Dan Mateescu

Dynamics and Stability of a Flexible Cylinder in a Narrow Coaxial Cylindrical Duct Subjected to Annular Flow

This paper considers analytically the dynamics of a flexible cylinder in a narrow coaxial cylindrical duct, subjected to annular flow. The viscous forces are derived by a systematic, albeit approximate, solution of the Navier-Stockes equations, which accounts for the unsteady viscous effects. The inviscid forces are not derived via the slender-body approximation, and hence the analysis is also applicable to bodies of relatively small length-to-radius ratio. The dynamics and stability of typical systems with fixed ends is investigated, concentrating mainly on viscous effects and comparing the results with those of previous work

The Unsteady Potential Flow in an Axially Variable Annulus and Its Effect on the Dynamics of the Oscillating Rigid Center-Body

This paper presents an analytical investigation of the unsteady potential flow in a narrow annular passage formed by a motionless rigid duct and an oscillating rigid center-body, both of axially variable cross section, in order to determine the fluid-dynamic forces exerted on the center-body. Based on this theory, a first-approximation solution as well as a more accurate solution are derived for the unsteady incompressible fluid flow. The stability of the center-body is investigated, in terms of the aerodynamic (or hydrodynamic) coefficients of damping, stiffness and inertia (virtual mass), as determined by this theory. The influence of various system parameters on stability is discussed.

Unsteady viscous effects on the annular-flow-induced instabilities of a rigid cylindrical body in a narrow duct

The dynamics and stability of a rigid cylindrical body oscillating about a hinge in a coaxial cylindrical duct containing flowing fluid are considered in this paper. A previously developed analytical model, in which the fluid-dynamic forces exerted on the oscillating cylinder had been determined by means of inviscid flow theory, is extended to take into account the unsteady viscous effects of a real fluid flow, in an approximate manner. It is shown that there exists a critical location of the hinge: if the cylinder is supported upstream of that point, then it remains stable at all flow velocities; however, if it is supported downstream of that location, then negative-damping oscillatory instability is possible for flow velocities sufficiently large to overcome the positive restraining mechanical damping of the system. The critical location of the hinge depends on the relative width of the annular passage with respect to its mean radius. By comparing inviscid and viscous flow theory results, it is shown that inviscid theory generally underestimates stability, by a margin increasing with the narrowness of the annulus.

Optimal atmospheric trajectory for aerogravity assist with heat constraint

In a previous paper, the authors addressed the problem of augmenting gravity assist by using planetary atmospheric maneuvering such that the heliocentric velocity of the spacecraft after flyby was maximized. The optimization was carried out by using Pontryagins maximum principle. The results showed that the heating rate was high during maneuvering of the spacecraft through the atmosphere. Therefore in the present paper, a heating rate constraint is imposed on the atmospheric trajectory for aerogravity assist (AGA). A comparison of AGA for Venus and Mars is given that shows the overall superiority of using Mars over Venus. The results also show that AGA with heat constraint gives slightly lower heliocentric velocity than AGA without heat constraint, particularly when the drag polar based on the Newtonian theory for the hypersonic regime is used in numerical calculations. This study concludes that AGA is possible with moderate planetocentric velocity.

Mars-Jupiter Aerogravity Assist Trajectories for High-Energy Missions

AMarsaerogravity assist forhigh-energy missions (solare yby and Pluto missions )is considered. Itisfound that for high-energy missions, Mars aerogravity assist alone will involve very high Earth launch energy and heating rate. The potential of an alternative technique of combining Mars aerogravity assist with Jupiter gravity assist for high-energy missions is examined. The analysis shows that the use of this technique could reduce Earth launch energy and heating rate by 50 and 85%, respectively. Some problems regarding the actual implementation of this alternative technique are also discussed.

Optimal atmospheric trajectory for aero-gravity assist

Abstract Gravity assist of a planet can be enhanced by an alternate technique called aero-gravity assist (AGA). The latter uses atmospheric maneuvering around the planet to increase the deflection angle, which leads to an increase in the velocity gain. This paper presents a realistic mathematical model for AGA. Optimal atmospheric trajectory is synthesized so as to maximize the heliocetric velocity of the spacecraft. Optimization is carried out using Pontryagins maximum principle. The atmospheric trajectory is such that the spacecraft flies mostly at the maximum lift-to-drag ratio ( E∗ ) so as to minimize the loss of kinetic energy and increase the angle of rotation around the planet. The heating rates have also been calculated, which show that high lift coefficient, corresponding to E∗ , results in lower heating rate.

Analysis of flexible-membrane and jet-flapped airfoils using velocity singularities

A method for predicting the flow past thin airfoils in incompressible potential flow is presented. This method makes use of special singularities that directly represent the complex conjugate perturbation velocity in the plane of the airfoil. The method is developed first for rigid cambered airfoils and is then extended to problems for which it is particularly suitable, namely, the flow past flexible impervious membranes and past airfoils with jet flaps.

AERODYNAMIC ANALYSIS OF AIRFOILS AT VERY LOW REYNOLDS NUMBERS

This paper presents an efficient numerical method for the flows past airfoils at very low Reynolds numbers, which are of interest for the micro-aircraft applications and the unmanned air vehicles (UAV). The flows at very low Reynolds numbers are dominated by viscous effects and by very thick boundary layers, with few computational or experimental results available for airfoils. The present analysis is based on a pseudo-time integration method using artificial compressibility to accurately solve the Navier-Stokes equations. This is done in a rectangular computational domain obtained by a coordinate transformation fiom the physical flow domain around the airfoil. The method uses a central differencing scheme on a stretched staggered grid in the computational domain. This method is first successfully validated for the flows with multiple separation regions past a downstream-facing step by comparison with previous experimental and computational results at very low Reynolds numbers between 400 and 1200. Then the method is used to obtain solutions for several NACA airfoils at very low Reynolds numbers between 400 and 6000. The present airfoil solutions are validated by comparison with the numerical results obtained by Kunz & Kroo for Reynolds numbers between 1000 and 6000 (no results were available for Reynolds numbers smaller than 1000). A very good agreement has been found between the two sets of results. The decrease in the low Reynolds number was found to lead to a marked increase in the pressure coefficient on the airfoil, which is more pronounced towards the trailing edge and for thinner airfoils.

A theoretical model compared with experiments for the unsteady pressure on a cylinder oscillating in turbulent annular flow

Abstract The unsteady turbulent flow around a cylindrical centre-body oscillating ina narrow duct is considered in this paper in support of the flow-induced vibration studies of such systems. A simplified analysis was developed for smooth geometries in order to assess the importance of the unsteady turbulent effects. To validate the theoretical solution, experimental investigations were conducted to measure the circumferential variation of the unsteady pressure on the oscillating centre-body in turbulent annular flow. Good agreement was found to exist between the theoretical solution and the experimental results for both the amplitude and phase of the measured unsteady pressure on the oscillating cylinder.

Theoretical Solutions for Unsteady Flows Past Oscillating Flexible Airfoils Using Velocity Singularities

A new method of solution is presented for the analysis of unsteady e ows past oscillating airfoils. This method is based on the derivation of the singular contributions of the leading edge and ridges (points where the airfoil boundary conditions change ) in the solutions of the e uid velocity and pressure coefe cient. The method has been validated by comparison with the results obtained by Theodorsen and by Postel and Leppert for rigid airfoil and aileron oscillations in translation and rotation. An excellent agreement was found between the present solutions and these previous results. This method has then been used to obtain solutions for the e exural oscillations of the e exible airfoils, e tted or not with oscillating e exible ailerons, which are of interest for the aeroelastic studies. The aerodynamic stiffness, damping, and virtual (or added) mass contributions in the solutions of the unsteady pressure distribution, lift coefe cient, and moment coefe cient are specie cally determined. An analysis of the relative magnitude of the quasi-steady and vortex shedding contributions in the aerodynamic coefe cients is also presented. In all cases studied, this method led to very efe cient and simple analytical solutions in closed form.