7

Mechanical Sciences

The panel met on December 20-21, 2018, at the National Academies of Sciences, Engineering, and Medicine (National Academies) facility in Washington, D.C., to review the In-House Laboratory Independent Research (ILIR) program projects in mechanical sciences conducted in 2018 at the U.S. Army Aviation and Missile Research, Development, and Engineering Center (AMRDEC). The panel received an overview presentation on AMRDEC and technical presentations describing the AMRDEC projects. During each presentation, the panel engaged in question-and-answer sessions with the presenter and a general discussion with AMRDEC staff after the panel had formulated initial impressions and developed additional questions during its closed-session deliberations, conducted after the AMRDEC staff had concluded their presentations.

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Project: Can Leading Edge Adverse Pressure Gradient be Quantified to Serve as a Precursor to Compressible Dynamic Stall Onset?

AMRDEC’s internal effort to study the mechanisms behind the vorticity flux when vorticity is shed off a lifting surface was described in this project’s presentation. Prior research effort showed that reducing the leading edge pressure gradient by using a variable drop leading edge preserves lift and mitigates negative pitch, thereby maintaining lifting surface performance.

The study was approached by considering the budget equation for the vorticity flux; however, it was not made clear whether the relative importance of all of the terms in the budget equation had been considered, or whether further benefits could be obtained by exploring the unsteady surface velocity or the Coriolis term. One way to improve the study would be to perform dimensional analysis of the vorticity flux budget equation to assess whether further benefits can be obtained from the unsteady surface

velocity and Coriolis terms. Additionally, companion supplementary numerical simulations could be of great value to this experimental research.

Overall, this project represents a well thought out effort and is well designed. The research addresses a well-defined problem. The scope of the study is feasible and has potential impact, and the approach to the research is methodical and scientific. This project is basic research and is relevant to the interests of AMRDEC.

Project: Effect of Blade Number and Solidity on Rotor Hover Performance

The intent of this project is to better understand the impact of solidity and the number of blades on the performance of relatively high solidity prop rotors. These rotors may have low aspect ratios because of design constraints. The research addresses some basic questions and is, essentially, a study comparing an inviscid lift model with an unstated drag model, with data from rather crude experiments. The presentation shown during the review addressed an elegant fundamental problem of hover rotor performance, the understanding of which may have been lost in time. This is a relatively new and very brief effort that started 6 months ago at a commitment of approximately 25 percent of the researcher’s time. The work appears to be well thought out and of value, especially for relatively low speed cases.

The presented numerical analysis was inspired by blade element theory conclusions about the importance of blade number on performance. The effort uses state-of-the-art tools and a new set of useful test data specifically designed to allow testing at the same rotor solidity while varying the number of blades. The results, which showed good agreement in most cases with test data, clearly showed the impact on figure of merit with the number of blades. The data were properly corrected for aspect ratio (Ar) using a rational wing theory approach. This allowed for a correction in profile drag, which would be very important for small radius rotors. The presenter suggested that additional work evaluating the impact of CD₀ as a function of angle of attack (Alpha) would be valuable and needs to be undertaken for further insight (e.g., a quasi-steady flat plate at varying angles of attack).

The work enhances the understanding of the interplay of blade number and solidity on the rotor hover performance. The major conclusion of the presentation—that induced power is highly dependent on the number of blades—makes sense. This conclusion refutes the classic experiments by Knight and Hefner,¹ which suggest that blade number can be eliminated as an independent parameter.

The researcher needs to make it clear that the number of blades and twist very often depends on nonaerodynamic factors such as dynamics (e.g., rotor-induced vehicle vibration) and acoustics. More work is needed to properly categorize profile drag, especially for low aspect ratio rotors. The presenter’s heuristic conclusion that profile drag is ~ N 1/A _R⁴ needs to be further developed and could prove to be a valuable addition to scientific knowledge.

The work would benefit from an additional literature search and computational fluid dynamics (CFD) to enhance understanding. Additional information as to the source and origin of the test data is highly desired.

Furthermore, classical aerodynamic theory for aspect ratio effects works quite well in unsteady flows, even for small O(2) aspect ratios, so the principal investigator (PI) may need to look for another cause. The method is inviscid (like a panel method) and therefore needs a drag model. No details of the drag model were given, and the PI needs to investigate possibly better drag models (in similar work the drag is approximately proportional to the characteristic inclination angle).

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¹ M. Knight and R.A. Hefner, 1937, Static thrust analysis of the lifting airscrew, NACN TA 626.

While the research exposed some interesting observations, they were not explained well during the review. The presentation slides needed much refinement to allow enhanced understanding, which would be much improved if supplemented by a written document. The researchers reported publications; however, collaborations were not reported.

Project: Flow Field Measurements to Characterize Wing Vortex Interactions

This project aims to study the interaction of a vortex with a wing, where the vortex is generated as a trailing vortex from a vertical wing and interacts with a horizontal wing at some distance downstream. This was one of the few presentations shown during the review for which the objectives were stated clearly and were stated in terms of developing a deeper scientific understanding rather than just being procedural. The research appeared to be soundly based. Although the setup is not new, the use of particle image velocimetry (PIV) (and the possible use of thermal particle image velocimetry [TPIV]) will provide better insight into the general problem of vortex-wing interaction.

The presentation lacked some details, such as the description of the flow control method, which was mentioned but not described. The lack of symmetry in the circulation distributions were dismissed by the researcher as being a consequence of three-dimensionality, but there needed to be more justification, possibly with experimental evidence. The high level of blockage was also of some concern, and there could have been more efforts at reducing this by changing the profile of the trolley, using a false floor, or perhaps using another method.

A data report was mentioned during the review, but it was not referenced. Publications or plans for publication were not given. One collaborator was mentioned as working on flow control, but the flow control part of the project was not reported in this presentation.

Attempts to determine spanwise and streamwise velocity would be beneficial. In some cases, a three-dimensional (3D) picture will be needed to explain such interactions. In the future, studies with the generation of more than one vortex can be justified. This can be expected to occur with rotary-wing applications.

Additional questions include the following: How will the quantification work? How was the flow control applied? Why did it not work? Why was it useful to measure the vortex near the surface of the wing?

Project: Investigation of Corotating and Counterrotating Coaxial Rotor Performance

The presentation outlined a short numerical study (using the Rotorcraft Comprehensive Analysis System [RCAS]) on the rotor hover performance for counterrotating and corotating coaxial rotors. The work used a physics-based model in comprehensive analysis tools to help separate the rotor wake and relative interference affects. The study has systematically investigated the problem by taking the rotors to the limit of separation (a single rotor with twice the number of blades or an isolated rotor with half the number). The idea to separate out induced and profile power makes sense, and the numerical approach allows one to separate the induced power into self-induced and interference components, which greatly helped the understanding of the phenomenon. This contribution alone was interesting and enlightening. Additionally, the analysis was able to identify the importance (or relative lack) of the swirl effect when compared to the index angle and rotor separation, for both directions of rotation.

The presentation builds on a new set of rotor test data, developed by the University of Texas at Austin, which was utilized for comparison with the RCAS-based analytical predictions. The approach enhanced understanding by elegantly separating the interference-induced velocity from self-induced velocity. This approach yielded a major and surprising finding—that the interference of the upper rotor

on the lower rotor is much less than intuitively thought. It is also clear that the upper rotor does better overall. Additionally, the work shows a much larger thrust at smaller blade index angles. The presentation shown during the review indicates that the work was done at one twist distribution (actually, no twist or untwisted blades) and at one axial separation between rotors.

The overall results make sense when compared with simple momentum theory. Specifically, the induced power factor Kint for interference developed from momentum theory is given by Leishman² as Kint= (Pi) coax/(2Pi) isolated=1.281 when the two rotors are at identical thrust. This ratio compares reasonably well with the overall interference factor. Leishman states that value becomes 1.219 when the rotors are at constant torque.

This project is applicable to the Army future vertical lift (FVL) (capability sets 1-3) program and will be of significant value during the Army’s efforts to renew its current aging helicopter fleet. More specifically, the current Future Attack and Reconnaissance Aircraft (FARA) and Joint Multirole (JMR) demonstration programs are expected to utilize some form of coaxial rotors and would benefit from this work. A better understanding of the performance for these rotors is a relevant and important effort. The results need to be compared to the experimental data of Harrington³ and Dingledein.⁴ Additionally, this effort needs to be expanded for different ΔZ/R, an extremely important design parameter, as well as different twist distributions (including those that are nonlinear). Continued collaboration with the University of Texas at Austin experimental team seems appropriate. Additionally, the work could be further expanded to include the forward flight impact on performance and, if possible, vibratory hub moment.

Project: Time Resolved Boundary Layer State and Surface Streamline Measurements of Rotating Aerodynamic Surfaces via Infrared Thermography

The aim of the project is to visualize laminar-to-turbulent transitions on a rotor blade both in hover and in forward flight. This is a high-caliber experiment that makes difficult measurements in a unique, large-scale facility. It is a good example of high-quality basic research carried out in a government facility that is not generally available to other researchers conducting similar basic research in the universities.

The method is long-wave infrared (IR) photography. The particular contribution of this project is to make time-resolved measurements using fast cameras, which are now available (2 kHz). The results were reported for Reynolds numbers 0.2 to 1.6 × 10⁶.

The work was well planned and well executed. This is one of the more impressive projects in the overall program. The PI presented high-quality visualizations, and he has shared information with people who model transition. In this respect, it would be good to share this information with the broader community beyond in-house or near-in-house groups.

Additionally, there is a possibility of an opportunity missed in that the PI could explore more in-depth studies of transition in unsteady flows, or possibly investigate tripping methods for such flows, which have the potential to make new and fundamental contributions to the understanding of transition. There also appears to be a good synergy with the project on development of a hub-based camera system to observe and measure rotorcraft blade deformation project, and this could be highlighted.

This is a good example of how basic research can be performed in the AMRDEC environment. That is, it focuses on the basic aspects of a problem that is important to the Army, it takes advantage of a unique

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² J.G. Leishman, 2000, Principles of Helicopter Aerodynamics, New York: Cambridge University Press, p. 103.

³ R.D. Harrington, 1951, Full-scale-tunnel investigation of the static-thrust performance of a coaxial helicopter rotor, NACA TN 2318, March.

⁴ R.C. Dingeldein, 1954, Wind-tunnel studies of the performance of multi-rotor configurations, NACA TN 3236, August.

facility, and it provides a powerful technique that illuminates a fundamental physical phenomenon. More collaborations and further interactions with universities and other centers would be very beneficial.

Project: Development of a Hub-Based Camera System to Observe and Measure Rotorcraft Blade Deformation

Motivated by the need to obtain independent high-resolution blade deformation data to address persistent discrepancies between flight test data and aeromechanics computations, this project aims to develop a hub-based camera system to measure blade deformation through the use of off-the-shelf cameras and algorithms for image processing. The project takes advantage of off-the-shelf components and algorithms for the image processing in the rotating system. The study builds on the work of Boden and Stasicki⁵ on an image pattern correlation technique (IPCT), which was applied to propellers. The main differences between this study and that of Boden and Stasicki is the more challenging environment experienced within this study by rotor blades that are undergoing large deformations.

A detailed technical approach has been developed with selected stereo cameras in a self-contained, fully wireless packaging, which is to be installed in an existing Army rotor system’s hardware (e.g., pressure sensitivity painting [PSP] rotor blades). This will be first tested at the National Aeronautics and Space Administration’s (NASA’s) Langley Research Center (LaRC) Rotor Test Cell before going to the 14’ × 22’ subsonic tunnel. The next steps would involve the detailed design and fabrication of the camera system mounting hardware and benchtop testing. The fact that this study uses a generic rotor system in collaboration with NASA LaRC will allow the AMRDEC researchers to publish and share their studies with the rotorcraft community at large.

The success of this project will enable a deeper understanding of blade deformation, particularly its twist distribution, as well as transient loading for hover and forward flight conditions. Since strain gauges can only measure blade deformations at specific locations, optical techniques can give full span results with some reasonable (1/4”) resolution. Therefore, this study opens up opportunities for further studies in the appropriateness of beam-based structural modeling of blades in aeromechanics codes and the assessment of the need for three-dimensional (3D) finite element method (FEM) modeling. Furthermore, it can enable the recovery of blade sectional properties under known loading conditions that will further support the improvement of blade cross-sectional analysis codes.

This project supports the needs of AMRDEC and can have significant impact on the understanding of the sources of discrepancies between aeromechanic analysis and flight test data through directly measuring blade deformation in time. In the past, the counterpart of these measurements (i.e., aerodynamics measurements) has been conducted for unsteady aerodynamic loads. Most of the fundamental challenges the project faces from a measurement technique standpoint have already been addressed in the literature.

The project will address several issues related to the implementation of such systems that in practice still need to be solved. Among them, the project will evaluate the availability of image processing algorithms and camera technology for time-resolved measurements (1 degree of azimuth resolution) in the rotating system.

To strengthen the fundamental nature of the work and increase its chance of success, clear scientific goals need to be defined and stated in the description of the project—for example, a goal to explore the actual blade twist in forward flight (a topic of continuous controversy) and understand how twist develops at different azimuth angles and advance ratios. Additionally, the researchers could explore

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⁵ DRL Aerospace Center, The World Seen From a Propeller Hub, https://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10081/151_read-10753/#/gallery/15372, accessed January 2019.

the connections of the planned test for this system to the boundary layer measurement project and the use of the infrared cameras for position measurements. Furthermore, the measuring techniques being developed under this project could be of immediate value for future vertical lift (FVL) participants, and it needs to be made available to them soon.

The PI is leaving for a new position, and it is unclear who will be taking the lead on this study. It will be necessary to appoint a new lead to make sure this project comes to a successful conclusion according to its original plan.

Project: Development of Tomographic Particle Image Velocimetry (PIV) Technique for Rotor Applications

The project aims to develop a tomographic particle image velocimetry (PIV) system in a large volume (1’ × 1’ × 1’). The aim is to develop a system of parameters that allow the TPIV technique to be applied confidently to investigate vortical structures in three-dimensional flows. However, it was unclear what candidate flows were of particular interest. The research program was not well articulated during the review (the objectives of the program took up 75 percent of the presentation), and most of the research activities were confined to the development of the system. The descriptions on this latter part were vague, and there could have been more discussion about the limitations of the system. For example, is a sampling rate of 1 Hz sufficient? Is a volume of 1’ × 1’ × 1’ enough to see anything of interest? For instance, in the blade vortex example, this volume would not be enough to capture more than one vortex, so vortex interactions given as a motivation could not be studied. The solutions offered to possible problems (e.g., more laser power, faster cameras and more of them, and new processing power) will all be very expensive. Future plans are very ambitious (four new fast cameras and a new high-powered high-speed laser), and may be beyond the resources made available for this project.

Some research interactions were mentioned, but they appeared superficial. The need for better analysis capabilities is evident, but there are undoubtedly ways that this can be solved within the Army network. There were no publications reported; the project was started in FY17, and it seems to have already consumed 2.56 staff years. All in all, this was a rather disappointing presentation, heavy on justification and light on detailed plans and implementation.

Project: Strand Mesh Generation Using Minimum Distance Fields

CFD is an indispensable tool used by engineers to design and analyze flow and heat transfer in and about aerospace vehicles. Virtually all CFD codes require a computational mesh for discretization of the governing partial differential equations. High-quality mesh generation can be time consuming, and the solution accuracy depends strongly on the quality of the mesh. There are several commercial vendors that offer standalone mesh generation software or as part of a general CFD package.

AMRDEC’s internal effort for development of strand mesh generation software was presented. The strands are lines that are orthogonal to the wall surface and enable one to compute normal derivatives accurately (e.g., for wall integration or wall models). They are easy to build because they do not strictly require the neighboring strands to be at a specific distance.

Interpolation is used to connect the strands to an underlying Cartesian mesh and to other strands to compute derivatives parallel to the walls. Conceptually, it is similar to chimera and overlapping grids. The main advantage is that it is easy to automate the generation of the strand, and it also requires less memory than unstructured solvers. The main disadvantage is the introduction of further interpolation errors and potential loss of physical conservation laws.

Overall, the grid generation methodology needs to be integrated with the discrete operators employed in the CFD code. For example, the choice of numerical discretization operators at the interface of the meshes can have a significant effect on accuracy or on the introduction of artificial flow disturbances. For high-fidelity simulations, the grid quality is paramount. Although the motivation for this mesh generation activity is to provide an automated black-box type mesh generator for the CFD user, caution needs to be exercised with regard to the sensitivity of the solution to grid parameters, and mesh convergence studies need to always be required before accepting the numerical solution. Mesh quality optimization is on the research team’s agenda for 2019; however, the approach and metrics used for mesh quality and robustness were not discussed during the review.

General agreement was demonstrated with solutions obtained with other mesh generation techniques (e.g., unstructured/Cartesian) in use in the Helios code and other CFD codes. However, no clear advantage (e.g., solution accuracy and cost) of the strand mesh technique was demonstrated in comparison to, for example, flight measurements. It would also have been instructive to show quantitative results from NASA’s high-lift workshop and comparison with other codes, including fully unstructured codes. Although it is important to reduce the burden of mesh generation on the design engineer, some control of the mesh parameters and a metric for mesh quality needs to be provided to the CFD user.

Additionally, the researchers need to examine the sensitivity of flow solution convergence, accuracy, and cost-to-grid parameters using a suite of canonical problems and by comparisons to experimental data. They also need to demonstrate the advantages of the strand mesh generation technology compared to other mesh generation software, including those offered by commercial vendors.

Project: Development of Three-Dimensional Fluidic Oscillators for Active Flow Control

This project intends to develop and test 3D fluidic oscillators; the two-dimensional versions have already been in use for a long time. This research makes two design contributions: the coupling of sweeping jet oscillators to produce in-phase and out-of-phase oscillations and the implementation of fluidic oscillators in three dimensions. Both aspects appear to be novel, or at least near the forefront of research in this area.

The project’s objective was to explore the possibility of producing longitudinal vortex structures. It was not made clear during the review how longitudinal vortex structures would enable flow control except for delaying separation, and the example problem was to produce a rotating jet. It was also not made clear how the oscillators could be activated with a variable phase, since that would appear to need some addressable valves. It was unclear how the boundary layer interaction study would be conducted and what results might be expected.

Despite this lack of clarification, this project is pursuing an innovative line of research with a possibly high level of payoff. The research addresses a well-defined problem, and the experimental program is well designed. The PI is studying the usage, design, and fabrication of synchronized unsteady jets in boundary layer flow to generate counterrotating streamwise vortices, which are highly effective to control the flow when introduced via passive control devices. The PI has a good collaboration with the U.S. Military Academy through its summer program and capstone design projects, which has led in the past to student-authored papers. These publications include presentations at Scitech 2018 and Aviation 2018.

This study is relevant to the Army’s needs and has high potential to control and reduce rotorcraft adverse force. The PI has a clear path to the fabrication of the nozzle and actuators, as well as to studying the necessary criteria for synchronization and nozzle flow rates. The scope of this study is feasible, and its potential impact is significant. The approach to this study is also methodical and scientific. This project is basic research and is relevant within the interest of AMRDEC.

Clarification is needed regarding the following questions: What would the orientation of the jets be? What are the researchers hoping to produce? What kind of interaction are the researchers hoping to show? How do these actuators compare to other forms of control in terms of C_\mu⁶ and the momentum ratio for the jet? Additionally, the researchers could examine the effect of momentum ratio for the jet and quantify the amount of power that is necessary for an optimal outcome.

Project: Investigation of the Relaxation of Residual Stresses in Rotorcraft Gears

Shot peening is known to increase the surface compressive residual stress of a metallic material, improving its fatigue life and crack resistance and impacting the design allowable. On the other hand, rotorcraft gear design does not take credit for the shot peening benefits in the gears due to the potential for compressive residual stress relaxation during normal operating bending fatigue. Since transmissions account for a significant weight fraction of a rotorcraft, being able to account for the shot peening benefits in the definition of gear design allowables can have a considerable impact on vehicle performance.

This project aims to investigate the effects of residual compressive stress in gear fatigue life. It has set forward the hypothesis that the compressive residual stresses from shot peening will not relax after cyclic bending fatigue at normal operating conditions (defined in terms of temperature range, load cycles, and other factors). For that, a series of tests were defined for a flat coupon and for gears, all using X53 material. These tests were set to obtain data on residual stress relaxation during normal operating loads. Of the results obtained, the main conclusion is that the level of compressive stresses added by the shot peening process after 10 load cycles remained in the part and, therefore, did not relax up to 1 million cycles. The initial relaxation was much more pronounced in the gears, by 35-45 percent drop, than in the flat coupons. This was attributed to the difficulties in shot peening close to the gear root. Nothing beyond these results seems conclusive based on the data presented.

This study has been ongoing for approximately 1.5 years, and it involves multiple partners (i.e., NASA Glenn, DANTE Solutions, and the Army’s Aviation Engineering Directorate) in what seems to be a healthy collaboration. The team plans to update current certification standards for rotorcraft gears, which would be a very useful outcome of this effort. However, if this is to be used to alter the design allowable, the variability of the current processes needs to be carefully quantified and understood. The residual stress results obtained so far show such a large variability among coupons and gears that it is not clear that any effect can be assigned to the shot peening process in production parts. A deeper understanding of the sources of the variability and ways to mitigate it, along with the generalization of such conclusions for material systems other than the X53, would pose an important and well-defined fundamental project to be undertaken.

This is certainly an important topic of study that can have a very positive impact in rotorcraft performance. However, as the project is currently defined, it is more a test program than a fundamental research one. As such, it would be more appropriate to consider this an applied research project.

AMRDEC Crosscutting Findings

In addressing the goals of their efforts, the PIs within AMRDEC often take the first step but not the logical next step. For example, in the application of thermography in the project Time Resolved Boundary Layer State and Surface Streamline Measurements of Rotating Aerodynamic Surfaces via Infrared

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⁶ C_\mu describes a momentum coefficient, which is a measure of the actuation force compared to the dynamic force exerted by the flow.

Thermography, the PI makes an excellent contribution to the measurement of transition but then does not take the next step to further the research effort—that is, use this information to help improve the understanding and modeling of transition. There needs to be more of a sense of curiosity to move such basic research projects forward.

Not all of these projects are basic research. The aims of the projects were often in line with basic research, but sometimes the focus of the research was not. The projects Development of Tomographic PIV Technique for Rotor Applications and Investigation of the Relaxation of Residual Stresses in Rotorcraft Gears fell into this category.

The impact of the research was not often explicitly stated during the review. For example, the following questions were not often explicitly addressed: How have the PIs communicated their findings? Have they influenced any other research group or any basic research effort? They also need to describe more specifically their publications, honors received, invitations received to lecture, hosting of students, collaborators, and their engagement in society activities.

It was often not made clear how connected the PIs were to other researchers in their field. More evidence of such collaborations needs to be documented, and if such collaborations were marginal or nonexistent, the PIs need to be encouraged to seek them out, especially within other parts of Army basic research and with other laboratories and universities. Over the years, interactions with the U.S. universities have largely been limited to two universities with rotorcraft centers supported by the Army. Collaborations with other universities that conduct world-class research in several areas of interest to AMRDEC need to be considered. Such collaborations would also have the indirect effect of enhancing the pool of bright and diverse talent for subsequent employment in the Army laboratories. In some cases, NASA collaboration was implied during the review, but it was not explicitly described.

Recommendation: To enhance its projects AMRDEC should pursue greater collaboration with other RDECs and Army laboratories and universities.

Recommendation: To advance greater scientific understandings that will enhance its projects, AMRDEC should promote interactions of its researchers with the broader scientific community.