Session L04: Aerodynamics: General

On the characteristics of pressure fluctuations around stall on a thick full-scale airfoil at high Reynolds number

Instantaneous contributions to the normal force on a thick airfoil at high Reynolds number  are determined over a range of angles of attack. In order to establish the possible loss of statistical invariance in the spanwise direction, the investigation is based on two rows of pressure sensors around the airfoil, both located close to its center. Evidence of statistical asymmetry between the chords is observed for angles of attack corresponding to the  onset of separation.  The pressure fluctuations on both chords are characterized by a local maximum that is located directly upstream of the separation point. Abrupt temporal changes are observed in the evolution of the local maximum fluctuations and are found to be anti-correlated in the spanwise direction, which confirms the observations made by Neunaber et al. (Wind Energy 2022). POD analysis along the chord shows that these  dynamics can be attributed to a dominant spatial mode that evolves continuously with the angle of attack.   

Effect of Vortex Impingement on the Aerodynamic Performance of a NACA 0010 Airfoil

This study examines the impact of vortex impingement on the aerodynamic performance of a NACA 0010 airfoil. The experimental setup involves vortex rings generated by a pulsed jet from a 2-inch nozzle at a nozzle exit Mach number of 0.1. These vortex rings impinge on the airfoil located three nozzle exit diameters downstream. The pulsation frequency, quantified using the non-dimensional Strouhal number (St), was varied from 0.06 to 0.2 to explore the full range of realistic conditions. The Reynolds numbers based on the nozzle exit diameter and the pulsed jet average exit velocity is 103,000. The measurements involve both force and flow field measurements using Particle Image Velocimetry (PIV). The results demonstrate significant and non-trivial variations in the lift coefficient at different Strouhal numbers. At St = 0.06, the lift coefficient was measured to be approximately 0.3, which increased to 0.6 at St = 0.2. These findings indicate that the airfoil's lift generation capabilities are highly influenced by the frequency of the impinging vortex. The drag coefficient also exhibits pronounced fluctuations due to vortex impingement. At St = 0.06, the drag coefficient was found to be approximately 0.04, which increased to 0.2 at St = 0.2. These results highlight the impact of vortex impingement on the airfoil's aerodynamic characteristics, leading to substantial changes in lift and drag forces. The obtained insights provide valuable guidance for designing airfoils in engineering applications such as wind turbine blades and aircraft wings, where maximizing lift and minimizing drag are essential for efficiency and performance.   

The Aerodynamics of Crescent Wings in Backward and Forward Sweep Orientations

Crescent wings are typically curved, backward swept wings of moderate aspect ratio and can be observed in fishes, birds and some aircraft. However, the crescent shape with a forward sweep also exists in some tropical fish. Here we provide results of numerical simulations using ANSYS-FLUENT on a NACA 0012 airfoil at a Re of 5.8e5 for backward swept, straight (i.e. unswept) and forward swept configurations. For unswept, mild, medium and strongly swept backward configurations our computational results agree well with existing experimental lift and drag measurements from 0 to 16 degrees AoA, which includes the sudden stall region. The simulations then provide new results for these four cases at up to 60 degrees AoA and all show stall recovery with a second lift peak at about 45 degrees AoA. Simulations of mild, medium and strongly swept forward cases generally show less lift pre-stall but more lift post-stall when compared to the backward swept cases. Notably, the stall behavior becomes much less sudden versus AoA for the three forward swept configurations, with minimal sudden stall for the most strongly swept case. In all seven cases studied, past 12 degrees AoA the flow becomes increasingly complex with leading edge separations, separation bubbles, and positive and negative vortices distributed along the span. These vortices are strongly dependent on the orientation of the leading edge of the wing - convex or concave relative to the oncoming flow - and hence on backward or forward sweep orientation of the wing.

    

Improvement of Aerodynamic Performance in Floatplane Wings with Installation of Winglets

Wing tip structures have become a standard design solution in modern airliners due to their ability to improve aerodynamic efficiency, reduce fuel consumption and consequently improve the environmental impact introduced by air transport. Their ability to interact with and modify vortex sheets also influences flight dynamics. This work focuses on the use of winglets in floatplanes to both improve aerodynamic efficiency and modify flight dynamics. Floatplanes are versatile aircraft suited to operate in remote regions where other means of transportation are not available. However, the floats, which confer the ability to take-off and land in water, introduce undesirable aerodynamic effects in flight, which are normally adjusted with the use of a set of tail-installed surfaces. CFD simulations using ANSYS CFX were carried out at flight conditions to assess the performance of winglets on a tapered NACA23012 wing. A model of the wing was tested in a wind tunnel for relevant angles of attack and speeds of 5-35 m/s. A second wing design (rectangular NACA23017 root and tapered NACA23012 tip) was additively manufactured in Nylon PA-12 for further tests. The wing lift and drag have been evaluated. The effects of the previously mentioned modification in the control forces and stability on this aircraft will be considered, and compared with the effectiveness of other more commonly used control measures, and how the aerodynamic efficiency of the proposed aircraft has changed. The main results will be presented and discussed.   

Covert-Inspired Flaps as Flight Control Surfaces for BWB aircrafts

Recently, there has been renewed interest in Blended Wing Body (BWB) aircraft because they provide reduced drag, larger lifting surfaces, and increased cargo volume compared to conventional configuration airplanes. However, the lack of a separate empennage section reduces the overall control authority available for stability and agility and complicates the flight control laws. In this paper, we present a novel control effector inspired by a feather system in birds known as the covert. The authors have previously investigated the effect of covert-inspired flaps on a two-dimensional wing section and show that the bioinspired flaps can result in 1.5 times more effective yaw control than a conventional collocated flap surface. In this presentation, we will extend the previous results to a three-dimensional wing section to investigate the effect of the covert-inspired flaps on the flow field and its resulting roll, pitch, and yaw control authority when it is deflected asymmetrically on the right and left wings. More specifically, we present results from wind tunnel experiments in Princeton University's low turbulence wind tunnel, where we systematically vary the flap location and deflection angles. Moreover, we will vary the flow angles (i.e., the angle of attack and side slip angle) and their rates to represent different flight maneuvers. Results will include flow field measurements from time-resolved PIV, force measurements, and data-driven models that relate the measured aerodynamic forces to the flap and flow parameters.   

Normal Force and Pitch Moment for a 6:1 Prolate Spheroid at Moderate Reynolds Numbers

Ideally, air- and waterborne vehicles would be designed primarily with computers using computational fluid dynamics (CFD) tools to predict the forces acting on the vehicle.  Despite significant advancements in computational power in recent decades, three-dimensional flow over doubly-curved bodies cannot currently be modeled with the desired level of accuracy in an acceptable amount of time.  Additional experimental observations are one pathway to improve computational design tools. 

The prolate spheroid has long been a favored reference for the development of CFD tools.  It is simple to describe mathematically, yet the flow around it is highly three-dimensional, featuring laminar to turbulent transition, laminar and turbulent separation, reattachment, and multiple sets of trailing vortices, depending on the flow conditions.  These phenomena provide a range of specific features against which to compare numerical results.  However, gross measures, including forces and moments, provide a first-order reference in the validation process.

The normal force and pitch moment were measured for a 6:1 prolate spheroid in the large towing tank facility at the U.S. Naval Academy for Reynolds numbers from 0.4-9×10⁶ and angles of attack from 2.5° to 20°.  The results of this work agree well with previous studies and expand the range of Reynolds numbers significantly.  A detailed description of materials and methods is provided to support CFD model validation.   

Filtered Lifting Theory and Application to Wind Turbine Blades

We present a new formulation of the filtered lifting line theory that can be applied to wings with significant changes in chord along the span, such as wind turbine blades. The new formulation can be used as a correction in large-eddy simulations of wind turbine blades using the actuator line model. The correction provides accurate blade loading when using coarse resolutions. We show how the solutions of coarse simulations with the correction match the results from fine resolutions with an optimal actuator line model. The filtered lifting line theory correction can significantly lower the computational expense of large-eddy simulations with an actuator line while providing accurate loading along the blades.   

Effects of Reynolds number on the asymmetric wake characteristics of a notchback Ahmed body

The reduction of aerodynamic drag of green vehicles such as electric and hydrogen vehicles is crucial for increasing driving range and market acceptance. The notchback Ahmed body is a simplified model of a sedan that can be used to understand the generation mechanisms of drag on ground vehicles to develop effective flow control strategies. However, unlike the square-back Ahmed body, the wake characteristics of the notchback are not well-understood. In this study, the effects of Reynolds number on the wake characteristics of a notchback Ahmed body with an effective backlight angle β = 17.8° are investigated using Reynolds-Averaged Navier-Stokes (RANS). The height-based Reynolds number was varied from 5×10³ to 5×10⁴ to demonstrate the effects of low-speed urban and highway driving conditions. To select the best model for predicting the asymmetric wake structure of the 17.8° notchback Ahmed body, detailed assessments of 12 eddy-viscosity (EV) and four second-moment closure turbulence models were performed. The predictions based on Reynolds number of 5×10⁴ were compared with the experimental results of Sims-Williams et al. 2011 and large-eddy simulations (LES) of He et al. 2021. Interestingly, the Spalart-Allmaras one-equation EV model was the only model that accurately predicted the asymmetric wake structure, and the aerodynamic coefficients were similar to the experiment and LES. Accordingly, the Spallart-Allmaras model was chosen for the Reynolds number study. The results showed that, as the Reynolds number decreases, the wake structure transitions from the asymmetric to symmetric state and this was associated with a significant increase in drag. The critical Reynolds number for the transition was found to be 1×10⁴.   

Experimental Investigation of the Wake Flow and Drag of a Luge Sled

The sport of luge is one of two Olympic sports that is timed to the 1/1000 sec. Because of the small time differences between finishers, detailed understanding of the physics is critical for success, particularly when designing equipment. Drag is an important factor of a luge run and past competitions have shown that modification of the wake flow can create significant competitive advantages. In this work we investigate the aerodynamic performance of a sled using drag measurements and high speed PIV measurements in the near wake of a luge sled. The PIV measurements reveal that the flow remains attached to the athletes body up to the neck. Two regions of reverse flow are observed on the upper and lower surfaces of the neck. The flow separates from the top of the helmet and small vortices are formed along the trailing shear layer. A low frequency, large spatial scale "flapping" of this shear layer is also observed. The wake itself was observed to be highly turbulent. The mean velocity profile in the wake showed a significant deficit in the region of the helmet, but also a significant wake deficit near to the ground plane   

The role of seams in the aerodynamics of baseballs

Spherical sports balls can generate forces due to asymmetrical transition to turbulence of the boundary layer. These forces, in turn, alter the movement of the ball. This is commonly seen in football (soccer) as well as in cricket. This effect has often incorrectly been used to explain baseball movement. We will show that in addition to causing transition to turbulence, baseball seams can act as spoilers, causing boundary layer separation closer to the front of the ball than normal. This is unique to baseballs. In fact, this effect has much more to do with baseball movement because baseball seams are arranged such that the boundary layer is nearly always turbulent everywhere on the ball at separation. In this presentation, we will show that in addition to these effects, seams can also cause turbulence transition of the boundary layer, or fail to cause transition in a narrow range of orientations. These conclusions are based upon measurements of the velocity field around a baseball in flight using particle image velocimetry. Lastly, our recent measurements on spheres roughened with macroscopic elements reveal that spin-sensitive drag is due to the wake of individual roughness elements. This may explain the reason that baseball drag is sensitive to spin. It is also possible that the seams contribute to lift due to spin.