$args = array(
        'post_type' => array('product'),
        'posts_per_page' => 15,
        'orderby' => 'date',
        'order' => 'DESC',
        'meta_query' => array(
            'relation' => 'OR',
            array( // Simple products type
                'key'=> '_sale_price',
                'value' => 0,
                'compare' => '>',
                'type' => 'numeric'
            ),
            array( // Variable products type
                'key' => '_min_variation_sale_price',
                'value' => 0,
                'compare' => '>',
                'type' => 'numeric'
            )
        )
      );
      $sale = new WP_Query($args);