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DOE OSTI2025

A randomized sketching trust-region secant method for low-memory dynamic optimization

The numerical solution of dynamic optimization problems is often limited by the memory required to store the state trajectory, which is used to evaluate the objective function and its derivatives. Recently, [R. Muthukumar et al., SIAM Journal on Optimization 31(2), pp. 1242–1275 (2021)] introduced a trust-region method for dynamic optimization that employs randomized sketching to compress the state trajectory, resulting in inexact derivative computations. By adaptively learning the sketch rank, the trust-region algorithm achieves rigorous convergence guarantees. Here, we extend this approach to use secant Hessian approximations. Due to the randomness introduced by the sketch, the traditional secant update formulae can produce poor Hessian approximations. In particular, the difference of two gradients, computed from two different sketches, may be inconsistent. To overcome this, we employ a sketched approximation of the Hessian application, in lieu of computing the gradient difference. We numerically demonstrate the improved stability of this approach on an example from PDE-constrained optimization.

dynamic optimization↗

DOE OSTI2026

Acquisitive root exploration strategies help maintain higher peak sap flux rates during summer drought, but more root biomass does not

Roots are responsible for soil water uptake, yet little is known about how variation in fine-root traits relates to whole-tree water movement, particularly during periods of drought. By combining a 3-yr dataset monitoring sap flow rates with measures of fine-root biomass, length, and morphology across 10 tree species, we addressed hypotheses concerning the role of fine-root system size and morphology in determining tree responses to summer drought as well as potential changes in their relative importance under more severe droughts. Greater fine-root biomass and length did not enable trees to maintain high rates of sap flow during drought, whereas the morphological traits, specific root length and specific root area, were linked to sustained sap flow rates during drought. We found that all species, irrespective of root traits, progressively reduced their sap flow under more extreme drought conditions, although more acquisitive root morphology was still associated with smaller reductions. Our results run counter to long-standing assumptions that larger root systems are better able to access soil water and maintain photosynthetic activity during drought. Instead, we find evidence that root morphology at least partially determines the capacity for water uptake and movement as soil moisture declines.

drought↗

DOE OSTI2025

Extrusion‐Spheronization of Mock Energetic Materials

The primary method for producing plastic bonded explosive (PBX) granules, or “prills”, has remained relatively unchanged for 70 years despite the complex nature of the process. In this work, we demonstrate the feasibility of using an extrusion‐spheronization technique to produce prills for PBX applications. We begin by detailing an inert formulation with similar properties of PBXs and then demonstrate the extrusion‐spheronization processing of these materials. A study is then performed where the spheronization process time of the extruded materials is varied and the resultant prills are morphologically characterized. Further, these prills are then pressed into high‐density articles and subject to compression testing to elucidate trends in process, properties, and performance. It was found that, for our formulation (95 wt.% melamine/5 wt.% polymer binder) and process, a spheronization time of 60 s yielded relatively uniform particles that exhibited improved poured, tapped, and pressed densities. Mechanical strength did not have a strong trend with process time as all spheronized materials had similar peak compression stress at failure. After further optimization, extrusion‐spheronization may be a promising path for future PBX formulation.

extrusion spheronization↗