M. L. Shur

Detached-eddy simulation of an airfoil at high angle of attack

We present the first true applications of Detached-Eddy Simulation (DES), in the sense of being three-dimensional. DES was defined in 1997 with hopes of combining the strengths of Reynolds-averaged methods and of Large-Eddy Simulations, in a non-zonal manner, to treat separated flows at high Reynolds numbers. We first simulate isotropic turbulence, to check the concept in LES mode and set its adjustable constant. Smooth inertial ranges are obtained up to the cutoff in the spectra. We then treat an airfoil in the challenging regime of massive separation and do so very successfully, in that lift and drag are within 10% of the experimental results at all angles of attack, to 90°. Such an accuracy is not achieved with traditional modelling, even unsteady, which gives up to 40% error. Cost puts a pure LES of the same flow (at Reynolds number 105 and beyond) out of reach on any computer, yet we use personal computers for the DES, and about 200,000 grid points. On the other hand, grid refinement, domain-size and Reynolds-number studies have not been completed yet. Hysteresis in the 15 - 25° range has not been addressed.

Physical and Numerical Upgrades in the Detached-Eddy Simulation of Complex Turbulent Flows

A new formulation of Detached-Eddy Simulation (DES) based on the k-ω RANS model of Menter (M-SST model) is presented, the goal being an improvement in separation prediction over the S-A model. A new numerical scheme adjusted to the hybrid nature of the DES approach and the demands of complex flows is also presented. The scheme functions as a fourth-order centered differentiation in the LES regions of DES and as an upwind-biased (fifth or third order) differentiation in the RANS and outer irrotational flow regions. The capabilities of both suggested upgrades in DES are evaluated on a set of complex separated flows.

Towards the prediction of noise from jet engines

Abstract We are in the initial stages of development of a “non-empirical” numerical tool for jet-noise prediction in the airline industry, ultimately to treat complex nacelles and nozzles. The non-empirical demand leads to compressible large-eddy simulations, followed by post-processing to produce the far-field sound. Here we treat a simple cold jet with an axisymmetric geometry. The simulations leave out the subgrid-scale model (which causes too much dissipation on the present grid as is often the case in transitional flows), use slightly upwind-biased high-order differencing, and are preliminary in that a grid-refinement has not yet been performed. We do not use any unsteady forcing. The initial instability remains grid-sensitive, but the region with developed turbulence gives accurate statistics. The sound seen in the simulations is also realistic. The far-field sound calculations use the Ffowcs Williams–Hawkings (FWH) equation with a control surface that encloses the turbulence as much as possible, and the outside quadrupoles omitted. We focus on the influence of the surface location and the problem of closing the FWH surface at the outflow of the simulation. Though many physical and numerical issues are only partly resolved, the agreement with experiment is quite good for the sound’s level, directivity, and spectral content.

Flow and Noise Predictions for Single and Dual-Stream Beveled Nozzles

Numerical simulations of the flowfield and noise of single and staggered dual, round, and beveled nozzles are carried out, with the goal of gaining insights into the flow features that are responsible for noise generation and mitigation, and ultimately arriving at better designs. For aircraft applications, the geometry of the nozzles must be satisfactory both for aerodynamic and acoustic performance. The importance of taking the nozzle internal flow into account in the simulations instead of using simple inflow profiles, especially for complex geometries of current interest, is emphasized. A two-step Reynolds-averaged Navier-Stokes/large-eddy simulation methodology is developed and applied to several nozzle geometries allowing a new level of complexity in the geometry. The predictions of the gross thrust measures by Reynolds-averaged Navier-Stokes computations are in very good agreement with experimental measurements. The spectral predictions from large-eddy simulation are also in good agreement with measured data, for a wide range of jet conditions. The measured azimuthal variations in the noise field from beveled nozzles are reproduced by large-eddy simulation. The spectra from dual-stream nozzles are also well predicted. Preliminary efforts at establishing the link between flow and noise are presented. The interpretation of the flow/noise connection is not straightforward and is a big challenge; it is far from clear how one can establish cause and effect. The ability to relate changes in flow to noise may remain an open issue for the foreseeable future.

Three-Dimensionality in Reynolds-Averaged Navier-Stokes Solutions Around Two-Dimensional Geometries.

The flow over two-dimensional geometries is studied via unsteady numerical simulations that are three dimensional, with periodic conditions applied along the spanwise coordinate. This framework is well accepted for direct numerical simulations (DNS), large-eddy simulations, and detached-eddy simulations (DES), but is here combined with standard Reynolds-averaged Navier-Stokes (RANS) turbulence models. This strategy, which is not new, is referred to as unsteady RANS (URANS). Limited previous evidence suggested that, in URANS, three dimensionality is suppressed by high eddy-viscosity levels. However, three dimensionality proves fairly easy to sustain with adequate initial conditions, in all three cases studied here: stalled airfoil, circular cylinder, and a rounded square, except that for one case three dimensionality failed to last from random-based initial perturbations and was sustained only when using a DES field as initial condition.

Modeling the Interaction of a Vortex Pair with the Ground

The results of a numerical investigation of the interaction between an aircraft vortex wake (vortex pair) and the ground during the takeoff and landing phases are presented. The calculations were performed within the framework of the time-dependent two-dimensional Reynolds-averaged Navier-Stokes equations using the Spalart-Shur turbulence model generalizing the turbulent viscosity transport model of Spalart and Allmaras to the case of the flows with curved streamlines and rotation. Similar calculations were carried out on the basis of the original Spalart-Allmaras model and the k-ω model of Menter. Some new qualitative and quantitative data on the distinctive features of the phenomenon under consideration are obtained.

Numerical Study of Wind-Tunnel Walls Effects on Transonic Airfoil Flow

Thee owpasttheRoyalAircraftEstablishment (RAE)2822airfoilisstudiedbythree-dimensionalcomputational e uid dynamics (CFD), which includestheviscous side walls of thewind-tunnel test, with an aspect ratio of3. Three turbulence models, two aspect ratios, and several grids are used, as well as two treatments of the e oor and ceiling (neither one actually representing slotted surfaces ) and small Mach number and incidence adjustments. The results deviate from two-dimensional results sufe ciently to revisit the quantitative conclusions and the ranking of turbulence models that were made from two-dimensional CFD in the 1980s and 1990s. However, contrary to our hopes, the three-dimensional effects fail to improve the pressure recovery after shock-induced separation in the more dife cult case 10, so that all of the turbulence models we tried still fail to match measurements by modern standards,evenwithMachnumberand angle-of-attackadjustments.Theunseparated case6producesvery similar trends. With present levels of computerpower, tests with fully documented three-dimensional solid-wall boundary conditions appear most desirable, but axisymmetric test cases can already be quite useful.

Tailored Nozzles for Jet Plume Control and Noise Reduction

A synergistic utilization of computational simulations with experimental measurements is employed to develop dual-stream nozzle geometries that provide jet noise reduction with the concurrent ability to control the orientation of the jet plumes, so as to minimize the thrust degradation associated with low-noise designs. The geometries consist of round primary and secondary nozzles, beveled primary nozzles, modified secondary nozzles, and combinations thereof. Specifically, the secondary nozzle is altered internally to provide the same deflection as a beveled primary in dual-stream exhaust geometry. The cross-sectional profiles are similar, but the bevel deflects the jet towards the short lip, whereas the modified secondary deflects the jet in the opposite direction. It is possible to eliminate/minimize the deflection of the total thrust vector through a judicious combination of the bevel and the modified secondary; numerical simulations facilitate this objective. The aeroacoustic characteristics of four beveled nozzles with bevel angles of 18o, 24o, 30o and 36o, and two modified secondary nozzles with weak and strong flow effects, have been established in a wind-tunnel test, with simultaneous measurement of thrust and noise. The magnitude of the noise reduction increases with increasing primary jet velocity and decreases with increasing flight Mach number. There is a gradual erosion of noise benefit as the azimuthal angle is increased from 0o (below the long lip of bevel). There is a benefit in EPNL for all the nozzle geometries evaluated in this investigation. The combinations of modified secondary nozzles with bevel24 and bevel30 provide the largest reduction in EPNL over a wide range of freestream Mach number, with a small thrust penalty. The noise benefit varies from ~2.5 EPNdB at Mt=0.0, to ~2.0 EPNdB at Mt=0.20, and ~1.2 EPNdB at Mt=0.28. The design approach developed and evaluated here seems promising vis-a-vis practical applications, requiring only relatively limited modifications to an existing design.

DDES and IDDES of tandem cylinders.

The paper presents an overview of the authors contribution to the BANC-I Workshop on the flow past tandem cylinders (Category 2). It includes an outline of the simulation approaches, numerics, and grid used, the major results of the simulations, their comparison with available experimental data, and some preliminary conclusions. The effect of varying the spanwise period in the simulations is strong for some quantities, and not others.

Improved Embedded Approaches

In contrast to the non-zonal, DES-like, hybrid approaches, in which a transition from RANS to LES relies upon a natural instability of separated shear layers in massively separated flows, the zonal RANS-LES (actually, RANS – Wall Modelled LES or RANS-WMLES) hybrids imply the presence of a sharp interface between the flow regions treated by RANS and LES. The location of this interface may be arbitrarily specified by the user based on their understanding of the flow physics, available computational resources or the objectives of the simulation, e.g., a need for unsteady flow characteristics.