Aerodynamic Code Research Articles

Application of an aerodynamic code to marine propellers

The vortex lattice method successfully applied in the past to helicopter rotors is now applied to marine propellers. Two cases are presented: advancing coefficient.7 with uniform and non-uniform upstream velocity behind a vortex generator. The results presented here concern the evolution of the thrust coefficient with the azimuth angle, the spanwise distribution of the thrust and the wake

Acoustic radiation from a thin airfoil in nonuniform subsonic flows

Noise resulting from the interaction of a three dimensional gust is modeled directly from unsteady aerodynamic codes. This paper provides bench mark results for the case of a thin airfoil for comparison with the more complex case of a loaded airfoil. It is shown that the acoustic pressure pattern strongly depends on the value of a certain reduced frequency. The effects of an oblique gust are examined and shown to significantly reduce the acoustic power for certain gust parameters range. The Acoustic power versus the gust frequency exhibits a maximum. High frequency and non-compact source effects are also investigated.

Transonic flutter analysis using time-linearization aerodynamics

Introduction O VER the past decade there has been considerable progress in the development of numerical simulation techniques for unsteady transonic flow calculations. This activity was motivated by the need to supplement the expensive and time-consuming wind-tunnel investigations and flight tests with inexpensive, fast, and efficient computer simulation programs to accurately predict flutter boundaries and many other important aeroelastic phenomena in the transonic regime. Although the basic fluid motion in unsteady aerodynamics is governed by the Navier-Stokes (NS) equations, numerical simulations of the complete NS equations for general threedimensional flow problems are not yet sufficiently developed for flutter applications. Computational methods in threedimensional unsteady transonic flows concentrate mainly on the transonic small disturbance (TSD) equation or full potential (FP) equation. The major advantage of the simpler model using TSD or FP approach over those based on Euler and NS formulation is the large reduction in computational time and memory requirements. In Ref. 1, the TSD code UST3D was described. This code utilizes the time-linearization approach for solving the transonic small disturbance equation. It was demonstrated that the technique could be used with confidence for computations of three-dimensional unsteady transonic flows over an isolated thin wing. An improved version of the UST3D program was given in Ref. 2 and results from a validation study were presented. This transonic aerodynamics code was incorporated into the Institute for Aerospace Research (IAR) Flutter Analysis Program. This Note presents some results from an aerodynamic and flutter analysis of the AGARD 445.6 wing which was recommended by AGARD as a standard configuration for code validation.

Automatic differentiation of advanced CFD codes for multidisciplinary design

Automated multidisciplinary design of aircraft and other flight vehicles requires the optimization of complex performance objectives with respect to a number of design parameters and constraints. The effect of these independent design variables on the system performance criteria can be quantified in terms of sensitivity derivatives which must be calculated and propagated by the individual discipline simulation codes. Typical advanced CFD analysis codes do not provide such derivatives as part of a flow solution; these derivatives are very expensive to obtain by divided (finite) differences from perturbed solutions. It is shown here that sensitivity derivatives can be obtained accurately and efficiently by using the ADIFOR source translator for automatic differentiation. In particular, it is demonstrated that the 3D, thin-layer Navier-Stokes, multigrid flow solver called TLNS3D is amenable to automatic differentiation in the forward mode even with its implicit iterative solution algorithm and complex turbulence modeling. It is significant that, using computational differentiation, consistent discrete nongeometric sensitivity derivatives have been obtained from an aerodynamic 3D CFD code in a relatively short time, e.g. O (man-week) not O (man-year).

Performance effects of irregular communication patterns on massively parallel multiprocessors

We conduct a detailed study of the performance effects of irregular communication patterns on the CM-2. We characterize the communication capabilities of the CM-2 under a variety of controlled conditions. In the process of carrying out our performance evaluation, we develop and make extensive use of a parameterized synthetic mesh. In addition we carry out timings with unstructured meshes generated for aerodynamic codes and a set of sparse matrices with banded patterns of nonzeros. This benchmarking suite stresses the communication capabilities of the CM-2 in a range of different ways. Our benchmark results demonstrate that it is possible to make effective use of much of the massive concurrency available in the communication network.

Calculation of steady and unsteady pressures on wings at supersonic speeds with a transonic small-disturbance code

A transonic unsteady aerodynamic and aeroelasticity code has been developed for application to realistic aircraft configurations. The new code is called CAP-TSD which is an acronym for Computational Aeroelasticity Program - Transonic Small Disturbance. The CAP-TSD code uses a time-accurate approximate factorization algorithm for solution of the unsteady transonic small-disturbance equation that is efficient for solution of steady and unsteady transonic flow problems including supersonic freestream flows. The new code can treat complete aircraft geometries with multiple lifting surfaces and bodies. Applications to wings in supersonic freestream flow are presented. Comparisons with selected exact solutions from linear theory are presented showing generally favorable results. Calculations for both steady and oscillatory cases for the F-5 and RAE tailplane models are compared with experimental data and also show good overall agreement. Selected steady calculations are further compared with a steady flow Euler code.

Observations of Dynamic Stall Phenomena Using Liquid Crystal Coatings

Novel, shear stress-sensitive/temperature-insensitive liquid crystal coatings have been applied to the surface of an oscillating airfoil in order to ascertain the unsteady fluid physics associated with the dynamic-stall process. Surface microtufts and laser sheet/smoke-particle flow visualization were used to compare the liquid-crystal results. Boundary-layer transition and turbulent separation locations were measured as a function of geometric angle of attack. The results obtained are compared with Eppler (1980) aerodynamic design code predictions.

Static aeroelastic tailoring for oblique wing lateral trim

Due to its asymmetrical configuration, an oblique wing aircraft may exhibit a roll-trim imbalance. To correct this imbalance, control surfaces, such as ailerons, might be deflected or built-in twists might be added. This paper explores an alternative method of achieving oblique wing roll trim. This is the concept of applying composite tailoring to the oblique wing for lateral trim. The practical use of this concept to trim an oblique wing is demonstrated through analysis of a realistic wing by a static aeroelastic computational procedure. The computational method includes the full-potential transonic aerodynamic code FLO22 and a Ritz structural plate program that is used to model the stiffness created by symmetrical, but unbalanced, advanced composite wing skins. Analysis results indicate that asymmetric composite tailoring reduces the aileron deflection needed for roll equilibrium of the oblique wing. Furthermore, the use of aeroelastic tailoring for lateral trim can reduce control surface hinge moments and drag, resulting in performance benefits when compared to an untailored wing. At the same time, however, wing-skin stresses are greater when tailoring is used, leading to potential design tradeoffs. As such, an integrated design approach would be required to evaluate the true impact of aeroelastic tailoring on the overall performance of an oblique wing.

Analytical method for the ditching analysis of an airborne vehicle

A simple analytical method has been introduced for aerohydrodynamic load analysis of an airborne configuration during water ditching. The method employs an aerodynamic panel code, based on linear potential flow theory, to simulate the flow of air and water around an aircraft configuration. The free surface separating the air and water region is represented by doublet sheet singularities. Although all the theoretical load distributions are computed for air, provisions are made to correct the pressure coefficients obtained on the configuration wetted surfaces to account for the water density. As an analytical tool, the Vortex Separation Aerodynamic (VSAERO) code is chosen to carry out the present investigation. After assessing the validity of the method, its first application is to analyze the water ditching of the Space Shuttle configuration at a 12 degree attitude.

Static aeroelastic analysis of a three-dimensional oblique wing

Requirements for the aeroelastic analyses of an oblique-wing aircraft have posed serious problems because available analysis programs cannot be applied to asymmetric configurations in the subsonic through the low supersonic speed range and the effects of the wing leading edge suction forces are completely disregarded. Therefore, a capability to perform static aeroelastic analyses of an oblique wing at arbitrary skew positions was developed based on the framework of the MSC/NASTRAN static aeroelastic analysis. By the means of DMAP alterations, a portion of the subsonic static aeroelastic analysis scheme was modified to insert an aerodynamic influence coefficient matrix created externally by the NASA Ames Research Center aerodynamic panel codes. The modified scheme can cover the subsonic as well as the supersonic range for both symmetric and asymmetric configurations. Static aeroelastic responses of the oblique wing are studied at two skew angles and, in particular, the capability to calculate three-dimensional camber effects on the aerodynamic properties of the wing is investigated. Various aerodynamic coefficients of the rigid oblique wing are computed for two Mach numbers, 0.7 and 1.4, and the angle of attack is varied from −5 through 15°. Also, the wing flexibility effects on the aerodynamic coefficients and the displacement are examined at a Mach number of 0.7 for a 45° swept wing.

Surface pressure measurements on a body subject to vortex wake interaction

Wind-tunnel experiments were conducted to examine the interaction effects between a rotor and an idealized airframe in forward flight. Mean and unsteady pressures were measured on the airframe suface for various flight speeds. Strong rotor-wake interactions with the airframe create large excursions in the mean pressure distribution. Extreme fluctuations in the unsteady component of pressure are also observed. The flowfield features that cause these effects are identified and discussed. It is essential that a comprehensive aerodynamic interaction code account for these features if accurate predictions of mean and unsteady airloads on the fuselage are to be obtained. b c Cp Cp' Cp» CT H/R P

Unsteady transonic flow calculations for realistic aircraft configurations

A transonic unsteady aerodynamic and aeroelasticity code has been developed for application to realistic aircraft configurations. The new code is called CAP-TSD which is an acronym for Computational Aeroelasticity Program - Transonic Small Disturbance. The CAP-TSD code uses a time-accurate approximate factorization (AF) algorithm for solution of the unsteady transonic small-disturbance equation. The AF algorithm is very efficient for solution of steady and unsteady transonic flow problems. It can provide accurate solutions in only several hundred time steps yielding a significant computational cost savings when compared to alternative methods. The new code can treat complete aircraft geometries with multiple lifting surfaces and bodies including canard, wing, tail, control surfaces, launchers, pylons, fuselage, stores, and nacelles. Applications are presented for a series of five configurations of increasing complexity to demonstrate the wide range of geometrical applicability of CAP-TSD. These results are in good agreement with available experimental steady and unsteady pressure data. Calculations for the General Dynamics one-ninth scale F-16C aircraft model are presented to demonstrate application to a realistic configuration. Unsteady results for the entire F-16C aircraft undergoing a rigid pitching motion illustrated the capability required to perform transonic unsteady aerodynamic and aeroelastic analyses for such configurations.

Design of High-Performance Fans Using Advanced Aerodynamics Codes

In the recent past, the performance of transonic fans has been significantly improved. In addition, through the extensive use of advanced aerodynamic computation codes, the development time required has been considerably reduced. Methods used range from the definition of airfoils in quasi-three-dimensional flow with boundary layer optimization to the analysis of three-dimensional inviscid flow for stage operation at the design point and in off-design conditions. Such a set of methods was used to design the fan blade of the CFM56-5 engine to a very high performance level. This paper will discuss the optimization of rotor and stator airfoils, the assessment of off-design performance, and the operational stability of this fan. A detailed comparison of full-size component test data with computation results shows the validity of these methods and also identifies those areas where research is still required.

Recent advances in transonic computational aeroelasticity

A transonic unsteady aerodynamic and aeroelasticity code called CAP-TSD has been developed for application to realistic aircraft configurations. The code permits the calculation of steady and unsteady flows about complete aircraft configurations for aeroelastic analysis in the flutter critical transonic speed range. The CAP-TSD code uses a time-accurate approximate factorization (AF) algorithm for solution of the unsteady transonic small-disturbance potential equation. The paper gives an overview of the CAP-TSD code development effort and presents results which demonstrate various capabilities of the code. Calculations are presented for several configurations including the General Dynamics one-ninth scale F-16C aircraft model and the ONERA M6 wing. Calculations are also presented from a flutter analysis of a 45° sweptback wing which agree well with the experimental data. The paper presents descriptions of the CAP-TSD code and algorithm details along with results and comparisons which demonstrate these recent developments in transonic computational aeroelasticity.

Supersonic airfoil optimization

A procedure for the optimization of supersonic airfoils is presented. A nonlinear, full-potential solver (NCOREL) is coupled with a numerical optimizer (CONMIN). The NCOREL code is a three-dimensional supersonic marching method, however, only two-dimensional (conical) results are used for the airfoil problem. Each airfoil evaluated in the NCOREL aerodynamic code is composed of a linear combination of user-specified basis airfoils. The weighting factors of the basis airfoils plus the angle of attack form the design variable vector. The inviscid drag coefficient is minimized subject to lift and crossflow Mach number gradient constraints. The optimization strategy is found to be extremely important for the efficient determination of an optimum airfoil.

Assessment of two fast aerodynamic codes for guided projectiles

Covers advancements in spacecraft and tactical and strategic missile systems, including subsystem design and application, mission design and analysis, materials and structures, developments in space sciences, space processing and manufacturing, space operations, and applications of space technologies to other fields.

Prediction of subsonic wind tunnel mounting system interference

This paper describes the use of an advanced aerodynamic low-order panel code (VSAERO) in predicting the effects of various model mounting system components in the Boeing Vertol low-speed wind tunnel. A long static probe, mounted on the wind tunnel centerline, was used in obtaining the experimental data. In general, there is good agreement between the theoretical and experimental changes in pressure due to the mounting systems. This analytical technique can now confidently be used to estimate mounting system pressure gradient effects on a static probe that can be applied with the other known tunnel corrections in producing higher quality experimental results for new configurations.

Interactive graphics for geometry generation - A program with a contemporary design

The aerodynamic design process is now strongly influenced by the application of advanced computational tools due mainly to recent advances in computer capabilities. Today's aerodynamic analysis codes require large amounts of geometric data to define the surfaces in sufficient detail for proper analysis. A program has been developed to aid the engineer in generating geometric models for computational aerodynamics codes. The program, Interactive Graphics for Geometry Generation (I3G), is not dramatically different on the outside from other interactive graphics programs of this type. It is, however, different on the inside, i.e., in how it performs its functions. I3G is capable of accepting input from a variety of geometry sources and can support the generation of geometric models for many different computational codes. User-friendly operation encourages program use. Graphics devices ranging from sophisticated systems to low-cost display-only terminals can be easily supported by I3G. The program's modular structure allows new modeling capabilities to be readily incorporated. A system for indexing and retrieving existing geometric models is available to the user. In summary, flexibility is the key word behind I3G's basic design and structure. This feature allows modifications and enhancements to be readily made as needs change or arise. Succinctly, it is a program designed to last.

Assessment of preliminary prediction techniques for wing leading-edge vortex flows at supersonic speeds

The applicability of existing theoretical models for predicting wing leading edge separated flow characteristics for high-lift supersonic flight was examined with regard to previous experimental force, pressure and flow visualization data. Correlations of data on uncambered delta wings revealed that the upper surface normal force and minimum pressure coefficients decreased nonlinearly with increasing angles of attack. The attainable vacuum pressure decreased with increasing Mach number, while the lower surface normal force increased nonlinearly with increasing speed. The LISTAR and VORCAM linear theory aerodynamic codes generated predictions for comparison with the data. LISTAR displayed better agreement with measured vortex strength and position and lifting characteristics than did VORCAM. The Euler code SWINT was ill-suited to calculating wing performance in separated flows at high lift and low supersonic speeds.

Impact of fuselage incidence on the supersonic aerodynamics of two fighter configurations

The results of experimental and theoretical investigations into the effect of fuselage upwash on fighter aircraft wing performance are reported. Wind tunnel trials were performed on 4 percent scale models of two supersonic fighters. The trials were run at Mach 1.6-2.0, an Re of 2,000,000 and at angles of attack (AOA) of -4 to 20 deg. Measurements were made of lift, drag and pitching moments. Two linearized theory supersonic aerodynamic prediction codes, PAN AIR and the SDAS lift analysis, were used to predict the same aerodynamic coefficients. The fuselage AOA augmented the lift and pitching moment at 0, 2 and 5 deg. The contribution mainly arose from the fuselage-induced upwash. The PAN AIR code gave superior data for the fuselage aerodynamics and effects, although both codes accurately predicted the overall lift and moment increments due to the fuselage AOA.