Characterisation of thermal boundary layers in acceleration

Musehane, Ndivhuwo M. (2023) Characterisation of thermal boundary layers in acceleration. Doctoral thesis, Northumbria University.

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Abstract

This research explored the thermal boundary layer on flat plates and blunt bodies experiencing moderate to high acceleration and deceleration in stationary air. The primary aim of the research is to evaluate differences in thermal boundary layer profiles and external flow field characteristics caused by moderate to high acceleration and deceleration of flat plates and blunt bodies with the corresponding steady-state profiles at the same instantaneous Mach numbers.

Using two-dimensional numerical models of flat plates and blunt bodies, the emphasis is on subsonic and hypersonic Mach numbers. OpenFoam is employed as the computational fluid dynamics software, and a custom solver for handling the unsteady conditions during acceleration and deceleration was developed based on the non-inertial conservation equations for mass, momentum, and energy derived in this research.

In subsonic cases, steady state simulations were conducted at velocities ranging from 10m=s to 70m=s, whereas hypersonic cases were simulated at Mach numbers ranging from 4 to 7. Using unsteady simulations, the flat plate and blunt body were accelerated at moderate (10g) to high (100000g) acceleration magnitudes, beginning at the lower velocity, and decelerated at moderate (0-10g) to high (-100000g) deceleration magnitudes, beginning at the higher velocity, over the same range of velocities. Flat plates were set at different wall temperature conditions to confirm the generalisability of thermal boundary layer profiles for subsonic and hypersonic regimes.

During acceleration at subsonic speeds, the skin friction coefficient and Nusselt numbers were found to be greater than their respective steady state profiles at the same velocities, but the opposite was true during deceleration at subsonic velocities. These differences were explained by the non-inertial thermal boundary layer equations derived in the present research and the response of the thermal boundary layer classified as weak (Type I), moderate (Type II), and strong (Type III). Differences between the unsteady and steady thermal boundary layer characteristics at subsonic speeds were found to be a function of the favourable or adverse pressure gradient exerted on the boundary layer by the external flow. Minimal variations were identified between the steady and unsteady thermal boundary layer characteristics in the hypersonic regime, and only a weak favourable pressure gradient (Type I) was observed. Moreover, acceleration/deceleration dependent behaviour of the bow shock was only detected at high acceleration magnitudes above the existing range of applications.

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