ng_vol: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/lib64/mpich/bin/ng_vol - from package: netgen-mesher-mpich
/usr/lib/mpich/bin/ng_vol - from package: netgen-mesher-mpich
/usr/lib64/openmpi/bin/ng_vol - from package: netgen-mesher-openmpi
/usr/lib/openmpi/bin/ng_vol - from package: netgen-mesher-openmpi
/usr/bin/ng_vol - from package: netgen-mesher

You may install the required package with command yum install {package}

Error in other languages:
ng_vol: bevel nie gevind nie
ng_vol: comandă negăsită
ng_vol: comando não encontrado
ng_vol: commande introuvable
ng_vol: command not found
ng_vol: käsku ei ole
ng_vol: không tìm thấy lệnh
ng_vol: komanda nerasta
ng_vol: Kommando nicht gefunden.
ng_vol: kommandot finns inte
ng_vol: komut yok
ng_vol: nie znaleziono polecenia
ng_vol: níor aimsíodh an t-ordú
ng_vol: no se encontró la orden
ng_vol: no s'ha trobat l'ordre
ng_vol: opdracht niet gevonden
ng_vol: parancs nem található
ng_vol: perintah tidak ditemukan
ng_vol: príkaz nenájdený
ng_vol: příkaz nenalezen
ng_vol: команда не найдена
ng_vol: командата не е открита
ng_vol: 命令找不到
ng_vol: コマンドが見つかりません