This study investigated the applicability of Particle Image Velocimetry (PIV) as a velocity measurement technique for use in wind tunnel flows. To carry out the investigation, a small scale wind tunnel was designed and built to be used specifically with PIV. The tunnel employed a novel contraction geometry which was compared to six other contraction designs using a computational fluid dynamics (CFD) software package. The wind tunnel configuration allowed for full optical access in the test section to allow for PIV measurements in three dimensions. The calibration and characterization of the flow quality within the wind tunnel were performed using PIV. Velocity measurements were obtained in the empty test section to assess the degree of uniformity, alignment, and turbulence at various test speeds. The longitudinal velocities were found to deviate by an average of 1. 8% along any given velocity profile. The flow was found to be well aligned with the test section walls, deviating by no more than &plusmn;0. 20° in most cases. As well, the turbulence levels in the test section were found to be low, with average intensities of 2. 0% and 0. 5% in the longitudinal and transverse directions, respectively. Following the characterization of the flow in the empty wind tunnel, a square cylinder was placed in the test section and PIV measurements were performed at a Reynolds number of 21,400. Mean velocities and turbulence intensities measured around the square cylinder were found to compare well with previous works conducted at similar Reynolds numbers in water flows. As a final validation of the wind tunnel/PIV system, measurements were made of the flow over a 1:18 scale Formula One racecar model at a free stream velocity of 40 <i>m/s</i>. The PIV system collected a large quantity of velocity information around the model, providing insight into the aerodynamic aspects of racecars such as downforce devices and vehicle draughting. The experiments performed in this study led to the conclusion that PIV is indeed a measurement technique with high potential for use in small wind tunnels, providing more spatially resolved velocity data than any other known measurement technique. The advancement of digital camera technology will make PIV a more practical measurement technique for use in larger wind tunnels as well.

This study investigated the applicability of Particle Image Velocimetry (PIV) as a velocity measurement technique for use in wind tunnel flows. To carry out the investigation, a small scale wind tunnel was designed and built to be used specifically with PIV. The tunnel employed a novel contraction geometry which was compared to six other contraction designs using a computational fluid dynamics (CFD) software package. The wind tunnel configuration allowed for full optical access in the test section to allow for PIV measurements in three dimensions. The calibration and characterization of the flow quality within the wind tunnel were performed using PIV. Velocity measurements were obtained in the empty test section to assess the degree of uniformity, alignment, and turbulence at various test speeds. The longitudinal velocities were found to deviate by an average of 1. 8% along any given velocity profile. The flow was found to be well aligned with the test section walls, deviating by no more than &plusmn;0. 20° in most cases. As well, the turbulence levels in the test section were found to be low, with average intensities of 2. 0% and 0. 5% in the longitudinal and transverse directions, respectively. Following the characterization of the flow in the empty wind tunnel, a square cylinder was placed in the test section and PIV measurements were performed at a Reynolds number of 21,400. Mean velocities and turbulence intensities measured around the square cylinder were found to compare well with previous works conducted at similar Reynolds numbers in water flows. As a final validation of the wind tunnel/PIV system, measurements were made of the flow over a 1:18 scale Formula One racecar model at a free stream velocity of 40 <i>m/s</i>. The PIV system collected a large quantity of velocity information around the model, providing insight into the aerodynamic aspects of racecars such as downforce devices and vehicle draughting. The experiments performed in this study led to the conclusion that PIV is indeed a measurement technique with high potential for use in small wind tunnels, providing more spatially resolved velocity data than any other known measurement technique. The advancement of digital camera technology will make PIV a more practical measurement technique for use in larger wind tunnels as well.

Dynamic stall has proven to be a complex problem in helicopter aerodynamics because it limits the helicopter flight regime. Dynamic stall is characterized by drastic increases in lift and a delay of stall due to rapid pitching motions of aerodynamic surfaces. Prediction and control of dynamic stall requires an understanding of the leading edge flow structure. An investigation was conducted of dynamic stall near the leading edge of a large-scale Sikorsky SSC-A09 airfoil, dynamically pitching about its quarter chord, under realistic helicopter flight conditions (M_infinity = 0.1, k = 0.1, Re_c = 1.0 x 10^6). A testing model with a chord of 0.46 m and a span of 2.13 m was designed and constructed for experimentation in the Dynamic Stall Facility at Texas A&M University. Particle image velocimetry data were recorded for the first 15% of the airfoil chord. Mean velocities, Reynolds stresses, and vorticity were computed. Analyses revealed that during the upstroke, stall onset is delayed in the leading edge region and the first indications of separation are observed at 18 degree angle of attack. The edge of the boundary layer has been characterized for alpha = 18 degrees. The roles of the Reynolds stresses and vorticity are examined.

Zugl.: Karlsruhe, Univ., Diss., 2009

Zugl.: Aachen, Techn. Hochsch., Diss., 2009.

An experimental research program was designed to study laminar flows through and over models of porous media with or without inertial effects. The models used were made up of circular or square rods arranged to cover solid volume fraction ϕ ranging from 0.03 to 0.49, and filling fraction h / H ranging from 0.34 to 1 of the test channel. In this way, the ratios of the depth of the test section to the porous medium pore H / l ranged from 5.75 to 18.25. Three types of model porous media were tested: (1) two-dimensional ‘horizontal’ models, having rod axes aligned along the span of the channel in a staggered or non-staggered fashion; (2) three-dimensional ‘vertical’ models with rod axes aligned in the transverse direction; and (3) three-dimensional ‘mesh’ models with rod axes aligned along both transverse and spanwise directions. Using a pressure-driven viscous fluid, the bulk Reynolds number Rebulk was varied from 0.1 to 10.3. Velocity measurements were obtained using particle image velocimetry at various streamwise-transverse planes of the test section. Differential pressure measurements were also obtained using electronic transducers. These measurements were used to determine relevant governing equations for the flow through the porous media; to characterize the effects of ϕ rod shape and arrangement, h / H, H / l, porous media dimensionality, and Rebulk on the flow; and to predict the flow at the porous medium-free flow interface. The Izbash and quadratic Forchheimer equations were respectively found to describe well the flow through two- and three-dimensional porous media. Penetration of the free flow into the porous medium varied with ϕ and rod arrangement, but was nearly independent of the rod shape. At the interface between the porous medium and the free flow, h / H and H / l effects were found to be counteractive. Penetration was highest for the vertical models compared with the mesh and horizontal models. Inertial dependence of interfacial flow was weak when porous medium conditions were considered. The interfacial flow was found to follow a dose response formulation with a predictable slip coefficient.

A Horizontal Axis Wind Turbine (HAWT) experiences imbalanced loads when it operates under yaw loads. For each blade element of the aerodynamically imbalanced rotor, not only is the angle of attack unsteady, but also the corresponding incident velocity, a fact usually unfairly ignored. For the unsteady angle of attack, a pitch oscillating airfoil has been studied experimentally and numerically when 3.5×10⁴<Re<10⁵. For small wind tunnel airfoils, Particle Image Velocimetry (PIV) was utilized to determine the aerodynamic loads and the pressure field where other measurement techniques are either intrusive or very challenging. For dynamic airfoils in highly separated flow fields, i.e., deep dynamic stall phenomena, loads were calculated successfully based on the control-volume approach by exploring ways to reduce the level of uncertainties in particular for drag estimation. Consecutive high resolution PIV velocity fields revealed that increasing the reduced frequency was followed by an enriched vortex growth time and phase delay as well as a reduced number of vortices during upstroke motion. Moreover, the locations of the vortices after separation were influenced by each other. Laminar separation bubble height also showed a reducing trend as the reduced frequency increased. The nature of the vortex sheet vortices before stall were explored in two Reynolds numbers, with and without laminar separation bubbles, at low angles of attack. For all cases, a vortex sheet was the result of random vortex sheding while a longer vortex sheet was more favorable for lift augmentation. A wake study and averaged drag calculation at low angles of attack were also performed with Laser Doppler Anemometry (LDA) for Re=10⁵. For the unsteady incident velocity, longitudinal freestream oscillations have been studied numerically, since experimental study of an unsteady freestream is challenging. In this regard, the streamwise freestream velocity and pitch angle of incidence oscillated with the same frequency in a wide range of phase differences. Changing the phase difference caused variation of the results, including significantly augmented and dramatically damped dynamic stall loads, both increasing and decreasing trends for vortex growth time during phase increase and shifted location of the maximum loads. The results showed strong dependency on the velocity and acceleration of the freestream during dynamic stall and the dynamic stall characteristics differed significantly from those of the steady freestream states. The results also demonstrated consistent trends regardless of the airfoil shape and the Reynolds number while Re=10⁵ and 10⁶. The vortex study presented here not only provides information about the unsteady aerodynamic forces, but also knowledge regarding airfoil noise generation and distributed flow for downstream objects beyond wind turbine applications.

An experimental research program was designed to study laminar flows through and over models of porous media with or without inertial effects. The models used were made up of circular or square rods arranged to cover solid volume fraction ϕ ranging from 0.03 to 0.49, and filling fraction h / H ranging from 0.34 to 1 of the test channel. In this way, the ratios of the depth of the test section to the porous medium pore H / l ranged from 5.75 to 18.25. Three types of model porous media were tested: (1) two-dimensional ‘horizontal’ models, having rod axes aligned along the span of the channel in a staggered or non-staggered fashion; (2) three-dimensional ‘vertical’ models with rod axes aligned in the transverse direction; and (3) three-dimensional ‘mesh’ models with rod axes aligned along both transverse and spanwise directions. Using a pressure-driven viscous fluid, the bulk Reynolds number Rebulk was varied from 0.1 to 10.3. Velocity measurements were obtained using particle image velocimetry at various streamwise-transverse planes of the test section. Differential pressure measurements were also obtained using electronic transducers. These measurements were used to determine relevant governing equations for the flow through the porous media; to characterize the effects of ϕ rod shape and arrangement, h / H, H / l, porous media dimensionality, and Rebulk on the flow; and to predict the flow at the porous medium-free flow interface. The Izbash and quadratic Forchheimer equations were respectively found to describe well the flow through two- and three-dimensional porous media. Penetration of the free flow into the porous medium varied with ϕ and rod arrangement, but was nearly independent of the rod shape. At the interface between the porous medium and the free flow, h / H and H / l effects were found to be counteractive. Penetration was highest for the vertical models compared with the mesh and horizontal models. Inertial dependence of interfacial flow was weak when porous medium conditions were considered. The interfacial flow was found to follow a dose response formulation with a predictable slip coefficient.

Stuttgart, Univ., Diss., 2008.

Thesis (Ph. D.)--University of Missouri--Rolla, 2007. / "UTC R155." Title from PDF title screen. Includes bibliographical references (p. 130-135). Also available online.