Experimental Investigation of Compressor Cascade Wake-Induced Transition 359 Figure 1. Compressor Cascade V103-220 EIZ (Erzeuger Instationaerer Zustroemung, see Fig. 2) and its constructional principles are explained by Acton and Fottner (1996) in more detail. The cylin- drical steel bars create a far wake very similar to the one produced by an actual airfoil (Pfeil and Eifler 1976). Preliminary tests showed that the wakes shed by bars of 2 mm diameter are representative for the wakes of the V103 pro- file geometry regarding the wake width. The distance ratio between the bars and the cascade inlet plane is about x/l = 0.35 (see Fig. 1). Two different bar pitches of 40 mm and 120 mm were used. The belt mechanism drives the bars with speeds of up to 40 m/s. However, the maximum bar speed for the present investigation is 20 m/s, thus generating Strouhal numbers between 0.22 and 0.66 for the investigated test cases. Figure 2. Wake generator (EIZ) with installed compressor cascade 360 It should be noted that the maximum bar speed together with the axial ve- locities is still too slow to produce a Strouhal number and inlet velocity tri- angle representative for modern compressors. The wakes enters the cascade passage almost parallel to the blades. Therefore the data acquired with this setup cannot be transferred directly to real turbomachines. The measurements should be considered as basic investigations of the unsteady multimode transi- tion process. As the main purpose of the present experimental investigations is to obtain a deeper unterstanding of the flow phenomena and to provide a sound database for the validation of unsteady numerical flow solvers and particular transition models, the angle of the incoming wake is of minor importance. 2.3 Test facility The experiments were carried out in the High Speed Cascade Wind Tunnel of the University of the Federal Armed Forces Munich, which is an open-loop test facility located inside an evacuable pressure tank (Fig. 3). Mach and Reynolds number in the test section can be varied independently by lowering the pressure level inside the tank and keeping the total temperature constant by means of an extensive cooling set-up, therefore allowing to simulate real turbomachinery conditions (Sturm & Fottner 1985). All test were performed with a constant total temperature of 303 K. The turbulence intensity of the inlet flow is adjusted by fitting a turbulence grid upstream of the nozzle. Figure 3. High Speed Cascade Wind Tunnel Experimental Investigation of Compressor Cascade Wake-Induced Transition 361 2.4 Measuring techniques 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 cas- cade 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 mea- surements. 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 µm 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 sen- sors 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 sam- pling 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 re- quire an extensive calibration procedure. The quasi-wall shear stress QWSS is determined by the output voltage E and the output voltage under zero flow conditions E 0 , which is measured subsequent to the unsteady measurements, according to Eq. (1) QWSS = constant · τ w 1 3 = E 2 − E 2 0 E 2 0 (1) The wake passing effects were studied for 5 wakes produced by 5 identi- cal bars, which could be ensured due to a once-per-revolution trigger mecha- nism. 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 ran- dom and periodic signals. The time-dependent signal b is composed of a peri- odic component ˜ b and the turbulent component b according to Eq. (2) 362 b = ˜ b + b ˜ b(t)= 1 N · N i=1 b i (t) (2) In case of the hot-film sensor measurements, a total of N = 300 ensem- bles 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) rep- resents the anemometer output voltage. To be able to compare the hot film sensors, the resulting values were normalized with the anemometer voltage at zero flow, thereby eliminating the influence of manufacturing differences be- tween the gauges. RMS(t)= 1 N N i=1 b i (t) − ˜ b(t) 2 (3) Skewness(t)= 1 N N i=1 b i (t) − ˜ b(t) 3 1 N N i=1 b i (t) − ˜ b(t) 2 3/2 (4) 3. Results All measurements in the present investigation were performed at the design conditions with an inlet Mach number of Ma 1 = 0.67 and an inlet Reynolds number of Re 1 = 450.000. To get an impression of the cascade flow, 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 sen- sors, are shown. The unsteady runs are performed at a bar pitch of t bar =40 mm and t bar = 120 mm at bar speeds of u bar = 20 m/s, resulting in Strouhal numbers of Sr 1 =0.66andSr 1 = 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 t bar = 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 x ax /l ax = 0.40, is somewhat reduced compared to the steady case, but still existent. Experimental Investigation of Compressor Cascade Wake-Induced Transition 363 Figure 4. Isentropic profile Mach number distribution The ensemble-averaged time traces of the unsteady pressure fluctuations 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 wave- like 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 x ax /l ax = 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 x ax /l ax = -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 β 1 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, 364 Figure 5. Ensemble-averaged time traces of pressure fluctuations 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 inflow 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 inflow conditions. As the flow 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 flow velocity also affects the velocity triangle and results in a periodic increase of the inflow angle of about ∆β = 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 Experimental Investigation of Compressor Cascade Wake-Induced Transition 365 Figure 6. Unsteady inflow conditions (ensemble averaged) values. To identify the movement of the transition point, the dash-dotted white lines in the RMS diagrams, representing zero skewness, are used. The transi- tion point under steady inflow 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 7a. Ensemble averaged RMS voltage, t bar =40mm Figure 7b. Quasi wall shear stress, t bar =40mm The flow development takes place along a wake-induced path and a path between two wakes. Following the wake path, a wake-induced transitional flow 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 366 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 influ- 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 voltage, t bar = 120 mm Figure 7d. Quasi wall shear stress, t bar = 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 cas- cade 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 ex- tent of the laminar separation bubble characterized by minimum values in the QWSS distribution. Every wake passing, the transitional flow regime (B) pre- vents 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 lam- inar 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. Experimental Investigation of Compressor Cascade Wake-Induced Transition 367 4. 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 flow caused by the relative motion of rotor and stator rows. The experiments were carried out at the design condi- tions 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 peri- odically change of the blade loading, which is accompanied by large pressure fluctuations with high amplitudes. The reduction in flow velocity also affects the velocity triangle and results in a periodic increase of the inflow angle of about ∆β = 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 dou- ble 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 high- speed 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, Part 3, pp. 225-237, Part 4, pp. 128-139. Hodson, H.P., Huntsman, I., Steele, A.B. (1994). An Investigation of Boundary Layer Develop- ment 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. 2 368 Mailach, R., Vogeler, K. (2003).Aerodynamic Blade Row Interaction in an Axial Compressor, Part I: 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., Eifler, 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 Tran- sition 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 Compres- sor Stator: Incidence and Clocking Effects: Part I – Experimental Data. ASME Journal of Turbomachinery, Vol. 121, pp. 398-407. [...]... Cicatelli, G and Sieverding, C H (1997) The effect of vortex shedding on the unsteady pressure distribution around the trailing edge of a turbine blade J of Turbomachinery, 119 :81 0– 81 9 Craft, T J., Launder, B E., and Suga, K (1996) Development and application of a cubic eddyviscosity model of turbulence Int J Heat and Fluid Flow, 17:1 08 115 Gerrard, J H (1961) An experimental investigation of the oscillating... wakes of the airfoils strongly infl uence the fl field ow downstream, and the varying incidence even causes fl uctuating fl separations ow in the blade rows downstream Keywords: Axial Compressor, Multistage, Unsteady Flow, Off-Design, Experimental Investigation 369 K C Hall et al (eds.), Unsteady Aerodynamics, Aeroacoustics and Aeroelasticity of Turbomachines, 369– 380 © 2006 Springer Printed in the Netherlands... turbulent scales interact or not with 381 K C Hall et al (eds.), Unsteady Aerodynamics, Aeroacoustics and Aeroelasticity of Turbomachines, 381 –393 © 2006 Springer Printed in the Netherlands 382 macroscopic vortices If yes, LES must be performed to simulate vortex shedding process, if no, URANS can be enough Experimental results (Sieverding et al., 2003) on turbine blade, and presented results, are based... many authors at low (Han and Cox, 1 983 , Cicatelli and Sieverding, 1997) and high speed (Sieverding et al., 2003) The experimental results present quantitative information on unsteady pressure, velocity, and temperature Sieverding’s results are a precious source of analysis and validation of numerical simulations The presented results try to evaluate the capabilities of URANS and LES to predict these... too small value of the turbulent kinetic energy Analyses of URANS and LES Capabilities to Predict Vortex Shedding 385 Figure 3 VKI turbine Left: Comparison of vortex location Right: Comparison of vortex density evolution at the center of vortex (fine grid) Figure 1-right shows the density iso-contours for the experiment and for the linear and non-linear k − ω models The vortex locations and the shock... 17 000 2.03 1.30 1. 28 1.22 13.40 920.00 17 000 2.03 1.31 1.26 1.23 13.66 996.14 17 000 2.29 1.35 1.32 1. 28 13.35 1155. 68 the compressor are summarized in Table 1 The compressor has a nominal total pressure ratio of 2.03, and a mass fl of 13.4 kg/s at a rotational speed of ow 17000 RPM With a circumferential tip speed of 345 m/s, the maximum relative Mach number is 0 .89 at the tip of the first rotor Although... oscillating lift and drag of a circular cylinder shedding turbulent vortices J Fluid Mech., 11:244–256 Graftieaux, L., Michard, M., and Grosjean, N (2001) Combining piv, pod and vortex identification algorithms for the study of unsteady turbulent swirling fl ows Meas Sci Technol., 12:1422–1429 Han, L S and Cox, W R (1 983 ) A visual study of turbine blade pressure side boundary layer J of Eng for Power,... The probe shown in Fig 2 (left hand side) was developed and manufactured at the Institute of Jet Propulsion and Turbomachinery The head diameter is 2 mm, and the probe is equipped with an Entran EPIH112 fast response semiconductor pressure transducer It is supplied by constant current and calibrated by an independent variation of pressure and temperature The approximation of the characteristics is realized... governing the turbulent energy k and the dissipation ω The evaluation of the Reynolds tensor and of the turbulent viscosity are carried out by different turbulence models: the linear model of Wilcox, 1993b, the low-Reynolds model Analyses of URANS and LES Capabilities to Predict Vortex Shedding 383 Figure 1 VKI turbine Left: Mach numbers contours Right: Comparison of density isocontours [Upper : linear... calculation of the total energy of the periodic pressure fl uctuations generated by the rotor blades Doing this for each measuring plane, the infl uence of the pressure fl uctuations of each blade row can be recognized in terms of their upstream and downstream infl uence It was concluded that Experimental Unsteady Secondary Flow Phenomena in an Axial Compressor 373 the infl uence of the rotor blades on the unsteady . Multistage, Unsteady Flow, Off-Design, Experimental Inves- tigation 369 Unsteady Aerodynamics, Aeroacoustics and Aeroelasticity of Turbomachines, 369– 380 . © 2006 Springer. Printed in the Netherlands. (eds.), et. unterstanding of the flow phenomena and to provide a sound database for the validation of unsteady numerical flow solvers and particular transition models, the angle of the incoming wake is of minor. pressure ratio of 2.03, and a mass flow of 13.4kg/s at a rotational speed of 17000 RPM. With a circumferential tip speed of 345 m/s, the maximum rela- tive Mach number is 0 .89 at the tip of the first