The Viscosity Parameter for Late-type Stable Be Stars

Using hydrodynamic principles we investigate the nature of the disk viscosity following the parameterization by Shakura & Sunyaev adopted for the viscous decretion model in classical Be stars. We consider a radial viscosity distribution including a constant value, a radially variable α assuming a power-law density distribution, and isothermal disks, for a late-B central star. We also extend our analysis by determining a self-consistent temperature disk distribution to model the late-type Be star 1 Delphini, which is thought to have a nonvariable, stable disk as evidenced by Hα emission profiles that have remained relatively unchanged for decades. Using standard angular momentum loss rates given by Granada et al., we find values of α of approximately 0.3. Adopting lower values of angular momentum loss rates, i.e., smaller mass loss rates, leads to smaller values of α. The values for α vary smoothly over the Hα emitting region and exhibit the biggest variations nearest the central star within about five stellar radii for the late-type, stable Be stars.

Optimization of wind augmenters for urban power generation

Wind power is the most available renewable energy source to date due to the relatively low costs and advances in the field. Consequently, there is a high demand for innovative wind technology. Furthermore, providing energy near consumers, such as in inner city dwellings and urban settings, provides a more efficient and more reliable energy source. The use of architecture to augment wind energy extraction is still unresolved and the area of research is still in its infancy. The few studies conducted have shown substantial benefits by using buildings to collect wind to increase the power efficiency of wind turbines beyond the Betz limit. This study utilized computational fluid dynamics to analyze building shapes to optimize wind turbine power production. Results indicate an increase in power of up to approximately 4-8 times compared with that for the undisturbed free stream flow. Furthermore, a porous medium was used to simulate the momentum loss due to the presence of the wind turbine. The trends remained similar despite the momentum loss caused by the presence of the wind turbine. The porous medium results showed an increase of power approximately 2-3 times. The study extended the geometry to 3D to support the 2D results. The test case indicated the 3D results had a higher performance in comparison to 2D due to the 3D interactions of the vortex shedding dampening the variance of velocity in the gap region. Furthermore, a certain geometry performs better at different angles of attack providing the optimal geometry will be specifically tailored to the typical wind directions associated with the desired building location.

Optimization of wind augmenters for urban power generation

Wind power is the most available renewable energy source to date due to the relatively low costs and advances in the field. Consequently, there is a high demand for innovative wind technology. Furthermore, providing energy near consumers, such as in inner city dwellings and urban settings, provides a more efficient and more reliable energy source. The use of architecture to augment wind energy extraction is still unresolved and the area of research is still in its infancy. The few studies conducted have shown substantial benefits by using buildings to collect wind to increase the power efficiency of wind turbines beyond the Betz limit. This study utilized computational fluid dynamics to analyze building shapes to optimize wind turbine power production. Results indicate an increase in power of up to approximately 4-8 times compared with that for the undisturbed free stream flow. Furthermore, a porous medium was used to simulate the momentum loss due to the presence of the wind turbine. The trends remained similar despite the momentum loss caused by the presence of the wind turbine. The porous medium results showed an increase of power approximately 2-3 times. The study extended the geometry to 3D to support the 2D results. The test case indicated the 3D results had a higher performance in comparison to 2D due to the 3D interactions of the vortex shedding dampening the variance of velocity in the gap region. Furthermore, a certain geometry performs better at different angles of attack providing the optimal geometry will be specifically tailored to the typical wind directions associated with the desired building location.

Life after eruption VIII: The orbital periods of novae

The impact of nova eruptions on the long-term evolution of Cataclysmic Variables (CVs) is one of the least understood and intensively discussed topics in the field. A crucial ingredient to improve with this would be to establish a large sample of post-novae with known properties, starting with the most easily accessible one, the orbital period. Here we report new orbital periods for six faint novae: X Cir (3.71 h), IL Nor (1.62 h), DY Pup (3.35 h), V363 Sgr (3.03 h), V2572 Sgr (3.75 h) and CQ Vel (2.7 h). We furthermore revise the periods for the old novae OY Ara, RS Car, V365 Car, V849 Oph, V728 Sco, WY Sge, XX Tau and RW UMi. Using these new data and critically reviewing the trustworthiness of reported orbital periods of old novae in the literature, we establish an updated period distribution. We employ a binary-star evolution code to calculate a theoretical period distribution using both an empirical and the classical prescription for consequential angular momentum loss. In comparison with the observational data we find that both models especially fail to reproduce the peak in the 3 – 4 h range, suggesting that the angular momentum loss for CVs above the period gap is not totally understood.

Numerical modelling of the wind over forests: roughness versus canopy drag

Parameterizing the effect of vertically-distributed vegetation through an effective roughness (z0,eff) – whereby momentum loss through a three-dimensional foliage volume is represented as momentum loss over an area at one vertical level – can facilitate the use of forest data in flow models, to any level of detail, and simultaneously reduce computational cost. Results of numerical experiments and comparison with observations show that a modelling approach based on z0,eff can estimate wind speed and turbulence levels over forested areas, at heights of interest for wind energy applications (∼60 m and higher), but only above flat terrain. Caution must be exercised in the application of such a model to zones of forest edges. Advanced flow models capable of incorporating local (distributed) drag forces are recommended for complex terrain covered by forest.

The effects of surface fossil magnetic fields on massive star evolution – II. Implementation of magnetic braking in mesa and implications for the evolution of surface rotation in OB stars

ABSTRACT The time evolution of angular momentum and surface rotation of massive stars are strongly influenced by fossil magnetic fields via magnetic braking. We present a new module containing a simple, comprehensive implementation of such a field at the surface of a massive star within the Modules for Experiments in Stellar Astrophysics (mesa) software instrument. We test two limiting scenarios for magnetic braking: distributing the angular momentum loss throughout the star in the first case, and restricting the angular momentum loss to a surface reservoir in the second case. We perform a systematic investigation of the rotational evolution using a grid of OB star models with surface magnetic fields (M⋆ = 5–60 M⊙, Ω/Ωcrit = 0.2–1.0, Bp = 1–20 kG). We then employ a representative grid of B-type star models (M⋆ = 5, 10, 15 M⊙, Ω/Ωcrit = 0.2, 0.5, 0.8, Bp = 1, 3, 10, 30 kG) to compare to the results of a recent self-consistent analysis of the sample of known magnetic B-type stars. We infer that magnetic massive stars arrive at the zero-age main sequence (ZAMS) with a range of rotation rates, rather than with one common value. In particular, some stars are required to have close-to-critical rotation at the ZAMS. However, magnetic braking yields surface rotation rates converging to a common low value, making it difficult to infer the initial rotation rates of evolved, slowly rotating stars.