Russell M. Cummings

Assessment of Sting Effect on X-31 Aircraft Model Using CFD

Adam Jirásek et al. — Wed, 09 Jun 2010 16:13:28 PDT

The article describes the computational evaluation of the effect a belly mounted sting on the aerodynamics characteristics of an X-31 wind tunnel model. The investigation includes results of steady and time-dependent CFD analysis of the X-31 model with and without sting. The values of the steady lift and pitching moment coefficients as well as the pitching moment of model undergoing a manuever are compared to each other and to the wind tunnel data. The results of this analysis show the detrimental effect of sting on stability of the vortical structure above the wing as well as a substantial increase of noise in pitching moment data.

Lessons Learned from the Numerical Investigations on the VFE-2 Configuration

Willy Fritz et al. — Wed, 09 Jun 2010 16:13:28 PDT

The Second International Vortex Flow Experiment provided a variety of experimental data for a 65° delta wing with sharp and blunt/rounded leading edges. Flow measurements including forces and moments, surface pressures, Pressure Sensitive Paint measurements, and off-surface flow variables from Particle Image Velocimetry were made available for comparisons with computational simulations. A number of test cases were chosen for simulation by seven numerical groups, and a summary of their results is presented here. The ability of computational fluid dynamics to predict such flow features as the dual primary vortex system found on the blunt leading edge configuration and a shock/vortex interaction for the sharp leading edge are assessed. While computational simulation has made great strides in recent years, there are still areas where further improvement can be made, including in turbulence modeling, transition modeling, and the ability to accurately compute unsteady flows.

Improved Methodologies for Maneuver Design of Aircraft Stability and Control Simulations

Adam Jirásek et al. — Wed, 09 Jun 2010 16:13:27 PDT

With many modern fighter aircraft experiencing unpredicted flight dynamics during flight tests, recent research has focused on developing methodologies for incorporating computational fluid dynamics into the aircraft development process. The goal of this approach is to identify configurations susceptible to stability and control issues early in the design process. Previous research has primarily focused on full aircraft configurations, however, to increase the rate of development the current study focused on a two-dimensional NACA0012 airfoil. The two-dimensional NACA0012 airfoil has the advantage of reducing the computational cost by orders of magnitude compared to full scale aircraft simulations, while still providing complicated aerodynamics at high angles of attack. Computationally predicted lift coefficients from a number of newly developed training maneuvers were used to generate reduced order aerodynamic loads models. For evaluation, these models were compared to generated static and dynamic validation data. Methods of improving both the computational training maneuver and the reduced order modeling approach are suggested.

Shock Effects on Delta Wing Vortex Breakdown

Lucy A. Schiavetta et al. — Wed, 09 Jun 2010 16:13:26 PDT

It has been observed that delta wings placed in a transonic freestream can experience a sudden movement of the vortex breakdown location as the angle of incidence is increased. The chapter reports on the use Computational Fluid Dynamics (CFD) to examine this behaviour in detail. The study shows that a shock-vortex interaction is responsible. The balance of the vortex strength and axial flow, and the shock strength, is examined to provide an explanation of the sensitivity of the breakdown location. Limited experimental data is available to supplement the CFD results in certain key respects, and the ideal synergy between CFD and experiments for this problem is considered.

Computational Investigation Of Slot Blowing For Fuselage Forebody Flow Control

Scott M. Murman et al. — Wed, 09 Jun 2010 16:13:26 PDT

This paper presents a computational investigation of a tangential slot blowing concept for generating lateral control forces on an aircraft fuselage forebody. The effects of varying both the jet width and jet exit velocity for a fixed location slot are analyzed. This work is aimed at aiding researchers designing future experimental and computational models of tangential slot blowing. The primary influence on the resulting side force of the forebody is seen to be the jet mass flow rate. This influence is insensitive to different combination of slot widths and jet velocities over the range of variables considered. Both an actuator plane and an overset grid technique are used to model the tangential slot. Tile overset method successfully resolves the details of the actual slot geometry, extending the generality of the numerical method. The actuator concept predicts side forces similar to those produced by resolving the actual slot geometry.

Numerical Solutions for the CAWAPI Configuration on Unstructured Grids at USAFA, United States

Scott A. Morton et al. — Wed, 09 Jun 2010 16:13:25 PDT

Numerical Solutions for the VFE-2 Configuration on Unstructured Grids at USAFA, United States

Russell M. Cummings et al. — Wed, 09 Jun 2010 16:13:24 PDT

The numerical simulation of the flow for the VFE-2 configuration with rounded leading edges is presented. For the numerical simulation the Cobalt Code was used, which uses a cell-centered unstructured hybrid mesh approach. Several numerical results are presented for the steady RANS equations as well as for DES and DDES hybrid approaches. Within this paper the focus is related to the dual primary vortex flow topology, especially the sensitivity of the flow to angle of attack and Reynolds number effects. Reasonable results are obtained with both steady RANS and SA-DDES simulations. The results are compared and verified by experimental data, including surface pressure and pressure sensitive paint results. The impact of transition on the resulting flow field is also assessed, and recommendations for improving future simulations are made

Airplane Design and the Biomechanics of Flight – A More Completely Multi- Disciplinary Perspective

John H. McMasters et al. — Wed, 09 Jun 2010 16:03:04 PDT

Aeronautics is usually presumed to have started as a formal engineering discipline somewhere in historical time between the mythological experiments of Daedalus and his ill-fated son, Icarus; and the dreams and schemes of Leonardo da Vinci during the Italian Renaissance. As reviewed in this paper, “aeronautics” has a far longer history, extending over a period of about 300 million years beginning with the evolution of the ability of insects to fly. With the advent of the success of the Wright brothers, technologists quickly turned their attention from the inspirations and lessons provided by natural models of flying machines to a more practical quest for increasingly dramatic improvements in speed, range and altitude performance far beyond the limits of what muscles and flapping wings could provide. Based on recent work done by the first author in support of the NASA/DARPA Morphing Aircraft Structures Program, a purpose of this paper is to demonstrate in broader terms some of the numerous, very rich sources of inspiration such multi-disciplinary explorations continue to offer both the engineering practitioner and educator.

Experimental Verification of the Aerodynamics of Stream Thrust Probes

Robert S. Hiers III et al. — Wed, 09 Jun 2010 16:03:03 PDT

Determining the local stream thrust (a vector quantity) from a measured pitot pressure (a scalar quantity) requires either knowledge of the flow direction or a probe shape that compensates for flow direction. This compensation ideally would make the measured pressure directly proportional to the component of momentum along the probe axis. The flow angle sensitivity required to resolve this component of momentum was previously determined theoretically. A proposed probe nose shape was analyzed using computational fluid dynamics (CFD) and was found to produce a flow angle sensitivity close to the required sensitivity. In the current work, the proposed nose shape was tested in a wind tunnel at Mach numbers 1.67, 2.45, and 3.48 at angles of attack from 0 to 15 deg. The test results indicate that the flow angle sensitivity of the proposed nose shape agrees with the required sensitivity to within 1 percent up to a flow angle of 15 deg.

Detached-Eddy Simulation of Slat and Flap Aerodynamics for a High-Lift Wing

Russell M. Cummings et al. — Wed, 09 Jun 2010 16:03:03 PDT

Three-dimensional multi-element wings are simulated to investigate slat and flap aerodynamics using Detached-Eddy Simulation. The computations are performed by solving the Navier-Stokes equations on unstructured grids. All of the computed cases include the main wing with a half-span flap deflected to 39 degrees and a three-quarter-span slat deflected to 6 degrees. Computations of the model, which simulates a landing configuration at 10 degrees angle of attack and a chord-based Reynolds number of 3.7 million, are validated with surface pressure measurements acquired at the NASA Ames 7- by 10-Foot Wind Tunnel. The results increase the computational knowledge of how to accurately model the flow physics of a multi-element wing with three-dimensional flow by using Detached-Eddy Simulation.

Application of Volterra Functions to X-31 Aircraft Model Motion

United States Air Force Academy et al. — Wed, 09 Jun 2010 16:03:02 PDT

Recent advances towards an efficient computational method for accurately determining the stability and control characteristics of an aircraft are discussed and critiqued. The present approach with greatest promise is to reduce the number of high-fidelity CFD simulations by using Volterra functions Reduced Order Modeling. This type of reduced order model is a predictive model which has a unique training maneuver -a unit impulse. The advantage of such an approach is the fast prediction of the aerodynamic characteristics of an aircraft. This article presents the results of application of the Volterra functions ROM for prediction of linear movement of a 2D airfoil and of an X-31 aircraft model. The Volterra ROM predicted well normal and axial force which are linear or weakly non-linear and was in a fairly good agreement with pitching moment as long as the pitching moment predictions were weakly non-linear.

The Influence of Viscosity and Surface Curvature on the Pressure Distribution of a Stream Thrust Probe

Renee Pasman et al. — Wed, 09 Jun 2010 16:03:02 PDT

Determining the local stream thrust (a vector quantity) from a measured pitot pressure (a scalar quantity) requires either knowledge of the flow direction, or a probe shape that compensates for flow direction. This compensation would ideally make the measured pressure directly proportional to the component of momentum along the probe axis. The flow angle sensitivity required to resolve this component of momentum was determined theoretically previously. A proposed probe nose shape was analyzed using CFD and found to produce flow angle sensitivity close to the required sensitivity. The proposed nose shape was also tested in a wind tunnel at Mach 1.67, 2.45, and 3.48 at angles of attack from 0 to 15 degrees. The test results indicate that the flow angle sensitivity of the proposed nose shape agrees with the required sensitivity to within 1% up to a flow angle of 15°. The current work extends the original theoretical development for the optimum nose shape to include viscous affects and surface curvature. The new second-order theory agrees well with experimental results for both the stream thrust probe as well as other, independent data. Further work can be done to refine the theory.

Detached-Eddy Simulation of the Vortical Flowfield about the VFE-2 DeltaWing

Russell M. Cummings et al. — Wed, 09 Jun 2010 16:03:01 PDT

The numerical simulation of the flow around a 65° delta wing configuration with rounded leading edges is presented. For the numerical simulation the Cobalt Code uses a cell-centered unstructured hybrid mesh approach. Several numerical results are presented for the steady RANS equations as well as for DES and DDES hybrid approaches. The simulations are done as part of the NATO RTO/AVT 113 working group focusing on experimental and numerical research on delta wing configurations with rounded leading edges. Within this paper the focus is related to the dual primary vortex flow topology, especially the sensitivity of the flow to angle of attack and Reynolds number effects. Reasonable results are obtained with both steady RANS and SA-DDES simulations. The results are compared and verified by experimental data, including surface pressure and pressure sensitive paint results. The impact of transition is assessed, and recommendations for improving future simulations are made.

Some Systemic Issues in the Development of the Aerospace Industry Technical Workforce of the Future

John H. McMasters et al. — Wed, 09 Jun 2010 16:03:01 PDT

This paper is a continuation of the authors’ previous examinations of a suite of issues surrounding the putative decline in aeronautics in this country. The purpose of this paper is to discuss three specific issues believed to be of particular importance to the future of our industry. The first is the question of how many engineers we may need in our future as we confront the problem of an aging workforce and the globalization of our industry. The second is the question of what skills and abilities these engineers will need to possess as the overall industry continues to evolve. Finally, the need for more systems-oriented, multidisciplinary-skilled talent is addressed. A basic message of the paper carried on from earlier writings is that while aeronautics may indeed be a “maturing industry” (at least in some major traditional product areas), there is much that we can and should do to create a vision of our future as vivid as that which has driven our past as a means to attract and develop the talent needed to assure the future of our enterprise. Without this talent, few of the major technological advances that can be currently foreseen can come to fruition.

Experiences in Accurately Predicting Time-Dependent Flows

Russell M. Cummings et al. — Wed, 09 Jun 2010 16:03:00 PDT

As computational fluid dynamics matures, researchers attempt to perform numerical simulations on increasingly complex aerodynamic flows. One type of flow that has become feasible to simulate is massively separated flow fields, which exhibit high levels of flow unsteadiness. While traditional computational fluid dynamic approaches may be able to simulate these flows, it is not obvious what restrictions should be followed in order to insure that the numerical simulations are accurate and trustworthy. Our research group has considerable experience in computing massively separated flow fields about various aircraft configurations, which has led us to examine the factors necessary for making high-quality time-dependent flow computations. The factors we have identified include: grid density and local refinement, the numerical approach, performing a time-step study, the use of sub-iterations for temporal accuracy, the appropriate use of temporal damping, and the use of appropriate turbulence models. We have a variety of cases from which to draw results, including delta wings and the F-18C, F-16C, and F-16XL aircraft. Results show that while it is possible to obtain accurate unsteady aerodynamic computations, there is a high computational cost associated with performing the calculations. Rules of thumb and possible shortcuts for accurate prediction of massively separated flows are also discussed.

Deformation of Unstructured Viscous Grids

Denis B. Kholodar et al. — Wed, 09 Jun 2010 16:03:00 PDT

A mesh deformation algorithm for unstructured grids is presented. It is designed for high Reynolds number flow problems. Such grids are employed in aerodynamic and aeroelastic studies of wings or complete aircraft configurations in flows where the viscous effects are important. Given a surface deformation, the method efficiently recalculates new locations of high aspect ratio cells that make up the viscous layers of the grid and then deforms the inviscid part of the grid using an established method based on a torsional spring analogy technique. Results are presented for monitoring the deterioration of the quality of the grid during subsequent deformation steps for aeroelastic studies as well as to ensure the time efficiency of the method. Results for grid deformation of a 1.4 million cell AGARD 445.6 wing grid designed for flow at high Reynolds numbers due to typical deformations are also presented. Finally, a discussion of the parallelization performance and comparison of the running time of the mesh deformation algorithm to that used by the flow solver is made.

F- 16XL Unsteady Simulations for the CAWAPI Facet of RTO Task Group AVT- 113

Scott A. Morton et al. — Wed, 09 Jun 2010 16:02:59 PDT

This work represents the USAF Academy portion of a culmination of three years of cooperative research in the Cranked Arrow Wing Aerodynamics International (CAWAPI) RTO Task Group, AVT-113. The objective of the group was to compute high resolution CFD simulations of a subset of the conditions created in the CAWAP flight test program managed by NASA Langley researchers and others. Seven flight conditions were chosen with four of them at symmetric conditions of medium to high angle of attack and subsonic Mach numbers, one symmetric condition at a transonic low angle of attack condition, and two conditions at medium angle of attack and subsonic Mach number but with positive and negative sideslips. The emphasis of the USAF Academy team was to explore unsteady effects and the ability of current methods to predict them. Very good agreement with flight test was found in almost all cases and the unsteadiness was documented with flowfield visualization and unsteady surface pressure coefficient data.

DES Turbulence Modeling on the C-130 Comparison between Computational and Experimental Results

Malcom P. Claus et al. — Wed, 09 Jun 2010 16:02:58 PDT

This paper represents the results from the initial phase of a research program to determine the flow characteristics of the C-130 Hercules transport aircraft. The initial phase of the program consists of evaluation and comparison of the flow-field obtained from flow visualization methods. Specifically CFD (Computational Fluid Dynamics) results are compared with experimental Hot Wire results produced by wind tunnel tests on the C-130 in clean configuration. This paper outlines the results to date and provides a description of further work. The CFD element of this research features the use of Detached Eddy Simulation (DES) in order to extend its use as a reliable method for use on complex flow-fields. DES combines the efficiency of a Reynolds-averaged turbulence model near the wall with the fidelity of LES (Large Eddy Simulation) in separated regions. Because of the LES treatment in separated regions, it provides more accurate descriptions of the geometry-dependant, three-dimensional unsteady motions resulting in regions of massive separation. The computational aspect of the research is performed at the US Air Force Academy, with subsequent wind tunnel tests (Hot Wire) being undertaken in France at ENSICA.

A Multifaceted Approach to the AIAA Foundation Undergraduate Team Aircraft Design Competition

Russell M. Cummings et al. — Wed, 09 Jun 2010 16:02:58 PDT

A multifaceted, novel approach was used to help students create entries for the AIAA Foundation Undergraduate Team Aircraft Design Competition. Each entry involved the design, analysis, construction, and testing of the aircraft. Three groups were involved for each airplane: the first group was responsible for the airframe, the second group was responsible for the engine, and the third group was responsible for the construction and flight testing of a radio-controlled flying model. There was an overall Chief Executive Officer who insured that engine-airframe integration issues were addressed. Students from a variety of majors, both technical and non-technical, participated in various aspects of the project. This approach is seen as one method to give students a multidisciplinary approach to design and problem solving.

Multidisciplinary Applications of Detached-Eddy Simulation to Separated Flows at High Reynolds Numbers

Scott A. Morton et al. — Wed, 09 Jun 2010 16:02:57 PDT

We focus on multidisciplinary applications of detached-eddy simulation (DES), principally flight mechanics and aeroelasticity. Specifically, the lateral instability (known as abrupt wing stall) of the preproduction F/A-18E is reproduced using DES, including the unsteady shock motion. The presence of low frequency pressure oscillations due to shock motion in the current simulations and the experiments motivated a full aircraft calculation, which showed low frequency high-magnitude rolling moments that could be a significant contributor to the abrupt wing stall phenomenon. DES is also applied to the F-18 high angle of attack research vehicle (HARV) at a moderate angle of attack to reproduce the vortex breakdown leading to vertical stabilizer buffet. Unsteady tail loads are compared to flight test data. This work lays the foundation for future deforming grid calculations to reproduce the aero-elastic tail buffet seen in flight test. Solution based grid adaption is used on unstructured grids in both cases to improve the resolution in the separated region.

Previous DoD Challenge work has demonstrated the unique ability of the DES turbulence treatment to accurately and efficiently predict flows with massive separation at flight Reynolds numbers. DES calculations have been performed using the Cobalt code and on unstructured grids, an approach that can deal with complete configurations with very few compromises. A broad range of flows has been examined in previous Challenge work, including aircraft forebodies, airfoil sections, a missile afterbody, vortex breakdown on a delta wing, and the F-16 and F-15E at high angles-of-attack. All DES predictions exhibited a moderate to significant improvement over results obtained using traditional Reynolds-averaged models and often excellent agreement with experimental/flight-test data. DES combines the efficiency of a Reynolds-averaged turbulence model near the wall with the fidelity of Large-Eddy Simulation (LES) in separated regions. Since it uses Large-Eddy Simulation in the separated regions, it is capable of predicting the unsteady motions associated with separated flows. The development and demonstration of improved methods for the prediction of flight mechanics and aeroelasticity in this Challenge is expected to reduce the acquisition cost of future military aircraft.