In this article a prototype application of an interactive CFD-based computational steering system is presented, connecting a high performance computer (HPC) with a Virtual Reality (VR) environment. During the preliminary investigation of indoor air flow phenomena the user is able to interact on line, e.g. by modifying the geometry of the simulation scenario during the simulation itself. To allow for a comfortable analysis of complex fluid flow configurations a VR system consisting of a stereoscopic projection unit and a wand device is supported. The underlying CFD simulation is processed on a high performance supercomputer in order to enable a real-time simulation that instantly reacts on user interaction by an ongoing update of flow data. Based on the Lattice-Boltzmann Method the computation kernel shows good parallel efficiency on the Hitachi SR8000 HPC. The development of this online-CFD simulation of turbulent indoor flow in CAD-generated virtual rooms, which can be derived from a building product model, is presented in its current state.