Wake resonance mechanisms in bluff body interactions

Abstract

When inhomogeneous upstream turbulence impinges on a dowstream bluff body many complicated interactions occur including: modification of the properties of the upstream turbulence; buffeting of the body; and modification of the bluff body flow regime near the body. Some features of the interactions are similar to the effects obtained with homogeneous upstream turbulence (eg grid turbulence) however the presence of coherent structures can lead to a wake resonance condition in which the quasi-periodic coherent structures force a quasi-resonant response of the wake of the downstream body. This resonance can have significant effects on the flow near the body and the bluff body loading. The buffeting and wake resonance components of the interaction of inhomogeneous turbulence with downstream bluff bodies have been studied using analytical, numerical and experimental techniques (Ramsay 1988a). This paper discusses the wake resonance mechanisms in the context of the interaction of turbulent shear flows (eg wake) with downstream bluff bodies. For weak inhomogeneous upstream turbulence it is possible to extend the methods of Rapid Distortion Theory (RDT) (Hunt 1973) to obtain a linear approximation to the vorticity dynamics of the inhomogeneous interaction. The extended RDT analysis; representation of the coherent and incoherent parts of the upstream inhomogeneous turbulent fields in an appropriate mathematical form; and other details of the analysis have been discussed previously (Ramsay 1988b, 1988c). This analysis can be used to investigate the interaction when the wake resonance is weak. For conditions of strong wake resonance experimental methods must be used. Some progress has also been made with investigations using numerical models of the wake resonance interaction. The results of the analytical and experimental studies show that the interaction of the inhomogeneous upstream turbulence is well described by linear theory using equivalent homogeneous turbulence parameters (integral scale and intensity) when the scale of the upstream coherent structures is well removed from the wake resonance range. Near the wake resonance condition nonlinear behaviour becomes dominant and the linear approximation breaks down. The range of strong wake resonance interactions has been experimentally delineated and occurs for a narrow range of scales of upstream coherent structures approximately equal to the scale of the structures in the wake of the downstream bluff body wake. Over this range of scales the character and magnitude of the response varies considerably from the predictions of the equivalent linear homogeneous theory.