Turbulent gravitational convection from a point source in a non-uniformly stratified environment

Abstract

We examine the turbulent gravitational convection which develops above a point source of buoyant fluid in a stably stratified environment in which the buoyancy frequency varies with height according to N²=N²_s (z/z_s)^β. This generalizes the classical model of turbulent buoyant plumes rising through uniform and uniformly stratified environments originally developed by Morton et al. (1956). By analogy, the height of rise of a plume with initial buoyancy flux F_s has the form H_p= A_pε_p^−1/2F_s^1/4N_s^−3/4h_p (λ, β) where ε_p is the entrainment constant for plume motion, A_p is an O(1) constant, and the non-dimensional plume height, h_p is a function of &λ=A_pε_p^−1/2F_s^1/4N_s^−3/4/z_s and β.

In the case β>0, the stratification becomes progressively stronger with height, and so plumes are always confined within a finite distance above the origin. Furthermore, the non-dimensional height of rise h decreases with λ. In contrast, in the case β<0, the stratification becomes progressively weaker with height, and so the non-dimensional plume height increases monotonically with λ. For slowly decaying stratification, β>−8/3, the motion is confined within a finite distance above the source. However, for each value of β with β<−8/3, there is a critical value λ_c(β) such that for λ<λ_c a plume is confined to a region near the source while for λ[ges ]λ_c the motion is unbounded. In the unbounded case, the motion asymptotes to the solution for a buoyant plume rising through a uniform environment, with asymptotic buoyancy flux F_∞(λ)<F_s. We show that in the limiting case λ=λ_c, dividing bounded and unbounded motion, as z→∞ the plume asymptotes to a new similarity solution of the second kind which describes the motion of a plume in a non-uniformly stratified environment. These similarity solutions are unstable in the sense that small perturbations to the initial conditions result in very different behaviour far from the source.

Analogous results for an instantaneous release of buoyant fluid from a point source, which forms a thermal, are also presented. The model is applied to describe the motion of plumes and thermals in the upper ocean and in naturally ventilated buildings since in both cases the stratification is typically non-uniform.