Session W01: Aerodynamics: General (10:00am - 10:45am CST)

Coherent structures in the wake of a large depth-ratio wall-mounted rectangular cylinder at high incidence angles

Coherent structures in the wake of a long wall-mounted rectangular cylinder is examined by changing the incidence angle using Large Eddy Simulations at Re$=$250-1000. The rectangular cylinder has a depth-ratio of DR$=$4.15 and an aspect-ratio of AR$=$0.83, which is placed in a uniform boundary flow at 10 different incidence (yaw) angles between 0$^{\mathrm{o}}\le $ i $\le $ 45$^{\mathrm{o}}$ with 5$^{\mathrm{o}}$ increments. Initially, the numerical results are validated and verified using similar studies in literature. Preliminary results show that changes in the incidence angle alters the distribution of surface pressure on the body, and vorticity in the wake. Furthermore, the strength of the upwash and downwash flow, which can alter the cylinder wake structures and their orientation, considerably changes as the incidence angle is increased. Moreover, the wake starts to experience weak unsteadiness as the incident angle approaches 40$^{\mathrm{o}}$. This unsteadiness strengthens at 45$^{\mathrm{o}}$ with the flow acting fully unsteadily at 90$^{\mathrm{o}}$. At i$=$45$^{\mathrm{o}}$, there are several dominant frequencies including a low frequency that leads to transitioning periods between steady and unsteady wake. Moreover, the results show that the projected width of cylinder can adequately scale the integral flow parameters of the cylinder.   

The aerodynamics of dart

Throwing darts is a popular sport, but very little is known about the aerodynamics of dart which enables the dart to pierce the board. We study the dart trajectories with high-speed imaging which reveals the delicate in-flight stabilization effected by the wings. Further investigation of the aerodynamic loads and the flow field around the dart is conducted using wind tunnel tests and numerical simulations. The aerodynamic force increases significantly with the angle of attack and is used for determining stability derivatives required for trajectory prediction using the equations of motion. The predicted trajectories agree quantitatively with the high-speed imaging footage. The laser-sheet flow visualization at different sections reveals multiple pair of counter-rotating vortices which strengthen with increasing angle of attack and eventually, the onset of vortex breakdown leads to a stall. The stabilizing moment generated by the wings is the key to maintaining equilibrium in the pitching plane and the dart landing at an apt angle to pierce the target. In addition, we observe that the spin on the dart has a negligible effect on the trajectory.   

Assessment of wind's impact on a golf ball's trajectory and gameplay

Golf is a high-speed ball sport with speeds exceeding 80m/s and thus aerodynamics plays a crucial role. The aerodynamic force depends on the relative airspeed of the ball and a gentle wind can deflect the ball's trajectory sufficiently to thwart the player's strategic gameplay. A moderate headwind of 5m/s can shorten the trajectory of a typical approach shot by up to 20{\%}. This could be catastrophic, considering that the deviation due to inconsistency in launch conditions is only 4{\%}. We assess wind's impact on golf using the equations of motions and case studies on 3 golf courses. First, we present the effects of wind on the trajectory due to variation in wind direction, golf club selection and the ball's aerodynamics characteristics. Thereafter, we describe adjustments and their effectiveness to counter the effects of wind. Finally, we explore how wind impacts gameplay strategies by simulating the series of golf shots in the Pebble beach, Muirfield and TPC Sawgrass golf courses having heroic, strategic and penal design styles respectively. In the presence of wind, penal courses tend to become prohibitive for all players, a heroic course might favour the experts whereas a~strategic course forces the player to pick a certain route over the other.   

Vortex force map for a smart rotor.

The vortex force map method for a single body is extended to multi-body viscous flows and applied to a smart rotor configuration. The flow-independent vortex force maps for each individual part of the smart rotor at different deployments are designed for the purpose of identifying the contribution of a given vortex in the flow field to body force, and defining the positive and negative force-generating critical regions or directions. These vortex force maps combined with the pre-known velocity and vorticity field are used to obtain the unsteady forces acting on each part of the smart rotor starting from rest. The results are compared with those from computational fluid dynamics. It is found that the dominant force is the pressure force. And for the main airfoil and the flap, the force variation against time is closely related to the evolution of the vortex structure near the whole configuration and near the flap, respectively.   

Modifications To Conventional Jet Flaps using Liquid Sprays

Conventional jet flaps or blown flaps use high velocity jets to replicate the effects of a conventional flap. The performance of jet flaps is primarily influenced by the jet momentum ratio which is a function of exit jet density and velocity. In this study the conventional jet flap is modified by replacing the gas-phase jet with a sprayed-liquid jet. This modification is inspired by fire-fighting aircraft which reported increases in lift during liquid-dumping maneuvers. Previous research has shown that these fire-fighting maneuvers yield increases to the lift coefficient due to the water volume altering the external flow around the aircraft. A CFD approach consisting of a Dispersed Multi-Phase solver is used to examine the interaction of a sprayed-liquid jet in the context of a conventional jet flap to further expand upon these observations and potentially improve the conventional jet flap in certain settings. In this study two parametric studies including: (1) the chordal jet-flap placement position and (2) the liquid-jet momentum ratio at a fixed jet-flap location are conducted on an NACA 0012 airfoil operating at a Reynolds number of 500,000 and a 5 degree angle of attack. Results show that the presence of the sprayed, liquid-jet flap results in an increase in lift coefficient and decrease in drag coefficient. Furthermore, it is possible to achieve thrust producing configurations with very large jet momentum coefficients..   

Unsteady versus Quasi-Steady Aerodynamic Response of Finite Aspect Ratio Wings in Surging Flow

The time-varying forces and moments of finite aspect ratio wings in an unsteady streamwise flow are examined to better identify unsteady versus quasi-steady effects. Experimental data was collected for four NACA 0015 wings with semi-span aspect ratios of 1, 2, 3, and 4 in low Reynolds Number flows between 50,000 and 150,000. The surging flow amplitude was varied between 10 and 30 percent of the mean velocity for reduced frequencies between 0.005 and 0.1. Both low angles of attack and angles of attack near stall demonstrate pronounced hysteresis loops for particular reduced frequencies at large surging amplitudes. The static lift curves at these angles of attack also showed a large Reynolds Number dependence, indicating potential quasi-steady phenomena at work. The measured hysteresis loops are compared to results interpolated from the static data to highlight which regions of the examined parameter space exhibit unsteady vs. quasi-steady aerodynamic behavior. This material is based upon work supported by the Air Force Office of Scientific Research under award number FA9550-18-1-0311 and through the 2020 AFRL Summer Faculty Fellowship Program.   

Flow characteristics of a hovering quadrotor UAV in ground effect

For a hovering quadrotor UAV near the ground, both the rotor interaction and ground effect play important roles in determining the flow characteristics of the UAV. To investigate the effect of the ground height and the distance between the rotors of the quadrotor UAV on the wake structure, we perform PIV measurements with varying the distance between rotors (0.13 $\le $ d/R $\le $ 2.37, where d is the distance between the adjacent rotor tips and R is the radius of the rotor) and the height of the UAV above the ground (0.31 $\le $ h/R $\le $ 7.0, where h is the distance between the rotor tip-path plane and the ground). When out of ground effect (h/R $\ge $ 5.0), as d/R decreases, the rotor wake deflects more towards the center of UAV. On the other hand, when in ground effect (h/R $\le $ 2.0), at a large d/R, the weak recirculating flow is formed between the rotors. The recirculating flow becomes stronger with decreasing d/R, entering the vortex ring state. With further decreasing d/R, the strong fountain flow is generated between the rotors. Some more details will be discussed in the presentation.   

Geometric Control Theory Applied to the Analysis of a Pitching-Plunging Airfoil

The geometric control theory is the application of differential geometry to control theory. It provides a means to study nonlinear dynamical systems that evolve on curvy spaces or manifolds. First developed in the 1970s, the geometric control theory is capable of capturing the higher order effects that are usually neglected in the classical linear theories, allowing for the discovery of unconventional force generation mechanisms. The aim of this study is to exploit the benefits of the geometric control theory in the field of unsteady aerodynamics. To do so, the aerodynamics of a two-dimensional pitching-plunging airfoil are formulated in a control framework using a state-space system. This reduced-order model (ROM) provides only the dynamics of some chosen output variables; it does not aim for the reconstruction of the entire flow field. The system is rich enough to capture the main physical aspects of the flow, but also compact to permit an analytic study of the results. The geometric control theory is then applied to analyze the behavior of the outputs, in this case the lift and drag forces. The results are studied in a fluid dynamics framework to determine if the unsteady motion of the airfoil can provide an enhancement or reduction of the mentioned parameters.