David Richter

Simulations of three-dimensional viscoelastic flows past a circular cylinder at moderate Reynolds numbers

The authors would like to acknowledge the Army High Performance Computing Research centre for Agility, Survivability and Informatics, Award No. W91IN F-072-0027, High Performance Technologies Inc and Department of the Army (Prime) for partial financial and computational support. In addition, this research has been funded in part by a King Abdullah University of Science and Technology (KAUST) research grant under the KAUST Stanford Academic Excellence Alliance program. Any opinions, findings and conclusions or recommendations expressed in this paper are those of the authors and do not necessarily reflect the views of the KAUST University.

Momentum transfer in a turbulent, particle-laden Couette flow

A point-force model is used to study turbulent momentum transfer in the presence of moderate mass loadings of small (relative to Kolmogorov scales), dense (relative to the carrier phase density) particles. Turbulent Couette flow is simulated via direct numerical simulation, while individual particles are tracked as Lagrangian elements interacting with the carrier phase through a momentum coupling force. This force is computed based on the bulk drag of each particle, computed from its local slip velocity. By inspecting a parameter space consisting of particle Stokes number and mass loading, a general picture of how and under what conditions particles can alter near-wall turbulent flow is developed. In general, it is found that particles which adhere to the requirements for the point-particle approximation attenuate small-scale turbulence levels, as measured by wall-normal and spanwise velocity fluctuations, and decrease turbulent fluxes. Particles tend to weaken near-wall vortical activity, which in turn, ...

Evidence of spray-mediated air-sea enthalpy flux within tropical cyclones

It has long been conjectured that spray ejected from the high-wind ocean surface enhances air/sea enthalpy fluxes, but a lack of observational data, particularly at wind speeds exceeding 20 m s −1 , has prevented either confirmation or refutation of this hypothesis. The current study has two aims: first, to provide an estimate of surface enthalpy fluxes obtained from dropsonde data and second, to provide evidence of spray-mediated enthalpy transfer. These are accomplished first by assuming that Monin-Obukhov similarity is satisfied throughout the bottom 100 m of the high-wind boundary layer, then by focusing on the enthalpy flux HK rather than its transfer coefficient CK . The scaling of HK with wind speed in observational data sets reveals similarities to spray-mediated fluxes predicted by a newly developed surface flux model, in contrast with measurements made in a laboratory setting. This behavior supports the claim that surface enthalpy fluxes are dominated by spray within tropical cyclones.

Modification of near-wall coherent structures by inertial particles

Direct numerical simulations are combined with two-way coupled Lagrangian point particles to study the effect of Reynolds number on particle-turbulence interaction. Turbulent planar Couette flow is simulated at a constant dispersed phase mass loading of ϕ m = 0.25 for particle Stokes numbers of St K = [O(1), O(10), O(100)] (based on the Stokes time scale of the particle and the Kolmogorov time scale of the flow) and bulk Reynolds numbers of Re b = [8100, 24000, 72000] (based on the plate velocity difference and separation distance). Statistics of swirling strength |λ ci | are used to evaluate the impact of particles on near-wall motions which are responsible for turbulent, wall-normal momentum transport. Instantaneously, the number of high-strength swirling motions near the wall decreases significantly in the presence of particles, and this trend is enhanced with increasing Re b . Conditional averages are computed using linear stochastic estimation, providing the average structures responsible for ejection events near the wall. These conditional eddies are weakened substantially by the presence of the dispersed phase, and this effect is again enhanced with increasing Re b . We propose a mechanism where particles, by interfering with the hairpin regeneration process near the wall, can influence turbulent fluxes in a way that increases with Re b despite only having direct interaction with scales on the same order as their small physical size. At the same time, turbulent momentum flux concentrated at higher wavenumbers with increasing Re b allows small particles to be effective agents for altering turbulent transport.

Turbulence modification by inertial particles and its influence on the spectral energy budget in planar Couette flow

Two-way coupled, particle-laden simulations are performed in turbulent Couette flow with the purpose of investigating the spectral extent of the particle influence on the turbulent energy cascade in wall-bounded flows. Direct numerical simulation of the carrier phase is performed in conjunction with the Lagrangian point-particle approximation for particles of three distinct inertia ranges: StK=O(1),O(10),O(100). Simulations are also performed at three increasing Reynolds numbers (Reτ≈125,325,900) to determine the longevity of these effects as the scale separation between large and small motions is increased. A spectral decomposition of the turbulent kinetic energy (TKE) budget shows that two simultaneous effects of particles are occurring: first, the mere presence of particles causes a reduction of TKE production across nearly the entire wavenumber range, where the particle Stokes number only determines the magnitude of this reduction; second, the direct energy exchange term between the carrier and disper...

The Sea Spray Contribution to Sensible Heat Flux

Direct numerical simulations (DNS) of turbulent Couette flow are combined with Lagrangian point-particle tracking to investigate the effects of a dispersed phase on bulk passive heat transport when the two phases can exchangebothmomentumandsensibleheat. Theidealizedsetupallows afixednumberofparticles,withoutthe influence of gravity, to be transported by carrier-phase motions across the mean velocity and temperature gradients that exist between the solid boundaries of turbulent Couette flow. In this way, the setup serves as a model of spray in a shear-dominated layer in the immediate vicinity of the water surface and provides insight into the ability of spray to enhance sensible heat fluxes. The authors find that the dispersed phase contributes arelativelylargeamountofverticalheattransport andincreases thetotalheatfluxacross thedomainby25%or greater. Particles that accumulate in regions associated with wall-normal ejections efficiently carry heat across the channel. Furthermore, the authors find that the relative contribution of the dispersed-phase heat flux becomes larger with Reynolds number, suggesting an importance at atmospheric scales.

An Assessment of the Flux Profile Method for Determining Air–Sea Momentum and Enthalpy Fluxes from Dropsonde Data in Tropical Cyclones

AbstractAn analysis of the reliability of using dropsonde profile data to compute surface flux coefficients of momentum and heat is performed. Monin–Obukhov (MO) similarity theory forms the basis for the flux profile method, where mean profiles of momentum, temperature, and moisture are used to estimate surface fluxes, from which bulk flux coefficients can then be determined given surface conditions. The robustness of this method is studied in terms of its sensitivity to internal, method-based parameters, as well as the uncertainty due to variability in the measurements and errors in the estimates of surface conditions, particularly sea surface temperature. In addition, “virtual sondes” tracked through a high-resolution large-eddy simulation of an idealized tropical cyclone are used to evaluate the flux profile method’s ability to recover known surface flux coefficients given known, prescribed surface conditions; this provides a test of whether or not MO assumptions are violated and under which regions th...

Particle stresses in dilute, polydisperse, two-way coupled turbulent flows.

Direct numerical simulations are performed of turbulent planar Couette flow which are seeded with two-way coupled particles at low volume concentration. Based on an understanding of the development of particle stress (horizontal momentum carried vertically on average by the particle phase) in monodisperse systems at various particle Stokes numbers, several bidisperse and continuously polydisperse systems are simulated which are chosen to understand how flows containing blends of particle Stokes numbers can be effectively modeled in the dilute regime. Under noninteracting conditions, the particle stresses from particles with different inertia and different feedback stresses are shown to be linearly additive, providing a convenient method for effectively representing dispersed phase stress in polydisperse systems. While this is true, it is demonstrated that a single effective particle size is in general not sufficient at representing the entire mixture.

Ocean spray: An outsized influence on weather and climate

Because the production, behavior, and life span of seawater droplets are complex, measuring and modeling them require a wide range of interdisciplinary techniques.

Influence of Evaporating Droplets in the Turbulent Marine Atmospheric Boundary Layer

Sea-spray droplets ejected into the marine atmospheric boundary layer take part in a series of complex transport processes. By capturing the air-droplet coupling and feedback, we focus on how droplets modify the total heat transfer across a turbulent boundary layer. We implement a high-resolution Eulerian–Lagrangian algorithm with varied droplet size and mass loading in a turbulent open-channel flow, revealing that the influence from evaporating droplets varies for different dynamic and thermodynamic characteristics of droplets. Droplets that both respond rapidly to the ambient environment and have long suspension times are able to modify the latent and sensible heat fluxes individually, however the competing signs of this modification lead to an overall weak effect on the total heat flux. On the other hand, droplets with a slower thermodynamic response to the environment are less subjected to this compensating effect. This indicates a potential to enhance the total heat flux, but the enhancement is highly dependent on the concentration and suspension time.