Jen-Ping Chen

Pulsed Jets Laminar Separation Control Using Instability Exploitation

The physics of control by pulsed blowing on a NACA 643-618 natural laminar flow airfoil is studied using hot-film anemometry. Measurements in the uncontrolled separated shear layer indicate that vortex shedding is taking place due to the Kelvin–Helmholtz-type inviscid instability. Steady and pulsed external acoustic excitation is used as well to decouple the frequency content of the perturbation from the vorticity introduced by the jet. Acoustic control introduces either the most unstable frequency or harmonics of carrier frequency in the most unstable range, which amplify exponentially in the separation region yielding a significant delay of the separation location to approximately 55% chord. Experimental data suggest that pulsed jets introduce higher-order harmonics of the low-frequency pulsing as well, amplifying the natural disturbances in the laminar separation. Phase-averaged wavelet analysis is used to study the control physics within a single pulsing period. It is shown that a jet-induced high-fre...

Numerical Simulations of Vortex Generating Jets on Low Pressure Turbine Blades

The combination of low Reynolds number (~20,000) and adverse pressure gradient can induce laminar separation in low pressure turbine sections of a gas turbine propulsion system. Total pressure loss generated by the separation zone has a negative impact on overall engine efficiency. Vortex generating jets have been shown to mitigate laminar separation and offer the ability to actively adapt to varying flight conditions. Various fluid dynamics mechanisms have been observed in jet-affected flow fields such as primary vorticity and turbulent transition. Understanding the appearance and behavior of these mechanisms will aid future implementation of this flow control system into flight hardware. Direct numeric simulations were evaluated for use in analyzing these flow fields. Benchmark data in the blade wake and near the jet hole were obtained from hot film anemometry, particle image velocimetry, and various pressure measurements. Uncontrolled blade wakes behave comparably to experimental observations in terms of t ime-averaged velocity and pressure fields. Steady state and pulsed flow control simulations are shown to successfully mitigate separation effects. Separated shear layer dynamics occurring in response to pulsed jets are observed and discussed. The results s how the presence of non-linear shear layer distortion just prior to separation zone elimination. Short duration, low blowing ratio pulsed jets were shown to be just as effective as steady blowing jets.

Designing a High Fidelity Wake Simulator for Research Using Linear Cascades

Owing to the extensive use of wake generators in the study of turbine and compressor airfoils in linear cascades, a study was undertaken to determine the most accurate model for the wakes generated by upstream blade rows. Velocity (PIV) measurements were taken to compare wake properties of several bluff bodies with different cross sections to the wake of an ultra high lift low pressure turbine profile. These measurements were taken at two Reynolds numbers, a low and a high one to simulate a separated and attached wake, respectively, for both the blade and two of the shape configurations. The L1A turbine blade profile was determined to shed a wake typical of high lift turbine blade profiles. It is shown that the wake of the turbine blade is highly dependent on Reynolds number. In order to make an appropriate comparison, all bluff body data were extracted along a plane parallel to the equivalent inlet plane of a rotor stage in the stationary frame of reference. It was found that no single rod shape matched all of the blade wake characteristics. From among the shapes used in this study, a 30° isosceles wedge placed 6 diameters upstream of the cascade inlet in the axial direction and skewed 15° from the rod relative flow was found to yield the closest match for the low Re case due to the asymmetry in the velocity and Reynolds shear stresses in this wake compared with the wake of the low pressure turbine blade. The same configuration placed 10 diameters upstream yielded the best comparison to a higher Re, more attached L1A wake. Large Eddy Simulations of various shapes largely corroborate the experimental findings.Copyright

Implicit Large Eddy Simulation of a Stalled Low-Pressure Turbine Airfoil

Time-accurate numerical simulations were conducted on the aft-loaded L1A low-pressure turbine airfoil at a Reynolds number of 22,000 (based on inlet velocity magnitude and axial chord length). This flow condition produces a nonreattaching laminar separation zone on the airfoil suction surface. The numerical code TURBO is used to simulate this flow field as an implicit large eddy simulation (ILES). Generally, good agreement was found when compared to experimental time-averaged and instantaneous flow measurements. The numerical separation zone is slightly larger than that in the experiments, though integrated wake loss values improved from Reynolds-averaged Navier–Stokes (RANS)-based simulations. Instantaneous snapshots of the numerical flow field showed the Kelvin Helmholtz instability forming in the separated shear layer and a large-scale vortex shedding pattern at the airfoil trailing edge. These features were observed in the experiments with similar sizes and vorticity levels. Power spectral density analyses revealed a global passage oscillation in the numerics that was not observed experimentally. This oscillation was most likely a primary resonant frequency of the numerical domain.

Implicit Large Eddy Simulation of a Stalled Low Pressure Turbine Airfoil

Time-accurate numerical simulations were conducted on the aft-loaded L1A low pressure turbine airfoil at a Reynolds number of 22,000 (based on inlet velocity magnitude and axial chord length). This flow condition produces a non-reattaching laminar separation zone on the airfoil suction surface. The numerical code TURBO is used to simulate this flow field as an Implicit Large Eddy Simulation. Generally good agreement was found when compared to experimental time-averaged and instantaneous flow measurements. The numerical separation zone is slightly larger than that in the experiments, though integrated wake loss values improved from RANS-based simulations. Instantaneous snapshots of the numerical flow field showed the Kelvin Helmholtz instability forming in the separated shear layer and a large-scale vortex shedding pattern at the airfoil trailing edge. These features were observed in the experiments with similar sizes and vorticity levels. Power spectral density analyses revealed a global passage oscillation in the numerics that was not observed experimentally. This oscillation was most likely a primary resonant frequency of the numerical domain.Copyright

The Role of Three-Dimensional Interactions in Fluidic Control of Shock-Induced Separation on a Transonic Turbine Blade

An experimental and numerical investigation is conducted to assess the fluid dynamic mechanisms of control by vortex-generator jets for shock-induced separation in a highly loaded low pressure turbine (LPT) blade. Two- and three-dimensional steady RANS computations are performed to evaluate their ability to reproduce the main features of such a complex flow. The test blade is part of a compressible LPT cascade that exhibits shock-induced separation at an exit Mach number of 0.8. Active flow control is implemented through a spanwise row of discrete vortex-generator jets (VGJs) located on the suction surface. The control performance of VGJs in these transonic conditions has an optimum blowing ratio beyond which losses increase. Three-dimensionalities in the flow field are established by discrete VGJ-boundary layer interaction as suggested by Particle-Image Velocimetry (PIV) acquisitions at different spanwise locations. Blade pressure distributions and wake total pressure losses are acquired to evaluate the control performance and compared with calculations. Two-dimensional numerical investigations by RANS simulations suggest that the effect of increased expansion over the passage is a product of massflow injection only. Three-dimensional RANS results are interrogated to give a more detailed representation of the flow features around the jets, such as the jet vortex dynamics and spanwise modulation of the potential field. The analysis of this experimental and numerical information identifies the mechanisms contributing to the performance of skewed jets for control of shock induced separation in a highly loaded LPT blade. The results provide indications on the accuracy of RANS simulations, identifying the challenges of using RANS (2D or 3D) to solve such complex flows.Copyright

Steady VGJ Flow Control on a Highly Loaded Transonic LPT Cascade: Effects of Compressibility and Roughness

A new high-speed linear cascade has been developed for low-pressure turbine (LPT) studies at The Ohio State University. A compressible LPT profile is tested in the facility and its baseline performance at different operating conditions is assessed by means of isentropic Mach number distribution and wake total pressure losses. Active flow control is implemented through a spanwise row of vortex-generator jets (VGJs) located at 60% chord on the suction surface. The purpose of the study is to document the effectiveness of VGJ flow control in high-speed compressible flow. The effect on shock-induced separation is assessed by Mach number distribution, wake loss surveys and shadowgraph. Pressure Sensitive Paint is applied to understand the three dimensional flow and shock pattern developing from the interaction of the skewed jets and the main flow. Data show that with increasing blowing ratio the losses are first decreased due to separation reduction, but losses connected to compressibility effects become stronger due to increased passage shock strength and jet orifice choking; therefore the optimum blowing ratio is a tradeoff between these counteracting effects. The effect of added surface roughness on the uncontrolled flow and on flow control behavior is also investigated. At lower Mach number turbulent separation develops on the rough surface and a different flow control performance is observed. Steady VGJs appear to have control authority even on a turbulent separation but higher blowing ratios are required compared to incompressible flow experiments reported elsewhere. Overall, the results show a high sensitivity of steady VGJs control performance and optimum blowing ratio to compressibility and surface roughness.Copyright

Exploitation of Subharmonics for Separated Shear Layer Control on a High-Lift LPT Using Acoustic Forcing

The mechanism of separation control by sound excitation is investigated on the aft loaded LPT blade profile, the L1A, which experiences a large boundary layer separation at low Reynolds numbers. Previous work by the authors has shown that on a laminar separation bubble such as that experienced by the front-loaded L2F profile, sound excitation control has its best performance at the most unstable frequency of the shear layer due to the exploitation of the linear instability mechanism. The different loading distribution on the L1A increases the distance of the separated shear layer from the wall and the exploitation of the same linear mechanism is no longer effective in these conditions. However, significant control authority is found in the range of the first subharmonic of the natural unstable frequency. The amplitude of forced excitation required for significant wake loss reduction is higher than that needed when exploiting linear instability, but unlike the latter case, no threshold amplitude is found. The fluid-dynamics mechanisms under these conditions are investigated by PIV measurements. Phase-locked PIV data gives insight into the growth and development of structures as they are shed from the shear layer and merge to lock into the excited frequency. Unlike near-wall laminar separation sound control, it is found that when such large separated shear layers occur, sound excitation at subharmonics of the fundamental frequency is still effective with high Tu levels.Copyright