A computational method for the analysis of acoustic radiation from turbofan inlets

A computational method is presented for the analysis of the noise radiation from turbofan inlets. The method developed considers the effect of mean flow and can be used at high frequencies. The techniques for generating the grid, solving the acoustic equations, applying radiating conditions on the far-field boundary, imposing inlet-fan interface conditions as well as solving the steady compressible flow equations are embodied in the Inlet Acoustic Analysis Method.

The theoretical basis, formulated for 3-D acoustics within an axisymmetric domain, considers the effect of non-uniform mean flow. The discretization of the field equations is done using a finite volume type differencing. This leads to a block tridiagonal system of equations which is then efficiently solved.

A new and powerful method is developed for the application of radiating conditions. A layer potential representation is used in obtaining numerically local radiating conditions. The locally radiating conditions, developed using the single layer source representation, can be used even at the interior eigenvalues. Using this technique, the radiating conditions can be applied very close to the inlet, and hence the computational efficiency can be significantly increased.

The irrotationality conditions for the axisymmetric compressible flow are discretized for solving the mean flow field. An iterative scheme is developed to solve for the stream function, the density, and the speed of sound.

The inlet-fan interface conditions are incorporated to properly specify the source of noise. The noise source is either directly specified or the interface potential distribution is split into a combination of an imposed right traveling disturbance and an unknown combination of left traveling disturbances that come out as part of the solution process.

The grid generation procedure utilizes algebraic transformations as well as the grid blending technique. This process is automated to accommodate variations in the grid geometry.

Several aspects of the numerical techniques are verified by demonstration problems and comparison with analytical solutions. The noise radiation from a typical inlet with a center-body at different frequencies and circumferential modes is presented including cases with mean flow. The results for a Bellmouth inlet are compared with experimental data and are in good agreement. The noise radiation at a typical blade passing frequency for an inlet with a center-body is also presented.