Session Q03: Aerodynamics: Control/ Vehicles

A computationally-augmented wind tunnel with irrotational gust generation for low-speed aerodynamics

Large-amplitude flow disturbances, such as gusts, can drastically change the aerodynamic loads on machines. Active flow control can mitigate or exploit these disturbances, but developing an effective control strategy is challenging, partly because the interactions are often non-linearly dependent on the gust amplitude. As a result, the parameter space is often too large to explore by wind tunnel experiments alone. One solution is to create a surrogate computational model (a digital twin) of the experiments that can augment the experimental data and model conditions outside the scope of the experiments. To ensure that the model is faithful to the true disturbed flow conditions, it should be able to assimilate data from experiments. We present a digital twin of a low-speed aerodynamics wind tunnel equipped with a system for irrotational gust generation. The digital twin couples a solver for the viscous flow without the wind-tunnel walls with one for a potential flow to correct the normal velocity at those walls, including the gust-generating suction flow. Using an ensemble Kalman filter, we infer the true time-varying gust conditions from the experiments. We compare the simulated gust response to the response predicted by classical aerodynamics and a low-order vortex model.   

Parameter-varying aerodynamics model of unsteadily pitched wing in low reynolds number high mach flow conditions

As the Martian environment combines very low atmospheric density with a lower than Earth speed of sound, rotorcrafts such as NASA’s Ingenuity craft operate in a unique compressible low Reynolds Number (Re) flow regime. Through previous work conducted by the authors, this flow regime showcases novel dynamic shock-vortex interactions during dynamic pitching motions of an airfoil and wing. Observations showcased this interaction caused variations in the flow field and ultimately the aerodynamic loading on a wing different to what is typically observed in more common flow regimes. In the presented work, a method for modeling these load variations on a 3D wing will be demonstrated using reduced order modeling techniques to formulate a control-oriented model of the aerodynamic loads. The reduced order modeling approach will utilize high-fidelity LES simulations of a NACA 0012 wing in dynamic flight conditions which maximize the available information in the aerodynamic loading time series. These time histories will be used in the derivation of a linear parameter varying (LPV) state space model suitable for control. Optimal selection of flight conditions and modeling performance under arbitrary motions will be showcased against existing approaches such as the Leishmen-Beddoes model.

    

Experimental study for wake manipulation of structured porous square cylinder

The purpose of this study is to experimentally explore the role of permeability of a structured porous cylinder with respect to the wake structure shed from the cylinder and its corresponding aerodynamic features. To this end, an addictive manufacturing technique was utilized to fabricate the porous cylinder that was comprised of a simple cubic lattice structure.  This unique manufacturing technique coupled with a simplified lattice structure allows to decouple the permeability of the porous cylinder from the porosity. Therefore, in this work, a fundamental and systemic study was performed to reveal the influence of permeability of the porous cylinder on the flow pattern behind the cylinder and its resulting aerodynamic force acting on the cylinder.

In this study, permeabilities of porous cylinders which porosity was fixed at 0.7 were carefully measured in an open-loop pipe flow system with a highly accurate differential pressure transmitter and a thermal mass flow meter. Based on these permeabilities, flow pattern induced by structured porous square cylinders (D=40 mm) and their aerodynamic features at moderate Reynolds number were acquired by particle-image velocimetry and three-component balance unit, respectively.

    

Boxfish and Delta Wings: A Parametric Study of the Effect of Thickness Distribution on Delta Wing Stability

Delta wing aircraft aerodynamics are largely characterized by twin vortices that shed from the leading edge of the wing. These leading edge vortices (LEVs) are also observed in nature on several bony fishes in the Ostraciidae family known as boxfishes. Obvious differences in these two geometries raise questions about the impact of geometry on the growth and development of the LEV and subsequent impacts on aero/hydro-dynamic performance and stability. In particular, what influence does the contour of the top surface of a delta wing have on the development of the LEV over the body? How can the geometry of the upper surface/fuselage be tailored to yield desirable aerodynamic characteristics? Additionally, these questions are relevant to a growing debate in the literature about the hydrodynamic function of boxfishes’ hard outer shells as they pertain to swimming dynamics. Hydrodynamic stability and performance of a range of delta wing geometries will be characterized in a novel water tunnel. Initial dye visualizations will inform particle image velocimetry (PIV) and force balance data collection.

This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1745301 and the Center for Autonomous Systems and Technologies at the California Institute of Technology.

    

Micro Flyer Requirements for Urban Wind Field Observations

Real-time weather forecasting to improve the safety of low altitude aircraft operations for advanced air mobility, particularly within urban settings, is vital for the integration of real-time operations from autonomous systems, such as drones and urban air taxis. Solving this includes in part developing autonomous systems capable of conducting observations accurately and reliably and determining the number and frequency of required observations and the sensitivity of these observations in data sparse regions of the lower atmosphere. Miniature sensor systems such as micro-air vehicles and bio-inspired flying sensors, both insect and seed based, have the opportunity to revolutionize atmospheric measurements through distributed flying sensor swarms. While these have largely been studied in wind tunnels or benign environments, within the urban canopy exists a complex flow field of a highly transient and three-dimensional nature. In order to design systems that will be optimized for this environment, we must understand the interplay between the urban wind fields and various types of micro flyers utilized for urban observations. This will drive aerodynamic inspiration and design of the systems, such as maneuvering systems to control the swarm dispersal in the atmosphere.

    

Wake interactions of a small Unmanned Air Vehicle with a high-speed moving ground vehicle

Small Unmanned Air Vehicles (UAVs) are particularly susceptible to unsteady flow environments and high incoming flow velocities. This work presents an experimental investigation of the aerodynamics of a small UAV maneuvering near a simplified fast-moving vehicle. A representative vehicle and quad-rotor UAV have been constructed with the ability to vary their positioning relative to one another in a low speed recirculating wind tunnel. High speed planar Particle Image Velocimetry (PIV) was used to measure the UAV wake interaction with the wake structures shed by the vehicle. This presentation will present the flow field evolution from slow to high speed landing interactions to affecting UAV landing maneuvers. A variety of relative positions and vehicle wake structure interactions will be shown.   

Attenuation of the unsteady loading on a high-rise building at different angles of attack using pulsed jet forcing

With the severity and frequency of significant weather events increasing, it is critical to investigate methods for alleviating unsteady wind loading for high-rise buildings. This study numerically investigates the 3D flow structures around a canonical high-rise building immersed in an atmospheric boundary layer at different oncoming wind angles, using large eddy simulations. The effect of the downwash flow near the top of the building on the spanwise vortex shedding is analysed. Exploiting this idea, a synthetic jet located on the top surface is used as an open loop active actuation to suppress the building's side-force fluctuations when exposed to oncoming wind variations via enhancing the downwash flow. Actuation across different frequencies is investigated numerically, and its effect on the time-averaged flow fields and the unsteady flow structures for cases with different oncoming wind angles explored. To the authors' knowledge, this work presents a first attempt at employing active actuation to control the three-dimensional wake of a high-rise building exposed to differing oncoming wind directions

    

Author not Attending

In recent years, more and more researches have been carried out on the bi-stable wake behind a square back Ahmed body. This bi-stability is a long time scale of the order of 1000H/U_∞ flow phenomenon. It represents the random shift of the recirculation region between two reflectional symmetry-breaking positions which is mutually symmetric with respect to the vertical central plane. However, previous studies on the bistable wake has focused on testing with uniform freestream conditions and neglected real-world road turbulence effects. In this work, the effect of inflow turbulence on the bi-stable wake behind a square back Ahmed body is numerically explored by IDDES. The turbulence intensity (I≈2%~10%) and the integral length scale of turbulence (L_x≈0.38H~2H) are varied using the Synthetic Eddy Method. The results shown that the boundary layer developing around the body together with the structure of the wake is strongly altered by the free-stream turbulence where both the length of the recirculation and the shear layer characteristics are modified, leading to a complex variation trend of the drag coefficient C_D. When the length scales of the fluctuations fully encompass the length scales of the relevant wake flow structures behind the body (L_x≈1H~2H), increasing the inflow turbulence intensity will gradually accelerate the switching of bi-stability wake. In addition, the drag coefficient C_D rises with the increase of the turbulence intensity. For the small integral length scale (L_x≈0.38H), a weakly effect on both the switching frequecy of bi-stability wake and C_D has been noted until the turbulence intensity is high enough to change the boundary layer at the rear edge and the shear layer dramatically. These findings are of major importance for guiding the design of the next generation of control strategies for drag reduction, most of which neglect the influence of inflow conditions.