MH
M. Henger
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Reynolds-Averaged Navier-Stokes (RANS) turbulence models are in widespread use in the commercial application of Computational Fluid Dynamics (CFD), but lack the capability to accurately compute unsteady flows and aeroacoustics due to its implied averaging of turbulent effects. In order to compute these kinds of flows, Large Eddy Simulation (LES) may be applied. However, this comes with a large computational cost. Hybrid RANS/LES aims to combine the turbulence-resolving aspects of LES, while attempting to approach the computational efficiency of RANS by applying a RANS turbulence model near walls. The X-LES turbulence model developed at NLR has been successfully applied to massively separated flows. A future application of the method is to obtain aeroacoustic predictions of turbulent noise near sharp edges and wakes. In anticipation of this goal, this work aims to assess the accuracy of the X-LES model when it resolves part of the near-wall turbulent spectrum of Turbulent Channel Flow (TCF).
Multiple TCF simulations have been performed at a variety of friction Reynolds numbers. It was found that the presence of resolved turbulent stress in the RANS-domain is not detrimental to the velocity profile in this domain, as it still conforms to the laws governing the viscous sublayer and the log-layer. The presence of so-called superstreaks causes a flow system where streamwise ribbons of high eddy viscosity, originating from within the RANS-domain, are transported away from the wall into the LES-domain. Here they contribute to the lack of development of turbulent stress, leading to an undesirable increase in velocity in the log-layer known as the Log-Layer Mismatch (LLM). Applying stochastic forcing led to a reduction in the LLM and broke up the large streamwise regions of eddy viscosity. It remains unclear in which proportion the disappearance of the high eddy-viscosity regions, and the stochastic forcing itself, contributed to the reduction in LLM.
An attempt has been made in formulating a consistent hybrid RANS/LES framework in which the effects of turbulence on the mean and fluctuating flow are governed by separate turbulence models throughout the entire domain. The initial results are presented in this report, which may be used to further develop the model in the future. ...
Multiple TCF simulations have been performed at a variety of friction Reynolds numbers. It was found that the presence of resolved turbulent stress in the RANS-domain is not detrimental to the velocity profile in this domain, as it still conforms to the laws governing the viscous sublayer and the log-layer. The presence of so-called superstreaks causes a flow system where streamwise ribbons of high eddy viscosity, originating from within the RANS-domain, are transported away from the wall into the LES-domain. Here they contribute to the lack of development of turbulent stress, leading to an undesirable increase in velocity in the log-layer known as the Log-Layer Mismatch (LLM). Applying stochastic forcing led to a reduction in the LLM and broke up the large streamwise regions of eddy viscosity. It remains unclear in which proportion the disappearance of the high eddy-viscosity regions, and the stochastic forcing itself, contributed to the reduction in LLM.
An attempt has been made in formulating a consistent hybrid RANS/LES framework in which the effects of turbulence on the mean and fluctuating flow are governed by separate turbulence models throughout the entire domain. The initial results are presented in this report, which may be used to further develop the model in the future. ...
Reynolds-Averaged Navier-Stokes (RANS) turbulence models are in widespread use in the commercial application of Computational Fluid Dynamics (CFD), but lack the capability to accurately compute unsteady flows and aeroacoustics due to its implied averaging of turbulent effects. In order to compute these kinds of flows, Large Eddy Simulation (LES) may be applied. However, this comes with a large computational cost. Hybrid RANS/LES aims to combine the turbulence-resolving aspects of LES, while attempting to approach the computational efficiency of RANS by applying a RANS turbulence model near walls. The X-LES turbulence model developed at NLR has been successfully applied to massively separated flows. A future application of the method is to obtain aeroacoustic predictions of turbulent noise near sharp edges and wakes. In anticipation of this goal, this work aims to assess the accuracy of the X-LES model when it resolves part of the near-wall turbulent spectrum of Turbulent Channel Flow (TCF).
Multiple TCF simulations have been performed at a variety of friction Reynolds numbers. It was found that the presence of resolved turbulent stress in the RANS-domain is not detrimental to the velocity profile in this domain, as it still conforms to the laws governing the viscous sublayer and the log-layer. The presence of so-called superstreaks causes a flow system where streamwise ribbons of high eddy viscosity, originating from within the RANS-domain, are transported away from the wall into the LES-domain. Here they contribute to the lack of development of turbulent stress, leading to an undesirable increase in velocity in the log-layer known as the Log-Layer Mismatch (LLM). Applying stochastic forcing led to a reduction in the LLM and broke up the large streamwise regions of eddy viscosity. It remains unclear in which proportion the disappearance of the high eddy-viscosity regions, and the stochastic forcing itself, contributed to the reduction in LLM.
An attempt has been made in formulating a consistent hybrid RANS/LES framework in which the effects of turbulence on the mean and fluctuating flow are governed by separate turbulence models throughout the entire domain. The initial results are presented in this report, which may be used to further develop the model in the future.
Multiple TCF simulations have been performed at a variety of friction Reynolds numbers. It was found that the presence of resolved turbulent stress in the RANS-domain is not detrimental to the velocity profile in this domain, as it still conforms to the laws governing the viscous sublayer and the log-layer. The presence of so-called superstreaks causes a flow system where streamwise ribbons of high eddy viscosity, originating from within the RANS-domain, are transported away from the wall into the LES-domain. Here they contribute to the lack of development of turbulent stress, leading to an undesirable increase in velocity in the log-layer known as the Log-Layer Mismatch (LLM). Applying stochastic forcing led to a reduction in the LLM and broke up the large streamwise regions of eddy viscosity. It remains unclear in which proportion the disappearance of the high eddy-viscosity regions, and the stochastic forcing itself, contributed to the reduction in LLM.
An attempt has been made in formulating a consistent hybrid RANS/LES framework in which the effects of turbulence on the mean and fluctuating flow are governed by separate turbulence models throughout the entire domain. The initial results are presented in this report, which may be used to further develop the model in the future.
Finding Venusian Volcanoes
Project Matryoshka
Bachelor thesis
(2016)
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M.P. den Boer, A.J.D. de Bruin, C.S. Ganani, A.A. Gunessee, S.F.M. Harmsen, M. Henger, J.J.X. Huang, T.C. Kroes, Y. Luan, J.N. Weinmiller, D.M. Stam, S. Teixeira De Freitas, Z. Hong
This report details the design of a mission aimed to find and analyse active Venusian volcanoes, if they exist. These volcanoes are interesting because active volcanism would significantly contribute to the understanding of the Venusian atmosphere, its extreme climate and geological processes. This knowledge would in turn help us understand Earth better. The design is based on the concept selected previously in the Midterm report and consists of five vehicles: a spacecraft, an aeroshell, an aircraft and two landers. The spacecraft with aeroshell will be launched into a Hohmann transfer orbit to Venus in 2023. Upon arrival, the satellite will map the surface, and find the most promising region for volcanic activity. It will then deploy the aeroshell containing the aircraft and landers. The satellite then changes its orbit to one that allows for it to act as a relay between the Venusian vehicles and Earth. After entry and having slowed down sufficiently to deploy a parachute, the first lander will be dropped. This lander will act as a reference for the lander inside the aircraft. Next, the aircraft is deployed after which it will start following flight tracks that allow for it to stay in the Sunlight. These tracks are designed by taking into consideration the power systems, thermal system and propulsion system, and then optimising such that the electronics do not overheat and that the battery size is reasonable. While flying, the aircraft will take measurements to locate volcanoes. Once a very promising location is found, the aircraft will deploy the second lander from an altitude of about 32 km. This lander will then descend further down and land on the surface where it will perform measurements. Combining the measurements of all vehicles it is expected that the mission can also complete a number of secondary objectives to further improve the knowledge of Venus...
...
This report details the design of a mission aimed to find and analyse active Venusian volcanoes, if they exist. These volcanoes are interesting because active volcanism would significantly contribute to the understanding of the Venusian atmosphere, its extreme climate and geological processes. This knowledge would in turn help us understand Earth better. The design is based on the concept selected previously in the Midterm report and consists of five vehicles: a spacecraft, an aeroshell, an aircraft and two landers. The spacecraft with aeroshell will be launched into a Hohmann transfer orbit to Venus in 2023. Upon arrival, the satellite will map the surface, and find the most promising region for volcanic activity. It will then deploy the aeroshell containing the aircraft and landers. The satellite then changes its orbit to one that allows for it to act as a relay between the Venusian vehicles and Earth. After entry and having slowed down sufficiently to deploy a parachute, the first lander will be dropped. This lander will act as a reference for the lander inside the aircraft. Next, the aircraft is deployed after which it will start following flight tracks that allow for it to stay in the Sunlight. These tracks are designed by taking into consideration the power systems, thermal system and propulsion system, and then optimising such that the electronics do not overheat and that the battery size is reasonable. While flying, the aircraft will take measurements to locate volcanoes. Once a very promising location is found, the aircraft will deploy the second lander from an altitude of about 32 km. This lander will then descend further down and land on the surface where it will perform measurements. Combining the measurements of all vehicles it is expected that the mission can also complete a number of secondary objectives to further improve the knowledge of Venus...