Stage Matching Investigation Rig

The DPIV measurements were acquired on the U. S. Air Force’s Stage Matching Investigation (SMI) rig. It is a high-speed, highly-loaded compres­sor consisting of three blade-rows: a wake generator, rotor, and stator as shown in Fig. 5. The rig was designed so that the wake generator to rotor axial spac­ing and the wake generator blade count could be varied. The axial spacings were denoted as "close", "mid", and "far". The mid and far spacings repre­sent typical axial gaps found in operational fans and compressors. However, the current generation of high performance fans and compressors are being designed with the goal of minimizing blade-row spacing in order to increase performance and reduce compressor length and thus weight. The wake genera­tor blade count could be set to 12, 24, or 40, or the rig could be run without any wake generators (identified as the "clean inlet" configuration). Table 1 gives the wake generator to rotor axial spacings normalized by the wake generator chord.

1.1 Compressor Stage and Wake Generators

The rotor and stator were designed by Law and Wennerstrom [12]. A sum­mary of the SMI stage aerodynamic design parameters is given in Table 2. The purpose of the wake generators was to create wakes typically found in modern-technology, highly-loaded, low-aspect-ratio fan and compressor front stages. In general, these wakes are turbulent and do not decay as rapidly as

Table 1. Wake Generator-Rotor Axial Spacing

Spacing ax/c ax/c ax/c (tip)

Close 0.13 0.10 0.14

Mid 0.26 0.26 0.26

Far 0,55 0.60 0.52

ax = axial spacing c = wake generator chord

Table 2. SMI Aerodynamic Design Parameters




Number of Airfoils



Aspect Ratio – Average



Inlet Hub/Tip Ratio



Flow/Annulus Area,



Tip Speed, Corrected m/s


Mrel LE Hub



Mrel LE Tip



Max D Factor



LE Tip Dia., m



wakes from high-aspect-ratio stages with lower loading. The wake generators were designed with the intent of producing a two-dimensional representation of wakes measured at the exit of a high-pressure-ratio, low-aspect-ratio fan stage reported by Creason and Baghdadi [13]. A two-dimensional represen­tation was desired in order to isolate the effect of different wake parameters during the experiment.

Details of the Wake Generator (WG) design were presented by Gorrell et al. [14]. In summary, the WG’s are uncambered symmetric airfoils that do not turn the ft>w. They have a small leading edge and a blunt trailing edge. This shape creates a large base drag and no swirl. Solidity is held constant from hub to tip by varying the chord, the intent being to hold spanwise loss and wake width constant.

Calibration of the WG’s showed this was the case except near the end – walls. The calibration procedure, instrumentation, and results are found in

Refs. [15] and [16]. From those results, the widening of the wake from close to far spacing was clearly evident. Wake depth was deepest at close spacing and became shallower at mid and far spacing. The wake width was nearly constant from hub to case. This confirmed the intent of the wake generator design to produce a two-dimensional wake profile. The wake is constant in the circum­ferential and radial directions but not in the streamwise direction. Also evident from rake measurements near the endwalls was the boundary layer growth as the spacing increased from close to far.

From calculated velocity profiles, it was observed that the wake depth was similar at the hub and case and deepest near mid span. Wake decay analyzed by Chriss et al. [16] showed that the SMI wake generator wakes demonstrated similar trends to that compared in the literature.

Due to the blunt trailing edge of the wake generator, its wakes may be wider than what would be produced from a normally cambered stator airfoil, but wake measurements for comparison are not found in the open literature. Re­gardless of the wake thickness, the loss produced was very near the design intent and well within the range typically found in highly loaded stators.