Ghader Ghorbaniasl

Validation of a time domain formulation for propeller noise prediction

The present paper is oriented at the definition of a series of validation test cases for the far-field subsonic formulation of Farassat in the time domain, solving the Lighthill analogy for aeroacoustics in the Ffowcs William-Hawkings formulation for far-field noise predictions. The focus will be on the definition, validation and verification of four test cases for far-field acoustic propagation. The test cases are: (i) a rotating rotor with a pre-defined body force, (ii) a translating and rotating small disk case with a moving observer, (iii) a conventional helicopter rotor case, (iv) and the consistency test known as the Isom thickness noise. The acoustic pressure signature, sound pressure level and directivity have been calculated for each test case. Graphical outputs and comparisons of results with those of analytical solutions are presented.

Acoustic Velocity Formulation for Sources in Arbitrary Motion

This paper deals with the acoustic velocity field simulation generated by interaction of flow with moving bodies. Starting from the Ffowcs Williams and Hawkings equation, an analytical formulation of the acoustic velocity is derived for sources in arbitrary motion. This makes the imposition of the boundary condition on a (rigid) scattering surface much more straightforward, as, if the traditional pressure formulation is used, then the pressure gradient must be calculated. Computational results for a pulsating sphere, dipole source, and a propeller case with subsonic tips verify this formulation.

A robust and efficient stepwise regression method for building sparse polynomial chaos expansions

Abstract Polynomial Chaos (PC) expansions are widely used in various engineering fields for quantifying uncertainties arising from uncertain parameters. The computational cost of classical PC solution schemes is unaffordable as the number of deterministic simulations to be calculated grows dramatically with the number of stochastic dimension. This considerably restricts the practical use of PC at the industrial level. A common approach to address such problems is to make use of sparse PC expansions. This paper presents a non-intrusive regression-based method for building sparse PC expansions. The most important PC contributions are detected sequentially through an automatic search procedure. The variable selection criterion is based on efficient tools relevant to probabilistic method. Two benchmark analytical functions are used to validate the proposed algorithm. The computational efficiency of the method is then illustrated by a more realistic CFD application, consisting of the non-deterministic flow around a transonic airfoil subject to geometrical uncertainties. To assess the performance of the developed methodology, a detailed comparison is made with the well established LAR-based selection technique. The results show that the developed sparse regression technique is able to identify the most significant PC contributions describing the problem. Moreover, the most important stochastic features are captured at a reduced computational cost compared to the LAR method. The results also demonstrate the superior robustness of the method by repeating the analyses using random experimental designs.

A self-adjusting flow dependent formulation for the classical Smagorinsky model coefficient

In this paper, we propose an efficient formula for estimating the model coefficient of a Smagorinsky model based subgrid scale eddy viscosity. The method allows vanishing eddy viscosity through a vanishing model coefficient in regions where the eddy viscosity should be zero. The advantage of this method is that the coefficient of the subgrid scale model is a function of the flow solution, including the translational and the rotational velocity field contributions. Furthermore, the value of model coefficient is optimized without using the dynamic procedure thereby saving significantly on computational cost. In addition, the method guarantees the model coefficient to be always positive with low fluctuation in space and time. For validation purposes, three test cases are chosen: (i) a fully developed channel flow at Re τ=180,395, (ii) a fully developed flow through a rectangular duct of square cross section at Re τ=300, and (iii) a smooth subcritical flow past a stationary circular cylinder, at a Reynolds nu...

Prediction of near- and far-field noise generated by contra-rotating open rotors

An innovative aeroacoustic prediction method for Contra-Rotating Open Rotors (CROR) based on the nonlinear harmonic method (NLH) for the CFD computations and on the Ffowcs Williams and Hawkings (FW-H) equations for the far-field acoustic propagation is described. This method is tested on a generic 8 × 8 puller CROR at typical take-off conditions. The outstanding efficiency of the prediction method in terms of CPU cost is demonstrated in this study. An analysis of the aerodynamic interactions between the two rotors as well as of the aeroacoustic field radiated in the far-field is presented, allowing for an improved understanding of noise generation mechanisms.

Analytical acoustic pressure gradient prediction for moving medium problems

In this paper, an acoustic pressure gradient formula capable of accounting for constant uniform flow effects is suggested. Acoustic pressure gradient calculation is key for acoustic scattering problems, because it may be used to evaluate the hardwall boundary condition. Realistic cases of rotating machines may be evaluated in a moving frame of reference and as such, an acoustic pressure gradient formula capable of accounting for constant uniform flow effects finds significant application. A frequency domain formulation was thus derived for periodic noise source motion located in a moving medium. The suggested formula is mathematically compact and easy to implement. It may offer us significant advantages when tonal noise emissions are dominant, thus finding application potential in acoustic scattering problems in rotating machines in a constant uniform flow. Moreover, the formula contains no Doppler factor, thus facilitating noise prediction for sources in supersonic motion.

VALIDATION AND APPLICATION OF AN HIGH-ORDER SPECTRAL DIFFERENCE METHOD FOR FLOW INDUCED NOISE SIMULATION

The main goal of this paper is to develop an efficient numerical algorithm to compute the radiated far field noise provided by an unsteady flow field from bodies in arbitrary motion. The method computes a turbulent flow field in the near fields using a high-order spectral difference method coupled with large-eddy simulation approach. The unsteady equations are solved by advancing in time using a second-order backward difference formulae scheme. The nonlinear algebraic system arising from the time discretization is solved with the nonlinear lower–upper symmetric Gauss–Seidel algorithm. In the second step, the method calculates the far field sound pressure based on the acoustic source information provided by the first step simulation. The method is based on the Ffowcs Williams–Hawkings approach, which provides noise contributions for monopole, dipole and quadrupole acoustic sources. This paper will focus on the validation and assessment of this hybrid approach using different test cases. The test cases used are: a laminar flow over a two-dimensional (2D) open cavity at Re = 1.5 × 103 and M = 0.15 and a laminar flow past a 2D square cylinder at Re = 200 and M = 0.5. In order to show the application of the numerical method in industrial cases and to assess its capability for sound field simulation, a three-dimensional turbulent flow in a muffler at Re = 4.665 × 104 and M = 0.05 has been chosen as a third test case. The flow results show good agreement with numerical and experimental reference solutions. Comparison of the computed noise results with those of reference solutions also shows that the numerical approach predicts noise accurately.

Study of the sediment transport over flat and wavy bottom using large-eddy simulation

In the present paper the sensitivity of the flow and of sediment transport to bottom roughness is studied. First, a thorough numerical investigation of smooth-bottom channel flow at Re τ = 395 is performed using large-eddy simulations (LESs). A dynamic version of the wall-adapted local eddy-viscosity (WALE) model is used for this study, whereas the subgrid-scale (sgs) diffusion stress is based on a gradient hypothesis. The computed results compare well with direct numerical simulation (DNS) data and experiments. Next, rough-bottom cases, with the bottom having a sinusoidal, wavy shape are considered. It was found that the wavy bottom has a strong influence on the flow field and that the sediment transport is highly sensitive to the bottom waviness, in particular for larger wave heights. It is shown that for light and low-concentrated sediment the Rouse theory is also valid in the case of a wavy bottom, mainly in the outer zone. Finally, it is found that the turbulent-Schmidt-number profiles are not very sensitive to the wave height.

Validation and application of a far-field time domain formulation for fan noise prediction

The present paper is oriented at the definition of a series of validation test cases for the far-field subsonic formulation of Farassat in the time domain, solving the Lighthill analogy for aeroacoustics in the Ffowcs William-Hawkings formulation for far-field noise predictions. The focus will be on the definition, validation and verification of five test cases for far-field acoustic propagation. The test cases are: (i) a rotating rotor with a pre-defined body force, (ii) a translating and rotating small disk case with a moving observer, (iii) a conventional helicopter rotor case, (iv) the consistency test known as the Isom thickness noise, (v) and a small sized industrial fan. The acoustic pressure signature, sound pressure level and directivity have been calculated for each test case. Graphical outputs and comparisons of results with analytical solution are presented for the four analytical cases and a comparison with results from another code is used for the industrial fan case.

Acoustic Velocity Formulation for Kirchhoff Data Surfaces

This paper deals with the derivation of an analytical time-domain formulation for the prediction of the acoustic velocity field generated by moving bodies in a medium at rest, according to the Kirchhoff method. The present formulation can be implemented in acoustic pressure codes based on the Farassats Kirchhoff formula for arbitrary moving bodies, thus allowing direct and fast calculation of the acoustic velocity field in scattering problems. For validation purposes, four test cases are considered, namely a three-dimensional monopole, dipole and quadrupole source, as well as a monopole in uniform flow. Comparison of the results with the analytical solutions proves the remarkable accuracy of the present formulation.