Variable property DNS of differentially heated cavities filled with air

• Published in: International journal of heat and mass transfer 2020-03, Vol.149, p.119259, Article 119259
• Main Authors: Demou, Andreas D., Grigoriadis, Dimokratis G.E.
• Format: Article
• Language: English
• Subjects: Aerodynamics; Approximation; Aspect ratio; Boundary layers; Boussinesq approximation; Computational fluid dynamics; Computer simulation; Differentially heated cavity; Direct numerical simulation; Disruption; Holes; Laminar-turbulent transition; Low-Mach approximation; Oberbeck–Boussinesq approximation; Relocation; Temperature distribution; Temperature gradients; Transition points; Variable properties
• ISSN: 0017-9310; 1879-2189
• DOI: 10.1016/j.ijheatmasstransfer.2019.119259

•Non Oberbeck–Boussinesq (NOB) effects investigated for different aspect-ratios.•Higher aspect-ratios amplify NOB effects on temperature statistics.•Flow dynamics intensify on heated wall and weaken on the cooled wall.•NOB effects cause an asymmetric relocation of the transition points.•Relocation is explained based on the interaction of plumes and boundary layers. Direct numerical simulations (DNS) of the thermally-driven flow of air inside differentially heated cavities (DHC) are presented. The variation of air properties is taken into account using the low-Mach approximation, to obtain solutions outside the limits of the Oberbeck–Boussinesq (OB) approximation. The effects of the cavity’s aspect-ratio A and large temperature differences ΔT were investigated over the range 1  ≤ A ≤  8 and ΔT ≤  720 K. The Rayleigh and Prandtl numbers were set at Ra=109 and Pr=0.7, considering a reference temperature of T0=600 K. Time-averaged and r.m.s. statistics of the temperature field along with other derived quantities are presented and discussed. It is found that the time-averaged temperature fields vary significantly between non-Oberbeck–Boussinesq (NOB) and OB cases for larger aspect-ratios. This is also true for the instantaneous temperature and temperature r.m.s. fields which demonstrate significant qualitative differences. In addition, as the temperature difference increases and the property variations become more intense, the boundary layers at the vertical walls become turbulent at lower positions. The relocation of the laminar-turbulent transition point is found to be strongly dependent on the cavity’s aspect-ratio. An explanation of this phenomenon is given, based on the disruption of the vertical boundary layers by horizontal streams originating from the downstream part of the vertical walls.