We present direct numerical simulations of developing turbulent channel flows subjected to thermal expansion or contraction downstream of a heated or cooled wall. Using different constitutive relations for viscosity we analyse the response of variable property flows to streamwise acceleration/deceleration by separating the effect of streamwise acceleration/deceleration from the effect of wall-normal property variations. We demonstrate that, beyond a certain streamwise location, the flow can be considered in a state of 'quasi-equilibrium' regarding semilocally scaled variables. As such, we claim that the development of turbulent quantities due to streamwise acceleration/deceleration is localized to the region of impulsive heating/cooling, while changes in turbulence occurring farther downstream can be attributed solely to property variations. This finding allows us to study turbulence modulation in accelerating/decelerating flows using the semilocal scaling framework. By investigating the energy redistribution among the turbulent velocity fluctuations, we conclude that a change in bulk streamwise velocity has a non-local effect which originates from the change in mean shear and modifies the energy pathways through velocity-pressure-gradient correlations. On the other hand, the wall-normal property gradients have a local effect and act through the modification of the viscous dissipation. We show that it is possible to superimpose and compare the two different effects when using the semilocal scaling framework.

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The present work investigates the modeling of turbulent heat transfer in flows where radiative and convective heat transfer are coupled. In high temperature radiatively participating flows, radiation is the most relevant heat transfer mechanism and, due to its non-locality, it causes counter intuitive interactions with the turbulent temperature field. These so-called Turbulence-Radiation Interactions (TRI) largely affect the temperature field, modifying substantially the turbulent heat transfer. Therefore, in the context of modeling (RANS/LES), these interactions require a closure model. This work provides the inclusion of TRI in the modeling of the turbulent heat transfer by adopting a unique approach which consists in approximating the fluctuations of the radiative field with temperature fluctuations only. Based on this approximation, coefficients of proportionality are employed in order to close the unknown terms in the relevant model equations. A closed form of all radiation-temperature-velocity correlation is explicitly derived depending on the chosen turbulent heat transfer model. This model is applied to a standard two-equation turbulent heat transfer closure and used to reproduce results obtained with high-fidelity DNS simulations. While a standard approach (i.e., neglecting TRI) is not able to correctly predict the DNS data, the new model's results shows exceptional agreement with the high-fidelity data. This clearly proves the validity (and the necessity) of the proposed model in non-reactive, radiative turbulent flows.

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Radiative heat transfer has a large influence in engineering systems, especially when the temperature involved is elevated. For this reason a correct assessment of heat transfer in presence of radiation is of great importance in high temperature and pressure equipment such as combustion chambers, heat exchangers as well as reentry vehicles and rockets. This thesis presents the results of innovative coupled radiative heat transfer and turbulence simulations, which aim at investigating turbulence-radiation interactions and the effect of radiation in the turbulent heat transfer process. The simulations are performed employing heterogeneous high performance computing systems in which the radiative heat transfer is solved on graphical processing units while the fluid flow is solved on CPUs. @en

The present work investigates gray and non-gray gas turbulence-radiation interactions (TRI) in a turbulent channel flow bounded by two isothermal hot and cold walls. Cases of various optical thicknesses are examined using Direct Numerical Simulations (DNS), coupled with a computationally efficient Monte Carlo radiative transfer solver. Several novel concepts are presented which not only allow to uniquely characterize but also to accurately model TRI for a wide range optical properties in non-reacting flows. First, we propose linear relations between fluctuations in radiative quantities (emission, incident radiation and absorption coefficient) and temperature fluctuations, where the coefficients of proportionality are solely functions of averaged quantities (e.g. emission fluctuations E′ can be related with temperature θ in the following way, E^′=f_E(θ¯)θ^′). The validity of these linear relations is supported by an excellent agreement with DNS for all considered gray gas cases. Using these linear relations it is possible to show that gray gas TRI can be fully characterized without accounting for fluctuations in absorption coefficient. Second, TRI for non-gray gases is investigated and the developed concepts are extended to account for the spectrally varying absorption coefficient. In particular, the derived linear relations are used to show that the influence of a wavelength dependent κ manifests itself in an increase of the “effective” optical thickness of the flow. A new turbulence based spectral averaging is proposed that results in a mean κ, which uniquely characterizes TRI of non-gray participating media. Finally, we apply our models to estimate classical TRI (impact of fluctuations in radiative quantities on the mean radiative source) and a perfect agreement with DNS is observed. We anticipate that the proposed formulations also have the potential to allow for a better characterization in TRI, where strong temperature fluctuations are present, such as in combustion applications. Yet, this needs to be explored in future studies.

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We implemented a fast Reciprocal Monte Carlo algorithm to accurately solve radiative heat transfer in turbulent flows of non-grey participating media that can be coupled to fully resolved turbulent flows, namely to Direct Numerical Simulation (DNS). The spectrally varying absorption coefficient is treated in a narrow-band fashion with a correlated-k distribution. The implementation is verified with analytical solutions and validated with results from literature and line-by-line Monte Carlo computations. The method is implemented on GPU with a thorough attention to memory transfer and computational efficiency. The bottlenecks that dominate the computational expenses are addressed, and several techniques are proposed to optimize the GPU execution. By implementing the proposed algorithmic accelerations, while maintaining the same accuracy, a speed-up of up to 3 orders of magnitude can be achieved.

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The present work consists of an investigation of the turbulence radiation interaction (TRI) in a radiative turbulent channel flow of grey gas bounded by isothermal hot and cold walls. The optical thickness of the channel is varied from 0.1 to 10 to observe different regimes of TRI. A high-resolution finite volume method for radiative heat transfer is employed and coupled with the direct numerical simulation (DNS) of the flow. The resulting effects of TRI on temperature statistics are strongly dependent on the considered optical depth. In particular, the contrasting role of emission and absorption is highlighted. For a low optical thickness the effect of radiative fluctuations on temperature statistics is low and causes the reduction of temperature variance through the dissipating action of emission. On the other hand, while increasing optical thickness to relatively high levels, the dissipation of temperature variance is balanced, at low wavenumbers in the turbulence spectrum, through the preferential action of absorption, which increases the large-scale temperature fluctuations. A significant rise in the effect of radiation on the temperature variance can be observed as a consequence of the reduction of radiative heat transfer length scales.

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