Turbulence and separation induced pressure fluctuations on a finite circular cylinder — application of a linear unsteady strip theory

Abstract

Abstract In a turbulent stream, the fluctuating pressures on bluff bodies are mainly caused by the oncoming turbulence. Further unsteadiness is engendered by the wake turbulence and by the vortex shedding. Although the gross properties of the flow around a body and in particular the separated flow are affected by the intensity and the scale of the incident turbulence, these sources of pressure fluctuations are thought to be statistically independent. On this basis, the physical system of a turbulent flow around a body is approximated by an artificial black box model. This model corresponds to a constant parameter linear system with multiple stationary random inputs, i.e. turbulent velocity fluctuations far upstream the body. The model output is equal to an instantaneous surface pressure, which is subdivided into a turbulence induced contribution and an extraneous noise term to account for the residual fluctuations. By means of velocity/pressure cross spectral densities (i) significant velocity fluctuations, (ii) the associated admittance functions and (iii) the conditioned and residual pressure spectra are identified experimentally for the flow around a surface mounted circular cylinder in an atmospheric boundary layer flow. In a first approximation, the longitudinal u′(t) and lateral velocity fluctuation ν′(t) of a single stream filament are thought to be the only decisive inputs to the multivariate system (linear unsteady strip theory). The latter induces maximum spectral amplitudes in the mid-frequency range, whereas the longitudinal component achieves maximum effectiveness at the lowest frequencies. However, significant deviations between the (u, ν)-conditioned and the factual pressure spectrum remain. Depending on the circumferential angle and in a certain frequency range, vortex shedding contributes up to 45% to the total pressure RMS-values.