Abstract: The technique of large-eddy simulation (LES) continues to play an important role in the numerical simulation of fluid dynamic processes in engineering and scientific applications. This paper will review and discuss a few of the various schemes for LES applied to the incompressible Navier-Stokes equations and to the scalar advection-diffusion equation. In particular, subgrid models based on deconvolution will be discussed. An interesting connection between the tensor diffusivity model, a particular version of a deconvolution model, and Lagrangian particle methods for the vorticity transport equation and for the scalar convection-diffusion equation will be explored. In addition, the possibility of using super-resolution, i.e. recovering fine-scale information knowing only coarse-scale information, in LES will be investigated.

ID: CaltechAUTHORS:20160930-083031310

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Abstract: A simple low-order model is derived for developing flight control laws for controlling the longitudinal dynamics of an aircraft using synthetic jet type actuators. Bi-directional changes in the pitching moment over a range of angles of attack are effected by controllable, nominally-symmetric trapped vorticity concentrations on both the suction and pressure surfaces near the trailing edge. Actuation is applied on both surfaces by hybrid actuators that are each comprised of a miniature obstruction integrated with a synthetic jet actuator to manipulate and regulate the vorticity concentrations. In previous work, a simple model was derived from a reduced order vortex model that includes one explicit nonlinear state for fluid variables and can be easily coupled to the rigid body dynamics of an aircraft. This paper further simplifies this model for control design. The control design is based on an output feedback adaptive control methodology that illustrates the effectiveness of using the model for achieving flight control at a higher bandwidth than achievable with typical static actuator assumptions. A unique feature of the control design is that the control variable is a pseudo-control based on regulating a control vortex strength. Wind tunnel experiments on a unique dynamics traverse verify that tracking performance is indeed better than control designs employing standard actuator modeling assumptions.

ID: CaltechAUTHORS:20191008-111635395

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Abstract: A simple low-order model is derived to determine the flow forces and moments on an airfoil that arbitrarily pitches and plunges with the presence of synthetic jet actuation for the use in an adaptive closed-loop control scheme. The low-order model captures the attached flow response of an airfoil in the presence of synthetic jet actuators near the trailing edge. The model includes two explicit non-linear states for fluid variables and can be easily coupled to the rigid body dynamics of the system. The model is validated with high fidelity numerical simulations and experiments. The low-order model agreement with experiments is good for low reduced frequency pitching. The agreement to numerical simulations is also good for reduced frequencies that are an order of magnitude higher than those attainable in experiments.

ID: CaltechAUTHORS:20200324-125732644

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Abstract: The evolution of initially weak structures of vorticity as they evolve in an incompressible turbulent flow is investigated. Such objects are candidates for important structures in the inertial range and in the dissipation range of scales. As these structures are strained by the flow, fine-scales of vorticity are produced along the direction of maximum compression with a consequent flow of energy to the high wavenumbers. It is shown that, under certain circumstances, the self-energy spectrum of such a structure may be time-averaged, producing a fractional power law. The exponent of the power law depends on the ratio of the first two Lyapunov exponents of the strain tensor.

No.: 71
ID: CaltechAUTHORS:20200304-094728369

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Abstract: Recent developments of the 3-D Lagrangian vortex element method for bluff body flows are presented. In this approach attached boundary layer regions are modelled using infinitely thin vortex sheets while Lagrangian vortex elements are used for the separation regions and the wake. Preliminary results for the flow past a simplified generic truck geometry are presented. Further developments, aimed at the development of a hybrid Eulerian-Lagrangian solver, are briefly introduced.

No.: 19 ISSN: 1613-7736

ID: CaltechAUTHORS:20200205-150626934

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Abstract: An experimental technique is introduced for deducing the unsteady fluid forces, F(t), on elastically vibrating structures using their oscillation trace y(t) , and the free stream velocity U. This new technique involves an accurate modeling of the elastic structure as an ordinary differential equation and employing time-dependent oscillation traces, y(t) and/or the acceleration a(t), to extract the side force on the structure. Filtering the oscillation signals and modeling the non-linear damping terms in the structural equation turn out to be the challenges of this technique. This technique may be applied in force studies relating vortex patterns and the the unsteady forces as well as methods for predicting or modeling fluid forces and amplitudes. Although the results are not compared with any independent technique, the unsteady lift coefficient traces show high values of above three as observed by other researchers.

ID: CaltechAUTHORS:20130930-085939898

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Abstract: The flow around two leap-frogging vortex rings is studied using the dynamical systems approach with a view to estimating the mixing taking place. The controlling parameter is the ratio of the initial radii of the rings to the axial distance between them. Small initial distances correspond to very little mixing, with interesting dynamical features in the region close to the rings. As the initial distance increases, the mixing region increases rapidly upto some point, and for larger distances the level of mixing remains approximately the same.

No.: 515
ID: CaltechAUTHORS:20200715-091033504

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Abstract: I have a rather provocative question to ask the speakers. What I am wondering is: When you apply the proper orthogonal decomposition - I know this is a procedure you use and it is probably standard - you remove the mean flow and you only look at the perturbations. Why do not you include the mean flow, because if you did you would suppress the cubic terms, and also you would be getting something that would be closer to what people call coherent structures, such as hairpin vortices.

No.: 357
ID: CaltechAUTHORS:20141201-140640640

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Abstract: Our task is to describe recent progress and present prospects for research in turbulence using numerical methods. This is quite a challenge, because we know that Liepmann has not been a strong advocate of computational turbulence, or "compulence," as it has sometimes been called -- the incompressible version presumably being "incompulence." We want to recognize several objectives in exploring turbulence by numerical methods, and to order these in priority as follows: 1. To gain understanding and insight into the physics of turbulence, so as to complement the insight obtained from experiments and analysis; for example, by helping the experimentalist to understand what it is that has been observed experimentally, and by helping the analyst to explore solution space in more detail. 2. To provide special "data" for guiding, evaluating, and calibrating simpler predictive models of turbulent flows. 3. To predict turbulent flows by numerical simulation.

No.: 320
ID: CaltechAUTHORS:20190605-093421211

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