A Test Case for the Numerical Investigation of Wake Passing Effects on a Highly Loaded LP Turbine Cascade Blade

2001 ◽  
Author(s):  
Peter Stadtmüller ◽  
Leonhard Fottner
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Suction Side ◽  
Test Case ◽  
Turbine Cascade ◽  
Hot Wire ◽  
Data Set ◽  
Lp Turbine ◽  
Wake Passing ◽  
Highly Loaded

The paper presents a compilation of experimental data on the effects of wake-induced transition on a highly loaded LP turbine cascade intended to be used for further numerical work. Although the underlying physics is not yet completely understood, the benefits of wake passing are already known and employed in the design process of modern gas turbines. For further optimizations, the next step seems to be now to enable numerical simulations detailed enough to capture the major effects while being as uncomplicated as possible at the same time to be cost-effective. The experimental results constituted in this systematic investigation are available for download and should serve as a basic data set for future calculations with different turbulence and transition models, thereby shedding some light on the complexity and modeling required for a suitable numerical treatment of the wake-induced transition process. The data introduced in this test case was acquired using a turbine cascade called T106D-EIZ with increased blade pitch compared to design point conditions in order to achieve a higher loading. A large separation bubble forms on the suction side and allows to study boundary layer development in great detail. The upstream blade row was simulated by a moving bar type wake generator. The measurements comprise hot wire data of the bar wake characteristics in the cascade inlet plane (velocity deficit and turbulence level), boundary layer surveys with surface-mounted hot films sensors and a hot wire probe at various locations and measurements of the total pressure loss coefficient. Unsteady pressure transducers are embedded into the suction side of a cascade blade and in a wake rake to resolve the local pressure distributions over time. They yield quantitative values easily comparable to the output of numerical simulations. The objective of this paper is to enable and to invite interested researchers to validate their code on the data set. From the extensive test program, a very limited number of operating points have been selected to focus the work. The standardized data files include a “reference” case with an exit Reynolds number of 200.000 and an exit Mach number of 0.4 as well as two points with higher Mach or lower Reynolds number for constant wake passing frequencies and background turbulence levels.

Unsteady Wake-Induced Boundary Layer Transition in High Lift LP Turbines

1998 ◽  
Vol 120 (1) ◽  
pp. 28-35 ◽  
Author(s):  
V. Schulte ◽  
H. P. Hodson
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Turbine Blade ◽  
Reynolds Numbers ◽  
Boundary Layer Development ◽  
Unsteady Wake ◽  
Lp Turbine ◽  
Wake Passing ◽  
Layer Development ◽  
Highly Loaded

The development of the unsteady suction side boundary layer of a highly loaded LP turbine blade has been investigated in a rectilinear cascade experiment. Upstream rotor wakes were simulated with a moving-bar wake generator. A variety of cases with different wake-passing frequencies, different wake strength, and different Reynolds numbers were tested. Boundary layer surveys have been obtained with a single hotwire probe. Wall shear stress has been investigated with surface-mounted hot-film gages. Losses have been measured. The suction surface boundary layer development of a modern highly loaded LP turbine blade is shown to be dominated by effects associated with unsteady wake-passing. Whereas without wakes the boundary layer features a large separation bubble at a typical cruise Reynolds number, the bubble was largely suppressed if subjected to unsteady wake-passing at a typical frequency and wake strength. Transitional patches and becalmed regions, induced by the wake, dominated the boundary layer development. The becalmed regions inhibited transition and separation and are shown to reduce the loss of the wake-affected boundary layer. An optimum wake-passing frequency exists at cruise Reynolds numbers. For a selected wake-passing frequency and wake strength, the profile loss is almost independent of Reynolds number. This demonstrates a potential to design highly loaded LP turbine profiles without suffering large losses at low Reynolds numbers.

scholarly journals Unsteady Wake-Induced Boundary Layer Transition in High Lift LP Turbines

1996 ◽  
Author(s):  
Volker Schulte ◽  
Howard P. Hodson
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Turbine Blade ◽  
Reynolds Numbers ◽  
Boundary Layer Development ◽  
Unsteady Wake ◽  
Lp Turbine ◽  
Wake Passing ◽  
Layer Development ◽  
Highly Loaded

The development of the unsteady suction side boundary layer of a highly loaded LP turbine blade has been investigated in a rectilinear cascade experiment. Upstream rotor wakes were simulated with a moving-bar wake generator. A variety of cases with different wake-passing frequencies, different wake strength and different Reynolds-numbers were tested. Boundary layer surveys have been obtained with a single hot-wire probe. Wall shear stress has been investigated with surface-mounted hot-film gauges. Losses have been measured. The suction surface boundary layer development of a modern highly loaded LP turbine blade is shown to be dominated by effects associated with unsteady wake-passing. Whereas without wakes the boundary layer features a large separation bubble at a typical cruise Reynolds-number, the bubble was largely suppressed if subjected to unsteady wake-passing at a typical frequency and wake strength. Transitional patches and becalmed regions, induced by the wake, dominated the boundary layer development. The becalmed regions inhibited transition and separation and are shown to reduce the loss of the wake-affected boundary layer. An optimum wake-passing frequency exists at cruise Reynolds-numbers. For a selected wake-passing frequency and wake-strength, the profile loss is almost independent of Reynolds-number. This demonstrates a potential to design highly loaded LP turbine profiles without suffering large losses at low Reynolds-numbers.

The Effect of Pulsed Blowing on the Boundary Layer of a Highly Loaded Low Pressure Turbine Blade

2013 ◽  
Author(s):  
Marion Mack ◽  
Roland Brachmanski ◽  
Reinhard Niehuis
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Mach Number ◽  
Suction Side ◽  
Reynolds Numbers ◽  
Low Pressure ◽  
Trailing Edge ◽  
Low Pressure Turbine ◽  
Wide Range ◽  
Highly Loaded

The performance of the low pressure turbine (LPT) can vary appreciably, because this component operates under a wide range of Reynolds numbers. At higher Reynolds numbers, mid and aft loaded profiles have the advantage that transition of suction side boundary layer happens further downstream than at front loaded profiles, resulting in lower profile loss. At lower Reynolds numbers, aft loading of the blade can mean that if a suction side separation exists, it may remain open up to the trailing edge. This is especially the case when blade lift is increased via increased pitch to chord ratio. There is a trend in research towards exploring the effect of coupling boundary layer control with highly loaded turbine blades, in order to maximize performance over the full relevant Reynolds number range. In an earlier work, pulsed blowing with fluidic oscillators was shown to be effective in reducing the extent of the separated flow region and to significantly decrease the profile losses caused by separation over a wide range of Reynolds numbers. These experiments were carried out in the High-Speed Cascade Wind Tunnel of the German Federal Armed Forces University Munich, Germany, which allows to capture the effects of pulsed blowing at engine relevant conditions. The assumed control mechanism was the triggering of boundary layer transition by excitation of the Tollmien-Schlichting waves. The current work aims to gain further insight into the effects of pulsed blowing. It investigates the effect of a highly efficient configuration of pulsed blowing at a frequency of 9.5 kHz on the boundary layer at a Reynolds number of 70000 and exit Mach number of 0.6. The boundary layer profiles were measured at five positions between peak Mach number and the trailing edge with hot wire anemometry and pneumatic probes. Experiments were conducted with and without actuation under steady as well as periodically unsteady inflow conditions. The results show the development of the boundary layer and its interaction with incoming wakes. It is shown that pulsed blowing accelerates transition over the separation bubble and drastically reduces the boundary layer thickness.

Recognition of Structures Leading to Transition in a Low Pressure Turbine Cascade: Effect of Reduced Frequency

2019 ◽  
Author(s):  
D. Lengani ◽  
D. Simoni ◽  
M. Ubaldi ◽  
P. Zunino ◽  
F. Bertini
Keyword(s):  
Boundary Layer ◽  
Suction Side ◽  
Low Pressure ◽  
Turbine Cascade ◽  
Turbulent Structures ◽  
Time Resolved ◽  
Low Pressure Turbine ◽  
Reduced Frequency ◽  
Displacement Thickness ◽  
Wake Passing

Abstract The boundary layer developing over the suction side of a low pressure turbine cascade operating under unsteady inflow conditions has been experimentally investigated. Time-resolved Particle Image Velocimetry (PIV) measurements have been performed in two orthogonal planes, the blade to blade and a wall parallel plane embedded within the boundary layer, for two different wake reduced frequencies. Proper Orthogonal Decomposition (POD) has been used to analyze the data and to provide an interpretation of the most significant flow structures for each phase of the wake passing cycle. To this purpose, a POD based procedure that sorts the data synchronizing the measurements of the two planes has been developed. Phase averaged data are then obtained for both cases. Moreover, once properly sorted, POD has been applied to sub-ensembles of data at the same relative phase within the wake passing cycle. Detailed information on the most energetic turbulent structures at a particular phase are obtained with this procedure (called phased POD), overcoming the limit of classical phase average that just provides a statistical representation of the turbulence field. Furthermore, the synchronization of the measurements in the two planes allows the computation of the characteristic dimension of boundary layer structures that are responsible for transition. These structures are often identified as vortical filaments parallel to the wall, typically referred to as boundary layer streaks. The largest and most energetic structures are observed when the wake centerline passes over the rear part of the suction side, and they appear practically the same for both reduced frequencies. The passing wake forces transition leading to the breakdown of the boundary layer streaks. Otherwise, the largest differences between the low and high reduced frequency are observed in the calmed region. The post-processing of these two planes further allowed us to compute the spacing of the streaks and make it non-dimensional by the boundary layer displacement thickness observed for each phase. The non-dimensional value of the streaks spacing is about constant, irrespective of the reduced frequency.

Unsteady Surface Pressures Due to Wake Induced Transition in a Laminar Separation Bubble on a LP Turbine Cascade

2003 ◽  
Author(s):  
Rory Stieger ◽  
David Hollis ◽  
Howard Hodson
Keyword(s):  
Boundary Layer ◽  
Steady Flow ◽  
Coherent Structures ◽  
Separation Bubble ◽  
Pressure Fluctuations ◽  
Separation Point ◽  
Blade Surface ◽  
Turbine Cascade ◽  
Lp Turbine ◽  
Wake Passing

This paper presents unsteady surface pressures measured on the suction surface of a LP turbine cascade that was subject to wake passing from a moving bar wake generator. The surface pressures measured under the laminar boundary layer upstream of the steady flow separation point were found to respond to the wake passing as expected from the kinematics of wake convection. In the region where a separation bubble formed in steady flow, the arrival of the convecting wake produced high frequency, short wavelength, fluctuations in the ensemble averaged blade surface pressure. The peak-to-peak magnitude was 30% of the exit dynamic head. The existence of fluctuations in the ensemble averaged pressure traces indicates that they are deterministic and that they are produced by coherent structures. The onset of the pressure fluctuations was found to lie beneath the convecting wake and the fluctuations were found to convect along the blade surface at half of the local freestream velocity. Measurements performed with the boundary layer tripped ahead of the separation point showed no oscillations in the ensemble average pressure traces indicating that a separating boundary layer is necessary for the generation of the pressure fluctuations. The coherent structures responsible for the large amplitude pressure fluctuations were identified using PIV to be vortices embedded in the boundary layer. It is proposed that these vortices form in the boundary layer as the wake passes over the inflexional velocity profiles of the separating boundary layer and that the rollup of the separated shear layer occurs by an inviscid Kelvin-Helmholtz mechanism.

Trailing Edge Thickness Impact on the Profile Losses of Highly Loaded Low Pressure Turbines Blades

2016 ◽  
Author(s):  
Jorge Parra ◽  
David Cadrecha ◽  
Ezequiel González ◽  
Benigno Lázaro
Keyword(s):  
Boundary Layer ◽  
Diffusion Rate ◽  
Lift Coefficient ◽  
Suction Side ◽  
Low Pressure ◽  
Full Potential ◽  
Hot Wire ◽  
Trailing Edge ◽  
Diffusion Factor ◽  
Highly Loaded

The losses breakdown of modern highly loaded low pressure turbines profiles shows that the trailing edge thickness can account for up to 20% of the overall profile loss depending on the thickness to pitch ratio highly affecting to the LPT overall performance. Additionally, this feature is of significant practical interest as the aerofoil mechanical behaviour and manufacturing costs are largely determined by the size of the trailing edge. Current trailing edge loss models are based on correlations derived from measurements on aerofoils very different with respect to the current state-of-the-art, they do not consider any effect of Reynolds number or lift coefficient, so it is questionable whether they are accurate enough for current applications and therefore an experimental validation campaign is required. The aim of the present experimental investigation is to examine the influence of that geometrical parameter on the unsteady Reynolds lapse characterization by means of four different low speed linear cascades varying the thickness from 50% to 200% of a nominal case. Cascades A, B and C (with small, nominal and large thickness) meet the same lift coefficient reducing the back surface diffusion factor due to the different velocity at the trailing edge because of the blockage generated by the trailing edge thickness. Cascade B2, with nominal thickness, is modified to meet the same diffusion factor as Cascade A to decouple the effect of the diffusion factor from the effect of the trailing edge thickness. Total pressure probes, Laser-Doppler and hot wire anemometry are used to characterize the suction side boundary layer just upstream from the trailing edge as well as the near wake developing close to the trailing edge. Additional characterisations are conducted at half chord downstream from the cascade trailing edge to evaluate its loss coefficient. Upstream located moving bars are used to simulate the incoming wakes shed by one upstream blade row. The hot wire measurements performed slightly upstream from the profile trailing edge are post-processed locked to the passage of the moving bars. The resulting data are analysed to characterise the temporal modulation of the suction side boundary layer momentum thickness by the incoming wakes. The measurements indicate that both the time-mean value and the phase-averaged distribution of the boundary layer integral parameters are largely determined by the diffusion rate of the profile. On the other hand, a negligible effect of the trailing edge thickness is observed for the same diffusion rate. The measurements conducted downstream from the profile, both close to its trailing edge and half chord downstream, illustrate the role of the trailing edge thickness on the initial wake development. The data is recorded for 60s with a sampling rate of 25kHz obtaining between 150 and 650 phase-locked datasets depending on the Reynolds No. Finally, the characterisation of the profile mix-out losses at the downstream plane is presented. The experimental results show that a significant reduction of losses can be achieved with thinner trailing edge, but, an increase in the number of aerofoils need to be allowed in order to get the full potential benefit of this strategy.

Investigation of Low Solidity LP Turbine Cascade With Flow Control: Part 2—Passive Flow Control Using Gurney-Flap

2010 ◽  
Author(s):  
Ping-Ping Chen ◽  
Wei-Yang Qiao ◽  
Hua-Ling Luo
Keyword(s):  
Boundary Layer ◽  
Flow Control ◽  
Separation Bubble ◽  
Suction Side ◽  
Boundary Layer Separation ◽  
Turbine Cascade ◽  
Passive Flow Control ◽  
Passive Flow ◽  
Gurney Flap ◽  
Lp Turbine

Numerical simulations were performed to investigate the effects of a passive flow control device named Gurney-flap on the laminar separation bubble and associated losses, aiming at assessing the feasibility of designing low solidity and highly-loaded LP turbine cascade with Gurney-flap. It was shown that with appropriate Gurney-flap the turbine cascade solidity could be decreased by 12.5% without loss increase. The deflection of the cascade mainstream due to Gurney flap can accelerate the flow at suction side of the adjacent blade, and decrease the adverse pressure gradient within the diffusion zone, which delay the boundary layer separation, thin the separation bubble and delay transition onset, contributing to reductions of both the separation-bubble-generated loss and the turbulent boundary-layer-generated loss. The numerical results indicate that the Gurney-flap height and type have significant impacts on the cascade performance, and the round Gurney-flap is the optimal flap type being the most effective for the reduction of flow losses.

Investigation of Low Solidity LP Turbine Cascade With Flow Control: Part 1—Active Flow Control Using Jet-Flap

2010 ◽  
Author(s):  
Ping-Ping Chen ◽  
Wei-Yang Qiao ◽  
Hua-Ling Luo ◽  
Farhan Ali Hashmi
Keyword(s):  
Boundary Layer ◽  
Flow Control ◽  
Boundary Layer Separation ◽  
Turbine Cascade ◽  
Suction Surface ◽  
Layer Separation ◽  
Turbine Cascades ◽  
Lp Turbine ◽  
The Impact ◽  
Highly Loaded

Increasing the airfoil lift and decreasing the solidity of turbine cascade are the effective ways to decrease blade count which lead to the reduction of weight and hardware cost of gas turbine in aircraft engine. The challenge with this effort is to prevent the flow separation on blade suction surface and to keep the efficiency at high levels. Recent investigations on the blade-flap have demonstrated dramatic reduction in the separation losses of turbine. It would be very attractive to integrate the blade-flap in the design of enhanced loaded turbine. The critical science that will enable this design innovation is a comprehensive understanding of the effect of flow control device on the boundary layer separation. The purpose of the present work was to investigate the impact of turbine cascade solidity on loss mechanisms (airfoil lift level) and to study the feasibility to develop low solidity and highly loaded LP turbine cascade blade using blade flap. This paper is the Part I of the study concerned with performance improvement of low solidity and highly loaded LP turbine cascade blade with jet-flap. The Part II is concerned with the Gurney-flap. Investigation on three turbine cascades with same type of airfoil but different solidity is presented in this paper. These turbine cascades are all constructed with the P&W LPTs highly loaded airfoil Pack B. Two dimensional steady Reynolds-averaged Navier-Stokes equations are solved for the flow of these cascades. It is shown that appropriate jet flap could decrease turbine cascade solidity about 12.5% without the considerable increase in loss, the flow deflection of the turbine cascade mainstream can be increased by jet-flap, and then contribute to increased blade loading. Because of the augmented deflection of the cascade mainstream, the flow velocity at suction side of the adjacent blade increases. This results in extension of the flow accelerating region and reduction of flow diffusion on the blade suction surface, consequently there is a delay in the boundary layer separation and/or makes the reattachment point advanced. In fact, the neighboring blade boundary layer flow is affected by the deflection of the mainstream, not on the flow of local boundary directly.

Experimental and Numerical Investigation of Wake-Induced Transition on a Highly Loaded LP Turbine at Low Reynolds Numbers

2000 ◽  
Author(s):  
Peter Stadtmüller ◽  
Leonhard Fottner ◽  
Andreas Fiala
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Good Description ◽  
Suction Side ◽  
Boundary Layer Transition ◽  
Special Focus ◽  
Transitional Region ◽  
Unsteady Boundary Layer ◽  
Low Reynolds ◽  
Lp Turbine

Experimental and numerical results of LP turbine cascade tests performed to investigate wake interaction effects on boundary layer transition are presented. The data obtained at different inlet flow angles, turbulence levels and Mach numbers are compared and discussed with special focus on low Reynolds number conditions. For the boundary layer, calculated propagation velocities of disturbances are introduced to explain the transition process on the suction side of the blade over time. Using a moving bar wake generator and surface-mounted hot films as well as surface pressure tappings, the effects of periodic wake passing were studied in the High-Speed Cascade Wind Tunnel on the aft-loaded LPT profile T106. Blade pitch was increased as compared with design point conditions to achieve a higher blade loading. As a result, a large separation bubble formed on the suction side of the surface and allowed unsteady boundary layer development to be studied in great detail. Starting at a characteristic Reynolds number, massive separation occurred on the suction side under steady state conditions, i.e. the boundary layer was unable to reach the back pressure at the trailing edge. By using the wake generator, it was possible to reduce this separation and thus decrease profile pressure losses by 50%. The primary objective of the study was to provide unsteady ensemble-averaged hot film data together with information on the wake induced path, sufficient for the validation of numerical simulations. Such a simulation of the experiment was conducted using the Unsteady Boundary Layer Interaction Method, which takes into account the influence of boundary layer displacement on the velocity distribution and the time-dependent turbulence level in the outflow. The computations provide a good description of the wall shear stress in the transitional region and are in good agreement with the experimental data. By plotting propagation directions of boundary layer disturbances in space-time diagrams, it is shown that one characteristic direction is deviated around the so-called becalmed region and the temporarily separated region into the wake-induced transitional region.

Separation in oscillating laminar boundary-layer flows

1971 ◽  
Vol 47 (1) ◽  
pp. 21-31 ◽  
Author(s):  
R. A. Despard ◽  
J. A. Miller
Keyword(s):  
Experimental Data ◽  
Boundary Layer ◽  
Experimental Investigation ◽  
Reynolds Number ◽  
Reverse Flow ◽  
Oscillation Amplitude ◽  
Hot Wire ◽  
Boundary Layer Flows ◽  
Laminar Boundary Layers ◽  
History Of

The results of an experimental investigation of separation in oscillating laminar boundary layers is reported. Instantaneous velocity profiles obtained with multiple hot-wire anemometer arrays reveal that the onset of wake formation is preceded by the initial vanishing of shear at the wall, or reverse flow, throughout the entire cycle of oscillation. Correlation of the experimental data indicates that the frequency, Reynolds number and dynamic history of the boundary layer are the dominant parameters and oscillation amplitude has a negligible effect on separation-point displacement.

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