Hsun H. Chen

Prediction of ice shapes and their effect on airfoil performance

Calculations of ice shapes and the resulting drag increases are presented for experimental data on an NACA 0012 airfoil. They were made with a combination of LEWICE and interactive boundary-layer codes for a wide range of conditions which include airspeed and temperature, the droplet size and liquid water content of the cloud, and the angle of attack of the airfoil. In all cases the calculated results account for the drag increase due to ice accretion and, in general, show good agreement with data.

Numerical method for predicting transition in three-dimensional flows by spatial amplification theory

An efficient second-order-accurate numerical method, based on Kellers boc scheme, is used to solve the Orr-Sommerfeld equation for three-dimensional incompressible flows. Transition is computed with the en method and with an eigenvalue formulation based on the saddle-point method of Cebeci and Stewartson. The frequencies needed in transition calculations are obtained from zarfs that correspond to three-dimensional neutral stability curves. The calculation method is evaluated in terms of experimental data on a swept wing and a prolate spheroid at an angle of incidence. In general, the results are in good agreement with measured values.

A turbulence model for iced airfoils and its validation

A turbulence model based on the extension of the algebraic eddy viscosity formulation of Cebeci and Smith developed for two dimensional flows over smooth and rough surfaces is described for iced airfoils and validated for computed ice shapes obtained for a range of total temperatures varying from 28 to -15 F. The validation is made with an interactive boundary layer method which uses a panel method to compute the inviscid flow and an inverse finite difference boundary layer method to compute the viscous flow. The interaction between inviscid and viscous flows is established by the use of the Hilbert integral. The calculated drag coefficients compare well with recent experimental data taken at the NASA-Lewis Icing Research Tunnel (IRT) and show that, in general, the drag increase due to ice accretion can be predicted well and efficiently.

Further consideration of the effect of curvature on the stability of three-dimensional flows

Abstract The effect of curvature on the stability and transition of three-dimensional incompressible flows is examined. The stability equations with and without curvature terms are solved with the temporal eigenvalue formulations used by Malik and Poll. Calculations are performed for flow over a yawed circular cylinder and results are compared with the calculations of Malik and Poll with and without curvature effects. Results show that the effect of curvature on transition is much smaller than claimed previously.

An interactive boundary-layer method for multielement airfoils

Abstract A calculation method for the aerodynamic prediction of multielement airfoils based on an interactive boundary-layer approach using an improved Cebeci–Smith eddy viscosity formulation is described. Results are first presented for single airfoils at low and moderate Reynolds numbers in order to demonstrate the need to calculate transition for accurate drag polar prediction and the ability of the improved Cebeci–Smith turbulence model to predict flows with extensive separation, and therefore to predict maximum lift coefficient. Results, in terms of pressure distributions and lift, drag and moment coefficients, are presented for a series of multielement airfoils with flaps and slats. The importance of the compressibility effects and the turbulence model on stall, and, again, the need to calculate the onset of transition, are demonstrated. Finally, recommendations are made for the preferred approach to predicting the aerodynamic performance of multielement airfoils for use as a practical and efficient design tool.

Analysis of iced wings

A method for computing ice shapes along the leading edge of a wing and a method for predicting its aerodynamic performance degradation due to icing is described. Ice shapes are computed using an extension of the LEWICE code which was developed for airfoils. The aerodynamic properties of the iced wing are determined with an interactive scheme in which the solutions of the inviscid flow equations are obtained from a panel method and the solutions of the viscous flow equations are obtained from an inverse three-dimensional finite-difference boundary-layer method. A new interaction law is used to couple the inviscid and viscous flow solutions. The application of the LEWICE wing code to the calculation of ice shapes on a MS-317 swept wing show good agreement with measurements. The interactive boundary layer method is applied to a tapered iced wing in order to study the effect of icing on the aerodynamic properties of the wing at several angles of attack.

A method for removing the coordinate singularity on bodies with blunt rounded noses at incidence

Abstract A general method for calculating the boundary-layer development in the stagnation region of bodies with blunt rounded noses at incidence and with planes of symmetry is investigated. The three-dimensional equations are expressed in a nonorthogonal body-oriented coordinate system and appropriate transformations are devised for the line of symmetry equations to remove the singularity appearing in the metric coefficients and geodesic curvatures. A novel procedure based on a quasi-three-dimensional form of the equations and the characteristic box scheme is used to compute the initial conditions on a coordinate line off the line of symmetry. Sample calculations are reported on the line of symmetry for three shapes which include one typical of those found on ships with bulbous bows. Results are also presented for the nose region of a prolate spheroid at incidence and the solutions are compared with the previous solutions of Cebeci, Khattab and Stewartson. The results demonstrate that the method, with its essential transformations and solution procedures, represents the boundary-layer development accurately and can be used for axisymmetric and nonaxisymmetric bodies with blunt rounded noses.