The current work presents experimental and computational investigations of fluid flow through a 3... more The current work presents experimental and computational investigations of fluid flow through a 37 element CANDU nuclear fuel bundle. Experiments based on Magnetic Resonance Velocimetry (MRV) permit three-dimensional, three-component fluid velocity measurements to be made within the bundle with sub-millimeter resolution that are non-intrusive, do not require tracer particles or optical access of the flow field. Computational fluid dynamic (CFD) simulations of the foregoing experiments were performed with the HYDRA-TH code using implicit large eddy simulation, which were in very good agreement with experimental measurements of the fluid velocity. Greater understanding has been gained in the evolution of geometry-induced inter-subchannel mixing, the local effects of obstructed debris on the local flow field, and various turbulent effects, such as recirculation, swirl and separation. These capabilities are not available with conventional experimental techniques or thermal-hydraulic codes. The overall goal of this work is to continue developing experimental and computational capabilities for further investigations that reliably support nuclear reactor performance and safety.
This study focuses on the qualitative effect of elevated ambient pressure on water droplets with ... more This study focuses on the qualitative effect of elevated ambient pressure on water droplets with a diameter of 2.4 mm, which accelerate 30 mm, under the influence of gravity, and impact onto a heated surface. A flat aluminum target was heated up to 400 °C (673.15 K) in order to cover nucleate boiling, transition and rebound impact regimes at 1 bar. The experiment is placed in a pressure chamber filled with air at pressures up to 24 bar. This is done to simulate conditions comparable to those of combustion chambers in modern engines. Observations of impact dynamics are made with a high-speed camera at 4000 fps and a high-speed LED stroboscope with flash duration of 300 to 400 ns. The lifetime of a droplet changes according to both the surface temperature and pressure in different boiling regimes. The regime borders shift according to the saturated vapor pressure of the fluid.
Recent experimental and computational studies of swimming hydrodynamics have contributed signific... more Recent experimental and computational studies of swimming hydrodynamics have contributed significantly to our understanding of how animals swim, but much remains to be done. Ten questions are presented here as an avenue to discuss some of the arenas in which progress still is needed and as a means of considering the technical approaches to address these questions. 1. What is the threedimensional structure of propulsive surfaces? 2. How do propulsive surfaces move in three dimensions? 3. What are the hydrodynamic effects of propulsor deformation during locomotion? 4. How are locomotor kinematics and dynamics altered during unsteady conditions? 5. What is the three-dimensional structure of aquatic animal vortex wakes? 6. To what extent are observed propulsor deformations actively controlled? 7. What is the response of the body and fins of moving animals to external perturbations? 8. How can robotic models help us understand locomotor dynamics of organisms? 9. How do propulsive surface...
The impact of chord-based Reynolds number on the formation of leading-edge vortices (LEVs) on uns... more The impact of chord-based Reynolds number on the formation of leading-edge vortices (LEVs) on unsteady pitching flat plates is investigated. The influence of secondary flow structures on the shear layer feeding the LEV and the subsequent topological change at the leading edge as the result of viscous processes are demonstrated. Time-resolved velocity fields are measured using particle image velocimetry simultaneously in two fields of view to correlate local and global flow phenomena in order to identify unsteady boundary-layer separation and the subsequent flow structures. Finally, the Reynolds number is identified as a parameter that is responsible for the transition in mechanisms leading to LEV detachment from an aerofoil, as it determines the viscous response of the boundary layer in the vortex–wall interaction.
ABSTRACT The present work addresses the combined influence of pressure variations and different a... more ABSTRACT The present work addresses the combined influence of pressure variations and different airflow velocities on the discharge intensity of plasma actuators. Power consumption, plasma length, and discharge capacitance were investigated systematically for varying pressure levels (p = 0.1–1 bar) and airflow velocities ( U ∞ = 0 − 100 m/s) to characterize and quantify the favorable and adverse effects on the discharge intensity. In accordance with previous reports, an increasing plasma actuator discharge intensity is observed for decreasing pressure levels. At constant pressure levels, an adverse airflow influence on the electric actuator performance is demonstrated. Despite the improved discharge intensity at lower pressure levels, the seemingly improved performance of the plasma actuators is accompanied with a more pronounced drop of the relative performance. These findings demonstrate the dependency of the (kinematic and thermodynamic) environmental conditions on the electric performance of plasma actuators, which in turn affects the control authority of plasma actuators for flow control applications.
ABSTRACT The increasing popularity and maturity of plasma actuators for many flow control applica... more ABSTRACT The increasing popularity and maturity of plasma actuators for many flow control applications requires a common standard for plasma actuator performance evaluation. In the present work, a comprehensive comparative study of existing and new evaluation measures is presented, based on results from identical plasma-actuator configurations. A power-flow diagram is introduced that covers the entire range of power stages from the energy source to the flow-control success. All individual power stages are explained, existing controversial definitions are clarified, and an evaluation guideline is applied to previously obtained data. Finally, the defined systematic analysis is applied to the results of a recently conducted plasma-actuator in-flight experiment.
In this work, an experimental study of spray impact onto a horizontal flat and rigid surface is p... more In this work, an experimental study of spray impact onto a horizontal flat and rigid surface is presented to specify the suitable measurement volume height above the surface in which the Phase-Doppler measurements are valid. Furthermore the influence of the input laser power on the characteristics of the impinging and secondary spray is studied in detail. The phase Doppler technique has then been used to characterise both the impacting and the secondary spray in terms of number flux, size distribution and velocities of the droplets above the target. A high-speed CCD camera has been used to characterize the splashing or rebounding droplets and to measure the average film thickness formed due to spray impact. In this paper some new considerations in Phase-Doppler measurements of spray-wall interaction are presented.
Public reporting burden for this collection of information is estimated to average 1 hour per res... more Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing the burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.
The temperature field within evaporating ethanol droplets is investigated, relying on the two-col... more The temperature field within evaporating ethanol droplets is investigated, relying on the two-color laser induced fluorescence (LIF) measurement technique and on a Direct Numerical Simulation (DNS). The configuration studied corresponds to a monodisperse droplet stream in a diffusion flame sustained by the droplet vapor. An experimental probe volume, small compared to the droplet size, is used to characterize the temperature field within the droplets, whereas DNS takes into account key aspects of the droplet heating and evaporation such as the non-uniform and transient stress, and the mass and heat transfer coefficients at the droplet surface. These investigations reveal that the frictional stresses are strongly reduced due to the small spacing between the droplets. They also show that the Marangoni effect has a significant influence on the internal motion and hence on the internal temperature field.
A phase averaged optical measurement technique is used for characterization of externally excited... more A phase averaged optical measurement technique is used for characterization of externally excited boundary-layer disturbances. Two-dimensional velocity fields of flat-plate boundary-layer flow are recorded via Particle Image Velocimetry to resolve the effect of Dielectric Barrier Discharge plasma actuators on Tollmien-Schlichting waves. These are artificially excited by a vibrating disturbance source flush-mounted with the surface of the flat plate. A delay-generator is used to trigger the image acquisition on predetermined phase angles of the excitation frequency. Phase averaging unveils the temporal and spatial evolution of the boundary-layer disturbances. Characteristics of the excited waves are quantified and compared to linear stability theory. The phase-locked technique allows the evaluation of plasma actuation on the excited waves. A reduction of the wave amplitude by 26% was recorded for a continuously operated, single Dielectric Barrier Discharge actuator. A comprehensive investigation of the actuator effect under variable flow velocities and pressure gradients is presented in this manuscript.
ABSTRACT In this investigation the flow in a generic swirl tube with a tangential double-inlet sw... more ABSTRACT In this investigation the flow in a generic swirl tube with a tangential double-inlet swirl generator and variable exit orifices was experimentally investigated. Using magnetic resonance velocimetry (MRV) three-dimensional, three-component velocity fields were measured for two different Reynolds numbers: 10,000 and 15,000, and for three different exit orifices. The swirl generator had a fixed geometry producing an initial swirl number of 1.6 for all cases. One major observation is the occurrence of a three-layered flow structure. An annular main flow was surrounded by a recirculation zone, as reported in previous literature. However, this recirculation zone – also of an annular shape – exhibited a third layer inside: a thin, high speed jet in the center of the tube with the same flow direction as the main flow. Therefore, the conventional classification of swirling flows into ring and recirculation zone, has to be extended by a core zone. This three-layered flow structure develops independently of the exit configuration. Helical structures were observed in the near-wall region for all cases investigated. Applying an eccentric exit orifice results in the occurrence of strong stationary helical structures not only in the near-wall region but also in the center of the tube. The results, deviating significantly from previous results in the literature, underline the need for more detailed research on the topic of cyclone type flows.
ABSTRACT A novel diagnostic approach to quantify discharge characteristics of dielectric barrier ... more ABSTRACT A novel diagnostic approach to quantify discharge characteristics of dielectric barrier discharge (DBD) plasma actuators is introduced and demonstrated for actuation in quiescent air and at high-speed conditions. Using the method a significant decrease of actuator performance is observed for operation at higher Mach numbers. As such, calibrations of the actuator obtained in quiescent air can be shown to strongly overestimate the exerted body force at higher Mach numbers, thus biasing numerical simulations of the actuator effects on the flow. Furthermore, the new approach for monitoring actuator discharge also provides clear guidelines for matching the impedance of the electrical system and for estimating the degree of system de-tuning as the air-flow velocity changes.
The current study is devoted to investigating velocity fields produced by Dielectric Barrier Disc... more The current study is devoted to investigating velocity fields produced by Dielectric Barrier Discharge (DBD) plasma actuators in quiescent air using a PIV system. The purpose of the study is to determine whether features in the velocity field can be recognized, which already allow direct conclusions about how effective the actuator might be for a particular flow control application. The parameter space investigated in the experiments comprises several electrode sizes, modulation frequencies and actuator voltages. Our interest is focussed at the present time on stabilization of boundary layers or delay of transition. To identify conducive induced velocity fields, we have chosen to examine the proper orthogonal decomposition of the velocity field and show that this representation can have direct physical interpretation of the influence exerted on the boundary layer. Comparing the present results to previous experience with various actuator configurations, we conclude that the following approach is viable and should be persude.
ABSTRACT This article addresses the direct experimental measurement of Tollmien-Schlichting waves... more ABSTRACT This article addresses the direct experimental measurement of Tollmien-Schlichting waves in a flat plate, laminar boundary layer excited by velocity perturbations; the free-stream velocity was 16 m/s, the excitation frequency 250 Hz. The two-dimensional velocity field in proximity of the flat plate was captured using a conventional PIV system; however the image recording was phase locked with the disturbance source and ensemble averaging was used to obtain mean velocity characteristics of the Tollmien-Schichting waves. In particular, after subtraction of the mean velocity, the characteristics of the excited waves in terms of isolines and streamlines were extracted, revealing that the investigated waves represented velocity deviations in the order of magnitude of 1 % of the undisturbed free-stream flow. This study is a prelude to the use of the same technique to visualize the affect of dielectric barrier discharge plasma actuators on the suppression of such Tollmien-Schlichting waves, which is difficult using other measurement techniques.
Particle Image Velocimetry (PIV) is used to characterize the flowfield induced by Dielectric-Barr... more Particle Image Velocimetry (PIV) is used to characterize the flowfield induced by Dielectric-Barrier Discharge (DBD) actuators, employed for flow control applications. Close-up imaging of a flat-plate boundary layer (BL) at freestream velocities between 1 m/s and 12m/s reveals characteristics of the boundary-layer profiles affected by actuator operation. High-speed 2D-2C PIV measurements visualize the transient flow after starting a DBD actuator in quiescent air. Vortical flow features during initiation of DBD actuation are identified and related to the mode of operation and geometry. A power-law is found to represent the vortex evolution and is a first step towards universal scaling rules for DBD actuators.
A new procedure of determining the time resolved capacitance of a plasma actuator during operatio... more A new procedure of determining the time resolved capacitance of a plasma actuator during operation is introduced, representing a simple diagnostic tool that provides insight into the phenomenological behavior of plasma actuators. The procedure is demonstrated by presenting example correlations between consumed electrical energy, size of the plasma region and the operating voltage. It is shown that the capacitance of a plasma actuator is considerably increased by the presence of the plasma; hence a system which has previously been impedance matched can be considerably de-tuned when varying the operating voltage of the actuator. Such information is fundamental for any attempts to increase the energy efficiency of plasma-actuator systems. A combined analysis of the capacitance, light emission, size of the plasma region, force production and power consumption is presented.
In this study the aerodynamic interactions between tandem pitching and plunging airfoils, inspire... more In this study the aerodynamic interactions between tandem pitching and plunging airfoils, inspired by dragonfly flight, were investigated at Re 30; 000. The primary parameter to be varied was the phase angle between fore and hindfoils. The aerodynamic interactions were investigated numerically using the unsteady Reynolds-averaged Navier-Stokes equations as well as experimentally in a wind tunnel with the aid of particle image velocimetry. The numerical simulations proved to be reliable in the prediction of the positions and intensities of the shed vortices. Two major aerodynamic interactions were detected. The first type of interaction occurred when the forefoil leading-edge vortex induced a separation on the lower surface of the hindfoil. This was observed for airfoil phasings with 60, 90, and 120. This vortex-induced separation exerted a strong suction forward and downward and was responsible for high, instantaneous values of thrust. The second type of interaction was observed when the forefoil trailing-edge vortex induced a separation on the upper surface of the hindfoil at airfoil phasings of 0, 30, and 60. This localized separation exerted a relative strong suction normal to the airfoil surface, increasing the lift as well as the drag. Moreover, this second form of vortex-induced separation over the airfoil upper surface allowed for instantaneous energy extraction on the hindfoil upstroke, i.e., energy recovery.
The current work presents experimental and computational investigations of fluid flow through a 3... more The current work presents experimental and computational investigations of fluid flow through a 37 element CANDU nuclear fuel bundle. Experiments based on Magnetic Resonance Velocimetry (MRV) permit three-dimensional, three-component fluid velocity measurements to be made within the bundle with sub-millimeter resolution that are non-intrusive, do not require tracer particles or optical access of the flow field. Computational fluid dynamic (CFD) simulations of the foregoing experiments were performed with the HYDRA-TH code using implicit large eddy simulation, which were in very good agreement with experimental measurements of the fluid velocity. Greater understanding has been gained in the evolution of geometry-induced inter-subchannel mixing, the local effects of obstructed debris on the local flow field, and various turbulent effects, such as recirculation, swirl and separation. These capabilities are not available with conventional experimental techniques or thermal-hydraulic codes. The overall goal of this work is to continue developing experimental and computational capabilities for further investigations that reliably support nuclear reactor performance and safety.
This study focuses on the qualitative effect of elevated ambient pressure on water droplets with ... more This study focuses on the qualitative effect of elevated ambient pressure on water droplets with a diameter of 2.4 mm, which accelerate 30 mm, under the influence of gravity, and impact onto a heated surface. A flat aluminum target was heated up to 400 °C (673.15 K) in order to cover nucleate boiling, transition and rebound impact regimes at 1 bar. The experiment is placed in a pressure chamber filled with air at pressures up to 24 bar. This is done to simulate conditions comparable to those of combustion chambers in modern engines. Observations of impact dynamics are made with a high-speed camera at 4000 fps and a high-speed LED stroboscope with flash duration of 300 to 400 ns. The lifetime of a droplet changes according to both the surface temperature and pressure in different boiling regimes. The regime borders shift according to the saturated vapor pressure of the fluid.
Recent experimental and computational studies of swimming hydrodynamics have contributed signific... more Recent experimental and computational studies of swimming hydrodynamics have contributed significantly to our understanding of how animals swim, but much remains to be done. Ten questions are presented here as an avenue to discuss some of the arenas in which progress still is needed and as a means of considering the technical approaches to address these questions. 1. What is the threedimensional structure of propulsive surfaces? 2. How do propulsive surfaces move in three dimensions? 3. What are the hydrodynamic effects of propulsor deformation during locomotion? 4. How are locomotor kinematics and dynamics altered during unsteady conditions? 5. What is the three-dimensional structure of aquatic animal vortex wakes? 6. To what extent are observed propulsor deformations actively controlled? 7. What is the response of the body and fins of moving animals to external perturbations? 8. How can robotic models help us understand locomotor dynamics of organisms? 9. How do propulsive surface...
The impact of chord-based Reynolds number on the formation of leading-edge vortices (LEVs) on uns... more The impact of chord-based Reynolds number on the formation of leading-edge vortices (LEVs) on unsteady pitching flat plates is investigated. The influence of secondary flow structures on the shear layer feeding the LEV and the subsequent topological change at the leading edge as the result of viscous processes are demonstrated. Time-resolved velocity fields are measured using particle image velocimetry simultaneously in two fields of view to correlate local and global flow phenomena in order to identify unsteady boundary-layer separation and the subsequent flow structures. Finally, the Reynolds number is identified as a parameter that is responsible for the transition in mechanisms leading to LEV detachment from an aerofoil, as it determines the viscous response of the boundary layer in the vortex–wall interaction.
ABSTRACT The present work addresses the combined influence of pressure variations and different a... more ABSTRACT The present work addresses the combined influence of pressure variations and different airflow velocities on the discharge intensity of plasma actuators. Power consumption, plasma length, and discharge capacitance were investigated systematically for varying pressure levels (p = 0.1–1 bar) and airflow velocities ( U ∞ = 0 − 100 m/s) to characterize and quantify the favorable and adverse effects on the discharge intensity. In accordance with previous reports, an increasing plasma actuator discharge intensity is observed for decreasing pressure levels. At constant pressure levels, an adverse airflow influence on the electric actuator performance is demonstrated. Despite the improved discharge intensity at lower pressure levels, the seemingly improved performance of the plasma actuators is accompanied with a more pronounced drop of the relative performance. These findings demonstrate the dependency of the (kinematic and thermodynamic) environmental conditions on the electric performance of plasma actuators, which in turn affects the control authority of plasma actuators for flow control applications.
ABSTRACT The increasing popularity and maturity of plasma actuators for many flow control applica... more ABSTRACT The increasing popularity and maturity of plasma actuators for many flow control applications requires a common standard for plasma actuator performance evaluation. In the present work, a comprehensive comparative study of existing and new evaluation measures is presented, based on results from identical plasma-actuator configurations. A power-flow diagram is introduced that covers the entire range of power stages from the energy source to the flow-control success. All individual power stages are explained, existing controversial definitions are clarified, and an evaluation guideline is applied to previously obtained data. Finally, the defined systematic analysis is applied to the results of a recently conducted plasma-actuator in-flight experiment.
In this work, an experimental study of spray impact onto a horizontal flat and rigid surface is p... more In this work, an experimental study of spray impact onto a horizontal flat and rigid surface is presented to specify the suitable measurement volume height above the surface in which the Phase-Doppler measurements are valid. Furthermore the influence of the input laser power on the characteristics of the impinging and secondary spray is studied in detail. The phase Doppler technique has then been used to characterise both the impacting and the secondary spray in terms of number flux, size distribution and velocities of the droplets above the target. A high-speed CCD camera has been used to characterize the splashing or rebounding droplets and to measure the average film thickness formed due to spray impact. In this paper some new considerations in Phase-Doppler measurements of spray-wall interaction are presented.
Public reporting burden for this collection of information is estimated to average 1 hour per res... more Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing the burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.
The temperature field within evaporating ethanol droplets is investigated, relying on the two-col... more The temperature field within evaporating ethanol droplets is investigated, relying on the two-color laser induced fluorescence (LIF) measurement technique and on a Direct Numerical Simulation (DNS). The configuration studied corresponds to a monodisperse droplet stream in a diffusion flame sustained by the droplet vapor. An experimental probe volume, small compared to the droplet size, is used to characterize the temperature field within the droplets, whereas DNS takes into account key aspects of the droplet heating and evaporation such as the non-uniform and transient stress, and the mass and heat transfer coefficients at the droplet surface. These investigations reveal that the frictional stresses are strongly reduced due to the small spacing between the droplets. They also show that the Marangoni effect has a significant influence on the internal motion and hence on the internal temperature field.
A phase averaged optical measurement technique is used for characterization of externally excited... more A phase averaged optical measurement technique is used for characterization of externally excited boundary-layer disturbances. Two-dimensional velocity fields of flat-plate boundary-layer flow are recorded via Particle Image Velocimetry to resolve the effect of Dielectric Barrier Discharge plasma actuators on Tollmien-Schlichting waves. These are artificially excited by a vibrating disturbance source flush-mounted with the surface of the flat plate. A delay-generator is used to trigger the image acquisition on predetermined phase angles of the excitation frequency. Phase averaging unveils the temporal and spatial evolution of the boundary-layer disturbances. Characteristics of the excited waves are quantified and compared to linear stability theory. The phase-locked technique allows the evaluation of plasma actuation on the excited waves. A reduction of the wave amplitude by 26% was recorded for a continuously operated, single Dielectric Barrier Discharge actuator. A comprehensive investigation of the actuator effect under variable flow velocities and pressure gradients is presented in this manuscript.
ABSTRACT In this investigation the flow in a generic swirl tube with a tangential double-inlet sw... more ABSTRACT In this investigation the flow in a generic swirl tube with a tangential double-inlet swirl generator and variable exit orifices was experimentally investigated. Using magnetic resonance velocimetry (MRV) three-dimensional, three-component velocity fields were measured for two different Reynolds numbers: 10,000 and 15,000, and for three different exit orifices. The swirl generator had a fixed geometry producing an initial swirl number of 1.6 for all cases. One major observation is the occurrence of a three-layered flow structure. An annular main flow was surrounded by a recirculation zone, as reported in previous literature. However, this recirculation zone – also of an annular shape – exhibited a third layer inside: a thin, high speed jet in the center of the tube with the same flow direction as the main flow. Therefore, the conventional classification of swirling flows into ring and recirculation zone, has to be extended by a core zone. This three-layered flow structure develops independently of the exit configuration. Helical structures were observed in the near-wall region for all cases investigated. Applying an eccentric exit orifice results in the occurrence of strong stationary helical structures not only in the near-wall region but also in the center of the tube. The results, deviating significantly from previous results in the literature, underline the need for more detailed research on the topic of cyclone type flows.
ABSTRACT A novel diagnostic approach to quantify discharge characteristics of dielectric barrier ... more ABSTRACT A novel diagnostic approach to quantify discharge characteristics of dielectric barrier discharge (DBD) plasma actuators is introduced and demonstrated for actuation in quiescent air and at high-speed conditions. Using the method a significant decrease of actuator performance is observed for operation at higher Mach numbers. As such, calibrations of the actuator obtained in quiescent air can be shown to strongly overestimate the exerted body force at higher Mach numbers, thus biasing numerical simulations of the actuator effects on the flow. Furthermore, the new approach for monitoring actuator discharge also provides clear guidelines for matching the impedance of the electrical system and for estimating the degree of system de-tuning as the air-flow velocity changes.
The current study is devoted to investigating velocity fields produced by Dielectric Barrier Disc... more The current study is devoted to investigating velocity fields produced by Dielectric Barrier Discharge (DBD) plasma actuators in quiescent air using a PIV system. The purpose of the study is to determine whether features in the velocity field can be recognized, which already allow direct conclusions about how effective the actuator might be for a particular flow control application. The parameter space investigated in the experiments comprises several electrode sizes, modulation frequencies and actuator voltages. Our interest is focussed at the present time on stabilization of boundary layers or delay of transition. To identify conducive induced velocity fields, we have chosen to examine the proper orthogonal decomposition of the velocity field and show that this representation can have direct physical interpretation of the influence exerted on the boundary layer. Comparing the present results to previous experience with various actuator configurations, we conclude that the following approach is viable and should be persude.
ABSTRACT This article addresses the direct experimental measurement of Tollmien-Schlichting waves... more ABSTRACT This article addresses the direct experimental measurement of Tollmien-Schlichting waves in a flat plate, laminar boundary layer excited by velocity perturbations; the free-stream velocity was 16 m/s, the excitation frequency 250 Hz. The two-dimensional velocity field in proximity of the flat plate was captured using a conventional PIV system; however the image recording was phase locked with the disturbance source and ensemble averaging was used to obtain mean velocity characteristics of the Tollmien-Schichting waves. In particular, after subtraction of the mean velocity, the characteristics of the excited waves in terms of isolines and streamlines were extracted, revealing that the investigated waves represented velocity deviations in the order of magnitude of 1 % of the undisturbed free-stream flow. This study is a prelude to the use of the same technique to visualize the affect of dielectric barrier discharge plasma actuators on the suppression of such Tollmien-Schlichting waves, which is difficult using other measurement techniques.
Particle Image Velocimetry (PIV) is used to characterize the flowfield induced by Dielectric-Barr... more Particle Image Velocimetry (PIV) is used to characterize the flowfield induced by Dielectric-Barrier Discharge (DBD) actuators, employed for flow control applications. Close-up imaging of a flat-plate boundary layer (BL) at freestream velocities between 1 m/s and 12m/s reveals characteristics of the boundary-layer profiles affected by actuator operation. High-speed 2D-2C PIV measurements visualize the transient flow after starting a DBD actuator in quiescent air. Vortical flow features during initiation of DBD actuation are identified and related to the mode of operation and geometry. A power-law is found to represent the vortex evolution and is a first step towards universal scaling rules for DBD actuators.
A new procedure of determining the time resolved capacitance of a plasma actuator during operatio... more A new procedure of determining the time resolved capacitance of a plasma actuator during operation is introduced, representing a simple diagnostic tool that provides insight into the phenomenological behavior of plasma actuators. The procedure is demonstrated by presenting example correlations between consumed electrical energy, size of the plasma region and the operating voltage. It is shown that the capacitance of a plasma actuator is considerably increased by the presence of the plasma; hence a system which has previously been impedance matched can be considerably de-tuned when varying the operating voltage of the actuator. Such information is fundamental for any attempts to increase the energy efficiency of plasma-actuator systems. A combined analysis of the capacitance, light emission, size of the plasma region, force production and power consumption is presented.
In this study the aerodynamic interactions between tandem pitching and plunging airfoils, inspire... more In this study the aerodynamic interactions between tandem pitching and plunging airfoils, inspired by dragonfly flight, were investigated at Re 30; 000. The primary parameter to be varied was the phase angle between fore and hindfoils. The aerodynamic interactions were investigated numerically using the unsteady Reynolds-averaged Navier-Stokes equations as well as experimentally in a wind tunnel with the aid of particle image velocimetry. The numerical simulations proved to be reliable in the prediction of the positions and intensities of the shed vortices. Two major aerodynamic interactions were detected. The first type of interaction occurred when the forefoil leading-edge vortex induced a separation on the lower surface of the hindfoil. This was observed for airfoil phasings with 60, 90, and 120. This vortex-induced separation exerted a strong suction forward and downward and was responsible for high, instantaneous values of thrust. The second type of interaction was observed when the forefoil trailing-edge vortex induced a separation on the upper surface of the hindfoil at airfoil phasings of 0, 30, and 60. This localized separation exerted a relative strong suction normal to the airfoil surface, increasing the lift as well as the drag. Moreover, this second form of vortex-induced separation over the airfoil upper surface allowed for instantaneous energy extraction on the hindfoil upstroke, i.e., energy recovery.
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