Usually one should use one Solr core per site (and language). If you have to index multiple sites into the same core the site hash makes sure that you still only get results for the correct site - that is the site the query was triggered from.
This TYPO3 CMS Extension disables the sitehash restrictions. Use at own peril.
In your TypoScript configuration add the following to remove the site hash.
plugin.tx_solr.query.disableSiteHash = 1