VISCOSITY IN SEAKEEPING

The paper addresses the issue of actuality in ship hydrodynamics: the estimation of ship’s linear and angular oscillations with respect to the state of equilibrium. The prediction of seakeeping properties raises a question about a relative importance of viscous and free-surface effects (Quérard et al. 2009), yet this question remains of more general importance in fluid mechanics, since it is related to the dynamic characteristics of objects/bodies immersed in a liquid. From a theoretical standpoint, the problem refers to flows with moving boundaries. It can also be considered in terms of fluid-structure interaction (FSI), however, not necessarily linked with the computation of the body deformation and stresses due to the flow. As the Author correctly notices, the computational solution to this problem in its full setup reveals to be extremely costly due to the 3D and unsteady nature of the fluid motion under turbulent flow conditions at nominally high Reynolds numbers (Re~109, as stated by the Author in Tab. 1) in presence of the free surface. For this reason, the full solution, or direct numerical simulation (DNS), of the governing Navier-Stokes (N-S) equations at these Re will remain unfeasible in the foreseeable future; see, e.g., Pozorski (2017) for an estimation of the DNS capability in simple wall-bounded turbulent flows. The situation gets even worse in ship hydrodynamics when a DNS of fluid flow would need to be coupled to the dynamics of the rigid body (of complex geometry, usually).

Asymptotic Weyl double copy

A characteristic value formulation of the Weyl double copy leads to an asymptotic formulation. We find that the Weyl double copy holds asymptotically in cases where the full solution is algebraically general, using rotating STU supergravity black holes as an example. The asymptotic formulation provides clues regarding the relation between asymptotic symmetries that follows from the double copy. Using the C-metric as an example, we show that a previous interpretation of this gravity solution as a superrotation has a single copy analogue relating the appropriate Liénard-Wiechert potential to a large gauge transformation.

Quantifying the Distorting Effect of Rapid Guessing on Estimates of Coefficient Αlpha

An underlying threat to the validity of reliability measures is the introduction of systematic variance in examinee scores from unintended constructs that differ from those assessed. One construct-irrelevant behavior that has gained increased attention in the literature is rapid guessing (RG), which occurs when examinees answer quickly with intentional disregard for item content. To examine the degree of distortion in coefficient alpha due to RG, this study compared alpha estimates between conditions in which simulees engaged in full solution (i.e., do not engage in RG) versus partial RG behavior. This was done by conducting a simulation study in which the percentage and ability characteristics of rapid responders as well as the percentage and pattern of RG were manipulated. After controlling for test length and difficulty, the average degree of distortion in estimates of coefficient alpha due to RG ranged from -.04 to .02 across 144 conditions. Although slight differences were noted between conditions differing in RG pattern and RG responder ability, the findings from this study suggest that estimates of coefficient alpha are largely robust to the presence of RG due to cognitive fatigue and a low perceived probability of success.

Quantifying the Distorting Effect of Rapid Guessing on Estimates of Coefficient Αlpha

An underlying threat to the validity of reliability measures is the introduction of systematic variance in examinee scores from unintended constructs that differ from those assessed. One construct-irrelevant behavior that has gained increased attention in the literature is rapid guessing (RG), which occurs when examinees answer quickly with intentional disregard for item content. To examine the degree of distortion in coefficient alpha due to RG, this study compared alpha estimates between conditions in which simulees engaged in full solution (i.e., do not engage in RG) versus partial RG behavior. This was done by conducting a simulation study in which the percentage and ability characteristics of rapid responders as well as the percentage and pattern of RG were manipulated. After controlling for test length and difficulty, the average degree of distortion in estimates of coefficient alpha due to RG ranged from −.04 to .02 across 144 conditions. Although slight differences were noted between conditions differing in RG pattern and RG responder ability, the findings from this study suggest that estimates of coefficient alpha are largely robust to the presence of RG due to cognitive fatigue and a low perceived probability of success.

Effect of constant collision mean free time on the boundary layer of the active collisional warm plasma

The plasma boundary layer is analyzed for a plasma in contact with a conducting plain surface where the ion temperature is comparable with the electron temperature and the plasma pressure is sufficiently high. The variations of electrical potential from the plasma-presheath boundary to the wall is studied using the fluidal formalism of plasma in three approaches; plasma and sheath asymptotic solutions and full solution. In the full solution approach, fluidal equations lead to a singularity when the ion velocity reaches the ion thermal speed. It is shown that removing the singularity causes a well-defined eigenvalue problem and leads to smooth solutions for the model equations. Some of the applicable aspects such as the floating velocity and density of ions, the floating electrical potential and an estimation of the floating thickness of the boundary layer are obtained. The dependency of these quantities on the ionization degree, the ion temperature and ion-neutral collision is examined too.