I Naqavi

Immersed-boundary methods for general finite-difference and finite-volume Navier-Stokes solvers

We present an immersed-boundary algorithm for incompressible flows with complex boundaries, suitable for Cartesian or curvilinear grid system. The key stages of any immersed-boundary technique are the interpolation of a velocity field given on a mesh onto a general boundary (a line in 2D, a surface in 3D), and the spreading of a force field from the immersed boundary to the neighboring mesh points, to enforce the desired boundary conditions on the immersed-boundary points. We propose a technique that uses the Reproducing Kernel Particle Method [W.K. Liu, S. Jun, Y.F. Zhang, Reproducing kernel particle methods, Int. J. Numer. Methods Fluids 20(8) (1995) 1081-1106] for the interpolation and spreading. Unlike other methods presented in the literature, the one proposed here has the property that the integrals of the force field and of its moment on the grid are conserved, independent of the grid topology (uniform or non-uniform, Cartesian or curvilinear). The technique is easy to implement, and is able to maintain the order of the original underlying spatial discretization. Applications to two- and three-dimensional flows in Cartesian and non-Cartesian grid system, with uniform and non-uniform meshes are presented.

Modelling and Experimental Study Into the Laser Assisted Nitriding of Ti-6Al-4V Alloy

Laser gas assisted processing offers advantages over commercial nitriding processes due to precise operation and local treatment. In the present study, laser gas assisted nitriding of Ti-6Al-4V alloy is considered. Temperature field is simulated using a nonconduction laser heating model, in which the phase change is accommodated. Temperature dependent nitrogen diffusion coefficient is used when computing the nitrogen concentration distribution in the substrate material. Thermal stresses due to temperature gradient are predicted in the solid region of the substrate material. Experimentation is carried out in an effort to nitride the Ti-6Al-4V alloy surface employing a pulsed Nd:YAG laser Laser treated surfaces are examined using SEM, XRD, and XPS. It is found that the temperature gradient reduces sharply while the nitrogen concentration gradient reduces gradually inside the substrate material. The depth of the nitride layer almost matches the depth of the melt layer. It is observed from the experimental study that the nitride compound in the surface region of the substrate material is TiN and neither microcracks nor microvoids are observed in the nitrided region.

Large Eddy Simulation for Turbines: Methodologies, Cost and Future Outlooks

Flows throughout different zones of turbines have been investigated using large eddy simulation (LES) and hybrid Reynolds-averaged Navier–Stokes-LES (RANS-LES) methods and contrasted with RANS modeling, which is more typically used in the design environment. The studied cases include low and high-pressure turbine cascades, real surface roughness effects, internal cooling ducts, trailing edge cut-backs, and labyrinth and rim seals. Evidence is presented that shows that LES and hybrid RANS-LES produces higher quality data than RANS/URANS for a wide range of flows. The higher level of physics that is resolved allows for greater flow physics insight, which is valuable for improving designs and refining lower order models. Turbine zones are categorized by flow type to assist in choosing the appropriate eddy resolving method and to estimate the computational cost.

Laser pulse heating and thermal stress developments: elastoplastic analysis

Abstract In the present study, laser non-conduction limited heating of steel is considered. Temperature and thermal stress fields are computed for the two-dimensional axisymmetric heating situation. The phase change processes (melting and evaporation) are accommodated in the energy equation with appropriate boundary conditions. In order to account for the plastic behaviour of the substrate material, the elastoplastic analysis is employed in the stress field calculations. An experiment was conducted to validate the cavity shape predicted. It is found that temperature rises at a fast rate in the surface region and energy conduction in the axial direction dominates over its counterpart, which takes place in the radial direction. Stress levels exceeding the yield strength of the substrate material resulted in the region close to the cavity surface. The cavity size predicted agrees well with the experimental findings.

Laser Nanosecond Pulse Heating of Surfaces and Thermal Stresses

Laser pulse heating of metallic surfaces finds wide application in industry because of the precision of operation and localized heating of the substrate material. The thermal stresses are developed because of the high temperature gradient generated in the region irradiated by a laser beam. The level of stresses developed becomes important during the laser surface treatment and annealing process. In this study, the laser nanosecond pulse heating of a metallic substrate is considered. Energy transport and thermal stress equations are solved numerically for step input intensity pulses. Because the heating process is axisymmetric, the cylindrical coordinate system is employed. The temperature and stress fields inside the substrate material are computed. It is found that in the early heating period, the temperature rises rapidly in the surface vicinity of the substrate material. As the heating progresses, diffusional energy transport becomes important, in which case the rise of temperature in the surface vicinity attains almost a steady value. The axial stress component is tensile, the radial stress component is compressive, while the tangential stress component is compressive in the region close to the symmetry axis and it becomes tensile as the distance from the symmetry axis increases.

LES for Turbines: Methodologies, Cost and Future Outlooks

Flows throughout different zones of turbines have been investigated using Large Eddy Simulation (LES) and hybrid Reynolds-Averaged Navier-Stokes-LES (RANS-LES) methods and contrasted with RANS modelling, more typically used in the design environment. Cases studied include low and high-pressure turbine cascades, real surface roughness effects, internal cooling ducts, trailing edge cut-backs and labyrinth and rim seals. Evidence is presented that shows that LES and hybrid RANS-LES produces higher quality data than RANS/URANS for a wide range of flows. The higher level of physics that is resolved allows greater flow physics insight which is valuable for improving designs and refining lower order models. Turbine zones are categorised by flow type, to assist in choosing the appropriate eddy resolving method and to estimate computational cost.© 2013 ASME

Predictive LES for jet aeroacoustics - current approach and industrial application

The major techniques for measuring jet noise have significant drawbacks, especially when including engine installation effects such as jet–flap interaction noise. Numerical methods including low order correlations and Reynolds-averaged Navier–Stokes (RANS) are known to be deficient for complex configurations and even simple jet flows. Using high fidelity numerical methods such as large eddy simulation (LES) allows conditions to be carefully controlled and quantified. LES methods are more practical and affordable than experimental campaigns. The potential to use LES methods to predict noise, identify noise risks, and thus modify designs before an engine or aircraft is built is a possibility in the near future. This is particularly true for applications at lower Reynolds numbers such as jet noise of business jets and jet-flap interaction noise for under-wing engine installations. Hence, we introduce our current approaches to predicting jet noise reliably and contrast the cost of RANS–numerical-LES (RANS–NLES) with traditional methods. Our own predictions and existing literature are used to provide a current guide, encompassing numerical aspects, meshing, and acoustics processing. Other approaches are also briefly considered. We also tackle the crucial issues of how codes can be validated and verified for acoustics and how LES-based methods can be introduced into industry. We consider that hybrid RANS–(N)LES is now of use to industry and contrast costs, indicating the clear advantages of eddy resolving methods.

Far-field noise prediction for jets using large-eddy simulation and Ffowcs Williams–Hawkings method

Large-eddy simulations are performed for hot and cold jets with and without a flight stream. The acoustic and flight stream Mach numbers are 0.875 and 0.3, respectively. The temperature ratios for the hot and cold jets are 2.7 and 1.0, respectively. The mean flow field results are in good agreement with the measurements. The Ffowcs Williams–Hawkings equation is used to predict far-field noise. Several axisymmetric Ffowcs Williams–Hawkings surfaces at increasing radial distances are used. They show that the surfaces closer to the jet can be affected by the hydrodynamic pressure. It is important to close the Ffowcs Williams–Hawkings surfaces at the ends to account for all the acoustic signals emanating from the jet. In this work, 11 end discs are used at the downstream end of the Ffowcs Williams–Hawkings surface. It is found that the simple averaging processes to cancel hydrodynamic sound at the end discs are insufficient for slowly decaying jets. In such cases, a partially closed disc can be a better choice. To remove hydrodynamic signals, a filtering scheme for the end discs is suggested. For slowly decaying jets, this gives better results.

Far Field Noise Prediction for Subsonic Hot and Cold Jets Using Large-Eddy Simulation

Large eddy simulations are performed for hot and cold single stream jets with an acoustic Mach number of (Ma = Vj/a∞ = 0.875). The temperature ratio (Tj/T∞) for the hot jet is 2.7 and for the cold jet it is 1.0. Grids with 34 million points are used. The simulation results for the flow field are in encouraging agreement with the mean velocity and Reynolds stress measurements. The Ffowcs Williams-Hawkings (FW-H) equation is used to predict the far-field noise. In this study four cylindrical FW-H surfaces around the jet at various radial distances from the centreline are used. The FW-H surfaces are closed at the downstream end with multiple endplates. These endplates are at x = 25.0D – 30.0D with Δ = 0.5D apart. It is shown that surfaces close to jet get affected with pseudo sound. To avoid pseudo sound, surfaces must be placed in the irrotational region. To account for all the acoustic signals end plates are necessary. However, a simple averaging process to cancel pseudo sound at the end plates is not sufficient.Copyright

Cost-effective hybrid RANS-LES type method for jet turbulence and noise prediction

Jets at higher Reynolds numbers have a high concentration of energy in small scales in the nozzle vicinity. This is challenging for large-eddy simulation, potentially placing severe demands on grid density. To circumvent this, we propose a novel procedure based on well-known Reynolds number (Re) independent of jets. We reduce the jet Re while rescaling the boundary layer properties to maintain incoming boundary layer thickness consistent with high Re jet. The simulations are carried out using hybrid large-eddy simulation type of approach which is incorporated by using near-wall turbulence model with modified properties. No subgrid scale model is used in these simulations. Hence, they effectively become numerical large-eddy simulation with Reynolds-averaged Navier–Stokes covering the full boundary layer region. The noise post-processing is carried out using the Ffowcs-Williams-Hawking approach. The simulations are made for Mach numbers (M) of 0.75 and 0.875 (cold and hot). The results for the overall sound pressure level are observed to be within 2–3% of the measurements, and directivity of sound is also captured accurately for both the cases. Hence, the low Re simulations can be more beneficial in saving time and cost while providing reasonably accurate results.