Repository of works by Caltech published authors.
Type = Conference or Workshop Item
Mostrando recursos 1 - 20 de 454
Effects of a gain-based optimal forcing on turbulent channel flow - Sharma, A.; Moarref, R.; Luhar, M.; Goldstein, D. B.; McKeon, B. J.
The effects of introducing a two- or three-dimensional streamwise traveling wave type body forcing in a turbulent channel flow at Re_τ = 180 are investigated using direct numerical simulations (DNS). The optimal forcing shape (that is, the forcing that leads to the most amplified or
highest-gain velocity response) is obtained from the resolvent analysis of McKeon & Sharma (2010) for wave-numbers and wave-speed representative of the near-wall cycle. The velocity response due to imposed forcing obtained from DNS is found to agree well with resolvent
analysis predictions at small forcing amplitude. The changes in mean velocity, shear stress and kinetic energy are...
Scaling in Wall Turbulence: Scale Separation and Interaction (Invited Paper) - McKeon, B. J.
High Reynolds number pipe flow data are used to demonstrate the importance of several conditions related to scale separation that are either assumed in the classical theories
or may be used in light of recent results in wall turbulence to infer a minimum Reynolds number condition above which scaling results may be suitable for extrapolation. Results from the Princeton Superpipe have suggested Re_τ > 5000 as the minimum Reynolds number
for which key properties of pipe flow reach a “fully-developed” condition, based on observations of streamwise mean and turbulent velocity structure. Additional values related to finer constraints on the structural development are...
Subsonic Boundary-Layer Wavefront Spectra for a Range of Reynolds Numbers - Smith, Adam E.; Gordeyev, Stanislav; Saxton-Fox, Theresa; McKeon, Beverley
Aero-optic measurements of turbulent boundary layers were performed in wind tunnels at the University of Notre Dame and California Institute of Technology for heated walls at a
range of Reynolds numbers. Temporally resolved measurements of wavefronts were collected at a range of Mach numbers between 0.03 and 0.4 and the range of Re_θ between
1,700 and 20,000. Wavefront spectra for both heated and un-heated walls were extracted and compared to demonstrate that wall heating does not noticeably alter the shape of
wavefront spectra in the boundary layer. The effect of Reynolds number on the normalized spectra was also presented, and an empirical spectral...
Perturbation Energy Production in Pipe Flow over a Range of Reynolds Numbers using Resolvent Analysis - Sharma, A. S.; McKeon, B. J.
The response of pipe flow to physically realistic, temporally and spatially continuous(periodic) forcing is investigated by decomposing the resolvent into orthogonal forcing and response pairs ranked according to their contribution to the resolvent 2-norm. Modelling the non-linear terms normally neglected by linearisation as unstructured forcing permits qualitative extrapolation of the resolvent norm results beyond infinitesimally small perturbations to the turbulent case. The concepts arising have a close relationship to input output transfer function analysis methods known in the control systems literature. The body forcings that yield highest disturbance energy gain are identified and ranked by the decomposition and a worst-case...
A Quadratic Programming Approach to Quasi-Static Whole-Body Manipulation - Shankar, Krishna; Burdick, Joel W.; Hudson, Nicolas H
This paper introduces a local motion planning method for robotic systems with manipulating limbs, moving bases (legged or wheeled), and stance stability constraints arising from the presence of gravity. We formulate the problem of selecting local motions as a linearly constrained quadratic program (QP), that can be solved efficiently. The solution to this QP is a tuple of locally optimal joint velocities. By using these velocities to step towards a goal, both a path and an inverse-kinematic solution to the goal are obtained. This formulation can be used directly for real-time control, or as a local motion planner to connect...
Application of a laser induced fluorescence model to the numerical simulation of detonation waves - Mével, Rémy; Davidenko, Dmitry; Pintgen, Florian; Austin, Joanna; Shepherd, Joseph
Although numerical simulations have become an essential tool for detonation dynamics understanding, most of the
physical constraints that exist within experiments are not accounted for which makes difficult a comparison. In the
present study, a simple laser induced fluorescence model has been implemented and used to post-treat detonation wave
numerical simulation results and allow a direct comparison with previous experimental visualizations of detonations in
hydrogen-oxygen-diluent mixtures. The model is first applied to steady one-dimensional simulation results obtained with
detailed chemistry. The effects on the fluorescence intensity of the model parameters are examined to underline the
dominant processes. The model is then applied to unsteady two-dimensional simulation...
On network coding and routing in dynamic wireless multicast networks - Ho, Tracey; Jin, Jia-Qi
We compare multicast network coding and routing
for a time-varying wireless network model with interference-
determined link capacities. We use dynamic back pressure
algorithms that are optimal for intra-session network coding
and routing respectively. Our results suggest that under such conditions, the gap in multicast capacity between network coding and routing can decrease relative to a collision-based wireless model with fixed link capacities.
Dry deposition of nitrogen containing species - McRae, Gregory J.; Russell, Armistead G.
Nitrogen oxides (NO_x) emissions and the oxidation products formed by photochemical interactions in the atmosphere are responsible for a significant fraction of both dry and wet acid deposition fluxes. In his paper a vertically-resolved, Lagrangian trajectory model is used to predict the diurnal variation of: NO, NO_2, NO_3, HONO, HONO_2, HO_2NO_2, RONO, RONO_2, RO_2NO_2, N_2O_5 and PAN over an urban airshed. Particular attention is given to the fate of nitric acid and its reaction with gaseous ammonia to form, aerosol phase, ammonium nitrate. A simple model for estimating the deposition fluxes of these species is also presented. A study of...
Stresses in Ytterbium Silicate Multilayer Environmental Barrier Coatings - Stolzenburg, F.; Almer, J.; Lee, K. N.; Harder, B. J.; Faber, K. T.
The internal stresses of plasma-sprayed multilayer ytterbium disilicate environmental barrier
coatings were measured using microfocused high-energy X-rays in a transmission geometry.
Stresses were measured for as-sprayed and ex-situ heat-treated ytterbium disilicate topcoats at
room temperature and during in-situ heating and cooling experiments. In-situ loading
experiments were also performed on the topcoat in order to establish its elastic constants. The
ytterbium disilicate was found to have a relatively low coefficient of thermal expansion resulting
in compressive stresses of approximately 100 MPa throughout the topcoat. In-situ heating
experiments revealed a statistically significant stress relaxation in the ytterbium disilicate topcoat
upon thermal cycling to temperatures above 1300°C, indicating the onset of...
Turbulence and Diffusion as Factors in Sediment Transportation - Vanoni, Vito A.; Hsu, En-Yun
The importance of turbulence in the transportation of sediment has long been recognized by hydraulic engineers and research workers in fluid flow. The turbulence contains the cross components of flow that transport or diffuse sediment from the bed into the main body of the stream. Without this turbulent motion no sediment could be picked up from the bed and there would be no muddy
streams. Transport would be limited to the small amount resulting from the rolling and sliding of particles at the bed under the action of viscous shear.
Speed faults in computation by chemical reaction networks - Chen, Ho-Lin; Cummings, Rachel; Doty, David; Soloveichik, David
Chemical reaction networks (CRNs) formally model chemistry in a well-mixed solution. Assuming a fixed molecular population size and bimolecular reactions, CRNs are formally equivalent to population protocols, a model of distributed computing introduced by Angluin, Aspnes,
Diamadi, Fischer, and Peralta (PODC 2004). The challenge of fast computation by CRNs (or population protocols) is to ensure that there is never a bottleneck "slow" reaction that requires two molecules (agent states) to react (communicate), both of which are present in low (O(1))
counts. It is known that CRNs can be fast in expectation by avoiding slow reactions with high probability. However, states may be...
Real Fluid Effects on an Accelerated Sphere Before Boundary-Layer Separation - Mellsen, S. B.; Waugh, J. G.; Ellis, A. T.
Studies were made on the apparent increase in mass on acceleration (added mass) of a sphere accelerated from rest and before boundary-layer separation, in cylinders
of various diameters filled with water or oil. From a comparison of theoretical and experimentally obtained added masses, the following conclusions were drawn: In the
absence of wall effects on the boundary layer, the wall shear stress over elements of the sphere can be approximated by the solution for the flat plate moving parallel to itself and the potential flow over the elements outside the boundary layer. The impulse on the elements is obtained by integration with...
Note on the Application of Cascade Theory to Design of Axial-Flow Pumps - Linhardt, H. D.; Acosta, A. J.
Theoretical and experimental results are presented which reassure the usefulness of two-dimensional cascade theories to the design of axial flow pumps. For this purpose it is necessary to include the effect of the blade thickness upon the impeller flow which has been found to be
responsible for reported discrepancies between predictions of thin airfoil theories and the performance of axial-flow pumps characterized by high stagger angle and low aspect ratio.