Measurement of turbulent dispersion behind a fine cylindrical heat source in a weakly sheared flow

Abstract

Turbulent structure in a turbulent scalar dispersion field behind a fine cylindrical heat source in a weakly sheared flow is experimentally investigated and previous computational turbulence models for third-order scalar transport terms in the second-order turbulence equations are assessed with the present data.The mean temperature and r.m.s. temperature profiles are found to be almost Gaussian even in the uniform shear layer. Decay of the peak temperature, mean dispersion and half-widths of the mean temperature and the r.m.s. temperatures are well correlated with corresponding data on the scalar dispersion behind an elevated line heat source in the turbulent boundary layer.Normalized streamwise heat flux $\overline{u\theta}$ changes appreciably with the downstream distance owing to the influence of the uniform mean shear, whereas normalized vertical heat flux $\overline{v\theta}$ remains the same with the downstream distance. The timescale ratio R between temperature and velocity fluctuations varies from 0.3 to 1.3 across the stream and it asymptotes to a value 0.5 at far downstream.Assessment of previous models for third-order moments with the present data reveals that application of a composite timescale between the dynamic timescale and the thermal timescale to the simplest gradient transport model yields a better overall prediction performance than any existing models, including Lumley's algebraic model equations for the moments. It was found that the timescale for the streamwise transports of $\overline{u\theta}$ and $\overline{\theta^2}$ is larger than that of lateral transports of $\overline{v\theta}$ and $\overline{\theta^2}$.In addition, since the experiment isolates the effect of uniform mean shear on the turbulent scalar transport, experimental data accumulated by the present study will be useful for further development of more refined second-order turbulence models for non-isothermal turbulent flows.

Application of a Reynolds Stress/Heat Flux Model to the Turbulent Thermal Dispersion Behind a Line Heat Source in a Uniformly Sheared Flow

“…They found the The velocity field as well as the mean and fluctuating temperature fields were documented in detail. At nearly the same time, a similar experiment was performed by Chung and Kyong [5] in a weakly sheared homogeneous turbulent flow. They measured the streamwise and vertical heat fluxes and third-order moments behind an elevated line heat source.…”

“…As addressed in the Introduction, the experimental results of Chung and Kyong [5] are employed for comparison. In their experiment, a fine cylindrical heat source was located in a weakly sheared flow.…”

“…As compared with the wake pattern in the mean velocity profile, the wake of the turbulent fluctuations persists comparatively longer owing to production of the turbulent kinetic energy by an amount -uvdU/dy [5]. In the usual two-dimensional wake behind a cylinder the u' distribution has two peaks at both sides of the wake even in the self-preservation region [27].…”

“…In an effort to analyze model performance, a simple gradient transport model is also employed for comparison. The predicted numerical results are compared with the experimental results of Chung and Kyong [5]. The model performance is shown to be generally satisfactory.…”

“…In applications of turbulent diffusion to passive scalar transport, it is sometimes necessary to make the complex turbulent problems simple. One example is the diffusion of a passive scalar from a continuous line source placed in a uniformly sheared homogeneous turbulent flow [5][6][7].…”