Dezso Gyorgyfalvy

Bio: Dezso Gyorgyfalvy is an academic researcher. The author has contributed to research in topics: Drag & Cabin pressurization. The author has an hindex of 2, co-authored 2 publications receiving 57 citations.

Possibilities of Drag Reduction by the Use of Flexible Skin

Abstract: A flexible aerodynamic surface is considered as a possible means of delaying transition and reducing skin-friction drag. The results of an extensive analytical study on boundary-layer instability and transition in incompressible Blasius-flow over a flexible surface are presented. A simple flexible skin model consisting of a taut membrane and an elastic base has been considered. The mechanical behavior of such a skin is controlled by the mass, the wave propagation velocity, the stiffness, and the damping. The analysis shows that with proper selection of the surface characteristics, the transition can be significantly delayed through reduced amplification rates, even though the critical Reynolds number for instability is only slightly increased. The extent of transition delay and the required surface properties are delineated. The theoretically possible drag reduction is most significant within the Reynolds number range of 3 to 50 X 10 6. Hence, potential fields of application of flexible skins would be in sailplanes, helicopters, small and medium subsonic airplanes, hydrofoils, torpedoes, speed boats, and miniature submarines. Nomenclature a = amplification factor, imaginary part of the complex wave velocity cr = phase velocity, real part of the complex wave velocity Com = wave propagation velocity in the membrane, (T/M)l/<2 d = damping coefficient, Eq. (21) D = damping constant, lb-sec/ft3 KM = mass parameter, Eq. (13) Ks = stiffness parameter, Eq. (15) KD = damping parameter, Eq. (17) m = mass coefficient, Eq. (18) M = membrane mass per unit area, Ib-sec2/ft3 Rx = Reynolds number based on length, Um x/v Rd = Reynolds number based on boundary-layer thickness S = compression stiff ness of base material, Ib/ft3 T = membrane tension, Ib/ft t/oo = freestream velocity, fps x = distance from leading edge along the plate, ft a. = dimensionless wave number, kd /3r = disturbance frequency, a.cr d = boundary-layer thickness, ft d* = boundary-layer displacement thickness, ft v — kinematic viscosity of fluid, ft2/sec p = density of fluid, Ib-sec2/ft3 coo = dimensionless cutoff frequency, (S/M)ll2d/Uo3 w* = circular frequency of disturbance, sec"1 ior = dimensionless circular frequency, (3r/Rd = co^v/U m2

Effect of pressurization on airplane fuselage drag.

Abstract: Cabin pressurization in transport airplanes tends to form bulges in the fuselage skin between the frames and stringers, and the resulting wavy surface is liable to produce a higher aerodynamic drag A flight test study has been made to determine the effect of pressurization on the fuselage drag of a Boeing 720 jetliner The study is based on boundary-layer measurements, made by pitot-static rakes at several points of the fuselage in both pressurized and unpressurized conditions The effect of pressurization is manifested in an increased boundary-layer momentum thickness The analysis indicates that the drag coefficient of the fuselage is increased by approximately 5% as a result of the pressurization It appears, however, that only a lesser part of this drag increment is caused by the skin bulging, and the greater part is caused by leakage of the pressurized air or exhaust-air associated with the pressurization system

The hydrodynamic stability of flow over Kramer-type compliant surfaces. Part 1. Tollmien-Schlichting instabilities

Abstract: The hydrodynamic stability of flows over Kramer-type compliant surfaces is studied. Two main types of instability are considered. First, there are those which could not exist without viscosity, termed Tollmien–Schlichting Type Instabilities (TSI). Secondly, there are Flow-Induced Surface Instabilities (FISI), that depend fundamentally on surface flexibility and could exist with an inviscid fluid flow. Part 1, the present paper, deals mainly with the first type. The original Kramer experiments and the various subsequent attempts to confirm his results are reviewed, together with experimental studies of transition in flows over compliant surfaces and theoretical work concerned with the hydrodynamic stability of such flows.The Kramer-type compliant surface is assumed to be an elastic plate supported by springs which are modelled by an elastic foundation. It is also assumed that the plate is backed by a viscous fluid substrate having, in general, a density and viscosity different from the mainstream fluid. The motion of the substrate fluid is assumed to be unaffected by the presence of the springs and is determined by solving the linearized Navier–Stokes equations. The visco-elastic properties of the plate and springs are taken into account approximately by using a complex elastic modulus which leads to complex flexural rigidity and spring stiffness. Values for the various parameters characterizing the surface properties are estimated for the actual Kramer coatings.The boundary-layer stability problem for a flexible surface is formulated in a similar way to that of Landahl (1962) whereby the boundary condition at the surface is expressed in terms of an equality between the surface and boundary-layer admittances. This form of the boundary condition is exploited to develop an approximate theory which determines whether a particular change to the mechanical properties of the surface will be stabilizing or destabilizing with respect to the TSI. It is shown that a reduction in flexural rigidity and spring stiffness, an increase in plate mass, and the presence of an inviscid fluid substrate are all stabilizing, whereas viscous and visco-elastic damping are destabilizing.Numerical solutions to the Orr–Sommerfeld equation are also obtained. Apart from Kramer-type compliant surfaces, solutions are also presented for the rigid wall, for the spring-backed tensioned membrane with damping, previously considered by Landahl & Kaplan (1965), and for some of the compliant surfaces investigated experimentally by Babenko and his colleagues. The results for the Kramer-type compliant surfaces on the whole confirm the predictions of the simple theory. For a free-stream speed of 18 m/s the introduction of a viscous substrate leads to a complex modal interaction between the TSI and FISI. A single combined unstable mode is formed in the case of highly viscous substrate fluids and in this case increased damping has a stabilizing effect. When the free-stream speed is reduced to 15 m/s the modal interaction no longer occurs. In this case the effects of combined viscous and visco-elastic damping are investigated. It is found that damping tends to have a strong stabilizing effect on the FISI, in the form of travelling-wave flutter, but a weaker destabilizing effect on the TSI. The opposing effects of damping on the two modes of instability forms the basis of a possible explanation for Kramer's empirical observation of an optimum substrate viscosity. Results obtained using the e9 method also indicate that a substantial transition delay is theoretically possible for flows over Kramer's compliant coatings.

The stability of boundary-layer flow over single- and multi-layer viscoelastic walls

Abstract: In this paper, we are concerned with the linear stability of zero pressure-gradient laminar boundary-layer flow over compliant walls which are composed of one or more layers of isotropic viscoelastic materials and backed by a rigid base. Wall compliance supports a whole host of new instabilities in addition to the Tollmien-Schlichting mode of instability, which originally exists even when the wall is rigid. The perturbations in the flow and the compliant wall are coupled at their common interface through the kinematic condition of velocity continuity and the dynamical condition of stress continuity. The disturbance modes in the flow are governed by the Orr-Sommerfeld equation using the locally-parallel flow assumption, and the response of the compliant layers is described using a displacement-stress formalism. The theoretical treatment provides a unified formulation of the stability eigenvalue problem that is applicable to compliant walls having any finite number of uniform layers; inclusive of viscous sublayer. The formulation is well suited to systematic numerical implementation. Results for single- and multi-layer walls are presented. Analyses of the eigenfunctions give an insight into some of the physics involved. Multi-layering gives a measure of control over the stability characteristics of compliant walls not available to single-layer walls. The present study provides evidence which suggests that substantial suppression of disturbance growth may be possible for suitably tailored compliant walls.

Progress on the Use of Compliant Walls for Laminar-Flow Control

Abstract: Thispaperreviewsand assessestherecentprogresstoward making theuseofcompliantwallsa practicalmethod of laminar-e ow control. Three main areas are covered. First, the current understanding of the vitally important e ow-induced surface instabilities is assessed. Some new results are included. Then the optimization of multiplepanel compliant walls is considered. New numerical simulation results are included showing that short compliant panels are very effective in suppressing Tollmien ‐Schlichting waves. It is found that for marine applications appropriately designed multiple-panel compliant walls are capable of suppressing Tollmien ‐Schlichting waves to indee nitely high Reynolds numbers. Finally, the feasibility of using compliant walls for laminar-e ow control in aeronautical applications is assessed. It is found that, although there is no reason in principle why compliant walls cannot be used in air, in practice exceptionally delicate walls are required to obtain the necessary match between thee uid and structural inertias. Theresulting lack ofrobustness forsuch walls isdeemed to make them completely impractical for aeronautical applications.