<?php

 // Query all of the nids of the research content type.
  $nids = db_select('node', 'n')
    ->fields('n', array('nid'))
    ->condition('type', 'research', '=')
    ->execute()
    ->fetchCol();

  foreach ($nids as $nid) {
  // The task now is to delete all revisions except the most recent 20 belonging to each of the nodes in the $nodes array.

  // We probably need a loop here that can loops through the $nodes array and then run the following on each node in the array

    // As written, the code below would delete all revisions belonging to each node, we need to limit that to those after the most recent 20.
    // Possible way to limit the array of revisons to delete: $revstodelete = array_slice($vid, < 19); to return revisions after the first 20.
    $node = node_load($nid, TRUE);
    $vids = \Drupal::entityManager()->getStorage('node')->revisionIds($node);
    $vids = array_slice($vids, 0, -20);
    foreach($vids as $vid){
      if ($node->vid !== $vid) {
        \Drupal::entityTypeManager()->getStorage('node')->deleteRevision($vid);
      }
    }
  }