Applications of the unsteady vortex-lattice method in aircraft aeroelasticity and flight dynamics

Unsteady aerofoil flows are often characterized by leading-edge vortex (LEV) shedding. While experiments and high-order computations have contributed to our understanding of these flows, fast low-order methods are needed for engineering tasks. Classical unsteady aerofoil theories are limited to small amplitudes and attached leading-edge flows. Discrete-vortex methods that model vortex shedding from leading edges assume continuous shedding, valid only for sharp leading edges, or shedding governed by ad-hoc criteria such as a critical angle of attack, valid only for a restricted set of kinematics. We present a criterion for intermittent vortex shedding from rounded leading edges that is governed by a maximum allowable leading-edge suction. We show that, when using unsteady thin aerofoil theory, this leading-edge suction parameter (LESP) is related to the term in the Fourier series representing the chordwise variation of bound vorticity. Furthermore, for any aerofoil and Reynolds number, there is a critical value of the LESP, which is independent of the motion kinematics. When the instantaneous LESP value exceeds the critical value, vortex shedding occurs at the leading edge. We have augmented a discrete-time, arbitrary-motion, unsteady thin aerofoil theory with discrete-vortex shedding from the leading edge governed by the instantaneous LESP. Thus, the use of a single empirical parameter, the critical-LESP value, allows us to determine the onset, growth, and termination of LEVs. We show, by comparison with experimental and computational results for several aerofoils, motions and Reynolds numbers, that this computationally inexpensive method is successful in predicting the complex flows and forces resulting from intermittent LEV shedding, thus validating the LESP concept.

Discrete-vortex method with novel shedding criterion for unsteady aerofoil flows with intermittent leading-edge vortex shedding

Nonlinear aeroelastic analysis is essential for high-altitude long-endurance (HALE) aircraft. In the current paper, we have presented a computational aeroelastic tool for nonlinear-aerodynamics/nonlinear-structure interaction. Specifically, a consistent nonlinear time-domain aeroelastic methodology has been integrated via tightly coupling a geometrically exact nonlinear intrinsic beam model and the generalized unsteady vortex-lattice aerodynamic model with vortex roll-up and free wake. The effects of discrete gust as well as flow separation at various angles of attack from attached flow to the stall and poststall ranges are also included in the nonlinear aerodynamic model. A HALE-wing model is analyzed as a numerical example. The trim angle of attack is first found for the wing, and the results show that aeroelastic instability could occur at higher angles of attack. The HALE-wing model under the trim condition is then analyzed for various gust profiles to which it is subject. It is found that for certain gust levels, the elastic deformations of the HALE wing tend to become unstable: notably, the in-plane deflections become very significant. It is noted for the unstable solution of the HALE wing that the flow may be well beyond the stall range. An engineering approach with the use of the nonlinear sectional lift is attempted to consider such stall effects.

Nonlinear-Aerodynamics/Nonlinear-Structure Interaction Methodology for a High-Altitude Long-Endurance Wing

Continuous photocatalytic, electrocatalytic and photo-electrocatalytic degradation of a reactive textile dye for wastewater-treatment processes: Batch, microreactor and scaled-up operation

This work investigates the effect of aerodynamic interference in the coupled nonlinear aeroelasticity and flight mechanics of flexible lightweight aircraft at low speeds. For that purpose, a geometrically exact composite-beam formulation is used to model the vehicle flexible-body dynamics by means of an intuitive and easily linearizable representation based on the displacement and Cartesian rotation vectors. The aerodynamics are modeled using the unsteady vortex-lattice method, which captures the instantaneous shape of the lifting surfaces and the free inviscid wake, including large deformations and interference effects. This results in a framework for simulation of high aspect ratio planes that provides a medium-fidelity representation of flexible-aircraft dynamics with a modest computational cost. Previous independent studies on the structural-dynamics and aerodynamics modules are complemented here with the integrated simulation methodology, including vehicle trim, and linear and nonlinear time-domain solutions. A numerical investigation is next presented on a simple wing-fuselage-tail configuration, assessing the interference effects between wing wake and horizontal tail, and the downwash due to the proximity of the wake is shown to play a significant role in the longitudinal dynamics of the vehicle. Finally, a brief discussion of direct wake-tail encounters is included to show the limitations of the approach.

Assessment of Wake-Tail Interference Effects on the Dynamics of Flexible Aircraft

Numerical simulation of unsteady motion of wing through undisturbed fluid is formulated using Unsteady Vortex Lattice Method. Singularities in the form of vortex ring elements are distributed over the surface of the wing mean camber and remain fixed in space. The wake vortex ring singularities are free to deform under the action of velocity field induced by the combination of moving wing and deforming wake vortex ring elements. CL, CDi , Γ distribution etc., are calculated for different cases of unsteady motion. A free-wake algorithm is developed based on the higher order numerical integration schemes. Minimum cut-off distance to avoid r−1 numerical singularity is obtained from Rankine vortex model. No-normal flow boundary condition is imposed over the surface of mean camber and the resulting MN × MN equations are solved for Γ using LU decomposition.

A Numerical Study of the Unsteady Motion of a Wing using N-Body Approximation

Morphing of wing surfaces during flight can significantly improve the aerodynamic performance of a flight vehicle. The potential benefits of morphing a diamond-back wing design for vehicles operating over a wide range of Mach numbers are explored. As a first step, the aerodynamic characteristics of a diamond-back wing are compared to those of equivalent unswept and swept wings. Numerical simulations have been carried out using commercial CFD software, Fluent, across subsonic, transonic, and supersonic flight regimes to understand the advantages and disadvantages associated with differences in induced and interference components of drag. Longitudinal aerodynamic characteristics and flow visualization are reported to bring out the advantages of a diamond-back wing over a traditional swept wing at higher flight speeds. Keywords: morphing, diamond-back, subsonic, transonic, supersonic, high-speed, CFD 

Aerodynamic Investigation of a Novel Diamond-Back Morphing Wing Configuration

Comparisons of the development of flow over a cylinder with a 20° cone nose and a cylinder with an ogive nose, which represent typical heat-shield configurations are studied using CFD and experiments at transonic Mach numbers. The Cp plots are studied to locate expansion or separation. Experiments are carried out at M = 0.8, 0.9, 0.95 and 1.1 and Re ≈ 2.45 × 106. Computations are carried out using the commercial package, FLUENT 6.3. Inadequate spatial resolution of pressure ports in experiments as well as limitations of the CFD tool result in some differences in experimental and CFD results.

https://iopscience.iop.org/article/10.1088/1742-6596/822/1/012071/pdf

Rinku Mukherjee

Papers

A Numerical Study of the Unsteady Motion of a Wing using N-Body Approximation

Aerodynamic Investigation of a Novel Diamond-Back Morphing Wing Configuration

Development of Flow over Blunt-Nosed Slender Bodies at Transonic Mach Numbers

Reduction of Induced Drag in Con guration Flight using Wing Twist at Post-Stall Angles of Attack

Numerical Morphing of a Rectangular Wing to Prevent Flow Separation