Transient Performance of Separated Flows: Characterization and Active Flow Control

2018 ◽  
Vol 141 (1) ◽  
Author(s):  
J. Saavedra ◽  
G. Paniagua
Keyword(s):  
Boundary Layer ◽  
Flow Control ◽  
Active Flow Control ◽  
Boundary Layer Separation ◽  
Low Pressure ◽  
Separated Flows ◽  
Mean Flow ◽  
Recirculation Bubble ◽  
Layer Separation ◽  
Inlet Conditions

The aerothermal performance of the low-pressure turbine in unmanned aerial vehicles is significantly abated at high altitude, due to boundary layer separation. Different flow control strategies have been proposed to prevent boundary layer separation, such as dielectric barrier discharges (DBD) and synthetic jets. However, the optimization of the control approach requires a better characterization of the separated regions at transient conditions. The present investigation analyzes the behavior of separated flows, reporting the inception and separation length, allowing the development of efficient flow control methods under nontemporally uniform inlet conditions. The development of separated flows was investigated with numerical simulations including Unsteady Reynolds average Navier–Stokes (URANS) and large Eddy simulations (LES). The present research was performed on a wall-mounted hump, which imposes a pressure gradient representative of the suction side of low pressure turbines. Through sudden flow accelerations, we looked into the dynamic response of the shear layer detachment as it is modulated by the mean flow evolution. Similarly, we studied the behavior of the recirculation bubble under periodic disturbances imposed at various frequencies ranging from 10 to 500 Hz, at which the Reynolds number oscillates between 40,000 and 180,000. As a first step into the flow control, we added a slot to allow flow injection and ingestion upstream of the separation inception. Exploring the behavior of the separated region at different conditions, we defined the envelope for its periodic actuation. We found that by matching the actuator frequency with the frequency response of the separated region, the performance of the actuation is boosted.

Transient Performance of Separated Flows: Characterization and Active Flow Control

2018 ◽  
Author(s):  
J. Saavedra ◽  
G. Paniagua
Keyword(s):  
Boundary Layer ◽  
Flow Control ◽  
Boundary Layer Separation ◽  
Low Pressure ◽  
Separated Flows ◽  
Shear Stresses ◽  
Recirculation Bubble ◽  
Control Approach ◽  
Layer Separation ◽  
Inlet Conditions

The aerothermal performance of the low pressure turbine in UAVs’ is significantly abated at high altitude, due to boundary layer separation. During past years different flow control strategies have been proposed to prevent boundary layer separation, such as dielectric barrier discharges, synthetic jets, vortex generators. However, the optimization of the control approach requires a better characterization of the separated regions at several frequencies. The present investigation analyzes the behavior of separated flows, and specifically reports the inception, reattachment and separation length, that allows the development of more efficient methods to manipulate flow separation under non-tempo-rally uniform inlet conditions. The development of separated flows under sudden flow accelerations or pulsating inlet conditions were investigated with series of numerical simulations including Unsteady Reynolds Average Navier Stokes and Large Eddy Simulations. The present research was performed on a wall mounted hump, which imposes an adverse pressure gradient representative of the suction side of low pressure turbines. The heat transfer and wall shear stresses were fully documented, as well as the flow velocity and temperature profiles at different axial locations to characterize the near wall flow properties and the thermal boundary layer. Through a sudden flow acceleration we looked into the dynamic response of the shear layer detachment as it is modulated by the mean flow evolution. Similarly, we studied the behavior of the recirculation bubble under periodic disturbances imposed by sinusoidal inlet total pressure signals at various frequencies ranging from 10 to 500 Hz. During each period the Reynolds number oscillates between 40000 and 180000 (based on a characteristic length of 0.1 m). Finally, as a first step into the flow control approach we added a slot in our geometry to allow flow injection and ingestion just upstream of the separation inception. Exploring the behavior of the separated region at different slot pressure conditions we defined the envelope for its periodic actuation. Thanks to that analysis, we found that matching the actuator frequency with the frequency response of the separated region the performance of the actuation is boosted.

Design of a Low Solidity Flow-Controlled Stator With Coanda Surface in a High Speed Compressor

2008 ◽  
Author(s):  
Y. Guendogdu ◽  
A. Vorreiter ◽  
J. R. Seume
Keyword(s):  
Boundary Layer ◽  
Flow Control ◽  
High Speed ◽  
Design Method ◽  
Active Flow Control ◽  
Suction Side ◽  
Boundary Layer Separation ◽  
Synthetic Jets ◽  
Layer Separation ◽  
Active Flow

Aerofoil active flow control has been attempted to increase the permissible loading of boundary layers in gas turbine components. Steady suction and blowing, pulsing and synthetic jets are all means to remove low energy flow, replace momentum deficits, or promote mixing to inhibit boundary layer separation. A curved surface near the trailing edge (“Coanda surface”) is another technique used to control aerofoil boundary layer separation. This paper presents the design of a stator with active flow control for a high speed compressor using a Coanda surface. The Coanda surface is located behind an injection slot on the aerofoil suction side of the first stage of a four-stage high speed research compressor. The design method and the present results are based on steady numerical calculations. The design intent is to reduce the number of vanes. This active flow control is used to maintain the flow exit angle of the reference stator despite the resulting increase in stator loading. It is shown that the solidity of the flow-controlled stator can be decreased by 25% with a blowing rate of 0.5% of the main mass flow.

scholarly journals Control of Laminar Boundary-Layer Separation Using Steady and Harmonic Vortex Generator Jets

2018 ◽  
Author(s):  
Aria Alimi ◽  
Olaf Wünsch
Keyword(s):  
Boundary Layer ◽  
Flow Control ◽  
Laminar Boundary Layer ◽  
Active Flow Control ◽  
Vortex Generator ◽  
Boundary Layer Separation ◽  
Transition To Turbulence ◽  
Momentum Exchange ◽  
Layer Separation ◽  
Vortex Generator Jets

Active flow control of canonical laminar separation bubbles by steady and harmonic vortex generator jets (VGJs) was investigated using direct numerical simulations. Both control strategies were found to be effective in controlling the laminar boundary-layer separation. However, the present results indicate that using the same blowing amplitude, harmonic VGJs were more effective and efficient in reducing the separated region than the steady VGJs considering the fact that the harmonic VGJs use less momentum than the steady case. For steady VGJs, longitudinal structures formed immediately downstream of injection location led to formation of hairpin-type vortices causing an earlier transition to turbulence. Symmetric hairpin vortices were shown to develop downstream of the forcing location for the harmonic VGJs as well. However, the increased control effectiveness for harmonic VGJs flow control strategy is attributed to the fact that shear-layer instability mechanism was exploited. As a result, disturbances introduced by VGJs were strongly amplified leading to development of large-scale coherent structures, which are very effective in increasing the momentum exchange, thus, limiting the separated region.

scholarly journals Control of Laminar Boundary-Layer Separation Using Steady and Harmonic Vortex Generator Jets

2018 ◽  
Author(s):  
Aria Alimi ◽  
Olaf Wünsch
Keyword(s):  
Boundary Layer ◽  
Flow Control ◽  
Laminar Boundary Layer ◽  
Active Flow Control ◽  
Vortex Generator ◽  
Boundary Layer Separation ◽  
Transition To Turbulence ◽  
Momentum Exchange ◽  
Layer Separation ◽  
Vortex Generator Jets

Active flow control of canonical laminar separation bubbles by steady and harmonic vortex generator jets (VGJs) was investigated using direct numerical simulations. Both control strategies were found to be effective in controlling the laminar boundary-layer separation. However, the present results indicate that using the same blowing amplitude, harmonic VGJs were more effective and efficient in reducing the separated region than the steady VGJs considering the fact that the harmonic VGJs use less momentum than the steady case. For steady VGJs, longitudinal structures formed immediately downstream of injection location led to formation of hairpin-type vortices causing an earlier transition to turbulence. Symmetric hairpin vortices were shown to develop downstream of the forcing location for the harmonic VGJs as well. However, the increased control effectiveness for harmonic VGJs flow control strategy is attributed to the fact that shear-layer instability mechanism was exploited. As a result, disturbances introduced by VGJs were strongly amplified leading to development of large-scale coherent structures, which are very effective in increasing the momentum exchange, thus, limiting the separated region.

scholarly journals Analysis of the Effect of Vortex Generator Spacing on Boundary Layer Flow Separation Control

2019 ◽  
Vol 9 (24) ◽  
pp. 5495 ◽  
Author(s):  
Xin-kai Li ◽  
Wei Liu ◽  
Ting-jun Zhang ◽  
Pei-ming Wang ◽  
Xiao-dong Wang
Keyword(s):  
Boundary Layer ◽  
Wind Tunnel ◽  
Kinetic Energy ◽  
Flow Control ◽  
Flow Separation ◽  
Lift Coefficient ◽  
Boundary Layer Separation ◽  
Wind Tunnel Experiments ◽  
Flow Separation Control ◽  
Layer Separation

During the operation of wind turbines, flow separation appears at the blade roots, which reduces the aerodynamic efficiency of the wind turbine. In order to effectively apply vortex generators (VGs) to blade flow control, the effect of the VG spacing (λ) on flow control is studied via numerical calculations and wind tunnel experiments. First, the large eddy simulation (LES) method was used to calculate the flow separation in the boundary layer of a flat plate under an adverse pressure gradient. The large-scale coherent structure of the boundary layer separation and its evolution process in the turbulent flow field were analyzed, and the effect of different VG spacings on suppressing the boundary layer separation were compared based on the distance between vortex cores, the fluid kinetic energy in the boundary layer, and the pressure loss coefficient. Then, the DU93-W-210 airfoil was taken as the research object, and wind tunnel experiments were performed to study the effect of the VG spacing on the lift–drag characteristics of the airfoil. It was found that when the VG spacing was λ/H = 5 (H represents the VG’s height), the distance between vortex cores and the vortex core radius were approximately equal, which was more beneficial for flow control. The fluid kinetic energy in the boundary layer was basically inversely proportional to the VG spacing. However, if the spacing was too small, the vortex was further away from the wall, which was not conducive to flow control. The wind tunnel experimental results demonstrated that the stall angle-of-attack (AoA) of the airfoil with the VGs increased by 10° compared to that of the airfoil without VGs. When the VG spacing was λ/H = 5, the maximum lift coefficient of the airfoil with VGs increased by 48.77% compared to that of the airfoil without VGs, the drag coefficient decreased by 83.28%, and the lift-to-drag ratio increased by 821.86%.

Effect of Unsteady Wakes on Boundary Layer Separation on a Very High Lift Low Pressure Turbine Airfoil

2010 ◽  
Author(s):  
Ralph J. Volino
Keyword(s):  
Boundary Layer ◽  
Boundary Layer Separation ◽  
Low Pressure ◽  
Freestream Turbulence ◽  
Suction Surface ◽  
High Lift ◽  
Layer Separation ◽  
Low Pressure Turbine ◽  
Wake Passing ◽  
Very High

Boundary layer separation has been studied on a very high lift, low-pressure turbine airfoil in the presence of unsteady wakes. Experiments were done under low (0.6%) and high (4%) freestream turbulence conditions on a linear cascade in a low speed wind tunnel. Wakes were produced from moving rods upstream of the cascade. Flow coefficients were varied from 0.35 to 1.4 and wake spacing was varied from 1 to 2 blade spacings, resulting in dimensionless wake passing frequencies F = fLj-te/Uave (f is the frequency, Lj-te is the length of the adverse pressure gradient region on the suction surface of the airfoils, and Uave is the average freestream velocity) ranging from 0.14 to 0.56. Pressure surveys on the airfoil surface and downstream total pressure loss surveys were documented. Instantaneous velocity profile measurements were acquired in the suction surface boundary layer and downstream of the cascade. Cases were considered at Reynolds numbers (based on the suction surface length and the nominal exit velocity from the cascade) of 25,000 and 50,000. In cases without wakes, the boundary layer separated and did not reattach. With wakes, separation was largely suppressed, particularly if the wake passing frequency was sufficiently high. At lower frequencies the boundary layer separated between wakes. Background freestream turbulence had some effect on separation, but its role was secondary to the wake effect.

scholarly journals Visualization of boundary layer separation and passive flow control on airfoils and bodies in wind-tunnel and in-flight experiments

2012 ◽  
Vol 25 ◽  
pp. 01078
Author(s):  
Lukas Popelka ◽  
Jana Kuklova ◽  
David Simurda ◽  
Natalie Souckova ◽  
Milan Matejka ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Wind Tunnel ◽  
Flow Control ◽  
Boundary Layer Separation ◽  
Passive Flow Control ◽  
Layer Separation ◽  
Passive Flow

The Effects of Periodic Wakes on Boundary Layer Separation of Low-Pressure Turbine Using Large Eddy Simulation

2016 ◽  
Author(s):  
Xiaodi Wu ◽  
Fu Chen ◽  
Yunfei Wang
Keyword(s):  
Boundary Layer ◽  
Flow Field ◽  
Boundary Layer Separation ◽  
Low Pressure ◽  
Layer Separation ◽  
Low Pressure Turbine ◽  
Eddy Simulation ◽  
Instantaneous Flow Field ◽  
Large Eddy ◽  
Scale Structures

For low-pressure turbine, the unsteady disturbances are dominated by relative motions between rotors and stators and the unsteady flow is closely associated with aerodynamic efficiency of low-pressure turbine and engine performance. One of its most important manifestations is the boundary layer separation on the turbine blades by the passing wakes produced by upstream rows of blades. Hence, accurate prediction of the flow physics at low Reynolds number conditions is required to effectively implement flow control techniques which can help mitigate separation induced losses. The present paper concentrates on simulations for boundary layer separation of low-pressure turbine cascade under periodic wakes. In this paper, a multiblock computational fluid dynamics (CFD) code of compressible N-S equations is developed for predicting the phenomenon of boundary layer separation, transition and reattachment using large eddy simulation (LES) in the field of turbomachinery. The large-scale structures can be directly obtained from the solution of the filtered Naiver-Strokes equations and the small-scale structures are modeled by dynamic subgrid-scale model of turbulence. Firstly, unsteady boundary layer separation on a flat plate with adverse pressure gradient is simulated under periodic inflow. The time-averaged field, the phase-averaged field and the instantaneous flow field are presented and analyzed. The separation bubble becomes unstable and the location of transition moves back and forth due to vortex shedding. Secondly, a stator of turbomachinery which is influenced by wakes periodically passing is simulated. The results of the numerical simulations are discussed and compared with experimental data. For the instantaneous flow field, it seems that the spanwise vortices induced by upstream wakes are the primary reason of the initial roll-up of the shear layer and the Kelvin-Helmholtz instability plays an important role in the transition to turbulence which is observed in the separated flow.

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