How to refine a contour to get a smooth shape on OpenCV in Python?

My task is to detect the boundary of a fluid flow and draw the associated contour. I have successfully detected the boundary and drawn the contour. The contour gets slightly deformed towards the top due to an irregularity in the image (presence of a feeder pipe). I wish to refine the contour so that I get a circular shape. Also, I wish to fill any gaps in the contour so that I have a closed circular form. This is my code:

contours, hierarchy = cv2.findContours(mask1, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_NONE)
Circle = []

if len(contours) != 0:
    for contour in contours:
        if cv2.contourArea(contour)>500:
            cv2.drawContours(cropped_img, contour,-1, (0,0,255), 2)
            Circle = np.append(Circle,[contour])

cv2.imshow("original", cropped_img)
cv2.waitKey(0)

This is what I have managed to get

I want the distortion towards the top to be removed and the contour closed into a circular form