Measurements of microclimates in beds in relation to the climatic requirements of house dust mites

House dust mites are animals of a size less than 0.5 mm that can live in beds, carpets and furniture feeding on skin scales. They are a common source of allergy in, e.g., Scandinavia, where their major habitat is in beds. Previous studies show that in drier environments the house dust mite occurrence is lower, and persons with dust mite allergy have fewer symptoms during the winter when the indoor relative humidity (RH) is low. There have also been attempts to alter the microclimatic conditions in beds to reduce the house dust mite occurrence. The microclimatic conditions in beds have been studied both in field measurements and by modeling but there is still a lack of knowledge of how the hygrothermal material properties of the mattress and bedding affect the environmental conditions for the house dust mites. This paper presents diurnal temperature and RH variations in mattresses and beddings under normal use measured for two different mattress types. The climatic results from the beds have been compared to microclimatic requirements that govern the house dust mite activity levels. This study is a part of a multidisciplinary project aiming to find technical solutions for reduction of house dust mites in bedrooms by environmental control

Numerical Simulation of Tip Vortex Cavitation Inception

Tip vortex cavitating flow is known as challenging to study. The objective of this paper is to investigate the effective parameters in numerical modelling of tip vortex cavitation (TVC) inception through the comparison of three different models. The models are (1) a commonly used homogenous mixture model, in which inception is based on pressure drop criterion; (2) a Lagrangian bubble model, in which cavitation is initiated from free nuclei in the liquid; and (3) a hybrid Eulerian-Lagrangian model, in which the cavities are initiated based on the pressure drop criterion, but the growth of initially small cavities are modelled using the more accurate Lagrangian equations. The simulations are conducted on the tip vortex flow around an elliptical foil. The results show that the commonly applied pressure drop assumption is not a sufficient criterion for cavitation inception. Also, it is seen that the water quality and nuclei transport towards the vortex core influence the cavity pattern at inception

Improved prediction of sheet cavitation inception using bridged transition sensitive turbulence model and cavitation model

Sheet cavitation inception can be influenced by laminar boundary layer flow separation under Reynolds numbers regimes with transitional flow. The lack of accurate prediction of laminar separation may lead to massive over-prediction of sheet cavitation under certain circumstances, including model scale hydrofoils and marine propellers operating at relatively low Reynolds number. For non-cavitating flows, the local correlation based transition model, γ − Reθ transition model, has been found to provide predictions of laminar separation and resulting boundary layer transition. In the present study, the predicted laminar separation using γ − Reθ transition model is bridged with a cavitation mass transfer model to improve sheet cavitation predictions on hydrofoils and model scale marine propellers. The bridged model is developed and applied to study laminar separation and sheet cavitation predictions on the NACA16012 hydrofoil under different Reynolds numbers and angles of attack. As a reference case, the open case of the PPTC VP1304 model scale marine propeller tested on an inclined shaft is studied. Lastly as an application case, the predictions of cavitation on a commercial marine propeller from Kongsberg is presented for model scale conditions. Simulations using the bridged model and the standard unbridged approach with k − ω SST turbulence model are performed using the open-source package OpenFOAM, both using the Schnerr–Sauer cavitation mass transfer model, and the respective results are compared with available experimental results. The predictions using the bridged model agree well compared to experimental measurements and show significant improvements compared to the unbridged approach

Numerical investigation of pressure pulse predictions for propellers mounted on an inclined shaft

In the presented study, two high-skew model scale marine propellers were tested in the cavitation tunnel and the induced pressure pulses were measured during the test. Propeller shaft was inclined about 10 degrees to create blade load variations. The cavitation pattern were recorded using high speed videos. The open-source package openFOAM and commercial package Star-ccm+ are used as simulation tools to predict pressure pulses numerically. By using the fully turbulent SST k − ω model, the predicted wetted flow pressure pulse levels agreed well compared to experimental measurements, but together with Schnerr-Sauer\ua0cavitation mass transfer model, massive cavitation was predicted which lead to inaccurate pressure pulse predictions.\ua0The transition sensitive turbulence model γ − Re θ model\ua0is used to study the cases, and simulation results reveal\ua0the existence of laminar-transition zone and vortex structures on the propeller blades. Attempts are made to linking correlation-based separation region from the transition model and the cavitation model, and good predictions of cavitation pattern are achieved but the predicted pressure pulses levels are merely improved

Numerical assessment of cavitation erosion risk using incompressible simulation of cavitating flows

In this paper, a numerical method to assess the risk of cavitation erosion is proposed, which can be applied to incompressible simulation approaches. The method is based on the energy description of cavitation erosion, which considers an energy transfer between the collapsing cavities and the eroded surface. The proposed framework provides two improvements compared with other published methods. First, it is based on the kinetic energy in the surrounding liquid during the collapse instead of the potential energy of collapsing cavities, which avoids the uncertainty regarding the calculation of the collapse driving pressure in the potential energy equation. Secondly, the approach considers both micro-jets and shock-waves as the mechanisms for cavitation erosion, while previous methods have taken into account only one of these erosion mechanisms. For validation, the proposed method is applied to the cavitating axisymmetric nozzle flow of Franc et al. (2011), and the predicted risk of cavitation erosion is compared with the experimental erosion pattern. This comparison shows that the areas predicted with high erosion risk agree qualitatively well with the experimental erosion pattern. Furthermore, as the current method can be used to study the relationship between the cavity dynamics and the risk of cavitation erosion, the hydrodynamic mechanism responsible for the high risk of cavitation erosion at the inception region of the sheet cavity is investigated in detail. It is shown for the first time that the risk of cavitation erosion in this region is closely tied to the separation of the flow entering the nozzle

Moisture penetration in a chair seat as a response to daily RH variations in the indoor air

In the indoor environment there are a number of materials with potential to act as moisture buffers including both building materials and furnishing materials. For daily moisture variations in the indoor air furniture with upholstery can play an important role as moisture buffers. Material properties and calculation models describing the response to moisture variations in the ambient climate for these material combinations are limited. In this project the moisture properties for a chair seat with a wool fabric and plastic foam padding were determined. The moisture penetration in the chair seat was measured using small temperature and relative humidity sensors. A numerical calculation model describing the step-response as well as the response to ramp variations is described. A comparison between measurements and theoretical calculations was performed. The difficulties with determination of material properties for highly permeable materials are also discussed as well as suitable methods and special considerations

Comparative analysis of tip vortex flow using RANS and LES

The current study focuses on the numerical analysis of tip vortex flows, with the emphasis n the investigation of turbulence modelling effects on tip vortex prediction. The analysis includes comparison of RANS and LES methods at two different mesh resolutions. Implicit LES, ILES, modelling is employed here to mimic the turbulent viscosity. In RANS, the two equation k-ω SST model is adopted. In order to also address possible benefits of using streamline curvature variations in RANS, two curvature correction methods proposed for k-ω SST are tested, and compared. ILES results show very good agreement with the experimental observations. The predicted vortex in ILES is also stronger than RANS predictions. ILES has predicted accelerated vortex core axial velocity very well, while tested RANS models under predict the axial velocity. Adoption of curvature correction has not improved the tip vortex prediction, even though it has reduced the turbulent viscosity at the vortex core