Turbulent jet characteristics for axisymmetric and serrated nozzles

Turbulent jet large eddy simulations (LES) are performed at Mach 0.9 and Reynolds number around 106 . Implicit large-eddy simulation (ILES) is employed, namely omitting explicit subgrid scale models. The Reynolds-averaged Navier–Stokes (RANS) solution is blended into the near wall region. This makes an overall hybrid LES-RANS approach. A Hamilton–Jacobi equation is applied to remove the disparate turbulence length scales implied by hybridization. Computations are contrasted for a baseline axisymmetric (round) nozzle and a serrated (or chevron) nozzle with high bending and penetration. Jet characteristics for both nozzles are studied in detail with well documented experimental data compared. The chevron effects are demonstrated by comparing both solutions using the same mesh resolution and flow conditions. Higher order velocity moments with potential for aeroacoustic modeling and noise prediction, such as the two-point velocity spatial correlations, are also explored. Numerical simulations presented in this study utilize an in-house flow solver with improved parallel scalability and efficiency by means of data packeting and a scheduling algorithm similar to the Round Robin scheduling

Hybrid LES-RANS study on square cylinder unsteady heat transfer

Flow passing a heated square cylinder is investigated using a hybrid LES-RANS approach on unstructured grids at a moderate Reynolds number of 22, 050. The implicit SGS is applied for LES and two turbulence models are tested for near-wall RANS: the Spalart-Allmaras model and the SST k-! model. Both models combined with the LES present good predictions of the time- and phase-averaged velocity profiles on a 4-million-cell grid. Results of the LES-SST approach agree better with the experimental data especially at locations close to the cylinder surface and this leads to improved surface convective heat transfer compared to LES-SA. Grid convergence study shows that grid resolution in the near-wall region and on the cylinder surfaces is important in resolving the unsteady convective heat transfer. Results of velocity field and surface heat transfer from the fine grid with 8 million cells compare favourably with the experimental data and show significant improvement over that of the medium and coarse grids. Analysis of turbulent statistics is performed by means of energy spectra and anisotropy invariants of the Reynolds stress tensor. Proper orthogonal decomposition (POD) is used to identify the vortex shedding phases. It is shown that the POD based phase-averaging produces more accurate velocity profiles than the conventional pressure-signal based method

A hybrid LES-RANS validation of effusion cooling array measurements

In this work, an effusion cooling array is studied using a hybrid LES-RANS approach under combustor representative conditions. The surface adiabatic cooling effectiveness is examined. Promising results are obtained from the hybrid LES approach based on an 18-million-cell grid. The turbulent flow field is studied in order to investigate the effects on the coolantmainstream mixing, as well as the distribution of the coolant film. It is found that the freestream turbulence leads to early breakdown of the coolant jets at the 1st and 2nd row of coolant holes, while such effects diminish in the downstream region due to the turbulent structures in the mixed mainstream. As a result, the surface adiabatic cooling effectiveness stays low in the first few rows, but keeps growing and reaches a high value in the downstream rows

Hybrid LES-RANS study of an effusion cooling array with circular holes

In this paper, a multi-row effusion cooling configuration with scaled gas turbine combustor conditions is studied numerically. The distribution of the coolant film is examined by surface adiabatic cooling effectiveness (ACE). Simulation results have shown that the accuracy of cooling effectiveness prediction is closely related to the resolution of turbulent flow structures involved in hot-cold flow mixing, especially those close to the plate surface. The formation of the coolant film in the streamwise direction is investigated. It is shown that the plate surface directly downstream the coolant holes are covered well by the coolant jets, while surface regions in between the two columns of the coolant holes could not be protected until the coolant film is developed sufficiently in the spanwise direction in the downstream region. More detailed study has also been carried out to study the time-averaged and time-dependent flow field. The relation between the turbulent flow structures and coolant film distribution are also examined. The Kelvin-Helmholtz instability in the upper and lower coolant jet shear layer, is found to have the same frequency of around 8000Hz, and is independent of the coolant hole position. Additionally, it is suggested by the spectral coherence analysis that those unsteady flow structures from the lower shear layer are closely related to the near-wall flow temperature, and such effect is also independent of the coolant hole position

Near-wall modelling and free-stream turbulence effects on square cylinder unsteady heat transfer

Unsteady flow passing a heated square cylinder has been investigated using a hybrid LES-RANS approach at a moderate Reynolds number of 22, 050. Two near-wall RANS models are blended smoothly to the LES region. The two models applied have successfully reproduced time- and phase-averaged flow field. Encouraging convective heat transfer has been predicted. An increase in the convective heat flux is found on the cylinder top/bottom surface with imposed free-stream turbulence. More accurate prediction of velocity and Nusselt number profiles has been made by the use of LES-k! model

Parabolized stability analysis of jets issuing from serrated nozzle

Parabolized stability analysis of jets issuing from serrated nozzl

A coupled LES-APE approach for jet noise prediction

Despite the continuing advances of computing power, the state-of-the-art direct numerical simulations of jet noise are still restricted to simple problems. Additionally, many established CFD codes are potentially too dissipative for propagating acoustic waves. Hence, alternative methods are needed for complex realistic configurations. Traditionally, the combination of Large-Eddy Simulations (LES) with surface integral methods has been widely used by the research community to predict jet noise in the far-field. However, its application in complex installed configurations poses quite a challenging task in defining a suitable surrounding integral surface. Furthermore, these methods only provide information of single observer locations. In the present work, a coupled LES-APE (Acoustic Perturbation Equations) strategy is presented as an alternative to traditional methods. Two different solvers are used. The LES is performed with a density based, second order finite volume solver with the σ-subgrid-scale model. The APE code is a high-order Discontinuous Galerkin spectral/hp finite element solver. It solves the APE which does not suffer from instability issues related to the more common Linearised Euler Equations (LEE). Moreover, the APE is advantageous by means of a filtered source, propagating only true sound. It is also planned to apply this methodology to more realistic configurations such as coaxial nozzles and jet-wing-flap interaction cases

Unsteady heat and mass transfer of a blunt leading edge using hybrid LES-RANS

A novel wall-proximity based hybrid LES-RANS approach has been applied for the study of a blunt leading edge flat plate with constant wall heat flux. The Reynolds number of the flow is about 10,200 based on half of the plate thickness. Results of the simulation are examined by comparing the velocity and heat transfer profiles with relevant measurements. Analysis has been carried out to study the relation between the unsteady flow features and the surface heat transfer

Effects of free-stream and near-wall turbulence on jet shear layer

Effects of free-stream and near-wall turbulence on jet shear laye

Computational investigation into the sensitivity of a simplified vehicle wake to small base geometry changes

For vehicles with a squareback geometry, for example Sports Utility Vehicles (SUVs), base pressure drag is a large contributor to overall drag. Simple passive techniques, such as tapering, can reduce drag significantly but at a large aesthetic and functional cost. Therefore, very small base geometry changes have been investigated. An experimentally validated methodology has used Detached Eddy Simulations (DES) to obtain time-averaged and instantaneous data; allowing the effect of horizontal base slats on global forces and wake structures to be presented. The small geometry modifications have caused substantial changes to the base pressure distribution with the main mechanisms of change being identified and observed close to the model surfaces. A region of separation is seen below each slat corresponding to reduced pressure whilst high pressure regions attributed to stagnation are increased. The combined effect is a statistically significant drag reduction of 4 counts (1 count = 0.001 CD) when a slat is added at 3/4 of the base height. The results show the scope for very small changes to a simplified road vehicle, in areas that have not previously been explored, to reduce overall drag with minimal aesthetic penalties. This understanding provides the impetus for new approaches in real vehicle development