The photon veto system in the NA62 experiment

The NA62 experiment at CERN SPS aims to collect O(100)K+ → π+νν events in two years of data taking with a S/B ratio of about 10:1. The Branching Ratio (BR) for this decay is ∼ 10 −11 and can be predicted with minimal theoretical uncertainties, making it a sensitive probe for New Physics. The guiding principles for the construction of the NA62 detectors are an accurate particle ID, precise timing and excellent veto efficiency. In particular, the veto inefficiency for photons from K+ → π+π0 decays should be smaller than 10−8. The photon veto system of NA62 consists of three detectors covering different angular regions: Large Angle Veto (LAV), Liquid Krypton calorimeter (LKr) and Small Angle Veto (SAV). The status of the project and present preliminary results from the recent tests will be reviewed

A multi region adjoint-based solver for topology optimization in conjugate heat transfer problems

This work presents an exploration of fluid region optimization within coupled fluid–thermal problems of industrial significance, namely the design of a cooling plate for the thermal management of Printed Circuit Boards (PCB) of electrical propulsion systems. The Topology Optimization technique has been employed through a in-house developed multi-region adjoint solver and a set of customized boundary conditions, allowing the sensitivity computation independently on the problem size. The technique involves the integration of solid material into the computational domain to induce modifications in flow dynamics. This alteration aims to minimize a multi-objective function that considers both the skin temperature and the mechanical power dissipation caused by fluid movement across the domain. The obtained sensitivity values were then employed in optimizing material distribution through the Method of Moving Asymptotes. The derived material distribution was further post-processed to extract the newly optimized configuration of the system. This enabled a thorough evaluation of the optimization methodology's performance and its effectiveness in enhancing the system's overall efficiency

An Adjoint‐Based Solver with Adaptive Mesh Refinement For Efficient Design of Coupled Thermal‐Fluid Systems

A multi-objective continuous adjoint strategy based on the superposition of boundary functions for topology optimization of problems where the heat transfer must be enhanced and the dissipated mechanical power controlled at the same time, has been here implemented in a Finite Volume (FV), incompressible, steady flow solver supporting a dynamic Adaptive Mesh Refinement (AMR) strategy. The solver models the transition from fluid to solid by a porosity field, that appears in the form of penalization in the momentum equation; the material distribution is optimized by the Method of Moving Asymptotes (MMA). AMR is based on a hierarchical non-conforming h-refinement strategy and is applied together with a flux correction to enforce conservation across topology changes. It is shown that a proper choice of the refinement criterium favors a mesh-independent solution. Finally, a Pareto front built from the components of the objective function is used to find the best combination of the weights in the optimization cycle. Numerical experiments on two- and three-dimensional test cases, including the aero-thermal optimization of a simplified layout of a cooling system, have been used to validate the implemented methodology

Coherent Near-Wall Structures and Drag Reduction by Spanwise Forcing

The effect of streamwise-traveling waves of spanwise wall velocity (StTW) on the quasistreamwise vortices (QSV) populating the near-wall region of turbulent channels is studied via a conditional averaging technique applied to flow snapshots obtained via direct numerical simulation. The analysis by Yakeno, Hasegawa, and Kasagi [Phys. Fluids 26, 085109 (2014)], where the special case of spatially uniform wall oscillation (OW) was considered, is extended to the general case of StTW, which yield both reduction and increase of turbulent skin-friction drag. StTW are found to significantly impact the wall-normal distribution of the vortex population. The conditionally averaged velocity field around the vortices shows that the contributions of the QSV to the quadrant Reynolds shear stresses change significantly during the control cycle. On the one hand, as for OW, the suppression of Q2 events (with upwelling of low-speed fluid away from the wall) dominates the drag-reduction process. On the other hand, the enhancement of Q2 and also Q4 events (with downwelling of high-speed fluid toward the wall) is related to drag increase. Based on the link identified between the phase changes of the Reynolds stresses and the principal directions of the rate-of-strain tensor induced by the StTW, a predictive correlation for drag reduction by StTW is proposed which uses physically significant parameters to overcome the shortcomings of existing models

Structure function tensor equations with triple decomposition

Exact budget equations are derived for the coherent and stochastic contributions to the second-order structure function tensor. They extend the anisotropic generalised Kolmogorov equations (AGKE) by considering the coherent and stochastic parts of the Reynolds stress tensor, and are useful for the statistical description of turbulent flows with periodic or quasi-periodic features, like e.g. the alternate shedding after a bluff body. While the original AGKE describe production, transport, inter-component redistribution and dissipation of the Reynolds stresses in the combined space of scales and positions, the new equations, called $\varphi$AGKE, contain the phase $\varphi$ as an additional independent variable, and describe the interplay among the mean, coherent and stochastic fields at the various phases. The newly derived $\varphi$AGKE are then applied to a case where an exactly periodic external forcing drives the flow: a turbulent plane channel flow modified by harmonic spanwise oscillations of the wall to reduce drag. The phase-by-phase action of the oscillating transversal Stokes layer generated by the forcing on the near-wall turbulent structures is observed, and a detailed description of the scale-space interaction among mean, coherent and stochastic fields is provided thanks to the $\varphi$AGKE