Academic literature on the topic 'Unsteady mode'

Due to concerns about the impacts of carbon emissions on the environment, the security of supply of electricity and the likelihood of achieving "peak-oil" in the near future, governments have legislated to reduce reliance on fossil fuels. An attractive alternative is power obtained from tidal currents, and the coast of the British Isles is especially hydraulically active. Tidal energy converters typically resemble wind turbines however, unlike wind turbines, they are expected to operate in an environment which is singularly hostile, and will also be expected to generate power in non-ideal operating conditions. This thesis is concerned with the ability to model individual and groups of tidal devices including their mutual interactions. The ability to capture unsteady inflow conditions at realistic array spacing requires preservation of turbine wakes over a sufficiently large range at spatial resolutions and over time durations which are not feasible using standard computational fluid dynamics software. This thesis has combined methodologies developed for helicopter wake modelling with techniques used in naval architecture for modelling thick maritime propellers into a computational tool. The particular formulation of the Navier-Stokes equations employed allows the determination of the unsteady pressure and force distributions on a turbine rotor due to the effects of a neighbouring device, even if it is operating some significant distance upstream. The constituents of the method of this thesis are developed and applied to "proof-of-principle" studies. These include flow past static and oscillating 2-D aerofoils and past a 3-D wing, wind turbine and tidal turbine configuration. The results from these studies demonstrate that the model is convergent and capable of capturing the time dependant forces on these devices, and by comparison with analytical or experimental results, or via inter-model comparison begins the process of calibration and validation of the model. The method is then applied to flow past groups of turbines in various array configurations, and a coaxial, contra-rotating device. The outcome of this work is a decision making tool which can be used to improve success and reduce risk in tidal power array planning, optimise device configurations and is translatable back into rotorcraft or naval architecture usage.

A method for predicting unsteady, subsonic aeroelastic responses has been developed. The technique accounts for aerodynamic nonlinearities associated with angles of attack, vortex-dominated flow, static deformations, and unsteady behavior. The angle of attack is limited only by the occurrence of stall or vortex bursting near the wing. The fluid and the wing together are treated as a single dynamical system, and the equations of motion for the structure and flowfield are integrated simultaneously and interactively in the time domain. The method employs an iterative scheme based on a predictor-corrector technique. The aerodynamic loads are computed by the general unsteady vortex-lattice method and are determined simultaneously with the motion of the wing. Because the unsteady vortex-lattice method predicts the wake as part of the solution, the history of the motion is taken into account; hysteresis is predicted. Two models are used to demonstrate the technique: a rigid wing on an elastic support experiencing plunge and pitch about the elastic axis, and an elastic wing rigidly supported at the root chord experiencing spanwise bending and twisting. The method can be readily extended to account for structural nonlinearities and/or substitute aerodynamic load models. The time domain solution coupled with the unsteady vortex-lattice method provides the capability of graphically depicting wing and wake motion.<br>Ph. D.

This study is concerned with the recovery of fluid mechanical modes that can be used to describe the physical properties of unsteady separated flows. The flow configuration of interest is a spanwise homogeneous NACA 0015 airfoil with leading edge laminar separation and turbulent recirculation. An in-depth understanding of the unsteady flow dynamics and fluid mechanical stability properties, can assist in the future development of more efficient separation control strategies. In order to provide a richer understanding of the physics, the flow fields are numerically generated, and characterised at various key Reynolds numbers leading up to the target turbulent case. Proper Orthogonal Decomposition modes are recovered to most efficiently represent the unsteady scales of motion, and linear stability modes are sought to identify how a perturbation will evolve in this unsteady environment. The generation of the Proper Orthogonal Decomposition modes can require very large amounts of data, and the current study presents a means of recovering these modes using parallel computation. To enable the stability analysis, a means of performing the calculation in steady two-dimensional flows of semi-complex geometry has been developed. The corrections required to perform the stability analysis in unsteady turbulent flows has also been identified by using a non-linear eddy viscosity model to close the triple decomposition stability equations. It is intended that the means of recovering these fluid mechanical modes can assist in the future development of reduced order models necessary for the control of unsteady separated flows<br>Cette étude s’intéresse à la détermination de modes pouvant être utilisés en mécanique des fluides pour décrire les propriétés physiques d'écoulements instationnaires décollés. La configuration d'écoulement qui nous intéresse est un profil d'aile NACA 0015 transversalement homogène caractérisé par un décollement laminaire au bord d'attaque et une zone de recirculation turbulente. Comprendre en profondeur la dynamique instationnaire de l'écoulement et ses propriétés de stabilité peut aider à améliorer l'efficacité de futures stratégies de contrôle de décollement. Afin de mieux appréhender la physique, l'écoulement est d’abord simulé puis caractérisé pour plusieurs valeurs du nombre de Reynolds allant jusqu’au régime turbulent. On retrouve alors que les modes obtenus par décomposition orthogonale aux valeurs propres (Proper Orthogonal Decomposition) représentent de manière efficace les échelles instationnaires du mouvement. Par ailleurs, les modes de stabilité linéaire sont recherchés afin d'identifier comment une perturbation évolue dans un environnement instationnaire. La détermination des modes de Proper Orthogonal Decomposition pouvant nécessiter une grande quantité de données, cette étude présente un moyen de les évaluer par calcul parallèle. Pour permettre l'analyse de stabilité, il a fallu développer des programmes permettant de réaliser les calculs pour un écoulement stationnaire bidimensionnel en géométrie semi-complexe. Les corrections nécessaires pour effectuer l'analyse de stabilité dans des écoulements turbulents instationnaires ont aussi été identifiés en utilisant un modèle de viscosité tourbillonnaire non linéaire pour fermer les équations de stabilité en décomposition triple. La détermination de ces modes en mécanique des fluides doit aider le développement futur de modèles réduits nécessaires au contrôle d'écoulement instationnaire décollé

A new unsteady rotor/fuselage interactional aerodynamics model has been developed. This model loosely couples a Generalized Dynamic Wake Theory (GDWT) to a Navier-Stokes solution procedure. This coupling is achieved using a newly developed unsteady pressure jump boundary condition in the Navier-Stokes model. The new unsteady pressure jump boundary condition models each rotor blade as a moving pressure jump which travels around the rotor azimuth =and is applied between two adjacent planes in a cylindrical, non-rotating grid. Comparisons are made between predictions using this new model and experiments for an isolated rotor and for a coupled rotor/fuselage configuration.<br>Ph. D.

Thesis (M.S. in Mechanical Engineering)--Naval Postgraduate School, June 2009.<br>Thesis Advisor(s): Chandrasekhara, M. S. "June 2009." Description based on title screen as viewed on July 10, 2009. Author(s) subject terms: Unsteady Aerodynamics, UCAV Maneuvers, 2D-unsteady flows. Includes bibliographical references (p. 43-44). Also available in print.

The present study focuses on the sediment deposition and consequent dredging issues in Lower Ohio River at the Olmsted Locks and Dam area-River mile (RM)-964.4 during the ongoing in-the-wet construction methodology. The study reach is between Locks and Dam 53 (RM 962.6) at upstream, and RM 970 at downstream. One dimensional (1-D) HEC-RAS numerical modeling in conjunction with Arc-GIS was employed. Stream flow measurements, velocity, incoming sediment concentration, bed gradation, and annual hydrographic survey data acquired from public archives of USGS and USACE Louisville District were used as inputs. The model was subjected to the 1-D quasi-unsteady and completely unsteady sediment transport module, available in the latest HEC-RAS 5.0 Beta release. Calibration and validation of the hydrodynamic and sediment models were performed using measured water surface elevation, velocity, and sediment loads at measured sections. Post-model calibration and validation, deposition to excavated cross-sections for future dam shells at Olmsted was predicted, which warrants dredging. The study attempted to analyze the sediment transport trend with the focus on depositionat Olmsted Locks and Dam area using the sensitivity analysis approach of transport capacity functions. Moreover, the capability of 1-D HEC-RAS quasi-unsteady and completely unsteady models were assessed in prediction of sediment deposition in the construction area (dam shells excavation area). A temporal deposition prediction model was developed that can potentially replace the current ad-hoc approach used to determine the dredging schedule. Likewise, a representative environmental risk associated with sedimentation in the study area was examined. The model can potentially be used as a decision support tool to analyze the long term impact of sedimentation in the vicinity of Olmsted Locks and Dam if further updates on the river bathymetry, and specific field data are supplemented to the model.

Simulating the complex physics and dynamics associated with unsteady aeroelastic systems is often attempted with high-fidelity numerical models. While these high-fidelity approaches are powerful in terms of capturing the main physical features, they may not discern the role of underlying phenomena that are interrelated in a complex manner. This often makes it difficult to characterize the relevant causal mechanisms of the observed features. Besides, the extensive computational resources and time associated with the use these tools could limit the capability of assessing different configurations for design purposes. These shortcomings present the need for the development of simplified and reduced-order models that embody relevant physical aspects and elucidate the underlying phenomena that help in characterizing these aspects. In this work, different fluid and aeroelastic systems are considered and reduced-order models governing their behavior are developed. In the first part of the dissertation, a methodology, based on the method of multiple scales, is implemented to show its usefulness and effectiveness in the characterization of the physics underlying the system, the implementation of control strategies, and the identification of high-impact system parameters. In the second part, the unsteady aerodynamic aspects of flapping micro air vehicles (MAVs) are modeled. This modeling is required for evaluation of performance requirements associated with flapping flight. The extensive computational resources and time associated with the implementation of high-fidelity simulations limit the ability to perform optimization and sensitivity analyses in the early stages of MAV design. To overcome this and enable rapid and reasonably accurate exploration of a large design space, a medium-fidelity aerodynamic tool (the unsteady vortex lattice method) is implemented to simulate flapping wing flight. This model is then combined with uncertainty quantification and optimization tools to test and analyze the performance of flapping wing MAVs under varying conditions. This analysis can be used to provide guidance and baseline for assessment of MAVs performance in the early stages of decision making on flapping kinematics, flight mechanics, and control strategies.<br>Ph. D.

The simulation tool ROBOOST, in use at the Alma Propulsion Lab of the University of Bologna – Forlì Campus, exploits a hybrid ballistic model 0D-1D. The need of a complete Q1D model for the entire combustion time, from motor start-up to burn out arised. The present work is devoted to the development and test of a Q1D unsteady ballistic model for solid rocket motors performance prediction. The newly developed code, called SOL1D, is written in Matlab environment and is capable of predicting the time and space evolution of all the main thermodynamic variables during the solid rocket motor combustion process. The model has been tested and validated on a BARIA motor, thus demonstrating its adherence to experimental data. SOL1D paves the way for future works aimed at simulating performances of actual launchers.

Steady and unsteady flow over a generic Suboff submarine model is studied. The skin-friction magnitudes are measured by using hot-film sensors each connected to a constant temperature anemometer. The local minima in the skin-friction magnitudes are used to obtain the separation locations. Steady static pressure measurements on the model surface are performed at 10° and 20° angles of attack. Steady and unsteady results are presented for two model configurations: barebody and sail-on-side case. The dynamic plunge-pitch-roll model mount (DyPPiR) is used to simulate the pitchup maneuvers. The pitchup maneuver is a linear ramp from 1° to 27° in 0.33 seconds. All the tests are conducted at ReL=5,500,000 with a nominal wind tunnel speed of 42.7±1 m/s. Steady results show that the flow structure on the leeward side of the barebody can be characterized by the crossflow separation. In the sail-on-side case, the separation pattern of the non-sail region follow the barebody separation trend closely. The flow on the sail side is strongly affected by the presence of the sail and the separation pattern is different from the crossflow separation. The flow in the vicinity of the sail-body junction is dominated by the horseshoe type separation. Unsteady results of the barebody and the non-sail region of the sail-on-side case show significant time lags between unsteady and steady crossflow separation locations. These effects produce the difference in separation topology between the unsteady and steady flowfields. A first-order time lag model approximates the unsteady separation locations reasonably well and time lags are obtained by fitting the model equation with the experimental data. The unsteady separation pattern of the sail side does not follow the quasi-steady data with a time lag and the unsteady separation structure is different from the unsteady crossflow separation topology observed for the barebody and the non-sail region of the sail-on-side case.<br>Master of Science