Repository of works by Caltech published authors.
Type = Conference or Workshop Item
Mostrando recursos 1 - 20 de 464
Decoupled formal synthesis for almost separable systems with temporal logic specifications - Livingston, Scott C.; Prabhakar, Pavithra
We consider the problem of synthesizing controllers automatically for distributed robots that are loosely coupled using a formal synthesis approach. Formal synthesis entails construction of game strategies for a discrete transition system such that the system under the strategy satisfies a specification, given for instance in linear temporal logic (LTL). The general problem of automated synthesis for distributed discrete transition systems suffers from state-space explosion because the combined state-space has size exponential in the number of subsystems. Motivated by multi-robot motion planning problems, we focus on distributed systems whose interaction is nearly decoupled, allowing the overall specification to be decomposed...
Flare-Induced Interaction Lengths in Supersonic, Turbulent Boundary Layers - Roshko, A.; Thomke, G. J.
Experimental results are presented for the effects of Mach number, Reynolds number, and corner angle on flare-induced separation of a supersonic, turbulent boundary layer. In particular, measurements were obtained for the variation with flare angle, α, of the ratio ℓ_0/δ_0 of the upstream interaction length to the boundary-layer thickness at the beginning of the interaction for Mach numbers 2≤M≤4.5, boundary-layer thickness Reynolds numbers 10^5 < R_δ < 10^6, and adiabatic wall conditions. The model consisted of a hollow cylinder of 12-in. diameter and 51-in. length. Flares of angle 9°≤α≤40° were attached to the cylinder model at either of two location,...
A two-layer calculation for the initial interaction region of an unseparated supersonic turbulent boundary layer with a ramp - Rosen, R.; Roshko, A.; Pavish, D. L.
The interaction of an unseparated supersonic turbulent boundary layer with a compression corner produces an extremely rapid rise in pressure at the corner, followed by a more gradual increase to the final pressure. In this paper, the flow in the corner region is analyzed by an integral method with the objective of predicting the initial pressure rise. Comparisons with experimental pressure rise data are presented for cases covering supersonic and hypersonic flows of practical interest. Also presented are some calculations and comparisons of downstream pressure distributions obtained by using the predicted corner rise as the first point in an existing...
Structure of turbulent shear flows - A new look - Roshko, Anatol
For many years experimental research in turbulence was devoted to the measurement of various correlations and special functions which had evolved from the statistical theories and from engineering computing methods based on the hierarchy of Reynolds equations. A recent change in direction toward a more deterministic description of turbulent structure has been initiated by the discovery of large coherent structures in several turbulent shear flows. The new point of view suggests that with every shear flow (jet, boundary layer, mixing layer, etc.) is associated an identifiable, characteristic structure; the development of the flow is controlled by the interactions of these...
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...