10:30
Large Eddy Simulation 1
Chair: Elisabetta De Angelis
10:30
15 mins
|
Influence of the tip gap size on the development of the tip-leakage vortex using Large Eddy Simulations
Jean Decaix, Guillaume Balarac, Matthieu Dreyer, Mohamed Farhat, Cécile Münch
Abstract: In hydraulic turbines, the tip-leakage vortex is responsible for flow instabilities and for promoting erosion due to cavitation. To better understand the flow in the tip region, LES computations are carried out to compute the flow around a NACA0009 blade including the gap between the tip and the wall. The influence of the gap size is investigated by computing two gap widths.
The validation of the results is performed by comparisons with experimental data. The simulations are also used to investigate the flow in the tip gap region. Depending on the gap width, the vortex flow topology differs from one case to the other. At large gap widths, the tip-leakage vortex merges with the tip-separation vortex. On the contrary, at small gap widths, the tip-leakage vortex move upward and no tip-separation vortex is clearly identified. Part of these observations are validated by comparisons with experimental visualizations of the cavitating tip-leakage vortex.
|
10:45
15 mins
|
Truncation of scales by relaxation
Roel Verstappen
Abstract:
This paper is about a relaxation model for large-eddy simulation of turbulent flow that truncates the too small scales of motion by making sure that they do not get energy from the larger eddies. To verify that a box filter is introduced and the relaxation parameter is determined in such a way that the production of small, box-fitting scales is counteracted by the modeled dissipation. This dissipation-production balance is worked out with the help of Poincar\'{e}'s inequality, which results in a relaxation model that depends on the invariants of the velocity gradient. This model is discretized and equipped with a Schumann filter. It is successfully tested for isotropic turbulence as well as for turbulent channel flow
|
11:00
15 mins
|
Non-equilibrium near wall velocity profiles in the flow around a cylinder mounted on a flat plate
Wolfgang Schanderl, Michael Manhart
Abstract: We consider fully turbulent flow around a cylinder mounted on a flat plate. We performed a highly resolved Large Eddy Simulation using zonally refined grids around the cylinder and a fully turbulent inflow condition. The flow is characterized by strongly non-equilibrium near wall velocity profiles which deviate from the equilibrium law of the wall. We assess the prediction of wall shear stresses in those regions which has an impact on wall modeling and prediction of erosion and scour development.
|
11:15
15 mins
|
Going beyond eddy viscosity: Finding a minimal representation of subgrid-scale stresses in large-eddy simulation
Maurits Silvis, Roel Verstappen
Abstract: In the current study we aim to go beyond the dissipative description of turbulent flows that is provided by eddy viscosity models for large-eddy simulation. As a starting point, we consider a general subgrid-scale model that is nonlinear in the velocity gradient. To reduce the number of degrees of freedom of the model, we propose a first-principles-based procedure to find a minimal representation of subgrid-scale stresses. Then, several criteria to determine the dependence of model coefficients on flow properties are detailed. Ultimately, this would lead to a better understanding of the role of different nonlinear model terms in the description of turbulent flows.
|
11:30
15 mins
|
Testing the Coupled Wake Boundary Layer model with LES of turbulent flow in widely spaced wind farms
Richard Stevens, Dennice Gayme, Charles Meneveau
Abstract: The Coupled Wake Boundary Layer (CWBL) model combines a classical wake model with a ``top-down'' boundary layer model through two-way coupling to combine the strengths of these two analytical modeling approaches. The wake model part of the CWBL model captures the effects of the relative turbine positioning and the growth of wakes due to turbulence while the ``top-down'' part describes the interactions between the wind-farm and the turbulent atmospheric boundary layer. Previously, the CWBL model has been shown to provide improved predictions compared to the results obtained from classical wake and ``top-down'' models for the power production of aligned and staggered wind-farm configurations with turbine spacing of up to $8$ turbine diameters. In addition the CWBL model has been validated against detailed LES results and field measurements for the Horns Rev and Nysted wind-farms. Here we will compare the CWBL model predictions for wind-farms with very large inter-turbine spacings with results from new large eddy simulations to verify the validity of the CWBL model in that regime.
|
11:45
15 mins
|
Wavenumber-frequency spectra in the logarithmic layer of wall turbulence
Michael Wilczek, Richard J.A.M. Stevens, Charles Meneveau
Abstract: We study space-time correlations of wall-bounded turbulence in terms of wavenumber-frequency spectra of the streamwise velocity component. The spectra are obtained from Large Eddy Simulations (LES), which provide a full space-time record of the flow. We find that the frequency distributions exhibit a Doppler shift, which is a consequence of mean flow advection, as well as a considerable Doppler broadening, consistent with the Kraichnan-Tennekes random sweeping hypothesis. For wall-bounded turbulence, both of these effects vary with the wall distance and are closely related to the logarithmic behavior of the mean velocity profile and the velocity fluctuation profiles.
We incorporate these observations into a simple analytical model for the wavenumber-frequency spectrum based on an advection equation featuring advection of the small-scale velocity fluctuations with a mean and a large-scale random-sweeping velocity. The model is found to be in very good agreement with the LES data. Potential applications of the model spectrum, e.g., to quantify the spatio-temporal structure of fluctuations in wind energy conversion, will be discussed.
|
|