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Mansour, N. N.

Publications and source records attributed to Mansour, N. N..

At least 55 records · Page 3
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NASA NTRS1993

The three-dimensional evolution of a plane wake

In the past three decades, linear stability analysis has led to a comprehensive understanding of the linear stages of transition in plane wakes. Our understanding of the nonlinear and turbulent stages is less developed. Nonlinear theory developed by Papageorgiou and Smith was used to study the long-wavelength regime in wakes. The nonlinear and turbulent stages were investigated experimentally, and few numerical studies examined the early nonlinear stages of forced wakes. The evolution of three dimensional disturbances in an incompressible wake is investigated using direct numerical simulations. The instantaneous three-dimaensional structures and corresponding statistics are presented.

Maekawa, H.↗
NASA NTRS1993

Rapid Distortion Analysis and Direct Simulation of Compressible Homogeneous Turbulence at Finite Mach Number

The effect of rapid mean compression on compressible turbulence at a range of turbulent Mach numbers is investigated. Rapid dist'ortion theory (RDT) and direct numerical simulation results for the case of axial (one-dimensional) compression are used to illustrate the existence of two distinct rapid compression regimes. These regimes - the nearly solenoidal and the 'pressure-released' - are defined by a single parameter involving the timescales of the mean distortion, the turbulence, and the speed of sound. A general RDT formulation is developed and is proposed as a means of improving turbulence models for compressible flows. In contrast to the well-documented observation that 'compressibility' (measured, for example, by the turbulent Mach number) is often associated with a decrease in the growth rate of turbulent kinetic energy, we find that under rapid distortion compressibility can produce an amplification of the kinetic energy growth rate. We also find that as the compressibility increases, the magnitude of the pressure-dilation correlation increases, in absolute terms, but its relative importance decreases compared to the magnitude of the kinetic energy production.

Cambon, C.↗
NASA NTRS1993

Compressibility Effects on the Growth and Structure of Homogeneous Turbulent Shear Flow

Compressibility effects within decaying isotropic turbulence and homogeneous turbulent shear flow have been studied using direct numerical simulation. The objective of this work is to increase our understanding of compressible turbulence and to aid the development of turbulence models for compressible flows. The numerical simulations of compressible isotropic turbulence show that compressibility effects are highly dependent on the initial conditions. The shear flow simulations, on the other hand, show that measures of compressibility evolve to become independent of their initial values and are parameterized by the root mean square Mach number. The growth rate of the turbulence in compressible homogeneous shear flow is reduced compared to that in the incompressible case. The reduced growth rate is the result of an increase in the dissipation rate and energy transfer to internal energy by the pressure-dilatation correlation. Examination of the structure of compressible homogeneous shear flow reveals the presence of eddy shocklets, which are important for the increased dissipation rate of compressible turbulence.

Blaisdell, G. A.↗
NASA NTRS1992

Rapid distortion analysis and direct simulation of compressible homogeneous turbulence at finite Mach number

The effect of rapid mean compression on compressible turbulence at a range of turbulent Mach numbers is investigated. Rapid distortion theory (RDT) and direct numerical simulation results for the case of axial (one-dimensional) compression are used to illustrate the existence of two distinct rapid compression regimes. These regimes are set by the relationships between the timescales of the mean distortion, the turbulence, and the speed of sound. A general RDT formulation is developed and is proposed as a means of improving turbulence models for compressible flows.

Cambon, C.↗
NASA NTRS1992

Microburst modelling and scaling

A microburst can be modeled by releasing a volume of fluid that is slightly heavier than the ambient fluid, allowing it to fall onto a horizontal surface. Vorticity develops on the sides of this parcel as it descends and causes it to roll up into a turbulent vortex ring which impinges on the ground. Such a model exhibits many of the features of naturally occurring microbursts which are a hazard to aviation. In this paper this model is achieved experimentally by releasing a volume of salt water into fresh water from a cylindrical dispenser. When care is taken with the release the spreading rate of the surface outflow is measurable and quite repeatable despite the fact that the flow is turbulent. An elementary numerical approximation to this model, based on inviscid vortex dynamics, has also been developed. A scaling law is proposed which allows experiments with different fluid densities to be compared with each other and with the numerical results. More importantly the scaling law makes it possible to compare the model results with real microbursts.

Lundgren, T. S.↗
NASA NTRS1992

Modeling the dissipation-rate equation with the aid of direct simulation data

The epsilon-budget was computed from the direct simulation data (DNS) of Kim (1990) for developed channel flow at Re(tau) = 395. The relative magnitude of the terms in the epsilon-equation is shown with the aid of scaling arguments, and the parameter governing this magnitude is established. The modeling of the terms in the equation is then addressed in the context of eddy-viscosity k-epsilon models. Some existing models for the sum of all source and sink terms in the epsilon-equation are tested against DNS data, and an improved model is proposed on the basis of these data.

Rodi, W.↗
NASA NTRS1992

Theoretical and computational study of rotating isotropic turbulence

The various flow regimes that occur in rotating isotropic turbulence and their associated turbulence structure are studied. Direct numerical simulations are conducted for moderate and rapid rotation rates and comparisons are made with the predictions of generalized Eddy Damped Quasi-Normal Markovian (EDQNM) approximations. It is shown that at high rotation rates the nonlinear transfer terms remain small and that the development of the spectrum is through pure viscous decay. This shows that the effect of rapid rotation on the turbulence kinetic energy is through the shutting off of the production term in the dissipation rate equation. At moderate and relatively rapid rates, the rotation causes a discernible anisotropy to develop in the integral length scale with a mild trend toward a two-dimensionalization of the flow. As the flow decays, the Rossby number decreases, leading again to the shutting off of the nonlinear transfer terms. For extremely rapid rotation rates, the anisotropies in the integral length scales are small and direct numerical simulation results are in good agreement with rapid distortion theory.

Mansour, N. N.↗
NASA NTRS1991

Vortex ring bubbles

Toroidal bubbles with circulation are studied numerically and by means of a physically motivated model equation. Two series of computations are performed by a boundary-integral method. One set shows the starting motion of an initially spherical bubble as a gravitationally driven liquid jet penetrates through the bubble from below causing a toroidal geometry to develop. The jet becomes broader as surface tension increases and fails to penetrate if surface tension is too large. The dimensionless circulation that develops is not very dependent on the surface tension. The second series of computations starts from a toroidal geometry, with circulation determined from the earlier series, and follows the motion of the rising and spreading vortex ring. Some modifications to the boundary-integral formulation were devised to handle the multiply connected geometry. The computations uncovered some unexpected rapid oscillations of the ring radius. These oscillations and the spreading of the ring are explained by the model equation which provides a more general description of vortex ring bubbles than previously available.

Lundgren, T. S.↗
NASA NTRS1991

The use of direct numerical simulation data in turbulence modeling

Direct numerical simulations (DNS) of turbulent flows provide a complete data base to develop and to test turbulence models. In this article, the progress made in developing models for the dissipation rate equation is reviewed. New scaling arguments for the various terms in the dissipation rate equation were tested using data from DNS of homogeneous shear flows. Modifications to the epsilon-equation model that take into account near-wall effects were developed using DNS of turbulent channel flows. Testing of new models for flows under mean compression was carried out using data from DNS of isotropically compressed turbulence. In all of these studies the data from the simulations was essential in guiding the model development. The next generation of DNS will be at higher Reynolds numbers, and will undoubtedly lead to improved models for computations of flows of practical interest.

Mansour, N. N.↗
NASA NTRS1991

Compressibility effects on the growth and structure of homogeneous turbulent shear flow

Direct numerical simulations of compressible homogeneous turbulent shear flow are used to provide insight into compressibility effects on turbulence. The simulations show a reduction in the growth rate of the turbulence compared to the incompressible case. Examination of the turbulent kinetic energy budget shows that the reduced growth rate is due to an increase in the dissipation rate due to the divergence of the velocity and to the pressure-dilatation correlation which acts to transfer energy between internal energy and kinetic energy. The structure of the turbulence is also examined. Visualizations of the flowfields reveal the presence of eddy shocklets. These shock structures are important contributors to the increased dissipation of compressible turbulence. A mechanism for the generation of the shocks is suggested.

Blaisdell, G. A.↗
NASA NTRS1991

Simulation and modeling of homogeneous compressible turbulence under isotropic mean compression

Compressible homogeneous turbulence subjected to isotropic mean compression is simulated numerically for high and low turbulent Mach numbers, at various compression rates. It is found that at low Mach numbers, the effects of viscosity variations on the development of the turbulent kinetic energy can be significant. A model consistent with the invariance of the Navier-Stokes equations to spherical compression, that takes into account variable viscosity effects is proposed. At high Mach numbers, the distribution of energy between the acoustic and solenoid fields is found to depend upon initial conditions. The simulation results are also used to evaluate two-equation compressible turbulence models.

Coleman, G. N.↗
NASA NTRS1990

Low Reynolds number kappa-epsilon modeling with the aid of direct simulation data

The constant C(sub mu) and the near-wall damping function f(sub mu) in the eddy-viscosity relation of the kappa-epsilon model are evaluated from direct numerical simulation (DNS) data for developed channel and boundary layer flow at two Reynolds numbers each. Various existing f(sub mu) model functions are compared with the DNS data, and a new function is fitted to the high-Reynolds-number channel flow data. The epsilon-budget is computed for the fully developed channel flow. The relative magnitude of the terms in the epsilon-equation is analyzed with the aid of scaling arguments, and the parameter governing this magnitude is established. Models for the sum of all source and sink terms in the epsilon-equation are tested against the DNS data, and an improved model is proposed.

Rodi, W.↗
NASA NTRS1990

One-equation near-wall turbulence modeling with the aid of direct simulation data

The length scales appearing in the relations for the eddy viscosity and dissipation rate in one-equation models were evaluated from direct numerical simulation data for developed channel and boundary-layer flow at two Reynolds numbers each. To prepare the ground for the evaluation, the distribution of the most relevant mean-flow and turbulence quantities is presented and discussed with respect to Reynolds-number influence and to differences between channel and boundary-layer flow. An alternative model is also examined in which bar-(v'(exp 2))(exp 1/2) is used as velocity scale instead of k(exp 1/2). With this velocity scale, the length scales now appearing in the model follow very closely a linear relationship near the wall so that no damping is necessary. For the determination of bar-v'(exp 2) in the context of a one-equation model, a correlation is provided between bar-(v'(exp 2))/k and bar-(u'v')/k.

Rodi, W.↗
NASA NTRS1990

Direct numerical simulation of a three-dimensional turbulent boundary layer

The effects of transverse strain on an initially two-dimensional turbulent boundary layer are studied in a direct numerical simulation of a planar channel flow with impulsively started transverse pressure gradient. Consistent with experiments in three-dimensional boundary layers, the simulation shows a decrease in the Reynolds shear stress with increasing transverse strain. Also, the directions of the Reynolds shear stress vector and the mean velocity gradient vector were found to differ. In addition, the simulation shows a drop in the turbulent kinetic energy. Terms in the Reynolds stress transport equations were computed. The balances indicate that the decrease in turbulent kinetic energy is a result of a decrease in turbulence production, along with an increase in turbulent dissipation. Intuitive reasoning and current turbulence models would predict an increase in kinetic energy along with increases in production and dissipation rates as a result of increased mean-flow strain rate. Later in the evolution of the flow, both turbulence production and dissipation increase.

Moin, P.↗
NASA NTRS1990

Modeling of near-wall turbulence

An improved k-epsilon model and a second order closure model is presented for low Reynolds number turbulence near a wall. For the k-epsilon model, a modified form of the eddy viscosity having correct asymptotic near wall behavior is suggested, and a model for the pressure diffusion term in the turbulent kinetic energy equation is proposed. For the second order closure model, the existing models are modified for the Reynolds stress equations to have proper near wall behavior. A dissipation rate equation for the turbulent kinetic energy is also reformulated. The proposed models satisfy realizability and will not produce unphysical behavior. Fully developed channel flows are used for model testing. The calculations are compared with direct numerical simulations. It is shown that the present models, both the k-epsilon model and the second order closure model, perform well in predicting the behavior of the near wall turbulence. Significant improvements over previous models are obtained.

Shih, T. H.↗
NASA NTRS1989

Propagation of a premixed flame in a divided-chamber combustor

Experimental observations on the propagation of lean premixed ethylene-air flames in a divided-chamber combustion vessel have been compared with the results of numerical simulations based on a flame sheet-vortex dynamics model in axisymmetric coordinates. Flame speeds were found to increase from 10-24 cm/s as the equivalence ratio was varied from 0.5-0.65 in the experiments. Using the associated increase in gas velocity with equivalence ratio, the estimated Reynolds number in the experiment was changed from 1870 to 8090. Good agreement between experimental and theoretical results was obtained for the prechamber flame propagation rates and for the spatial and temporal development of the flame in the main combustion chamber at the lowest Reynolds number.

Cattolica, R. J.↗
NASA NTRS1989

Numerical simulation of a three-dimensional turbulent boundary layer

The effects of transverse strain on an initially two-dimensional turbulent boundary layer are studied in a direct numerical simulation of a planar channel flow with impulsively started transverse pressure gradient. Consistent with experiments in three-dimensional boundary layers, the simulation shows a drop in the Reynolds shear stress with increasing transverse strain. Also, the directions of the Reynolds shear stress vector and the mean velocity gradient vector were found to differ. In addition, the simulation shows a drop in the turbulent kinetic energy. The terms in the budget of the Reynolds stress were computed. In general, the individual terms do not show a significant change. However, an increase in dissipation rate rather than reduction in total production appears to be responsible for the drop in turbulent kinetic energy.

Moin, P.↗