Modification of the modal characteristics of a square cylinder wake obstructed by a multi-scale array of obstacles

An experimental study was undertaken to investigate the changes on the turbulent wake of a confined square cylinder, caused by the introduction of three multi-scale arrays of obstacles. The arrays were introduced upstream, downstream and around the square cylinder, using the same obstacles in all cases. The results show that changes on the confinement caused by the introduced elements produce an increase of the shedding frequencies and on the energy contribution of the leading modes obtained by a Proper Orthogonal Decomposition, POD. The results also show that different modifications of the wake properties are observed if the obstacles are located either upstream or downstream of the square cylinder. When the obstacles are mainly located upstream it is possible to observe an increase in the peak magnitude of Turbulent Kinetic Energy in the wake. These changes are associated to a redistribution of the energy contribution of the POD modes. For larger regions obstructed upstream, the leading modes reduce their energy contribution, while the higher order modes increase it. It is also observed that an increased reattachment length can be obtained by locating most of the array downstream of the square cylinder. All these observations are discussed in terms of the expected interactions of flow structures. From the results it is possible to conclude that the general spatial patterns of the POD modes are not changed importantly by the introduction of the arrangements compared with the unconfined case and previously reported confined cases with lower Reynolds number. The most relevant changes for the confined obstructed case analysed here can be found in the shedding frequency and the modal energy distribution.