Measuring techniques

All measurements in the present investigation were performed at the design conditions with inlet Mach number of Mal = 0.67 and an inlet Reynolds number of Rel = 450.000. To get an impression of the cascade fbw, the mean blade loading in terms of the isentropic profile Mach number distribution is plotted in Fig. 4. Both steady and unsteady inflow conditions, measured with conventional static pressure tappings technique and fast-response Kulite sen­sors, are shown. The unsteady runs are performed at a bar pitch of tbar = 40 mm and tbar = 120 mm at bar speeds of ubar = 20 m/s, resulting in Strouhal numbers of Srl = 0.66 and Srl = 0.22 based on axial inlet velocity.

The differences compared to the steady inflow case are due to a reduced time-mean inflow velocity. The velocity deficit in the wake lowers the mean value resulting in lower velocities on the blade surface. This is more obvious for the small bar pitch of tbar = 40 mm, where additionally a further change in inlet flow angle compared to the steady case occurs. The mean Kulite data (filled symbols) show an excellent agreement with the values obtained from the static pressure tappings.

At unsteady inlet flow conditions, the separation bubble on the suction side starting at about xax/lax = 0.40, is somewhat reduced compared to the steady case, but still existent.

steady flow static pressure tappings

Kulite probes static pressure tappings

Kulite probes

= 120 mm. = 20 nvs

Kulite probes

Figure 4. Isentropic profile Mach number distribution

The ensemble-averaged time traces of the unsteady pressure ffictuations are displayed in Fig. 5. For clarity reasons, only four axial chord positions on the suction side are shown for each bar pitch. The loactions of these four Kulite – Sensors are shown in Fig. 1.

In case of the bar pitch 40 mm, the first sensors located in the acceleration part of the suction side register only small pressure peaks due to incoming wakes, while with increasing streamwise distance, the amplitude of the wave­like fluctuations raises. There is also a slight phase shift in the Kulite signals detectable. The ensemble-averaged pressure fluctuations for the high bar pitch 120 mm indeed show strong variations in time and amplitude starting right from the start. Therefore the wake passing leads to a periodically change of the blade loading. Sensor seven, which is located at the beginning of the separation bubble, displays a distinct maximum in pressure fluctuations and a saw tooth distribution. The pressure signals of the last Kulite sensor, located at xax/lax = 65.5% in the turbulent part of the boundary layer, show several peaks during one wake passing period.

To provide a comprehensive unsteady data set for numerical modeling of wake passing, the inflow conditions for the cascade have to be investigated in detail. Triple hot wire measurements were taken up-stream of the cascade inlet at about xax/lax = -0.16. Results for both bar pitches are shown in Fig. 6, where the normalized inflow velocity, the turbulence level Tu and the inflow angle ві are plotted for four bar passing periods t/T. The velocity deficit in the wake reaches about 12% of the inflow velocity. In case of the low bar pitch,

Figure 5. Ensemble-averaged time traces of pressure flictuations

the turbulence level rises from about 6% background level to 9.5 % in the bar wake. The distribution correlates with the velocity during the wake passing period. Compared to steady infbw conditions with a freestream turbulence intensity of 3.5%, the overall turbulence intensity in the unsteady case (bar pitch = 40 mm) is substantially larger. The turbulence level in case of the high bar pitch of 120 mm rises from about 4% to 9.5% in the bar wake, but in contrast to the case with low bar pitch, the turbulence intensity decreases very slowly to a value comparable with steady infbw conditions. As the fbw velocity is nearly constant during most part of the wake passing period, the turbulence level decrease must be caused by the decay of turbulence. Due to the high bar pitch, the absolute time between two bar wakes is large enough for a decay process until the next wake arrives. This could also explain the high background level in case of the lower bar pitch 40 mm, because the following wake arrives before the turbulence is completely decayed. The reduction in ft>w velocity also affects the velocity triangle and results in a periodic increase of the infttw angle of about Дв = 2° during every wake passing. The wake width can be easily extracted from the figures.

The results of the hot-film measurements in terms of space-time diagrams of ensemble averaged normalized RMS values and ensemble averaged quasi wall shear stress (QWSS) are shown in Figure 7 a-d. The data is mapped only qualitatively, where dark regions indicate maximum and light areas minimum

values. To identify the movement of the transition point, the dash-dotted white lines in the RMS diagrams, representing zero skewness, are used. The transi­tion point under steady infbw conditions is shown as a dotted vertical line. To illustrate the wake-induced transition process, different regions representative for various boundary layer states are marked in the figures similar to Halstead et al. (1997).

Figure 7b. Quasi wall shear stress, tbar = 40 mm

The few development takes place along a wake-induced path and a path between two wakes. Following the wake path, a wake-induced transitional few regime (B) emerges, where early transition is forced as can be seen in the RMS values and the white zero skewness line (Fig. 7 a, b). The migration of the transition point covers about 25% of the surface length. The path between two wakes remains still laminar (A). The transitional region (B) is followed in

time by a stable calmed region (D) with decreasing RMS values. The calmed region is able to delay the onset of transition in the path between two wakes (E). The transition point moves periodically downstream in the region infli – enced by calming effects (D) as compared with steady inflow conditions. The regions (C) and (F) are turbulent up to the trailing edge, but the boundary layer properties significantly in time.

Figure 7c. Ensemble averaged RMS Figure 7d. Quasi wall shear stress, tbar

voltage, tbar = 120 mm =120 mm

The RMS plots reveal, that the wake-induced transitional region (B) exhibits a double peak of high RMS values, which might be caused by shedded vortices in the wake. The wake vortices seem to be not mixed out as they enter the cas­cade inlet plane, although the inlet turbulence distribution in the wake region (Fig. 6) does not clearly show any double peaks indicating vortex shedding. However, the wake width in the RMS diagrams corresponds to the results of the triple hot wire measurements displayed in Fig. 6. In the investigations of Teusch et al. (1999) one can also find double peaks in the RMS distribution for the high Reynolds number test case. The space-time diagram of quasi wall shear stress on the suction side surface allows identifying the location and ex­tent of the laminar separation bubble characterized by minimum values in the QWSS distribution. Every wake passing, the transitional ft>w regime (B) pre­vents the formation of a separation bubble and transition takes place via bypass mode. The laminar separation is also suppressed by the calmed region (D). In case of the high bar pitch 120 mm, a region of undisturbed transition via lam­inar separation bubble exists between two wakes. As the bar pitch is reduced to 40 mm, this undisturbed region almost disappears. The separation bubble is getting smaller and still exists. The location of the transition point is shifted somewhat downstream in case of the low bar pitch.

3. Conclusions

Detailed experimental investigations focusing on wake-induced transition were performed in a highly loaded linear compressor cascade using different measurement techniques. Cylindrical bars moving parallel to the cascade inlet plane simulate the periodically unsteady ft>w caused by the relative motion of rotor and stator rows. The experiments were carried out at the design condi­tions of the compressor cascade using two different bar pitches of the wake generator.

In case of the high bar pitch of 120 mm, the passing wakes lead to a peri­odically change of the blade loading, which is accompanied by large pressure fluctuations with high amplitudes. The reduction in ft>w velocity also affects the velocity triangle and results in a periodic increase of the inflow angle of about Ав = 2° during every wake passing. The background turbulence level in case of the low bar pitch is significant larger compared to the higher bar pitch case, but the maximum turbulence value is uneffected by variation of the bar pitch.

For both bar pitches, the separation bubble is periodically reduced, but still existent. The migration of the transition point covers about 25% of the surface length. The RMS values in the wake-induced transitional region exhibit a dou­ble peak. This might be caused by shedded vortices in the wake, which are not mixed out as they enter the blade passage.

The measurements are intended as a contribution to the validation process of unsteady codes.

Acknowledgments

The authors wish to acknowledge the support of the Deutsche Forschungs Gemeinschaft (DFG) for the research program partly reported in this paper.

References

Acton, P. and Fottner, L. (1996). The generation of instationary flow conditions in the high­speed cascade wind tunnel. 13th Symposium on Measuring Techniques in Transonic and Supersonic Flow in Cascades and Turbomachines.

Halstead, D. E., Wisler, D. C., Okiishi, T. H., Walker, G. J., Hodson, H. P., Shin, H. W. (1997). Boundary layer development in axial compressors and turbines: Part 1-4. ASME Journal of Turbomachinery, Vol. 119, Part 1, pp. 114-127, Part 2, pp. 426-444, Part3, pp. 225-237, Part 4, pp. 128-139.

Hodson, H. P., Huntsman, I., Steele, A. B. (1994). An Investigation of Boundary Layer Develop­ment in a Multistage LP Turbine. Journal of Turbomachinery, Vol. 116, pp. 375-383 Hourmouziadis, J. (2000).Das DFG-Verbundvorhaben Periodisch Instationaere Stroemungen in Turbomaschinen. DGLR Paper JT2000-030

Lakshminarayana, B., Poncet, A. (1974)A method of measuring three-dimensional rotating wakes behind turbomachines. J. of Fluids Engineering, Vol. 96, No. 2

Mailach, R., Vogeler, K. (2003).Aerodynamic Blade Row Interaction in an Axial Compressor, PartI: Unsteady Boundary Layer Developmen. ASME-GT2003-38765

Mayle, R. E. (1991). The role of laminar-turbulent transition in gas turbine engines. ASME Journal of Turbomachinery, Vol. 113, pp. 509-537

Pfeil, H., Eilfer, J. (1976).Turbulenzverhaeltnisse hinter rotierenden Zylindergittern. Forschung im Ingenieurwesen, Vol. 42, pp. 27-32

Schobeiri, M. T., Read, K., Lewalle, J. (1995). Effect of unsteady wake passing frequency on boundary layer transition: experimental investigation and wavelet analysis. ASME Paper 95-GT-437

Sturm, W., Fottner, L. (1985). The High-Speed Cascade Wind Tunnel of the German Armed Forces University Munich. 8th Symp. on Meas. Techn. for Transonic and Supersonic Flows in Cascades and Turbomachines, Genoa

Teusch, R., Brunner, S., Fottner, L. (2000). The Influence of Multimode Transition Initiated by Periodic Wakes on the Profile Loss of a Linear Compressor Cascade. ASME Paper No. 2000-GT-271

Teusch, R., Swoboda, M., Fottner, L. (1999). Experimental Investigation of Wake-Induced Tran­sition in a Linear Compressor Cascade with Controlled Diffusion Blading. ISOABE-Paper IS-7057

Walker, G. J., Hughes, J. D., Solomon, W. J. (1999). Periodic Transition on an Axial Compres­sor Stator: Incidence and Clocking Effects: Part I – Experimental Data. ASME Journal of Turbomachinery, Vol. 121, pp. 398-407.

The experimental data acquired provides time-averaged as well as time – resolved information regarding the boundary layer development on the suction side of a compressor blade. The time-averaged loading of the compressor cas­cade was measured by means of conventional static pressure tappings on both the suction and the pressure side at mid-span connected to a Scanivalve system. These pneumatic data were recorded via computer control and represent mean values. The time-resolved compressor profile loading was determined using 10 Kulite fast-response absolute pressure sensors embedded into the suction side of the center blade. For each Kulite sensor a static calibration in the range of 50 to 350 hPa has been performed inside the pressure tank prior to the mea­surements.

To document the unsteady inflow conditions, 3D hot-wire measurements were performed in the cascade inlet plane. The probe employed in the present investigation consists of three sensing tungsten wires of 5 ^m diameter with a measuring volume of approximately 1 mm in diameter. The relative error of the hot-wire velocity is estimated to be less than 5%; the absolute angle deviation is less than 1°. To measure the qualitative distribution of unsteadiness and the quasi wall shear stress on the suction side, surface mounted hot-film sensors are used. The entire length of the suction surface is covered with an array of 36 gauges at midspan with their spacing varying between 2.5 and 5 mm. The sen­sors consist of a 0.4 mm thin nickel film applied by vapor deposition process onto a polyamide substrate. They were operated by a constant-temperature anemometer system in sets of 12 sensors and logged simultaneously at a sam­pling frequency of 50 kHz.

As shown e. g. by Hodson (1994), the boundary layer characteristics can be derived directly from the anemometer output and do not necessarily re­quire an extensive calibration procedure. The quasi-wall shear stress QWSS is determined by the output voltage E and the output voltage under zero fbw conditions E0, which is measured subsequent to the unsteady measurements, according to Eq. (1)

I E2 -El

QWSS = constant ■ tw з = ——— (1)

E0

The wake passing effects were studied for 5 wakes produced by 5 identi­cal bars, which could be ensured due to a once-per-revolution trigger mecha­nism. Processing of the raw hot-wire and hot-film measurement data for the unsteady case was done using the PLEAT technique (Phase Locked Ensemble Averaging Technique, Lakshminarayana et al., 1974) in order to separate ran­dom and periodic signals. The time-dependent signal b is composed of a peri­odic component b and the turbulent component b’ according to Eq. (2)

b = b + b’

N (2)

Kt) = jrY. m

i=l

In case of the hot-film sensor measurements, a total of N = 300 ensem­bles was logged and evaluated for quasi-wall shear stress (see Eq. 1), random unsteadiness RMS (Eq. 3) and skewness (Eq. 4), where the variable b(t) rep­resents the anemometer output voltage. To be able to compare the hot film sensors, the resulting values were normalized with the anemometer voltage at zero fbw, thereby eliminating the inflience of manufacturing differences be­tween the gauges.