This study attempts to quantify the decay rates of stratified wakes in active oceanic environments, characterized by the presence of intermittent turbulence and double-diffusive convection. Of particular interest is the possibility of utilizing standard oceanographic microstructure measurements as a means of wake identification and analysis. The investigation is based on a series of direct numerical simulations of wakes produced by a sphere uniformly propagating in stratified two-component fluids. We examine and compare the evolution of wakes in fluid systems that are (i) initially quiescent, (ii) double-diffusively unstable, and (iii) contain preexisting turbulence. The model diagnostics are focused primarily on the dissipation of turbulent kinetic energy (ε) and thermal variance (χ). The analysis of decay patterns of ε and χ indicates that microstructure generated by an object of D = 0.6 m in diameter moving at the speed of U = 0.02 m/s could be detected, using modern high-resolution profiling instruments, for 0.5–0.7 h. The detection period depends on environmental conditions; convective overturns are shown to be particularly effective in terms of dispersion of microscale wake signatures. The extrapolation of model results to objects of ∼10 m in diameter propagating with speeds of ∼10 m/s suggests that the microstructure-based wake detection is feasible for at least 4 h after the object’s passage through the monitored areas. The overall conclusion from our study is that the measurement of microscale signatures of turbulent wakes could represent a viable method for hydrodynamic detection of propagating submersibles.

This study explores the dynamics and statistical patterns of coherent long-lived vortices spontaneously forming in bluff body wakes. The analysis is based on a series of two-dimensional direct numerical simulations performed for a wide range of Reynolds numbers. We demonstrate that the majority of coherent vortices beyond the recirculation zone are well represented by the canonical Lamb–Oseen solution. This observation is used to develop a low-order census of long-lived eddies in terms of their core sizes and vorticity magnitudes. We demonstrate that the increase in the Reynolds number (Re) leads to the systematic reduction in the initial core radii (r0), whereas the core vorticity (ζ0) increases. These dependencies exhibit singular behavior in the inviscid limit (Re → ∞), which is captured by the proposed explicit relations for r0(Re) and ζ0(Re).

Limitations of, and improvements to, the beam-column design procedures of SNAME Bulletin 5-5 are described. Results from 25 inelastic FEM analyses are used to define the basecase failure surface for a LeTourneau tear-drop chord section of full bay length. A further 16 cases were run for k=0.5 and reverse curvature for establishing a more realistic upper bound on capacity. Supplementary studies were used to validate a knocked-down stress-strain curve, and to show that using base case results is conservative for more complex situations. Introduction The IADC Jack-up Committee has funded several projects to improve SNAME Technical and Research Bulletin 5-5A (ref. 1), including this project on the capacity of LeTourneau type chords which, among other things, addresses the conservatism in the beam-column formulation for singly symmetric sections. Based on preliminary results from this study (ref. 2), corrections and enabling language have been included in the second edition of SNAME 5-5. The beam-column interaction equations in AISC-LRFD and SNAME ¶8.1.4.1 give a reasonable lower bound fit to the plastic interaction surface, for doubly symmetric column sections. For significant axial load (>20% of capacity), the generic interaction is shown in Eq. (1). (Mathematical equation available in full paper) Normally the designer refers axial loads to the elastic neutral axes of a member for ease of correlation with global finite element analyses. However, with different yield strengths present in the cross section, maximum axial capacity occurs when the load is applied at a different location; this is the so-called center of squash. Moments from frame analysis must be adjusted to the new reference, being particularly careful to get the signs right. Figure 1 shows a cut through the interaction envelop for axial load equal to half of the squash load (axial compressive force causing full yielding of the section). In the prescribed formula, off-axis bending is handled with the exponent "?". Linear addition of the two bending stresses (exponent of 1.0) applies to sections that fail as soon as they yield at the corners. Quadratic interaction (exponent of 2.0) would apply for circular sections. All other sections, doubly and singly symmetric (e.g. Type 4 Chords), are presumed to fall between these limits. This formulation misses important aspects of jack-up leg chords having a singly symmetric "tear-drop" shape. In elastic stress-based design, negative Mx helps My, as it relieves axial compression on the back plate. Several large classes of jackup rigs have been successfully designed to exploit this behavior. The fully plastic section envelope also shows offaxis capacity exceeding that on the adjoining axis for this quadrant. The inelastic behavior of realistic beam columns falls between the elastic and plastic section envelopes, reflecting important inelastic P-delta effects. SNAME Bulletin 5-5A essentially prescribes linear addition of the two bending effects, with only a little relief at the bulge, when the exponent is derived from their Figure 8.4. The basic form of the mandated interaction equation is incapable of giving extra off-axis capacity, even with an exponent of infinity.

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