Douglas G. MacMartin

Rotorcraft retreating blade stall control

Flow control to avoid or delay rotorcraft retreating blade stall can be an enabling technology for future high performance rotorcraft. Aerodynamic experiments and computations have indicated that appropriate unsteady excitation can delay boundary layer separation and stall on airfoils. Work is in progress to determine the control requirements for helicopter rotor blades at full scale Mach numbers, Reynolds numbers, and with unsteady pitching motions. Compact, powerful, and efficient flow actuation and control systems will be needed. Three actuation concepts were favorably evaluated during initial studies: electromechanical directed synthetic jets (DSJ), periodic flow modulation, and plasma actuation. Electromechanical DSJ and plasma actuators are being developed further and will be evaluated in full scale pitching blade section experiments. These experiments will determine the required control authority, validate the actuator concepts, and study open and closed loop control approaches. Computational studies are being performed of the combined external and actuator flow fields to determine preferred actuation geometries and operating points. System analyses are being used to quantify the benefits for representative aircraft configurations and missions. Copyright ÿ2000 by United Technologies Corporation. Published by the American Institute of Aeronautics and Astronautics, Inc., with Permission. Nomenclature A airfoil pitch rate, U c 2 / α&

Control of uncertain structures using an H(infinity) power flow approach

A technique is described for generating guaranteed stable control laws for uncertain, modally dense structures with collocated sensors and actuators. By ignoring the reverberant response created by reflections from other parts of the structure, a dereverberated mobility model can be developed that accurately models the local dynamics of the structure. This is similar in many respects to a wave-based model, but can treat more general structures, not only those that can be represented as a collection of waveguides. This model can be determined directly from transfer function data using an analysis technique based on the complex cepstrum. In order to minimize the effect of disturbances propagating through the structure, the power dissipated by the controller is maximized in an //<» sense. This guarantees that the controller is positive real and, thus, that the system will remain stable for any uncertainty, provided that the power flow is correctly modeled. The approach is demonstrated for two examples. The resulting controllers are much more effective than simple collocated rate feedback.

A multi-model assessment of regional climate disparities caused by solar geoengineering

Global-scale solar geoengineering is the deliberate modification of the climate system to offset some amount of anthropogenic climate change by reducing the amount of incident solar radiation at the surface. These changes to the planetary energy budget result in differential regional climate effects. For the first time, we quantitatively evaluate the potential for regional disparities in a multi-model context using results from a model experiment that offsets the forcing from a quadrupling of CO2 via reduction in solar irradiance. We evaluate temperature and precipitation changes in 22 geographic regions spanning most of Earthʼs continental area. Moderate amounts of solar reduction (up to 85% of the amount that returns global mean temperatures to preindustrial levels) result in regional temperature values that are closer to preindustrial levels than an un-geoengineered, high CO2 world for all regions and all models. However, in all but one model, there is at least one region for which no amount of solar reduction can restore precipitation toward its preindustrial value. For most metrics considering simultaneous changes in both variables,

MODEL-BASED CONTROL OF CAVITY OSCILLATIONS, PART II: SYSTEM IDENTIFICATION AND ANALYSIS

Experiments using active control to reduce oscillations in the flow past a rectangular cavity have uncovered surprising phenomena: in the controlled system, often new frequencies of oscillation appear, and often the main frequency of oscillation is split into two sideband frequencies. The goal of this paper is to explain these effects using physics-based models, and to use these ideas to guide control design. We present a linear model for the cavity flow, based on the physical mechanisms of the familiar Rossiter model. Experimental data indicates that under many operating conditions, the oscillations are not self-sustained, but in fact are caused by amplification of external disturbances. We present some experimental results demonstrating the peak-splitting phenomena mentioned above, use the physics-based model to study the phenomena, and discuss fundamental performance limitations which limit the achievable performance of any control scheme.

Studying geoengineering with natural and anthropogenic analogs

Solar radiation management (SRM) has been proposed as a possible option for offsetting some anthropogenic radiative forcing, with the goal of reducing some of the associated climatic changes. There are clearly significant uncertainties associated with SRM, and even small-scale experiments that might reduce uncertainty would carry some risk. However, there are also natural and anthropogenic analogs to SRM, such as volcanic eruptions in the case of stratospheric aerosol injection and ship tracks in the case of marine cloud albedo modification. It is essential to understand what we can learn from these analogs in order to validate models, particularly because of the problematic nature of outdoor experiments. It is also important to understand what we cannot learn, as this might better focus attention on what risks would need to be solely examined by numerical models. Stratospheric conditions following a major volcanic eruption, for example, are not the same as those to be expected from intentional geoengineering, both because of confounding effects of volcanic ash and the differences between continuous and impulsive injection of material into the stratosphere. Nonetheless, better data would help validate models; we thus recommend an appropriate plan be developed to better monitor the next large volcanic eruption. Similarly, more could be learned about cloud albedo modification from careful study not only of ship tracks, but of ship and other aerosol emission sources in cloud regimes beyond the narrow conditions under which ship tracks form; this would benefit from improved satellite observing capabilities.

Control and alignment of segmented-mirror telescopes: matrices, modes, and error propagation

Starting from the successful Keck telescope design, we construct and analyze the control matrix for the active control system of the primary mirror of a generalized segmented-mirror telescope, with up to 1000 segments and including an alternative sensor geometry to the one used at Keck. In particular we examine the noise propagation of the matrix and its consequences for both seeing-limited and diffraction-limited observations. The associated problem of optical alignment of such a primary mirror is also analyzed in terms of the distinct but related matrices that govern this latter problem.

Dynamics and Control of Shock Motion in a Near-Isentropic Inlet

Inlet pressure recovery of supersonic aircraft could be improved using a near-isentropic inlet with only a weak normal shock aft of the throat; however, such an inlet is highly susceptible to unstart. Small perturbations can move the shock ahead of the throat, where it is unstable. The dynamics of the inlet and shock are analyzed using a low-order model that captures both the nonlinear shock motion and inlet acoustic propagation. This model allows parametric exploration of both the potential and limitations of using control to stabilize actively the shock, including actuator authority as a function of location, actuator authority, and bandwidth requirements, and sensor requirements. A simple control law is shown to be sufficient to stabilize the shock motion.

Dynamics of the Coupled Human-climate System Resulting from Closed-loop Control of Solar Geoengineering

If solar radiation management (SRM) were ever implemented, feedback of the observed climate state might be used to adjust the radiative forcing of SRM in order to compensate for uncertainty in either the forcing or the climate response. Feedback might also compensate for unexpected changes in the system, e.g. a nonlinear change in climate sensitivity. However, in addition to the intended response to greenhouse-gas induced changes, the use of feedback would also result in a geoengineering response to natural climate variability. We use a box-diffusion dynamic model of the climate system to understand how changing the properties of the feedback control affect the emergent dynamics of this coupled human–climate system, and evaluate these predictions using the HadCM3L general circulation model. In particular, some amplification of natural variability is unavoidable; any time delay (e.g., to average out natural variability, or due to decision-making) exacerbates this amplification, with oscillatory behavior possible if there is a desire for rapid correction (high feedback gain). This is a challenge for policy as a delayed response is needed for decision making. Conversely, the need for feedback to compensate for uncertainty, combined with a desire to avoid excessive amplification of natural variability, results in a limit on how rapidly SRM could respond to changes in the observed state of the climate system.

Solar geoengineering to limit the rate of temperature change.

Solar geoengineering has been suggested as a tool that might reduce damage from anthropogenic climate change. Analysis often assumes that geoengineering would be used to maintain a constant global mean temperature. Under this scenario, geoengineering would be required either indefinitely (on societal time scales) or until atmospheric CO2 concentrations were sufficiently reduced. Impacts of climate change, however, are related to the rate of change as well as its magnitude. We thus describe an alternative scenario in which solar geoengineering is used only to constrain the rate of change of global mean temperature; this leads to a finite deployment period for any emissions pathway that stabilizes global mean temperature. The length of deployment and amount of geoengineering required depends on the emissions pathway and allowable rate of change, e.g. in our simulations, reducing the maximum approximately 0.3°C per decade rate of change in an RCP 4.5 pathway to 0.1°C per decade would require geoengineering for 160 years; under RCP 6.0, the required time nearly doubles. We demonstrate that feedback control can limit rates of change in a climate model. Finally, we note that a decision to terminate use of solar geoengineering does not automatically imply rapid temperature increases: feedback could be used to limit rates of change in a gradual phase-out.

Robust control design and implementation on the Middeck Active Control Experiment

This paper presents a coherent methodology for robust controller synthesis for the Middeck Active Control Experiment (MACE): a Shuttle program scheduled for flight on STS-67 in February 1995. The experiment has been designed to investigate the extent to which the on-orbit behavior of a precision-controlled spacecraft can be predicted and controlled using analysis and ground testing prior to launch. A goal of the flight experiment is to demonstrate good payload pointing performance using active controllers designed based on the predicted structural dynamics. For systems with complicated control topologies and large model uncertainties, this requires a systematic control design methodology. The results from preliminary ground-based control experiments are used in this paper to present such a design technique and to illustrate how it can be applied to future flight experiments. This control design methodology is then used to develop controllers that obtain a 22 dB improvement in the performance metric on the current MACE hardware.