Frank Jacobitz

On the structure and dynamics of sheared and rotating turbulence : Direct numerical simulation and wavelet-based coherent vortex extraction

The influence of rotation on the structure and dynamics of sheared turbulence is investigated using a series of direct numerical simulations. Five cases are considered: turbulent shear flow without rotation, with moderate rotation, and with strong rotation, where the rotation configuration is either parallel or antiparallel. For moderate rotation rates an antiparallel configuration increases the growth of the turbulent kinetic energy, while the parallel case reduces the growth as compared to the nonrotating case. For strong rotation rates decay of the energy is observed, linear effects dominate the flow, and the vorticity probability density functions tend to become Gaussian. Visualizations of vorticity show that the inclination angle of the vortical structures depends on the rotation rate and orientation. Coherent vortex extraction, based on the orthogonal wavelet decomposition of vorticity, is applied to split the flow into coherent and incoherent parts. It was found that the coherent part preserves the...

Exploring the Basic Principles of Electric Motors and Generators With a Low-Cost Sophomore-Level Experiment

In order to meet changing curricular needs, an electric motor and generator laboratory experience was designed, implemented, and assessed. The experiment is unusual in its early placement in the curriculum and in that it focuses on modeling electric motors, predicting their performance, and measuring efficiency of energy conversion. While subfractional-horsepower electric motors and a primitive, but unique, small-scale dynamometer were used, experimental results proved to be reliable, accurate, and repeatable. The change in student knowledge and confidence in the application of that knowledge was assessed and shown to have increased significantly in both cases.

Waves in stably stratified turbulent flow

Two approaches for the identification of internal gravity waves in sheared and unsheared homogeneous stably stratified turbulence are investigated. First, the phase angle between the vertical velocity and density fluctuations is considered. It is found, however, that a continuous distribution of the phase angle is present in both weakly and strongly stratified flows. Second, a projection onto the solution of the linearized inviscid equations of motion of unsheared stratified flow is investigated. Solutions of the fully nonlinear viscous Navier–Stokes equations are thus decomposed into vertical wave modes and horizontal vortical modes. However, this decomposition includes some ‘turbulent’ motions in the ‘wave’ field when the Froude number is non-zero. In fact, for Fr > 5 the flow kinetic energy is nearly equally split between the turbulent and wave fields for all wave numbers, as for unstratified flows. This calls into question the adequacy of this decomposition, at least for weak or moderate stratification.

A Comparison of the Turbulence Evolution in a Stratified Fluid With Vertical or Horizontal Shear

The evolution of homogeneous turbulence in a stratified shear flow is studied using direct numerical simulations. Vertically stably stratified flows with uniform vertical shear and with uniform horizontal shear are considered and compared. The Richardson number is varied to include unstratified and strongly stratified cases. For a given value of the Richardson number, the turbulent velocity and density fluctuations were found to be significantly larger in the horizontal shear case due to an increased turbulence production rate that is not directly influenced by buoyancy. Eddy viscosity and eddy diffusivity coefficients are about an order of magnitude larger in the horizontal shear case. Volume visualization shows internal wave development in the strongly stratified cases with both vertical or horizontal shear. A vanishing buoyancy flux, however, is observed in the vertical shear case only. In decaying vertical shear cases, the Ozmidov scale restricts the size of vertical overturns. However, such a restric...

On the structure and dynamics of sheared and rotating turbulence: Anisotropy properties and geometrical scale-dependent statistics

This study is based on a series of nine direct numerical simulations of homogeneous turbulence, in which the rotation ratio f / S of Coriolis parameter to shear rate is varied. The presence of rotation stabilizes the flow, except for a narrow range of rotation ratios 0 f / S1. The main mechanism for the flow’s destabilization is an increased turbulence production due to increased anisotropy. Reynolds stress and the dissipation rate anisotropy tensors have been evaluated and provide a reference for newly defined anisotropy measures. Wavelet-based directional energies capture the properties of velocity gradients. The intermittency of the flow in different directions is quantified with scale-dependent directional flatness. Scale-dependent helicity probability distribution functions allow one to statistically characterize the geometry of the motion at different scales. Small scales are found locally to be predominantly helical, while large scales are not since they tend to two-dimensionalization for cases with growing turbulent kinetic energy. Joint probability distribution functions show that the signs of velocity helicity and vorticity helicity are strongly correlated. This indicates that vorticity helicity tends to diminish velocity helicity.

Exploring Three-Phase Systems and Synchronous Motors: A Low-Voltage and Low-Cost Experiment at the Sophomore Level

In order to meet changing curricular and societal needs, a three-phase system and synchronous motor laboratory experience for sophomore-level students in a wide variety of engineering majors was designed, implemented, and assessed. The experiment is unusual in its early placement in the curriculum, and in that it focuses primarily on basic understanding of balanced three-phase systems and synchronous motor operating principles. While a low-voltage three-phase system and subfractional-horsepower electric motors were used, experimental results proved to be reliable, accurate, and repeatable. Changes in student knowledge and confidence in the application of that knowledge was assessed and shown to have increased significantly in each case.

Hollow-Fiber Cartridges: Model Systems for Virus Removal From Blood

Aethlon Medical is developing an extracorporeal blood filter as a therapeutic device designed to remove viruses and toxins from the blood of patients. The Hemopurifier is a modified hollow-fiber plasmapheresis cartridge containing an affinity matrix in the extra capillary space. The matrix contains a high mannose specific lectin as the active capture agent. The flow configuration of the device is that of Starling flow. The filter is designed to clear viruses and toxins from blood, delaying illness so the patient’s immune system can fight off the virus. Results to date indicate the efficient removal of a variety of enveloped viruses including HIV, HCV and poxviruses with in vitro evidence indicating the ability to capture Dengue fever virus, measles, mumps, influenza, Ebola and Marburg. Possible additional targets include bioweapons such as smallpox and bacterial toxins. A schematic of the use of the filter in a therapeutic application is shown in figure 1. In order to optimize the design of such a filter, the fluid mechanics of the device is modeled analytically and investigated experimentally. Additional information can be found in Tullis et al. [1], Tullis et al. [2], and Duffin and Tullis [2].Copyright

Application of the Large-Eddy Approach to the Simulation of Turbulence in Uniform Shear Flow

ABSTRACT In the Rogallo approach to the simulation of homogeneous turbulence in uniform shear flow, the equations of motion are solved in a reference frame that is moving with the mean flow. This choice of reference frame allows the application of periodic boundary conditions to the fluctuating velocity components and the use of a highly accurate spectral scheme for the spatial discretization. However, as time is advanced, the reference frame becomes more and more skewed and a regridding of the computational domain using the periodic structure of the velocity components is required. This regridding procedure introduces aliasing errors that are removed in direct numerical simulation. This study addresses the application of this approach to large-eddy simulation of turbulence in uniform shear flow. Results are compared between direct numerical simulation and large-eddy simulation.

On helical multiscale characterization of homogeneous turbulence

The helical properties of five prototypical homogeneous turbulent flows are investigated: statistically steady forced isotropic turbulence, decaying isotropic turbulence, decaying rotating turbulence, growing sheared turbulence, and growing rotating sheared turbulence with a rotation ratio f/S=+0.5. The five turbulent flows were originally studied using direct numerical simulations, and well-developed flow fields are chosen for this analysis. For comparison, a solenoidal uncorrelated Gaussian random field is included in the analysis as a sixth case. An orthogonal wavelet decomposition is used to study the scale-dependent properties of the cases. It is found that flows with growing turbulent kinetic energy and turbulent motion at large scales show a maximum in the relative kinetic helicity probability distribution functions (PDFs) at zero, corresponding to a trend to local two-dimensionalization of the flow with vorticity and velocity tending to be perpendicular. Flows with decaying turbulent kinetic energ...

A Wind Tunnel as a Senior/Capstone Design Project: Laboratory Enhancement and Assessment of Student Learning

Senior-level engineering students spend their final year designing and constructing challenging engineering-based capstone design projects in groups of two or more. The main objective of the senior design project is to give students first-hand knowledge of how the engineering design process works in a simulated industrial environment. Specifically, it is meant to familiarize students with the key objectives of senior design: design, construction, and testing. In addition to satisfying the learning objectives of senior design, a wind tunnel was constructed to enhance the fluid mechanics laboratory. The completion of the wind tunnel satisfied the two major goals of the project: completion of the senior design course objectives, and laboratory enhancement. In order to demonstrate the effectiveness of the students learning the fundamentals of design, construction, and testing, student surveys were completed and reported.