Nonlinear Tidal Flows in Short-Period Planets

I discuss two related nonlinear mechanisms of tidal dissipation that require finite tidal deformations for their operation: the elliptical instability and the precessional instability. Both are likely to be important for the tidal evolution of short-period extrasolar planets. The elliptical instability is a fluid instability of elliptical streamlines, such as in tidally deformed non-synchronously rotating or non-circularly orbiting planets. I summarise the results of local and global simulations that indicate this mechanism to be important for tidal spin synchronisation, planetary spin-orbit alignment and orbital circularisation for the shortest period hot Jupiters. The precessional instability is a fluid instability that occurs in planets undergoing axial precession, such as those with spin-orbit misalignments (non-zero obliquities). I summarise the outcome of local MHD simulations designed to study the turbulent damping of axial precession, which suggest this mechanism to be important in driving tidal evolution of the spin-orbit angle for hot Jupiters. Avenues for future work are also discussed

Nonlinear hydrodynamical evolution of eccentric Keplerian discs in two dimensions: Validation of secular theory

We perform global two-dimensional hydrodynamical simulations of Keplerian discs with free eccentricity over thousands of orbital periods. Our aim is to determine the validity of secular theory in describing the evolution of eccentric discs, and to explore their nonlinear evolution for moderate eccentricities. Linear secular theory is found to correctly predict the structure and precession rates of discs with small eccentricities. However, discs with larger eccentricities (and eccentricity gradients) are observed to precess faster (retrograde relative to the orbital motion), at a rate that depends on their eccentricities (and eccentricity gradients). We derive analytically a nonlinear secular theory for eccentric gas discs, which explains this result as a modification of the pressure forces whenever eccentric orbits in a disc nearly intersect. This effect could be particularly important for highly eccentric discs produced in tidal disruption events, or for narrow gaseous rings; it might also play a role in causing some of the variability in superhump binary systems. In two dimensions, the eccentricity of a moderately eccentric disc is long-lived and persists throughout the duration of our simulations. Eccentric modes are however weakly damped by their interaction with non-axisymmetric spiral density waves (driven by the Papaloizou-Pringle instability, which occurs in our idealised setup with solid walls), as well as numerical diffusion.AJB is supported by the Leverhulme Trust and Isaac Newton Trust through the award of an Early Career Fellowship. The early stages of this research were supported by STFC through grants ST/J001570/1 and ST/L000636/1.This is the author accepted manuscript. The final version is available from Oxford University Press via http://dx.doi.org/10.1093/mnras/stw58

Non-linear evolution of the tidal elliptical instability in gaseous planets and stars

Tidally distorted rotating stars and gaseous planets are subject to a well-known linear fluid instability – the elliptical instability. It has been proposed that this instability might drive enough energy dissipation to solve the long-standing problem of the origin of tidal dissipation in stars and planets. But the non-linear outcome of the elliptical instability has yet to be investigated in the parameter regime of interest, and the resulting turbulent energy dissipation has not yet been quantified. We do so by performing three-dimensional hydrodynamical simulations of a small patch of a tidally deformed fluid planet or star subject to the elliptical instability. We show that when the tidal deformation is weak, the non-linear outcome of the instability leads to the formation of long-lived columnar vortices aligned with the axis of rotation. These vortices shut off the elliptical instability, and the net result is insufficient energy dissipation to account for tidal dissipation. However, further work is required to account for effects neglected here, including magnetic fields, turbulent convection and realistic boundary conditions

Turbulent viscosity acting on the equilibrium tidal flow in convective stars

Convection is thought to act as a turbulent viscosity in damping tidal flows and in driving spin and orbital evolution in close convective binary systems. This turbulent viscosity should be reduced, compared to mixing-length predictions, when the forcing (tidal) frequency $| {\omega }_{t}|$ exceeds the turnover frequency ω cv of the dominant convective eddies. However, two contradictory scaling laws have been proposed and this issue remains highly disputed. To revisit this controversy, we conduct the first direct numerical simulations of convection interacting with the equilibrium tidal flow in an idealized global model of a low-mass star. We present direct computations of the turbulent effective viscosity, ν E , acting on the equilibrium tidal flow. We unexpectedly report the coexistence of the two disputed scaling laws, which reconciles previous theoretical (and numerical) findings. We recover the universal quadratic scaling ${\nu }_{E}\propto {(| {\omega }_{t}| /{\omega }_{{cv}})}^{-2}$ in the high-frequency regime $| {\omega }_{t}| /{\omega }_{{cv}}\gg 1$. Our results also support the linear scaling ${\nu }_{E}\propto {(| {\omega }_{t}| /{\omega }_{{cv}})}^{-1}$ in an intermediate regime with $1\leqslant | {\omega }_{t}| /{\omega }_{{cv}}\lesssim { \mathcal O }(10)$. Both regimes may be relevant to explain the observed properties of close binaries, including spin synchronization of solar-type stars and the circularization of low-mass stars. The robustness of these two regimes of tidal dissipation, and the transition between them, should be explored further in more realistic models. A better understanding of the interaction between convection and tidal flows is indeed essential to correctly interpret observations of close binary stars and short-period planetary orbits

Teaching intercultural skills in the multicultural classroom

This paper evaluates the adaptation of the ExcelL Intercultural Skills Program to an international human resource management course, and critically examines the effectiveness of integrating case study methods with the skills based ExcelL program. The ExcelL program is a theory-driven and evidence-based group program that utilises classroom cultural diversity, student experiences and role-plays to develop specific intercultural skills. The primary purpose of the study was to assess the usefulness of ExcelL within a sample of 85 third-year university business students. Four methodologies were used: (1) questionnaires, (2) case studies, (3) course evaluation survey and (4) focus groups. After completing the program, participants demonstrated improvements in intercultural competencies required in business contexts as well as increased self-confidence and feelings of self-efficacy in cross-cultural situations. Additionally, participants demonstrated improved identification and articulation of cross-cultural business problems and formulation of appropriate intervention strategies to address these problems

Theory and Simulations of Rotating Convection

We study thermal convection in a rotating fluid in order to better understand the properties of convection zones in rotating stars and planets. We first derive a mixing-length theory for rapidly rotating convection, arriving at the results of Stevenson via simple physical arguments. The theory predicts the properties of convection as a function of the imposed heat flux and rotation rate, independent of microscopic diffusivities. In particular, it predicts the mean temperature gradient, the rms velocity and temperature fluctuations, and the size of the eddies that dominate heat transport. We test all of these predictions with high resolution three-dimensional hydrodynamical simulations of Boussinesq convection in a Cartesian box. The results agree remarkably well with the theory across more than two orders of magnitude in rotation rate. For example, the temperature gradient is predicted to scale as the rotation rate to the four-fifths power at fixed flux, and the simulations yield 0.75 ± 0.06. We conclude that the mixing-length theory is a solid foundation for understanding the properties of convection zones in rotating stars and planets

Preferences in recruitment and selection in a sample of Australian organisations

The diversity of the Australian workforce has increased over the last 25 years, particularly with the increase in the number of migrants from South-east Asia. Despite the knowledge, skill and expertise that many of these immigrants possess prior to migrating, a high proportion of immigrants experience significant difficulties obtaining employment commensurate with their abilities. This paper is an exploratory study of the preferences of managers, recruitment specialists and migrants regarding recruitment and selection practices in selected Australian cities. Surprisingly, the findings revealed similarities between the three groups (managers, migrants and recruitment consultants). Thus, this study adds to the debate about the impact of cross-cultural expectations in recruitment and selection in Australian organisations

The views of managers and recruitment specialists in relation to migrants' opportunities in recruitment and selection processes in Australia

Australia’s population is one of the world’s most culturally and linguistically diverse. With declining fertility rates, the overall intake of immigrants will continue to increase with the proportion of Asian immigrants predicted to rise significantly over the next 50 years. Despite many of these immigrants holding recognised qualifications and previous work experience, a high proportion are faced with unemployment or employment in positions, which are not commensurate with their knowledge and abilities. This paper which examines the views of managers and recruitment specialists is part of a larger study investigating the experiences of South-east Asian migrants in recruitment and selection in Australia. Surveys were administered to Master of Business Administration students who are currently employed in management positions. Due to the increasingly popular trend of outsourcing HR activities, 10 Recruitment Consultants were interviewed with questions based upon the managers’ questionnaire. The responses to questions relating to the difficulties that migrants are faced with in recruitment and selection, and recommendations as to what migrants may do regarding behaviour and training, were analysed. Overall, communication and cultural issues were seen as central to the difficulties experienced by migrants in recruitment and selection processes and therefore carved the basis upon which recommendations were formed. The findings of this study have implications for human resource professionals and employment support agencies who are actively involved with recruitment and selection in general, or who specialise in assisting migrants with job seeking

The role of communication in recruitment and selection in Australia

In Australia, unemployment and under-employment continue to be significant issues experienced bymigrants from non-English-speaking backgrounds (NESB). Despite these immigrants having relevant qualifications, skills, and work experience, a high proportion have difficulties in obtaining employment in Australian organisations. Communication or language proficiency has been identified as one of the primary causal factors. Moreover, studies have identified the importance of communication as a key competency sought by managers and recruiters in all job applicants. Given the importance of making employment decisions more objective and measurable to meet anti-discrimination requirements, this study examines the perceptions of managers and recruitment specialists about the communication skills that are essential for successful recruitment and selection. Subsequently, areas for inteNention are highlighted

Non-linear tides in a homogeneous rotating planet or star: Global modes and elliptical instability

We revisit the global modes and instabilities of homogeneous rotating ellipsoidal fluid masses, which are the simplest global models of rotationally and tidally deformed gaseous planets or stars. The tidal flow in a short-period planet may be unstable to the elliptical instability, a hydrodynamic instability that can drive tidal evolution. We perform a global (and local WKB) analysis to study this instability using the elegant formalism of Lebovitz & Lifschitz. We survey the parameter space of global instabilities with harmonic orders ℓ ≤ 5, for planets with spins that are purely aligned (prograde) or anti-aligned (retrograde) with their orbits. In general, the instability has a much larger growth rate if the planetary spin and orbit are anti-aligned rather than aligned. We have identified a violent instability for anti-aligned spins outside of the usual frequency range for the elliptical instability (when n/Ω ≲ -1, where n and Ω are the orbital and spin angular frequencies, respectively) if the tidal amplitude is sufficiently large. We also explore the instability in a rigid ellipsoidal container, which is found to be quantitatively similar to that with a realistic free surface. Finally, we study the effect of rotation and tidal deformation on mode frequencies. We find that larger rotation rates and larger tidal deformations both decrease the frequencies of the prograde sectoral surface gravity modes. This increases the prospect of their tidal excitation, potentially enhancing the tidal response over expectations from linear theory. In a companion paper, we use our results to interpret global simulations of the elliptical instability.AJB is supported by the Leverhulme Trust and Isaac Newton Trust through the award of an Early Career Fellowship. The early stages of this research were supported by STFC through grants ST/J001570/1 and ST/L000636/1. HB was supported by a studentship funded by STFC and Trinity College, Cambridge. We would like to thank the referee, Jeremy Goodman, for several suggestions which have allowed us to improve the paper.This is the author accepted manuscript. The final version is available from Oxford University Press via http://dx.doi.org/10.1093/mnras/stw70