1,720,977 research outputs found
Detached eddy simulation of a supersonic flow over an open cavity
No full textA supersonic flow over an open cavity is numerically studied using
the RANS and DES approaches. The results analysis and the comparison with
experiments show that the DES approach is able to captured correctly
the unsteady phenomenology. On the contrary, the RANS approach shows all its
limitations providing only steady state solutions
COMPRESSIBILITY CORRECTION FOR THE SPALART-ALLMARAS MODEL IN FREE-SHEAR FLOWS
No full textThis paper presents a correction of the Spalart-Allmaras
turbulence model to account for the compressibility effects in mixing layer flows. Unlike other corrections proposed for the K- epsilon model, the present correction does not need the knowledge of the turbulent Mach number and, therefore, can be applied to those turbulence models, like the Spalart-Allmaras one, which do not integrate directly the turbulent kinetic energy equation. To explore the validity of the proposed correction, four mixing layer flows
and four supersonic backward facing step flows, covering a wide range
of flow conditions, were selected and computed using both the standard
and the corrected Spalart-Allmaras model. The analysis of the numerical
results and their comparison with the experimental data
show that the proposed correction produces a significant improvement of
the numerical predictions
EQUILIBRIUM AND NONEQUILIBRIUM MODELING OF HYPERSONIC INVISCID FLOWS
No full textHypersonic flows about ellipses at high angles of attack are analyzed by means of nonequilibrium, equilibrium and inert gas models. It is shown that the technique proposed for the nonequilibrium model, based on a streamline integration of the chemical rate equations, provides accurate results and allows precise computation of the stagnation point conditions, where equilibrium must be attained. Compared with the nonequilibrium one, the equilibrium model underpredicts the bow shock stand-off distance and overpredicts the wall temperature, whereas the inert gas model can only be used as a crude approximation for evaluating the wall pressure
FAST EULER SOLVER FOR NONEQUILIBRIUM STEADY FLOWS
No full textThe Moretti's Fast Solver, based on the implicit solution of four compatibility
equations, has shown to produce some unphysical behaviour in the strong
relaxation layers occurring in reacting flows. In the paper a new formulation
is presented which overcomes these drawbacks, while retaining the Riemann
variable formulation which allows an efficient implicit integration of four
separate bidiagonal equations. Moreover the scheme is extended to non
orthogonal grids and its accuracy is raised to second order. The improvement
of the solution quality is shown through the application to both reacting and
non reacting flows over blunt bodies at hypersonic speeds
WAVE REFLECTION ON POROUS WALLS: NUMERICAL MODELLING AND APPLICATION TO TRANSONIC WIND TUNNELS
No full textTransonic wind tunnel tests are often affected by wave reflections
stemming from the edge of the test section that may be attenuated by
using perforated end walls. In this work, a numerical model is
presented that includes a novel boundary condition for thick walls,
that can be used to enhance perforated wind tunnel test section
designs.
An in-house finite volume scheme for compressible inviscid flows is
enhanced by the addition of a new perforated wall boundary condition.
This wall condition is based on the assumption that the wall
thickness is larger than the perforation size or diameter, which is
common of transonic wind tunnels with single leaf walls. The model
was validated against a simple oblique shock reflection test case.
The model predictions were shown to be an improvement with respect
to those from a benchmark perforated wall boundary condition that
assumes a small wall thickness in relation to the perforation size.
A numerical and experimental study was conducted of the end wall
effects in a chocked nozzle turbine linear cascade, discharging at
Mach 1.27. Without a perforated wall, the cascade displays strong
end wall trailing edge shock reflections, giving a pitchwise
non-periodic discharge. A numerical model flow
with a 50% void ratio tailboard indicated some regain in flow
periodicity, as verified by experiment. The thick wall model better
reproduced the flow features documented in the flow visualisations
than the benchmark model, indicating that this is a better boundary
condition to enhance perforated tailboard designs for compressible
model flows
On the Role of Vibrational Excitation in Hypersonic Flow Computation
No full textThe paper investigates the influence of the thermochemical modeling on the flow-field properties for both compressing and expanding flows. For the nitrogen hypersonic flow over a cylinder a comparison between a model neglecting vibrational energy and two-temperature Park‘s nonequilibrium model has been performed in a wide variety of experimental and free flight conditions. It is shown that the differences are confined in a thin layer behind the bow shock while the overall flow properties are slightly affected. For the expanding air flows inside hypersonic facilities, besides the two models mentioned above, vibrational equilibrium and CVDV model have been compared. It was found that vibrational nonequilibrium can not be disregarded but no discernible differences have been observed between Park‘s and CVDV model results, unless the enthalpy is very large
Campo aerodinamico nella zona centrale di una centrifuga molto allungata sottoposta a gradiente assiale di temperatura
No full textStoichiometry of hemolysis by the polyene antibiotic lucensomycin.
No full textThe stoichiometry of hemolysis by the polyene antibiotic lucensomycin was investigated. It appears that hemolysis occurs only when a relatively high fraction (probably between 15 and 40%) of the cholesterol sites in the erythrocyte membrane have combined with the polyene. Also in phospholipid-cholesterol vesicles the increase of permeability requires occupancy of 40-50% of the existing cholesterol sites. As for the possible cooperative effect in the hemolytic process, it is probable that several (at least 9-10) lucensomycin-cholesterol adducts must interact on each side of the membrane to form an aqueous channel; the distribution of these adducts in the erythrocyte membrane occurs, however, apparently at random
NONEQUILIBRIUM HYPERSONIC INVISCID STEADY FLOWS
No full textA numerical method for the solution of nonequilibrium flows about blunt bodies is presented. The method is
based on the splitting in two parts of the reactive Euler equations: the gasdynamic operator (mass and momentum
equations) and the chemical operator (energy and species conservation equations). The gasdynamic operator is discretized
on a body- and shock-fitted grid, and integrated in diagonalized form by means of a semi-implicit technique. The
chemical operator is integrated along the streamlines by means of an implicit technique with variable step size. A detailed
chemical nonequilibrium model is adopted, while vibrational energy is assumed in equilibrium. The shock is modeled
with a shock-fitting technique. Nonequilibrium flows about cylinders are computed in order to demonstrate the capability
of the present method, both to achieve high resolution in chemical relaxation layers and to overcome the stiffness in nearequilibrium
conditions without resulting in cumbersome calculations. Numerical results are presented and compared with
experimental data
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