Comparison of Reacting DDES and LES CFD Simulation Methodologies for a Dual Inlet Ramjet Engine Combustor

Turbulent flow at Reynolds numbers 5 · 104 to 106 around the NREL S826 airfoil used for wind turbine blades is simulated using delayed detached eddy simulation (DDES). The 3D domain is built as a replica of the low speed wind tunnel at the Norwegian University of Science and Technology (NTNU) with the wind tunnel walls considered as slip walls. The subgrid turbulent kinetic energy is used to model the sub-grid scale in the large eddy simulation (LES) part of DDES. Different Reynoldsaveraged Navier-Stokes (RANS) models are tested in ANSYS Fluent. The realizable k - ∈ model as the RANS model in DDES is found to yield the best agreement of simulated pressure distributions with the experimental data both from NTNU and the Technical University of Denmark (DTU), the latter for a shorter spanwise domain. The present DDES results are in excellent agreement with LES results from DTU. Since DDES requires much fewer cells in the RANS region near the wing surface than LES, DDES is computationally much more efficient than LES. Whereas DDES is able to predict lift and drag in close agreement with experiment up to stall, pure 2D RANS simulations fail near stall. After testing different numerical settings, time step sizes and grids for DDES, a Reynolds number study is conducted. Near stall, separated flow structures, so-called stall cells, are observed in the DDES results.

The flow over the linear low-pressure turbine cascade MTU-T161 at Re = 90,000 is analyzed using Delayed Detached Eddy Simulations (DDES). At this operating point, the low Reynolds number and the high loading of the blade result in a separation bubble and a separation-induced transition of the flow over the suction side. The utilized DDES method is based on a vorticity-based formulation to calculate the subgrid length scales and it incorporates the one-equation γ-transition model. The computational model of the MTU-T161 cascade consists of one blade passage, including the diverging viscous sidewalls. To reproduce realistic operating conditions and to mimic the experiments, synthetic turbulence is prescribed at the inlet of the computational domain. Several studies are performed to assess the accuracy and performance of the DDES one-equation γ-transition model against experimental data and a benchmark LES. The primary focus is on the prediction of the separation and the separation-induced transition mechanism. First of all, a systematic grid convergence study is conducted and grid criteria are derived in order to ensure a satisfactory agreement of the flow metrics, such as isentropic Mach number, friction coefficient distribution and total pressure wake losses at mid-span with experimental data. Furthermore, a detailed analysis of the DDES model parameters, such as shielding function and subgrid length scale is presented and the effect of these parameters on the prediction accuracy of the separation bubble region is analyzed. The analysis of the suction side boundary layer indicates that the turbulent kinetic energy should be resolved and modeled properly in order to represent the separation bubble correctly. In particular, the correct prediction of the separated shear layer above the separation bubble is of utmost importance. The results of the simulations reveal higher demands on grid resolution for such transitional flows than typically have been reported in the literature for turbulent boundary layers. This higher demand on grid resolution results in more expensive simulations than Reynolds-averaged Navier-Stokes (RANS). Nevertheless, DDES requires less computing time than wall-resolved Large Eddy Simulations (LES). Additionally, the results of the transitional DDES model are compared to DDES without a transition model, RANS eddy viscosity model and a reference LES. The results show that the DDES approach needs to be coupled with a transition model, such as the one-equation γ-transition model, in order to capture the flow topology over a highly loaded turbine blade correctly. The benefit of the DDES one-equation γ-transition model becomes particularly evident when predicting the separated shear layer, transition process and the subsequent reattachment. The RANS eddy viscosity turbulence and transition models applied within our study are not able to predict the aforementioned mechanisms accurately. For highly loaded turbine blades in particular, the accurate prediction of flow separation and potential reattachment is crucial for the aerodynamic design of turbines, since large parts of the total pressure loss are generated in the separated region. For this reason, the DDES one-equation γ-transition model can be a good compromise in terms of predictive accuracy and computational costs.

The flow over the linear low-pressure turbine cascade MTU-T161 at Re = 90,000 is analyzed using delayed detached eddy simulations (DDES). At this operating point, the low Reynolds number and the high loading of the blade result in a separation bubble and a separation-induced transition of the flow over the suction side. The utilized DDES method is based on a vorticity-based formulation to calculate the subgrid length scales, and it incorporates the one-equation γ-transition model. The computational model of the MTU-T161 cascade consists of one blade passage, including the diverging viscous sidewalls. To reproduce realistic operating conditions and to mimic the experiments, synthetic turbulence is prescribed at the inlet of the computational domain. Several studies are performed to assess the accuracy and performance of the DDES one-equation γ-transition model against experimental data and a benchmark large eddy simulations (LES). The primary focus is on the prediction of the separation and the separation-induced transition mechanism. First of all, a systematic grid convergence study is conducted and grid criteria are derived in order to ensure a satisfactory agreement of the flow metrics, such as isentropic Mach number, friction coefficient distribution, and total pressure wake losses at mid-span with experimental data. Furthermore, a detailed analysis of the DDES model parameters, such as shielding function and subgrid length scale, is presented and the effect of these parameters on the prediction accuracy of the separation bubble region is analyzed. The analysis of the suction side boundary layer indicates that the turbulent kinetic energy should be resolved and modeled properly in order to represent the separation bubble correctly. In particular, the correct prediction of the separated shear layer above the separation bubble is of utmost importance. The results of the simulations reveal higher demands on grid resolution for such transitional flows than typically have been reported in the literature for turbulent boundary layers. This higher demand on grid resolution results in more expensive simulations than Reynolds-averaged Navier–Stokes (RANS). Nevertheless, DDES requires less computing time than wall-resolved LES. Additionally, the results of the transitional DDES model are compared to DDES without a transition model, an RANS eddy viscosity model, and a reference LES. The results show that the DDES approach needs to be coupled with a transition model, such as the one-equation γ-transition model, in order to capture the flow topology over a highly loaded turbine blade correctly. The benefit of the DDES one-equation γ-transition model becomes particularly evident when predicting the separated shear layer, the transition process, and the subsequent reattachment. The RANS eddy viscosity turbulence and transition models applied within our study are not able to predict the aforementioned mechanisms accurately. For highly loaded turbine blades in particular, the accurate prediction of flow separation and potential reattachment is crucial for the aerodynamic design of turbines, since large parts of the total pressure loss are generated in the separated region. For this reason, the DDES one-equation γ-transition model can be a good compromise in terms of predictive accuracy and computational costs.

Delayed detached eddy simulation (DDES) has been proved to be suitable for the numerical simulation of massively separated flow. Whereas, there are still some drawbacks in the treatment of gray area, which is the transition zone between Reynolds-Averaged Navier–Stokes (RANS) and large eddy simulation (LES). In this paper, a modified DDES with shear layer adapted (SLA) subgrid length scale was employed, which takes advantage of the peculiarities of flow and grid topology in the initial shear layer, it can rapidly destabilize the separated shear layer and accelerate RANS to LES transition. To evaluate the performance of modified DDES versus conventional DDES, two typical separated flows are considered, they are the flow over backward-facing step with fixed geometry-induced separation and wall-mounted hump with non-fixed pressure-induced separation. The fifth-order Adaptive Dissipative Compact Scheme (ADCS) is also formulated to reduce numerical dissipation in grey area. The results show that the gray area can be slightly alleviated by ADCS, but it cannot be effectively mitigated with conventional DDES model. The visualizations of instantaneous flow reveal that the modified DDES is capable of unlocking the Kelvin–Helmholtz instability rapidly and accelerating the transition to resolved turbulence in the initial shear layer, which is strongly delayed by conventional DDES. The time-averaged pressure and skin friction coefficients show the mitigation of delayed transition as well. The distributions of mean velocity and Reynolds stress of modified DDES exhibit a rapid development in the initial shear layer; thus, more turbulent structures can be distinguished and the accuracy of results can be enhanced.

The main objective of this article is to investigate the capability of the flamelet progress variable (FPV) model to capture the extinction processes observed in under-ventilated fire scenarios. To this end, large eddy simulation (LES) of the methane line fire plumes in oxygen-reduced environments down to global extinction, investigated experimentally at the University of Maryland (UMD), is performed. Two experimental burner configurations, that differ by the presence (anchored) or not (non-anchored) of an oxygen anchor to stabilise the flame base, are considered leading to two different extinction modes. Both the FPV and the steady laminar flamelet (SLF) model coupled with a presumed filtered density function (FDF) are considered. The Rank Correlated Full Spectrum k-distribution (RCFSK) model is used as a gas radiative property model. In both non-anchored and anchored scenarios, the FPV model reproduces with fidelity the evolution of the fire plume structure, radiative loss, and combustion efficiency with decreasing down to global extinction, without introducing any adjustable constant. The extinction in the non-anchored scenario occurs owing to flame-based detachment coupled to the generation of a buoyancy-driven vortex and is found to be very sensitive to the grid resolution in the near burner region. The present results suggest that these processes can be adequately resolved with a spatial resolution of 2.5 mm in this region. The SLF model, for its part, provides reliable predictions comparable to the FPV as long as no local extinction/re-ignition process occurs.

ABSTRACTMassively unsteady separated flow past a four-wheel rudimentary landing gear is computed based on three different numerical approaches. The methods include Delayed Detached Eddy Simulation (DDES) and the Unsteady Reynolds-Averaged Navier–Stokes (URANS) method based on the two-equation Shear Stress Transport (SST) model as well as Large Eddy Simulation (LES) based on the dynamic model. Using computational fluid dynamics software ANSYS® CFX®, surface features of both the mean and unsteady flows are studied and compared with experimental data, such as time-averaged pressure, surface flow patterns, sound pressure level and so on, while flow field characteristics like instantaneous vorticity and turbulent kinetic energy are obtained to assess the quality of different numerical methods. The accuracy of DDES in predicting the landing gear flow is assessed both aerodynamically and acoustically from an engineering point of view. As expected, URANS can predict the attached flow near the wall well, but fails to obtain reasonable fluctuations in detached regions. Owing to poor near-wall grid resolution, LES predicts some non-physical separation in the area where the flow was originally attached, which adds superfluous turbulence fluctuations. The results of DDES have the advantages of both, and are in good agreement with the experimental results, which characterize the unsteady properties of the flow better. The feasibility of the CFX-DDES method is demonstrated in predicting the unsteady flow for landing gears with moderate grid scales. What's more, because of its numerical robustness and low dissipation, DDES also obtains better near-field noise distribution which shows the potential in noise prediction for engineering applications. Additionally, a detailed analysis aiming at exploring the troublesome mechanisms of noise source generation is also exhibited using DDES. It shows the possibility that strongly unsteady interactions between vortices and landing gear structures may contribute to noise generation.

Delayed detached eddy simulation of pedestrian-level wind around a building array – The potential to save computing resources

This numerical investigation explores supersonic flow over a wall-mounted cylinder using Large Eddy Simulations (LES), unsteady Reynolds Averaged Navier-Stokes (RANS), Delayed Detached Eddy Simulation (DDES), and hybrid RANS/LES approaches. The LES was obtained using a well-validated high-order Navier-Stokes flow solver employing a hybrid 6-order compact spatial discretization and 2-order Roe scheme. An 8-order low-pass spatial filter was used to regularize the flow. The RANS and hybrid RANS/LES solutions were obtained with a 2-order k − turbulence model in conjunction with the high-order flow solver employed in the LES. Additional RANS and DDES results were obtained using a 5-order WENO scheme available in the OVERFLOW code. Results compare the characteristics of both time-mean and instantaneous solutions using the four turbulence modeling approaches. Overall, RANS solutions display favorable agreement with time-mean LES flow field structure and boundary layer characteristics. Unfortunately, both the DDES and hybrid RANS/LES approaches developed significantly longer separation regions upstream of the cylinder. In the hybrid RANS/LES approach, the length of the upstream separation shock was driven by the location chosen to transition from the RANS to hybrid RANS/LES model. For the DDES approach, the upstream separation shock shifted to a location very near the inflow boundary. Both the DDES and hybrid RANS/LES methodologies developed larger transitional/laminar separation regions because the upstream shock-wave/boundary-layer interaction was unable to force the development of small-scale turbulence structures necessary to overcome the reduction in eddy viscosity.

CFD simulation of the wind environment around an isolated high-rise building: An evaluation of SRANS, LES and DES models

A range of popular hybrid Reynolds-averaged Navier–Stokes -large eddy simulation (RANS-LES) methods are tested for a cavity and two labyrinth seal geometries using an in-house high-order computational fluid dynamics (CFD) code and a commercial CFD code. The models include the standard Spalart–Allmaras (SA) and Menter shear stress transport (SST) versions of delayed detached eddy simulation (DDES) and the Menter scale adaptive simulation (SAS) model. A recently formulated, enhanced, variant of SA-DDES presented in the literature and a new variant using the Menter SST model are also investigated. The latter modify the original definition of the subgrid length scale used in standard DDES based on local vorticity and strain. For all geometries, the meshes are considered to be hybrid RANS-LES adequate. Very low levels of resolved turbulence and quasi-two-dimensional flow fields are observed for the standard DDES and SAS models even for the test cases here that contain obstacles, sharp edges, and swirling flow. Similar findings are observed for both the commercial and in-house high-order CFD codes. For the cavity simulations, when using standard DDES and SAS, there is a significant under prediction of turbulent statistics compared with experimental measurements. The enhanced versions of DDES, on the other hand, show a significant improvement and resolve turbulent content over a wide range of scales. Improved agreement with experimental measurements is also observed for profiles of the vertical velocity component. For the first labyrinth seal geometry swirl velocities are more accurately captured by the enhanced DDES versions. For the second labyrinth seal geometry, the mass flow coefficient prediction using the enhanced models is significantly improved (up to 30%). Standard, industrially available hybrid RANS-LES models, when applied to the present canonical cases can produce little to no resolved turbulent content. The standard SA- and Menter-based DDES models can yield lower levels of eddy viscosity when compared to equivilent steady RANS simulations which means that they are not operating as RANS or LES. It is recommended that hybrid RANS-LES models should be extensively tested for specific flow configurations and that special care is exercised by CFD practitioners when using many of the popular hybrid RANS-LES models that are currently available in commercial CFD packages.

Tip leakage vortex (TLV) has a large impact on compressor performance and should be accurately predicted by computational fluid dynamics (CFD) methods. New approaches of turbulence modeling, such as delayed detached eddy simulation (DDES), have been proposed, the computational resources of which can be reduced much more than for large eddy simulation (LES). In this paper, the numerical simulations of the rotor in a low-speed large-scale axial compressor based on DDES and unsteady Reynolds-averaged Navier–Stokes (URANS) are performed, thus improving our understanding of the TLV dynamic mechanisms and discrepancy of these two methods. We compared the influence of different time steps in the URANS simulation. The widely used large time-step makes the unsteadiness extremely weak. The small time-step shows a better result close to DDES. The time-step scale is related to the URANS unsteadiness and should be carefully selected. In the time-averaged flow, the TLV in DDES dissipates faster, which has a more similar structure to the experiment. Then, the time-averaged and instantaneous results are compared to divide the TLV into three parts. URANS cannot give the loss of stability and evolution details of TLV. The fluctuation velocity spectra show that the amplitude of high frequencies becomes obvious downstream from the TLV, where it becomes unstable. Last, the anisotropy of the Reynolds stress of these two methods is analyzed through the Lumley triangle to see the distinction between the methods and obtain the Reynolds stress. The results indicate that the TLV latter part in DDES is anisotropic, while in URANS it is isotropic.

Numerical investigation of a pitching airfoil undergoing dynamic stall using Delayed Detached Eddy Simulations

This dissertation describes an alternate formulation for Delayed Detached Eddy Simulation or DDES. Detached Eddy Simulation (DES) falls under the category of hybrid RANS/LES models where a single turbulence model functions as either a RANS (Reynolds-Averaged NavierStokes) or an LES (Large Eddy Simulation) model. Certain fundamental issues were identified in the original DES formulation, which led to revised formulations such as the Delayed DES (DDES) and Improved DDES (IDDES) with increasing complexity, which negatively impacted the readability of the model. This is the motivation to explore an alternate formulation for DES which aims to correct the issues found in the original DES, while at the same time being simple and easy to understand. Towards this end, the eddy viscosity formulation in a given RANS model is modified such that it mimics the Smagorinsky LES subgrid viscosity expression when the model is in eddy simulation mode. The resemblance of the resulting DES formulation to the Smagorinsky model allows the implementation of a dynamic procedure to compute the model constant, similar to the dynamic Smagorinsky model. This was found to improve the model performance in several cases. The description of this alternate DES formulation and the implementation of a dynamic procedure in this model will be the major focus of this dissertation.

A range of popular hybrid Reynolds averaged Navier-Stokes - large eddy simulation (RANS-LES) methods are tested for cavity and labyrinth seal flows using an in-house high-order computational fluid dynamics (CFD) code and a commercial CFD code. The models include the Spalart-Allmaras (SA) and Menter SST variants of delayed detached eddy simulation (DDES), the Menter scale adaptive simulation (SAS) model, and a new enhanced variant of SA-DDES recently presented in the literature. The latter modifies the original definition of the subgrid length-scale used in DDES based on local vorticity and strain. For both geometries, the meshes are hybrid RANS-LES adequate. Very low levels of resolved turbulent content are observed for both the cavity and labyrinth seal flows for all models apart from the enhanced version of DDES. Similar findings are observed for both the commercial and in-house CFD codes. For both cases most models essentially produce a quasi-two-dimensional flow field with minimal resolved content. For the cavity simulations there is a significant under prediction of turbulent statistics. The enhanced version of SA-DDES shows a significant improvement and resolves turbulent content over a wide range of scales. Improved agreement with experimental measurements is also observed. It is recommended that extreme care should be taken where hybrid RANS-LES simulations are essentially steady but have lower than RANS levels of eddy viscosity.

In the present study, the subcritical flow past a generic side mirror on a base plane is investigated at the Reynolds number of 5.2×105 using delayed detached eddy simulation (DDES) turbulence model. Asides from the capability of capturing main features of the large recirculation vortex in the wake of the side mirror and the front horseshoe vortex, the accuracy of DDES estimation of recirculation length is significantly increased by over 20%, compared to the detached eddy simulation (DES) estimation using the same grid. And DDES prediction of pressure coefficient at the trailing edge of the mirror is in good agreement with the experiments, which is more accurate than both DES and large eddy simulation (LES) results. The results verify the capacity of DDES turbulence model to solve the turbulent flow around the side mirror. This is a key foundation for possible future study of full simulation of external flow field of vehicle.