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<br />~ypes of Laboratory Airflows <br />The laboratory simulation of transport and dispersion over <br />irregular terrain presents several problems which may be generalized <br />as follows: <br />1) The physical limitations of the laboratory facilities <br />necessitate adopting certain restrictive assumptions, <br />2) the problem of similitude between model and prototype, <br />3) the problem of obtaining proper measurements of the pertinent <br />parameters in the laboratory facility as well as the field, and <br />4) the problem of verifying the model results with actual field <br />measurements. <br />Three general types of airflow can be generated in a laboratory <br />fad li ty: <br />1) Neutral airflow, where static stability is assumed neutral and <br />the pressure field is determined by the geometry of the terrain <br />features. If the terrain features are sharp, the flow patterns <br />are not influenced by viscous forces and Reynold's number differ- <br />ences between the model and prototype. Irreversibility in the flow <br />(as well as turbulence) is usually the result of separation eddies, <br />which appear on the lee side of obstacles. <br />2) Barostromatic* airflow, where the air is stably stratified <br />due to density or temperature stratification. This type of airflow <br />is generally quasi-laminar and with proper density stratification <br />gravity waves and "hydraulic" jumps occur. Large vertical temper- <br />ature gradients and low flow velocities are required in order to <br />produce this type of flow in a laboratory facility. <br />3) Unstable airflow, where the air is heated from below producing <br />thermal convection cells throughout the flow medium (Ref. 12). <br /> <br />; <br /> <br />~ <br /> <br />Restriction on Laboratory Airflows <br />In order that the flow in any laboratory model should be of value <br />in interpreting or predicting the 'observed flow in the atmosphere, it <br />is essential that the two flow systems should by dynamical, thermally <br />and kinematially similar. This means that it must be possible to <br />describe the flow in the two systems by the same equations after <br />appropriate adjustments of the units of length, time and other variables. <br />Several difficulties arise in attempting to generate a physical <br />model which will be similar to the actual atmosphere. The difficulties <br />are principally due to the limitations of the laboratory facility in <br />reproducing a scaled-down model atmosphere. The problem requires a <br />simplification of the basic equations of the atmosphere** by a set of <br />restrictive assumptions. In this study the following restrictions were <br />placed upon the atmospheric flow and boundary conditions in order to <br />make laboratory simulation possible: <br /> <br />* Word derived from Greek and adopted by R. S. Scorer as representing <br />an airflow which exhibits density stratification. For the purpose of <br />this study it represents airflow with stable thermal stability in the <br />upper levels and near-neutral thermal stability in the lower levels. <br /> <br />** See Appendix <br /> <br />~ <br /> <br />18 <br />