],
        this.x - radius,
        this.y - radius,
        size,
        size);
    };
        
    return {
    
        init: function(canvas, w, h) {

            particles = [];
            grid      = [];
            close = [];
            textures  = [];
        
            var canvas 	  = document.getElementById(canvas);
			ctx   	      = canvas.getContext('2d');
			canvas.height = h || window.innerHeight;
			canvas.width  = w || window.innerWidth;
			width         = canvas.width;
			height        = canvas.height;

            var meta_canvas    = document.createElement("canvas");
            meta_canvas.width  = width;
            meta_canvas.height = height;
            meta_ctx           = meta_canvas.getContext("2d");

            for(var i = 0; i < GROUPS.length; i++) {

                var colour;

                if(GROUP_COLOURS[i]) {
                    colour = GROUP_COLOURS[i];
                } else {

                    colour =
                    'hsla(' + Math.round(Math.random() * 360) + ', 80%, 60%';
                }

                textures[i] = document.createElement("canvas");
                textures[i].width  = radius * 2;
                textures[i].height = radius * 2;
                var nctx = textures[i].getContext("2d");

                var grad = nctx.createRadialGradient(
                    radius,
                    radius,
                    1,
                    radius,
                    radius,
                    radius
                    );

                grad.addColorStop(0, colour + ',1)');
                grad.addColorStop(1, colour + ',0)');
                nctx.fillStyle = grad;
                nctx.beginPath();
                nctx.arc(radius, radius, radius, 0, Math.PI * 2, true);
                nctx.closePath();
                nctx.fill();
            }
            
            canvas.onmousedown = function(e) {
				mouse.down = true;
				return false;
			};
            
			canvas.onmouseup = function(e) {
				mouse.down = false;
				return false;
			};

			canvas.onmousemove = function(e) {
    var rect = canvas.getBoundingClientRect();
  mouse.x = e.clientX - rect.left;
  mouse.y = e.clientY - rect.top;
				return false;
			};
            
            num_x = Math.round(width / spacing) + 1;
            num_y = Math.round(height / spacing) + 1;
            
            for (var i = 0; i < num_x * num_y; i++) {
                grid[i] = {
                    length: 0,
                    close: []
                }
            }
            
            for (var i = 0; i < GROUPS.length; i++ ) {
                for (var k = 0; k < GROUPS[i]; k++ ) {
                    particles.push(
                        new Particle(
                            i,
                            radius + Math.random() * (width - radius * 2),
                            radius + Math.random() * (height - radius * 2)
                            )
                        );
                }
            }

            num_particles = particles.length

            play = true;
            run();
        },

        stop: function() {
            play = false;
        }
    
    };
    
}();

fluid.init('c', 800, 376);

document.getElementById('reset').onmousedown = function() {
    fluid.stop();
    setTimeout(function(){fluid.init('c', 800, 366)}, 100);
}

style.css

*{
  margin: 0;
  padding: 0;
  overflow:hidden;
}

body {
 background:#333; 
}

#reset {
  position: absolute;
  left:50%;
  top:10px;
  margin-left:-400px;
  background: #444;
  border: 1px solid #555;
  color: #888;
  padding: 6px 10px;
  cursor: pointer;
  opacity: 0.8;
}

#c {
  width:800px;
  height:376px;
  margin:0 auto;
  display:block;
}

#info {
  position:absolute;
  left:-1px;
  top:-1px;
  width:auto;
  max-width:380px;
  height:auto;
  background:#f2f2f2;
  border-bottom-right-radius:10px;
}

#top {
  background:#fff;
  width:100%;
  height:auto;
  position:relative;
  border-bottom:1px solid #eee;
}

p {
  font-family:Arial, sans-serif;
  color:#666;
  text-align:justify;
  font-size: 16px;
  margin:10px;
}

a {
  font-family:sans-serif;
  color:#444;
  text-decoration:none;
  font-size: 20px;
}

#site {
  float:left;
  margin: 10px;
  color: #38a;
}

#close {
  float:right;
  margin: 10px;
}