Browsing by Subject "Aerospace Engineering and Mechanics"

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Accuracy of UAV Pitot-Static System
(2012-04-18) Carlson, Ryan
Though they are unnecessary for the structural integrity of the aircraft, an error in the pitot-static system can cause inaccuracies in flight and eventually lead to aircraft failure. Through a series of tests, the University of Minnesota’s UAV pitot-static system error was calculated and graphed. This allows the pressure sensors built into the pitot-static system to be calibrated for error, which prevents harm to the aircraft and operators.
Adaptive mesh refinement and cut-cell algorithms for DSMC simulation of hypersonic flows.
(2010-05) Zhang, Chonglin
Adaptive mesh refinement (AMR) and cut-cell algorithms were developed for a 3-level Cartesian mesh based Direct Simulation Monte Carlo (DSMC) implementation. The simple and efficient AMR algorithm adapts the cell size to the local mean free path of the flow field. Variable time step technique was implemented together with the AMR algorithm to set a time step consistent with the local mean collision time. The control of simulation particles through the use of variable time step was also illustrated. The cut-cell method decouples the flow field Cartesian mesh and the triangulated surface mesh representing any object inside the flow field. Two key aspects of the cut-cell method: cut-cell sorting and volume calculation were discussed in detail. The 3-level embedded Cartesian mesh combined with AMR and variable time step allows increased flexibility for precise control of local mesh size and time step, both vital for accurate and efficient DSMC simulation. Hypersonic flow simulations were conducted to highlight the performance of AMR, variable time step and cut-cell algorithms. Three dimensional simulation of Planetary probe reproduced the experimental heat flux measurement.
Asymptotic models in magnetostriction with application to design of sensors.
(2012-04) Krishnan, Shankar Narayan
Magnetostrictive wires of diameter in the nanometer scale have been proposed for application as acoustic sensors [Downey et al., 2008], [Yang et al., 2006]. The sensing mechanism is expected to operate in the bending regime. In the first part of this work, we derive a variational theory for the bending of magnetostrictive nanowires starting from a full 3-dimensional continuum theory of magnetostriction. We recover a theory which looks like a typical Euler-Bernoulli bending model but includes an extra term contributed by the magnetic part of the energy. The solution of this variational theory for an important, newly developed magnetostricitve alloy called Galfenol ¡ cf. [Clark et al., 2000] ¢ is compared with the result of experiments on actual nanowires ¡ cf. [Downey, 2008] ¢ which shows agreement. In the next part of this thesis, Multilayered wires of diameter in the nanometer scale with periodic layering of non-magnetic copper and ferromagnetic galfenol segments are studied. The numerical computation of the physics of magnetization for such geometries is very costly computationally. We use the theory of periodic homogenization to understand the overall behavior of such structures. We first determine a “homogenized theory” after which this “homogenized model” is used to study the nucleation and stability of staturated states. Thus we get a broad generalization of what is known in the magnetic literature as the “fanning model” first introduced in [Jacobs and Bean, 1955] for a chain of spheres geometry. Some further numerical work on computing M vs H curves for such geometries is also presented.
Austenite-Martensite phase transformation and magneto-mechanical studies on nickel manganese gallium single crystal alloy.
(2010-05) Wu, Yiming
The results of experiments on austenite-martensite phase transformation and magneto- mechanical behavior of a NiMnGa ferromagnetic shape-memory alloy are reported. This alloy undergoes a cubic to tetragonal martensitic phase transformation at about 5 oC and both phases are ferromagnetic. These experiments were conducted using a Magneto- Mechanical Testing Machine (MMTM) that is capable of simultaneously applying a uni- axial load and a magnetic ¯eld that can be varied in a plane containing the mechanical loading axis. One of the main accomplishments of this work was the extension of these measurements to tensile loads. Two types of experiments were conducted. First a set of experiments were performed to determine the e®ects of applied loads and magnetic ¯elds on the transformation temperature of the alloy. The e®ect of compression on the phase transformation is about 0.2 to 0.4 K/MPa, while the e®ect of tension is only 0.1 to 0.2 K/MPa. These results agree with predictions made using the Clausius-Clapeyron equation. The e®ects of applied magnetic ¯eld are more complicated and do not follow the simple tends predicted by this equation. Observations of the microstructure that forms during transformation agree reasonably well with the predictions of the crystal- lographic theory of martensite. The second set of experiments was conducted to measure the behavior of the alloy un- der a constant load and applied magnetic ¯elds. These experimental results are directly applicable to using this material in an actuator. Several di®erent magnetic ¯elds paths were used to determine if this has an e®ect on the strains observed. Measurements of the specimen average magnetization where also made using Hall probes to measure the stray ¯eld produced by the specimen during these experiments. These results have some portions of the strain-magnetization curves that are linear, which is what is pre- dicted by models that have both of these quantities directly related to variant volume fractions. Two of the key parameters for actuators that were measured are blocking stress and work output. It was found that the maximum work output occurs for a small tensile bias load due to the reduced e®ect of specimen demagnetization in this case. The blocking stress in tension appears to be just above the largest tensile stress of 3 MPa that could be applied, which is similar to the value in compression. The values of these two parameters compare well to values in the literature for compression, while the tensile results are the results to be reported.
Computational Fluid Dynamics Validation of Supersonic Parachute Analysis
(2012-04-18) Bowell, Tanner
For many years now humans have been exploring outer space for a number of reasons. Whether the goal is to send a satellite into orbit or to explore other planets, we have always been interested in discovering more about our universe. One of the most studied planets over the years has been Mars. There is interest in discovering if there is, or ever has been, any life there as well as determining if it is an adequate planet for inhabitance. Regardless of the desire, the best way to study the planet is to of course go there. Numerous Mars Rovers have been sent to land on Mars in the hopes of learning more about the terrain, atmosphere and biology. One issue that arises is the limitation as to where the rover can land due to the extremely high entry velocities. So far, the Rovers have only landed in areas equivalent to 2 kilometers below our sea level. NASA has interest in the ability to land in areas of higher elevation to explore unseen land. In addition to landing on higher ground, the Mars Science Laboratory (MSL) of NASA recently launched a Rover that is much heavier than in the past. A possible solution to land a heavier Rover on higher ground is to deploy the parachute much earlier, at velocities up to Mach 2.5 (two and a half times the speed of sound). As a result, extreme pressure fluctuations occur due to a highly unsteady wake that is shed off of the Rover capsule and interacts with the shockwaves that form at the parachute canopy3. The unsteady interaction forces fluid inside the canopy which then escapes, causing the canopy to partially collapse on itself. This cycle repeats itself over and over during descent4. These findings have come from previous research and experimental results. Professor Graham Candler from the University of Minnesota and his research team developed a numerical method to model a supersonic parachute entry scenario using US3D Computational Fluid Dynamics (CFD) software. After the previous study they refined the method with the intent to reexamine the cases. The goal of this research project is to run these cases and analyze the results.
Computational Studies of Rotations and Quaternions
(2011-07-19) Tuttle, Joseph
As a fundamental description of motion, rotations are important aspects of kinematics and engineering. All rotations can be described either as vector rotations, in which a series of vectors are rotated, or frame rotations, in which the entire frame is rotated. Each type of rotation can be expressed through three main notations. The first is the directional cosine matrix or DCM. Through matrix multiplication of the DCM and a vector, the resultant vector is produced. By applying the product of three DCMs, the Aerospace Rotation sequence can be expressed in terms of the Euler Angles. These three angles can describe the orientation of the body in almost every position. A series of rotations can also be expressed as a single rotation with a known angle and axis of rotation. With these two parameters, the rotation can be expressed through quaternions. Through hyper complex operations, quaternions offer another method of calculating both frame and vector rotations. Each of these representations are investigated and related through computational means. Although each representation has their own advantages and disadvantages, quaternions are very significant for applications with a known axis of rotation. An example is proving the intersection of any plane and a double cone produces the conic sections. To eliminate a variable and express the intersection in two dimensions, a rotation must be applied. By calculating the axis and angle of rotation, the required rotation is found by method of quaternions.
Computational study of nonequilibrium chemistry in high temperature flows.
(2010-11) Doraiswamy, Sriram
Recent experimental measurements in the reflected shock tunnel CUBRC LENS-I facility raise questions about our ability to correctly model the recombination processes in high enthalpy flows. In the carbon dioxide flow, the computed shock standoff distance over the Mars Science Laboratory (MSL) shape was less than half of the experimental result. For the oxygen flows, both pressure and heat transfer data on the double cone geometry were not correctly predicted. The objective of this work is to investigate possible reasons for these discrepancies. This process involves systematically addressing different factors that could possibly explain the differences. These factors include vibrational modeling, role of electronic states and chemistry-vibrational coupling in high enthalpy flows. A state-specific vibrational model for CO2, CO, O2 and O system is devised by taking into account the first few vibrational states of each species. All vibrational states with energies at or below 1 eV are included in the present work. Of the three modes of vibration in CO2, the antisymmetric mode is considered separately from the symmetric stretching mode and the doubly degenerate bending modes. The symmetric and the bending modes are grouped together since the energy transfer rates between the two modes are very large due to Fermi resonance. The symmetric and bending modes are assumed to be in equilibrium with the translational and rotational modes. The kinetic rates for the vibrational-translation energy exchange reactions, and the intermolecular and intramolecular vibrational-vibrational energy exchange reactions are based on experimental data to the maximum extent possible. Extrapolation methods are employed when necessary. This vibrational model is then coupled with an axisymmetric computational fluid dynamics code to study the expansion of CO2 in a nozzle. The potential role of low lying electronic states is also investigated. Carbon dioxide has a single excited state just below the dissociation limit. CO and O recombine exclusively to this excited state and then relaxes to the ground electronic state. A simple model is proposed to represent the effect of this intermediate state in the recombination process. Preliminary results show that this excited electronic state is a potential reason for increased shock standoff distance observed in LENS facility.The general role of chemistry-vibrational coupling in modeling recombination dominated flows is also investigated. A state-specific model is developed to analyze the complex chemistry-vibration coupling present in high enthalpy nozzle flows. A basic model is formulated assuming molecules are formed at a specific vibrational level and then allowed to relax through a series of vibration-vibration and vibration-translation processes. This is carried out assuming that the molecules behave as either harmonic or anharmonic oscillators. The results are compared with the standard vibration-chemistry model for high enthalpy nozzle flows. Next, a prior recombination model that accounts for the rotational-vibrational coupling is used to obtain prior recombination distribution. A distribution of recombining states is obtained as a function of the total energy available to the system. The results of this model are compared with recent experiments. Additionally, a reduced model is formulated using the concepts of the state-specific model. The results of this reduced model is compared with the state specific model.
Control of jets in cross.
(2010-07) Sau, Rajes
We use direct numerical simulations to study control of jets in cross ow by axial pulsing. Our main idea is that pulsing generates vortex rings; the effect of pulsing on jets in crossflow can therefore be explained by studying the behavior of vortex rings in crossflow. A method is proposed that allows optimal values of pulsation frequency, modulation and energy to be estimated a priori. This is accomplished in the following three stages. First, direct numerical simulation is used to study the mixing of a passive scalar by a vortex ring issuing from a nozzle into stationary fluid. The ‘formation number’ (Gharib et al. 1998), is found to be 3.6. Simulations are performed for a range of stroke ratios encompassing the formation number, and the effect of stroke ratio on entrainment, and mixing is examined. When the stroke ratio is greater than the formation number, the resulting vortex ring with trailing column of fluid is shown to be less effective, at mixing and entrainment. As the ring forms, ambient fluid is entrained radially into the ring from the region outside the nozzle exit. This entrainment stops once the ring forms, and is absent in the trailing column. The rate of change of scalar containing fluid is studied for its dependence on stroke ratio. This rate varies linearly with stroke ratio until the formation number, and falls below the linear curve for stroke ratios greater than the formation number. This behavior is explained by considering the entrainment to be a combination of that due to the leading vortex ring, and that due to the trailing column. For stroke ratios less than the formation number, the trailing column is absent, and the size of the vortex ring increases with stroke ratio, resulting in increased mixing. For stroke ratios above the formation number, the leading vortex ring remains the same, and the length of the trailing column increases with stroke ratio. The overall entrainment decreases as a result. Next, direct numerical simulation is used to study the effect of crossflow on the dynamics, entrainment and mixing characteristics of vortex rings issuing from a circular nozzle. Three distinct regimes exist, depending on the velocity ratio and stroke ratio. Coherent vortex rings are not obtained at velocity ratios below approximately 2. At these low velocity ratios, the vorticity in the crossflow boundary layer inhibits roll–up of the nozzle boundary layer at the leading edge. As a result, a hairpin vortex forms instead of a vortex ring. For large stroke ratios and velocity ratio below 2, a series of hairpin vortices are shed downstream. The shedding is quite periodic for very low Reynolds numbers. For velocity ratios above 2, two regimes are obtained depending upon the stroke ratio. Lower stroke ratios yield a coherent asymmetric vortex ring, while higher stroke ratios yield an asymmetric vortex ring accompanied by a trailing column of vorticity. These two regimes are separated by a transition stroke ratio whose value decreases with decreasing velocity ratio. For very high values of the velocity ratio, the transition stroke ratio approaches the ‘formation number’ defined by Gharib et al. (1998). In the absence of trailing vorticity, the vortex ring tilts towards the upstream direction, while the presence of a trailing column causes it to tilt downstream. This behavior is explained. Then, we study the mixing behavior of pulsed jets in crossflow using direct numerical simulations. The pulse is a square wave and the simulations consider several jet velocity ratios and pulse conditions. We study the effects of pulsing, and explain the wide range of optimal pulsing conditions found in experimental studies of the problem. Vortex rings in crossflow exhibit three distinct flow regimes depending on stroke ratio and ring velocity ratio. The simulations of pulsed transverse jets show that at high velocity ratios, optimal pulse conditions correspond to the transition of the vortex rings produced by pulsing between the different regimes. At low velocity ratios, optimal pulsing conditions are related to the natural timescale on which hairpin vortices form. An optimal curve in the space of stroke ratio and velocity ratio is developed. Data from various experiments are interpreted in terms of the properties of the equivalent vortex rings and shown to collapse on the optimal curve. The proposed regime map allows the effects of experimental parameters such as pulse frequency, duty cycle, modulation, and pulse energy to all be predicted by determining their effect on the equivalent stroke and velocity ratios. The thesis also discusses work towards the development of Large Eddy Sim- ulation (LES) methodology to predict mixing in very high Reynolds number turbulent flows. We propose a novel estimation procedure to model the subgrid velocity for LES. The subgrid stress is obtained directly from the estimated subgrid velocity. The model coefficients for the subgrid velocity are obtained by imposing constraints on resulting ensemble-averaged subgrid dissipation and local subgrid kinetic energy. The subgrid dissipation may be obtained through either eddy–viscosity models or a new dynamic model for dissipation. The subgrid kinetic energy may be obtained either from the dynamic Yoshizawa model or a modeled transport equation. We also extend the estimation procedure to LES of passive scalar transport and propose an estimation model for subgrid scalar concentration. The subgrid flux is computed directly from the estimated subgrid velocity and estimated subgrid scalar. The model coefficient for the subgrid scalar is obtained by constraining mean scalar dissipation which is provided by an eddy–diffusivity approach. The velocity and scalar estimation models are applied to decaying isotropic turbulence with an uniform mean scalar gradient and good results are obtained. Realistic backscatter is also predicted. A dynamic model for subgrid scale dissipation is proposed. The dissipation is modeled using invariants of strain–rate tensor. The proposed dynamic approach uses a second level test filter and the model coefficient is obtained using two scalar and propose an estimation model for subgrid scalar concentration. The subgrid flux is computed directly from the estimated subgrid velocity and estimated subgrid scalar. The model coefficient for the subgrid scalar is obtained by constraining mean scalar dissipation which is provided by an eddy–diffusivity approach. The velocity and scalar estimation models are applied to decaying isotropic turbulence with an uniform mean scalar gradient and good results are obtained. Realistic backscatter is also predicted. A dynamic model for subgrid scale dissipation is proposed. The dissipation is modeled using invariants of strain–rate tensor. The proposed dynamic approach uses a second level test filter and the model coefficient is obtained using two scalar identities. We show that this approach can also be used to obtain the Smagorinsky model coefficient for subgrid stress. This is an alternative to Germano’s dynamic procedure where the single model constant is obtained by minimizing the error in a tensor identity, the Germano identity error
Dependence of Fire Detection Systems to Mass Concentrations and Particle Size Distributions
(2012-04-18) Hart, Colin
Combustion of common household items containing plastic and cellulosic solid and liquid materials results in flaming and smoldering fires. Products of combustion of these fires include H2O, CO2, energy release, visible or infrared radiation, gaseous hydrocarbons, and solid and semi-volatile particles. Commercial and household smoke detectors rely on ionization and photoelectric techniques to detect particles generated by flaming and smoldering fires and the National Fire Protection Association recommends using both techniques in parallel to improve detection capabilities. The purpose of this study is to improve the understanding of how the response of commercial smoke detectors depends on the physical and chemical characteristics of smoke particles, i.e. varying number concentration and size distribution. Our data, in turn, will support the development, analysis, and optimization of the complete fire detection systems used by commercial aircrafts in the future.
Dynamic flight envelope assessment with flight safety applications.
(2010-12) Pandita, Rohit
Aircraft have a manufacturer prescribed operating flight envelope for safe operation. exceeding these limits can result in unrecoverable departures or even structural failure. Numerous commercial aircraft accidents in the past have been attributed to loss-of-control (LOC) resulting from exceeding the safe operating flight envelope. Hence, real-time knowledge of the safe operating flight envelope is essential for safe flight operation, a problem known as dynamic flight envelope assessment. This dissertation explores dynamic flight envelope assessment from a control theoretic perspective. Two notions of the flight envelope, namely, the reachable sets and the region-of-attraction analysis are investigated. The NASA generic transport model (GTM) aircraft dynamics is used as an application problem. Linear and nonlinear techniques for flight envelope assessment are formulated in the linear matrix inequality (LMI) and sum-of-squares (SOS) framework, respectively. LMI and SOS problems are computationally tractable convex optimization problems for which many semi-definite programming solvers are available. This thesis also investigated fault detection and isolation strategies. Commercial jet transport aircrafts make extensive use of active controls. Faults or failures in the flight control system (FCS) elements like sensors or control effectors can lead to catastrophic failure. Model-based fault detection and isolation (FDI) filters can provide analytical redundancy by reliably detecting such faults in the system. Practical application of model-based FDI filters is limited so far due to poor performance, false alarms and missed detection arising out of uncertain dynamics of the aircraft, effect of nonlinearities in the system and the influence of closed-loop controllers. An application of closed-loop metrics to assess worst case FDI filter performance in the presence of a controller and uncertain dynamics is presented. Longitudinal GTM dynamics are considered. An H∞ FDI filter and a geometric filter design are compared using the metrics and the results validated through simulation. This research was expanded to include synthesis, real-time implementation and flight validation of robust FDI filters for a small uninhabited aerial vehicle. The influence of different closed-loop controllers on FDI filter performance is investigated. A comparison is presented between simulation of the predicted FDI filter performance and flight experiment results.
Dynamic Modeling and Simulation of an Autonomous Underwater Vehicle (AUV)
(2021) Orpen, Kevin;
Autonomous Underwater Vehicles (AUVs) have been in development in recent decades to address the difficulties and high costs of oceanic exploration, with applications including marine life monitoring, search and rescue operations, and wreck inspection. An underwater robot developed by the Interactive Robotics and Vision (IRV) Laboratory at the University of Minnesota is LoCO, a Low Cost Open-Source AUV. LoCO seeks to assist in a number of underwater applications while reducing the current high cost of entry into underwater robotics. One aspect of this underwater vehicle that is integral to its capacity as an AUV is the modeling of its dynamics, and each new AUV comes with unique geometries spanning various propulsion control methods for specializing in different underwater tasks. This thesis seeks to establish an underwater dynamic model for the robot, implement the model in a simulated setting so as to provide testing opportunities before field deployment, and compare the effectivity of the model to collected experimental data. This, in turn, will lead to the efficient development of its autonomous systems and capability to assist in underwater operations. Within this research, the dynamic models have been produced and geometry-dependent coefficients have been derived for LoCO. A simulator for the robot has also been developed that can interface with onboard software. Though the simulation agrees relatively well with experimental data collected for LoCO’s forward motion, there are still other motion modes that require further investigation. Overall, this dynamic foundation will provide for future control system and other autonomous development to further its underwater capabilities.
An Efficient Formation Flight Simulator with Extensions to Unsteady Maneuvers
(2018) Humbert, Peter;
A quasi-steady formation flight simulator is implemented in MATLAB by following a previously published methodology for calculating aerodynamic influence among lifting surfaces. The methodology extends Prandtl’s lifting-line theory both to multiple lifting surfaces and to lifting surfaces with complex geometries. The simulator presented herein is capable of calculating the forces acting on multiple lifting surfaces and on those with various geometric properties, including taper, sweep, and dihedral. A description of the simulator’s implementation is offered, as are several demonstrations of its correctness. Numerous examples of its practical uses are then provided. The quasi-steady model’s use of the section lift coefficient then allows it to be hybridized with an empirical Theodorsen model for unsteady pitching, which in turn allows us to formulate an expectation for the manner in which the unsteady, sinusoidal pitching of a leading wing affects a trailing wing. The pitching motion is found to reduce the trailing wing’s efficiency appreciably, but the reduction behaves asymptotically. Though not implausible, this result would need to be validated by experiment, offering one of many opportunities for further work. Computational efficiency is central to both the quasi-steady and hybrid methodologies. The former only depends on the geometry of the formation flight scenario, thereby avoiding calculations at points between the wings, and the latter similarly avoids the the usual requirement of calculating vortex panel dynamics.
Evolution of eddies and packets in turbulent boundary layers.
(2011-03) Gao, Qi
The objective of this study was to improve understanding of the population distribution and evolution of eddies and eddy packets in turbulent boundary layers using experimental methods. To effectively identify vortical structures, an advanced vortex identification algorithm was developed based on swirl strength, and the real eigenvector of the velocity gradient tensor as an indicator of eddy orientation. The new method applied on the streamwise/spanwise plane had a good performance on vortex identification, which was tested by two Direct Numerical Simulation (DNS) of channel flow at Re#28; = 590 and 934, and one Dual-Plane Particle Image Velocimetry (PIV) data set of boundary layer at Re#28; = 2480. The effect of spatial resolution was studied. Although it was found that relatively coarse resolution of 24.5 wall units (Dual-Plane PIV) caused underestimation of velocity gradients, which resulted in underestimated magnitudes of several derived variables, such as swirl strength, vorticity and circulation, the statistical results of vortex population distribution had a good agreement between DNS and Dual-Plane PIV data sets. A new method of scaling the swirl was proposed based on the invariants of the characteristic equation of the velocity gradient tensor, which could minimize the effects of data resolution among different data sets. A volumetric PIV technique, Tomographic PIV (TPIV), was applied to investigate vortical structures. Although the TPIV resolution was too coarse to resolve the smallest eddies accurately, it worked very well for identifying larger eddies and packets. Population distributions of eddy orientation, size, circulation and convection velocity in the logarithmic region were obtained from both numerical and experimental data. It was found that the elevation angle and size of eddies increased with increasing wall-normal distance, while the eddy circulation decreased with increasing wall-normal distance. The mean convection velocity of eddies was normally #24;97% of the local mean. Joint PDFs of eddy radius and circulation yielded a peak fitting curve of an exponential function a(r+)b. The exponent b was found equal to 2.2 for all data sets in the logarithmic region, while the coefficient a was proportional to the mean magnitude of vorticity which was dependent on the resolution. The difference between vorticity vector and the real eigenvector was documented, and this was thought to be caused by the local shear motion. The volumetric data from TPIV gave strong support for the local shear hypothesis showing that the eigenvector is a better indicator of eddy orientation. Eddy packets and flow evolution were studied using flying TPIV data in the lower (z+ = 100 #24; 300) and upper (z+ = 300 #24; 500) logarithmic region for Re#28; = 2480. Long slow regions (> 0:6#14;) surrounded by eddy pairs were often seen in both locations (#24;10% of all instantaneous fields). The width of the long slow regions was #24;400-500 viscous units (#24;0.16-0.2#14;) at z+ #25; 200 and #24;650- 750 viscous units (#24;0.26-0.3#14;) at z+ #25; 400. Pairs of hairpin legs propagated mostly at velocity of 0.92U+ for both wall-normal locations. The streamwise spacing of vortex pairs in the packets was about 300 viscous units at z+ #25; 200 and 150 viscous units at z+ #25; 400. The spanwise spacing of two long slow regions was typically larger than 0.5#14;. One case at the lower wall-normal location showed that a long slow region stably lasted at least #1;t+ #25; 2300 and traveled about 15.5#14;, while one long slow region lasted #1;t+ #25; 1500 and traveled about 11.5#14; at the upper location. Deforming, meandering, merging, and breaking of these long slow regions was observed. Interaction of neighboring eddies was also observed. Oscillation of hairpin legs in the spanwise direction showed that, whenever they became closer, they appeared to become stronger, and vice versa. Eddies with strong circulation showed greater stability over time. The eddies with less than 10% circulation variation over #1;t+ = 78 had mean circulation of 500 at the lower location and 350 at the upper location.
Experimental study of control laws for supercavitating vehicles.
(2012-03) Hjartarson, Arnar
Supercavitation is when a cavity is made to envelop a submerged body. Supercavitation can be used to achieve an order of magnitude reduction in drag on underwater vehicles. Supercavitating vehicles can reach unprecedented speeds underwater. Supercavitation has reportedly been used to create underwater vehicles that reach speeds of 370 km/h, which is significantly faster than the fastest traditional submarine vehicles. Methods and technologies to control and maneuver supercavitating vehicles are actively being researched. The efforts to develop control strategies and assess the effectiveness of control effectors are hampered by a lack of access to working test beds and operational vehicles. This thesis describes the development and testing of an experimental test bed for validating the performance of control strategies for supercavitating vehicles. The test bed addresses the need for an experimental platform that enables researchers to test candidate control algorithms and associated technologies on a real, physical, supercavitating system. The test bed was used to evaluate the performance of feedback control systems on a model supercavitating vehicle in a water tunnel. Two controllers were developed using H∞ control design techniques and evaluated on the test bed. The validity of the hydrodynamic model that the control designs were based on was established, and a comparison and partial validation of their performance was obtained in water tunnel experiments. The experiments demonstrated that a test bed of this kind can be used to evaluate control algorithms, and study the effects of active control systems on a supercavitating vehicle.
Finite element modeling of articular cartilage at different length scales
(2012-04) Chiravarambath, Sidharth Saktan
The composition and structure of articular cartilage (AC) are inhomogeneous within the tissue and vary throughout its depth. Its extracellular matrix can be considered as a fiber-reinforced composite solid consisting of a dense stable network of collagen fibers embedded in a proteoglycan (PG) gel. Several studies have shown that this specialized structure plays a vital role in the mechanical function of AC. In pathological conditions, such as osteoarthritis (OA), degeneration of cartilage due to changes in mechanical properties is observed. Osteoarthritis is the most common cause of disability in the elderly and affects more than 20 million people in the USA alone. The focus of this work is to understand the mechanical response of AC using finite element models using ABAQUS, a commercial FEA package that is widely used in the field of cartilage mechanics. This is done at two different scales - the macroscale and the mesoscale. At the macroscale, AC is considered as a homogeneous isotropic poroviscoelastic (PVE) material saturated by the interstitial fluid (water). Indentation tests are performed on cartilage from the mouse tibia plateau using two different sized flat-ended conical indenters with flat-end diameters of 15 μm and 170 μm. A finite element (FE) model of the test is developed and the PVE parameters identified by using inverse methods to minimize the errors between FE simulated and test data. Data from the smaller indenter is first used to fit the viscoelastic (VE) parameters, on the basis that for this tip size the gel diffusion time (approximate time constant of the poroelastic (PE) response) is of the order of 0.1 s, so that the PE response is negligible. These parameters are then used to fit the data from the larger indenter for the PE parameters, using the VE parameters extracted from the data from the smaller indenter. At the mesoscale the inhomogeneities of AC need to be addressed to understand the microstructural behavior of AC. The problem of interest in this part of the work is to understand the mechanical role of interfibrillar cross-links (IFLs), if they exist, suspected in AC and most collagenous tissues. A 3D FE model of AC meso-structure motivated by the parallel fibril geometry of the mid and deep zones of the patella is developed consisting of a PE matrix, unidirectional, bilinear fibrils (different stiffness in tension and compression), and the IFLs. Parametric studies are then performed for the model in simulated compression tests along the fibril direction and the effect of the IFLs and matrix are predicted and compared. Results suggest presence of IFLs would increase the effective modulus in compression. This is due to maintaining organization of the fibrils into a network due to IFLs imparting stability to the network by preventing early bending of fibrils and effectively reducing the Poisson effect. Finally, with a set of literature based parameters, compression tests for AC using the mesomodel show that removing the cross-links results in a significant (43%) drop in the effective compressive modulus, suggesting resolution necessary to experimentally detect the IFLs. At the mesoscale, the IFLs would play the mechanical role of stabilizing the fibril network and enhancing its stiffness.
Force control of a highly inertial specimen.
(2010-05) Saari, Byron John
Many servo-hydraulic control applications require high bandwidth realtime force tracking. Some specimen dynamics greatly limit the force bandwidth obtainable with standard PID loops. In these cases, the force control bandwidth is much lower than its position control counterpart. When the specimen includes resonant modes in the frequency range of interest, these resonant poles result in complex zeros in the open loop transfer function, which adversely affect the loop shape. These low frequency complex zeros are a consequence of the “highly inertial specimen” characteristics where the frequency of the zeros is inversely proportional to the square root of the mass. This paper presents a servo-hydraulic model to study this problem and design control laws which are implemented on an experimental test bench. An evaluation of a dynamic inversion controller shows a lack in robustness to model uncertainties. Included in this paper is the design of an H-infinity loop shaping controller which exhibits high gain disturbance rejection and robust stability with an order of magnitude improvement in the force reference tracking bandwidth. The controller also includes an inverse model prefilter, which increases the force reference tracking bandwidth higher than the displacement control (without pre-filter) tracking bandwidth.
Improved Yaw Control Strategies Using LIDAR Preview and Alternative Algorithms
(2012-04-18) Carlson, Caleb
Large Eddy Simulation of crashback in marine propulsors.
(2011-06) Jang, Hyunchul
Crashback is an operating condition to quickly stop a propelled vehicle, where the propeller is rotated in the reverse direction to yield negative thrust. The crashback condition is dominated by the interaction of the free stream flow with the strong reverse flow. This interaction forms a highly unsteady vortex ring, which is a very prominent feature of crashback. Crashback causes highly unsteady loads and flow separation on the blade surface. The unsteady loads can cause propulsor blade damage, and also affect vehicle maneuverability. Crashback is therefore well known as one of the most challenging propeller states to analyze. This dissertation uses Large-Eddy Simulation (LES) to predict the highly unsteady flow field in crashback. A non-dissipative and robust finite volume method developed by Mahesh et al. (2004) for unstructured grids is applied to flow around marine propulsors. The LES equations are written in a rotating frame of reference. The objectives of this dissertation are: (1) to understand the flow physics of crashback in marine propulsors with and without a duct, (2) to develop a finite volume method for highly skewed meshes which usually occur in complex propulsor geometries, and (3) to develop a sliding interface method for simulations of rotor-stator propulsor on parallel platforms. LES is performed for an open propulsor in crashback and validated against experiments performed by Jessup et al. (2004). The LES results show good agreement with experiments. Effective pressures for thrust and side-force are introduced to more clearly understand the physical sources of thrust and side-force. Both thrust and side-force are seen to be mainly generated from the leading edge of the suction side of the propeller. This implies that thrust and side-force have the same source - the highly unsteady leading edge separation. Conditional averaging is performed to obtain quantitative information about the complex flow physics of high- or low- amplitude events. The events for thrust and side force show the same tendency. The conditional averages show that during high amplitude events, the vortex ring core is closer to the propeller blades, the reverse flow induced by the propeller rotation is lower, the forward flow is higher at the root of the blades, and leading and trailing edge flow separations are larger. The instantaneous flow field shows that during low amplitude events, the vortex ring is more axisymmetric and the stronger reverse flow induced by the vortex ring suppresses the forward flow so that flow separation on the blades is smaller. During high amplitude events, the vortex ring is less coherent and the weaker reverse flow cannot overcome the forward flow. The stronger forward flow makes flow separation on the blades larger. The effect of a duct on crashback is studied with LES. Thrust mostly arises from the blade surface, but most of side-force is generated from the duct surface. Both mean and RMS of pressure are much higher on inner surface of duct, especially near blade tips. This implies that side-force on the ducted propulsor is caused by the blade-duct interaction. Strong tip leakage flow is observed behind the suction side at the tip gap. The physical source of the tip leakage flow is seen to be the large pressure difference between pressure and suction sides. The conditional average for high amplitude event shows consistent results; the tip leakage flow and pressure difference are significantly higher when thrust and side-force are higher. A sliding interface method is developed to allow simulations of rotor-stator propulsor in crashback. The method allows relative rotations between different parts of the computational grid. Search algorithm for sliding elements, data structures for message passing, and accurate interpolation scheme at the sliding interface are developed for arbitrary shaped unstructured grids on parallel computing platforms. Preliminary simulations of open propulsor in crashback show reasonable performance.
Linear and nonlinear analysis of susceptibility of F/A-18 flight control laws to the falling leaf mode.
(2010-05) Chakraborty, Abhijit
The F/A-18 Hornet aircraft with the original flight control law exhibited an out-of-control phenomenon known as the falling leaf mode. The falling leaf mode went undetected during the validation and verification stage of the flight control law. Several F/A-18 Hornet aircraft were lost due to the falling leaf mode and this led NAVAIR and Boeing to redesign the flight control law. The revised flight control law exhibited successful suppression of the falling leaf mode during flight tests with aggressive maneuvers. Prior to performing expensive flight tests, the flight control law is extensively validated and verified by performing linear robustness analysis at different trim points and running many Monte-Carlo simulations. Additional insight can be gained by using nonlinear analyses. This thesis compares the two flight control laws using standard linear robustness analyses, nonlinear region-of-attraction analyses and Monte Carlo simulations. The classical linear robustness analyses, i.e. gain and phase margin, does not indicate any significant improvement in robustness properties of the revised control law over the baseline design. However, advanced linear robustness analyses, i.e, the #22; and worst-case analysis, indicate that the revised design is better able to handle the cross-coupling and variations in the dynamics than the baseline design. However, it can be difficult to interpret these results since the falling leaf motion is a truly nonlinear dynamical phenomenon. Thus nonlinear analyses tools provide useful insight into the susceptibility of both control laws to the falling leaf motion. The results of the nonlinear analyses indicate that the revised flight control law has considerably better stability properties than the baseline design and less susceptible to the falling leaf motion.
A method of simulating fluid structure interactions for deformable decelerators.
(2010-11) Gidzak, Vladimyr Mykhalo
A method is developed for performing simulations that contain fluid-structure interactions between deployable decelerators and a high speed compressible flow. The problem of coupling together multiple physical systems is examined with discussion of the strength of coupling for various methods. A non-monolithic strongly coupled option is presented for fluid-structure systems based on grid deformation. A class of algebraic grid deformation methods is then presented with examples of increasing complexity. The strength of the fluid-structure coupling is validated against two analytic problems, chosen to test the time dependent behavior of structure on fluid interactions, and of fluid on structure interruptions. A one-dimentional material heating model is also validated against experimental data. Results are provided for simulations of a wind tunnel scale disk-gap-band parachute with comparison to experimental data. Finally, a simulation is performed on a flight scale tension cone decelerator, with examination of time-dependent material stress, and heating.

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