Detached-eddy simulation (DES) was first proposed in 1997 and first used in 1999, so its full history can be surveyed. A DES community has formed, with adepts and critics, as well as new branches. The initial motivation of high–Reynolds number, massively separated flows remains, for which DES is convincingly more capable presently than either unsteady Reynolds-averaged Navier-Stokes (RANS) or large-eddy simulation (LES). This review discusses compelling examples, noting the visual and quantitative success of DES. Its principal weakness is its response to ambiguous grids, in which the wall-parallel grid spacing is of the order of the boundary-layer thickness. In some situations, DES on a given grid is then less accurate than RANS on the same grid or DES on a coarser grid. Partial remedies have been found, yet dealing with thickening boundary layers and shallow separation bubbles is a central challenge. The nonmonotonic response of DES to grid refinement is disturbing to most observers, as is the absence of...

Detached-Eddy Simulation

Review of numerical simulations for high-speed, turbulent cavity flows

Simulation has been widely used for performance prediction and optimum design of refrigeration systems. A brief review on history of simulation for vapour-compression refrigeration systems is done. The models for evaporator, condenser, compressor, capillary tube and envelop structure are summarized. Some developing simulation techniques, including implicit regression and explicit calculation method for refrigerant thermodynamic properties, model-based intelligent simulation methodology and graph-theory based simulation method, are presented. Prospective methods for future simulation of refrigeration systems, such as noise-field simulation, simulation with knowledge engineering methodology and calculation methods for nanofluid properties, are introduced briefly.

Recent developments in simulation techniques for vapour-compression refrigeration systems

Further to our previous large-eddy simulation (LES) of flow over a staggered array of uniform cubes, a simulation of flow over random urban-like obstacles is presented. To gain a deeper insight into the effects of randomness in the obstacle topology, the current results, e.g. spatially-averaged mean velocity, Reynolds stresses, turbulence kinetic energy and dispersive stresses, are compared with our previous LES data and direct numerical simulation data of flow over uniform cubes. Significantly different features in the turbulence statistics are observed within and immediately above the canopy, although there are some similarities in the spatially-averaged statistics. It is also found that the relatively high pressures on the tallest buildings generate contributions to the total surface drag that are far in excess of their proportionate frontal area within the array. Details of the turbulence characteristics (like the stress anisotropy) are compared with those in regular roughness arrays and attempts to find some generality in the turbulence statistics within the canopy region are discussed.

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Large-Eddy Simulation of Flows over Random Urban-like Obstacles

This paper deals with the experimental validation of an analytical trailing-edge noise model dedicated to low-speed fans operating in free field. The model is intrinsically related to the aerodynamics of the blades and should lead to a useful fast-running tool to be included in a blade-design process in an industrial context. The investigations are made on a two-bladed low-speed axial fan without shroud, installed inside an anechoic room. The blades are instrumented with two sets of embedded small-size microphones (2.5 mm diam), and the wall-pressure signals are acquired via a slip ring mounted on the fan axis. The chord-based Reynolds number is about 200,000, and the tip Mach number about 0.07. The data base is completed by far-field measurements made with a single microphone on a moving support. The analytical model is based on a previously published extension of Amiet's trailing-edge noise theory. A blade is split into several strips in the spanwise direction, and the model is applied to each strip. For this the input data are interpolated from the measurements performed with the aforementioned sets of microphones. The trailing-edge noise model is more reliable for observer positions within ±30° from the fan-rotation plane.

/pdf/rotating-blade-trailing-edge-noise-experimental-validation-2urrc0ocou.pdf

Rotating Blade Trailing-Edge Noise: Experimental Validation of Analytical Model

A set of simulations has been conducted on the 30P30N high-lift three-element airfoil using a hybrid Reynolds-averaged Navier?Stokes-large-eddy simulation (RANS?LES) method on multiblock structured and Cartesian-prismatic unstructured grids. These simulations using the SST-IDDES (improved delayed detached-eddy simulation) model have been shown to agree well with previous experimental and numerical investigations in terms of mean quantities, as well as spectra and flow structures. It has also been shown that results from a Cartesian-prismatic unstructured mesh generated in only a few days were able to predict both mean and instantaneous flow features to an acceptable level. However, there were clearly areas where improvements to the mesh, numerical scheme, and turbulence model could improve the correlation to both the experimental data and the multiblock structured mesh. It was demonstrated from the grids used in this study that IDDES exhibited a sensitivity to the near-wall meshing.

/pdf/flow-dynamics-past-a-30p30n-three-element-airfoil-using-24fhmb641h.pdf

Flow dynamics past a 30P30N three-element airfoil using improved delayed detached-eddy simulation

This paper focuses on the application of CFD to the prediction of radial compressor aeroacoustics. It concentrates mainly on automotive turbocharger operations in the low mass-flow range where blade leading-edge and tip separation reduce the compressor performance and induce transient flow behaviour. Whereas the blade-passing is tonal and at high frequency, usually beyond the human hearing range, transience in the flow are turbulence-dominated, broad-band in nature, and in magnitude a significant source of aeroacoustics which appears well within the range of peak human hearing (1–5kHz). Other noise sources occur due to distortions in the flow upstream of the compressor face, and rotating stall.The simulation methodology enumerated here pays attention to all the above flow-induced aeroacoustics. Due consideration is given to turbulence modelling, to ensure that both the narrow-band and broad-band sources are directly resolved in the CFD. Appropriate discretisation practices are adopted, so as to capture both turbulent-convection and sound-propagation mechanisms. Pressure-wave non-reflective boundary conditions are applied to the computational boundaries to remove any artificial resonances in the domain.STAR-CCM+, the commercial CFD code used here, was previously benchmarked against experimental data for the same compressor under ideal installation conditions, then the compressor performance assessed under real installation conditions [1]. The main foci of the studies reported here are to exploit possible improvements in modelling of the device performance and efficiency curves using more detailed wall modelling, comparing low-y+ versus high-y+ wall resolution, and to explore the viability for transient CFD calculations to capture the noise sources in the compressor at the challenging low mass flow end of the performance characteristic.© 2012 ASME

Simulation of Radial Compressor Aeroacoustics Using CFD

Two rectangular cavity configurations at Mach 0.85 are investigated with the objective of assessing the extent to which 3D CFD with advanced turbulence modeling is capable of predicting narrowband and broadband flow noise. A non-linear, two-equation, eddy-viscosity model run in unsteady mode (URANS) is compared with Detached Eddy Simulation (DES) on a cavity with a L/D ratio of 5, representing cavity flow in so-called shear layer mode. Detailed experimental data for this cavity, configured with and without doors, provides a valuable opportunity to compare predictions of the spectra at many points along the cavity ceiling and band limited amplitude along the cavity length.

CFD Prediction of Narrowband and Broadband Cavity Acoustics at M=0.85

Validation of a New Hybrid CAA Strategy and Application to the Noise Generated by a Flap in a Simplified HVAC Duct

Transient CFD acoustic source prediction and CA propagation studies of a double diaphragm orifice illustrate complex flow and aeroacoustics phenomena, combining shedding structures (tonal noise) and large eddy structures (broadband noise). Such studies are necessary for aircraft climate control systems because such components introduce nonlinearities in the system characterization. In this paper we investigate, through modeling, the noise signature of a given double diaphragm configuration comparing against microphone measurements the predicted noise spectra in the near field (source region) and the predicted propagated sound in the far field, some distance downstream of the source region. A second configuration is then assessed, with the spacing between the orifices doubled, so as to derive confidence that the modeling is accurate in both absolute and differential terms. A novel method is presented for estimating the mesh frequency cut-off from a steady-state CFD calculation; such an estimate gives valuable insight a priori into the local mesh refinement required for a transient CFD calculation to resolve the frequency range of interest. The work described in this paper is part of a wider evaluation into the use of CAA methods for aircraft climate control systems.

Fred Mendonça

Papers

Flow dynamics past a 30P30N three-element airfoil using improved delayed detached-eddy simulation

Simulation of Radial Compressor Aeroacoustics Using CFD

CFD Prediction of Narrowband and Broadband Cavity Acoustics at M=0.85

Validation of a New Hybrid CAA Strategy and Application to the Noise Generated by a Flap in a Simplified HVAC Duct

Aeroacoustic Simulation of Double Diaphragm Orifices in an Aircraft Climate Control System