Marilyn J. Smith

Evaluation of computational algorithms suitable for fluid-structure interactions

The objective was to identify and mathematically evaluate suitable methods to transfer information between nonlinear computational fluid dynamics (CFD) and computational structural dynamics (CSD) grids. This data transfer is vital in the field of computational aeroelasticity, where the interpolation method between the two grids can easily be the limiting factor in the accuracy of an aeroelastic simulation. The data to be transferred can include a variety of field variables, such as deflections, loads, pressure, and temperature. For a method to be suitable, it is important that it provide a smooth, yet accurate transfer of data for a wide variety of functional forms that the data may represent. An extensive literature survey was completed that identified current algorithms in use, as well as other candidate algorithms from different implementations, such as mapping and CAD/CAM. The performance of the various methods was assessed on a number of analytical functions, followed by a series of applications that have been or are currently being studied using nonlinear CFD methods coupled with linear representations of the CSD equations (equivalent plate/shell mode shapes and influence coefficient matrices). Two methods, multiquadric-biharmonic and thin-plate spline, are shown to be the most robust, cost-effective, and accurate of all of the methods tested.

An evaluation of computational algorithms to interface between CFD and CSD methodologies

Abstract : Methods to transfer information from Computational Fluid Dynamics (CFD) to Computational Structural Dynamics (CSD) and vice versa have been evaluated. The data include geometry deformations, slopes and loads. The methods were evaluated for accuracy, robustness and case of use. Both interpolations and extrapolations were evaluated. Six methods were analyzed: Infinite Plate Splines, Multiquadrics, Thin Plate Splines, Non-Uniform B-Splines, Finite Plate Splines and Inverse Isoparametric Mapping. Of these, the Thin Plate Splines were the most accurate and robust, followed closely by Multiquadrics. The Infinite Plate Spline Method, although widely used, is not recommended.

Application of Circulation Control to Advanced Subsonic Transport Aircraft, Part I: Airfoil Development

An experimental/ analytical research program was undertaken to develop advanced versions of circulation control wing (CCW) blown high-lift airfoils, and to address specific issues related to their application to subsonic transport aircraft. The primary goal was to determine the feasibility and potential of these pneumatic airfoils to increase high-lift system performance in the terminal area while reducing system complexity. A four-phase program was completed, including 1) experimental development and evaluation of advanced CCW high-lift configurations, 2) development of effective pneumatic leading-edge devices, 3) computational evaluation of CCW airfoil designs plus high-lift and cruise capabilities, and 4) investigation of the terminal-area performance of transport aircraft employing these airfoils. The first three phases of this program are described in Part I of this article. Applications to the high-lift and control systems of advanced subsonic transport aircraft and resulting performance are discussed in the continuation of this article, Part II. Experimental lift coefficient values approaching 8.0 at zero incidence and low blowing rates were demonstrated by two-dimensional CCW configurations that promised minimal degradation of the airfoils performance during cruise. These results and experimental/CFD methods will be presented in greater detail in the following discussions.

Hybrid RANS-LES Turbulence Models on Unstructured Grids

ow eld in an unstructured legacy RANS computational uid dynamics code. The hybrid method consists of a blending of the k ! SST RANS model with a one-equation LES model for the subgrid-scale turbulent kinetic energy (k sgs ). Unstructured grids provide better resolution of complex geometries which is the motivation for extending this method. Correlations include theoretical data, experimental data and computational results with RANS turbulence models.

Revolutionary Physics-Based Design Tools for Quiet Helicopters

Abstract : A computational research program was performed at Georgia Institute of Technology, Penn State University, and at Northern Arizona University to develop a set of first-principles based computational modeling tools for analyzing and designing advanced helicopter configurations. The approach involved incorporation of advanced numerical algorithms and turbulence models in OVERFLOW 2, development of advanced comprehensive analyses (DYMORE and RCAS) that are seamlessly coupled to the flow analysis, modeling of rotor noise characteristics using an advanced acoustics prediction tool (PSU-WOPWOP) that is seamlessly coupled to the flow analysis and the comprehensive analyses, and validation and application of the integrated suite of tools for current generation (UH-60, BO105) and next generation configurations. Under the Phase I-B extension, assessment of this suite of tools is being performed by Ga Tech and Penn State for the Boeing MD-900 model rotor (MDART), an actively controlled rotor (SMART), and the Comanche rotor blade (as an option).

Application of Circulation Control to Advanced Subsonic Transport Aircraft, Part II: Transport Application

An experimental/ analytical research program was undertaken to develop advanced versions of circulation control wing (CCW) airfoils and to address specific issues related to the application of these blown high-lift devices to subsonic transport aircraft. The primary goal was to determine the feasibility and potential of these pneumatic configurations to increase high-lift system performance in the terminal area while reducing system complexity and aircraft noise. A four-phase program was completed, including 1) experimental development and evaluation of advanced CCW high-lift configurations; 2) development of effective pneumatic leading-edge devices; 3) computational evaluation of CCW airfoil designs plus high-lift and cruise capabilities; and 4) the investigation of the terminal-area performance of transport aircraft employing these airfoils. The first three phases were presented in Part I of this article. This segment, Part II, describes the fourth phase of the program. Experimental lift coefficient values approaching 8.0 at zero incidence were demonstrated by two-dimension al CCW configurations and were reported in Part I. These were used to predict 70-80% reductions in takeoff and landing distances for a three-dimensional advanced subsonic transport configuration employing a simplified pneumatic high-lift system. These results and the methodology used to obtain them will be presented in greater detail in the following discussions.

Analysis of Rotor-Fuselage Interactions Using Various Rotor Models

Accurate prediction of the rotor and fuselage interaction is essential for the design and analysis of modern rotorcraft. A variety of Navier-Stokes based methodologies have been employed in the past to simulate these effects. The purpose of this study is to examine the merits of some of the simplified techniques of modeling the rotor and their influence on the physics of the overall rotor/fuselage interaction problem. Specifically, a constant actuator disk, varying actuator disk, and blade element actuator disk are considered. The computational results are compared with wind tunnel data obtained on various rotorcraft models. The constant actuator disk is found to be inadequate for most applications, but can be easily improved upon by allowing for pressure variations about the blade radius and azimuth.

Recent Improvements to a Hybrid Method for Rotors in Forward Flight

Ahybrid Navier ‐Stokes/full potential solver has been developed for the efe cient prediction of three-dimensional unsteady viscous e ow phenomena that occur over helicopter rotors in forward e ight. The method combines a Navier‐Stokes analysis near the blade, modeling the viscous e ow and near wake with a potential e ow analysis in the far e eld, modeling inviscid isentropic e ow. A grid motion module has been developed to account for the blade motion and elastic deformations. Free and prescribed wake models have been developed to account for the tip vortex effects once the vortex leaves the viscous e ow region and enters the potential e ow region. Sample results are presented for a two-bladed AH-1G rotor in descent and for a UH-60A rotor in high-speed forward e ight. Comparisons with experiments, e ight test data, and other numerical simulations are given.

Unstructured Overset Mesh Adaptation with Turbulence Modeling for Unsteady Aerodynamic Interactions

Schemes for anisotropic grid adaptation for dynamic overset simulations are presented. These approaches permit adaptation over a periodic time window in a dynamic flowfield so that an accurate evolution of the unsteady wake may be obtained, as demonstrated on an unstructured flow solver. Unlike prior adaptive schemes, this approach permits grid adaptation to occur seamlessly across any number of grids that are overset, excluding only the boundary layer to avoid surface manipulations. A demonstration on a rotor/fuselage-interaction configuration includes correlations with time-averaged and instantaneous fuselage pressures, and wake trajectories. Additionally, the effects of modeling the flow as inviscid and turbulent are reported. The ability of the methodology to improve these predictions is confirmed, including a vortex/fuselage-impingement phenomenon that has before now not been captured by computational simulations. The adapted solutions exhibit dependency based on the choice of the feature to form the...

Extension and Exploration of a Hybrid Turbulence Model on Unstructured Grids

M OST fluid flows of engineering interest are turbulent, and while numerous advances have been made in the numerical solution of the Navier–Stokes equations, known as computational fluid dynamics (CFD), turbulent flows still present challenges for today’s methods. Turbulent flows are characterized by a very wide range of scales in both time and space.Most of the kinetic energy of a turbulent flow is stored in the large-scale structures of the flow. In contrast, kinetic energy is dissipated as heat at the smallest scales. Although the much more computationally intensive large eddy simulations (LES) and direct numerical simulations (DNS) are performed in research environments, simulations that resolve the Reynolds-averaged Navier–Stokes (RANS) equations are still required for rapid engineering results. The size of the smallest length scales, and therefore the maximum allowable CFD grid spacing to completely resolve all turbulence, is inversely proportional to Re. So for a three-dimensional simulation, the number of grid points must scale with Re [1]. Because such fine grids and small time steps are not practical with today’s computers, compromises must be made that approximate certain aspects of the physics. Most CFD methods solve the steady or unsteady RANS (URANS) equations, as illustrated in Fig. 1. Essentially, only the mean flow is solved on the computational mesh, and the turbulent physics are replaced by closure models of varying sophistication. URANS–based methods include simple algebraic models like Baldwin–Lomax, one-equation models like Baldwin–Barth and Spalart–Allmaras, and two-equation models such as k-! and k-! shear stress transport (SST). These turbulence models tend to be heuristic and rely on nonphysical constants that are “tuned” to specific flows, such as airfoils at low angles of attack. That is, the results from the model are iteratively compared against experimental data, and the constants are adjusted until the computational results agree with the experiment. Although they perform well for those cases, they fail to accurately predict unsteady flows dominated by viscous effects, as in the case of static and dynamic stall [2,3] or bluffbody flows. Hybrid RANS/LESmethods provide away to achieve some of the advantages of LES for separated and highly vortical flowfields while retaining the computational efficiency of URANS methods. Baurle et al. [4] developed the idea that URANS and LES methods can be linearly blended by some smooth function to form a hybrid model. This idea of blending was used in 2006 by Sanchez-Rocha et al. [5], who used the k-! SST RANSmodel as the basis for a hybrid method within an existing LES code to resolve wall-bounded turbulence. Sanchez-Rocha et al. demonstrated this capability with simulations of a NACA 0015 airfoil in static and dynamic stall at a Reynolds number of 1 10. A more common approach is to incorporate an LES model into a URANS code to capture subgrid scales, so that where grid resolution permits, LES-like results are obtained. Strictly speaking, LES requires that the resolved scales extend into the inertial subrange. These scales, and those smaller, are more universal, meaning that their physics depends less on the particular geometry. Such universal scales are more amenable to modeling, since even when heuristics are used, they should be valid for a larger range of flows. The drawback of LES is that in order to directly solve for the larger turbulent scales, it requiresmuch finer grids and smaller time steps than URANS computations. In most engineering applications, LES remains far too computationally expensive for routine use.On coarse grids that are usually only suitable forURANS applications, hybrid methods of this variety can capture larger turbulent eddies in the interior of the flow (away from boundaries), thus providing a better approximation of the true physics than URANS alone. Detached eddy simulation (DES) is a commonhybrid formulation, inwhich the turbulence near walls ismodeledwithMenter’s k-!SST [6] or Spalart–Allmaras [7] URANS turbulence models. However, it must be noted that DESmodels are separate and distinct from hybrid methods like those of Sanchez-Rocha et al. [5] in that they lack a dedicated subgrid-scale model. Instead, the URANS equations perform “double-duty” as theLESmodel bymodifying a length scale used in the destruction terms [8,9]. For this effort, the RANS–LES hybrid model developed by Sanchez-Rocha et al. [5,10] has been extended and evaluated within an unstructured CFD methodology. Comparisons with experimental data, as well as published LES and structured CFD simulations, were used to verify and expand the base of knowledge of unstructured hybrid RANS–LES methods. Emphasis is placed on the improvement of the aerodynamic performance quantities (forces and moments) through more accurate prediction of the pressure distribution and separation location on the cylinder.