On the Achievable Rates of Multihop Virtual Full-Duplex Relay Channels

We study a multihop "virtual" full-duplex relay channel as a special case of a general multiple multicast relay network. For such channel, quantize-map-and-forward (QMF) (or noisy network coding (NNC)) achieves the cut-set upper bound within a constant gap where the gap grows {\em linearly} with the number of relay stages $K$. However, this gap may not be negligible for the systems with multihop transmissions (i.e., a wireless backhaul operating at higher frequencies). We have recently attained an improved result to the capacity scaling where the gap grows {\em logarithmically} as $\log{K}$, by using an optimal quantization at relays and by exploiting relays' messages (decoded in the previous time slot) as side-information. In this paper, we further improve the performance of this network by presenting a mixed scheme where each relay can perform either decode-and-forward (DF) or QMF with possibly rate-splitting. We derive the achievable rate and show that the proposed scheme outperforms the QMF-optimized scheme. Furthermore, we demonstrate that this performance improvement increases with $K$.Comment: To be presented at ISIT 201

Regulation-Structured Dynamic Metabolic Model Provides a Potential Mechanism for Delayed Enzyme Response in Denitrification Process

In a recent study of denitrification dynamics in hyporheic zone sediments, we observed a significant time lag (up to several days) in enzymatic response to the changes in substrate concentration. To explore an underlying mechanism and understand the interactive dynamics between enzymes and nutrients, we developed a trait-based model that associates a community’s traits with functional enzymes, instead of typically used species guilds (or functional guilds). This enzyme-based formulation allows to collectively describe biogeochemical functions of microbial communities without directly parameterizing the dynamics of species guilds, therefore being scalable to complex communities. As a key component of modeling, we accounted for microbial regulation occurring through transcriptional and translational processes, the dynamics of which was parameterized based on the temporal profiles of enzyme concentrations measured using a new signature peptide-based method. The simulation results using the resulting model showed several days of a time lag in enzymatic responses as observed in experiments. Further, the model showed that the delayed enzymatic reactions could be primarily controlled by transcriptional responses and that the dynamics of transcripts and enzymes are closely correlated. The developed model can serve as a useful tool for predicting biogeochemical processes in natural environments, either independently or through integration with hydrologic flow simulators

Regulation-Structured Dynamic Metabolic Model Provides a Potential Mechanism for Delayed Enzyme Response in Denitrification Process

In a recent study of denitrification dynamics in hyporheic zone sediments, we observed a significant time lag (up to several days) in enzymatic response to the changes in substrate concentration. To explore an underlying mechanism and understand the interactive dynamics between enzymes and nutrients, we developed a trait-based model that associates a community’s traits with functional enzymes, instead of typically used species guilds (or functional guilds). This enzyme-based formulation allows to collectively describe biogeochemical functions of microbial communities without directly parameterizing the dynamics of species guilds, therefore being scalable to complex communities. As a key component of modeling, we accounted for microbial regulation occurring through transcriptional and translational processes, the dynamics of which was parameterized based on the temporal profiles of enzyme concentrations measured using a new signature peptide-based method. The simulation results using the resulting model showed several days of a time lag in enzymatic responses as observed in experiments. Further, the model showed that the delayed enzymatic reactions could be primarily controlled by transcriptional responses and that the dynamics of transcripts and enzymes are closely correlated. The developed model can serve as a useful tool for predicting biogeochemical processes in natural environments, either independently or through integration with hydrologic flow simulators

Experimental Validation of CFD-Based Correlations for 5 mm Louver- and Slit-Fin Heat Exchangers: Lessons Learned

This paper examines a recent effort to experimentally test copper-tube, aluminum-fin heat exchangers (HXs) to validate CFD-based correlations for air-side thermal-hydraulic performance of enhanced fins with small-diameter tubes (Sarpotdar et al., 2016). Sample HXs with 5 mm diameter tubes were prepared from four manufacturers and covered a range of unique slit- and louver-fin geometries, fin densities, and tube rows. Experimental data was collected for heat exchangers tested in the dry condition using water as the tube-side working fluid and air as the external fluid with velocities ranging from 1-4 m/s. Rather than following the conventional ε-NTU-based data reduction procedure outlined by Wang (2000), a finite-control-volume-based heat exchanger modeling software was used to reduce heat transfer coefficients from experimental data. The presented experimental results serve as a reference for the air-side performance of these 5 mm slit and louver fin HX geometries, but also validate the predictions of a CFD-based correlation that covers a much larger design space, including tube diameters from 3-5 mm. Correction factors were applied to the existing correlations based on experimental findings and the final correlations were able to predict 100% of all observed air-side pressure drops and 98% of observed heat transfer coefficients with 20% error less. The findings suggest that the CFD-based approach is suitable for developing correlations for air-side heat transfer and pressure drop. This method is highly desirable because of its ability to simulate a wide-ranging design space that could not be feasibly evaluated experimentally, including heat exchanger geometries that cannot be manufactured at present. However, some deviations between CFD predictions and experimental observations are evident and the work underscores the importance of experimental validation and tuning. It is hypothesized that factors which are not accounted for in the CFD model, including thermal contact resistances between fin and tube, may contribute toward the CFD correlation’s consistent over-prediction of observed heat transfer