Repository of works by Caltech published authors.
Type = Conference or Workshop Item
Mostrando recursos 1 - 20 de 460
Transmission of thin light beams through turbulent mixing layers - Wissler, John B.; Roshko, Anatol
This research investigates the effects of spanwise and streamwise coherent structures in a turbulent mixing layer on the deflection of a thin light beam which is transmitting transversely through the mixing layer from the high-speed side to the low speed side. Both equal and unequal density mixing layers of varying pressures and velocities are studied, using a lateral effect detector to dynamically track the motion of a He-Ne laser beam. Beam deflections in the streamwise direction are found to be associated mainly with the spanwise coherent structures; at low Reynolds Numbers the beam deflection is directly related to the part...
Effects of periodic forcing on mixing in turbulent shear layers and wakes - Roberts, F. A.; Roshko, A.
The effects of a periodic disturbance, applied to the one of the the free streams, on large-scale structure and mixing processes in chemically reacting
shear layers and wakes were investigated over a range of Reynolds numbers above and below the mixing transition range. Two different methods were employed to measure the amount of chemical product and thus the extent of molecular-scale mixing. Absorption by reacted phenolphthalein provided cross-stream average product thickness and laser induced fluorescence intensity provided the product concentration distribution. These methods provided, in addition, effective flow visualization of the large-scale structures and of their response to the periodic forcing....
An experimental study of the near wake of a two-dimensional hypersonic blunt body with mass addition - Collins, Donald J.; Lees, Lester; Roshko, Anatol
An experimental investigation of the steady, laminar near-wake flow field of a two-dimensional, adiabatic, circular cylinder with surface mass transfer has been made at a free-stream Mach number of 6.0. The pressure and mass-concentration fields associated with the transfer of argon, nitrogen or helium into the near wake were studied for mass transfer from the forward stagnation region, and from the base. For sufficiently low mass transfer rates from the base, for which a recirculating zone exists, the entire near-wake flow field correlates with the momentum flux, not the mass flux, of the injectant, and the mass-concentration field is determined...
The uses of coherent structure (Dryden Lecture) - Coles, Donald
The concept of coherent structure in turbulent flow is a revolutionary idea which is being developed by evolutionary means. The main objective of this review is to list some solid achievements, showing what can be done by using the concept of coherent structure that cannot be done without it. The nature of structure is described in terms of some related concepts, including celerity,
topology, and the phenomenon of coalescence and splitting of structure. The main emphasis is on the mixing layer, as the one flow whose structure is well enough understood so that technical applications are now being made in problems...
Low-Effort Specification Debugging and Analysis - Ehlers, Rüdiger; Raman, Vasumathi
Reactive synthesis deals with the automated construction of implementations of reactive systems
from their specifications. To make the approach feasible in practice, systems engineers need effective
and efficient means of debugging these specifications. In this paper, we provide techniques for
report-based specification debugging, wherein salient properties of a specification are analyzed, and
the result presented to the user in the form of a report. This provides a low-effort way to debug
specifications, complementing high-effort techniques including the simulation of synthesized implementations.
We demonstrate the usefulness of our report-based specification debugging toolkit by
providing examples in the context of generalized reactivity(1) synthesis.
Flying-hot-wire study of two-dimensional mean flow past an NACA 4412 airfoil at maximum lift - Coles, Donald; Wadcock, Alan J.
Hot-wire measurements have been made in the boundary layer, the separated region, and the near wake for flow past an NACA 4412 airfoil at maximum lift. The Reynolds number based on chord was about 1,500, 000. Special care was taken to achieve a two-dimensional mean flow. The main instrumentation was a flying hot wire; that is, a hot-wire probe mounted on the end of a rotating arm. The probe velocity was sufficiently high to avoid the usual rectification problem by keeping the relative flow direction always within a range of ±30
degrees to the probe axis. A digital computer was used...
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.