Recursos de colección

Caltech Authors (157.532 recursos)

Repository of works by Caltech published authors.

Group = GALCIT

Mostrando recursos 1 - 20 de 132

  1. Analytical Models for the Thrust of a Rotating Detonation Engine

    Shepherd, J.E.; Kasahara, J.
    Two models are proposed for rotating detonation engine performance. The first model is motivated by models of pulse detonation engine performance which are based on the pressure-time history within the detonation tube. The present work extends those ideas to treat rotating detonation engines with a control volume analysis that considers the forces within the combustion chamber. The key scaling parameters for this model are the height of the reactant layer just ahead of the detonation wave and the computed Chapman-Jouguet pressure and velocity. The thrust can be estimated using these parameters and a simple functional form of the pressure history...

  2. Coherent structures, uniform momentum zones and the streamwise energy spectrum in wall-bounded turbulent flows

    Saxton-Fox, Theresa; McKeon, Beverley J.
    Large-scale motions (LSMs) in wall-bounded turbulent flows have well-characterised instantaneous structural features (Kovasznay et al., J. Fluid Mech., vol. 41 (2), 1970, pp. 283–325; Meinhart & Adrian, Phys. Fluids, vol. 7 (4), 1995, pp. 694–696) and a known spectral signature (Monty et al., J. Fluid Mech., vol. 632, 2009, pp. 431–442). This work aims to connect these previous observations through the development and analysis of a representative model for LSMs. The model is constructed to be consistent with the streamwise energy spectrum (Monty et al. 2009) and amplification characteristics of the Navier–Stokes equations (McKeon & Sharma, J. Fluid Mech., vol....

  3. Incompressible variable-density turbulence in an external acceleration field

    Gat, Ilana; Matheou, Georgios; Chung, Daniel; Dimotakis, Paul E.
    Dynamics and mixing of a variable-density turbulent flow subject to an externally imposed acceleration field in the zero-Mach-number limit are studied in a series of direct numerical simulations. The flow configuration studied consists of alternating slabs of high- and low-density fluid in a triply periodic domain. Density ratios in the range of 1.05 ⩽ R ≡ ρ_1/ρ_2 ⩽ 10 are investigated. The flow produces temporally evolving shear layers. A perpendicular density–pressure gradient is maintained in the mean as the flow evolves, with multi-scale baroclinic torques generated in the turbulent flow that ensues. For all density ratios studied, the simulations attain...

  4. Turbulent flow over a long flat plate with uniform roughness

    Pullin, D. I.; Hutchins, N.; Chung, D.
    For turbulent boundary-layer flow under a uniform freestream speed U∞ over a plate of length L, covered with uniform roughness of nominal sand-grain scale k_s, the physical behaviors underlying two distinguished limits at large Re_L≡U∞L/ν are explored: the fully rough wall flow where k_s/L is fixed and the long-plate limit where Re_k≡U∞k_s/ν is fixed. For the fully rough limit it is shown that not only is the drag coefficient C_D independent of Re_L but that a universal skin-friction coefficient C_f and normalized boundary-layer thickness δ/k_s can be found that depends only on ks_/x, where x is the downstream distance. In...

  5. Molecular based temperature and strain rate dependent yield criterion for anisotropic elastomeric thin films

    Bosi, F.; Pellegrino, S.
    A molecular formulation of the onset of plasticity is proposed to assess temperature and strain rate effects in anisotropic semi-crystalline rubbery films. The presented plane stress criterion is based on the strain rate-temperature superposition principle and the cooperative theory of yielding, where some parameters are assumed to be material constants, while others are considered to depend on specific modes of deformation. An orthotropic yield function is developed for a linear low density polyethylene thin film. Uniaxial and biaxial inflation experiments were carried out to determine the yield stress of the membrane via a strain recovery method. It is shown that...

  6. Performance data of the new free-piston shock tunnel T5 at GALCIT

    Hornung, H.; Sturtevant, B.; Bélanger, J.; Sanderson, S.; Brouillette, M.; Jenkins, M.
    A new free piston shock tunnel has been constructed at the Graduate Aeronautical Laboratories at Caltec. Compression tube length is 30 m and diameter 300 mm. Shock tube length is 12 m and diameter 90 mm. Piston mass is 150 kg and maximum diaphragm burst pressure is 130 MPa. Special features of this facility are that the pressure in the driver gas is monitered throughout the compression process until well after diaphragm rupture, and that the diaphragm burst pressure can be measured dynamically. An analysis of initial performance data including transient behavior of the flow over models is presented.

  7. Hypervelocity flow over spheres

    Hornung, H.; Wen, C.; Candler, G.
    Some aspects of the principle of binary scaling of hypervelocity flows with chemical reactions are discussed and tested both numerically and experimentally. The experiments, obtained in a new free-piston shock tunnel, show the value and limitations of binary scaling in very good agreement with the numerical computations. The use of spherical models eliminates end-effect problems previously encountered with cylindrical models. Global quantities, such as the bow shock stand-off distance, follow binary scaling very well. The results include differential interferograms and surface heat transfer measurements of nitrogen, air and carbon dioxide flows.

  8. The Boundary Layer on a Sharp Cone in High-Enthalpy Flow

    Germain, P.; Hornung, H.
    The boundary layer on a 5 deg. half-angle cone is studied by measurement of the heat flux distribution and by qualitative flow visualisation. In the laminar layer, heat flux levels are higher in air than in nitrogen because of a larger heat release from oxygen recombination at the wall. By varying the specific reservoir enthalpy in air and N_2, and from measurements in CO_2, it is found that real-gas effects stabilize the boundary layer.

  9. Transverse Jet Mixing and Combustion Experiments in the Hypersonic Shock Tunnel T5 at GALCIT

    Bélanger, J.; Hornung, H.
    For ground simulation of the flows that occur in SCRAM-Jet propulsion of hyper-velocity vehicles in which the hydrogen fuel is used as a coolant, it is essential that not only the enthalpy and pressure of the air flow, but also that the pressure and speed of the hydrogen be reproduced. Both can be achieved in the free-piston shock tunnel T5 together with the new small, combustion-heated hydrogen injection shock tunnel at GALCIT. Critical elements that need to be examined are the combustion efficiency and the turbulent mixing rate, especially at the unavoidable zero-shear condition, where the fuel and air speeds...

  10. Laboratory study of a stationary oblique plunging breaker for surfboard testing

    Hornung, Hans; Killen, Peter
    Consider a water wave which is obliquely incident on a sloping beach. An observer looking along the wave front from the beach sees the portion of the wave closest to him to be already broken while that far from him is still in deep water and unbroken. In the transition region, the waveface reaches its maximum slope and this is therefore the part most suitable for the surfboard rider, who uses the waveface much like a skier uses a mountain. Because the wave breaks when the water depth reaches a particular value, the path of the break point, and therefore...

  11. Shock Tubes in Rarefied Gas Flow Research

    Coles, D.; Liepmann, H. W.; Roshko, A.; Sturtevant, B.
    In any real fluid motion there exists regions in space-time in which the fluid is far from thermodynamic equilibrium. The relative extent of these non-equilibrium regions is determined by the ratio of the molecular relaxation times and the corresponding length scales to the macroscopic time and space scales appropriate to the flow. Gas flow within such non-equilibrium regions is properly called "rarefied". In recent years the shock tube has become a rather efficient tool in the investigations of rarefied gas flows and I intend to illustrate progress in this use through a discussion of some very recent and typical investigations of the GALCIT...

  12. Rarefied gas dynamics

    Estermann, I.; Roshko, A.
    The program of this symposium, which was held in Paris 26–29 June, 1962, included papers on new developments in the theory of flow of neutral and partly ionized gases at low densities, interactions between gas molecules and solid surfaces, high-speed and thermal molecular beams, measurements of drag and heat transfer in the transition and free molecule flow regime, and new experimental techniques. About 25 of the 60 papers presented are discussed in the following pages; the complete program is reproduced as Appendix I.

  13. Instability and Turbulence in Shear Flows

    Roshko, A.
    Increasing attention is being paid to the large scale structure of turbulence and to the so-called “coherent” vortical structures which have been disclosed and studied for a number of turbulent shear flows in laboratory experiments and in numerical simulations. The coherent structures develop from the instability waves which create the flow; they portray the genesis of the turbulence in the primary instability of the global vorticity distribution. This is not in the sense of classical laminar instability, whichinitiates transition to turbulence, but as thedriving instability in the fully developed turbulence. That instability provides the link to the amplitude of the...

  14. Measurements of Entrainment and Mixing in Turbulent Jets

    Dahm, W. A.; Dimotakis, P. E.
    An experimental investigation of entrainment and mixing in the self-similar far field of an axisymmetric free turbulent jet in water is presented. Length and time scales for the flame length fluctuations of reacting jets are shown to be approximately equal to the local characteristic large scale length and time of the flow. It is also shown that instantaneous radial profiles of concentration across the jet do not resemble the mean concentration profile, indicating that the mean profile is a poor representation of the mixed fluid states within the jet. These instantaneous profiles also show that unmixed ambient fluid is transported...

  15. Interaction Effects on the Drag of Bluff Bodies in Tandem

    Roshko, A.; Koenig, K.
    The objective of this study is to obtain better understanding of the flow over two tandemly positioned bluff bodies in close enough proximity to strongly interact with each other. This interaction is often beneficial in that the drag of the overall system is reduced. Prototypes for this problem come from tractor-trailer and cab-van combinations, and from various add-on devices designed to reduce their drag. The primary object of the present investigation is an axisymmetric configuration which seems to have first been studied by Saunders (1966). A disc of diameter d_1 is coaxially placed in front of a flat-faced cylinder of diameter...

  16. Measurements in a Synthetic Turbulent Boundary Layer

    Arakeri, J. H.; Coles, D. E.
    Some measurements in a synthetic turbulent boundary layer (SBL) are reported. The main diagnostic tool is an X-wire probe. The celerity of the large eddies is determined to be 0.842 u∞. The mean properties of the SBL are reasonably close to those of a natural turbulent boundary layer. The large eddy in the SBL appears to be a pair of counter-rotating eddies in the stream direction, inclined at a shallow angle and occupying much of the boundary-layer thickness.

  17. The Structure and Control of a Turbulent Reattaching Flow

    Sigurdson, L. W.; Roshko, A.
    An experimental study was made of the effect of a periodic velocity perturbation on the separation bubble downstream of the sharp-edged blunt face of a circular cylinder aligned coaxially with the free stream. Velocity fluctuations were produced with an acoustic driver located within the cylinder and a small circumferential gap located immediately downstream of the fixed separation line to allow communication with the external flow. The flow could be considerably modified when forced at frequencies lower than the initial Kelvin-Helmholtz frequencies of the free shear layer, and with associated vortex wavelengths comparable to the bubble height. Reattachment length, bubble height,...

  18. Nonlinear forcing in the resolvent analysis of exact coherent states of the Navier-Stokes equations

    Rosenberg, Kevin; McKeon, Beverley J.
    The resolvent analysis of McKeon & Sharma (2010) recasts the Navier-Stokes equations into an input/output form in which the nonlinear term is treated as a forcing that acts upon the linear dynamics to yield a velocity response. The framework has shown promise with regards to producing low-dimensional representations of exact coherent states. Previous work has focused on a primitive variable output; here we show a velocity-vorticity formulation of the governing equations along with a Helmholtz decomposition of the nonlinear forcing term reveals a simplified input/output form in the resolvent analysis. This approach leads to an improved method for compact representations...

  19. Structure in the Near Field of the Transverse Jet

    Fric, T. F.; Roshko, A.
    Photographs of a jet issuing from a wall into a crossflow display the four types of vortical structures which exist in the near field: namely, the jet shear layer vortices, the nascent far field vortex pair, the near-wall horseshoe vortices, and a system of vortices in the wake of the jet. It is shown that the wake vorticity is not “shed” from the jet but is formed from vorticity which originated in the wall boundary layer. The sources of vorticity for the other types of structures are also briefly discussed.

  20. Chemically Reacting, Turbulent Shear Layer

    Breidenthal, R.
    A chemically reacting, turbulent shear layer between two streams was investigated in a new, blowdown water tunnel. The two streams contained dilute, aqueous solutions of diffusion-limited reactants (phenolphthalein and sodium hydroxide, respectively) which mixed in the layer and reacted to form a visible reaction product. Using optical densitometry techniques, the amount of product was measured as a function of Reynolds number. These results for water (Schmidt No. = 600) are compared with the previous mixing measurements of Konrad in a gaseous shear layer (Sc = 0.7) and the simple mixing method of Broadwell. The unique flow visualization provided by the...

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