I also found a file named related.phpin woocommerce\template\single-product\ :
$args = apply_filters('woocommerce_related_products_args', array(
'post_type' => 'product',
'ignore_sticky_posts' => 1,
'no_found_rows' => 1,
'posts_per_page' => $posts_per_page,
'orderby' => $orderby,
'post__in' => $related,
'post__not_in' => array($product->id)
) );
Not really strong in wordpress. This is all new to me so any ideas or pointer will be very appreciated.