B01: Active flow control of stator cascades at periodically-transient boundary conditions
Prof. D. Peitsch  (mail )
Prof. R. Liebich  (mail )
R.A.: Jan Mihalyovics, Dipl.-Ing. 
phone: +49 (0)30 314-29481
R.A.: Tobias Werder, M.Sc.  (mail)
phone: +49 (0)30 314-22941
R.A.: Carola Ebert, M.Sc.  (mail)
phone: +49 (0)30 314-21559
The main objective of subproject (SP) is to modify the flow by means of active flow control at a compressor stage in such a way that it meets the requirements of the periodically unsteady boundary conditions caused by pressure-increasing combustion concepts. Pressure-increasing combustion concepts are known to introduce additional unsteady pressure fluctuations into the flow field, which in turn promote separation phenomena, especially on the stator blades of a compressor stage. The compressor profiles investigated in this SP are characteristic industry relevant geometries, such as those that may be used in the mid stages of a future high-pressure compressor.
In general, it is found that strong three-dimensional secondary flow structures form in an annular blade grid at the blade-wall transition. Due to the higher loading, these are more pronounced on the hub side than on the casing side. In order to further develop the actuation and control concepts of the first funding period, this asymmetrical flow field is simulated by means of a contoured side walls in the in the low-speed linear cascade .
At the low-speed annular cascade, a 3D-optimized stator blading is developed to relieve the hub area, enabling a better point of attack for active flow control approaches. In addition to periodic throttling due to the pressure-increasing combustion concepts, the influence of the rotor-stator interaction on the active flow control is being investigated. Furthermore, the trailing edge blowing developed in SP B06  will be transferred to the low-speed annular cascade and tested in combination with the sidewall blowing. Additionally, experimental and numerical investigations of different concepts of a piezo adaptive blade will be validated and verified at the modified low-speed linear cascade. The findings from the low-speed linear cascade are used for the concept and design of a measuring section for the low-speed annular cascade. This test section shall enable later investigations with a suitable piezo-adaptive vane in a transient annular flow.
3rd funding period 2020 - 2024
Within the third funding period, the results of previous funding periods are to be transferred to realistic turbomachine conditions. In compressor stages, the incident flow conditions of a stator operating in the transonic Mach number range are Ma∞ > 0.5. In order to adapt the actuation system to these compressible flow conditions and to achieve effective flow control at high velocity ratios, it is necessary to generate actuator jet speeds in excess of Majet >1
- Fig.1: Numerical flow simulation of the internal flow field of the SWJ actuator (animated).
- © TUB
Preparatory work for the third funding period included the design and basic investigation of the functionality of self-switching fluidic actuators for their usage in compressible flows. The internal flow field of a Sweeping Jet (SWJ) actuator was investigated by means of numerical flow simulation and validated using experimental methods. It could be shown that the periodic oscillating output of the SWJ actuator is capable of reaching a Mach number of Mapeak = 1.6 at a characteristic switching frequency of f ≈ 1200Hz. The operation of the SWJ actuator is visualized by the result of a numerical simulation shown in Figure 1.
Conducting wind tunnel experiments the investigated actuator concept was used for active flow control investigations using a test section with the geometry of a half-diffusor ramp in transonic flow conditions. Using oil-flow visualization techniques and surface pressure measurements, it was shown that the pressure-induced flow separation in the mid-section of the half-diffusor ramp could be suppressed successfully using active flow control. Additionally, an advantageous influence onto the flow field was achieved.
2nd funding period 2016 - 2020
Low-speed linear cascade
In the second funding period, the low-speed linear cascade was extended to a 2.5D stator cascade by means of a one-sided contoured sidewall mounted into the measuring section. These measures aim to create an asymmetric flow in the 2.5D stator passage, which is comparable to the complex flow topology of the 3D-low-speed annular cascade. With this as a basis, the actuator concepts from the first funding period were adapted to the asymmetric vortex system.
- Fig 2: Pressure measurements on a blades suction side with (right) and without (left) AFC.
- © TUB
By using active flow control (AFC) on the contoured sidewall, the formation of the dominant corner vortex could be reduced and its expansion over the blade height was minimized. The positive influence of the secondary flow effects are causing an unblocking of the stator passage. As a result, the rise of static pressure at the trailing edge of the stator blades is increased when compared to the basic flow (see Figure 2). It could thus be shown that the asymmetrical, disturbed flow field in the 2.5D stator passage can successfully be stabilized by means of AFC.
In cooperation with subproject B06 , a suitable control concept for the active flow control system in the 2.5D stator passage is being added. For this purpose, a second actuator is implemented at each stator passage on the straight side wall, which additionally increases the pressure build-up.
Within the project, flow control methods are investigated both on an optimized and a non-optimized stator design in order to reduce areas of separation on these. Flow control is reached by the injection of pulsed compressed air, already used in previous periods of the CRC. In addition to this work, the trailing edge actuation, which was already developed in the first funding period on the linear cascade of subproject B06 , was adapted for an application in the low speed annular cascade.
- Fig 3: Instationary pressure field of the blade sucction side side during periodic throttling (animated).
- © TUB
In previous investigations on the low speed linear cascade, it was shown that a throttling of the compressor stator by sequential blocking of 90% of the stator passage causes significant incidence variations. In the stator passage of the low speed annular grid, this results in an open boundary layer separation (corner separation), which occurs on the suction side of a compressor blade both in the throttled and unthrottled case. The fluctuating expansion of the corner separation due to periodic throttling could be observed and quantified by pressure measurements on the suction side of the blade. It could also be found, that pulsed blowing introduced upfront of the onset of the corner separation (at 20% of the relative suction side coordinate s) significantly reduced the corner separation.
In the first funding period, the required turning for the stator grid of the 3D low speed annular cascade was generated by means of variable inlet guide vanes. Within the scope of a first design of a compressor rotor stage in the second funding period, an axial compressor rotor was systematically designed with an analytical solution for the radial equilibrium. Subsequently, a parameter study was carried out for several blade sections in order to select the most efficient profiles. With the help of a parametric blade model, the rotor was then improved by means of a response surface based pareto optimization. All investigated design candidates met the required outflow angle profile in a satisfactory manner and were superior to the initial design with respect to operating range and losses.
One design candidate achieved the maximum total pressure build-up at a mass flow rate of 8.0 kg/s and had significantly lower losses than the initial design, thus achieving the optimization goal.
In the first funding period of the CRC 1029, a piezo-adaptive compressor blade was developed within subproject B02. Using piezoelectric transducers bonded to the inside of a hollow controlled diffusion airfoil (CDA) as compressor stator blade, the blade is enabled to vibrate the nose part and thus react actively upon periodic incidence fluctuations.
To simulate these periodic pressure fluctuations, a throttling system was positioned in the wake of the low speed linear cascade of the aerodynamics department. The throttling device must cover a frequency range of 3 - 70 Hz, which corresponds to Strouhal numbers of 0.018 - 0.42 at an inflow velocity of 25 m/s and a chord length of 0.15 m. The piezo-adaptive compressor blade was manufactured from plastic using an additive manufacturing process. Thus, while the plastic blade operated at its natural frequency it had a sufficient influence on the flow which was throttled at the same frequency. A further advantage of the plastic blade is the greater deflection amplitude even beyond its natural frequency.
In the first wind-tunnel investigations using the new throttling system, it was found that high frequencies have a decreasing influence on the blade’s pressure distribution. Up to a throttling frequency of 10 Hz these influences are still clearly visible. With the aid of a closed-loop control system, the pressure disturbances on the blade surface can be reduced. Here, the excitation is not controlled by phase shift, but the maximum amplitudes of the pressure are sought and the phase is then adjusted accordingly. As soon as the blade is in phase with the disturbance, the measured pressure fluctuations are only half as large as with a phase shift of e.g. 180 degrees.
In preparation for a third funding period, an extension of this concept from a 2D application to a 3D low speed annular cascade was developed. For this purpose, a measuring section was designed, where the piezo-adaptive blades can be easily installed and removed. This measuring section is designed analogous to the 3D low speed annular cascade.
Brück, C., Mihalyovics, J. and Peitsch, D. (2018). Experimental Investigations On Highly Loaded Compressor Airfoils With Different Active Flow Control Parameters Under Unsteady Flow Conditions  GPPS North America Conference 2018, At Montrea
Brück, C., Tiedemann, C. and Peitsch, D. (2016). Experimental Investigations on Highly Loaded Compressor Airfoils With Active Flow Control Under Non-Steady Flow Conditions in a 3D-Annular Low-Speed Cascade  ASME Turbo Expo 2016: Turbomachinery Technical Conference and Exposition
Mihalyovics, J., Brück, C., Peitsch, D., Vasilopoulos, I. and Meyer, M. (2018). Numerical and Experimental Investigations on Optimized 3D Compressor Airfoils  ASME Turbo Expo 2018: Turbomachinery Technical Conference and Exposition, At Oslo, Norway, V02AT39A038.
Phan, T. D., Springer, P. and Liebich, R. (2017). Numerical Investigation of an Elastomer-Piezo-Adaptive Blade for Active Flow Control of a Nonsteady Flow Field Using Fluid–Structure Interaction Simulations . Journal of Turbomachinery, 091004.
Staats, M., Mihalyovics, J. and Peitsch, D. (2019). A Qualitative Comparison of Unsteady Operated Compressor Stator Cascades with Active Flow Control . Active Flow and Combustion Control 2018, 91–104.
Staats, M. and Nitsche, W. (2017). Experimental Investigations on the Efficiency of Active Flow Control in a Compressor Cascade With Periodic Non-Steady Outflow Conditions ASME Turbo Expo 2017: Turbomachinery Technical Conference and Exposition
Ebert, C., J. Mihalyovics, M. Staats, J. Weiss und D. Peitsch (2019). Numerical and Experimental Investigationsfor Super Sonic Active Flow Control in the transonic Mach regime . Proceedings of the 68th Deutscher Luft- und Raumfahrtkongress 2019, Bonn, 2019. Deutsche Gesellschaft für Luft- und Raumfahrt.
Mihalyovics, J., J. Gambel und D. Peitsch (2019). 3D Profile Design and Optimization of a Rotor for a Low SpeedAxial Compressor Wind Tunnel Proceedings of the 68th Deutscher Luft- und Raumfahrtkongress 2019, Bonn, 2019. Deutsche Gesellschaft für Luft- und Raumfahrt.
Tiedemann, C. u. A. Heinrich (2017). Increasing Blade Turning by Active Flow Control and Tandem Configurati-ons: A Comparison . Proceedings of the 23rd International Symposium on Air Breathing Engines (ISA-BE 2017): Economy, Efficiency and Environment. International Society of Air-breathing Engines
Staats, M. und W. Nitsche: Active Flow Control on a Non-steady Operated Compressor Stator Cascade by Means of Fluidic Devices. In: Dillmann, A., G. Heller, E. Krämer, C. Wagner, S. Bansmer, R. Radespiel und R. Semaan (Hrsg.): New results in numerical and experimental fluid mechanics XI, Bd. 136 d. Reihe Notes on Numerical Fluid Mechanics and Multidisciplinary Design, S. 337–347.Springer, Cham, 2018, ISBN 978-3-319-64519-3
V. Motta, L. Malzach, D. Peitsch, G. Quaranta: A Physically Consistent Reduced Order Model for Plasma Aeroelastic Control on Compressor Blades. In: ASME Turbo Expo 2018: Turbomachinery Technical Conference and Exposition, Volume 7C: Structures and Dynamics, 2018.
1st Funding period 2012 - 2016
flow control in a compressor stator under highly unsteady inflow and
The overall efficiency of modern gas turbines is approximately 40% and was raised in small increments only. One approach to increase efficiency currently under investigation includes the modification of the combustion process in the machine. By employing other combustion concepts such as pressure gaining combustion, the efficiency of a gas turbine could be increased.
In the design of a pulsed detonation engine can-annular combustion chambers are used, of which the inlets are closed with a mechanical shutter during combustion to prevent backflow. Closing of the combustion chamber, however, obstructs the flow in the preceding turbomachinery components, mainly on the stator vanes of the last compressor stage.
- Fig. 1: Cascade Testrig
- © TUB
Within the B01 project we conduct experiments on a periodically unsteady compressor stator flow of the type which would be expected in consequence of pulsed combustion. The experiments are conducted on a highly loaded compressor stator cascade consisting of seven stator blades. The cascade test rig, as it is shown in Figure 1, is operated under low speed conditions at a Reynolds number of Re = 600000. Additionally, a 3D annular wind tunnel will allow for an analysis of the flow at more turbomachinery-like conditions based on the knowledge gained by the 2D cascade.
On this setup a periodic outflow condition is imposed, inducing periodical choking of every passage at Strouhal numbers up to Sr = 0.04. Static pressure measurements on the blade surface and 2D/3C Particle Image Velocimentry (PIV) measurements qualify the baseline flow field under the influence of periodical choking with a focus on the development of the secondary flow structures in the passage and the formation of a laminar separation bubble on the blade. Measurements in the wake of the center blade with a five hole probe are evaluated to determine the phase dependent performance degradation of the stator passage. Figure 2 depicts the highly dynamic flow development in the measurement passage. The disturbance of the neighboring passages induces changes regarding the incidence angle and thereby induce periodic flow separation on the blade’s trailing edge.
- Fig. 2: Dynamic Passage Flow (animated)
- © TUB
Pulsed air jets emanating from rectangular orifices along the sidewalls of the passages are used for active flow control (AFC) purposes so far. The effect of AFC parameters can be evaluated in terms of reduction of total pressure loss and raise in static pressure across the center passage. It is found that the heavily disturbed uncontrolled flow field stabilizes under the influence of AFC. This implies favorable effect on total pressure loss and static pressure recovery. In future the integration of a blade actuator system will further increase the performance and hence the stability of the compressor. The highly dynamic flow structures regard different AFC parameters for every phase angle. In close cooperation with the B06 project within this collaborative research center 1029 we conduct experiments using different control algorithms.
- Fig. 3: Three-dimensional Test Rig
- © TUB
In addition to the 2D cascade test rig, rotating bars will be implemented in a three-dimensional test rig producing incoming wakes to the compressor stator. The wake generator is variable in the number of bars and speed to investigate different flow coefficients and blade passing frequencies. Thereby the interaction between the wakes and the periodic choking can be analyzed using independent parameter setting. The AFC concept used in the linear cascade will be adapted to the three-dimensional test rig.
M. Staats and W. Nitsche and I. Peltzer, 2015. “Active Flow Control on a Highly Loaded Compressor Cascade with Non-Steady Boundary Conditions”. In Active Flow and Combustion Control 2014, King, R., ed., Vol. 127 of Notes on Numerical Fluid Mechanics and Multidisciplinary Design. Springer International Publishing, pp. 23–37.
S. J. Steinberg, M. Staats, W. Nitsche, R. King, 2015. “Comparison of Iterative Learning and Repetitive Control Applied to a Compressor Stator Cascade”. In Active Flow and Combustion Control 2014, King, R., ed., Vol. 127 of Notes on Numerical Fluid Mechanics and Multidisciplinary Design. Springer International Publishing, pp. 39–53.
M. Staats and W. Nitsche, ASME TurboExpo2015 “Active Control of the Corner Separation on a Highly Loaded Compressor Cascade with Periodic non-Steady Boundary Conditions by Means of Fluidic Actuators”
S. J. Steinberg, M. Staats, R. King and W. Nitsche, ASME TurboExpo2015 “ Iterative Learning Active Flow Control Applied to a Compressor Stator Cascade with Periodic Disturbances“
SpeakerProf. Dr.-Ing. Dieter Peitsch
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+49 (30) 314 22954
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- Poster B01 (2nd period) 
- Poster B01 (linear cascade) 
- Poster B01 (annular cascade) 
- Poster B01 (piezo blade)