發送短信 : Head-on collisions of vortex rings upon round cylinders

   _____   _    _    _    _     _____     _____   
  / ___// | || | || | |  | ||  |  ___||  |__  //  
  \___ \\ | || | || | |/\| ||  | ||__      / //   
  /    // | \\_/ || |  /\  ||  | ||__     / //__  
 /____//   \____//  |_// \_||  |_____||  /_____|| 
`-----`     `---`   `-`   `-`  `-----`   `-----`