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We deal with a numerical realization, which is a numerical analysis methodology to reproduce real flows by integrating numerical simulation and measurement. It is difficult to measure or calculate field information of real three-dimensional unsteady flows due to the lack of an experimental field measurement method, as well as of a way to specify the exact boundary or initial conditions in computation. Based on the observer theory, numerical realization is achieved by a combination of numerical simulation, experimental measurement, and a feedback loop to the simulation from the output signals of both methods. The present paper focuses on the problem of how an inappropriate model or insufficient grid resolution influences the performance of the numerical realization in comparison with ordinary simulation. For a fundamental flow with the Karman vortex street behind a square cylinder, two-dimensional analysis is performed by means of numerical realization and ordinary simulation with three grid resolutions

The present paper deals with a fundamental study of aerodynamic drag reduction for a vehicle with a feedback flow control. As the first step, two-dimensional calculation was performed for a flow around a simplified vehicle model. The mechanism of unsteady drag was investigated in relation to the vortex shedding from the model. The location of the control flow nozzle was so determined that the control flow influences the drag most effectively. The key in designing the present feedback control is the definition of the output signal. Based on the physical consideration of the drag generation, the location of the output velocity measurement was changed within a limited region near the front windshield. A systematic calculation revealed that the output signal defined in a small region results in a significant drag reduction of 20% with respect to the case without control. The present feedback flow control is generally applicable to the drag reduction of the bluff body for which the drag is generated under the same mechanism of essentially two-dimensional vortex shedding.

This paper deals with a new flow analysis system, namely the hybrid wind tunnel ,w hich integrates the experimental measurement with a wind tunnel and a corresponding numerical simulation with a computer. Analysis here is performed for the fundamental flow with the Karman vortex street in a wake of a square cylinder. A specific feature of the hybrid wind tunnel is existence of the feedback signal to compensate the error in the pressure on the side walls of the cylinder and the feed-forward signal to adjust the upstream velocity boundary condition. Investigation is focused on evaluating the hybrid wind tunnel as a flow analysis methodology with respect to the ordinary simulation and the experiment. As compared with the ordinary simulation, the hybrid wind tunnel substantially improves the accuracy and the efficiency in the analysis of the flow. Especially, the oscillation of the flow due to the Karman vortex street reproduced with the hybrid wind tunnel exactly synchronizes with that of the experiment, while that with the ordinary simulation never behave like that. In comparison with the experiment, the hybrid wind tunnel provides more detailed information of the flow than the experiment does.