Browsing by Subject "Turbulence modeling"

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A viscous vorticity method for the prediction of turbulent flows around hydrofoils and propellers
(2023-12) You, Rui, Ph. D.; Kinnas, Spyros A.; Hodges, Ben R.; Sepehrnoori, Kamy; Bogard, David G.; Raja, Laxminarayan L.
Marine propellers operate in turbulent flows, presenting a significant challenge for numerical analysis and design. Conventional linear and potential flow theories have limitations in predicting turbulent flows. Viscous flow methods, though capable of handling turbulence by incorporating a turbulence model, are often computationally intensive. The primary objective of this dissertation is to develop an efficient viscous numerical solver for simulating turbulent flows around propellers. The VIScous Vorticity Equation (VISVE) numerical solver was previously developed to handle laminar flow problems. This dissertation aims to extend this solver to allow for turbulence modeling and cavitation. The focus is on implementing the k − ω SST turbulence model in both 2-D and 3-D versions of the VISVE solver. The turbulence model is discretized using the Finite Volume Method (FVM), and a hybrid OpenMP/MPI parallelization strategy is adopted to fully leverage High-Performance Computing (HPC) resources. The developed method is first applied to 2-D hydrofoils in turbulent flow, showcasing excellent agreement with predictions from a Reynolds-Averaged Navier-Stokes (RANS) solver. The influence of Reynolds number (Re) on vorticity, velocity, and turbulent viscosity is studied across a range of Re values. The turbulent VISVE model is also successfully applied to a hydrofoil with a cupped trailing edge, capturing turbulent separated flow at high Reynolds numbers. The method is then extended to 3-D analysis, demonstrating consistency with RANS solver results for infinitely long hydrofoils and rectangular wings. The turbulent VISVE solver is further employed to study propellers in turbulent flow, with a focus on a 5-bladed propeller (e.g., the NSRCD propeller 4381) under various loading conditions. In addition to investigating 2-D and 3-D turbulent flow scenarios, the 2-D solver is extended to address turbulent cavitating conditions. A pressure calculation scheme is proposed for 2-D turbulent and cavitating flow, and multiphase and cavitation models are implemented in the 2-D turbulent VISVE solver, using the mixture model. Scalability tests are conducted to optimize computational resources, enhancing the solver’s efficiency. Notably, the VISVE method offers advantages such as a significantly smaller computational domain and post-processing pressure calculation. Moreover, the computational grid size, unknowns, and computation time for the propeller system are significantly reduced by considering the effects of all other blades using the key blade’s solution from an earlier time step. Overall, this research contributes to the development of a robust and efficient numerical solver for simulating turbulent flows around hydrofoils and propellers. The implementation of turbulence models and parallelization strategies advances the understanding and analysis of turbulent propeller flows, with potential applications in marine propulsion design and performance analysis.
The influence of thunderstorm downbursts on wind turbine design
(2012-08) Nguyen, Hieu Huy, 1980-; Manuel, Lance
The International Electrotechnical Commission (IEC) standard 61400-1 for the design of wind turbines does not explicitly address site-specific conditions associated with anomalous atmospheric events or conditions. Examples of such off-standard atmospheric conditions include thunderstorm downbursts, hurricanes, tornadoes, low-level jets, etc. This study is focused on the simulation of thunderstorm downbursts using a deterministic-stochastic hybrid model and the prediction of wind turbine loads resulting from these simulated downburst wind fields. The wind velocity field model for thunderstorm downburst simulation is first discussed; in this model, downburst winds are generated separately from non-turbulent and turbulent parts. The non-turbulent part is based on an available analytical model (with some modifications), while the turbulent part is simulated as a stochastic process using standard turbulence power spectral density functions and coherence functions. Tower and rotor loads are generated using simulation of the aeroelastic response for models of utility-scale wind turbines. The main objective is to improve our understanding from the point of view of design so that we may begin to address transient events such as thunderstorm downbursts based on the simulations carried out in this research study. The study discusses as well the role of control systems (for blade pitch and turbine yaw), of models for representing transient turbulence characteristics, and of correlated demand and loads on multiple units in turbine arrays during thunderstorm downbursts.
Investigating and modeling turbulence using numerical simulations
(2023-03-13) Mohan, Prakash, Ph. D.; Moser, Robert deLancey; Dawson, Clint; Ghattas, Omar; Goldstein, David; Oliver, Todd
Turbulence is a complex fluid phenomenon that is present in high Reynolds number flows. It has a profound effect on the flows in which it occurs, and it is therefore important to understand and model its effects. It occurs in multiple domains from flows inside our bodies to ocean currents and atmospheric winds. The difficulty in modeling and simulating turbulence arises from the fact that it is comprised of a wide range of scales that interact with each other non-linearly. The field of turbulence still has many open problems — from fundamental questions about the underlying physics to enabling tractable engineering models. The Navier-Stokes equations are a reliable representation of turbulent flows and solving them with sufficient accuracy gives us the detailed turbulent flow field. These are called Direct Numerical Simulations (DNS) and are an invaluable tool to study the turbulence phenomenon. In this work, we first consider how DNS of forced isotropic turbulence can be used to study time predictability of turbulence using Lyapunov exponents. Further analysis of the DNS field shows that flow instabilities act on the smallest eddies, and that at any time, there are many sites of local instabilities. DNS, however, is generally too expensive for simulating practical flows. Alternatively, Large Eddy Simulations (LES), in which only the largest scales of turbulent motion are simulated, is more promising as an engineering tool. However, in the near-wall region the large, dynamically important eddies are on the order of viscous scales, which makes resolving them very expensive. It is therefore desirable to formulate an approach in which the near-wall region is modeled, leading to the so-called wall-modeled LES. Spectral analysis of DNS data indicates that thin-film type asymptotics is a promising approach to model the interactions between the near-wall layer and the outer flow. For this approach an asymptotic analysis of the filtered Navier-Stokes equations is pursued in the limit in which the horizontal filter scale is large compared to the thickness of the wall layer. In the second part of this work, we present a new wall model formulated using the asymptotic analysis and insights from DNS data.
Large eddy simulation analysis of non-reacting sprays inside a high-g combustor
(2012-08) Martinez, Jaime, master of science in engineering; Raman, Venkat; Clemens, Noel T
Inter-turbine burners are useful devices for increasing engine power. To reduce the size of these combustion devices, ultra-compact combustor (UCC) concepts are necessary. One such UCC concept is the centrifugal-force based high-g combustor design. Here, a model ultra-compact combustor (UCC) with fuel spray injection is simulated using large eddy simulation (LES) and Reynolds-Averaged Navier-Stokes (RANS) methodologies to understand mixing and spray dispersion inside centrifugal-based combustion systems. Both non-evaporating and evaporating droplet simulations were carried, as well as the tracking of a passive scalar, to explore this multiphase system. Simulation results show that mixing of fuel and oxidizer is based on a jet-in-crossflow system, with the fuel jet issuing into a circulating oxidizer flow stream. It is seen that a a high velocity vortex-like ring develops in the inner core of the combustor, which has enough momentum to obstruct the path of combustion products. There is minimal fuel droplet and vapor segregation inside the combustor and enhanced turbulent mixing is seen at mid-radius.
Modeling turbulence using optimal large eddy simulation
(2012-05) Chang, Henry, 1976-; Moser, Robert deLancey; Engquist, Bjorn; Ghattas, Omar; Hughes, Thomas J.; Raman, Venkat
Most flows in nature and engineering are turbulent, and many are wall-bounded. Further, in turbulent flows, the turbulence generally has a large impact on the behavior of the flow. It is therefore important to be able to predict the effects of turbulence in such flows. The Navier-Stokes equations are known to be an excellent model of the turbulence phenomenon. In simple geometries and low Reynolds numbers, very accurate numerical solutions of the Navier-Stokes equations (direct numerical simulation, or DNS) have been used to study the details of turbulent flows. However, DNS of high Reynolds number turbulent flows in complex geometries is impractical because of the escalation of computational cost with Reynolds number, due to the increasing range of spatial and temporal scales. In Large Eddy Simulation (LES), only the large-scale turbulence is simulated, while the effects of the small scales are modeled (subgrid models). LES therefore reduces computational expense, allowing flows of higher Reynolds number and more complexity to be simulated. However, this is at the cost of the subgrid modeling problem. The goal of the current research is then to develop new subgrid models consistent with the statistical properties of turbulence. The modeling approach pursued here is that of "Optimal LES". Optimal LES is a framework for constructing models with minimum error relative to an ideal LES model. The multi-point statistics used as input to the optimal LES procedure can be gathered from DNS of the same flow. However, for an optimal LES to be truly predictive, we must free ourselves from dependence on existing DNS data. We have done this by obtaining the required statistics from theoretical models which we have developed. We derived a theoretical model for the three-point third-order velocity correlation for homogeneous, isotropic turbulence in the inertial range. This model is shown be a good representation of DNS data, and it is used to construct optimal quadratic subgrid models for LES of forced isotropic turbulence with results which agree well with theory and DNS. The model can also be filtered to determine the filtered two-point third-order correlation, which describes energy transfer among filtered (large) scales in LES. LES of wall-bounded flows with unresolved wall layers commonly exhibit good prediction of mean velocities and significant over-prediction of streamwise component energies in the near-wall region. We developed improved models for the nonlinear term in the filtered Navier-Stokes equation which result in better predicted streamwise component energies. These models involve (1) Reynolds decomposition of the nonlinear term and (2) evaluation of the pressure term, which removes the divergent part of the nonlinear models. These considerations significantly improved the performance of our optimal models, and we expect them to apply to other subgrid models as well.