Bullet Pattern for Bullet Hell

Hi all, I'm designing a bullet hell game and already came up with some patterns. However, I'm stuck now trying to achieve a certain pattern. I'm trying to make the bullet stream accelerate into a spiral clockwise then slow down and spin counter clockwise. I manage to make it spiral clockwise but couldn't get it to change direction. Please help. Thanks.

 public IEnumerator Pattern006()
     {
         yield return new WaitForSeconds(nextShot);
 
         GameObject enemyShot = enemyShots[mainShot];
         float stepAngle = 360.0f - (360.0f / numberOfSpirals);
 
         if (nextShot <= countdown)
         {
             acceleration += rotationSpeed;
 
             for (int i = 0; i <= numberOfSpirals - 1; i++)
             {      
                 float shotDirX = shotSpawnPoint.position.x + Mathf.Sin(((angle + stepAngle * i) * Mathf.PI) / 180f);
                 float shotDirY = shotSpawnPoint.position.y + Mathf.Cos(((angle + stepAngle * i) * Mathf.PI) / 180f);
 
                 Vector3 shotMoveVector = new Vector3(shotDirX, shotDirY, 0f);
                 Vector3 shotDir = (shotMoveVector - shotSpawnPoint.position).normalized * shotSpeed;
 
                 GameObject shot = Instantiate(
                     enemyShot, 
                     shotSpawnPoint.position, 
                     Quaternion.Euler(0.0f, 0.0f, -(angle + stepAngle * i))
                 );
                 shot.GetComponent<Rigidbody>().velocity = new Vector3(shotDir.x, shotDir.y, 0.0f);
             } 
 
             angle += acceleration * Time.deltaTime;
 
             if (acceleration >= 1000.0f)
                 acceleration = 1000.0f;
 
             countdown = 0.0f;
         }
     }