Entrainment and turbulence characteristics of low-velocity isothermal and buoyant jets

[摘要] The current study examined the transition region of axisymmetric isothermal and buoyant jets of low Reynolds number, directed vertically upwards into a stagnant, unstratified ambient. These flows were examined experimentally and numerically. The region in which measurements were obtained allows examination of two types of transition occurring in the jet: from nozzle exit dominated to fully developed, and from momentum to buoyancy dominated flow. Velocity data were acquired using a two channel Laser-Doppler Anemometer for isothermal Reynolds numbers of 850 to 7405, and buoyant Froude numbers of 12 to 6425 and Reynolds numbers from 525 to 6500. Curve fit approximations of the data were developed by assuming polynomial similarity profiles for the measured quantities. Correlation equations were developed which allow prediction of the downstream velocity flow field and turbulent flow field as a function of the Reynolds number, Froude number and density ratio at the nozzle exit. Profile width and entrainment increased at low Reynolds number. Axial and radial velocity fluctuations were found to increase at low Reynolds number. The buoyant cases studied were found to have lower velocity fluctuations and significantly lower Reynolds stresses than isothermal cases of similar Reynolds number. Once comprehensive correlation equations were developed predicting mean and turbulent flow quantities, the basis was formed for development of a new turbulence model, the analytic turbulence model. This method involved substitution of correlation equation approximations to the mean flow quantities into the boundary layer forms of the governing equations. The turbulent terms were then solved for explicitly. It was shown that this method predicts the fully developed boundary layer turbulent flow field, and can be used as a criterion for a flow attaining boundary layer form. Comparison of turbulent values predicted analytically with those measured empirically revealed that the transition flow examined experimentally had not fully developed to boundary layer form. Testing of the turbulent correlation equations was performed by using them as a turbulence model in a finite difference numerical solution. While the correlation equations were representative of the turbulent flow field, they did not produce correct results when used in a marching, boundary layer simulation. As verified by the analytic turbulence model, this occurred because the transition flow had not yet reached fully developed boundary layer form.

[发布机构] Rice University