Jean-Marc Moschetta

Shock wave instability and the carbuncle phenomenon : same intrinsic origin?

The theoretical linear stability of a shock wave moving in an unlimited homogeneous environment has been widely studied during the last fty years. Important results have been obtained by D yakov (1954), Landau & Lifchitz (1959) and then by Swan & Fowles (1975) where the fluctuating quantities are written as normal modes. More recently, numerical studies on upwind nite dierence schemes have shown some instabilities in the case of the motion of an inviscid perfect gas in a rectangular channel. The purpose of this paper is rst to specify a mathematical formulation for the eigenmodes and to exhibit a new mode which was not found by the previous stability analysis of shock waves. Then, this mode is conrmed by numerical simulations which may lead to a new understanding of the so-called carbuncle phenomenon.

Robustness versus accuracy in shock-wave computations

Despite constant progress in the development of upwind schemes, some failings still remain. Quirk recently reported that approximate Riemann solvers, which share the exact capture of contact discontinuities, generally suffer from such failings. One of these is the odd – even decoupling that occurs along planar shocks aligned with the mesh. First, a few results on some failings are given, namely the carbuncle phenomenon and the kinked Mach stem. Then, following Quirk’s analysis of Roe’s scheme, general criteria are derived to predict the odd – even decoupling. This analysis is applied to Roe’s scheme, the Equilibrium Flux Method, the Equilibrium Interface Method and the AUSM scheme. Strict stability is shown to be desirable to avoid most of these flaws. Finally, the link between marginal stability and accuracy on shear waves is established.

Aerodynamic Performance of a Biplane Micro Air Vehicle

The present paper addresses the problem of improving the aerodynamic performance of a fixed-wing micro air vehicle under stringent maximum size constraints. Monoplane wing planforms are compared with biplane concepts using low-speed wind-tunnel measurements and numerical calculations including viscous effects. A parametric study of the effect of various geometrical parameters such as aspect ratio, gap, stagger, and wing planforms has been carried out in order to optimize a biplane micro air vehicle configuration in the low-Reynolds regime. Finally, wind-tunnel measurements including the influence of propellers indicate that biplane micro air vehicle configurations can drastically increase the overall aerodynamic performance over the classical monoplane fixed-wing concept.

ON THE PATHOLOGICAL BEHAVIOR OF UPWIND SCHEMES

Despite constant progress in the developement of upwind schemes, some failings still remain. Quirk recently reported that approximate Riemann solvers, which share the exact capture of contact discontinuities, generally suffer from such failings. One of them is the odd-even decoupling that occurs along planar shocks aligned with the mesh. Quirk proposed to test this shortcoming with the propagation of a planar shock in a duct. First, we give a few results on some failings. Then, following Quirks analysis of Roes scheme, general criteria are derived to predict the odd-even decoupling. This analysis is applied to Roes scheme, EFM Pullins scheme, EIM Macrossans scheme and AUSM Lious scheme. Strict stability is shown to be desirable to avoid must of these flaws.

A Fixed-Wing Biplane MAV for Low Speed Missions

Practical MAV missions, such as outdoor urban environment recognitions, simultaneously require a capability of both dashing to escape enemy fire and slowly loitering over a target in order to capture and transmit clear images to a ground station. Since an MAV intrinsically offers better payload and endurance capabilities than a rotorcraft of an equal size, fixed-wing MAVs can be considered as promising platforms to start with. The objective of this study is to investigate the possibility of developing a fixed-wing MAV which can both perform rapid translations and low-speed flights through urban canyons. A low-speed wind tunnel testing is conducted to compare several powered configurations including monoplane, biplane and tandem wing combinations. The testing also focuses on wing-propeller interactions. Results indicate that a positive-stagger biplane configuration powered by counter-rotating propellers placed in pusher position provides the best trade-off between a high-speed performance and a low-speed capability with a limited electric consumption. Consequently, a 30 cm-span MAV biplane prototype, named TYTO-30, has been designed and built. TYTO-30 is equipped with a 110g-payload which includes a video camera, navigation and autopilot system and has been flight tested successfully.

Energy-Harvesting Mechanisms for UAV Flight by Dynamic Soaring

Dynamic Soaring is a flying technique which extracts energy from an environment where wind gradients form, with the potential to increase the endurance of small unmanned vehicles. The feasibility to use dynamic soaring flight is questioned here; it requires the identification of energy-extraction mechanisms as well as accurate understanding of the way energy-harvesting performances are governed by trajectory constraints, vehicle characteristics and environment conditions. A three-dimensional energy-neutral trajectory is derived out of a specified optimization problem. Characteristic phases of flight are evidenced out of an overall closed trajectory. Simplified equations are used to evidence the physics behind energy transfers. Finally, overall energy-harvesting balance is studied through local variations of total energy along the path.

Numerical Investigation of Supersonic Vortical Flow About a Missile Body

Three-dimensional supersonic viscous flow computations including crossflow separation are presented for an ogive-cylinder at an angle of attack varying from 0 to 20 deg. Solutions are obtained for the parabolized Navier- Stokes equations from an algorithm that incorporates a fully upwind implicit approach. Results are presented for laminar flow assumption and include side-by-side comparison with wind-tunnel data. Excellent agreement is obtained between computations and wind-tunnel measurements atA/oo = 2 in terms of surface pressure distribution, body streamline pattern, and stagnation pressure contours in crossflow planes. Euler solutions can account for the formation of vortices arising from smooth-body boundary-layer separation if some local treat- ment, such as a Kutta condition along the separation line, is applied. A boundary-layer code is necessary to provide the separation-line location, and a weak-coupling technique between the two methods has to be used.12 However, great discrepancies between calculation and experimental data are observed on the location of the vortex cores, and the stagnation pressure loss in the vortices is generally underpredicted by Euler solvers. Because of their efficiency and ability to compute some of the complex phenomena involved in the aerodynamics exhibited by tactical missiles at angle of attack, the parabolized Navier-Stokes (PNS) equations are widely used in the design process. Solutions of these equations are obtained by march- ing in space rather than time and are therefore obtained much more efficiently than are solutions to the unsteady Navier-Stokes equa- tions. The PNS equations contain all of the terms of both Euler and boundary-layer equations and consequently the interaction between the viscous and inviscid portions of the flowfield is automatically taken into account. Furthermore, as long as the streamwise com- ponent of the velocity remains positive, the PNS equations are still valid for crossflow separation around missile bodies. The present investigation is directed toward applying an extended version of the PNS code TORPEDO3 to an ogive-cylinder configuration at MOO =2.Q,ReD = 1.61 x 10 5, and a varying from 0 to 20 deg. The flow conditions, chosen to correspond with the experimental case45 are such that the flow is assumed to be laminar. This configuration was chosen because it provides a considerable number of measured data such as oil flow patterns, surface pressure distributions, and stagnation pressure distributions in different crossflow planes. The objectives of this study are 1) to validate the current approach by comparing computational results with wind-tunnel data and 2) to provide a detailed description of the flowfield and aerodynamics governing the flow at angle of attack.

Optimization of a Biplane Micro Air Vehicle

The present paper addresses the problem of improving the aerodynamics performances of a fixed-wing Micro Air Vehicle (MAV) under stringent maximum size constraints. Monoplane wing planforms are compared with biplane concepts using low-speed wind tunnel measurements and numerical calculations including viscous effects. A parametric study of the effect of various geometrical parameters has been carried out in order to optimize a biplane micro-air vehicle configuration in the low-Reynolds regime. Finally, wind-tunnel measurements including the influence of propellers indicate that biplane MAV configurations can drastically increase the overall performances over the classical monoplane fixed-wing concept.

A Cure for the Sonic Point Glitch

Among the various numerical schemes developed since the ’80s for the computation of the compressible Euler equations, the vast majority produce in certain cases spurious pressure glitches at sonic points. This flaw is particularly visible in the computation of transonic expansions and leads lo unphysical “expansion shocks” when the flow undergoes rapid change of direction. The analysis of this flaw is presented, based on a series of numerical experiments. For Flux-Vector Splitting methods, it is suggested that it is not the order of differentiability of the numerical flux which is crucial but the way the pressure at an interface is calculated. A new way of evaluating the pressure at the interface is proposed, based upon kinetic theory, and is applied to most current available algorithms including Flux-Vector Splitting and Flux-Difference Splitting methods as well as recent hybrid schemes (AUSM, HUS).

Bioinspired wind field estimation—part 1: Angle of attack measurements through surface pressure distribution

One of the major challenges of Mini-Unmanned Aerial Vehicle flight is the unsteady interaction with turbulent environment while flying in lower levels of atmospheric boundary layer. Following inspiration from nature we expose a new system for angle of attack estimation based on pressure measurements on the wing. Such an equipment can be used for real-time estimation of the angle of attack during flight or even further building of wind velocity vector with additional equipment. Those information can find purpose in control and stabilization of the aircraft due to inequalities seen by the wing or even for various soaring strategies that rely on active control for energy extraction. In that purpose, flying wing aircraft has been used with totally four span-wise locations for local angle of attack estimation. In-flight angle of attack estimation from differential pressure measurements on the wing has been compared with magnetic sensor with wind vane. The results have shown that pressure ports give more reliable estimation of angle of attack when compared to values given by wind vane attached to a specially designed air-boom. Difference in local angle of attack at four span-wise locations has confirmed spatial variation of turbulence in low altitude flight. Moreover, theoretical law of energy dissipation for wind components described by Kaimal spectrum has shown acceptable match with estimated ones.