Direct measurements of aerodynamic and inertial contributions to unsteady forces on non-rigid objects in a flow

Abstract

A method is presented to separate inertial and aerodynamic contributions to the forces measured in experiments on a non-rigid object in a flow. In particular, it can be used to remove spurious inertial contributions on aerodynamic loads measured with a balance. The method builds on a multiple-input/multiple-output framework and uses conditional spectral analysis in order to provide frequency response functions, or alternatively impulse response functions, to link the inputs quantifying inertia on the object (any kind of acceleration measurements) and the unknown aerodynamic forces on the object to the outputs quantifying total forces on the object (aerodynamic balance or any kind of force measurements). The method provides an efficient way of separating inertial and aerodynamic force contributions on objects in a flow from relatively few acceleration measurements and total force measurements.

This methodology is assessed on a wall-mounted cylinder in a turbulent boundary layer having two principal bending modes contributing to important inertial forces. It performs well to separate the inertial force contributions of the different bending modes from the pure aerodynamic force contributions which align very closely to the reference aerodynamic pressure forces. Especially, the method allows to separate unambiguously the inertial and aerodynamic force contributions even when structural and aerodynamic resonances have matching frequencies. A second test-case of a model of high-rise building with more complex structural dynamics and less effort put in manufacturing is presented. The method performs also well but with a bit more discrepancies to the reference aerodynamic pressure forces. In this case, part of the inertial forces are not correctly accounted for depending on the relevance of the location and number of acceleration measurements on the object, and on the complexity of the structural dynamics of the object. Leads are provided and discussed to tackle these relative limitations.