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This paper presents a numerical method and its application for an assessment of the flow field ins ide a wind tunnel. A structured CFD solver with overset mesh t echnique is developed in order to simulate geometrically com plex configurations. Applying the developed solver, a wh ole transonic cascade wind tunnel is modelled and simul ated by a two-dimensional manner. The upstream and downstream periodicity of the cascade and the effect of the tu nnel wall on the unsteady flow field are focused on. From the st eady flow simulations, the existence of an optimum throttle p osition for the best periodicity for each tailboard angle is sh own, which provides appropriate aerodynamic characteristics of ideal cascades in the wind tunnel environment. Unsteady simulations with blade oscillation is also conducte d, and the difference in the influence coefficients between id eal and wind tunnel configurations becomes large when the pressu amplitude increases on the lower blades. INTRODUCTION A detailed knowledge of the characteristics of moti onexcited aerodynamic force is essential for understa nding and predicting an aeromechanical behaviour in turbomach inery. In order to measure motion-excited aerodynamic force ( often referred to as unsteady aerodynamic force), a numbe r of researches have been conducted so far using linear cascade wind tunnels [1]. A typical way for obtaining unste ady aerodynamic force is to measure the responses of fl ow field and aerodynamic force acting on the airfoils under pr scribed blade motion. Such data are used for validations of numerical model for predicting and optimizing blade vibration characteristics during the design stage of turbomac hinery [2]. The operating conditions of the wind tunnels are car fully controlled to realize flow field similar to ideal i nf nite cascade (i.e. pitchwise periodicity) before detailed aerody namic measurement [3-4]. Therefore, establishing a guideli ne for controlling the wind tunnel is beneficial for gathe ring data over a wide range of flow conditions. In addition, the cascade wind tunnels often have different geometrical detai ls from an ideal infinite cascade, such as finite number of ai r oils, tailboards, and suction mechanisms. Thus, the differ ences that can arise from these real geometrical features shou ld be known in detail when a comparison is made between w ind tunnel measurement and numerical simulation results . Some past studies focuses attention on the effects of wind tunnel geometry on steady flow fields and unsteady aerodynamic force characteristics. Lepicovsky et al. [5] showed the importance of tailboard geometry on the periodicity of steady flow field through experiment al and numerical assessments. Buffum and Fleeter [6] discu ssed the deterioration of uniformity in unsteady pressure co fficient for traveling-wave-mode oscillation, with focusing on propagating wave direction and its interaction with the wind tunnel wall. Later they reported the effect of acous tic mode in the wind tunnel on the measured aerodynamic influen ce coefficients [7]. Ott et al. [8] conducted a numeri cal study for extracting the effect of tailboard on the steady an d u steady flow field in a transonic turbine cascade. All the studies reported the tailboard or wind tunnel wall have sig nif cant effect on the steady and motion-excited flow fields . The aim of this study is to develop a numerical meth od for an assessment of the flow field inside a cascad e wind tunnel for establishing a basic procedure for contr olling its flow field. The modelling of whole wind tunnel and parametric study of its geometrical setup are enabl ed y using overset mesh technique. Using the developed method, t e steady and unsteady simulations of the whole wind t u nel are conducted with focusing on the periodicity of the c as ade section and the effects of tunnel wall on the motio n-excited flow field. TRANSONIC CASCADE WIND TUNNEL An analysis target for this study is a transonic ca scade wind tunnel in the University of Tokyo. This wind t unnel is designed for aeroelastic investigations of fan or c ompressor cascade, and it had been used for fundamental resea rches on

Numerical method for an assessment of steady and motion-excited flowfields in a transonic cascade wind tunnel