Shaping Pulses to Control Bistable Biological Systems

In this paper we study how to shape temporal pulses to switch a bistable system between its stable steady states. Our motivation for pulse-based control comes from applications in synthetic biology, where it is generally difficult to implement real-time feedback control systems due to technical limitations in sensors and actuators. We show that for monotone bistable systems, the estimation of the set of all pulses that switch the system reduces to the computation of one non-increasing curve. We provide an efficient algorithm to compute this curve and illustrate the results with a genetic bistable system commonly used in synthetic biology. We also extend these results to models with parametric uncertainty and provide a number of examples and counterexamples that demonstrate the power and limitations of the current theory. In order to show the full potential of the framework, we consider the problem of inducing oscillations in a monotone biochemical system using a combination of temporal pulses and event-based control. Our results provide an insight into the dynamics of bistable systems under external inputs and open up numerous directions for future investigation.Comment: 14 pages, contains material from the paper in Proc Amer Control Conf 2015, (pp. 3138-3143) and "Shaping pulses to control bistable systems analysis, computation and counterexamples", which is due to appear in Automatic

Modelling Natural Ventilation in Double Skin Facade

The assessment of the energy performance of buildings with Double Skin Facades (DSF) requires proper dynamic simulation tools, based on models capable of predicting heat and mass transfer in the DSF under variable boundary conditions, at the price of a reasonable computational effort. Many DSF simplified models have been developed and implemented in building simulation tools, but the validation of these tools is still an open issue, especially for the prediction of the mass flow rate in naturally ventilated DSF. The CFD modelling activity presented in this work aims at investigating the reliability of the assumptions and hypotheses employed in the simplified model, which was specifically developed for the dynamic simulation of heat transfer in buildings. Both the CFD and simplified models have been tested and evaluated on an experimental case study, using the database provided by a research program developed under IEA ECBCS Annex 43/SHC Task 34, reporting the results of a measurement campaign conducted on an a transparent naturally ventilated DSF tested in Denmark, in an experimental facility called "the Cube"

Modelling Mechanically Ventilated Double Skin Facades with Integrated Shading Device

Double skin fa\ue7ades (DSF), are typically composed by two transparent envelope elements separated by a ventilated airspace. Such a technology can be applied both in new and existing buildings and may combine architectural value with energy efficiency. In the most common mechanically ventilated configurations the DSF air inlet came from the indoor environment and the outlet air returns to the HVAC system. Integrated shading devices are positioned between the skins. The assessment of the energy performance of buildings with DSFs requires proper dynamic simulation tools, based on models capable of predicting DSF heat transfer under variable boundary conditions, at the price of a reasonable computational effort. Many DSF simplified models have been developed and implemented in building simulation tools, but the validation of these tools is still an open issue, especially in presence of shading devices. The CFD modelling activity presented in this work aims at supporting the assessment of a DSF simplified model, specifically developed for the dynamic simulation of heat transfer in buildings. Such a model is based on an integral approach to the vertical channel, which is assumed to be separated into two channels when the shading device is used. Averaged surface heat transfer coefficients, depending on the geometry and flow regime, are adopted in order to represent convection inside the channels, according to the available correlations. The dataset of a measurement campaign, which was performed in a twin test facility on a mechanically ventilated DSF adopting both Venetian and roller blinds, was used to validate both the CFD model developed for this study, and the implementation of a former simplified model suitable for building simulation. The CFD approach allows for an assessment of the assumptions and hypotheses employed by the simplified model. Moreover, the CFD analyses provide a deeper insight on important aspects such as, the presence and impact of recirculation, the development of velocity and temperature profiles

High altitude airship cabin sizing, pressurization and air conditioning

This paper aims at defining a design methodology for the global thermodynamic performance of a high altitude airship cabin. This design method applies to different systems, which could not use the traditional air conditioning plant layout based on bleed air intake from the compressor stage of jet engines. In the case of electrically propelled green vehicles and airships, other energy sources must be exploited. The MAAT EU FP7 project presents an innovative, energetically self sufficient, airship system based on cruiser-feeder architecture. Both the cruiser and feeder are fed by photovoltaic energy. The energy storage system by electrolysis and fuel cells with intermediate energy storage by hydrogen and oxygen is characterized by high temperature energy dispersions (about 800-1000°C for High temperature SOFC cells). This situation encourages the definition of a novel pressurization and air conditioning system. A preliminary cabin sizing with some structural considerations, an energetic evaluation of the thermal insulation of the cabin and a general balance of the energy production system are provided

Naturally ventilated double-skin façade in modeling and experiments

The modeling activity presented in this work aims at the assessment of a simplified model, named BS model, which was specifically developed for integration of DSF in Building Simulation. The BS model is based on a pressure loop and on an integral approach to the heat transfer along the vertical channel. It considers buoyancy as a function of the average temperature in the channel. The wind action is taken into account by means of wind pressure coefficients (Cp) on the fa\ue7ade openings. The focus of this study is the experimental validation of the modeling \u201ccore\u201d: the natural ventilation through the DSF. The validation is based on the dataset of the experimental campaign conducted on a DSF test facility, the \u201cCube\u201d, in Denmark, under IEA ECBCS ANNEX 43/SHC Task 34. Hourly simulations were performed with the BS model for the 15 days of the experimental campaign. A CFD modeling activity was also carried out on a selection of four cases, extracted from the experimental benchmark and representative of different temperature and pressure boundary conditions. The results show that the BS model presents a good level of agreement with the experimental data in predicting the mass flow rate and the heat removed by ventilation. Although the two experimental methods used to determine the airflow rate in the DSF cavity produce in many cases divergent results, it was possible to distinguish valid experimental results for comparison with the BS model. This was possible thanks to a thorough analysis of the experimental procedure together with the insight provided by the model into the determination of the driving wind and thermal differential pressures. In particular, by selecting only the measurements associated to sufficiently low wind fluctuations in the hourly averaged data, a good degree of correlation was found between the predicted total driving pressure and the flow measurements. Concerning the four cases investigated also by means of CFD, the agreement between the BS and CFD models is remarkable in terms of outlet temperatures and in the prediction of flow reversal

Mathematical model of a constructal Coanda effect nozzle

This paper analyses the ACHEON Coanda effect nozzle for aircraft propulsion, based on the dynamic equilibrium of two jet streams. The ACHEON concept, and, in particular, the HOMER nozzle, which is its main component, are presented, together with the literature milestones from which the idea originally stems. A subsystem analysis inspired by the principles of Constructal Theory is presented for the current architecture. A mathematical model of a 2D case of the system is developed, focusing on the combined effect of the mixing of the two streams and the Coanda adhesion over a convex surface. A validation of the model is also reported, based on 2D CFD analyses, under the hypothesis of incompressible flow. Results highlight that, in spite of its relative simplicity, the model produces accurate results

A fast algorithm for Direct Numerical Simulation of natural convection flows in arbitrarily-shaped periodic domains

A parallel algorithm is presented for the Direct Numerical Simulation of buoyancy-induced flows in open or partially confined periodic domains, containing immersed cylindrical bodies of arbitrary cross-section. The governing equations are discretized by means of the Finite Volume method on Cartesian grids. A semi-implicit scheme is employed for the diffusive terms, which are treated implicitly on the periodic plane and explicitly along the homogeneous direction, while all convective terms are explicit, via the second-order Adams-Bashfort scheme. The contemporary solution of velocity and pressure fields is achieved by means of a projection method. The numerical resolution of the set of linear equations resulting from discretization is carried out by means of efficient and highly parallel direct solvers. Verification and validation of the numerical procedure is reported in the paper, for the case of flow around an array of heated cylindrical rods arranged in a square lattice. Grid independence is assessed in laminar flow conditions, and DNS results in turbulent conditions are presented for two different grids and compared to available literature data, thus confirming the favorable qualities of the method

Development and calibration of a 1D thermo-fluid dynamic model of ventilation in tunnels

In complex, large civil infrastructures where ventilation has a crucial role for the safety of users in both normal operation and hazardous scenarios, the correct prediction of flow and heat transfer parameters is of fundamental importance. While full 3D simulation is applicable only to a limited extent, and the resort to 1D modeling is a common practice in both design and evaluation phases, the limitation of such models lies in the choice of transfer parameters, such as friction loss coefficients and heat transfer coefficients. In this work, we present an original approach based on the Finite Volume integration of the 1D flow and energy equations on a network of ducts, representing the ventilation system in the 11.6 km long Mont Blanc Tunnel with a spatial resolution of 10 m. The calibration of a set of friction loss coefficients against a rich experimental dataset collected throughout a dedicated set of in situ tests is of particular concern here, as it is carried out by means of genetic optimization algorithms. Predictions of the flow field are in remarkable agreement with the experimental data, with an overall RMS error of ± 0.42 m/s. Further refinements and possible parameter choices are also discussed