Energy Dissipation Rate (energy + dissipation_rate)

Distribution by Scientific Domains

Selected Abstracts

Small-scale fluid motion mediates growth and nutrient uptake of Selenastrum capricornutum

Summary 1. A fluid-flow reactor using submersible speakers was constructed to generate small-scale fluid motion similar to conditions measured in open water environments; flow was quantified by particle image velocimetry. Additionally a Couette-type rotating cylinder was used to generate shear flows; flow was quantified using an optical hotwire probe and torque measurements. Growth rates of the green alga Selenastrum capricornutum were determined from changes in cell counts and viability was tested using the fluorogenic probe fluoresceine diacetate. 2. Evidence that fluid motion directly affects growth rates was obtained as a significant difference between growth in a moving versus non-moving fluid. A near 2-fold increase in growth rate was achieved for an energy dissipation rate of , = 10,7 m2 s,3; a rate common in lakes and oceans. The onset of the viability equilibrium, identified as the day of the test period when the number of active cells equalled non-active cells, was delayed by 2 days for moving fluid conditions as compared with a non-moving fluid. 3. Nutrient uptake was determined by a decrease in the bulk fluid concentration and cellular phosphorus concentration was also estimated. The thickness of the diffusive sublayer surrounding a cell, a zone dominated by molecular diffusion, was estimated. Increasing fluid motion was found to decrease the thickness of this layer. The Sherwood number (ratio of total mass flux to molecular mass flux) showed that advective flux surrounding cells dominated molecular diffusion flux with regard to Péclet numbers (ratio of advective transport to molecular diffusion transport). Fluid motion facilitated uptake rates and resulted in increased growth rates, compared with no-flow conditions. The rate-of-rotation and the rate-of-strain in a moving fluid equally mediated the diffusive sublayer thickness surrounding the cells. Our study demonstrates that small-scale fluid motion mediates algal growth kinetics and therefore should be included in predictive models for algal blooms. [source]

CFD modeling of heat transfer in turbulent pipe flows

AICHE JOURNAL, Issue 9 2000
S. S. Thakre
Twelve versions of low Reynolds number k-, and two low Reynolds number Reynolds stress turbulence models for heat transfer were analyzed comparatively. Predictions of the mean axial temperature, the radial and axial turbulent heat fluxes, and the effect of Prandtl number on Nusselt number were compared with the experimental data. The model by Lai and So from the k-, group and Lai and So from the Reynolds stress group had the best overall predictive ability for heat transfer in turbulent pipe flow. The Lai and So model was attributed to its success in the predictions of flow parameters such as mean axial velocity, turbulent kinetic energy, eddy diffusivity, and the overall energy dissipation rate. The k-, models performed relatively better than the Reynolds stress models for predicting the mean axial temperature and the Nusselt number. This qualitative and quantitative study found the need for more sophisticated near-wall experimental measurements and the accuracy of the dissipation (of turbulent energy) and the pressure-scrambling models. [source]

On the orbital period modulation of RS CVn binary systems

Antonino F. Lanza
ABSTRACT The Applegate hypothesis proposed to explain the orbital period modulation of RS Canum Venaticorum (RS CVn) close binaries (Applegate 1992) is considered in the framework of a general model to treat the angular momentum exchanges within the convective envelope of a magnetically active star. This model assumes that the convection zone is strictly adiabatic and that the Taylor,Proudman balance holds, leading to an internal angular velocity constant over cylindrical surfaces co-axial with the rotation axis. It turns out that the angular velocity perturbations, whatever their origin, can be expressed in terms of the eigenfunctions of the equation of angular momentum conservation with stress-free boundary conditions. Moreover, a lower limit for the energy dissipation rate in a turbulent convection zone can be set, thanks to the extremal properties of the eigenfunctions. This approach allows to apply precise constraints on the amplitude and the radial profile of the angular velocity variations that are required to explain the observed orbital period changes in classical RS CVn binaries (i.e. with orbital period longer than 1,2 d and a subgiant secondary component). It is found that an angular velocity change as large as 10 per cent of the unperturbed angular velocity at the base of the stellar convection zone is needed. Such a large change is not compatible with the observations. Moreover, it would produce an energy dissipation rate much larger than the typical luminosities of the active components of RS CVn systems, except in the case that fast rotation and internal magnetic fields reduce the turbulent viscosity by at least 2 orders of magnitude with respect to the value given by the mixing-length theory. Therefore, the model proposed by Applegate should be rejected, at least in the case of classical RS CVn close binaries. Possible alternative models are briefly discussed, emphasizing the effects of intense magnetic fields (, 10 T) on the internal structure of magnetically active stars and the dynamics of close binary systems. [source]

The local isotropy hypothesis and the turbulent kinetic energy dissipation rate in the atmospheric surface layer

M. Chamecki
Abstract We test the applicability of the local isotropy hypothesis to surface-layer turbulent flow; turbulent velocities measured with a three-dimensional sonic anemometer are used for this purpose, and the predictions of local isotropy for the spectra, second- and third-order structure functions are assessed against measured data. Also investigated are scale interactions via the correlation between velocities and velocity increments, and the ability of isotropic spectral models to reproduce measured spectra. In general, second-order structure functions display a narrower inertial range than the corresponding spectra; both the known effects of path-averaging and the predictions of the spectral models show that the sonic anemometer is unable to resolve the whole inertial range, even at a measurement frequency of 60 Hz. We confirm previous results that unstable runs tend to be more isotropic, but find that, for third-order statistics, isotropy does not hold well for the data analysed. Turbulence intensity, and not atmospheric stability, plays a determining role on the correlation coefficient between velocities and velocity increments. The observed anisotropic behaviour has important implications for the calculation of the turbulent kinetic energy dissipation rate from Kolmogorov's four-fifths law, whose estimates are consistently smaller than those from the inertial range of the spectrum or the structure functions. Copyright © 2004 Royal Meteorological Society [source]

Representing energy drain in numerical weather prediction models induced by boundary layer sub-grid scale processes

C. G. Collier
Abstract The representation of sub-grid scale boundary layer processes is central to understanding the errors in numerical weather prediction model forecasts. Of particular importance are statistics associated with convective turbulence, notably the temporal and spatial variations of kinetic energy dissipation rate. In this paper we outline how a 1.5-micron Doppler lidar system may be used in this context, and propose an operational network of such systems for use in numerical forecasting. Copyright © 2009 Royal Meteorological Society [source]

Growth inhibition of dinoflagellate algae in shake flasks: Not due to shear this time!

Weiwei Hu
Abstract Large scale algae cultures present interesting challenges in that they exhibit characteristics of typical bacterial and animal cell cultures. One current commercial food additive, docosahexaenoic acid (DHA), is produced using the dinoflagellate algae, Crypthecodiniumcohnii. Like animal cell culture, the perceived sensitivity of algae culture to hydrodynamic forces has potentially limited the agitation and aeration applied to these systems. However, the high density cultivation of C. cohnii required for an economically feasible process inevitably results in high oxygen demand. In this study, we demonstrated what first appeared to be a problem with shear sensitivity in shake flasks is most probably a mass transfer limitation. We subsequently demonstrated the limit of chronic and rapid energy dissipation rate, EDR, that C. cohnii cells can experience. This limit was determined using a microfluidic device connected in a recirculation loop to a stirred tank bioreactor, which has been previously used to repeatedly expose animal cells to high levels of EDR. Inhibition of cell growth was observed when C. cohnii cells were subjected to an EDR of 5.9 × 106 W/m3 with an average frequency of 0.2/min or more. This level of EDR is sufficiently high that C. cohnii can withstand typically encountered hydrodynamic forces in bioprocesses. This result suggests that at least one dinoflagellate algae, C. cohnii, is quite robust with respect to hydrodynamic forces and the scale-up of process using this type of algae should be more concerned with providing sufficient gas transfer given the relatively high oxygen demand. © 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2010 [source]

The Sensitivity of the Dinoflagellate Crypthecodinium cohnii to Transient Hydrodynamic Forces and Cell-Bubble Interactions

Weiwei Hu
The increased interest in the benefits of omega-3 fatty acids for human health has resulted in the commercial development of the dinoflagellate Crypthecodiniumcohnii for production of docosahexaenoic acid (DHA). The growing market demand for DHA requires highly efficient, very large scale cultures of DHA. While the effects of hydrodynamic forces on dinoflagellates have been investigated for several decades, the majority of the work focused on the negative effects of oceanic turbulence on the population growth of environmentally important dinoflagellates. In contrast, significantly less is known on the effect of hydrodynamic forces encountered by algae in bioprocesses. Unlike other studies conducted on algae, this study employed a microfluidic, flow contraction device to evaluate the effect of transient hydrodynamic forces on C. cohnii cells. It was found that C. cohnii cells can sustain the energy dissipation rate of 5.8 × 107 W/m3 without lysis. However, an obvious sublethal effect, the loss of flagella, was observed at a lower level of 1.6 × 107 W/m3. Finally the cell-bubble interaction and the effect of bubble rupture were also explored to simulate the conditions of sparged bioreactors. [source]

Bubble Size Distribution in Oil-Based Bubble Columns

S.-S. Homayouni
Abstract A practical population balance model was used to evaluate the bubble size distribution in a bubble column. In addition, the bubble size distribution in the bubble column was measured at different gas velocities by photography and analysis of the pictures. Four types of liquid, i.e., water and three petroleum-based liquids, were used in the experiments. The gas phase was air. It was found that the existing models in the literature are not able to satisfactorily predict the experimentally measured bubble size distribution. The model can be corrected by applying a correction factor to the energy dissipation rate. The corrected model fits the experimental bubble size distribution considerably better than the existing models. The variation of this correction factor is reported for different systems at different gas velocities. [source]