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A modification of focused laser differential interferometry (FLDI) is demonstrated with an infrared tunable diode laser (TDL) to achieve simultaneous absorption spectroscopy (AS) measurements. Measurements from this absorbing-FLDI (A-FLDI) are shown for a Hencken burner plume. Initial comparison measurements are recorded using TDLAS and an electrical hygrometer. Raw voltage and estimated absorbance measurements illustrate that the technique detects five distinct absorbance peaks of the methane–air flame while retaining the expected behavior of typical FLDI. This modification furthers efforts to enable FLDI to conduct analysis of the pressures, temperatures, and molecular densities of flows. It also explores the potential for reducing path-integration (PI) effects in absorption spectroscopy and potentially enabling spatially and temporally resolved local flow measurements. Reductions in PI length as high as 82%–84% are observed. 

As the field of fluid dynamics progresses, the demand for sophisticated diagnostic methods to accurately assess flow conditions rises. In this work, resonantly ionized photoemission thermometry (RIPT) has been used to directly target and ionize diatomic nitrogen (N₂) to measure one-dimensional (1D) temperature profiles in a supersonic jet flow. This technique can be considered non-intrusive as the premise uses resonantly enhanced multiphoton ionization (REMPI) to target molecular nitrogen. This resonance excites N₂into absorption bands of the P, Q, and R rotational branches of N₂(b¹Π_u). The ideal (3 + 1) REMPI scheme excites from the ground state and ionizes N₂(b¹Π_u←X¹Σ_g⁺) where de-excitation results in photoemission from the first negative band of ionizedN₂⁺(B²Σ_u⁺→X²Σ_g⁺) as nitrogen returns to the ground state. The resulting emission can be observed using an intensified camera, thus permitting inference of the rotational temperature of ground-state molecular nitrogen. A linearly regressive Boltzmann distribution is applied based on previous calibration data for this technique to quantify the temperature along the ionized line. This work applies this technique to a pure N₂supersonic jet in cross-flow and counter-flow orientations to demonstrate N₂RIPT’s applications in a supersonic flow. Temperature variations are observed at different locations downstream of the exit in cross-flow, and axisymmetric in counter-flow, to generate profiles characterizing the flow dynamics. Due to the collisional effects resulting from the number density of N₂at higher pressures, a (3 + 2) REMPI scheme is observed throughout this text. 

We explore a Federated Reinforcement Learning (FRL) problem where agents collaboratively learn a common policy without sharing their trajectory data. To date, existing FRL work has primarily focused on agents operating in the same or ``similar" environments. In contrast, our problem setup allows for arbitrarily large levels of environment heterogeneity. To obtain the optimal policy which maximizes the average performance across all potentially completely different environments, we propose two algorithms: FedSVRPG-M and FedHAPG-M. In contrast to existing results, we demonstrate that both FedSVRPG-M and FedHAPG-M, both of which leverage momentum mechanisms, can exactly converge to a stationary point of the average performance function, regardless of the magnitude of environment heterogeneity. Furthermore, by incorporating the benefits of variance-reduction techniques or Hessian approximation, both algorithms achieve state-of-the-art convergence results, characterized by a sample complexity of $$O(\epsilon^{-\frac{3}{2}}/N)$$. Notably, our algorithms enjoy linear convergence speedups with respect to the number of agents, highlighting the benefit of collaboration among agents in finding a common policy. 

This paper presents an extensive parameter study of a non-intrusive and non-seeded laser diagnostic method for measuring one dimensional (1D) rotational temperature of molecular nitrogen (N₂) at 165 - 450 K. Compared to previous efforts using molecular oxygen, here resonantly ionized and photoelectron induced fluorescence of molecular nitrogen for thermometry (N₂RIPT) was demonstrated. The RIPT signal is generated by directly probing various rotational levels within the rovibrational absorption band of N₂, corresponding to the 3-photon transition of N₂(X¹Σ_g⁺,v=0→b¹Π_u,v^′=6) near 285 nm, without involving collisional effects of molecular oxygen and nitrogen. The photoionized N₂produces strong first negative band of N₂⁺(B²Σ_u⁺−X²Σ_g⁺) near 390 nm, 420 nm, and 425 nm. Boltzmann analyses of various discrete fluorescence emission lines yield rotational temperatures of molecular nitrogen. By empirically choosing multiple rotational levels within the absorption band, non-scanning thermometry can be accurately achieved for molecular nitrogen. It is demonstrated that the N₂RIPT technique can measure 1D temperature profile up to ∼5 cm in length within a pure N₂environment. Multiple wavelengths are thoroughly analyzed and listed that are accurate for RIPT for various temperature ranges.