Blair Perot

Laminar drag reduction in microchannels using ultrahydrophobic surfaces

A series of experiments is presented which demonstrate significant drag reduction for the laminar flow of water through microchannels using hydrophobic surfaces with well-defined micron-sized surface roughness. These ultrahydrophobic surfaces are fabricated from silicon wafers using photolithography and are designed to incorporate precise patterns of microposts and microridges which are made hydrophobic through a chemical reaction with an organosilane. An experimental flow cell is used to measure the pressure drop as a function of the flow rate for a series of microchannel geometries and ultrahydrophobic surface designs. Pressure drop reductions up to 40% and apparent slip lengths larger than 20 μm are obtained using ultrahydrophobic surfaces. No drag reduction is observed for smooth hydrophobic surfaces. A confocal surface metrology system was used to measure the deflection of an air–water interface that is formed between microposts and supported by surface tension. This shear-free interface reduces the ...

A moving unstructured staggered mesh method for the simulation of incompressible free-surface flows

A new moving staggered mesh discretization for the numerical simulation of incompressible flow problems involving free-surfaces is presented. The method uses the staggered mesh to obtain speed and conservation properties. Mesh motion provides a high quality mesh in the interior and cetailed resolution of the free-surface motion on the surface. Mesh flipping allows for optimal mesh connectivity to be maintained. The method uses an exact projection procedure which reduces the number of unknowns as well as satisfying the continuity constraint without solving a pressure Poisson equation. The implementation of surface tensior forces in the staggered mesh framework is discussed. The resulting method is tested against analytical solutions for liquid sloshing and free-surface channel flow. It is also demonstrated on the cases of droplet collision, three-dimensional sloshing, and turbulence next to a free-surface.

Turbulence modeling using body force potentials

Reynolds averaged Navier–Stokes (RANS) turbulence models are usually concerned with modeling the Reynolds stress tensor. An alternative approach to RANS turbulence modeling is described where the primary modeled quantities are the scalar and vector potentials of the turbulent body force—the divergence of the Reynolds stress tensor. This approach is shown to have a number of attractive properties, most important of which is the ability to model nonequilibrium turbulence situations accurately at a cost and complexity comparable to the widely used two-equation models such as k-e. Like Reynolds stress transport equation models, the proposed model does not require a hypothesized constitutive relation between the turbulence and the mean flow variables. This allows nonequilibrium turbulence to be modeled effectively. However, unlike Reynolds stress transport equation models, the proposed system of partial differential equations is much simpler to model and compute. It involves fewer variables, no realizability c...

Prediction of turbulent transition in boundary layers using the turbulent potential model

The turbulent potential model is applied to predict transition in flat plate boundary layers. The turbulent potential model is a non-equilibrium RANS model with the predictive capabilities of a Reynolds stress transport model but the cost and complexity of more popular two-equation models. It has been applied to numerous fully turbulent flows with great success. In this work the model is applied to bypass and natural transition of flat plate boundary layers. We include the effects of acoustic noise and pressure gradients and demonstrate the models ability to relaminarize.

Advances in turbulent mixing techniques to study microsecond protein folding reactions

Recent experimental and computational advances in the protein folding arena have shown that the readout of the one-dimensional sequence information into three-dimensional structure begins within the first few microseconds of folding. The initiation of refolding reactions has been achieved by several means, including temperature jumps, flash photolysis, pressure jumps, and rapid mixing methods. One of the most commonly used means of initiating refolding of chemically denatured proteins is by turbulent flow mixing with refolding dilution buffer, where greater than 99% mixing efficiency has been achieved within 10s of microseconds. Successful interfacing of turbulent flow mixers with complementary detection methods, including time-resolved Fluorescence Spectroscopy (trFL), Förster Resonance Energy Transfer, Circular Dichroism, Small-Angle X-ray Scattering, Hydrogen Exchange followed by Mass Spectrometry and Nuclear Magnetic Resonance Spectroscopy, Infrared Spectroscopy (IR), and Fourier Transform IR Spectroscopy, has made this technique very attractive for monitoring various aspects of structure formation during folding. Although continuous-flow (CF) mixing devices interfaced with trFL detection have a dead time of only 30 µs, burst phases have been detected in this time scale during folding of peptides and of large proteins (e.g., CheY and TIM barrels). Furthermore, a major limitation of the CF mixing technique has been the requirement of large quantities of sample. In this brief communication, we will discuss the recent flurry of activity in micromachining and microfluidics, guided by computational simulations, which are likely to lead to dramatic improvements in time resolution and sample consumption for CF mixers over the next few years.

A model for the dissipation rate tensor in inhomogeneous and anisotropic turbulence

A model for the dissipation rate tensor in anisotropic inhomogeneous turbulence is developed. By including terms that depend on gradients a dissipation model is developed that is exact in the limit of very strong inhomogeneity (such as near solid walls or free surfaces). Rapid distortion theory and equilibrium theory are used to motivate the anisotropic terms in the model. The resulting model has only one free constant (from the equilibrium theory) which is determined via comparisons with turbulent channel flow at Re=590. A priori tests of the model for two shear-free boundary layers, channel flow at lower Reynolds numbers, and a backward facing step are presented. Full simulations using the model in channel flow are also performed. Comparisons are made with a variety of existing tensor dissipation rate models.

Modeling return to isotropy using kinetic equations

Kinetic equations modeling the behavior of the velocity probability density function (PDF) in homogeneous anisotropic decaying turbulence are hypothesized and their implications for return-to-isotropy are investigated. Anisotropic turbulent decay is a parametrically simple but theoretically complex turbulent flow that is dominated by nonlinear interactions. The physical implications of the Bhatnagar–Gross–Krook model, a relaxation model, and the Fokker–Planck model for the “collision” term in the PDF evolution equation are analyzed in detail. Using fairly general assumptions about the physics, three different parameter-free return-to-isotropy models are proposed. These models are compared with experimental data, classical models, and analytical limits. The final model expression is particularly interesting, and can easily be implemented in classic Reynolds stress transport models.

Application of the turbulent potential model to complex flows

ABSTRACT The turbulent potential model is a RANS model that avoids modeling the Reynolds stress tensor. As a result it has the ability to obtain the physical accuracy of Reynolds stress transport equation models at a cost and complexity comparable to popular two equation models. The models ability to predict channel flow, free-shear layers, homogeneous shear flow, stagnation point flow, backward facing step flows, and boundary layers with and without strong adverse pressure gradients has been demonstrated previously. In the present study, the performance of the turbulent potential model is evaluated in a series of complex non-equilibrium turbulent flows. These include three-dimensional boundary layers, unsteady vortex shedding, rotating turbulent flows and boundary layer transition.

Modeling Separation and Reattachment Using the Turbulent Potential Model

ABSTRACT A new type of turbulence model has recently been proposed by the first author which involves transport equations for the scalar and vector potentials of the turbulent body force (the divergence of the Reynolds stress tensor). Theoretical analysis of this turbulent potential model suggests that the predictive accuracy of Reynolds stress transport equation models might be obtained at a cost comparable to state-of-the-art two-equation turbulence models. Initial tests of the model showed promising results for simple turbulent flows such as channel flow at various Reynolds numbers, boundary layers, mixing layers, rotating channel flow, and even transition. In order to understand the model’s behavior in more complex flow situations it is now tested on a number of more complex flows involving flow separation, reattachment and stagneation. Model predictions for two adverse pressure-gradient boundary layers are presented, one mild and one on the verge of separation. In addition, predictions for the backward facing step and a turbulent impinging jet are presented.

Eddy Collision Models for Turbulence

ABSTRACT Simple fluids such as gases and liquids are the result of collisions between molecules. More complex fluids, such as granular flows and colloidal suspensions (non-Newtonian fluids), result from the more complex collision (or interaction) behaviors of their constituent particles. In this paper it is demonstrated that collision rules can be constructed for large chunks of fluid material (eddies) such that the resulting collective system behaves like the mean (RANS) flow of a turbulent fluid. The collision model approach has a number of advantages over classic Reynolds stress transport (RST) models. For example, turbulent transport does not require a model and mathematical constraints like realizability are automatically satisfied. Using some ideas from lattice-Boltzmann methods and adaptive moving mesh algorithms for CFD it is shown that this modeling approach can be made computationally efficient and comparable in cost to classic Reynolds stress transport (RST) models. Finally, it is shown that the collisional approach to turbulence modeling can lead to some insights into turbulence and turbulence modeling that would probably not have been achieved via the traditional RST approach.