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Zvi Rusak - One of the best experts on this subject based on the ideXlab platform.

  • Transonic Flow of Moist Air Around a Thin Airfoil with Equilibrium Condensation
    Journal of Aircraft, 2001
    Co-Authors: Zvi Rusak
    Abstract:

    The two-dimensional and steady transonic e ow of atmospheric moist air with equilibrium condensation around a thin airfoil is investigated. The study is based on an asymptotic analysis and numerical simulations. A smalldisturbance model is developed to explore the nonlinear interactions between the near-sonic speed of the e ow, the small thickness ratio and angle of attack of the airfoil, and the small amount of mass of water vapor in the air. The condensation process of water vapor in the air is assumed to be isentropic. The similarity parameters that govern the e ow problem are provided. The e owe eld may be described by a modie ed transonic small-disturbance (TSD)equation that includes parameters that are related to the condensation process. Murman and Cole’ smethod (Murman, E. M., and Cole, J. D., “ Calculation of Plane Study Transonic Flows,” AIAA Journal , Vol. 9, No. 1, 1971, pp. 114 ‐121.)is used for the numerical solution of the modie ed TSD problem. The results show that the e ow of moist air is similar to the e ow of dry air with an effective freestream Mach number that is greater than the freestream Mach number of moist air. The present approach is used to study the aerodynamic performances of airfoils in atmospheric transonic e ight with humidity.

  • transonic flow of moist air around a thin airfoil with non equilibrium and homogeneous condensation
    Journal of Fluid Mechanics, 2000
    Co-Authors: Zvi Rusak
    Abstract:

    A new small-disturbance model for a steady transonic flow of moist air with non-equilibrium and homogeneous condensation around a thin airfoil is presented. The model explores the nonlinear interactions among the near-sonic speed of the flow, the small thickness ratio and angle of attack of the airfoil, and the small amount of water vapour in the air. The condensation rate is calculated according to classical nucleation and droplet growth models. The asymptotic analysis gives the similarity parameters that govern the flow problem. Also, the flow field can be described by a non-homogeneous (extended) transonic small-disturbance (TSD) equation coupled with a set of four ordinary differential equations for the calculation of the condensate (or sublimate) mass fraction. An iterative numerical scheme which combines Murman & Cole's method for the solution of the TSD equation with Simpson's integration rule for the estimation of the condensate mass production is developed. The results show good agreement with available numerical simulations using the inviscid fluid flow equations. The model is used to study the effects of humidity and of energy supply from condensation on the aerodynamic performance of airfoils

  • Numerical studies of transonic BZT gas flows around thin airfoils
    Journal of Fluid Mechanics, 1999
    Co-Authors: Chun-wei Wang, Zvi Rusak
    Abstract:

    Numerical studies of two-dimensional, transonic flows of dense gases of retrograde type, known as BZT gases, around thin airfoils are presented. The computations are guided by a recent asymptotic theory of Rusak & Wang (1997). It provides a uniformly valid solution of the flow around the entire airfoil surface which is composed of outer and inner solutions. A new transonic small-disturbance (TSD) equation solver is developed to compute the nonlinear BZT gas flow in the outer region around most of the airfoil. The flow in the inner region near the nose of the airfoil is computed by solving the problem of a sonic flow around a parabola. Numerical results of the composite solutions calculated from the asymptotic formula are compared with the solutions of the Euler equations. The comparison demonstrates that, in the leading order, the TSD solutions of BZT gas flows represent the essence of the flow character around the airfoil as computed from the Euler equations. Furthermore, guided by the asymptotic formula, the computational results demonstrate the similarity rules for transonic flows of BZT gases. There are differences between the self-similar cases that may be related to the error associated with the accuracy of the asymptotic solution. A discussion on the flow patterns around an airfoil at transonic speeds and at various upstream thermodynamic conditions is also presented. The paper provides important guidelines for future studies on this subject.

L. Dragos - One of the best experts on this subject based on the ideXlab platform.

  • A direct boundary integral equations method to subsonic flow with circulation past thin airfoils in ground effect
    Computer Methods in Applied Mechanics and Engineering, 1995
    Co-Authors: L. Dragos, Adrian Dinu
    Abstract:

    Abstract A direct method for solving the problem of subsonic flow past a lifting or non-lifting airfoil in ground effects is presented. The problem is formulated in terms of velocity and it does not make use of the potential or the stream function. The non-linear boundary condition is used. In this way, the solution is exact in the incompressible case for any shape of the airfoil (not necessarily thin). For the compressible case, we use linear equations of motion and the solution will be valid for thin airfoils. A set of Green functions that satisfies the boundary condition on the ground are deduced and an integral equation on the airfoil only is constructed. We solve the integral equation by means of a collocation method. The circulation is introduced as a parameter of the problem and it is related to the Kutta condition. The equal-pressure Kutta condition is used but the problem remains linear due to the formulation in velocities. In the incompressible case the solution for the circular body is compared with the exact solution, the coincidence being almost perfect. The other numerical experiments designed for the lifting profile NACA-4412, enlighten the compressibility and ground effects.

  • The application of the boundary integral equations method to subsonic flow with circulation past thin airfoils in a wind tunnel
    Acta Mechanica, 1994
    Co-Authors: L. Dragos, Anca Dinu
    Abstract:

    In this paper we apply the direct boundary integral equations method to subsonic flow with circulation past a thin airfoil in a wind tunnel. A set of Green's functions for the equations of the velocity perturbation is deduced. These functions together with the non-linear limit condition imposed just on the surface profile lead to a Fredholm integral equation of the second kind over the profile only. The integral formulation has the advantage not to truncate the flow domain and to save computing effort when numerical solving is performed, due to the lack of the tunnel walls discretisation. In the case of the incompressible fluid we use the exact equations of motion, which implies a valid solution for any shape of the profile, being not necessarily thin. In the case of the compressible fluid we use the linearized motion equations, which implies a valid solution only for thin airfoils. The integral equation obtained on the surface, together with the circulation integral formula are solved via a collocation method. The numerical tests made for the circular obstacle show a very good agreement with the theory. The numerical experiments on the NACA-4412 profile were made in order to determine the tunnel and the compressibility effects being compared to the unbounded flow.

Adrian Dinu - One of the best experts on this subject based on the ideXlab platform.

  • A direct boundary integral equations method to subsonic flow with circulation past thin airfoils in ground effect
    Computer Methods in Applied Mechanics and Engineering, 1995
    Co-Authors: L. Dragos, Adrian Dinu
    Abstract:

    Abstract A direct method for solving the problem of subsonic flow past a lifting or non-lifting airfoil in ground effects is presented. The problem is formulated in terms of velocity and it does not make use of the potential or the stream function. The non-linear boundary condition is used. In this way, the solution is exact in the incompressible case for any shape of the airfoil (not necessarily thin). For the compressible case, we use linear equations of motion and the solution will be valid for thin airfoils. A set of Green functions that satisfies the boundary condition on the ground are deduced and an integral equation on the airfoil only is constructed. We solve the integral equation by means of a collocation method. The circulation is introduced as a parameter of the problem and it is related to the Kutta condition. The equal-pressure Kutta condition is used but the problem remains linear due to the formulation in velocities. In the incompressible case the solution for the circular body is compared with the exact solution, the coincidence being almost perfect. The other numerical experiments designed for the lifting profile NACA-4412, enlighten the compressibility and ground effects.

Anya R. Jones - One of the best experts on this subject based on the ideXlab platform.

  • Reynolds Number Effects on Airfoils in Reverse Flow
    53rd AIAA Aerospace Sciences Meeting, 2015
    Co-Authors: Andrew H. Lind, Luke R. Smith, Joseph Milluzzo, Anya R. Jones
    Abstract:

    This work is aimed at providing an improved understanding of the impact of the radial Reynolds number distribution that exists in the reverse flow region of a helicopter operating at high advance ratios. Time-averaged sectional airloads and flow fields were measured experimentally for four airfoils in forward and reverse flow at Reynolds numbers between 3.3× 10 and 1.0× 10. Two airfoils with a sharp geometric trailing edge (NACA 0012 and NACA 0024) and two airfoils with a blunt geometric trailing edge (a 24 % thick elliptical airfoil, and a 26 % thick cambered ellipse airfoil) were tested. This work shows that the airloads for a NACA 0012 in reverse flow (a “thin” airfoil with a sharp aerodynamic leading edge) are insensitive to Reynolds number due to early flow separation. The airloads of thicker airfoils are found to be more sensitive to Reynolds number. In reverse flow, a NACA 0024 airfoil exhibits a decrease in the magnitude of the airloads with increasing Reynolds number for −3 ≤ −αrev ≤ 15deg. The lift curve of an elliptical airfoil becomes more linear with increasing Reynolds number. The character of the lift curve for the cambered ellipse airfoil in both forward and reverse flow changes drastically for Re ≥ 3.3 × 10. This includes a large shift in the zero-lift angle of attack. These results give insight to the design of high-speed helicopter rotor blades by examining the sensitivity of airloads to the range of Reynolds numbers encountered in the reverse flow region.

J. D. A. Walker - One of the best experts on this subject based on the ideXlab platform.

  • Unsteady separation from the leading edge of a thin airfoil
    Physics of Fluids, 1996
    Co-Authors: A. T. Degani, J. D. A. Walker
    Abstract:

    At high Reynolds numbers, the process leading to dynamic stall on airfoils initiates in the leading‐edge region. For thin airfoils, the local motion near rounded leading edges can be represented as flow past a parabola and when the mainstream flow is at an angle of attack to the airfoil, a portion of the boundary layer will be exposed to an adverse pressure gradient. Once the angle of attack exceeds a certain critical value, it is demonstrated that unsteady boundary‐layer separation will occur in the leading‐edge region in the form of an abrupt focused boundary‐layer eruption. This process is believed to initiate the formation of the dynamic stall vortex. For impulsively‐started incompressible flow past a parabola, a generic behavior is found to occur over a range of angles of attack, and a limit solution corresponding to relatively large angles is found. The separation in the leading‐edge region develops in a zone of relatively limited streamwise extent over a wide range of angles of attack. This suggests that localized control measures (such as suction) may possibly be effective at inhibiting separation.