Abstract This paper presents preliminary results of laboratory experiments designed to study the effects of specific volume variation as regards the basic physical mechanisms governing weakly compressible flows. Turbulent free convection developing in a large vertical open tunnel was investigated. Results were analyzed in the framework of an original formulation of the Navier–Stokes equations in which no simplifying assumptions regarding density variations were made, and where the terms representing specific volume fluctuation effects could be isolated and examined separately. Non-isovolume mechanisms were found to play a noticeable role in the transport equations of internal and turbulent energy, and had to be accounted for in budget analysis. In contrast, the analysis showed that the form of the transport equation of mean momentum obtained and used classically when the Boussinesq approximation applies could still be used in the present case, consistently with earlier observations in the atmosphere. Results put forward that the role played by non-isovolume mechanisms depends strongly on the mean velocity divergence and, with respect to the temperature field, on the dissipation rate of the temperature variance.