BACKGROUND: The hemorrhage risk of unruptured and untreated cerebral arteriovenous malformations (AVMs) has been shown to be higher for female patients than male patients in their child bearing ages. Although it has been neurosurgical practice to advise female patients in their childbearing ages to postpone pregnancy until proven AVM obliteration, there is no literature consensus regarding this potential hemorrhage risk increase. OBJECTIVE: To accurately quantify the risk increase for AVM hemorrhage during pregnancy. METHODS: This study is based on data from previous publications, consisting of known age at the first AVM hemorrhage in 3425 patients. The risk increase during pregnancy could be calculated from the difference in age distribution for the first AVM hemorrhage between male patients and female patients, taking the average pregnancy time per female into account. A comparison was also made with data for all hospital discharges (13 751) in Germany 2008 to 2018 with the diagnosis brain AVM. RESULTS: The average pregnancy and puerperium time was 1.54 years per female in the patient population, which was used to determine the annual AVM hemorrhage risk during pregnancy to be around 9%. The increased risk during pregnancy was further evidenced by analysis of a subgroup of 105 female patients, for which pregnancy status at the time of hemorrhage was known. CONCLUSION: The quantified annual risk for AVM hemorrhage during pregnancy is about 3 times higher than that of male patients at corresponding age. This provides an important basis for advising female patients with patent AVMs about the increased risk for hemorrhage that a pregnancy would entail.

In a recent study (Želi et al. in Bound Layer Meteorol 176:229–249, 2020), we have shown that the explicit algebraic Reynolds-stress (EARS) model, implemented in a single-column context, is able to capture the main features of a stable atmospheric boundary layer (ABL) for a range of stratification levels. We here extend the previous study and show that the same formulation and calibration of the EARS model also can be applied to a dry convective ABL. Five different simulations with moderate convective intensities are studied by prescribing surface heat flux and geostrophic forcing. The results of the EARS model are comparedto large-eddy simulations of Salesky and Anderson (J Fluid Mech 856:135–168, 2018). It is shown that the EARS model performs well and is able to capture the counter-gradient heat flux in the upper part of the ABL due to the presence of the non-gradient term in the relation for vertical turbulent heat flux. The model predicts the full Reynolds-stress tensor and heat-flux vector and allows us to compare other important aspects of a convective ABLsuch as the profiles of vertical momentum variance. Together with the previous studies, we show that the EARS model is able to predict the essential features of the ABL. It also shows that the EARS model with the same model formulation and coefficients is applicable over awide range of stable and moderately unstable stratifications.

We present a subgrid-scale (SGS) scalar variance model based on the explicit algebraic subgrid scalar flux model, EASSFM (8). The EASSFM is a dynamic mixed nonlinear tensor eddy diffusivity model, which is derived from the modeled transport equation of the SGS scalar flux. The explicit algebraic form is obtained using the weak equilibrium assumption. The resulting model improves the direction of the predicted SGS flux vector and enables the prediction of shear-induced SGS fluxes, in contrast with the eddy diffusivity model. The EASSFM has been used for large eddy simulation (LES) of turbulent channel flow with and without system rotation (8; 9) and has been found to improve LES predictions over the dynamic eddy diffusivity model. A priori analysis of the EASSFM using the filtered DNS data from a reacting turbulent wall-jet has been performed recently (6; 7), which also showed favorable results. In this study, we evaluate our SGS scalar variance model using the filtered DNS database of a turbulent reacting wall-jet, which is an extension of our previous study on reactive turbulent wall-jet flows (5; 7) to a larger simulation domain. The results show a good agreement between the filtered DNS and our model predictions for the passive and active scalars. This indicates that acceptable predictions of the SGS scalar variance can be obtained using the EASSFM with the new SGS scalar variance model.

For turbulent flows that are influenced by an active scalar, the Reynolds stresses and scalar flux are coupled in a complicated way, which makes it difficult to model these flows. A framework has been derived for obtaining explicit algebraic Reynolds-stress and scalar-flux models for two-dimensional mean flows with stratification. For the specific case of stably stratified parallel shear flows, the derived model was shown to give good results. As an extension of these results, two more cases are considered: unstable stratification in a horizontal channel and natural convection in a vertical channel. 

The concept of local isotropy [1,2] of passive and active scalar fields is addressed for a turbulent wall-jet. A plane wall-jet is formed when a jet flow is injected parallel and next to a solid surface. At the inlet of the computational domain, both a fuel component (active) and a passive scalar are injected through the jet stream, within the height h [3]. The mean profiles of the two scalars are shown in Fig. 1. The remainder of the inlet consists of a coflow with a velocity of the order of 10% of the jet flow and contains 50% of the oxidizer. The reaction forms in such a way that a considerable amount of fuel is consumed throughout the domain. The main objective of this study is to use the DNS-database to address the statistical characteristics of both active and passive scalars. In particular, we discuss the properties of skewness and flatness at large and small scales. The scalar statistics are interesting both for applied problems, when the large scale properties such as the scalar variance and the scalar flux are examined, and for fundamental ones, concerning the universal properties of the advected fields.

A recently developed explicit algebraic Reynolds-stress (EARS) model is validated for an idealized representation of the night-time high-latitude stably stratified atmospheric boundary layer. The simulations are made with four surface cooling rates that result in weakly to moderately stratified stable boundary layers. The predictions of the EARS model are compared to high-resolution large-eddy simulations (LES) of Sullivan et al. (J Atmos Sci 73(4):1815–1840, 2016). First- and second-order statistics are shown to be well predicted by the EARS model. The EARS model also predicts the horizontal turbulent fluxes and turbulence anisotropy and these compare well with the LES results. The sensitivity to the model coefficients is studied by comparing the EARS model results with LES results. Finally, we propose a new scaling for the production of turbulence kinetic energy and show that the EARS model captures the essential trends of the LES results for different cooling rates.

DNS have been performed of turbulent channel flow with spanwise rotation and bulk Reynolds number Re up to 30000. At moderate rotation rates Ro the flow on one side of the channel is approximately laminar, or has turbulent patches or oblique turbulent-laminar patterns. Intense cyclic bursts of turbulence with long time intervals are in some cases observed at sufficiently high Re and Ro. A linear analysis indicates that the turbulence bursts are initiated by a linear instability of plane waves aligned with the rotation axis. This linear instability can develop even if parts of the flow are strongly turbulent. 

Large-scale fully resolved direct numerical simulations (DNSs) have been performed with a high-order spectral element method to study the flow of an incompressible viscous fluid in a smooth circular pipe of radius R and axial length 25R in the turbulent flow regime at four different friction Reynolds numbers Reτ = 180, 360, 550 and 1000. The new data is compared to other simulation data sets, obtained in pipe, channel and boundary-layer geometry. The pressure is the variable that differs the most between the cases; a significantly higher mean and fluctuating pressure are observed in boundary layers that is linked to a stronger wake region. Critical assessment of the available DNS data is conducted in order to determine which difference or correspondence between the data sets are real and caused by physics, and which discrepancies are likely due to statistical or numerical inaccuracies. Furthermore, two-dimensional spectra of axial/streamwise velocity show an imprint of the large-scale motions from the outer layer in all canonical flows, however with different amplitude. 

Concerning wall resolved large-eddy simulation (LES), a considerable reduction of computational resources is achievable by employing the Explicit Algebraic subgrid scale model (EAM) (\cite{marstorp2009explicit}).LES of periodic hill is carried out using OpenFOAM with the EAM and a low-diffusive implementation that has been previously tested on a turbulent channel flow. The aim of the present study is to evaluate in a broad sense the influence of  the Reynolds number on the flow quantities.

Standard turbulence models for the atmospheric boundary layer (ABL) typically use boundary conditions based on the Monin-Obukhov similarity theory (MOST). This can lead to inconsistency between the boundary condition and the closure model. Here, we propose a new boundary-condition treatment of the stratified ABL, derived for the so-called explicit algebraic Reynolds-stress model. The boundary conditions correspond to the relations for vanishing buoyancy effects that are valid close to the ground. The solution for the stratified surface layer is in agreement with the surface scaling physics and MOST functions. This was validated in a simulation of an idealized diurnal cycle of the ABL based on the second Global Energy and Water cycle Experiment (GEWEX) Atmospheric Boundary Layer Study (GABLS2) case.