Abstract: TBA

Abstract: In this work a review and new observations of the flow developed after a multi-scale obstacle will be presented. The results of measurements of the flow developed after a fractal forcing in an open channel, reveals the signature of non-kolmogorov spectral behaviour in the flow. A discussion of the link between this observation and the role of Helicity on the Energy cascade will be discussed. Special emphasis will be given to the need of DNS simulations to describe the flow features as this non-kolmogorov behaviour questions the applicability of current eddy-resolving technique’s closures.

Abstract: In the context of mathematical modeling and numerical simulation for the marine energy, the project presented in this talk aims to construct a numerical model for wave propagation coupling a spectral model (WaveWatch III) and a non-linear dispersive (Boussinesq-type) model (BOSZ), which are more accurate in offshore and nearshore regions, respectively. By dividing a domain accordingly to the water depth, and managing the simultaneous execution of both models and the transmission of data between them, the objective is to perform simulations covering larger ranges of depths. In order to improve the exchange of information, define well-posed problems in each subdomain and obtain more accurate results, it is essential to define appropriate interface boundary conditions. Nevertheless, as the problem of generating and absorbing waves in Boussinesq-types models remains unsolved, BOSZ makes use of a source function method, which imposes limitations to the current coupled model. In this talk, we present our efforts on this issue, by the study on transparent boundary conditions (TBCs) for BOSZ. As TBCs in general cannot be computed analytically, we propose, via the resolution of a domain decomposition method, approximated operators depending on a set of coefficients to be optimized numerically. The well-posedness of the problem using such approximated TBCs is also investigated.

Abstract: Santiago experiences extreme pollution events during wintertime. These events are produced by frequent temperature inversions that lead to stable atmospheric conditions and high concentrations of particular matter from vehicles and domestic wood-burning emissions. The transport of this pollution outside of Santiago is strongly determined by the surrounding topography and thermally driven winds. Several studies have been conducted to analyze mechanisms responible for this transport such as valley-mountains circulation, venting mechanisms and intrusion events based on observations and numerical simulations. All these studies show that the thermally driven winds and vertical turbulence are able to carry urban pollutant from the basin to the top of the mountains.
As a first step to towards assessing the potential transport of urban pollutants from Santiago up to the Andean cryosphere, a one-week field campaign during July 2015 was conducted in Santiago and in a narrow mountain canyon to the south east of the center of Santiago. This study was performed on days with deteriorated air quality levels and winds were observed blowing a deep BC layer from the city into the nearby valley.
Result illustrating the performance of the CHIMERE model to simulate these intrusions of polluted air from Santiago into the nearby valley observed during the above mentioned field campaign will be presented. In addition, the model will be applied to study the transport of urban pollutants up into the Andes and its potential transport up to the glaciers.

Abstract: Firefighters could greatly benefit from a technology based on predictive fire simulations, able to assist their decision making process. CFD fire simulations typically take large periods of time to complete, and their results are thus not available in time to be of use during an emergency. Due to the high complexity of fire spread dynamics that arises from the interaction between solid and gas phase and the corresponding physical-chemical processes (e.g. pyrolysis), the spread of the fire cannot be predicted from first principles in real-time using contemporary computers, and has to be given as parameters to the model. Data can be incorporated into the model to characterise the fire. Inverse CFD modelling and tangent linearisation is used in the present study to assimilate data. It is shown that the growth rate of the fire and the location of its origin can be correctly and efficiently estimated using data such as sprinkler and smoke detector activation time. It is further shown that the estimated spread rate is not sensitive to fire origin location.

Abstract: A reliable representation of wind fields in urban areas is critical to accurately represent transport and fate of air pollutants at urban and regional scales. Santiago, Chile has been subject to intense air pollution episodes for decades; these episodes are characterized by accumulation of pollution when atmospheric stability is high, mixing height is low and winds are weak. Besides the presence of urban canopy, the complex topography of Santiago’s basin poses another challenge for the mesoscale modeling of wind fields, so it can be considered as a demanding test for any numerical weather prediction model.
The Weather Research and Forecasting Model and Chemistry (WRF-Chem) is applied to model the urban scale wind fields considering: a) urban canopy, b) complex topography and c) both. A statistical comparison of simulated and observed winds at a network of meteorological stations in Santiago is presented for the fall and winter season in 2014, and for different parametrizations of mixing height, urban canopy and topography. The WRF-Chem model was run in forecasting mode with several parametrizations and compared with the current configuration used for air quality forecasting in Central Chile by the Ministry of the Environment.

Abstract: Method of asymptotic partial decomposition of a domain (MAPDD) proposed and justified earlier for thin domains (rod structures, tube structures consisting of a set of thin cylinders) generates some special interface conditions between three-dimensional and one-dimensional parts (see[1-3]). In the case of fluid mechanics these conditions prescribe a pre-computed Poiseuille-type shape of a solution at the interface. Although it gives a high order precision for the non-steady three dimensional Navier-Stokes equations in tube structures without boundary layer in time, it is not applicable in the case of presence of the boundary layer in time. In the present talk we formulate and justify a precise junction condition for this general case. Moreover, for the reason of numerical implementation a Dirichlet-type interface condition (prescribed shape) for the velocity may be undesirable. So several alternative interface conditions involving pressure are proposed and justified: the closeness of solutions of these hybrid dimension problems to the solution of the complete three-dimensional setting is proved. Numerical implementation of these conditions in the case of 2D Stokes and Navier-Stokes equations is discussed.
1. Panasenko G., Method of asymptotic partial decomposition of domain, Mathematical Models and Methods in Applied Sciences, v. 8, No 1, 1998, 139-156.
2. Panasenko G., Partial asymptotic decomposition of domain: Navier-Stokes equation in tube structure , C.R. Acad. Sci. Paris, v. 326, Série IIb, 1998, pp. 893-898.
3. Panasenko G., Pileckas K., Asymptotic analysis of the non-steady Navier-Stokes equations in a tube structure.I. The case without boundary layer-in-time. Nonlinear Analysis, Series A, Theory, Methods and Applications, 122, 2015, 125-168.

Abstract: Bicuspid aortic valve (BAV) is the most common congenital cardiac defect. Early diagnosis of BAV may reduce the long-term risk of cardiovascular events. Until now, it is not clear the appropriated time to perform surgery to BAV patients. The standard metric used to plan the surgery, which is the aortic diameter, has been changed three times in the last 20 years. Such uncertain criteria for surgery motivates the development of more predictive aortic risk markers that can bring consistent guidelines to treat patient with BAV. Recent studies have found that BAV patients exhibit altered blood flow in the ascending aorta as outflow jet, helix and vortex flow. These hemodynamic alterations may lead to changes in hemodynamics parameters (HP) as wall shear stress (WSS), oscillatory shear index (OSI), vorticity, helicity density, viscous dissipation, energy loss and kinetic energy. However, only a few combinations of these parameters have been applied in the same cohort of patients. The objective of this work is to show the applicability of a finite element approach to obtain all these parameters in BAV patients using a single methodology, based on the analysis of 4D-flow data. We perform a finite-element least-squares projection method to process the velocities field obtained by 4D Flow MRI. A total of 14 healthy volunteers and 49 BAV-patients were included in the study. The 4D-flow data was acquired at Hospital Universitari Vall d'Hebron with a 1.5T GE-MR Signa Scanner and using the VIPR technique. The great advantage of the proposed approach is that from a single segmentation of the 4D-flow data we can obtain several quantitative HP along the entire aorta. Parameters such as WSS, vorticity, viscous dissipation, energy loss and kinetic energy were higly correlated in this cohort of patients (R² > 0.7). However, the eccentricity, the mean angle in axial direction, the mean backward flow fraction and the kinetic energy were the only parameters correlated with the aortic diameter (R² > 0.6) in the ascending aorta. Longitudinal studies may consider all those parameters to evaluate the risk factors on BAV patients. The methodology presented here represents an advanced hemodynamic description that may help to improve BAV patient’s management.

Abstract: Since the early nineties of the previous century, the Netherlands Aerospace Center NLR has developed a block-structured finite-volume Navier-Stokers solver. The state-of-the-art solver is fourth-order accurate on curvi-linear meshes. Specific hybrid RANS-LES turbulence models have been developed which mitigate the grey area issue of the standard hybrid models. Aero-acoustic and aero-elastic simulations are routinely performed at NLR. The current presentation will focus on two aero-elastic applications and the specific computational challenges that are posed by the applications.
The first application is an isolated helicopter rotor in forward flight, the well-known HART-II experiment. The highly flexible blades fly in their own wake, which necessitates the accurate resolution of said wake, in particular the tip vortices. High-order methods are essential, but even they require uniform meshes. A uniform mesh in the rotor disk is difficult to generate in a block-structured way. Therefore an automatic topology-based grid adaptation algorithm has been developed to generate such a mesh. The increase in vortex resolution will be demonstrated.
The second application concerns the gust encounter of a passenger aircraft. Gust encounters are defining load cases for many structural components of an aircraft. Gusts used in the certification process of aircraft are either continuous (‘turbulent’) gusts are discrete (‘one minus cosine’) gusts. Straightforward resolution of the gust requires high resolution in the farfield, significantly increasing the computational cost of otherwise ‘standard’ aero-elastic simulations. A new algorithm will be presented which resolves the gusts on meshes with the usual resolution for aircraft simulations.

Biography: Harmen van der Ven is a Senior Scientist at the Flight Physics and Loads department of the Netherlands Aerospace Centre in Amsterdam. His research interests are in the application of computational science to aerospace engineering, in specific the development of fast and accurate numerical algorithms for fluid dynamics, electromagnetics, and mechanical engineering. In the field of computational fluid dynamics, his main research focus is on efficient numerical algorithms for aeroelasticity with application to rotorcraft design.