for(int x=0; x < _size; x += 10){
    for(int y=0; y < _size; y += 10){
    double velX = Math.abs(_u[x + _size * y]);
    double velY = Math.abs(_v[x + _size * y]);
    g.drawLine(x, y, x+(int)(velX/20.0), y+(int)(velY/20.0));
    }
}