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NASA NTRS1994

Force override rate control for robotic manipulators

The work reported deals with the problem of operating a robot manipulator under a rate control mode while the end effector is not in contact with the external environment, and then switching to a force control mode when contact is made. The paper details how the modal changeover may be accomplished in a manner transparent to the operator, and will allow operator applied forces to be reflected at the robot end effector. A one degree of freedom demonstration system is used to illustrate the concept, which is then applied to a PUMA manipulator. Sample code for the implementation of the control is provided, experimental results show that the optimum setting for the gain is a function of the compliance of the end effector, and the compliance of the external constraint.

Driels, Morris↗

NASA NTRS2014

Section 508 Electronic Information Accessibility Requirements for Software Development

Section 508 Subpart B 1194.21 outlines requirements for operating system and software development in order to create a product that is accessible to users with various disabilities. This portion of Section 508 contains a variety of standards to enable those using assistive technology and with visual, hearing, cognitive and motor difficulties to access all information provided in software. The focus on requirements was limited to the Microsoft Windows® operating system as it is the predominant operating system used at this center. Compliance with this portion of the requirements can be obtained by integrating the requirements into the software development cycle early and by remediating issues in legacy software if possible. There are certain circumstances with software that may arise necessitating an exemption from these requirements, such as design or engineering software using dynamically changing graphics or numbers to convey information. These exceptions can be discussed with the Section 508 Coordinator and another method of accommodation used.

Pathways↗

NASA NTRS2011

Final Report for Intravenous Fluid Generation (IVGEN) Spaceflight Experiment

NASA designed and operated the Intravenous Fluid Generation (IVGEN) experiment onboard the International Space Station (ISS), Increment 23/24, during May 2010. This hardware was a demonstration experiment to generate intravenous (IV) fluid from ISS Water Processing Assembly (WPA) potable water using a water purification technique and pharmaceutical mixing system. The IVGEN experiment utilizes a deionizing resin bed to remove contaminants from feedstock water to a purity level that meets the standards of the United States Pharmacopeia (USP), the governing body for pharmaceuticals in the United States. The water was then introduced into an IV bag where the fluid was mixed with USP-grade crystalline salt to produce USP normal saline (NS). Inline conductivity sensors quantified the feedstock water quality, output water purity, and NS mixing uniformity. Six 1.5-L bags of purified water were produced. Two of these bags were mixed with sodium chloride to make 0.9 percent NS solution. These two bags were returned to Earth to test for compliance with USP requirements. On-orbit results indicated that all of the experimental success criteria were met with the exception of the salt concentration. Problems with a large air bubble in the first bag of purified water resulted in a slightly concentrated saline solution of 117 percent of the target value of 0.9 g/L. The second bag had an inadequate amount of salt premeasured into the mixing bag resulting in a slightly deficient salt concentration of 93.8 percent of the target value. The USP permits a range from 95 to 105 percent of the target value. The testing plans for improvements for an operational system are also presented.

McQuillen, John B.↗

NASA NTRS2007

Solutions Network Formulation Report. Visible/Infrared Imager/Radiometer Suite and Landsat Data Continuity Mission Simulated Data Products for the Great Lakes Basin Ecological Team

The proposed solution would simulate VIIRS and LDCM sensor data for use in the USGS/USFWS GLBET DST. The VIIRS sensor possesses a spectral range that provides water-penetrating bands that could be used to assess water clarity on a regional spatial scale. The LDCM sensor possesses suitable spectral bands in a range of wavelengths that could be used to map water quality at finer spatial scales relative to VIIRS. Water quality, alongshore sediment transport and pollutant discharge tracking into the Great Lakes system are targeted as the primary products to be developed. A principal benefit of water quality monitoring via satellite imagery is its economy compared to field-data collection methods. Additionally, higher resolution satellite imagery provides a baseline dataset(s) against which later imagery can be overlaid in GIS-based DST programs. Further, information derived from higher resolution satellite imagery can be used to address public concerns and to confirm environmental compliance. The candidate solution supports the Public Health, Coastal Management, and Water Management National Applications.

Estep, Leland↗

NASA NTRS1973

Study of design and control of remote manipulators. Part 2: Vibration considerations in manipulator design

An investigation is reported involving a mathematical procedure using 4 x 4 transformation matrices for analyzing the vibrations of flexible manipulators. Previous studies with the procedure are summarized and the method is extended to include flexible joints as well as links, and to account for the effects of various power transmission schemes. A systematic study of the allocation of structural material and the placement of components such as motors and gearboxes was undertaken using the analytical tools developed. As one step in this direction the variables which relate the vibration parameters of the arm to the task and environment of the arm were isolated and nondimensionalized. The 4 x 4 transformation matrices were also used to develop analytical expressions for the terms of the complete 6 x 6 compliance matrix for the case of two flexible links joined by a rotating joint, flexible about its axis of rotation.

Book, W. J.↗

NASA NTRS1973

Pogo suppression on space shuttle - early studies

Preliminary studies for pogo prevention on the shuttle vehicle are reported. The importance of the effect of oscillatory outflow from a hydroelastic tank is displayed in terms of excitation of normal modes for a structure containing that tank assuming its outlet is closed. Evaluation of an approximate propulsion frequency response at undamped feedline resonance reveals the conditions for which the contribution of tank outflow is destabilizing and also provides a criterion for identifying those structural modes which are of potential significance for system stability. Various finite-element and normal-mode models for hydraulic feedlines are evaluated relative to accuracy of admittances of a long line. A procedure is recommended for modeling a feed system to minimize the required number of second-order equations. Specific recommendations are made for the analytical estimation of pump cavitation compliance and a first estimate for the shuttle pumps is given. Weakness in past practices of pump testing are identified and a new three-phase program is proposed. Finally results of numerical studies on the early vehicle configuration are presented. It is concluded that an accumulator between the boost and main pump offers promise of higher effectiveness than one at the engine inlet.

Rubin, S.↗

NASA NTRS1994

Simulation study of the ROMPS robot control system

This is a report presenting the progress of a research grant funded by NASA for work performed from June 1, 1993 to August 1, 1993. The report deals with the Robot Operated Material Processing System (ROMPS). It presents results of a computer simulation study conducted to investigate the performance of the control systems controlling the azimuth, elevation, and radial axes of the ROMPS and its gripper. Four study cases are conducted. The first case investigates the control of free motion of the three areas. In the second case, the compliant motion in the elevation axis with the wrist compliant device is studied in terms of position accuracy and impact forces. The third case focuses on the behavior of the control system in controlling the robot motion along the radial axis when pulling the pallet out of the rack. In the fourth case, the compliant motion of the gripper grasping a solid object under the effect of the gripper passive compliance is studied in terms of position accuracy and contact forces. For each of the above cases, a set of PIR gains will be selected to optimize the controller performance and computer simulation results will be presented and discussed.

Nguyen, Charles C.↗

NASA NTRS2011

NASA-DoD Lead-Free Electronics Project. DRAFT Joint Test Report

The use of conventional tin-lead (SnPb) in circuit board manufacturing is under ever-increasing political scrutiny due to increasing regulations concerning lead. The "Restriction of Hazardous Substances" (RoHS) directive enacted by the European Union (EU) and a pact between the United States National Electronics Manufacturing Initiative (NEMI), Europe's Soldertec at Tin Technology Ltd. and the Japan Electronics and Information Technology Industries Association (JEITA) are just two examples where worldwide legislative actions and partnerships/agreements are affecting the electronics industry. As a result, many global commercial-grade electronic component suppliers are initiating efforts to transition to lead-free (Pb-free) in order to retain their worldwide market. Pb-free components are likely to find their way into the inventory of aerospace or military assembly processes under current government acquisition reform initiatives. Inventories "contaminated" by Pb-free will result in increased risks associated with the manufacturing, product reliability, and subsequent repair of aerospace and military electronic systems. Although electronics for military and aerospace applications are not included in the RoHS legislation, engineers are beginning to find that the commercial industry's move towards RoHS compliance has affected their supply chain and changed their parts. Most parts suppliers plan to phase out their non-compliant, leaded production and many have already done so. As a result, the ability to find leaded components is getting harder and harder. Some buyers are now attempting to acquire the remaining SnPb inventory, if it's not already obsolete. Original Equipment Manufacturers (OEMs), depots, and support contractors have to be prepared to deal with an electronics supply chain that increasingly provides more and more parts with Pb-free finishes-some labeled no differently than their Pb counterparts-while at the same time providing the traditional Pb parts. The longer the transition period, the greater the likelihood of Pb-free parts inadvertently being mixed with Pb parts and ending up on what are supposed to be Pb systems. As a result, OEMs, depots, and support contractors need to take action now to either abate the influx of Pb-free parts, or accept it and deal with the likely interim consequences of reduced reliability due to a wide variety of matters, such as Pb contamination, high temperature incompatibility, and tin whiskering. Allowance of Pb-free components produces one of the greatest risks to the reliability of a weapon system. This is due to new and poorly understood failure mechanisms, as well as unknown long-term reliability. If the decision is made to consciously allow Pb-free solder and component finishes into SnPb electronics, additional effort (and cost) will be required to make the significant number of changes to drawings and task order procedures. This project is a follow-on effort to the Joint Council on Aging Aircraft/Joint Group on Pollution Prevention (JCAA/JG-PP) Pb-free Solder Project which was the first group to test the reliability of Pb-free solder joints against the requirements of the aerospace and military community.

Kessel, Kurt↗

NASA NTRS1992

Flight 20 (STS-45) polysulfide gas path investigation

This report documents the results of the investigation into causes of gas paths on the 20A and 20B case-to-nozzle joints on STS-42. The investigation was conducted by the Investigation Board appointed by the senior vice president and general manager of Space Operations, Mr. R. E. Lindstrom, on 7 Feb. 1992. The probability of gas path occurrence in the nozzle-to-case-joint polysulfide had been identified during joint redesign. However, actual flight gas path incidence has been limited to RSRM-11 and the 20A and 20B segments. The blow-by condition on the 20A segment was a first time occurrence which was a special concern. The investigation covered all technical aspects associated with the gas path and blow-by conditions: materials and processing history, design requirements and as-built compliance to the design, thermal and structural analyses, computer modeling, and laboratory experimentation with the materials involved. The investigation was coordinated with Mr. Ken Jones at NASA Marshall in bi-weekly teleconferences. The Board also supported Dr. James C. Blair's independent NASA investigation team by providing copies of collected data, conducting requested analyses, and supporting several all-day teleconferences to provide understanding and resolve issues. The Dr. Blair support requirement was successfully concluded on 4 Mar. 1992.

Bjorkman, Rey C.↗

NASA NTRSDate not supplied

Limitations on the Use of Eye-Tracking Data to Understand Operator Awareness

In the last 20 years, a number of accidents and incidents in commercial aviation have pointed to poor flight crew awareness of basic flight path parameters (e.g., airspeed, bank, pitch). As a result, there is a desire to improve pilot situation awareness and how attention is allocated. Eye-tracking has been a commonly used measure of awareness; it can aid in understanding whether specific indications were fixated, and perhaps how a pilot gathered information (that is, which indications in which sequence). In this paper, we discuss limitations on what eye-tracking data can reveal about pilot awareness and understanding. First, previous studies (e.g., Sarter et al., 2007) have shown that fixation on an indication may not ensure awareness or understanding. Further, an operator may have awareness of information not fixated. Additional measures—such as self-report or control inputs—can help to better establish the extent of pilot awareness and understanding. Second, sequences of fixations (scan patterns) have also become a performance measure. While a small number of recognized scan patterns have been validated for a small set of parameters on the Primary Flight Display (PFD), scan patterns have not been identified to support the broader context of flight path management or flight operations. More important is to understand the full set of drivers underlying the larger pattern of eye fixations; this approach moves away from the idea of well-established scan patterns as a marker of skilled performance and gives a larger role to pilot cognition. Pilots have various reasons to direct attention to specific elements on the interface, such as: - feedback tied to control inputs - a check on compliance with flight path targets - a reaction to an alert or a call out - attempt to understand an unexpected indication - assess progress toward a flight path target The importance of cognition is further implicated in the finding that pilot interviews show that fixating typically is accompanied by expectation; generally, pilots have a strong expectation of what value or indication they will see, which allows more efficient integration of information and an ability to identify indications that suggest an alternative account of the current system state. We will describe a range of eye-tracking measures and how they should and should not be used.

aviation↗

NASA NTRS1963

Launch Vehicle Flight Report - Nasa Project Apollo Little Joe 2 Qualification Test Vehicle 12-50-1

The Little Joe II Qualification Test Vehicle, Model 12-50-1, was launched from Army Launch Area 3 {ALA-3) at White Sands Missile Range, New Mexico, on 28 August 1963. This was the first launch of this class of boosters. The Little Joe II Launch Vehicle was designed as a test vehicle for boosting payloads into flight. For the Apollo Program, its mission is to serve as a launch vehicle for flight testing of the Apollo spacecraft. Accomplishment of this mission requires that the vehicle be capable of boosting the Apollo payload to parameters ranging from high dynamic pressures at low altitude to very high altitude flight. The fixed-fin 12-50 version was designed to accomplish the low-altitude parameter. The 12-51 version incorporates an attitude control system to accomplish the high altitude mission. This launch was designed to demonstrate the Little Joe II capability of meeting the high dynamic pressure parameter for the Apollo Program. For this test, a boiler-plate version of the Apollo capsule, service module and escape tower were attached to the launch vehicle to simulate weight, center of gravity and aerodynamic shape of the Apollo configuration. No attempt was made to separate the payload in flight. The test was conducted in compliance with Project Apollo Flight Mission Directive for QTV-1, NASA-MSC, dated 3 June 1963, under authority of NASA Contract NAS 9-492,

Source record↗

NASA NTRS2017

Non-Linear Analysis of the NASA Super Pressure Balloons: Whole Flight Simulations

Tensys have a long-established background in the shape generation and load analysis of architectural stressed membrane structures. Founded upon their inTENS finite element analysis suite, these activities have broadened to encompass 'lighter than air' structures such as aerostats, airships, hybrid air vehicles and stratospheric balloons. Tensys have acted as consultants to the NASA Super Pressure Balloon (SPB) Program since 2004. Previous papers have focussed upon the application of inTENS to the overall structural and stability analysis of pumpkin type balloons as used in the SPB Program. Particular emphasis has been placed upon the ability to study both stress and stability at all stages of a flight. As the program has developed, increasing modelling fidelity has been introduced as design refinement has moved emphasis from the overall shape to the performance of individual details. Examples include the introduction of a contact capability into inTENS to better represent the separate cap layer of film. Localised investigations have considered the consequence of local geometric anomalies introduced during fabrication and the effects of debonding of the PBO tendon within its sleeve. Analysis to date has used a material model for the polyethylene shell film developed by Dr Rand of Winzen Engineering, supported by a program of fine resolution material tests at the Balloon Research and Development Laboratory (BRDL) at the GSFC Wallops Island Facility. Based upon work first reported by Schapery, this model has been incorporated into so-called 'snapshot' analyses within inTENS. For a given elapsed time point and temperature distribution, non-linear material properties are iteratively updated for the current stress state in each individual element until changes in those material properties are insignificant. This process has to be iterative as the film stresses and material properties are interdependent. Attempts to derive a time-stepping incremental viscoelastic capability using the Schapery Rand model encountered problems when dealing with the varying temperatures associated with balloon deployment, pressurisation and diurnal behaviour. An alternative approach has been developed by Pellegrino et al at Caltech, again with support from the NASA Balloon Program Office. This is a large strain non-linear viscoelastic model that includes film out-of-plane mechanical and thermal effects. Working from a unified base, this model can be utilised for time-stepping analyses in both a modulus or compliance mode, or a combination of both. This new capability is intended to enhance the current NASA balloon design process. This paper presents the implementation of the Caltech model into a specialist finite element suite, inTENS and its application to whole flight simulations.

Wakefield, David↗

NASA NTRS2024

2024 Software for NASA Science Mission Directorate Workshop Report

The 2024 Software for the NASA Science Mission Directorate Workshop was the first workshop of its kind in over 10 years. The numerous attendees and high level of interaction in this hybrid workshop portrayed the untapped interest and energy in the NASA SMD community about software. Over 100 takeaways were collected from the hosted discussions in four key areas - Communication, Communities, Funding and Clarity - as summarized in this report. One takeaway was representative of all four categories: “Software is not hardware; it is organic and needs a different model. You often don't know which components will be Open-Source reusable until later in the development cycle.” For the communication category, this takeaway motivates the reduction of silos by shifting towards a model that better supports community collaboration on common challenges and a more streamlined and improved software release process. For the communities category, the same statement points to forming communities of practice with varying scopes and a new approach to recognition and incentives for open-source software contributions. Concerning funding, this statement motivates a more sustained and flexible funding model that better supports the software foundation needed for NASA’s long-term success, including the collaboration and infrastructure a good foundation requires. The new approach to clarity motivated by this statement calls for significant changes to the software release process and related policies to streamline compliance, align those policies with the open-source science culture NASA is promoting and with each other, and simplify use of the cloud. It is time to recognize software as a foundational component of NASA with an organic nature not properly supported by current approaches. Different models are needed in all four areas to shift the NASA SMD software community and governance structures into a more efficient, open, and collaborative ecosystem - one that enables ground-breaking science and daring exploration into the coming decades

science↗

NASA NTRS2014

NASA's UAS Integration into the NAS: A Report on the Human Systems Integration Phase 1 Simulation Activities

In 2011 the National Aeronautics and Space Administration (NASA) began a five-year Project to address the technical barriers related to routine access of Unmanned Aerial Systems (UAS) in the National Airspace System (NAS). Planned in two phases, the goal of the first phase was to lay the foundations for the Project by identifying those barriers and key issues to be addressed to achieve integration. Phase 1 activities were completed two years into the five-year Project. The purpose of this paper is to review activities within the Human Systems Integration (HSI) subproject in Phase 1 toward its two objectives: 1) develop GCS guidelines for routine UAS access to the NAS, and 2) develop a prototype display suite within an existing Ground Control Station (GCS). The first objective directly addresses a critical barrier for UAS integration into the NAS - a lack of GCS design standards or requirements. First, the paper describes the initial development of a prototype GCS display suite and supporting simulation software capabilities. Then, three simulation experiments utilizing this simulation architecture are summarized. The first experiment sought to determine a baseline performance of UAS pilots operating in civil airspace under current instrument flight rules for manned aircraft. The second experiment examined the effect of currently employed UAS contingency procedures on Air Traffic Control (ATC) participants. The third experiment compared three GCS command and control interfaces on UAS pilot response times in compliance with ATC clearances. The authors discuss how the results of these and future simulation and flight-testing activities contribute to the development of GCS guidelines to support the safe integration of UAS into the NAS. Finally, the planned activities for Phase 2, including an integrated human-in-the-loop simulation and two flight tests are briefly described.

National Airspace System↗

NASA NTRS2020

Analysis of Pilot Monitoring Skills and a Review of Training Effectiveness

The commercial aviation industry world-wide has identified a need for improved pilot monitoring and awareness (e.g., FAA, 2013, ICAO, 2016). More specifically, aviation safety data indicate that failures in pilots’ flight path management (FPM) monitoring and awareness have contributed to a range of undesired outcomes: accidents, major upsets, and non-compliance with air traffic control (ATC) guidance. The Federal Aviation Administration (FAA) has further stated that these types of FPM failures are likely to worsen with the increasingly complex air traffic control systems and FPM concepts proposed for NextGen (https://www.faa. gov/nextgen/what_is_nextgen/) operations (e.g., see Hah et al., 2017). Adding to this complexity is the introduction of increasingly automated aircraft systems that can increase monitoring burdens. One potential mitigation for this situation is to enhance pilot training for effective monitoring. NASA Ames Research Center was asked to identify and evaluate training approaches that have the potential to enhance pilots’ ability to effectively monitor for FPM (with the result of improved awareness). The focus of this work is to identify, develop or validate training guidance to improve pilot monitoring/awareness regarding FPM and mitigate the recent trend of accidents and incidents, especially loss of control (LOC) events. The result of this work should be input for improved industry standards and FAA guidance to reduce the risk of incidents and accidents due to inadequate pilot monitoring/awareness. This is the first of three reports that were developed for this project.

aviation human factors↗

NASA NTRS1988

Correlation of macro and micro cardiovascular function during weightlessness and simulated weightlessness

The investigation of cardiovascular function necessarily involves a consideration of the exchange of substances at the capillary. If cardiovascular function is compromised or in any way altered during exposure to zero gravity in space, then it stands to reason that microvascular function is also modified. We have shown that an increase in cardiac output similar to that reported during simulated weightlessness is associated with a doubling of the number of post-capillary venules and a reduction in the number of arterioles by 35%. If the weightlessness of space travel produces similar changes in cardiopulmonary volume and cardiac output, a reasonable expectation is that astronauts will undergo venous neovascularization. We have developed an animal model in which to correlate microvascular and systemic cardiovascular function. The microcirculatory preparation consists of a lightweight, thermo-neutral chamber implanted around intact skeletal muscle on the back of a rat. Using this technique, the performed microvasculature of the cutaneous maximus muscle may be observed in the conscious, unanesthetized animal. Microcirculatory variables which may be obtained include venular and arteriolar numbers, lengths and diameters, single vessel flow velocities, vasomotion, capillary hematocrit anastomoses and orders of branching. Systemic hemodynamic monitoring of cardiac output by electromagnetic flowmetry, and arterial and venous pressures allows correlation of macro- and microcirculatory changes at the same time, in the same animal. Observed and calculated hemodynamic variables also include pulse pressure, heart rate, stroke volume, total peripheral resistance, aortic compliance, minute work, peak aortic flow velocity and systolic time interval. In this manner, an integrated assessment of total cardiovascular function may be obtained in the same animal without the complicating influence of anesthetics.

NASA Discipline Number 00-00↗

NASA NTRS2011

Summary Report for National Aeronautics Space Administration (NASA) and Centro Para Prevencao da Poluicao (C3P) 2011 International Workshop on Environment and Alternative Energy

The C3P &. NASA International Workshop on Environment and Alternative Energy was held on November 15-18, 2011 at the European Space Agency (ESA)'s Research and Technology Centre (ESTEC) in Noordwijk, The Netherlands. The theme of the workshop was "Global Collaboration in Environmental and Alternative Energy Strategies". The workshop was held at ESTEC's conference center. More than 110 individuals from eleven countries attended the workshop. For the first time since the inception of NASA-C3P workshops, a full day was dedicated to a student session. Fifteen students from around the globe gave oral presentations along with poster displays relating to the latest technologies in environmental and alternative energy strategies. Judges from NASA, C3P and ESA awarded plaques to the top three students. In addition to the students, thirty eight U.S. and international subject matter experts presented on the following general environmental-related topics: (1) Hazardous materials management and substitution in support of space operations (2) Emerging renewable and alternative energy technologies (3) Sustainable development and redevelopment (4) Remediation technologies and strategies The workshop also included a panel discussion on the topic of the challenges of operating installations across borders. Throughout the workshop, attendees heard about the scope of environmental and energy challenges that industry and governments face. They heard about technologies for increasing energy efficiency and increasing use of renewable energy. They learned about ways companies and government agencies are using materials, processes, goods and services in a manner more respectful with the environment and in compliance with health and safety rules. The concept of partnerships and their inherent benefits was evidenced throughout the workshop. Partnering is a key aspect of sustainability because sustainable development is complicated. Through formal presentations and side discussions, attendees commented on the need for continued exploration of joint projects of mutual interest.

Greene, Brian↗

NASA NTRS1993

Active stabilization to prevent surge in centrifugal compression systems

This report documents an experimental and analytical study of the active stabilization of surge in a centrifugal engine. The aims of the research were to extend the operating range of a compressor as far as possible and to establish the theoretical framework for the active stabilization of surge from both an aerodynamic stability and a control theoretic perspective. In particular, much attention was paid to understanding the physical limitations of active stabilization and how they are influenced by control system design parameters. Previously developed linear models of actively stabilized compressors were extended to include such nonlinear phenomena as bounded actuation, bandwidth limits, and robustness criteria. This model was then used to systematically quantify the influence of sensor-actuator selection on system performance. Five different actuation schemes were considered along with four different sensors. Sensor-actuator choice was shown to have a profound effect on the performance of the stabilized compressor. The optimum choice was not unique, but rather shown to be a strong function of some of the non-dimensional parameters which characterize the compression system dynamics. Specifically, the utility of the concepts were shown to depend on the system compliance to inertia ratio ('B' parameter) and the local slope of the compressor speedline. In general, the most effective arrangements are ones in which the actuator is most closely coupled to the compressor, such as a close-coupled bleed valve inlet jet, rather than elsewhere in the flow train, such as a fuel flow modulator. The analytical model was used to explore the influence of control system bandwidth on control effectiveness. The relevant reference frequency was shown to be the compression system's Helmholtz frequency rather than the surge frequency. The analysis shows that control bandwidths of three to ten times the Helmholtz frequency are required for larger increases in the compressor flow range. This has important implications for implementation in gas turbine engines since the Helmholtz frequencies can be over 100 Hz, making actuator design extremely challenging.

Epstein, Alan H.↗