Separation Control on High Lift Low-Pressure Turbine Airfoils Using Pulsed Vortex Generator Jets

Boundary layer separation control has been studied using vortex generator jets (VGJs) on a very high lift, low-pressure turbine airfoil. Experiments were done under low (0.6%) freestream turbulence conditions on a linear cascade in a low speed wind tunnel. 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. Cases were considered at Reynolds numbers (based on the suction surface length and the nominal exit velocity from the cascade) of 25000 and 50000. Jet pulsing frequency and duty cycle were varied. In cases without flow control, the boundary layer separated and did not reattach. With the VGJs, separation control was achieved. At sufficiently high pulsing frequencies, separation control was possible with low jet velocities and 10% duty cycle. At lower frequencies, a 50% duty cycle helped by separating the disturbances associated with the jets turning on and turning off, thereby doubling the frequency of separation control events above the pulsing frequency. Phase averaged velocity profiles and wavelet spectra of the velocity show the VGJ disturbance causes the boundary layer to reattach, but that it can re-separate between disturbances. When the disturbances occur at high enough frequency, the time available for separation is reduced, and the separation bubble remains closed at all times

WAVELET ANALYSIS OF TRANSITIONAL FLOW DATA UNDER HIGH FREE-STREAM TURBULENCE CONDITIONS

ABSTRACT Transitional flow data from boundary layers subject to strong acceleration (K as high as 9x10) and high free-stream turbulence (-8%) were analyzed using wavelet transforms. Wavelet analysis provides the energy content of a signal on both a frequency and instantaneous time basis. It differs from traditional Fourier spectral analysis, which can only provide the spectral energy on a time averaged basis. Instantaneous velocity data from intermittent, transitional boundary layers were segregated into turbulent and nonturbulent zones through conditional sampling. Wavelet analysis was used to determine the frequency content of the velocity fluctuations and turbulent shear stress in the two zones separately. The streamwise velocity fluctuations in the turbulent and non-turbulent zones appeared similar. This was attributed to the effect of the free-stream turbulence, which had the same effects on both zones. The wall-normal fluctuations and turbulent shear stress were of significantly higher magnitude and frequency in the turbulent zone. These results suggest that turbulence models should be based on transport quantities rather than turbulent kinetic energy. The regions just upstream and just downstream of turbulent zones were also analyzed, to check for possible important frequencies leading to the initiation of turbulence or characteristic of the "calm" zone trailing a turbulent spot. No distinct behavior was observed in either of these zones. Uncertainty values associated with the wavelet spectra are high due to the short data records available. Results are shown to be valid in spite of these uncertainties, however longer data records should be acquired in future studies

GT2003-38728 PASSIVE FLOW CONTROL ON LOW-PRESSURE TURBINE AIRFOILS

ABSTRACT Two-dimensional rectangular bars have been used in an experimental study to control boundary layer transition and reattachment under low-pressure turbine conditions. Cases with Reynolds numbers (Re) ranging from 25,000 to 300,000 (based on suction surface length and exit velocity) have been considered at low (0.5%) and high (8.5% inlet) free-stream turbulence levels. Three different bars were considered, with heights ranging from 0.2% to 0.7% of suction surface length. Mean and fluctuating velocity and intermittency profiles are presented and compared to results of baseline cases from a previous study. Bar performance depends on the bar height and the location of the bar trailing edge. Bars located near the suction surface velocity maximum are most effective. Large bars trip the boundary layer to turbulent and prevent separation, but create unnecessarily high losses. Somewhat smaller bars had no immediate detectable effect on the boundary layer, but introduced small disturbances which caused transition and reattachment to move upstream from their locations in the corresponding baseline case. The smaller bars were effective under both high and low free-stream turbulence conditions, indicating that the high free-stream turbulence transition is not simply a bypass transition induced by the free-stream. Losses appear to be minimized when a small separation bubble is present, so long as reattachment begins far enough upstream for the boundary layer to recover from the separation. Correlations for determining optimal bar height are presented. The bars appear to provide a simple and effective means of passive flow control. Bars which are large enough to induce reattachment at low Re, however, cause higher losses at the highest Re. Some compromise would, therefore, be needed when choosing a bar height for best overall performance

Separation Control on High Lift Low-Pressure Turbine Airfoils Using Pulsed Vortex Generator Jets

Boundary layer separation control has been studied using vortex generator jets (VGJs) on a very high lift, low-pressure turbine airfoil. Experiments were done under low (0.6%) freestream turbulence conditions on a linear cascade in a low speed wind tunnel. 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. Cases were considered at Reynolds numbers (based on the suction surface length and the nominal exit velocity from the cascade) of 25000 and 50000. Jet pulsing frequency and duty cycle were varied. In cases without flow control, the boundary layer separated and did not reattach. With the VGJs, separation control was achieved. At sufficiently high pulsing frequencies, separation control was possible with low jet velocities and 10% duty cycle. At lower frequencies, a 50% duty cycle helped by separating the disturbances associated with the jets turning on and turning off, thereby doubling the frequency of separation control events above the pulsing frequency. Phase averaged velocity profiles and wavelet spectra of the velocity show the VGJ disturbance causes the boundary layer to reattach, but that it can re-separate between disturbances. When the disturbances occur at high enough frequency, the time available for separation is reduced, and the separation bubble remains closed at all times

Synthetic Vortex Generator Jets Used to Control Separation on Low-Pressure Turbine Airfoils

Low-pressure turbine (LPT) airfoils are subject to increasingly stronger pressure gradients as designers impose higher loading in an effort to improve efficiency and lower cost by reducing the number of airfoils in an engine. When the adverse pressure gradient on the suction side of these airfoils becomes strong enough, the boundary layer will separate. Separation bubbles, particularly those that fail to reattach, can result in a significant loss of lift and a subsequent degradation of engine efficiency. The problem is particularly relevant in aircraft engines. Airfoils optimized to produce maximum power under takeoff conditions may still experience boundary layer separation at cruise conditions because of the thinner air and lower Reynolds numbers at altitude. Component efficiency can drop significantly between takeoff and cruise conditions. The decrease is about 2 percent in large commercial transport engines, and it could be as large as 7 percent in smaller engines operating at higher altitudes. Therefore, it is very beneficial to eliminate, or at least reduce, the separation bubble

Measurements in Transitional Boundary Layers Under High Free-Stream Turbulence and Strong Acceleration Conditions

Measurements from transitional, heated boundary layers along a concave-curved test wall are presented and discussed. A boundary layer subject to low free-stream turbulence intensity (FSTI), which contains stationary streamwise (Gortler) vortices, is documented. The low FSTI measurements are followed by measurements in boundary layers subject to high (initially 8%) free-stream turbulence intensity and moderate to strong streamwise acceleration. Conditions were chosen to simulate those present on the downstream half of the pressure side of a gas turbine airfoil. Mean flow characteristics as well as turbulence statistics, including the turbulent shear stress, turbulent heat flux, and turbulent Prandtl number, are documented. A technique called "octant analysis" is introduced and applied to several cases from the literature as well as to data from the present study. Spectral analysis was applied to describe the effects of turbulence scales of different sizes during transition. To the authors'knowledge, this is the first detailed documentation of boundary layer transition under such high free-stream turbulence conditions

Experimental Studies of Low-Pressure Turbine Flows and Flow Control

This report summarizes research performed in support of the NASA Glenn Research Center (GRC) Low-Pressure Turbine (LPT) Flow Physics Program. The work was performed experimentally at the U.S. Naval Academy faculties. The geometry corresponded to "Pak B" LPT airfoil. The test section simulated LPT flow in a passage. Three experimental studies were performed: (a) Boundary layer measurements for ten baseline cases under high and low freestream turbulence conditions at five Reynolds numbers of 25,000, 50,000, 100,000, 200,000, and 300,000, based on passage exit velocity and suction surface wetted length; (b) Passive flow control studies with three thicknesses of two-dimensional bars, and two heights of three-dimensional circular cylinders with different spanwise separations, at same flow conditions as the 10 baseline cases; (c) Active flow control with oscillating synthetic (zero net mass flow) vortex generator jets, for one case with low freestream turbulence and a low Reynolds number of 25,000. The Passive flow control was successful at controlling the separation problem at low Reynolds numbers, with varying degrees of success from case to case and varying levels of impact at higher Reynolds numbers. The active flow control successfully eliminated the large separation problem for the low Reynolds number case. Very detailed data was acquired using hot-wire anemometry, including single and two velocity components, integral boundary layer quantities, turbulence statistics and spectra, turbulent shear stresses and their spectra, and intermittency, documenting transition, separation and reattachment. Models were constructed to correlate the results. The report includes a summary of the work performed and reprints of the publications describing the various studies

Measurements in Film Cooling Flows With Periodic Wakes”.

ABSTRACT Film cooling flows subject to periodic wakes were studied experimentally. The wakes were generated with a spoked wheel upstream of a flat plate. Cases with a single row of cylindrical film cooling holes inclined at 35 degrees to the surface were considered at blowing ratios of 0.25, 0.50, and 1.0 with a steady freestream and with wake Strouhal numbers of 0.15, 0.30, and 0.60. Temperature measurements were made using an infrared camera, thermocouples, and constant current (cold wire) anemometry. Hot wire anemometry was used for velocity measurements. The local film cooling effectiveness and heat transfer coefficient were determined from the measured temperatures. Phase locked flow temperature fields were determined from cold wire surveys. Wakes decreased the film cooling effectiveness for blowing ratios of 0.25 and 0.50 when compared to steady freestream cases. In contrast, effectiveness increased with Strouhal number for the 1.0 blowing ratio cases, as the wakes helped mitigate the effects of jet liftoff. Heat transfer coefficients increased with wake passing frequency, with nearly the same percentage increase in cases with and without film cooling. The time resolved flow measurements show the interaction of the wakes with the film cooling jets. Near-wall flow measurements are used to infer the instantaneous film cooling effectiveness as it changes during the wake passing cycle

Flow Control on Low-Pressure Turbine Airfoils Using Vortex Generator Jets

Motivation - Higher loading on Low-Pressure Turbine (LPT) airfoils: Reduce airfoil count, weight, cost. Increase efficiency, and Limited by suction side separation. Growing understanding of transition, separation, wake effects: Improved models. Take advantage of wakes. Higher lift airfoils in use. Further loading increases may require flow control: Passive: trips, dimples, etc. Active: plasma actuators, vortex generator jets (VGJs). Can increased loading offset higher losses on high lift airfoils. Objectives: Advance knowledge of boundary layer separation and transition under LPT conditions. Demonstrate, improve understanding of separation control with pulsed VGJs. Produce detailed experimental data base. Test and develop computational models