When a bluff body is placed in a crossflow, the total temperature in its wake can become substantially less than the incoming one, as manifested by the fact that the recovery factor R on its rearmost surface takes negative values at high subsonic flow: this is the phenomenon referred to here as the Eckert-Weise effect. Although a vortex street has been a suspected cause, the issue of whether this is so, and what the mechanism is, has remained unsettled. In this experimental and theoretical investigation, we first examine the cause of the Eckert-Weise effect by enhancing the vortex shedding through acoustic synchronization: resonance between the vortex shedding and transversely standing acoustic waves in a wind tunnel. At the lowest synchronization, where a ringing sound emanates from the wind tunnel, R at the rearmost section of the cylinder is found to become negative even at a Mach number of 0.2; the base pressure (Cpb) takes dips correspondingly, indicative of the intensification of the vortex street. At this lowest acoustic resonance, the decrease of R and Cpb, uniform along the span, agrees with the expectation based on the spanwise uniformity of the lowest standing wave. At the next acoustic resonance where the standing wave now varies along the span, the corresponding dips in R and Cpb, non-uniform along the span, reveals an interesting ‘strip-theory’-like behaviour of the vortex intensities in the vortex street. These results correlating the change in R with Cpb confirm that the Eckert-Weise effect is indeed caused by the vortex shedding, the mechanism of which is examined theoretically in the latter half of the paper.A simple theoretical argument, bolstered by a full numerical simulation, shows that the time-varying static pressure field due to the vortex movement separates the instantaneous total temperature into hot and cold spots located around vortices; once time-averaged, however, the total temperature distribution conceals the presence of hot spots and takes the guise of a colder wake, the Eckert-Weise effect. Therefore the correct explanation of the Eckert-Weise effect, a time-averaged phenomenon, emerges only out of, and only as a superposition of, instantaneous total temperature separation around vortices. Such a separation is not confined to the outside of vortex cores; every vortex in its entirety becomes thermally separated. Nor is it limited to the far downstream equilibrium configuration of the Kármán vortex street but applies to the important near-wake vortices, and to any three-dimensional vortical structure as well. For low subsonic flows in particular, this dynamical explanation also leads to a similar separation of total pressure; these features may thus be potentially exploited as a general marker to identify and quantify vortices.
This paper presents an experimental and analytical investigation of compressor stability assessment during engine transient operation. A two-dimensional, linear, compressible, state-space analysis of stall-inception (Feulner et al., 1996, ASME J. Turbomach., 118, pp. 1–10) was modified to account for engine transients and deterioration, with the latter modeled as increased tip-clearance and flow blockage. Experiments were performed on large commercial aircraft engines in both undeteriorated and deteriorated states. Unsteady measurements of pressure in these test engines during rapid accelerations revealed the growth of pre-stall disturbances, which rotate at rotor speed and at approximately half rotor speed. These disturbances are stronger in deteriorated engines. The model showed that the signal at shaft speed was the first compressible system mode, whose frequency is near shaft speed, excited by geometric nonuniformities. The computed behavior of this mode during throttle transients closely matched engine data. The signal increased in strength as stall was approached and as the engine deteriorated. This work firmly establishes the connection between observed signals in the these engines and first principles stability models. [S0889-504X(00)01603-2]
This paper presents an experimental and analytical investigation of compressor stability assessment during engine transient operation. A 2-dimensional, linear, compressible, state-space analysis of stall-inception (Feulner et al. (1996)) was modified to account for engine transients and deterioration, with the latter modeled as increased tip-clearance and flow blockage. Experiments were performed on large commercial aircraft engines in both undeteriorated and deteriorated states. Unsteady measurements of pressure in these test engines during rapid accelerations revealed the growth of pre-stall disturbances, which rotate at rotor speed and at approximately half rotor speed. These disturbances are stronger in deteriorated engines. The model showed that the signal at shaft speed was the first compressible system mode, whose frequency is near shaft speed, excited by geometric nonuniformities. The computed behavior of this mode during throttle transients closely matched engine data. The signal increased in strength as stall was approached and as the engine deteriorated. This work firmly establishes the connection between observed signals in the these engines and first principles stability models.
State of the art turbomachinery flow analysis codes are not capable of predicting the viscous flow features within turbomachinery blade wakes. Until efficient 3D viscous flow analysis codes become a reality there is therefore a need for models which can describe the generation and transport of blade wakes and the mixing process within the wake. To address the need for experimental data to support the development of such models, high response pressure measurements and laser anemometer velocity measurements have been obtained in the wake of a transonic axial flow fan rotor.
The results of a test program incorporating a dual hot-wire aspirating probe to radially survey the exit flowfields of two axial-flow transonic fans are presented. The probe measures time-resolved total temperature and total pressure, from which the instantaneous isentropic efficiency is calculated. A technique for quantifying the randomness in an unsteady turbomachine flowfield is developed. Randomness is quantified by blade wake aperiodicity (variation of blade passage period) and non-uniformity (variation of blade wake shape). A method of ensemble averaging instantaneous data is presented which produces an identifiable blade passage wake profile even in a random flowfield where traditional techniques often fail. The flowfield randomness for the two fans is shown to correlate well with the respective tip losses. Results also show that the overall efficiency of the best blade on a fan rotor can be 0.5% higher than the performance of the rotor overall average or as much as 0.7% higher than the worst blade on the rotor.
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