Through high-fidelity numerical simulation based on the lattice Boltzmann method, we have conduct... more Through high-fidelity numerical simulation based on the lattice Boltzmann method, we have conducted an in-depth study on the heat and mass transport from an oblate spheroid neutrally suspended in a simple shear flow. In the simulation, the temperature and mass concentration are modeled as a passive scalar released at the surface of the spheroid. The fluid dynamics induced by the interaction between the carrier fluid and the suspended spheroid, as well as the resultant scalar transport process, have been extensively investigated. A coupled transport mechanism comprising several components of the flow around the oblate spheroid has been identified. The effects of the Reynolds number and the aspect ratio of the spheroid on the flow characteristics and scalar transport rate are examined. The variation of the nondimensional scalar transport rate suggests that the effect of spheroid shape on scalar transfer rate can be decoupled from the effects of Peclet and Reynolds numbers, which facil...
Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences
A quasi-steady-state model of the dissolution of a single prolate or oblate spheroidal particle h... more A quasi-steady-state model of the dissolution of a single prolate or oblate spheroidal particle has been developed based on the exact solution of the steady-state diffusion equation for mass transfer in an unconfined media. With appropriate treatment of bulk concentration, the model can predict the detailed dissolution process of a single particle in a container of finite size. The dimensionless governing equations suggest that the dissolution process is determined by three dimensionless control parameters, initial solid particle concentration, particle aspect ratio and the product of specific volume of solid particles and saturation concentration of the dissolved substance. Using this model, the dissolution processes of felodipine particles are analysed in a broad range of space of the three control parameters and some characteristics are identified. The effects of material properties indicated by the product of specific volume and saturation concentration are also analysed. The mo...
Through high-fidelity numerical simulation, the simple shear flow over regularly arranged micro p... more Through high-fidelity numerical simulation, the simple shear flow over regularly arranged micro pillars has been investigated. The essential issues to be addressed include the characteristics of a simple shear flow over quadrilateral array of micro pillars, the effect of fluid inertia on the basic flow pattern, and the decomposition of the complex surface friction. The results show that the flow is characterized by a series of microscale recirculating eddies in the gaps between the streamwise neighboring pillars. The recirculation of the micro eddies and the oscillation of the overhead flow climbing over the pillar tips create a local flow advection. At smaller Reynolds number, the fluid inertia is weak and the flow patterns are symmetrical about the pillar center. When the Reynolds number is sufficiently large, the fluid inertia takes effect and breaks the symmetrical patterns. The overhead flow tilts downward, forming a spiral long-range advection between the fluid flow above pillar array and the flow in the spaces among micro pillars. The local advection and long-range advection constitute the transport mechanism in wall-normal direction. On micro-structured walls, the total friction includes the reaction forces of micro pillars due to flow shear and flow pressure at pillar surfaces and the reaction force of bottom plane due to flow shear on bottom surface. For larger Reynolds numbers, fluid inertia prevents the fluid from flowing along the curved surface of micro pillars and reduces the equivalent shear stress of the pillar reaction force due to flow shear. At the same time, the fluid inertia makes the overhead flow impact the windward side of micro pillars more strongly and therefore increases the equivalent shear stress of the pillar reaction force due to flow pressure.
Day-to-day observations reveal numerous medical and social situations where maintaining physical ... more Day-to-day observations reveal numerous medical and social situations where maintaining physical distancing is either not feasible or not practiced during the time of a viral pandemic, such as, the coronavirus disease 2019 (COVID-19). During these close-up, face-to-face interactions, a common belief is that a susceptible person wearing a face mask is safe, at least to a large extent, from foreign airborne sneeze and cough droplets. This study, for the first time, quantitatively verifies this notion. Droplet flow visualization experiments of a simulated face-to-face interaction with a mask in place were conducted using the particle image velocimetry setup. Five masks were tested in a snug-fit configuration (i.e., with no leakage around the edges): N-95, surgical, cloth PM 2.5, cloth, and wetted cloth PM 2.5. Except for the N-95 mask, the findings showed leakage of airborne droplets through all the face masks in both the configurations of (1) a susceptible person wearing a mask for pr...
Recirculation in underexpanded supersonic jet flow with varying degrees of swirl was experimental... more Recirculation in underexpanded supersonic jet flow with varying degrees of swirl was experimentally investigated due to its potential for enhanced flame holding in supersonic combustion engines. Recirculation caused by swirl is free of solid walls, avoiding the challenge of wall cooling due to high heat fluxes. Schlieren imaging, planar laser Mie scattering, and planar particle image velocimetry were used to observe and quantify shock cell and vortex structure, and where applicable, to map velocity in and around the recirculation zone. Case study showed that different patterns of tangential injection, pressure ratios, and nozzle sizes were more or less favorable to flow recirculation. Various metrics of recirculation and shock structure were correlated with the onset, variability, characteristic length, and characteristic time of recirculation vortex. Statistical analysis of instantaneous velocities indicated that (1) recirculation velocity and recirculation zone width had standard deviations of ~ 30% and ~ 20%, respectively (in part due to unsteady precession of the vortex around the axis of mean flow); and (2) the average velocity within the recirculation zone was seen to be insensitive to the pressure ratio (and hence, to the fully expanded Mach number). The recirculation zone width was slightly increased by doubling the pressure ratio, within the range investigated. Thus, under conditions that most favored recirculation, the net effect of (1) and (2) was the formation of a toroidal vortex structure within which the average velocity was sufficiently slowed (by a factor of 3 relative to the bulk velocity) to measurably enhance flame holding in supersonic combustor applications.
Taylor vortices in a miniature mixed-flow rotodynamic blood pump were investigated using micro-sc... more Taylor vortices in a miniature mixed-flow rotodynamic blood pump were investigated using micro-scale particle image velocimetry (μ-PIV) and a tracer particle visualization technique. The pump featured a cylindrical rotor (14.9 mm diameter) within a cylindrical bore, having a radial clearance of 500 μm and operated at rotational speeds varying from 1000 to 12 000 rpm. Corresponding Taylor numbers were 700-101 800, respectively. The critical Taylor number was observed to be highly dependent on the ratio of axial to circumferential velocity, increasing from 1200 to 18 000 corresponding to Rossby numbers from 0 to 0.175. This demonstrated a dramatic stabilizing effect of the axial flow. The size of Taylor vortices was also found to be inversely related to Rossby number. It is concluded that Taylor vortices can enhance the mixing in the annular gap and decrease the dwell time of blood cells in the high-shear-rate region, which has the potential to decrease hemolysis and platelet activati...
A two-degree-of-freedom, up-scaled, robotic hummingbird model with rigid wings was used to simula... more A two-degree-of-freedom, up-scaled, robotic hummingbird model with rigid wings was used to simulate the hovering, flapping kinematics of a hummingbird to understand the flow structure around the wing and the underlying mechanism for dynamic force generation. Digital Particle Image Velocimetry (DPIV) method was applied to visualize the flow field at the mid-section of the wing over a range of Reynolds numbers (850 < Re < 13,000). It was observed that in the major duration of the stroke the leading edge vortex (LEV) was kept attached to the wing's suction side over the entire range of Reynolds numbers. It was also found that the rapid change of angle of attack at the beginning of each half stroke generated shedding of LEV. Time history of the lift and drag forces were measured to relate the dynamic response with the flow field development. The magnitudes of lift and drag forces increased with Reynolds number, however, the overall lift coefficient decreased for Re < 6,000....
Experiments on a scaled model wind turbine, designed using blade element momentum theory, were pe... more Experiments on a scaled model wind turbine, designed using blade element momentum theory, were performed under laminar inflow conditions with and without yaw. A detailed dataset containing wake structure variations under yawed inflow was obtained to provide useful validation data for certain classes of simulation codes. Phase locked PIV experiments performed at various blade orientations (phases) showed that the turbulence characteristics in the mid- to far-wake region is approximately axisymmetric. The power extracted by the model was obtained from the horizontal velocity deficit observed at the wake and compared with the power obtained from torque sensor measurements. Significant differences between these two measurements demonstrate the importance of losses due to viscous and turbulent dissipation.
Rotodynamic blood pumps (also known as rotary or continuous flow blood pumps) are commonly evalua... more Rotodynamic blood pumps (also known as rotary or continuous flow blood pumps) are commonly evaluated in vitro under steady flow conditions. However, when these devices are used clinically as ventricular assist devices (VADs), the flow is pulsatile due to the contribution of the native heart. This study investigated the influence of this unsteady flow upon the internal hemodynamics of a centrifugal blood pump. The flow field within the median axial plane of the flow path was visualized with particle image velocimetry (PIV) using a transparent replica of the Levacor VAD. The replica was inserted in a dynamic cardiovascular simulator that synchronized the image acquisition to the cardiac cycle. As compared to steady flow, pulsatile conditions produced periodic, transient recirculation regions within the impeller and separation in the outlet diffuser. Dimensional analysis revealed that the flow characteristics could be uniquely described by the nondimensional flow coefficient (F) and its time derivative (_ U), thereby eliminating impeller speed from the experimental matrix. Four regimes within the F-_ U plane were found to classify the flow patterns, well-attached or disturbed. These results and methods can be generalized to provide insights for both design and operation of rotodynamic blood pumps for safety and efficacy.
When leaves and strips of paper fall in air, their trajectories can be quite complicated. This gr... more When leaves and strips of paper fall in air, their trajectories can be quite complicated. This gravity-driven motion has drawn attention for ages in various research fields, e.g. aerospace engineering, meteorology, and biomechanics in breed dispersal, etc [1, 2]. Due to the geometry of leaves or papers, straight vertical falling path was very rare because tiny perturbation will be amplified quickly and results in aerodynamic force and moments which will change the falling motion significantly. Depending on the density ratio and dimensions of the falling plates, two types of motions were observed. The first is that the plate slides from side to side periodically while falling, which is called fluttering; the other type is called tumbling, in which case the plate maintains unidirectional rotation with respect to one of its horizontal primary axes and falls downward to one inclined direction. The first systematic study of the falling plate phenomenon dates back to Maxwell in 1940 [3]. He generalized some preliminary experiment results and qualitatively summarized the mechanism for different types of motion of falling paper strips. In 1964, Willmarth et al. systematically conducted the freely falling disk experiments with various types of circular disks [4]. With various combination of Reynolds number and non-dimensional moment of inertia, the falling disks’ motion can be stable or unstable. Two types of unstable motions were observed: periodic pitching (fluttering) and tumbling. The falling modes are characterized in a phase diagram with these two non-dimensional parameters. It is also found that, in case of fluttering motion, the pitching frequency is linearly related to the non-dimensional moment of inertia I∗, indicating its influence on the falling motion when Reynolds number is high (Re ∼ 104). In 1971, Smith et al. [5] performed experimental study about auto-rotation of the fixed wing to uncover the mechanism and dynamics of the autorotation phenomenon with different Reynolds and Strouhal numbers. Also, some preliminary experiments about freely falling autorotation disks are conducted to study the inner mechanism in the tumbling motion. In 1998, Belmonte et al. [6] conducted quasi-2-dimensional experiments with thin flat strips to study the transition from fluttering to tumbling. In their research the transition is found to be determined by the Froude number (similar physical meaning as the non-dimensional moment of inertia) and the transition occurs at Fr = 0.67±0.05. In addition, a phenomenological model related to the Froude number similarity is proposed to simulate the inertial lift and drag forces and successfully reproduced the motion. Recently in 2011, experimental studies on the 3 dimensional trajectory of the falling disks with low I∗ are conducted by Zhong et al. [7]. They observed 3 types of falling motions, which are governed by I∗ and Reynolds number, named as planar zigzag, transitional and spiral motion, respectively. Numerous theoretical models are also proposed and developed to predict the trajectories of falling plates. Unsteady nonlinear coupling of fluid and structure has brought inevitable complexity. In 1994, Tanabe et al. [8] modified the classical Kutta-Joukowski’s theorem for the 2-D falling plate problem and successfully revealed 5 types of paths for the falling plates: periodic rotation (PR), chaotic rotation (CR), chaotic fluttering (CF), periodic fluttering (PF) and simple perpendicular fall (SPF). In this model, different values of friction coefficient has been used. In addition, Mahadevan, [9] in 1999, summarized his experimental results about the steady tumbling paper cards in air and discovered that the autorotation frequency of the tumbling cards are related to its dimension by the law Ω T 1/2L−1, i.e. Ω = Ω(L/T 1/2), where Ω is the tumbling frequency, and T and L are the thickness and length of the paper card respectively. In 2005, Anderson, et al. [10] built a theoretical quasi-steady model by extending the Kutta-Joukowski’s theorem and used data fitting method to reproduce the dynamics response of plates, which yielded qualitative agreement with his experiment measurements. ∗PhD Candidate, Department of Mechanical and Aerospace Engineering †Assistant Professor, Department of Mechanical and Aerospace Engineering
49th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, 2011
The aerodynamics of a flight-worthy, radio controlled ornithopter is investigated using a combina... more The aerodynamics of a flight-worthy, radio controlled ornithopter is investigated using a combination of Particle-Image Velocimetry (PIV), load cell measurements, and high-speed photography of smoke visualizations. The lift and thrust forces of the ornithopter are measured at various flow speeds, flapping frequencies and angles of attack to characterize the flight performance. These direct force measurements are then compared with forces estimated using control volume analysis on PIV data. High-speed photography of smoke streaks is used to visualize the evolution of leading edge vortices, and to qualitatively infer the effect of wing deformation on the net downwash. Vortical structures in the wake are compared to previous studies on root flapping, and direct measurements of flapping efficiency are used to argue that the current ornithopter operates sub-optimally in converting the input energy into propulsive work. Nomenclature U Horizontal velocity component V Vertical velocity component St Strouhal number Re Reynolds number based on free-stream speed and chord length D Drag c Chord length s Wingspan T Thrust V Voltage Curre nt
Seminars in Thoracic and Cardiovascular Surgery: Pediatric Cardiac Surgery Annual, 2006
... Department of Surgery, University of Pittsburgh, Pittsburgh, PA. ¶ McGowan Institute for Rege... more ... Department of Surgery, University of Pittsburgh, Pittsburgh, PA. ¶ McGowan Institute for Regenerative Medicine, University of Pittsburgh, Pittsburgh, PA. Department of Biomedical Engineering, Carnegie Mellon University, Pittsburgh, PA. †† LaunchPoint, LLC, Goleta, CA. ...
41st AIAA Fluid Dynamics Conference and Exhibit, 2011
National Laboratory-Numerous studies have demonstrated the importance of the leading edge vortex ... more National Laboratory-Numerous studies have demonstrated the importance of the leading edge vortex (LEV) in enhancing lift production during hovering flight for a hummingbird. Almost all of these experiments have been performed under laminar inflow conditions without the presence of transient flow phenomena (e.g. gust). And yet, real-life ornithopters in the field have to routinely tackle gust and directional changes in the wind. In this talk, preliminary results from an investigation of the flow field modulation around a hummingbird wing under well-controlled gusty conditions are presented. Using a 2-degree of freedom robotic hummingbird model wing mounted on a translation stage, conditions replicating a gust impacting a wing are created at the NMSU water channel facility. Phase-locked PIV velocity measurements were obtained around the wing in the presence of gusts varying from 5-30% of the mean tangential wing velocity. These measurements, in combination with force and moment measurements from a six-axis load cell, are used to understand transient flow phenomena induced by the gust, and their effect on the net thrust and lift forces on the robot's wings over a range of Reynolds number (1400<Re<20000).
Through high-fidelity numerical simulation based on the lattice Boltzmann method, we have conduct... more Through high-fidelity numerical simulation based on the lattice Boltzmann method, we have conducted an in-depth study on the heat and mass transport from an oblate spheroid neutrally suspended in a simple shear flow. In the simulation, the temperature and mass concentration are modeled as a passive scalar released at the surface of the spheroid. The fluid dynamics induced by the interaction between the carrier fluid and the suspended spheroid, as well as the resultant scalar transport process, have been extensively investigated. A coupled transport mechanism comprising several components of the flow around the oblate spheroid has been identified. The effects of the Reynolds number and the aspect ratio of the spheroid on the flow characteristics and scalar transport rate are examined. The variation of the nondimensional scalar transport rate suggests that the effect of spheroid shape on scalar transfer rate can be decoupled from the effects of Peclet and Reynolds numbers, which facil...
Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences
A quasi-steady-state model of the dissolution of a single prolate or oblate spheroidal particle h... more A quasi-steady-state model of the dissolution of a single prolate or oblate spheroidal particle has been developed based on the exact solution of the steady-state diffusion equation for mass transfer in an unconfined media. With appropriate treatment of bulk concentration, the model can predict the detailed dissolution process of a single particle in a container of finite size. The dimensionless governing equations suggest that the dissolution process is determined by three dimensionless control parameters, initial solid particle concentration, particle aspect ratio and the product of specific volume of solid particles and saturation concentration of the dissolved substance. Using this model, the dissolution processes of felodipine particles are analysed in a broad range of space of the three control parameters and some characteristics are identified. The effects of material properties indicated by the product of specific volume and saturation concentration are also analysed. The mo...
Through high-fidelity numerical simulation, the simple shear flow over regularly arranged micro p... more Through high-fidelity numerical simulation, the simple shear flow over regularly arranged micro pillars has been investigated. The essential issues to be addressed include the characteristics of a simple shear flow over quadrilateral array of micro pillars, the effect of fluid inertia on the basic flow pattern, and the decomposition of the complex surface friction. The results show that the flow is characterized by a series of microscale recirculating eddies in the gaps between the streamwise neighboring pillars. The recirculation of the micro eddies and the oscillation of the overhead flow climbing over the pillar tips create a local flow advection. At smaller Reynolds number, the fluid inertia is weak and the flow patterns are symmetrical about the pillar center. When the Reynolds number is sufficiently large, the fluid inertia takes effect and breaks the symmetrical patterns. The overhead flow tilts downward, forming a spiral long-range advection between the fluid flow above pillar array and the flow in the spaces among micro pillars. The local advection and long-range advection constitute the transport mechanism in wall-normal direction. On micro-structured walls, the total friction includes the reaction forces of micro pillars due to flow shear and flow pressure at pillar surfaces and the reaction force of bottom plane due to flow shear on bottom surface. For larger Reynolds numbers, fluid inertia prevents the fluid from flowing along the curved surface of micro pillars and reduces the equivalent shear stress of the pillar reaction force due to flow shear. At the same time, the fluid inertia makes the overhead flow impact the windward side of micro pillars more strongly and therefore increases the equivalent shear stress of the pillar reaction force due to flow pressure.
Day-to-day observations reveal numerous medical and social situations where maintaining physical ... more Day-to-day observations reveal numerous medical and social situations where maintaining physical distancing is either not feasible or not practiced during the time of a viral pandemic, such as, the coronavirus disease 2019 (COVID-19). During these close-up, face-to-face interactions, a common belief is that a susceptible person wearing a face mask is safe, at least to a large extent, from foreign airborne sneeze and cough droplets. This study, for the first time, quantitatively verifies this notion. Droplet flow visualization experiments of a simulated face-to-face interaction with a mask in place were conducted using the particle image velocimetry setup. Five masks were tested in a snug-fit configuration (i.e., with no leakage around the edges): N-95, surgical, cloth PM 2.5, cloth, and wetted cloth PM 2.5. Except for the N-95 mask, the findings showed leakage of airborne droplets through all the face masks in both the configurations of (1) a susceptible person wearing a mask for pr...
Recirculation in underexpanded supersonic jet flow with varying degrees of swirl was experimental... more Recirculation in underexpanded supersonic jet flow with varying degrees of swirl was experimentally investigated due to its potential for enhanced flame holding in supersonic combustion engines. Recirculation caused by swirl is free of solid walls, avoiding the challenge of wall cooling due to high heat fluxes. Schlieren imaging, planar laser Mie scattering, and planar particle image velocimetry were used to observe and quantify shock cell and vortex structure, and where applicable, to map velocity in and around the recirculation zone. Case study showed that different patterns of tangential injection, pressure ratios, and nozzle sizes were more or less favorable to flow recirculation. Various metrics of recirculation and shock structure were correlated with the onset, variability, characteristic length, and characteristic time of recirculation vortex. Statistical analysis of instantaneous velocities indicated that (1) recirculation velocity and recirculation zone width had standard deviations of ~ 30% and ~ 20%, respectively (in part due to unsteady precession of the vortex around the axis of mean flow); and (2) the average velocity within the recirculation zone was seen to be insensitive to the pressure ratio (and hence, to the fully expanded Mach number). The recirculation zone width was slightly increased by doubling the pressure ratio, within the range investigated. Thus, under conditions that most favored recirculation, the net effect of (1) and (2) was the formation of a toroidal vortex structure within which the average velocity was sufficiently slowed (by a factor of 3 relative to the bulk velocity) to measurably enhance flame holding in supersonic combustor applications.
Taylor vortices in a miniature mixed-flow rotodynamic blood pump were investigated using micro-sc... more Taylor vortices in a miniature mixed-flow rotodynamic blood pump were investigated using micro-scale particle image velocimetry (μ-PIV) and a tracer particle visualization technique. The pump featured a cylindrical rotor (14.9 mm diameter) within a cylindrical bore, having a radial clearance of 500 μm and operated at rotational speeds varying from 1000 to 12 000 rpm. Corresponding Taylor numbers were 700-101 800, respectively. The critical Taylor number was observed to be highly dependent on the ratio of axial to circumferential velocity, increasing from 1200 to 18 000 corresponding to Rossby numbers from 0 to 0.175. This demonstrated a dramatic stabilizing effect of the axial flow. The size of Taylor vortices was also found to be inversely related to Rossby number. It is concluded that Taylor vortices can enhance the mixing in the annular gap and decrease the dwell time of blood cells in the high-shear-rate region, which has the potential to decrease hemolysis and platelet activati...
A two-degree-of-freedom, up-scaled, robotic hummingbird model with rigid wings was used to simula... more A two-degree-of-freedom, up-scaled, robotic hummingbird model with rigid wings was used to simulate the hovering, flapping kinematics of a hummingbird to understand the flow structure around the wing and the underlying mechanism for dynamic force generation. Digital Particle Image Velocimetry (DPIV) method was applied to visualize the flow field at the mid-section of the wing over a range of Reynolds numbers (850 < Re < 13,000). It was observed that in the major duration of the stroke the leading edge vortex (LEV) was kept attached to the wing's suction side over the entire range of Reynolds numbers. It was also found that the rapid change of angle of attack at the beginning of each half stroke generated shedding of LEV. Time history of the lift and drag forces were measured to relate the dynamic response with the flow field development. The magnitudes of lift and drag forces increased with Reynolds number, however, the overall lift coefficient decreased for Re < 6,000....
Experiments on a scaled model wind turbine, designed using blade element momentum theory, were pe... more Experiments on a scaled model wind turbine, designed using blade element momentum theory, were performed under laminar inflow conditions with and without yaw. A detailed dataset containing wake structure variations under yawed inflow was obtained to provide useful validation data for certain classes of simulation codes. Phase locked PIV experiments performed at various blade orientations (phases) showed that the turbulence characteristics in the mid- to far-wake region is approximately axisymmetric. The power extracted by the model was obtained from the horizontal velocity deficit observed at the wake and compared with the power obtained from torque sensor measurements. Significant differences between these two measurements demonstrate the importance of losses due to viscous and turbulent dissipation.
Rotodynamic blood pumps (also known as rotary or continuous flow blood pumps) are commonly evalua... more Rotodynamic blood pumps (also known as rotary or continuous flow blood pumps) are commonly evaluated in vitro under steady flow conditions. However, when these devices are used clinically as ventricular assist devices (VADs), the flow is pulsatile due to the contribution of the native heart. This study investigated the influence of this unsteady flow upon the internal hemodynamics of a centrifugal blood pump. The flow field within the median axial plane of the flow path was visualized with particle image velocimetry (PIV) using a transparent replica of the Levacor VAD. The replica was inserted in a dynamic cardiovascular simulator that synchronized the image acquisition to the cardiac cycle. As compared to steady flow, pulsatile conditions produced periodic, transient recirculation regions within the impeller and separation in the outlet diffuser. Dimensional analysis revealed that the flow characteristics could be uniquely described by the nondimensional flow coefficient (F) and its time derivative (_ U), thereby eliminating impeller speed from the experimental matrix. Four regimes within the F-_ U plane were found to classify the flow patterns, well-attached or disturbed. These results and methods can be generalized to provide insights for both design and operation of rotodynamic blood pumps for safety and efficacy.
When leaves and strips of paper fall in air, their trajectories can be quite complicated. This gr... more When leaves and strips of paper fall in air, their trajectories can be quite complicated. This gravity-driven motion has drawn attention for ages in various research fields, e.g. aerospace engineering, meteorology, and biomechanics in breed dispersal, etc [1, 2]. Due to the geometry of leaves or papers, straight vertical falling path was very rare because tiny perturbation will be amplified quickly and results in aerodynamic force and moments which will change the falling motion significantly. Depending on the density ratio and dimensions of the falling plates, two types of motions were observed. The first is that the plate slides from side to side periodically while falling, which is called fluttering; the other type is called tumbling, in which case the plate maintains unidirectional rotation with respect to one of its horizontal primary axes and falls downward to one inclined direction. The first systematic study of the falling plate phenomenon dates back to Maxwell in 1940 [3]. He generalized some preliminary experiment results and qualitatively summarized the mechanism for different types of motion of falling paper strips. In 1964, Willmarth et al. systematically conducted the freely falling disk experiments with various types of circular disks [4]. With various combination of Reynolds number and non-dimensional moment of inertia, the falling disks’ motion can be stable or unstable. Two types of unstable motions were observed: periodic pitching (fluttering) and tumbling. The falling modes are characterized in a phase diagram with these two non-dimensional parameters. It is also found that, in case of fluttering motion, the pitching frequency is linearly related to the non-dimensional moment of inertia I∗, indicating its influence on the falling motion when Reynolds number is high (Re ∼ 104). In 1971, Smith et al. [5] performed experimental study about auto-rotation of the fixed wing to uncover the mechanism and dynamics of the autorotation phenomenon with different Reynolds and Strouhal numbers. Also, some preliminary experiments about freely falling autorotation disks are conducted to study the inner mechanism in the tumbling motion. In 1998, Belmonte et al. [6] conducted quasi-2-dimensional experiments with thin flat strips to study the transition from fluttering to tumbling. In their research the transition is found to be determined by the Froude number (similar physical meaning as the non-dimensional moment of inertia) and the transition occurs at Fr = 0.67±0.05. In addition, a phenomenological model related to the Froude number similarity is proposed to simulate the inertial lift and drag forces and successfully reproduced the motion. Recently in 2011, experimental studies on the 3 dimensional trajectory of the falling disks with low I∗ are conducted by Zhong et al. [7]. They observed 3 types of falling motions, which are governed by I∗ and Reynolds number, named as planar zigzag, transitional and spiral motion, respectively. Numerous theoretical models are also proposed and developed to predict the trajectories of falling plates. Unsteady nonlinear coupling of fluid and structure has brought inevitable complexity. In 1994, Tanabe et al. [8] modified the classical Kutta-Joukowski’s theorem for the 2-D falling plate problem and successfully revealed 5 types of paths for the falling plates: periodic rotation (PR), chaotic rotation (CR), chaotic fluttering (CF), periodic fluttering (PF) and simple perpendicular fall (SPF). In this model, different values of friction coefficient has been used. In addition, Mahadevan, [9] in 1999, summarized his experimental results about the steady tumbling paper cards in air and discovered that the autorotation frequency of the tumbling cards are related to its dimension by the law Ω T 1/2L−1, i.e. Ω = Ω(L/T 1/2), where Ω is the tumbling frequency, and T and L are the thickness and length of the paper card respectively. In 2005, Anderson, et al. [10] built a theoretical quasi-steady model by extending the Kutta-Joukowski’s theorem and used data fitting method to reproduce the dynamics response of plates, which yielded qualitative agreement with his experiment measurements. ∗PhD Candidate, Department of Mechanical and Aerospace Engineering †Assistant Professor, Department of Mechanical and Aerospace Engineering
49th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, 2011
The aerodynamics of a flight-worthy, radio controlled ornithopter is investigated using a combina... more The aerodynamics of a flight-worthy, radio controlled ornithopter is investigated using a combination of Particle-Image Velocimetry (PIV), load cell measurements, and high-speed photography of smoke visualizations. The lift and thrust forces of the ornithopter are measured at various flow speeds, flapping frequencies and angles of attack to characterize the flight performance. These direct force measurements are then compared with forces estimated using control volume analysis on PIV data. High-speed photography of smoke streaks is used to visualize the evolution of leading edge vortices, and to qualitatively infer the effect of wing deformation on the net downwash. Vortical structures in the wake are compared to previous studies on root flapping, and direct measurements of flapping efficiency are used to argue that the current ornithopter operates sub-optimally in converting the input energy into propulsive work. Nomenclature U Horizontal velocity component V Vertical velocity component St Strouhal number Re Reynolds number based on free-stream speed and chord length D Drag c Chord length s Wingspan T Thrust V Voltage Curre nt
Seminars in Thoracic and Cardiovascular Surgery: Pediatric Cardiac Surgery Annual, 2006
... Department of Surgery, University of Pittsburgh, Pittsburgh, PA. ¶ McGowan Institute for Rege... more ... Department of Surgery, University of Pittsburgh, Pittsburgh, PA. ¶ McGowan Institute for Regenerative Medicine, University of Pittsburgh, Pittsburgh, PA. Department of Biomedical Engineering, Carnegie Mellon University, Pittsburgh, PA. †† LaunchPoint, LLC, Goleta, CA. ...
41st AIAA Fluid Dynamics Conference and Exhibit, 2011
National Laboratory-Numerous studies have demonstrated the importance of the leading edge vortex ... more National Laboratory-Numerous studies have demonstrated the importance of the leading edge vortex (LEV) in enhancing lift production during hovering flight for a hummingbird. Almost all of these experiments have been performed under laminar inflow conditions without the presence of transient flow phenomena (e.g. gust). And yet, real-life ornithopters in the field have to routinely tackle gust and directional changes in the wind. In this talk, preliminary results from an investigation of the flow field modulation around a hummingbird wing under well-controlled gusty conditions are presented. Using a 2-degree of freedom robotic hummingbird model wing mounted on a translation stage, conditions replicating a gust impacting a wing are created at the NMSU water channel facility. Phase-locked PIV velocity measurements were obtained around the wing in the presence of gusts varying from 5-30% of the mean tangential wing velocity. These measurements, in combination with force and moment measurements from a six-axis load cell, are used to understand transient flow phenomena induced by the gust, and their effect on the net thrust and lift forces on the robot's wings over a range of Reynolds number (1400<Re<20000).
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