Erik Fernandez

Separation Control on a Low-Pressure Turbine Blade using Microjets

Flow separation that occurs over low-pressure turbine blades at low Reynolds numbers has been a cause of concern due to its detrimental effect on engine performance. In the present study, the effect of microjet actuation, a low-mass, high-momentum device, is demonstrated for the elimination of separation on a low-pressure turbine blade over a wide range of Reynolds numbers using a range of complementary diagnostics, which include the surface pressure, velocity field, and wake-loss measurements. The U.S. Air Force Research Laboratory L1A low-pressure turbine blade used in this study is a highly aftloaded profile that experiences a nonreattaching separation at approximately 60% axial chord at low Reynolds numbers. Baseline blade pressure distributions as well as wake-loss coefficient measurements show that the blade experiences nonreattaching separation for Reynolds numbers based on axial chord less than 50,000 for a freestream turbulence intensity of 1% with steady inlet conditions. Microjet-based control ...

Active Separation Control on Highly Loaded LPT Blades using Microjets

The efficacy of separation control on an LPT using microjets was determined through the use of pressure and velocity field measurements over a wide range of Reynolds numbers. The AFRL designed L1A low pressure turbine blade, which was used in this study, is a highly aft-loaded profile which experiences a non-reattaching separation at approximately 62%Cx for low Reynolds Numbers. Microjet actuators have been shown to be very effective for separation control in subsonic conditions and a microjet array has been implemented to the L1A LPT at 60%Cx. Baseline blade pressure distributions as well as wake loss coefficient traverses show that the blade’s non-reattaching separation is present for Re<50000 for a freestream turbulence intensity of 1%. Microjets were activated at various blowing ratios for each Reynolds number of interest and were shown to eliminate the boundary layer separation. At Re≈20000, the minimum effective blowing ratio was B=3 whereas at Re≈40000, the minimum effective blowing ratio was B=1. By utilizing microjet control, the integrated loss coefficient dropped 500% from the no-control case at Re≈20000 and the Re≈40000 case had a drop of 700% from the no-control case. Lower blowing ratios were required as the experimental Reynolds number approached that of the blade’s transition Reynolds number which was found to be between Re≈50000 and Re≈60000. PIV velocity fields show a complete elimination of all reverse flow on the blade’s suction surface when the microjets were steadily actuated during low Reynolds number conditions. Turbulence statistics found through PIV also showed a dramatic reduction in urms and vrms unsteady quantities when the microjets were actuated. These results were achieved with low mass flux and compare favorably with other flow control methods for LPT separation reduction.

Near Wake Dynamics for an Ahmed Body with Active Flow Control

An Ahmed body with a 25° rear slant (rear window) is the base model used to investigate the use of steady microjets as a control method to redu ce separation. The actuator array is placed just downstream of the leading edge of the r ear slant in order to control the flow separation that occurs on this surface. Modificati ons to the flow field are studied over a wide range of control conditions in terms of momentum flux through the actuators. Attention is focused on the dynamics of the trailin g wake and the effect of separation control on the near wake characteristics. By controlling t he degree of separation changes to the primary shedding frequency as well as to the dominant wake structure were found to occur. The near wake was found to elongate by nearly 17% causing a pressure increase on the rear vertical base. This is thought to be a key compon ent to drag reduction on blunt body separation.

Effect of Microjet Spacing on the Control of a Highly Separated Flowfield

The effect of lateral jet-to-jet spacing in a microjet array has been examined in the context of a separation flow control application. The main mechanism responsible for separated flow reattachment from microjet actuation is the generation of streamwise vorticity due to the counter rotating vortex pair (CRVP) produced by the jet in cross-flow. These CRVPs entrain high momentum freestream fluid to the near wall regions, thus energizing the low momentum boundary layer. As the jet-to-jet spacing changes, the interaction between streamwise vortices and their impact on flow control effectiveness must be examined. Three microjet arrays with 6.25, 12.5, and 25 diameter spacings are investigated. Steady Reynolds Averaged Navier-Stokes (RANS) simulations modeling the microjets in a zero pressure gradient cross-flow, were performed for the three jet spacings. 12.5d and 25d spacing simulations revealed the presence of coherent streamwise vortices past 80 jet diameters. The 6.25d spacing however, is characterized by a rapid decay of streamwise vorticity downstream of injection with coherent vortices lasting only about 20 jet diameters downstream. This is attributed to enhanced interaction of CRVPs generated by adjacent microjets and, due to their close proximity, their rapid dissipation. This configuration is also characterized by a stretching of streamwise vorticity in the wall normal direction. Experimentally, a highly adverse pressure gradient was imposed on a flat plate for the same three microjet spacings. The effectiveness of each confinguation was examined for controlling the characteristic flow separation. Pressure distributions measured along the flat plate centerline demonstrate that each configuration (corresponding to three microjet spacings) was able to effectively reattach the separated flow. A jet to cross-flow velocity ratio of 10 was found to be effective for all three cases. Similarly, a steady momentum coefficient of about 0.7 was also found to be effective for all three cases. The analysis further reveals that the 25d microjet configuration was the most efficient for this particular application in that it produced the greatest benefit at minimal cost in terms of actuator mass flow rate. Velocity fields measured through Particle Image Velocimetry for the selected optimal control cases reinforced the results from the surface pressure distributions and simulations. As seen in the numerical results, the 6.25d jet configuration showed a substantial velocity deficit downstream of the injection location compared to the larger jet spacing configurations.

Flow Sensory Actuators for MAVs

A macro fiber piezoelectric composite has been studied for boundary layer management in Micro-Air Vehicles (MAVs). Specifically, self-sensing piezoelectric composite structures have been designed, fabricated, and tested in wind tunnel studies to quantify performance characteristics, such as the velocity field response to actuation, relevant for actively managing boundary layers (laminar and transition flow control). The dynamic properties of these Flow Sensory Actuators (FSA) were also evaluated in-situ. Results based on velocity field measurements and unsteady pressure measurements show that these macro fiber composites can sense the state of flow above the surface and have sufficient authority to manipulate the flow conditions. Control authority using open loop and closed loop control designs have also been investigated in trade-off studies to quantify performance enhancements versus power input and weight requirements (a critical driver in MAVs) relevant to this application.

Nonlinear Adaptive Approach to Microjet-Based Flow Separation Control

Boundary-layer separation, a critical phenomenon in the operation of aerodynamic surfaces, limits the performance of compressor and turbine blades, fixed and rotary wings, as well as bluff bodies moving through a fluid. Flow separation leads to increased drag, decreased lift, and unpredictable vibrations due to unsteadiness. On these systems, effective control of separation could provide greater maneuverability and performance, and reduced vibration. Separated flow is a macroscale phenomenon governed by complex flow interactions, but it can be controlled by microscale actuation. Recently, the emergence of closed-loop methods has enhanced robustness. Modern processors enable the use of sophisticated adaptive control methods that achieve separation control with adaptive models. This paper considers control of flow separation over a NACA-0025 airfoil using microjet actuators. Experimental results are presented for a novel approach to nonlinear model predictive control, referred to as adaptive sampling-based ...

An Experimental Study of Detailed Flow and Heat Transfer Analysis in a Single Row Narrow Impingement Channel

An experimental investigation of detailed flow and heat transfer in a narrow impingement channel was studied; the channel included 15 inline jets in a single row with a jet-to-target wall distance of 3 jet diameters. The spanwise length of the channel was 4 jet diameters, and a streamwise jet spacing of 5 jet diameters was considered for the current study. Both the flow physics and heat transfer tests were run at an average jet Reynolds number of 30,000. Temperature sensitive paint was used to study heat transfer at the target wall. Along with other parameters, jet-to-jet interaction in a narrow row impingement channel plays a significant role on heat transfer distribution at the side and target walls as the self-induced jet cross flow tends to bend the downstream jets. The present work shows detailed information of flow physics using Particle Image Velocimetry (PIV). PIV measurements were taken at planes normal to the target wall along the jet centerline for several jets. The flow field and heat transfer data was compared between the experiment and CFD in order to understand the relationship between flow characteristics and heat transfer. The experimental data gathered from PIV can be used as benchmark data for validating the current state of the art RANS turbulence models as well as for Large Eddy Simulation (LES).Copyright

Implementing Rotating Stall Control in a Radial Diffuser Using Microjet Arrays

This study is part of our effort to implement and refine microjet-based flow control in realistic and challenging applications. Our goal is to reduce/eliminate rotating stall in the radial diffuser of a production compressor used in commercial heating, ventilation, and air conditioning (HVAC) systems, using microjet arrays. We systematically characterize the flow using pressure and velocity field measurements. At low load conditions, the flow is clearly stalled over a range of RPM where the presence of two rotating stall cells was documented. Circular microjet arrays were integrated in the diffuser and the flow response to actuation was examined. The array closest to the initiation of stall cells was most effective in reattaching the flow. Control led to a very significant increase in the stall margin, reducing the minimum operational mass flow rate to 14% of the design flow rate, half of the original 28% flow rate before microjet control was implemented. The results will show that the parameters found be most effective in the simple configurations proved to be near-optimal for the present surge control application in a much more complex geometry. This provides us confidence that the lessons learned from prior studies can be extended to more complex configurations.

Separation Control of a Generic Airfoil using Longitudinal Ridges

An experimental investigation was carried out with an objective to enhance the flight envelop of a generic USA-35B wing at low speeds. The technique employed in this study is a pair of longitudinal ridges to take advantage of both leading edge protrusions and chordwise fences. Two-component particle image velocimetry (PIV) was used to measure the velocity field. In the present study, the ridge height and inter-ridge spacing were varied to obtain an optimal control configuration for maximum effectiveness in terms of flow separation region and attachment location. The results showed that the baseline airfoil exhibited a massive flow separation on the suction side at an angle of incidence of 12° and beyond. With the addition of optimal longitudinal ridges, the stall was significantly delayed and the flow was completely attached on the entire airfoil surface up to an angle of incidence of 16°. The study suggest that longitudinal ridges are simple in design, implementation and very effective for separation con...

Development of Velocity Profile Generating Screens for Gas Turbine Components

Laboratory experiments on components of complex systems such as gas turbines require many conditions to be met. Requirements to be met to simulate real world conditions include inlet flow conditions such as velocity profile. The methodology to be introduced designs a velocity profile generating screen through the use of perforated plates. The velocity profile generating screen is an array of jets arranged in a manner to produce annular sections of different solidities. In an effort to better understand the interaction between perforated plates of different solidities, an experimental data set is presented to characterize the plates. This includes identification of flow regions with characterization of flow dynamics though the analysis of velocity and turbulence decay. The aim of this characterization is to determine how the perforated plate solidity affects the velocity downstream and the location of the velocity profile being produced.