new_contents: #!/usr/bin/env python
  """
  This script generates a force and velocity vector diagram for a cross-flow
  turbine.
  """
  
  import gizeh as gz
  import numpy as np
  import matplotlib.pyplot as plt
  
  
  def gen_naca_points(naca="0020", c=100, npoints=100):
      """Generate points for a NACA foil."""
      x = np.linspace(0, 1, npoints)*c
      t = float(naca[2:])/100.0
      y = 5.0*t*c*(0.2969*np.sqrt(x/c) - 0.1260*(x/c) - 0.3516*(x/c)**2 \
              + 0.2843*(x/c)**3 - 0.1015*(x/c)**4)
      y = np.append(y, -y[::-1])
      x = np.append(x, x[::-1])
      points = [(x0, y0) for x0, y0 in zip(x, y)]
      return points
  
  
  def test_gen_naca_points():
      points = gen_naca_points()
      x = []
      y = []
      for p in points:
          x.append(p[0])
          y.append(p[1])
      fig, ax = plt.subplots()
      ax.plot(x, y, "o")
      ax.set_aspect(1)
      plt.show()
  
  
  def draw_foil(naca="0020", c=100):
      """Draw NACA 0020 foil."""
      points = gen_naca_points(naca, c)
      line = gz.polyline(points, close_path=False, stroke_width=2, xy=(300, 300))
      return line
  
  
  def main():
      canvas = gz.Surface(width=700, height=700)
      foil = draw_foil()
      foil.draw(canvas)
      canvas.write_to_png("cft-vectors.png")
  
  
  if __name__ == "__main__":
      main()
  
subject: Add script to draw NACA foil