SMI Performance

DPIV results are presented for close and far spacing configurations at 75% span, 100% corrected speed, peak efficiency. Of particular interest is the in­teraction between the wake generator wake and the rotor bow shock and the effect blade-row axial spacing has on the overall flow field. The median of 50 instantaneous images is plotted as it was found that it provided the clearest image of the flow field.

2.1 Close Spacing

Previous analysis of fbw visualization and DPIV results from the SMI rig (Refs. [17], [18], and [21]) have shown that at close spacing vortex shedding from the wake generator trailing edge is phase locked to the rotor blade pass frequency. The main source of the synchronization appears to be the strong pressure perturbation provided by the rotor bow shock to the wake generator trailing edge. At close spacing the instantaneous images of vortex shedding are similar for any given operating condition and blade delay. This is consistent with high response pressure measurements obtained on the wake generator sur­face, which showed a strong fhctuation in pressure at blade-passing frequency (7.7 kHz). Since the instantaneous DPIV data contains holes in velocity in­formation where seeding was not sufficient to obtain a correlation, it is more informative to look at the average fbw field where data intermittency can be minimized. Since the vortex shedding is phase locked to rotor passing, rotor phase locked averaging is possible without destroying the details of the veloc­ity field in this interaction region.

At close spacing DPIV measurements were made at blade delay intervals of 5 3ts giving 30 different rotor blade locations for one blade-pass period. Seven of the blade delays are shown in Fig. 5. The rotor bow shock is defined by the large velocity gradient and a change in fbw angle toward the shock. Stream­lines are drawn near the wake generator to highlight the wake motion at differ­ent rotor locations. The wavy motion of the wake is a result of a vortex being shed from the pressure or suction surface of the wake generator. This up and down motion continues as the wake convects downstream and interacts with the rotor bow shock and then is chopped by the rotor blade. Downstream of the rotor bow shock there is an expansion zone due to the ft>w accelerating around the rotor suction surface.

The DPIV images at blade delay 140 ^s and 20 ^s illustrates that the shock – wake interaction results in a wider and deeper wake downstream of the shock. At time 20 ^s the low velocity region downstream of the shock and within the wake moves up to 18% pitch suggesting that the shock-wake interaction has resulted in an increase in wake width.

From the plot at blade delay 140 ^s it is clear that the wake actually splits the shock into two distinct regions above and below the wake. It was also observed that the velocity magnitude at the wake generator trailing edge flic – tuates significantly depending on the location of the rotor bow shock. As the shock approaches the wake generator the velocity increases first near the wake generator pressure surface, then on the suction surface. Once the shock is sep­arated and propagates upstream the velocity magnitude decreases. Numerical analysis reported by Gorrell et al. [7] showed that the interaction of the wake generator trailing edge with the rotor bow shock causes the shock to turn more normal to the freestream flow. This phenomenon is also observed in the exper­imental data presented in Fig. 5.

Performance characteristics for the SMI rig are shown in Fig. 5. There was a significant difference in performance between each of the three spacings tested. Both the pressure ratio and efficiency characteristics decreased significantly as the blade-row axial spacing was reduced from far to close. The choking mass fbw rate decreased as the blade-row axial spacing was reduced. The difference in pressure ratio, efficiency, and mass fbw rate between the far and close spac­ing configurations was greater than the repeatability documented in Ref. [14]. Therefore it was concluded that the observed change in performance with axial blade-row spacing was real and not due to experimental measurement uncer­tainty.

2. DPIV System

The DPIV system used to obtain the measurements presented in this paper has been described in detail by Estevadeordal et al. [17]. Figure 5 contains schematics of the optical system. Two frequency-doubled Nd:YAG lasers are employed for instantaneous marking of the seed particles in the flow field. Combined by a polarizing cube or a beam combiner, the beams are directed through sheet-forming optics and illuminate the test section with a 2D plane of thickness ~1 mm. The scattering from the seed particles is recorded on a cross­correlation CCD camera with 1008 x 1018 pixels (Redlake ES1.0). The camera maximum repetition rate is 15 double exposures per second and was set to 10 Hz for synchronization with the laser repetition rate. The time delay between

Figure 2. SMI Performance, 24WG’s, 100% Corrected Speed

the lasers was typically 2 ^sec. For the present experiments where only a small area was to be captured, the camera offered sufficient resolution. A 105­mm Nikon lens was used. The magnification for the present experiments was 17 and 27 pixels/mm which corresponds to a viewing width of 59 mm (close spacing) and 37 mm (far spacing).

The laser-sheet delivery system consists of a probe inserted in an enlarged WG, light-sheet-forming optics, prisms, and probe holders for mounting the optics and for protecting them from contaminated seed materials. To mini­mize perturbations the modified WG was located two WGs below the WG that was centered at the receiving window. A receiving window made of chemi­cally strengthened glass allowed optical access to the region of interest. Fig­ure 5 shows schematic diagrams of the path for the laser system and the optical probe. Although the path was relatively long, the power required for laser-sheet

Figure 3. a) Schematic of optical path; b) Schematic of fbw features (drawn to scale) showing

DPIV delivery and receiving optics

illumination was very low (~10 mJ/pulse) because of the minimal amount of optics loses. The F stop of the 105 mm lenses was kept at 5.6 for these ex­periments; this allowed the laser power to be kept low which is important for the safety of the optical components in the optical probe as the beam starts focusing.

The shape of the laser sheet (thickness, width, focal distance) can be changed through various combinations of the spherical-lens focal length, the cylindrical – lens diameter, and the distance between them inside the WG as well as through external optics (a spherical lens) located in the laser path. The spanwise loca-

The length of the probe that was outside the WG could also be changed to provide further flexibility in moving the laser-sheet in the streamwise direction. The probe was set manually before each experiment.

The camera was aligned and focused on the laser sheet prior to each run. It was mounted on a tripod to minimize the effect of rig vibrations. To account for possible motion of the camera with respect to the laser sheet that might occur, the camera was positioned by means of a translation stage that was remotely controlled to allow small corrections in the camera location. Large changes with respect to the laser sheet could produce magnification changes that must
be taken into account. After every experiment, the laser-sheet and camera locations were verified for possible misplacements. In the present experiments, the only change required was slight refocusing, with negligible magnification effects.

The viewing window had the same curvature as the rotor housing (inner housing radius is 241.3 mm), was made of chemically strengthened glass, and had a thickness of 2 mm. The effect of window curvature and thickness was investigated by Copenhaver et al. [18] and found to have a negligible effect for the present CARL setup.

Several options for seeding the high-fhw (~ 16 kg/s) SMI rig in the CARL facility were evaluated [17], including the use of various seeding units and seed materials. Both local and global seeding was considered. The seed material used was sub-micron-size smoke particles generated from a glycerin and water mixture. During its use in the CARL facility, this system produced sufficient seed when the particles were introduced at the end of the settling chamber, before the contraction, and at the height of the receiving window. The machine can be remotely controlled. The seed material was introduced through a pipe of 50.4-mm diameter located under the contraction entrance.

The rotor one-per-revolution signal was used for triggering the synchroniza­tion system. A digital pulse generator (Stanford DG535) and a camera frame – grabber (National Instruments PCI-1424) were used.

Once the PIV images have been captured and digitized, the velocity field is obtained using cross-correlation techniques over interrogation domains of the images using DPIV software developed internally. The dimensions of each in­terrogation domain are dependent on particle density, estimated local velocity gradients, particle-image size, and desired spatial resolution. The peak of the correlation map corresponds to the average velocity displacement within the interrogation spot. An intensity-weighted peak-searching routine is used to de­termine the location of the peak to sub-pixel accuracy. To improve the signal – to-noise ratio in the correlation maps, a correlation-correction scheme [19] is applied wherein each map is multiplied by its immediate four neighborhoods. An overlapping of 75% is used to include much the same particles in the five maps that are multiplied to yield a single correlation map with lower noise. Zero padding is also employed for adding accuracy. The software includes a grid feature that allows selection of areas of the image to be processed. This permits removal of solid regions such as blades and WGs and also shadows from the processing areas. It also provides a choice of various correlation en­gines and correlation peak locators and incorporates several improvements to standard (single-pass) PIV techniques such as recursive estimation of the ve­locity field through a multipass algorithm for increasing resolution. Two passes with interrogation cells overlapping 75% were performed. The interrogation domains are overlapped by three-quarters the domain size to yield more vec­tors. The overlapping includes new particles in every subregion. Average rou­tines allow for removal of outliers beyond any number of standard deviations. Because of the strong phase-locked ft>w features, the median offers a valid, robust, and smooth statistical representation of the average velocity field [17].

Many factors are involved in the DPIV uncertainty-calculation process (laser, CCD, seeding, imaging, algorithms, oscilloscope, etc.). The highest uncer­tainty was found to be associated with the velocity calculation which involves Ax (the displacement in pixels of each interrogation region), At (the time in­terval between the two exposures), and the magnification of the digital image relative to the object (pix/m). The displacement in pixels obtained by peak – locator algorithms can provide sub-pixel accuracy (< 0.1 pixels) after correc­tion for various biases [20]. The At was adjusted to yield typical displacements of the main stream > 10 pixels, and the uncertainty is thus <1%. Values in the wake region, however, may have higher uncertainties due to the lower Ax. The maximum uncertainty in the At was calculated from the time interval between the two laser pulses with the aid of an oscilloscope (uncertainty 2%). It was found that this uncertainty increases with lower laser power and with lower At. A conservative number for the present experiments, which employed a At of about 2 ps and powers around 10 mJ, was found to be 1%. The magnification was measured using images of grids located in the laser-sheet plane to better than 1%. Combining these conservative measurements of uncertainty yields a maximum error of < 2% for the free-stream velocity and ~10% in the wake near the WG area.