Forcing Function Effects on Unsteady Aerodynamic Gust Response: Part 1—Forcing Functions

Abstract

The fundamental gust modeling assumption is investigated by means of a series of experiments performed in the Purdue Annular Cascade Research Facility. The unsteady periodic flow field is generated by rotating rows of perforated plates and airfoil cascades. In this paper, the measured unsteady flow fields are compared to linear-theory vortical gust requirements, with the resulting unsteady gust response of a downstream stator cascade correlated with linear theory predictions in an accompanying paper. The perforated-plate forcing functions closely resemble linear-theory forcing functions, with the static pressure fluctuations small and the periodic velocity vectors parallel to the downstream mean-relative flow angle over the entire periodic cycle. In contrast, the airfoil forcing functions exhibit characteristics far from linear-theory vortical gusts, with the alignment of the velocity vectors and the static pressure fluctuation amplitudes dependent on the rotor-loading condition, rotor solidity, and the inlet mean-relative flow angle. Thus, these unique data clearly show that airfoil wakes, both compressor and turbine, are not able to be modeled with the boundary conditions of current state-of-the-art linear unsteady aerodynamic theory.

An Investigation of Unsteady Impeller-Diffuser Interactions in a Centrifugal Compressor

“…Because an unsteady circumferential static pressure gradient is generated by potential effects due to impeller rotation, Section 1.2, with the velocity nonuniformity generated by viscous effects, the wake analysis does not directly account for it. Henderson (1991) has shown unsteady circumferential pressure gradients to exist in axial machines significant distances downstream of the rotor, and that wake behavior and downstream airfoil forced response are significantly different than in classical linear theory. In their experiment, linear theory type gusts are only generated with perforated plates which have minimal lift and no large areas of separation.…”

Inlet Guide Vane Wakes Including Rotor Effects

“…Vortical disturbances, convecting with the mean #ow, result from the viscous action at the blade surface. Fundamental vortical gust forcing functions have been considered by Henderson & Fleeter (1992); also, combined vortical}potential gust forcing functions have been considered by Manwaring & Wisler (1992) and Feiereisen et al (1993).…”

Combined-Simultaneous Gust and Oscillating Compressor Blade Unsteady Aerodynamics

“…A number of experiments have studied forcing ftmctions [1-3], with the key linear theory vortical gust characteristics identified [4]. When the experimental gust closely resembled a linear theory vortical gust, the correlation between the gust response data and predictions were excellent.…”

“…Another important finding was that the airfoil generated static pressure field was of equivalent magnitude to the vortical gust. Thus both the vortical and potential gust forcing functions must be considered in evaluating gust unsteady aerodynamic forced response [4].…”