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Analytical solutions for turbulent non-Boussinesq plumes
- PIERRE CARLOTTI, GARY R. HUNT
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- Journal:
- Journal of Fluid Mechanics / Volume 538 / 10 September 2005
- Published online by Cambridge University Press:
- 17 August 2005, pp. 343-359
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Analytical solutions are developed for non-Boussinesq turbulent plumes rising from horizontal area sources in unconfined quiescent environments of uniform density. The approach adopted follows and extends an earlier approach for Boussinesq plumes and replaces the non-Boussinesq area source of interest and located at $z\,{=}\,0$ with an idealized point source located at a virtual origin $z\,{=}\,z_v$ such that the flow above the idealized source approximates that from the actual source. Asymptotic analytical expressions are developed for the location of the virtual source that are valid for large vertical distances above the non-Boussinesq source. The non-Boussinesq source is characterized by a non-dimensional parameter $\Gamma_{\hbox{\scriptsize{\it nb}}}$ which is a measure of the relative strengths of the mass, momentum and density deficit fluxes at, or at a specified height above, the source. The vertical distance between the actual and virtual sources scales on the length scale $\ell$ that characterizes the height over which the flow is non-Boussinesq and expressions for $z_v/\ell$ are developed for lazy ($\Gamma_{\hbox{\scriptsize{\it nb}}}\,{>}\,1$) and forced plume ($\Gamma_{\hbox{\scriptsize{\it nb}}}\,{<}\,1$) sources. For pure-plume source conditions $\Gamma_{\hbox{\scriptsize{\it nb}}}\,{=}\,1$, and the virtual source provides an exact representation of the actual plume above $z\,{=}\,0$. The limiting cases of a nearly pure lazy plume and of a highly lazy plume are also explored analytically. For fire plumes, $\Gamma_{\hbox{\scriptsize{\it nb}}}$ is determined from the balance of fluxes immediately above the combustion region and a procedure for estimating these fluxes is given. Solutions expressing the dependence of the mass flux with height are also developed for the near-field flow regions and thereafter an approximation for the mass and momentum fluxes valid for all heights and for source conditions yielding $0\,{<}\,\Gamma_{\hbox{\scriptsize{\it nb}}}\,{<}\,\infty$ is deduced. Applications of the model may include plumes above fires and forced releases of highly buoyant gas into the atmosphere, for example, following the rupturing of a pressurized container vessel.
Length scales in wall-bounded high-Reynolds-number turbulence
- PIERRE CARLOTTI, PHILIPPE DROBINSKI
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- Journal:
- Journal of Fluid Mechanics / Volume 516 / 10 October 2004
- Published online by Cambridge University Press:
- 24 September 2004, pp. 239-264
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In this study, estimates of inhomogeneous integral scales are derived from rapid distortion theory (RDT) for the case of wall-bounded high-Reynolds-number turbulence and from large-eddy simulation (LES) of a neutrally stratified atmospheric boundary layer (ABL). As for any inhomogeneous flow, integral scales in different directions are introduced. Downward integral scales are introduced since they differ from the usual vertical integral scales because of the presence of the wall. The study concentrates on the length scales based on the vertical velocity, which are the most affected by blocking by the wall, which is assumed to be horizontal.
It is shown from RDT that the asymptotic behaviour of the integral length scales for small heights depends crucially on the spectrum power law $-2p$. When $2p>1$ there is always one length scale which does not scale with the distance to the wall $z$. Only the downward integral scale is proportional to $z$ for any $2p$. These results show that the assumption, often made in studies of boundary layers, that all the lengths are proportional to $z$, is not compatible with the assumption of a spectrum decaying according to Kolmogorov's law, but rather with a spectrum following a $-1$ power law. It is an encouraging result since there is now widespread theoretical, experimental and numerical evidence that such a $-1$ power-law subrange exists in the spectra of high-Reynolds-number wall-bounded turbulence, for eddies larger than $z$. The RDT results allow an interpretation of the vertical profiles of the integral length scales computed from the LES outputs: above the third grid point, the vertical profiles of the integral length scales have a linear shape, as expected for high-Reynolds-number turbulence and $2p=1$. Very close to the surface, the upward integral length scales decreases with $z$ because of the fast decay of the spectrum ($2p>2$) from the LES subgrid model.
The longitudinal-to-transverse integral length scale ratio is computed using RDT and LES. This ratio is interpreted as the aspect ratio of elongated near-wall large eddies, which are ubiquitous features of LES of boundary layers in which shear plays an important role in the dynamics. The LES shows that the longitudinal-to-transverse integral length scale ratio is an increasing function of $z$, ranging between 1 and 3, which is of the order of magnitude of the published theoretical value of 3.5. From RDT, the evolution with $z$ of the longitudinal-to-transverse integral length scale ratio means either that the velocity shear $\beta$ decreases with $z$ and the spectral power law $2p$ varies in a non-trivial manner, or if both the RDT and LES are valid that the scale of the large eddies is proportional to $\beta z$ with $\beta$ varying from 1.3 to about 4.