Ali Mahallati

Aerodynamics of a Low-Pressure Turbine Airfoil at Low-Reynolds Numbers: Part 1 — Steady Flow Measurements

This two-part paper presents a detailed experimental investigation of the laminar separation and transition phenomena on the suction surface of a high-lift low-pressure (LP) turbine airfoil, PakB. The first part describes the influence of Reynolds number, freestream turbulence intensity and turbulence length scale on the PakB airfoil under steady inflow conditions. The present measurements are distinctive in that a closely-spaced array of hot-film sensors has allowed a very detailed examination to be made of both the steady and unsteady behaviour of the suction surface boundary layer. In addition, this paper presents a technique for interpreting the transition process in steady, and periodically unsteady, separated flows based on dynamic and statistical properties of the hot-film measurements. Measurements were made at Reynolds number varying from 25,000 to 150,000 and for freestream turbulence intensities of 0.4%, 2% and 4%. Two separate grids were used to generate turbulence intensity of 4% with integral length scales of about 10% and 40% of the airfoil axial chord length. The first is comparable with the turbulence length scales expected in the engine and the second is considerably larger. While the higher levels of freestream turbulence intensity promoted earlier transition and a shorter separation bubble, the varying turbulence length scale did not have a noticeable effect on the transition process. The size of the separation bubble increased with decreasing Reynolds number, and under low freestream turbulence levels the bubble failed to reattach at low Reynolds numbers. As expected, the losses increased with the length of the separation bubble on the suction side of the airfoil, and increased significantly when the bubble failed to reattach.Copyright

CONTROLLING CORNER STALL SEPARATION WITH PLASMA ACTUATORS IN A COMPRESSOR CASCADE

This paper presents a numerical and experimental assessment of a plasma actuation concept for controlling corner stall separation in a highly loaded compressor cascade. CFD simulations were first carried out to assess actuator effectiveness and determine the best actuation parameters. Subsequently, experiments were performed to demonstrate the concept and confirmed the CFD tool validity at a Reynolds number of 1.5 × 105. Finally, the validated CFD tool was used to simulate the concept at higher velocities, beyond the experimental capability of existing plasma actuators. These results were used to obtain a preliminary scaling law that would allow approximation of the plasma actuation requirements at realistic operating conditions. Several configurations were examined, but the most effective setup was found to be when plasma actuators were mounted upstream of the separation point on both the suction surface and the endwall. Most of the improvement in total pressure loss stemmed from the suction surface actuator. Comparison with experimental data showed that the CFD simulations could capture the flow features and the effect of plasma actuation reasonably well. Simulations at higher flow velocities indicated that the required plasma actuator strength scales approximately with the square of the Reynolds number.

Aerodynamics of a Low-Pressure Turbine Airfoil at Low Reynolds Numbers—Part I: Steady Flow Measurements

This two-part paper presents a detailed experimental investigation of the laminar separation and transition phenomena on the suction surface of a high-lift low-pressure turbine airfoil, PakB. The first part describes the influence of Reynolds number, freestream turbulence intensity and turbulence length scale on the PakB airfoil under steady inflow conditions. The present measurements are distinctive in that a closely-spaced array of hot-film sensors has allowed a very detailed examination of the suction surface boundary layer behavior. In addition, this paper presents a technique for interpreting the transition process in steady, and periodically unsteady, separated flows based on dynamic and statistical properties of the hot-film measurements. Measurements were made in a low-speed linear cascade facility at Reynolds numbers between 25,000 and 150,000 at three freestream turbulence intensity levels of 0.4%, 2%, and 4%. Two separate grids were used to generate turbulence intensity of 4% with integral length scales of about 10% and 40% of the airfoil axial chord length. While the higher levels of turbulence intensity promoted earlier transition and a shorter separation bubble, turbulence length scale did not have a noticeable effect on the transition process. The size of the suction side separation bubble increased with decreasing Reynolds number, and under low freestream turbulence levels the bubble failed to reattach at low Reynolds numbers. As expected, the losses increased with the length of the separation bubble, and increased significantly when the bubble failed to reattach.

Effects of Scalloping on the Mixing Mechanisms of Forced Mixers With Highly Swirling Core Flow

This paper presents a detailed experimental and computational investigation of the effects of scalloping on the mixing mechanisms of a scaled 12-lobe turbofan mixer. Scalloping was achieved by eliminating approximately 70% of the lobe sidewall area. Measurements were made downstream of the mixer in a co-annular wind tunnel, and the simulations were carried out using an unstructured Reynolds averaged Navier–Stokes (RANS) solver, Numeca FINE/Hexa, with k-ω SST model. In the core flow, the swirl angle was varied from 0 deg to 30 deg. At high swirl angles, a three-dimensional separation bubble was formed on the lobes suction surface penetration region and resulted in the generation of a vortex at the lobe valley. The valley vortex quickly dissipated downstream. The mixer lobes removed most of the swirl, but scalloped lobes removed less swirl in the region of the scalloped notch. The residual swirl downstream of the scalloped mixer interacted with the vortices and improved mixing rates compared to the unscalloped mixer. Core flow swirl up to 10 deg provided improved mixing rates and reduced pressure and thrust losses for both mixers. As core flow swirl increased beyond 10 deg, the mixing rate continued to improve, but pressure and thrust losses declined compared to the zero swirl case. Lobe scalloping, in high swirl conditions, resulted in better mixing and improved pressure loss over the unscalloped mixer but at the expense of reduced thrust.

Experimental and Numerical Investigation of Corner Stall in a Highly-Loaded Compressor Cascade

Three-dimensional corner stall is one of the most important factors limiting compressor performance. This paper presents a complementary experimental and computational study of corner stall in a highly-loaded compressor cascade subjected to three inlet boundary layer thicknesses, two levels of freestream turbulence intensity and two Reynolds numbers. Experiments included seven-hole pressure probe traverses, airfoil loading and surface oil flow visualization. Measurements were supplemented with the numerical predictions from a commercially available CFD code. It was found that the low momentum boundary layer on the endwall was unable to overcome the large streamwise adverse pressure gradient in this high-lift profile and turned sharply towards the midspan due to the strong cross-passage pressure gradient. The corner stall, with distinct regions of three-dimensional reversed flow, started at 50% chord and occupied a large area of the suction surface as well as the downstream passage. Only a small region of the inlet boundary layer, very close to the endwall seemed to play a role in the corner stall. As such, the flow in the endwall region was found to be nearly independent of the inlet boundary layer thickness, freestream turbulence intensity and Reynolds number. Based on the endwall flow structures, a new topology of corner stall for compressor cascades with high airfoil diffusion factor and high flow turning has also been proposed.Copyright

Effects of Area Ratio and Mean Rise Angle on the Aerodynamics of Inter-Turbine Ducts

The present work, a continuation of a series of investigations on the aerodynamics of aggressive inter-turbine ducts (ITD), is aimed at providing detailed understanding of the flow physics and loss mechanisms in four different ITD geometries. A systematic experimental and computational study was carried out for varying duct mean rise angles and outlet-to-inlet area ratio while keeping the duct length-to-inlet height ratio, Reynolds number and inlet swirl constant in all four geometries. The flow structures within the ITDs were found to be dominated by the counter-rotating vortices and boundary layer separation in both the casing and hub regions. The duct mean rise angle determined the severity of adverse pressure gradient in the casing’s first bend whereas the duct area ratio mainly governed the second bend’s static pressure rise. The combination of upstream wake flow and the first bend’s adverse pressure gradient caused the boundary layer to separate and intensify the strength of counter-rotating vortices. At high mean rise angle, the separation became stronger at the casing’s first bend and moved farther upstream. At high area ratios, a 2-D separation appeared on the casing. Pressure loss penalties increased significantly with increasing duct mean rise angle and area ratio.Copyright

The effect of stiffening tabs on the performance of lobed mixers at off-design conditions

This paper presents a computational investigation of scaled turbofan lobed mixer stiffening tabs at low-speed off-design conditions. Stiffening tabs provide rigidity to the thin lobed mixer by connecting the mixer valley to the more rigid centrebody. Evidence shows that the tabs affect the flow structures of turbofan exhaust systems at off-design core inlet swirl conditions. Observations were made downstream of the mixer in simulations that were carried out with an unstructured RANS solver and the k-ω SST turbulence model. To model off-design conditions, the core flow swirl was increased from axial flow to 10° at the moderate case and 30° at the high swirl case. The tab geometry was shown to perturb some of the less involved flow mixing structures, streamwise vortices near the lobe valley. Simulations of geometries with the tabs displayed more uniform flow throughout the common nozzle with higher thrust outputs; however, these minor improvements are negated by higher total pressure losses.

Controlling Corner Stall Separation With Plasma Actuators in a Compressor Cascade

This paper presents a numerical and experimental assessment of a plasma actuation concept for controlling corner stall separation in a highly-loaded compressor cascade. CFD simulations were first carried out to assess actuator effectiveness and determine the best actuation parameters. Subsequently, experiments were performed to demonstrate the concept and confirmed the CFD tool validity at a Reynolds number of 1.5×105. Finally, the validated CFD tool was used to simulate the concept at higher velocities, beyond the experimental capability of existing plasma actuators. These results were used to obtain a preliminary scaling law that would allow approximation of the plasma actuation requirements at realistic operating conditions. Several configurations were examined, but the most effective setup was found to be when plasma actuators were mounted upstream of the separation point on both the suction surface and the endwall. Most of the improvement in total pressure loss stemmed from the suction surface actuator. Comparison with experimental data showed that the CFD simulations could capture the flow features and the effect of plasma actuation reasonably well. Simulations at higher flow velocities indicated that the required plasma actuator strength scales approximately with the square of the Reynolds number.Copyright

Isolating Effects of Area Ratio From Lobe Number for Turbofan Engine Exhaust Systems

This paper presents a numerical investigation of lobed mixer performance at experimentally validated low speed conditions and conditions representative of high speed engine operation. The purpose of this study was to first assess and understand how variations in bypass-to-core area ratio (AR) can affect engine performance, then isolate those effects to determine the efficacy of increasing the number of mixer lobes. The area ratio was manipulated via adjustment of the lobe crest and valley radiuses. No other geometric features were altered in any of the 5 mixers studied (12-lobe AR of 3, 2.5 and 3.5, 16-lobe AR of 3 and 18-lobe AR of 3). Results indicate that performance can be affected by area ratio. Low-speed results showed that pressure loss and thrust output were improved at lower area ratios. High speed results showed the opposite. This behavior is believed to be the result of a bypass-to-core momentum ratio difference between the two test conditions. These effects were avoided when studying the number of lobes by maintaining a constant area ratio. Results indicate that adding lobes enhanced exhaust mixing but hampered performance at low speed conditions. No appreciable performance difference was observed at high speed conditions. Fluid viscosity and associated viscous mixing losses are believed to be the parameters at fault for the reduced low-speed performance results.Copyright

Influence of Inlet Swirl on the Aerodynamics of a Model Turbofan Lobed Mixer

This paper presents a detailed experimental investigation of the influence of core flow inlet swirl on the mixing and performance of a 12-lobe un-scalloped turbofan mixer. Measurements were made downstream of the mixer in a co-annular wind tunnel. The core-to-bypass velocity ratio was set to 2:1, temperature ratio to 1.0, and pressure ratio to 1.03, giving a Reynolds number of 5.2×105 , based on the core flow inlet velocity and equivalent hydraulic diameter. In the core flow, the background turbulence intensity was raised to 5% and the swirl angle was varied using five vane geometries, with nominally uniform swirl angles of 0°, 5°, 10°, 20° and 30°. Flow measurements captured flow structures involved in the mixing process. Most of mixing took place immediately downstream of the exit nozzle. The vane wake slightly enhanced large scale mixing of streamwise vortices. At low swirl angles, mixing was found to be mainly due to the interaction between streamwise vortices and normal vortices. At high swirl angles, the lobed mixer acted similar to a guide vane and removed most of the inlet swirl between the crest and trough of the mixer. However, the upstream swirling flow persisted in the core region between the center-body and lobed mixer trough, causing a reverse flow zone downstream of the centre-body. As the reversed flow became larger with increasing swirl, the swirling flow in the core region moved radially outwards and further interacted with the outer region flow. The stronger interaction of streamwise vortices with normal vortex improved mixing from the trough to the crest of the lobed mixer. The balance between enhanced mixing and increased reversed flow downstream of the centre-body, resulted in increased overall total pressure losses with increasing inlet swirl angles.Copyright