Interactive Boundary-layer Method Research Articles

Wing aeroelasticity analysis based on an integral boundary-layer method coupled with Euler solver

An interactive boundary-layer method, which solves the unsteady flow, is developed for aeroelastic computation in the time domain. The coupled method combines the Euler solver with the integral boundary-layer solver (Euler/BL) in a “semi-inverse” manner to compute flows with the inviscid and viscous interaction. Unsteady boundary conditions on moving surfaces are taken into account by utilizing the approximate small-perturbation method without moving the computational grids. The steady and unsteady flow calculations for the LANN wing are presented. The wing tip displacement of high Reynolds number aero-structural dynamics (HIRENASD) Project is simulated under different angles of attack. The flutter-boundary predictions for the AGARD 445.6 wing are provided. The results of the interactive boundary-layer method are compared with those of the Euler method and experimental data. The study shows that viscous effects are significant for these cases and the further data analysis confirms the validity and practicability of the coupled method.

A Structured Mesh Euler and Interactive Boundary Layer Method for Wing/Body Configurations

To compute transonic flows over a complex 3D aircraft configuration, a viscous/inviscid interaction method is developed by coupling an integral boundary-layer solver with an Eluer solver in a “semi-inverse” manner. For the turbulent boundary-layer, an integral method using Green's lag equation is coupled with the outer inviscid flow. A blowing velocity approach is used to simulate the displacement effects of the boundary layer. To predict the aerodynamic drag, it is developed a numerical technique called far-field method that is based on the momentum theorem, in which the total drag is divided into three component drags, i.e. viscous, induced and wave-formed. Consequently, it can provide more physical insight into the drag sources than the often-used surface integral technique. The drag decomposition can be achieved with help of the second law of thermodynamics, which implies that entropy increases and total pressure decreases only across shock wave along a streamline of an inviscid non-isentropic flow. This method has been applied to the DLR-F4 wing/body configuration showing results in good agreement with the wind tunnel data.

An updated interactive boundary layer method for high Reynolds number flows

The quasi-simultaneous interactive boundary layer (IBL) method is improved with the iterative correction of an inviscid operator. The updated interactive boundary layer method (UIBL) presented in this work, uses the Hess–Smith panel method (HSPM) as an inviscid operator to update the outer flow calculation and the inviscid velocity in the interaction law (IL). The discretization of the Hilbert integral (HI) from the original method is modified to reduce the error introduced by the calculation of the HI in a restricted domain. The method is tested on a flat plate with a small indentation for two-dimensional, steady, incompressible and laminar flow. The UIBL method is capable to predict the flow separation and reattachment with good accuracy. The accuracy of the results is competitive with the numerical solution of the Navier–Stokes equations (NSE). Copyright © 2005 John Wiley & Sons, Ltd.

헬리콥터 로터 블레이드 예비설계를 위한 에어포일 공력 해석

헬리콥터 로터 블레이드 예비설계 단계에서 에어포일 선정과 분포의 기초 자료를 생성하기 위해 로터 에어포일 공력해석을 수행하였다.기존 헬리콥터의 에어포일 중 공력해석 대상으로 10개를 선정하고 블레이드 요소이론을 이용한 공력해석에 적합한 table 형태의 공력계수 자료를 생성하였다. 해석 비용을 고려, 풍동시험 대신 간단한 수학적 모델을 이용하여 모든 받음각 영역(<TEX>$-180^{o}\sim180^{o}$</TEX>)에서 공력특성 곡선(<TEX>$C_{l},C_{m},C_{d}$</TEX>)을 구성하였다. 공력특성곡선 구성에 필요한 각 에어포일의 필수 공력자료를 IBLM을 이용하여 구하였으며, 구성된 공력특성곡선은 실험결과와 정성적으로 일치하였다. 마지막으로, 에어포일 선정과 분포설계의 기준을 마련하기 위하여 각 에어포일의 양력계수, 모멘트계수를 상호 비교한 후 분류하였다. The aerodynamic analysis of helicopter rotor airfoils was performed to generate the basic data for selection and distribution of airfoils at the helicopter rotor blade preliminary design phase.10 airfoils were chosen among the existing rotor airfoils, and the tabulated aerodynamic coefficients which are proper for the aerodynamic analysis using blade element theory were generated. Considering analysis cost, the simple mathematical models were chosen before the wind tunnel test to generate the aerodynamic characteristic curves(<TEX>$C_{l},C_{m},C_{d}$</TEX>) in full AoA range(<TEX>$-180^{o}\sim180^{o}$</TEX>) including the reverse flow region. The essential data necessary to the generation of the complete curves were obtained by using the IBLM(Interactive Boundary Layer Method). The generated aerodynamic characteristic curves agree with experimental results qualitatively. Finally, the aerodynamic characteristics of all 10 airfoils were compared and classified according to their own lift or moment characteristics.

Small-Scale Roughness Effects on Laminar Separation

In this study an interacting boundary-layer (IBL) algorithm is used to investigate small-scale surface roughness effects on the laminar separation mechanism, where “small-scale” is intended to mean roughness fully contained within the boundary layer. Steady, laminar breakaway separation is computed for two-dimensional flow past a symmetric biconvex airfoil with small-scale roughness elements added to the surface. In this case the flow separation is generated at the trailing edge of a biconvex airfoil, but results are relevant to laminar separation points in general such as that occurring in the leading-edge region. The study is interested primarily in the laminar separation point, and not necessarily the entire bubble and downstream region. The use of the IBL method made it possible to achieve the required fine resolution in areas of interest. For some roughness geometries and flow conditions, up to 15000 grid points (over 4 million total grid points) were used in the streamwise direction to capture the resulting flow physics, which would still be time restrictive with a full Navier–Stokes algorithm. A number of different small-scale roughness configurations were evaluated including variations of roughness height, wavelength, distribution, and geometry. Results from this work show that small-scale roughness can alter the characteristics of the laminar separation point in low-speed flows.

Effect of ice on airfoil stall at high Reynolds numbers

Introduction I wing design it is very important to determine the maximum lift coefficient as accurately as possible, since this lift coefficient corresponds to the stall speed that is the minimum speed at which controllable flight can be maintained. Any further increase in angle of incidence will increase flow separation on the wing upper surface, resulting in a loss of lift and a large increase in drag. Despite the significant advances in computational fluid dynamics, our ability to predict the maximum lift coefficient and poststall behavior of single airfoils, wings, multielement airfoils, or wings is still not satisfactory. The chief culprit is the lack of an accurate turbulence model to represent flows with extensive separation. Inaccuracies of numerical solutions of the conservation equations at these flow conditions, as well as difficulties in modeling of flow near the trailing edge of an airfoil or wing, add to this dilemma. Recently Cebeci et al. described an interactive boundary-layer method, together with the e approach to the calculation of transition, for predicting stall and poststall behavior of airfoils at low and high Reynolds numbers. The turbulence model was based on the Cebeci-Smith algebraic eddy-viscosity formulation with improvements for strong pressure gradient effects and transitional flows at low Reynolds numbers. Comparison of calculated results for incompressible flows indicated good agreement with experiment for a wide range of Reynolds numbers. Preliminary calculations for low-Mach-number flows with this interactive method that applies compressibility corrections to the panel method also indicated good agreement with data and showed that at a Mach number of 0.30, the compressibility effect on (Q)max was not negligible. The ability of the calculation method of Ref. 1 to predict the effect of ice on airfoil stall at high Reynolds numbers is examined here, and the results are presented in the following section. The interactive boundary-layer method previously developed for iced airfoils and coupled to the LEWICE code for predicting ice shapes' is replaced with the calculation method of Ref. 1. Appropriate changes are made to this method, including roughness effects due to ice, and the results are presented in the following section.

Predicting stall and post-stall behavior of airfoils at low Mach numbers

An interactive boundary-layer method, together with the eapproach to the calculation of transition, is used to predict the stall and post-stall behavior of airfoils in incompressible and compressible flows at low freestream Mach numbers. Two separate inviscid methods—a panel method with compressibility corrections and a full potential method—are used to compute the external velocity distribution needed in the solution of the compressible boundary-layer equations. The turbulence model is based on the Cebeci-Smith algebraic eddy-viscosity formulation with improvements for strong pressure gradient effects. Comparison of calculated results with inviscid flow computed with a panel method indicate excellent agreement with experiment for a wide range of Reynolds numbers in incompressible flows. Comparison of calculated results obtained with inviscid flow computed with a full potential method also indicate excellent agreement with experiment for a wide range of angles of attack, including stall for compressible flows at low freestream Mach numbers. The study also shows that even though the compressibility corrections to the panel method are adequate at small-to-moderate angles of attack, they are not satisfactory at higher angles of attack, especially near stall.

Comparison of the Triple-Deck Theory, Interactive Boundary Layer Method, and Navier-Stokes Computation for Marginal Separation

The steady two-dimensional marginal separation of an incompressible boundary layer flow within a channel was solved independently by three different methods: the triple-deck method of marginal separation, the interactive boundary layer method, and the full Navier-Stokes computation. From comparison of the results between these three methods, the accuracy and appropriateness of each method was determined. The critical condition beyond which the steady marginal separation solution of triple-deck method does not exist was related to a physical phenomenon in which the separation bubble becomes unsteady. Factors such as Reynolds number and pressure gradient distribution which might influence the accuracy of the marginal separation solution were also investigated.

An efficient interacting boundary layer method for supersonic flow in the transonic regime

A quasi-simultaneous interacting boundary layer (IBL) method with alternating direction explicit acceleration is extended to include the nonlinear weak-shock branch of a nonlinear transonic viscous—inviscid interaction. This IBL scheme is valid for an inviscid flow which is supersonic everywhere as the free-stream Mach number (M −8) approaches one. Solutions computed by this method are compared with transonic triple-deck solutions for expansion ramps as M ∞ a ̊ 1 . The convergence rate of the method is found to be comparable to an identically accelerated scheme with a linear supersonic interaction law. Solutions for a parabolic hump on a flat plate are also given which show the development of a growing region of separation behind the hump as M ∞ a ̊ 1 .

An Inviscid-Viscous Interaction Approach to the Calculation of Dynamic Stall Initiation on Airfoils

An interactive boundary-layer method is described for computing unsteady incompressible flows over airfoils, including the initiation of dynamic stall. The inviscid unsteady panel method developed by Platzer and Teng is extended to include viscous effects. The solutions of the boundary-layer equations are obtained with an inverse finite-difference method employing an interaction law based on the Hilbert integral, and the algebraic eddy-viscosity formulation of Cebeci and Smith. The method is applied to airfoils subject to periodic and ramp-type motions and its abilities are examined for a range of angles of attack, reduced frequency, and pitch rate.

Indented plate problem revisited

AbstractAn interactive boundary layer method, together with the en approach to the calculation of transition, has been used to investigate the flow over an indented surface for which previous calculations had led to numerical instabilities. The results show two possible reasons for these numerical difficulties. First, it is shown that the gradients of wall shear become very steep at larger Reynolds numbers, particularly in the vicinity of reattachment. Extremely fine numerical grids are required to resolve these gradients. Secondly, and perhaps of greater importance, transition is shown to occur within the region of recirculation for all Reynolds numbers except for the lowest ones. Thus, the calculated flows downstream of the transition locations are fictitious and may be expected to deviate from the corresponding real flows by increasing amounts as the Reynolds number becomes larger. Calculations involving laminar, transitional and turbulent flow have been performed and confirm this conjecture.

Fortified LEWICE with viscous effects

A method for computing the prediction of ice shapes on airfoils and their effects on the airfoil lift and drag coefficients are described. The previously developed LEWICE code has been modified to avoid problems due to multiple stagnation points. The interactive boundary-layer method developed by Cebeci has been incorporated into LEWICE to improve the accuracy of predicting ice shapes as well as to compute the performance characteristics of iced airfoils. The paper also presents ice shapes calculated without and with viscous effects, the consequences for aerodynamic properties, particulary lift and drag, and evaluation of the time steps used in the ice accretion process.

Interactive boundary-layer method for unsteady airfoil flows - Quasisteady model

An interactive boundary-layer method previously developed and tested for steady flows is used here in a quasisteady manner to examine the evolution of the flow behavior of airfoils subject to harmonic oscillation and ramp-type motions. The calculations encompass the airfoil and wake flows at angles of attack that lead to separation. The results quantify the effects of the viscous boundary layer and wake on the variation of lift coefficient with angle of attack and reduced frequency. These effects are shown to be large at angles of attack that involve boundary-layer separation.

Calculation of transonic flows with separation past arbitrary inletsat incidence

A method of calculating transonic separated flows past arbitrary inlets at incidence and yaw is presented. The inviscid flow solver is based on the transonic potential equation, which is approximated and solved numerically on a boundary-conforming coordinate system. The multrigrid finite-volume method is used to solve the flow equation. The inviscid flow technique is coupled with an interactive boundary-layer method that uses an inverse formulation to calculate shock-induced separated flows

An alternative approach to linear and nonlinear stability calculations at finite Reynolds numbers

An extended version of the interactive boundary-layer approach which has been used widely in steady-flow calculations is applied here to the linear and nonlinear stability properties of channel flows and boundary layers in the moderate-to-large Reynolds-number regime. This is the regime of most practical concern. First, for linear stability the agreement found between the interactive approach and Orr-Sommerfeld results remains fairly close even at Reynolds numbers as low as about$\frac{1}{10}$of the critical value for plane Poiseuille flow, or$\frac{1}{5}$for Blasius flow. Secondly, nonlinear unsteady calculations and comparisons with full solutions obtained by enlarging the same method are also presented. Overall the work suggests that, at the finite Reynolds numbers where real interest lies, the dominant physical processes of instability in channel flow and boundary layers are of boundary-layer form, with interaction, and it suggests also an alternative numerical technique for determining those processes. This alternative technique uses the interactive boundary-layer method as the central means for obtaining full unsteady Navier-Stokes solutions.