Zifeng Yang

An Experimental Study of the Laminar Flow Separation on a Low-Reynolds-Number Airfoil

An experimental study was conducted to characterize the transient behavior of laminar flow separation on a NASA low-speed GA (W)-1 airfoil at the chord Reynolds number of 70,000. In addition to measuring the surface pressure distribution around the airfoil, a high-resolution particle image velocimetry (PIV) system was used to make detailed flow field measurements to quantify the evolution of unsteady flow structures around the airfoil at various angles of attack (AOAs). The surface pressure and PIV measurements clearly revealed that the laminar boundary layer would separate from the airfoil surface, as the adverse pressure gradient over the airfoil upper surface became severe at AOA ≥8.0 deg. The separated laminar boundary layer was found to rapidly transit to turbulence by generating unsteady Kelvin-Helmholtz vortex structures. After turbulence transition, the separated boundary layer was found to reattach to the airfoil surface as a turbulent boundary layer when the adverse pressure gradient was adequate at AOA 12.0 deg.

An Experimental Investigation on Aerodynamic Hysteresis of a Low-Reynolds Number Airfoil

( ) An experimental study was conducted to investigate the aerodynamic characteristics of a NASA low speed GA(W)-1 airfoil at the chord Reynolds number of ReC=160,000. Aerodynamic hysteresis was observed for the angles of attack close to the static stall angle of the airfoil. In addition to mapping surface pressure distribution around the airfoil, a high-resolution PIV system was used to make detailed flow field measurements to quantify the occurrence and behavior of laminar boundary layer separation and transition on the airfoil when aerodynamic hysteresis occurs. The flow field measurements were correlated with the airfoil surface pressure measurements to elucidate underlying fundamental physics. For the same angle of attack in hysteresis loop, the flow obtained along the increasing angle branch was found to result in an almost attached flow with small unsteadiness, higher lift and lower drag, whereas the one with decreasing angle of attack branch was associated with large unsteadiness, lower lift, and higher drag. The hysteresis was found to be closely related to the behavior of the laminar boundary layer separation and transition on the airfoil. The ability of the flow to remember its past history is believed to be responsible for the hysteretic behavior.

Visualization of the tip vortices in a wind turbine wake

In the present study, an experimental study was conducted to characterize the formation and the evolution of the helical tip vortices and turbulent flow structures in the wake of a horizontal axis wind turbine model placed in an atmospheric boundary layer wind. A high-resolution particle image velocimetry system was used to make detailed flow field measurements to quantify the time evolution of the helical tip vortices in relation to the position of the rotating turbine blades in order to elucidate the underlying physics associated with turbine power generation and fatigue loads acting on the wind turbines.Graphical abstract

Study of Trailing-Edge Cooling Using Pressure Sensitive Paint Technique

at five different blowing ratios between 0.4 and 1.6, both with and without moveable lands mounted on the trailingedge model. The measurement results indicate clearly that the blowing ratio and the existence of the lands would affect the film cooling effectiveness of the tailing-edge design significantly. The detailed film cooling effectiveness maps were correlated with the characteristics of the flow structures revealed from the particle image velocimetry measurements to elucidate underlying physics and to explore and optimize design paradigms for a better protection of the critical portions of turbine blades from the extremely hot environment.

An Experimental Investigation on the Wake Characteristics of a Wind Turbine in an Atmospheric Boundary Layer Wind

An experimental study is conducted to characterize the dynamic wind loads and evolution of the turbulent vortex and flow structures in the wake of a horizontal axis wind turbine (HAWT). In addition to measuring dynamic wind loads (both aerodynamic forces and moments) acting on a wind turbine model, a high-resolution Particle Image Velocimetry (PIV) system was used to make “free-run” and phase-locked flow field measurements to quantify the time evolution of the turbulence vortex and flow structures in the wake of the wind turbine model. The detailed flow field measurements were correlated with the dynamic wind load measurements to elucidate the underlying physics associated with power generation and fatigue loads acting on wind turbines operating in an atmospheric boundary wind.

Aerodynamic hysteresis of a low-Reynolds-number airfoil

L OW-REYNOLDS-NUMBER airfoil aerodynamics is important for bothmilitary and civilian applications. The applications include propellers, sailplanes, ultralight man-carrying/man-powered aircraft, high-altitude vehicles, wind turbines, unmanned aerial vehicles (UAVs), and micro air vehicles (MAVs). For the applications just listed, the combination of small length scale and low flight velocities results in airfoils operating at low chord Reynolds numbers of Re < 500; 000. It is well known that many significant aerodynamic problems occur below chord Reynolds numbers of about 500,000. Hysteresis phenomena have been found to be relatively common for roundnosed airfoils at low Reynolds numbers. Aerodynamic hysteresis of an airfoil refers to airfoil aerodynamic characteristics as it becomes history dependent, i.e., dependent on the sense of change of the angle of attack, near the airfoil stall angle. The coefficients of lift, drag, and moment of the airfoil are found to be multiple-valued rather than single-valued functions of the angle of attack. Aerodynamic hysteresis is of practical importance because it produces widely different values of lift coefficient and lift-to-drag ratio for a given angle of attack. It could also affect the recovery from stall and/or spin flight conditions. Whereas aerodynamic hysteresis associated with the pitchingmotion of airfoils (also known as dynamic stall) has been investigated extensively as summarized in the review article of McCorskey [1], hysteresis phenomena observed for static stall of an airfoil have received much less attention. Mueller [2] investigated the aerodynamic characteristics of Lissaman 7769 and Miley M06-13-128 airfoils at low Reynolds numbers, and found both airfoils produced hysteresis loops in the profiles of measured lift and drag forces when they operated below chord Reynolds numbers of 300,000. Based on qualitative flow visualization with smoke, he suggested that airfoil hysteresis is closely related to laminar boundary-layer transition and separation on the airfoils. Hoffmann [3] studied the aerodynamic characteristics of a NACA 0015 airfoil at a chord Reynolds number of 250,000, and hysteresis loopwas observed in themeasured coefficients of drag and lift. He also found that hysteresis was observed for low-freestream turbulence cases but disappeared for high-freestream turbulence cases. More recently, Mittal and Saxena [4] conducted a numerical study to predict the aerodynamic hysteresis near the static stall angle of a NACA 0012 airfoil in comparison with the experimental data of Thibert et al. [5]. In the present study, we report the measurement results of an experimental study to investigate aerodynamic hysteresis near the static stall angle of a low-Reynolds-number airfoil. In addition to mapping surface pressure distribution around the airfoil with pressure sensors, a high-resolution particle image velocimetry (PIV) system was used to make flowfield measurements to quantify the occurrence and behavior of boundary-layer transition and/or separation on the airfoil when aerodynamic hysteresis occurs. To the best knowledge of the authors, this is the first effort of its nature. The primary objective of the present study is to gain further insight into fundamental physics of aerodynamic hysteresis. In addition, the quantitative flowfield measurements will be used as the database for the validation of computational fluid dynamics (CFD) simulations of such complex phenomena for the optimum design of low-Reynoldsnumber airfoils.

An experimental investigation on the multiphase flows and turbulent mixing in a flat-panel photobioreactor for algae cultivation

Keeping an appropriate mixing state of the multiphase flows in photobioreactors (PBRs) is a key issue for the optimal design and operation of the PBRs. In the present study, an experimental investigation is conducted to quantify the turbulent mixing of multiphase flows inside a flat-panel PBR and its consequential effects on the performance of the PBR for algae cultivation. While a high-resolution particle image velocity (PIV) system is used to achieve detailed flow field measurements to quantify the unsteady behaviors of the multiphase flows and turbulent mixing inside the PBR, algae cultures are also grown in the same PBR under the same test conditions. Detailed flow field measurement results are correlated with the algae growth performance in order to elucidate the underlying physics and explore/optimize design paradigms. The measurement results reveal that even though the airflow rate that is supplied to the PBR plays a dominant role in determining the characteristics of the turbulent mixing in the PBR, the geometric positioning of the aeration inlets also significantly contributes to the turbulent mixing. These differences in turbulent mixing cause differences in algae productivity within the PBR, clearly effecting efficiency of the PBR.

An Experimental Investigation on the Flow Separation on a Low-Reynolds-Number Airfoil

An experimental investigation was conducted to study the transient behavior of the flow separation on a NASA low-speed GA (W)-1 airfoil at the chord Reynolds numbers of 68,000. A high-resolution PIV system was used to make detailed flow field measurements in addition to the surface static pressure distribution mapping around the airfoil. The measurement results visualized clearly that a separation bubble would be generated on the airfoil upper surface if the adverse pressure gradient is adequate. The length of the separation bubble could be up to 20% of airfoil chord length and its height only about 1% of the cord length. The transient behavior of the flow separation on the airfoil, which includes the “taking-off” of the laminar boundary layer from the airfoil surface at the separation point, the generation of unsteady Kelvin-Helmholtz vortex in the separated boundary layer, the rapid transition of the separated laminar boundary layer to turbulent flow, the reattachment of the turbulent flow to the airfoil surface to form separation bubble, and the burst of the separation bubble to cause airfoil stall, were elucidated clearly and quantitatively from the detailed flow field measurements.

An Experimental Investigation on the Wake Interference of Multiple Wind Turbines in Atmospheric Boundary Layer Winds

In this study, an investigation was carried out in an atmospheric boundary layer (ABL) wind tunnel to investigate the wake interferences of multiple wind turbines sited over a flat terrain in order to elucidate the underlying physics to optimize the design of wind turbines layout in wind farm for higher power yield and better durability. Firstly, the effects of the turbine spacing and the wind farm layout on the wake interferences were investigated among multiple wind turbines sited over a flat terrain. The characteristics of the surface winds (both mean velocity and turbulence profiles) were quantified to elucidate the interaction between atmospheric boundary layer and wind farms. The detailed flow field measurements were correlated with the dynamic wind loads as well as the power outputs of the wind turbine models in both aligned and staggered wind farms. In addition, the effects of different characteristics of the incoming atmospheric boundary layer on the performance of the individual wind turbines and on the array efficiency of different wind farm layouts were also investigated. The results obtained from the present study shed light on how complex aerodynamics and efficiency of different wind farms could be affected by different factors such as the wind farm configuration, turbine spacing, as well as incoming flow turbulence level.

Visualization of flow structures around a gable-roofed building model in tornado-like winds

Graphical Abstract