Flow model for velocity distribution in fixed porous beds under isothermal conditions

In a brief survey of the previous work the limitations of the modified Darcy equation and of the vectorial form of the Ergun equation are discussed. To include the effect of wall friction on the flows the viscous resistance term is added to the vectorial form of the Ergun equation. Using the generalized Ergun equation a one-dimensional formulation is presented for flow of fluids through packed beds taking into account the variation of porosity along the radial direction. It is found that there is a reasonable agreement between the numerical and the experimental results and it is observed that the variation of porosity with radial position has greater influence on channeling of velocity near the walls. For the assumption of constant porosity the velocity profiles exhibit similar nature as the plug flow profiles with a thin boundary layer near the wall. © 1979 Springer-Verlag

Effect of radiative transfer on the onset of convection in a porous medium

The effect of radiative transfer on thermal convection of a thin fluid-saturated densely packed porous layer bounded by stress-free radiating horizontal planes heated from below is studied using linear theory. The coefficients of absorption, emission and scattering are computed from packed bed properties using a two-flux model. The Milne-Eddington approximation is employed to determine approximate solutions valid for optically thin (transparent) and optically thick (opaque) gray media which absorb and emit thermal radiation. The effect of radiation parameters on the cell size and on the onset of convection are studied in detail using the Galerkin approximation. It is shown that the effect of radiation is to inhibit the onset of convection in a porous medium. The physical explanation for this is given, taking into account the increase in thermal conductivity due to the combined effects of the porosity of the medium and radiation

Histological Tracking into the Third Dimension: Evolution of Early Tumorigenesis in VHL Kidney

Using a novel three-dimensional (3D) approach, we tracked histological changes to elucidate the earliest stages of renal clear cell neoplasia in normal kidney tissue of patients with von Hippel-Lindau (VHL) disease. Tissue blocks of interest were procured, serially sectioned, and 3D reconstruction of the entirety of pathologic events was performed. The results reveal an abundance of foci with aberrant clear cell proliferation that initially develop along the tubular lining, but have the potential to aggregate within individual tubules. This stage is followed by the extension of clear cell aggregates beyond the tubular basement membrane, which allows for the recruitment of angiogenesis derived from interstitial vasculature. The results suggest that the most frequent pathologic event in VHL kidneys is the presence of isolated or aggregated clear cells within the tubular epithelium, potentially developing further into a protracted process of neoplasia. The abundance of independent pathologic events in VHL kidneys confirms developmental mechanisms to precede tumor initiation. To our knowledge, this is the first report demonstrating that tracking of histologic changes in the 3rd dimension enables the confirmation of the sequence of events from the earliest pathologic change in the VHL kidney to the neoplastic stage. This approach is not only useful for visualization and quantification of pathologic changes but also for targeted sampling allowing selective analysis of the earliest stages of clear cell carcinogenesis

Eddies: Fluid Dynamical Niches or Transporters?–A Case Study in the Western Baltic Sea

Fluid flows in the ocean have a strong impact on the growth and distribution of planktonic communities. In this case study, we applied a Lagrangian eddy detection and tracking tool and a transfer operator approach to data from a coupled hydrodynamical-chemical-biological model of the Western Baltic Sea and studied the effects of eddies on plankton in the blooming period March to October 2010. We investigated the residence times of water bodies inside these eddies, using a tracer analysis and found that eddies can act in two different ways: They can be transporters of an enclosed water body that embodies nutrients and the plankton community and export them from the coast to the open sea; and they can act as fluid dynamical niches that enhance the growth of certain species or functional groups by providing optimal temperature and nutrient composition

Pathology of the Nervous System in Von Hippel-Lindau Disease

Von Hippel-Lindau (VHL) disease is a tumor syndrome that frequently involves the central nervous system (CNS). It is caused by germline mutation of the VHL gene. Subsequent VHL inactivation in selected cells is followed by numerous well-characterized molecular consequences, in particular, activation and stabilization of hypoxia-inducible factors HIF1 and HIF2. The link between VHL gene inactivation and tumorigenesis remains poorly understood. Hemangioblastomas are the most common manifestation in the CNS; however, CNS invasion by VHL disease-associated endolymphatic sac tumors or metastatic renal cancer also occur, and their differentiation from primary hemangioblastoma may be challenging. Finally, in this review, we present recent morphologic insights on the developmental concept of VHL tumorigenesis which is best explained by pathologic persistence of temporary embryonic progenitor cells.

Activating KRAS Mutations in Arteriovenous Malformations of the Brain: Frequency and Clinicopathologic Correlation

Arteriovenous malformations (AVM) of the brain are considered congenital. Most AVMs are presumably sporadic, however rare familial cases occur and they may be observed in certain genetic disorders. We sought to determine the frequency of KRAS mutations and their association with clinicopathologic characteristics. We searched our neuropathology database from 2014–2017 for resected AVMs of the brain or dura mater. Twenty-one AVMs were tested (12 females, 9 males; average age: 32 years). KRAS mutations were found in 6/21 cases (28.5%). Five mutations were p.G12 V, and one p.G12C. The KRAS-mutant group contained 4 females and 2 males, with an average age of 28 years, compared to 34 years in the non-mutant group (P = .54). The average AVM size in the KRAS-mutant group was 3.9 cm, compared to 3.1 cm in the non-mutant group (P = .52). There were no histologic differences between KRAS-mutant and non-mutant cases. In summary, KRAS mutations occur in almost one third of brain AVMs. KRAS p.G12 V was the most common mutation identified. We also demonstrate the first reported instance of a KRAS p.G12C mutation in a brain AVM. The mean age of patients with KRAS-mutant AVMs was lower than the non-mutant group, and the mean size larger. Histologic characteristics were equally distributed between KRAS-mutant and non-mutant groups