Session E25: Aerodynamics: Fixed, Flapping and Rotating Wing I

Unsteady Forces and Flow Field on Perching Wings Close to the Ground.

During landing, birds often perform a perching maneuver that involves pitching their wing upward while decelerating to a complete stop. Such high angle of attack (AOA) maneuvers has been found to enhance the aerodynamic forces and assist in smooth landing. In this study, we want to explore how the rapid pitch-up maneuver during downward deceleration affects the evolution of flow fields and the unsteady flow fields. We consider two cases: one, where the wing only heaves downward during deceleration, and another, where the wing carries out both the heave down and pitch up motion during deceleration. In addition, we want to investigate how the wing sweep influences the perching maneuver. Results show that combined heaving and pitching resulted in higher peak forces, but the forces also decayed faster than heave only case. Our results indicate that for both the kinematics the instantaneous forces are higher in the case of the plate with a sweep. Our result also shows that the higher unsteady force on the swept plate was contributed by a stable leading-edge vortex.     

Experimental Measurement of the Interaction of an Oscillating Wing with a Vertical Gust

Recent computational studies have shown that the effects of a large vertical gust might be mitigated by oscillating wing motions. An experiment was developed to validate and further investigate the effectiveness of oscillating wings as a gust mitigation technique. A full span NACA 0012 wing model was pitched about 1/4 chord at a reduced frequency of k = 0.5. Oscillation amplitude was held fixed at α = +/- 4°.  The wing model interacted with the Actuated Recirculating Gust Generator for sUAS Studies (ARGGUS) developed at the Army Research Lab, which produced a gust with Gust Ratio (GR) of 0.3 at a Reynolds number of Re = 12,000. The experiments were designed to investigate three separate cases: the interaction of an oscillating wing with a gust, a pitch down response maneuver while oscillating to evaluate whether quasi-attached flow could be maintained through the gust interaction, and a test of gust approximation via superposition. The gust approximation case mimicked the effective angle of attack generated by the gust and superimposed that on the oscillating motion profile. Particle Image Velocimetry was used to capture the flow field and the underlying physics of the problem will be discussed.     

Streamwise gust alleviation: vortex flow modification using a rotating, swept tip on a rectangular wing

Drones and micro-air vehicles may often experience adverse weather conditions such as gusts, since these vehicles fly at low altitudes and through various landscapes. One of the key aerodynamic features of gusts is flow separation. We explore dynamic wing planform change for mitigating streamwise gusts where the velocity increase/decrease with time is a step function. A swept tip panel on a rectangular wing rotates inward during step-up gusts and outward during step-down to counter the gust effects. A quantitative flow measurement, Shake-the-Box is performed to investigate the unsteady gust flow around the AR = 4 wing and how it changes with the tip-panel actuation. The modification in vorticity field found from the experiment further justifies the load changes on the wing.   

The Interaction Between a Plunging Wing and Gusty Environment

There has been a great progress in unmanned vehicle's design and control, but these nature-inspired robots are still troubled while flying in gusty environments. However, birds and insects can easily stabilize themselves in complex gust environment. Therefore, we need to understand the gust-mitigation performance of flapping wings and flow physics of wing-gust interaction. A plunging wing, designed and built in-house, was tested in the presence of unsteady wake from a gust generator to investigate the effect of the flow disturbances on vorticity fields. The gust generator was made of multiple sinusoidally oscillating vanes placed in a closed-circuit water channel. The plunge amplitude and frequency of the oscillation were adjusted to bracket the range of Strouhal numbers relevant to the biological locomotion. Particle Image Velocimetry (PIV) was employed to quantitatively study the unsteady wake at different upstream flow speeds, oscillating frequencies, and amplitudes. We performed a flow measurement analysis to investigate the effects of vortical structures on the leading-edge vortex (LEV) of the wing. The results showed that the LEV is more strongly influenced by the gust structures at lower Strouhal numbers and the plunging wing with higher Strouhal numbers may have better performance in gust-mitigation. This research and its successive investigations may eventually lead to more efficient or better-performing Unmanned Aerial Vehicles, along with a better understanding of their fluid dynamics.   

Investigation of the Flow Around Symmetric and Cambered Airfoils Interacting with a Gust at Low Reynolds Numbers

Understanding the interaction of small Unmanned Aircraft Systems (sUAS) with vertical gusts is an important step in their development and use. Common methods for gust generation are highly transient and tend to create short-lived gust interactions. The Actuated Recirculating Gust Generator for sUAS Studies (ARGGUS) was developed to study the interaction of wings with vertical gusts of controllable length and magnitudes up to 30% of the free stream velocity at low Reynolds numbers. This work focuses on the flow around three airfoils entering and exiting vertical gusts created by the ARGGUS. Full-span NACA 0012, Eppler 387, and SD 5060 airfoils were held at static angles of attack during encounters with gusts of 20% and 30% of the free stream velocity at a Reynolds number of 12x10³. The wings also interacted with a gust of 20% of the free stream at a Reynolds number of 54.4x10³. Particle Image Velocimetry was used to capture the flow field around the airfoils. The flow around the NACA 0012 was compared to similar experiments and simulations along with the cambered wings. This allowed for an investigation into the impacts of camber during a gust encounter. Testing the Eppler and SD airfoil also allowed for analyzing gust effects on airfoils specifically designed for low Reynolds number operation.   

Characterization of the Wake of a Gust Generator for Unsteady Aerodynamics Applications

Gusts are common incidents in nature which can rapidly change the aerodynamic forces experienced by the birds, insects, and micro aerial vehicles (MAVs). Despite the great progress in their design and control, these machines are tremendously troubled while flying in turbulent environments due to rapid changes in forces that result in large safety and stability factors. An experimental investigation was conducted to characterize the gust environment generated by a cascade of sinusoidally oscillating vanes in a closed-circuit water channel. Particle Image Velocimetry (PIV) was employed to quantitatively study the wake of this gust generator at different upstream flow speeds, oscillating frequencies, and amplitudes. The vertical components of velocity and turbulence intensity were shown to be consistent for the range of measurement points chosen. The peak vertical velocity component downstream of the vanes was shown to be proportional to the maximum vane angle of attack but largely independent of reduced frequency. This gust generator is used to understand the effect of unsteady wake on the flow measurements of flapping wings which may eventually lead to more efficient or better-performing MAVs.   

The Effect of Twisting on a Heaving Flat Pate

Remotely operated aerial vehicles are used extensively by the military, industry, and civilians alike. Current research into unsteady flapping flight has been concerned with rigid foils. The purpose of this study is to investigate how a dynamically morphing foil affects the fluid-structure interactions of unsteady flapping locomotion. The effects of non-dimensional heaving amplitude and reduced frequency were studied. Two morphing modes were investigated: spanwise twisting in the direction of upward pitch (Mode A), and spanwise twisting in the direction of downward pitch (Mode B). Force sensor data showed that Mode A recovered some lift during the upstroke. Mode A also maintained near-constant lift during the transition between downstroke and upstroke, suggesting more stable locomotion. Particle Image Velocimetry (PIV) data showed that Mode A limits circulation during the downstroke, keeping C_d ≅ 0 at the cost of reduced lift. By contrast, Mode B was found to increase circulation during the downstroke, resulting in higher lift and drag. Force sensor data for Mode B showed that this effect is reversed during the upstroke, where it causes negative lift. The effects of morphing were found to be caused by changes in shear layer velocity that occur as a result of spanwise twisting.   

Not Participating

This study is motivated by understanding the galloping instability of rectangualr cylinders in the Reynolds number range 1,000 < Re <10,000 (based on the cylinder thickness). The particular focus of the investigation is to examine the presence of non quasi-steady behavior over a range of reduced velocity, oscillation amplitude and Reynolds number. To this end, experiments are conducted in a water tunnel to characterize the unsteady force acting on, and the boundary layer around a rectangular cylinder undergoing forced harmonic oscillation. The results show that non quasi-steady effects are present even at reduced velocities as high as 30 and oscillation amplitudes as small as 20% of the cylinder thickness. An analysis is conducted to understand the role of the non quasi-steady component of the force in gallopoing instability of the cylinder, and how this role is affected by key parameters.   

Lift Production of an Unsteady Self-Propelled Airfoil

In this research, we study the lift force and vortex formation of an oscillating, self-propelled NACA0012 airfoil. These experiments are enabled by the closed-loop force-feedback system of the Cyber-Physical Fluid Dynamics (CPFD) Facility (Mackowski & Williamson 2011). We employ combined heaving and asymmetric pitching motions on the airfoil, which generate both streamwise and transverse forces. Under its self-generated streamwise forces, the airfoil starts from rest and accelerates to an equilibrium cruising velocity. The asymmetry in pitching motion profile is introduced by a rotational offset, allowing the airfoil to produce a net lift force over a cycle, enabling the wing to carry the weight of a “payload”. Our parameter space is characterized by varying the heaving and pitching amplitudes. Contours of performance metrics form the basis for simple (yet remarkably useful) “Heave-Pitch Diagrams”. By exploring these diagrams, we observe the tradeoffs between lift, propulsion, and energy expenditure. We also investigate the vortex dynamics and flow physics that underpin the efficient production of coupled lift and propulsion, through PIV flow visualization.   

Experimental Investigation of Self-Propulsion in Pitching Airfoil

We experimentally investigate the self-propulsion characteristics of NACA0015 airfoil section by allowing the airfoil to propel in the longitudinal direction freely. We measure the self-propelling speed for different pitching amplitude and frequency for the airfoil pitching about three pitching points (from the leading edge to the mid-chord). The velocity increases monotonically with pitching frequency and amplitude. However, there is no further increase in the speed when the trailing edge excursion is more than half of the airfoil chord length. The distance traveled by the airfoil in terms of body length per oscillation is constant for all the frequencies but monotonically increases with amplitude and reaches a plateau. The wake exhibits different complex patterns across all the parameters, and it is characterized by a wide jet-like velocity profile with significant lateral momentum. Deflected vortex pairs are observed at lower speeds whereas, reverse BvK vortices are observed at moderate speeds. At higher speeds, many smaller vortices are shed, which coalesce into larger ones. These observations provide helpful insight into factors governing the speed and efficiency of self-propelling bodies.