James C. Hill

An Improved Hydrodynamic Model for Open Wet Transmission Clutches

A mathematical model describing the hydrodynamics of the flow within a disengaged wet transmission clutch is presented. The primary improvement of this model over the existing ones is the inclusion of the surface tension effect, which is expressed in the pressure equation as an additional term. The drag torque predicted by the model correlates well with the test data for nongrooved clutch packs. The significance of the surface tension in this type of flows is discussed as well.

Scalar dissipation and mixing in turbulent reacting flows

The mixing term that appears in probability density function‐based formulations of the statistical behavior of turbulent reacting flows was studied with the use of direct numerical simulations of an irreversible, second‐order, isothermal chemical reaction between initially segregated reactants in decaying, homogeneous turbulence. Three‐dimensional, time‐dependent pseudospectral calculations with 643 Fourier modes were made as well as were Monte Carlo simulations of the joint concentration pdf equations for coalescence–dispersion (C/D) models. It was found that the development of the scalar microscales depends largely on the initial concentration distributions (both isotropic and nonisotropic distributions were used) and is relatively independent of the reaction rate constant. Joint concentration probability density functions (pdf ’s) were compared to predictions of C/D models that make use of parameters evaluated from the nonreacting or inert system. Although the models predict proper variance decay when ...

Direct numerical simulation of turbulent flows with chemical reaction

Results from full turbulence simulations incorporating the effects of chemical reaction are compared with simple closure theories and used to reveal some physical insights about turbulent reacting flows. Pseudospectral methods for homogeneous turbulent flows with constant physical and thermal properties in domains as large as 643 Fourier modes were used for these simulations. For the case of nonpremixed flows involving a two-species, second-order, irreversible chemical reaction, it is found that the scalar dissipation microscale is only a weak function of the reaction rate and that chemical reaction contributes very little to the decay of the variance of the reactant concentration. Examination of local values of the velocity and concentration fields shows that the local reaction rate is highest in regions of the greatest rates of strain and that vorticity tends to align with the reaction zone. Finally, difficulties associated with the evaluation of multipoint pdfs and with the archival of time-dependent data from the threedimensional simulations are described.

ATOMIZATION IN A VENTURI SCRUBBER

High speed motion pictures were made or the breakup of a single jet of water in the throat of a venturi scrubber for various air and water jet velocities and different nozzle diameters. Atomization seemed to occur as the result of several different mechanisms, depending on the flow conditions, the most common mechanisms being breakup due to either capillary or acceleration waves and also breakup by “steady shear,” all of which have been observed by other investigators. The so-called “cloud-type” atomization proposed by Hesketh (1970) was not observed despite several attempts to achieve it. Measured values of pressure drop across the test section of the scrubber compare well with previously published results.

Mixing and chemical reaction in sheared and nonsheared homogeneous turbulence

Direct numerical simulations were made to examine the local structure of the reaction zone for a moderately fast reaction between unmixed species in decaying, homogeneous turbulence and in a homogeneous turbulent shear flow. Pseudospectral techniques were used in domains of 643 and higher wavenumbers. A finite-rate, single step reaction between non-premixed reactants was considered, and in one case temperature-dependent Arrhenius kinetics was assumed. Locally intense reaction rates that tend to persist throughout the simulations occur in locations where the reactant concentration gradients are large and are amplified by the local rate of strain. The reaction zones are more organized in the case of a uniform mean shear than in isotropic turbulence, and regions of intense reaction rate appear to be associated with vortex structures such as horseshoe vortices and fingers seen in mixing layers. Concentration gradients tend to align with the direction of the most compressive principal strain rate, more so in the isotropic case.

COLLECTION OF INERTIALESS PARTICLES ON SPHEROIDS AND SPHERES WITH ELECTRICAL FORCES AND GRAVITATIO

Collection efficiencies are predicted for the capture of fine, incrtialess, charged particles on a single spheroidal collector in a gaseous flow field by the action of coulombic and external electric field forces and gravity. With the flow and external fields parallel to the axis of symmetry, collection efficiencies for spheroidal collectors are found by determining particle trajectories. For three-dimensional nonsymmetric systems resulting from the flow being at an arbitrary angle to both the axis of symmetry and the external fields, collection efficiencies are found by determining particle fluxes to the collector. For single force cases, particle deposition is independent of collector geometry and, for point panicles, is the same for all stationary incompressible flows. Using a cellular model of flow around a sphere, droplet density in wet scrubbers is shown to have little hydrodynamic effect on single droplet collection efficiencies.

A simple chemical reaction in numerically simulated homogeneous turbulence

v Conservation equations for the concentrations of two species undergoing an irreversible, isothermal, second-order reaction were integrated along with the Navier-Stokes equations f o r a homogeneous, incompressible turbulent flow. A pseudospectral method developed by Kerr was used for domains of 32x32~32 points. Results of the simulations are used to examine closure theories of Toor and of Patterson.

Conditional statistics for passive-scalar mixing in a confined rectangular turbulent jet

Experimental results for the conditional statistics, such as the velocity conditioned on a conserved scalar and the scalar conditioned on velocity, in a confined liquid-phase rectangular jet are presented and analyzed for a data set collected using simultaneous particle image velocimetry and planar laser-induced fluorescence. The joint velocity-scalar probability density function (PDF) is not joint Gaussian in this flow, as the PDF of the conserved scalar is accurately described by a beta-PDF. The conditional mean velocity is found to agree with a linear model when the scalar is close to its local mean value. A gradient PDF model is found to give poor predictions for the streamwise conditional velocity. However, the improved gradient PDF model predicts both the streamwise and transverse conditional velocities well. A linear model for the scalar fluctuation conditioned on velocity is also tested against the experimental data, showing that this model only obtains good approximations when the joint velocity-...

Direct Numerical Simulation and Simple Closure Theory for a Chemical Reaction in Homogeneous Turbulence

The direct numerical simulation of turbulent flows serves as a useful test of simple closure theories, since one can examine the dynamics of the concentration and veloCity fields in more detail than in laboratory experiments and learn how the interaction of turbulent motion and molecular diffusion affects the overall reaction rate. A brief review of the most popular methods available for full turbulence simulations is presented, and a demonstration of the usefulness of direct numerical simulation is given for simple single-point closure theories (viz., those of Toor and of Patterson) applied to the irreversible, second-order chemical reaction of initially unmixed reactants.

Fossil and Active Turbulence

Stably stratified turbulence is commonly encountered in geophysical flows and can dramatically alter the evolutionary characteristics of the turbulence by introducing oscillatory behavior in both the scalar and velocity transport fields. Although only velocities in the vertical direction are subject to buoyancy forces, the horizontal components are also affected through nonlinear interaction with the vertical velocity components. If the length scales of the velocity field are small relative to those at which the buoyancy forces become strong, the turbulence is classified as “active” by Gibson [1986]. In buoyancy-dominated turbulence, some or all motions have inertial forces less than or equal to buoyant forces. There is also reduced turbulent scalar transport and dissipation compared with active turbulence of similar total energy. These ideas have been used by Gibson [1986] in his physical modeling of this problem to derive criteria for discriminating stably stratified turbulent flow which is buoyancy-dominated from inertially-dominated (active) turbulence. His results have received support from the experimental studies of Stillinger et al. [1983] and Itsweire et al. [1986].