| dc.description.abstract | A study of ‘Airflow over Mountains’ is carried out using a two-dimensional, stratified, time-dependent, nonlinear numerical model. Numerical experiments are conducted for high Froude numbers (Fr > 1) and low Froude numbers (Fr < 1). Results show that in the case of high Froude number flow, lee wave activities are almost negligible. However, nonlinear phenomena like hydraulic jump on the lee slope and jet-like flow in the lower levels on the lee side are observed.
Experiments for low Froude number cases give rise to well-marked lee waves and rotors. It is observed that for very low Froude numbers usually lee disturbances appear to move towards the upstream against the general flow. Upstream influences are observed in all these experiments. Nonlinear phenomena like hydraulic jump on the lee slope, rotors, and strong jet-like flow in the lower levels of the lee side are constant features in all the low Froude number flows. Also, strong upward and downward velocities of the order of two to three times the velocity of the initial mean flow are found close to the mountain, in the lower levels on windward side and lee side respectively in all these experiments.
A few experiments are conducted to study the effects of the lateral and top boundary conditions when applied separately as well as simultaneously. Results show that in the case of low Froude number flow, boundary conditions have very profound influence on the governing flow. However, more experiments have to be conducted to study the effect of boundary conditions more completely.
Study of the airflow over Western Ghats in India, during monsoon period, shows that no well-marked lee waves are seen in general in this period. However, the nonlinear phenomena like hydraulic jump on the lee slope and low-level jet-like flow on the lee side are observed in all the numerical experiments.
Finally, for a few cases rainfall amounts on the windward slope of the mountain are computed by assuming a saturated atmosphere and pseudo-adiabatic lapse rate. These computed rainfall amounts are then compared with the observed rainfall amounts and linear model results of R.P. Sarker. It is found that our rainfall computations are larger in all the cases studied. The discrepancy is mainly due to larger vertical velocities obtained in the lower levels of the numerical model. Neglect of frictional force, three-dimensional effects, and convective mixing most probably are the causes for the larger vertical velocities in the numerical model. Further experimentation is suggested with the inclusion of some of the above-said effects. | |
