Research Interests:
Turbulence modeling introduces one of the major sources of uncertainty in the prediction of aeronautical flows. This statement is even truer for supersonic flows for which physical insight is lacking and for which any modeling ideas that... more
Turbulence modeling introduces one of the major sources of uncertainty in the prediction of aeronautical flows. This statement is even truer for supersonic flows for which physical insight is lacking and for which any modeling ideas that do exist remain unvalidated because of lack of data. Physical understanding and proper modeling of turbulent supersonic flow have seen a new beginning, however, thanks to a number of direct simulations by several groups in the last decade. For instance, an important effect which has recently come to light in free shear flows i s a significant reduction with Mach number in the ability of the flow to produce turbulent shear stress from turbulent energy.
Radar has been proposed as one way to track wake vortices to reduce aircraft spacing. Radar echoes from aircraft wakes are usually interpreted qualitatively using Tatarski's theory of scattering by isotropic atmospheric turbulence. The... more
Radar has been proposed as one way to track wake vortices to reduce aircraft spacing. Radar echoes from aircraft wakes are usually interpreted qualitatively using Tatarski's theory of scattering by isotropic atmospheric turbulence. The present work predicts RCS by (1) Keeping the weak scattering approximation but dropping the assumptions of a far-field and a uniform incident wave, neither of which is generally valid for a coherent wake (2) Considering three simple mechanisms for the structure and magnitude of refractive index variations: (i) Radial density gradient in each vortex (ii) Adiabatic transport of atmospheric fluid in the oval surrounding the vortices (iii) 3D fluctuations in the vortex cores. For mechanism (ii) the predictions agree with available data. However, the predictions have a cut-off away from normal incidence which is not present in the measurements due possibly to 3D fluctuations in the oval. The reflectivity of mechanism (i) is comparable but cuts-off at frequencies lower than those considered in the experiment. Finally, we suggest that hot engine exhaust could increase RCS by 40 db and reveal vortex circulation, provided its mixing is prevented in the laminar vortices.
Research Interests:
A number of advantages result from using B-splines as basis functions in a Galerkin method for solving partial differential equations. Among them are arbitrary order of accuracy and high resolution similar to that of compact schemes but... more
A number of advantages result from using B-splines as basis functions in a Galerkin method for solving partial differential equations. Among them are arbitrary order of accuracy and high resolution similar to that of compact schemes but without the aliasing error. This work develops another property, namely, the ability to treat semi-structured embedded or zonal meshes for two-dimensional geometries. This can drastically reduce the number of grid points in many applications. Both integer and non-integer refinement ratios are allowed. The report begins by developing an algorithm for choosing basis functions that yield the desired mesh resolution. These functions are suitable products of one-dimensional B-splines. Finally, test cases for linear scalar equations such as the Poisson and advection equation are presented. The scheme is conservative and has uniformly high order of accuracy throughout the domain.
Research Interests:
The sound generated due to a localized flow over an infinite flat surface is considered. It is known that the unsteady surface pressure, while appearing in a formal solution to the Lighthill equation, does not constitute a source of sound... more
The sound generated due to a localized flow over an infinite flat surface is considered. It is known that the unsteady surface pressure, while appearing in a formal solution to the Lighthill equation, does not constitute a source of sound but rather represents the effect of image quadrupoles. The question of whether a similar surface shear stress term constitutes a true source of dipole sound is less settled. Some have boldly assumed it is a true source while others have argued that, like the surface pressure, it depends on the sound field (via an acoustic boundary layer) and is therefore not a true source. A numerical experiment based on the viscous, compressible Navier-Stokes equations was undertaken to investigate the issue. A small region of a wall was oscillated tangentially. The directly computed sound field was found to to agree with an acoustic analogy based calculation which regards the surface shear as an acoustically compact dipole source of sound.
Research Interests:
We sketch a scenario for primary accretion of chondrite parent bodies based on turbulent concentration of chondrule-size constituents into dense clumps, followed by slow gravitational contraction. We note key elements of the physics and... more
We sketch a scenario for primary accretion of chondrite parent bodies based on turbulent concentration of chondrule-size constituents into dense clumps, followed by slow gravitational contraction. We note key elements of the physics and describe the stati
Research Interests:
Several mechanisms have been identified that create dense particle clumps in the solar nebula. The present work is concerned with the gravitational collapse of such clumps, idealized as being spherically symmetric. Calculations using the... more
Several mechanisms have been identified that create dense particle clumps in the solar nebula. The present work is concerned with the gravitational collapse of such clumps, idealized as being spherically symmetric. Calculations using the two-fluid model are performed (almost) up to the time when a central density singularity forms. The end result of the study is a parametrization for this time, in order that it may be compared with timescales for various disruptive effects to which clumps may be subject. An important effect is that as the clump compresses, it also compresses the gas due to drag. This increases gas pressure which retards particle collapse and leads to oscillation in the size and density of the clump. The ratio of gravitational force to gas pressure gives a two-phase Jeans parameter, $J_t$, which is the classical Jeans parameter with the sound speed replaced by an the wave speed in a coupled two-fluid medium. Its use makes the results insensitive to the initial density ratio of particles to gas as a separate parameter. An ordinary differential equation model is developed which takes the form of two coupled non-linear oscillators and reproduces key features of the simulations. Finally, a parametric study of the time to collapse is performed and a formula (fit to the simulations) is developed. In the incompressible limit $J_t \to 0$, collapse time equals sedimentation time. As $J_t$ increases, the collapse time decreases roughly linearly with $J_t$ until $J_t \gtrsim 0.4$ when it becomes approximately equal to the dynamical time.
The persistence of condensation trails, "contrails", in the far-field of an aircraft wake is numerically analyzed. Contrails are ice clouds formed by condensation of exhaust water vapor in a cold atmosphere. Their evolution and... more
The persistence of condensation trails, "contrails", in the far-field of an aircraft wake is numerically analyzed. Contrails are ice clouds formed by condensation of exhaust water vapor in a cold atmosphere. Their evolution and persistence are controlled by the wake dynamics, atmospheric turbulence, as well as by background water vapor content. Under suitable atmospheric conditions, they may trigger the formation of cirrus clouds, thus having a potential climate impact on regional/global scales. The object of the present study is to reproduce the contrail evolution numerically in intermediate scales (between aircraft- and atmosphere- scales) where exhaust mixing and clouds microphysics are expected to control their dynamics and, eventually, their transition to cirrus clouds. The simulations are carried out using a two-phase approach to deal with both gas and ice phases. Large eddy simulations are used for the gas phase while a Lagrangian particles tracking approach is used for ice crystals. Mass transfer between the two phases is used to account for vapor condensation, by employing available ice microphysics models. Besides the physical description of the phenomenon, the results may be useful for "calibrating" source terms, representative of the environmental impact of aircraft-generated emissions, in global climate codes.
Research Interests:
Initial results from the direct numerical simulation (DNS) of compressible turbulent boundary layers will be presented. The spatially developing boundary layer is first transformed to a parallel shear layer using a transformation similar... more
Initial results from the direct numerical simulation (DNS) of compressible turbulent boundary layers will be presented. The spatially developing boundary layer is first transformed to a parallel shear layer using a transformation similar to that used by Spalart for an incompressible boundary layer. This allows us to avoid inflow and outflow boundary conditions, and to apply periodic boundary conditions in the streamwise and spanwise directions. The resulting equations are then solved using a mixed Fourier B-spline Galerkin method. One challenge to these highly accurate and non-dissipative numerics has been the occurrence of sharp density gradients, which require significantly more resolution than the incompressible case, especially during transients. The first simulation is at Mach 2.5 with a momentum thickness Reynolds number based on wall viscosity of R_θ'=825. The simulations are used to examine the physics of the compressible boundary layer and to compute turbulence statistics and terms in the budget equations. The turbulence statistics include: rms and mean profiles, energy spectra, and two-point correlations.
When analytes containing cationic components, such as proteins, are separated in fused silica capillaries or micro-chips, they adsorb strongly to the negatively charged walls. Broadened and highly asymmetric peaks in the detector signal... more
When analytes containing cationic components, such as proteins, are separated in fused silica capillaries or micro-chips, they adsorb strongly to the negatively charged walls. Broadened and highly asymmetric peaks in the detector signal is symptomatic of the presence of such wall interactions. Band broadening is caused by a reduction in wall zeta potential at the adsorbed sites which in turn introduces a shear into the electroosmotic flow which leads to Taylor dispersion. In this work, numerical solutions of the coupled electro-hydrodynamic equations for fluid flow and the transport equation for analyte concentration were undertaken in the limit of thin Debye layers. The calculations reproduce many of the qualitative effects of wall adsorption familiar from observation. Further, the simulation results are compared, and found to agree very well with a recently developed asymptotic theory (Ghosal, S., (2003) J. Fluid Mech., 491, 285--300.)
Research Interests:
ABSTRACT Several two-equation turbulence models are compared to data from direct numerical simulations (DNS) of the homogeneous elliptic streamline flow, which combines rotation and strain. The models considered include standard... more
ABSTRACT Several two-equation turbulence models are compared to data from direct numerical simulations (DNS) of the homogeneous elliptic streamline flow, which combines rotation and strain. The models considered include standard two-equation models and models with corrections for rotational effects. Most of the rotational corrections modify the dissipation rate equation to account for the reduced dissipation rate in rotating turbulent flows, however, the DNS data shows that the production term in the turbulent kinetic energy equation is not modeled correctly by these models. Nonlinear relations for the Reynolds stresses are considered as a means of modifying the production term. Implications for the modeling of turbulent vortices will be discussed.
A numerical technique for computations of turbulent flows is described. The technique is based on B-splines and allows grid embedding in physically significant flow regions. Numerical tests, which include solutions of nonlinear... more
A numerical technique for computations of turbulent flows is described. The technique is based on B-splines and allows grid embedding in physically significant flow regions. Numerical tests, which include solutions of nonlinear advection-diffusion equations and computations of flow over a circular cylinder at Reynolds numbers up to 300, indicate that the method is accurate and efficient. In computations of flow over a cylinder, the lift, drag, and base suction coefficients agree well with existing experimental data and previous numerical simulations.
Research Interests:
Direct numerical simulations are performed for homogeneous turbulence with a mean flow having elliptic streamlines. This flow combines the effects of rotation and strain on the turbulence. Qualitative comparisons are made with linear... more
Direct numerical simulations are performed for homogeneous turbulence with a mean flow having elliptic streamlines. This flow combines the effects of rotation and strain on the turbulence. Qualitative comparisons are made with linear theory for cases with high Rossby number. The nonlinear transfer process is monitored using a generalized skewness. In general, rotation turns off the nonlinear cascade; however, for moderate ellipticities and rotation rates the nonlinear cascade is turned off and then reestablished. Turbulence statistics of interest in turbulence modeling are calculated, including full Reynolds stress budgets.
Research Interests:
The formation of vortex rings generated through impulsively started jets is studied through using a piston/cylinder arrangement. For a wide range of piston stroke to diameter ratios (L/D), the DPIV results indicate that the flow field... more
The formation of vortex rings generated through impulsively started jets is studied through using a piston/cylinder arrangement. For a wide range of piston stroke to diameter ratios (L/D), the DPIV results indicate that the flow field generated by large L/D consists of a leading vortex ring followed by a trailing jet. The vorticity field of the formed leading vortex ring is disconnected from that of the trailing jet. On the other hand, flow fields generated by small stroke ratios show only a single vortex ring. The transition between these two distinct states is observed to occur at a stroke ratio of approximately 4, which, in this paper, is referred to as the "formation number". This number indicates the maximum circulation attainable by a vortex ring. The universality of this number was tested by generating vortex rings with different jet exit boundaries, as well as with various non- impulsive piston velocities. The mere existence of the "formation number" is intriguing since it hints at the possibility that nature uses this time scale for some evolutionary incentives such as optimum ejection of blood from the left atrium to the heart's left ventricle or locomotion process where ejection of vortices might have been utilized for the purposes of propulsion.
Research Interests:
Research Interests:
Research Interests:
A novel numerical method based on B-splines is developed for turbulence simulations on zonal grids. The technique permits fine meshes to be embedded in physically significant flow regions without placing a large number of grid points in... more
A novel numerical method based on B-splines is developed for turbulence simulations on zonal grids. The technique permits fine meshes to be embedded in physically significant flow regions without placing a large number of grid points in the rest of the computational domain. The method was tested in simulations of laminar flows over a circular cylinder at steady and unsteady Reynolds numbers. Information transfer between zones is accomplished efficiently without accumulation of errors in the regions of sudden grid changes. Numerical solutions on multi-zone grids were of the higher accuracy than those on a single zone grid with the same total number of grid points. The overhead cost associated with performing computations on zonal grids was negligible. The drag coefficient, the size of the recirculation bubble at Re=40 and the vortex shedding frequency at Re=80 and 100 agree well with the experimental data and previous simulations of these flows. The domain and grid size independence studies were performed. Three-dimensional large eddy simulations of a flow over a circular cylinder at the subcritical Reynolds number, Re=3900, are underway.
Research Interests:
The head-on collision of two identical axisymmetric viscous vortex rings is studied through direct simulations of the incompressible Navier-Stokes equations. The initial vorticity distributions considered are those of Hill's spherical... more
The head-on collision of two identical axisymmetric viscous vortex rings is studied through direct simulations of the incompressible Navier-Stokes equations. The initial vorticity distributions considered are those of Hill's spherical vortex and of rings with circular Gaussian cores, each at Reynolds numbers of about 350 and 1000. The Reynolds number is defined by Gamma/Nu, the ratio of circulation to viscosity. As the vortices approach each other by self-induction, the radii increase by mutual induction, and vorticy cancels through viscous cross-diffusion across the collision plane. Following contact, the vorticity distribution in the core forms a head-tail structure (for the cases considered). The characteristic time of vorticity annihilation is compared with that of a 3D collision experiment and 3D numerical simulations. It is found that the annihilation time is somewhat longer in the axisymmetric case than it is in the symmetry plane of the experiment and 3D numerical simulation. By comparing the annihilatiom time with a viscous timescale and a circulation timescale, it is deduced that both the strain rate due to local effects and to 3D vorticity realignment are important.
Research Interests:
The formation of vortex rings generated through impulsively started jets is studied experimentally. Utilizing a piston/cylinder arrangement in a water tank, the velocity and vorticity fields of vortex rings are obtained using digital... more
The formation of vortex rings generated through impulsively started jets is studied experimentally. Utilizing a piston/cylinder arrangement in a water tank, the velocity and vorticity fields of vortex rings are obtained using digital particle image velocimetry (DPIV) for a wide range of piston stroke to diameter (L/D) ratios. The results indicate that the flow field generated by large L/D consists of a leading vortex ring followed by a trailing jet. The vorticity field of the leading vortex ring formed is disconnected from that of the trailing jet. On the other hand, flow fields generated by small stroke ratios show only a single vortex ring. The transition between these two distinct states is observed to occur at a stroke ratio of approximately 4, which, in this paper, is referred to as the ‘formation number’. In all cases, the maximum circulation that a vortex ring can attain during its formation is reached at this non-dimensional time or formation number. The universality of this number was tested by generating vortex rings with different jet exit diameters and boundaries, as well as with various non-impulsive piston velocities. It is shown that the ‘formation number’ lies in the range of 3.6 4.5 for a broad range of flow conditions. An explanation is provided for the existence of the formation number based on the Kelvin Benjamin variational principle for steady axis-touching vortex rings. It is shown that based on the measured impulse, circulation and energy of the observed vortex rings, the Kelvin Benjamin principle correctly predicts the range of observed formation numbers.
Research Interests:
Research Interests:
A recent LES study of flow around a circulation control airfoil (Nishino et al., AIAA Paper 2010-347) is further extended, with a particular focus on the characteristics of a Coanda wall jet separating from a rounded trailing edge of the... more
A recent LES study of flow around a circulation control airfoil (Nishino et al., AIAA Paper 2010-347) is further extended, with a particular focus on the characteristics of a Coanda wall jet separating from a rounded trailing edge of the airfoil. A number of backward-tilted hairpin vortices are observed in the outer shear layer of the wall jet; these hairpins create a strong upwash between their legs and thereby lift the high-momentum jet flow below the hairpins upward. The LES results are then compared with 2D RANS simulations employing the Spalart-Allmaras model and Menter's SST model. The Reynolds shear stresses predicted by the two models are both found to be too small in the outer shear layer of the wall jet, resulting in smaller jet spreading rates, larger peak jet velocities, and hence stronger circulations around the airfoil compared with the LES.
Research Interests:
Research Interests:
Page 1. Calculation of the Turbulence Characteristics of Flow Around a Circulation Control Airfoil Using LES (Invited Paper) Takafumi Nishino∗ NASA Ames Research Center, Moffett Field, California, 94035 Seonghyeon Hahn ...
Page 1. Copyright 1996 Scientific American, Inc. Page 2. Below the towering cliffs of Ma-kapuu Beach on the island of Oahu, Hawaii, is a unique lab-oratory dedicated to the study of dol-phins. Project Delphis, run by the non ...
Research Interests: Animal Behavior, Dolphins, Air, Female, Animals, and 4 moreMale, Hawaii, Scientific American, and Motor activity
Research Interests:
Research Interests:
Research Interests:
ABSTRACT The sound generated due to a localized flow over a large (compared to the acoustic wavelength) plane no-slip wall is considered. It has been known since 1960 that for inviscid flow the pressure, while appearing to be a source of... more
ABSTRACT The sound generated due to a localized flow over a large (compared to the acoustic wavelength) plane no-slip wall is considered. It has been known since 1960 that for inviscid flow the pressure, while appearing to be a source of dipole sound in a formal solution to the Lighthill equation, is, in fact, not a true dipole source, but rather represents the surface reflection of volume quadrupoles. The subject of the present work--namely, whether a similar surface shear stress term constitutes a true source of dipole sound--has been controversial. Some have boldly assumed it to be a true source and have used it to calculate the noise in boundary-layer flows. Others have argued that, like the surface pressure, the surface shear stress is not a valid source of sound but rather represents a propagation effect. Here, a numerical experiment is undertaken to investigate the issue. A portion of an otherwise static wall is oscillated tangentially in an acoustically compact region to create shear stress fluctuations. The resulting sound field, computed directly from the compressible Navier-Stokes equations, is nearly dipolar and its amplitude agrees with an acoustic analogy prediction that regards the surface shear as acoustically compact and as a true source of sound. However, there is a correction that becomes noticeable for observers near wall-grazing angles as the computational domain size L(sub d) along the wall is increased. An estimate, validated by the simulations, shows that as L(sub d)[right arrow]infinity the correction to the sound in the Fraunhofer zone is proportional to [delta](sub bl)/[lambda] (the ratio of oscillatory boundary-layer thickness to acoustic wavelength) times a directivity factor that becomes large at angles close to grazing. For observers at such angles, Lighthill's acoustic analogy does not apply.
Research Interests:
ABSTRACT Recent work has described screech noise from a supersonic jet as being due to leakage of a wave that is otherwise trapped in the jet's interior. In that work, the simplest of many techniques used is ray tracing for a... more
ABSTRACT Recent work has described screech noise from a supersonic jet as being due to leakage of a wave that is otherwise trapped in the jet's interior. In that work, the simplest of many techniques used is ray tracing for a single shear-layer modeled as a row of Stuart vortices. In the present work, a lower row of vortices is added to form a plane jet. Instead of plotting ray paths, a technique of visualization analogous to streaklines is used that better corresponds to instantaneous density fields as observed, for instance, by the Schlieren method. This produces striking images that show leakage of waves at each internal reflection resulting in a row of acoustic sources as envisioned since the 1950s. However, the sources are not isotropic and each has a zone of silence in the downstream direction. Leakage creates a fold in the wave pattern internal to the jet which leads to fine scale features. Reported experiments have also observed fine scale features (described as splitting) in the shock-cell pattern; they may be related to those observed here. Internally reflected rays also undergo a diffusive process as they propagate down the jet. In particular, each successive internal reflection at an unsteady shear-layer scatters rays along a wider range of wave angle and makes them more susceptible to leakage at the next reflection. It also causes more downstream directivity for the more downstream sources. An important result is that as the Mach number Mj is varied, maxima in leakage rate and mean acoustic amplitude occur at (near) resonances between the Mach-wave and shear-layer periods. Maxima in sound pressure level versus Mj have also been reported for laboratory round jets. Finally, as the shear-layer thickness is increased, a minimum in the rate of leakage (correlated with a minimum in radiation amplitude) occurs due to the competing effects of increased shear-layer penetration versus reduced eddy passage frequency.
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
The formation of vortex rings generated through impulsively started jets is studied experimentally. Utilizing a piston/cylinder arrangement in a water tank, the velocity and vorticity fields of vortex rings are obtained using digital... more
The formation of vortex rings generated through impulsively started jets is studied experimentally. Utilizing a piston/cylinder arrangement in a water tank, the velocity and vorticity fields of vortex rings are obtained using digital particle image velocimetry (DPIV) for a ...
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
ABSTRACT Basis splines (B-splines) are basis functions for piecewise polynomials having a high level of derivative continuity. They possess attractive properties for complex flow simulations: they have compact support, provide a... more
ABSTRACT Basis splines (B-splines) are basis functions for piecewise polynomials having a high level of derivative continuity. They possess attractive properties for complex flow simulations: they have compact support, provide a straightforward handling of boundary conditions and grid nonuniformities, yield numerical schemes with high resolving power, and the order of accuracy is a mere input parameter. This paper reviews progress made in the development and application of B-spline numerical methods to computational fluid dynamics. Basic approximation properties of B-spline schemes are discussed, and their relationship with conventional numerical methods is reviewed. Some fundamental developments towards spline methods in complex geometries are covered. These include local interpolation methods, fast solution algorithms on Cartesian grids, block-structured discretization and compatible pressure bases for the Navier-Stokes equations. Finally, application of some of these techniques to the computation of viscous incompressible flows is presented.