| 1. |
- Andreasson, Patrik, et al.
(författare)
-
A note on a generalized eddy-viscosity hypothesis
- 1992
-
Ingår i: Journal of Fluids Engineering. - : ASME International. - 0098-2202 .- 1528-901X. ; 114:3, s. 463-466
-
Tidskriftsartikel (refereegranskat)abstract
- The standard eddy-viscosity concept postulates that zero velocity gradient is accompanied by zero shear stress. This is not true for many boundary layer flows: wall jets, asymmetric channel flows, countercurrent flows, for example. The generalized eddy-viscosity hypothesis presented in this paper, relaxes this limitation by recognizing the influence of gradients in the turbulent length scale and the shear. With this new eddy-viscosity concept, implemented into the standard k - ε model, predictions of some boundary layer flows are made. The modelling results agree well with measurements, where predictions with the standard eddy-viscosity concept are known to fail.
|
|
| 2. |
- Engelmark, Helen, et al.
(författare)
-
Numerical modelling of phase change in freezing and thawing unsaturated soil
- 1993
-
Ingår i: Nordic Hydrology. - : IWA Publishing. - 0029-1277 .- 1996-9694. ; 24:2-3, s. 95-110
-
Tidskriftsartikel (refereegranskat)abstract
- Problems associated with seasonal freezing and thawing processes occur in natural and man modified soil environments. To optimize various technical solutions in such areas, the importance of accurately predicting heat and water flow in seasonally freezing and thawing soils becomes obvious. A phase change between water and ice occurs in moist soils subjected to freezing and thawing. In concentrating on water transport phenomena in frozen soil, it can be demonstrated that the heat and moisture flow relationships are coupled to relationships for mass balance and phase change. A new numerical model was developed for handling the phase change process. In the new method, heat and mass transfer equations for soils subjected to both freezing and thawing (but without heaving and transport of mass in the air phase), are solved numerically. This new method for handling the phase change process is based on a total energy balance together with soil-water-freezing-characteristics (SWFC). The total energy balance of the frozen zone includes sensible and latent heat components. The phase change during a time increment is calculated, without any iteration, after the heat and water flow equations are solved for the same time increment (i.e., a two-step approach). Simulations of the three tests with the proposed new method were carried out with abrupt boundary conditions from time zero. Little difference between measured and calculated temperature and moisture content profiles resulted. However, a difference in the results at the cold end, between simulations with the new model and a previously developed model, shows that without any exact measurement of the moisture content at the cold end, no final conclusion about the real magnitude of the moisture content at the cold end can be drawn. A supercooling of the pore water was needed before ice formation could begin. This was due to moderate initial moisture content. The water flow toward the freezing front caused the moisture content to decrease in the unfrozen part of the soil column. Lesser amounts of water in this part, then required even lower temperatures before ice formation could start. Frost depths were not detectable with only temperature measurements. (See also W93-08925) (Lantz-PTT)
|
|
| 3. |
|
|
