Thomas Carolus

Large-Eddy Simulation and Trailing-Edge Noise Prediction of an Airfoil with Boundary-Layer Tripping

This paper deals with hybrid methods for trailing edge noise prediction of a single NACA 6512-63 airfoil at zero angle-of-attack. The procedure is based on two steps. First, an incompressible large-eddy simulaton (LES) of the airfoil trailing-edge flow is performed. Then, in a second step, the far-field acoustic pressure is predicted from the LES source terms using three different methods based on the acoustic analogy. These are Amiet’s and Ffowcs Williams & Hall’s trailing-edge noise theories and Curle’s compact dipole solution. Aerodynamic and acoustic results are then compared to experimental measurements performed in the aeroacoustic wind tunnel of the University of Siegen. For comparison purposes with experimental measurements, the large-eddy simulation includes wind tunnel installation effects by a choice of suitable inflow boundary conditions. The experimental chord-based Reynolds number is 1.9× 10 5 , which results in long laminar flow regions along the airfoil that lead to Tollmien-Schlichting instabilty waves with an associated additional tonal and broadband noise radiation. To avoid this additional noise source, the boundary layer of the airfoil had been tripped on both sides in the experiment. This tripping has been included in the numerical grid of the LES in its full complex configuration (forward facing serrations) and also as a simplified geometry (simple stair-step). Eventually three different LES computations are compared that differ in boundary condition and boundarylayer tripping modeling. The modeling effects are assessed with regard to the aerodynamic and aeroacoustic prediction capability of the respective LES approaches. The comparison stresses once more the necessity of accurate boundary conditions in the LES in order to arrive at comparable results with wind-tunnel measurements.

Airfoil Trailing-Edge Blowing: Broadband Noise Prediction from Large-Eddy Simulation

Trailing-edge blowing has in recent studies found to be a potential technique for broadband noise reduction in turbomachinery applications with rotor-stator arrangement. The key-idea is to inject fluid into the wake of the rotor in such a manner that the wake will become momentumless and the turbulence structure will be modified by the enhanced mixing process. Ultimately, this wake manipulation should lead to a reduced aeroacoustic response of the downstream stator-vane with the modified turbulent wake of the rotor. The study presented within this paper investigates the trailing edge blowing mechanism by numerical means in a simplified configuration. A large-eddy simulation on a NACA 6512-63 airfoil without and with trailing-edge blowing is undertaken to investigate the effect of blowing on wall-pressure statistics of the blowing airfoil and also its wake turbulence structure and characteristics. The broadband self-noise of the airfoil is predicted by Amiet’s trailing edge noise theory. It appears that two competing mechanisms exist: a potentially increased airfoil self-noise due to the blowing jet interacting with the jet slot-lip and the trailing edge, and a reduction in interaction noise through manipulation of the wake turbulence and early wake-turbulence decay. Overall this study provides a first insight into some relevant parameters concerning the potential for broadband noise reduction through trailing-edge blowing.

Airfoil Trailing Edge Noise Prediction from Large-Eddy Simulation: Influence of Grid Resolution and Noise Model Formulation

This study is concerned with the effect of a grid refinement in large-eddy simulation (LES) for airfoil trailing edge noise prediction using different model approaches. Three different LES studies are carried out with a difference either in the spanwise grid resolution or the computational spanwise domain size. The airfoil simulated is equipped with a forward-facing serration trip on both sides to trigger the transition from a laminar to turbulent boundary layer. Differences in the evolution of the wall-pressure traces are discussed and several different analytical trailing edge noise models are tested. In addition, the airfoil noise is predicted with a finite element solution to Lighthill’s equation.

Tonal fan noise of an isolated axial fan rotor due to inhomogeneous coherent structures at the intake

In spite of low circumferential Mach number and no obvious disturbances up- and downstream, the sound of axial fans is often dominated by distinctive tones at blade passing frequency (BPF) and its higher harmonics. Flow visualization at the air intake and a correlation analysis of measured blade pressure fluctuations reveal that quasi-stationary coherent structures exist in the inflow which may act as a source of the tones. They can partly be suppressed by a hemispherical inflow control device which supports this finding.

Experimental and Numerical Investigation of the Tip Clearance Noise of an Axial Fan

The aerodynamic and aeroacoustic performance of axial fans are strongly affected by the unavoidable tip clearance. Three identical fan impellers but with different tip clearance ratio were investigated. Details of the time averaged tip flow were analysed by a numerical RANS-simulation. Unsteady wall pressure fluctuations in the tip region of the rotating blades and on the interior wall of the duct type shroud and the overall sound radiated were measured. As the tip clearance is increased, overall fan pressure rise and efficiency drop, the onset of the rotating stall moves to higher flow rates and broadband noise is found to become dominant in the spectrum. The RANS simulation revealed a vortex system consisting of three different vortices in the tip region. Their strength and trajectories are controlled by the size of the tip clearance and the fan’s operating point. Measurements showed that these vortices impose tonal and — more pronounced — broadband pressure fluctuations on stationary and rotating walls in the vicinity of the blade tip. The tip vortex system is the main driver of the unsteady flow in the tip region. As tip clearance is increased the unsteady wall pressure becomes more developed and with it the sound radiated by the fan. Hence it is concluded that these vortex induced pressure fluctuations form the dipole source mechanism of the noise observed. This hypothesis is supported by a preliminary correlation analysis.Copyright

An Aerodynamic Design Methodology for Low Pressure Axial Fans With Integrated Airfoil Polar Prediction

A common blade design methodology for low solidity fan rotors is based on blade element theory combined with empirical airfoil lift and drag data. Often the required airfoil characteristics have to be estimated from existing wind tunnel data, roughly estimating the effects of Reynolds number and airfoil modifications such as trailing edge thickening. This contribution presents an extension of that methodology: Polar curves are computed during the fan design procedure and applied to each blade element. Reynolds and even Mach number as well as all geometrical features of the airfoil are fully taken into account. For that the public domain code XFOIL for analysis of subsonic isolated airfoils by Drela and Youngren has been integrated in an existing blade design code. The paper summarizes blade element theory and points out the interface where XFOIL data enter. A case study demonstrates how the airfoil specification affects the fan blade design. Two fan rotors for the same duty point but with NACA 4512 and FX60-126 airfoil blades are compared. Moreover, the effect of trailing edge bluntness on the blade shape is investigated.Copyright

Model-based selection of full-scale Wells turbines for ocean wave energy conversion and prediction of their aerodynamic and acoustic performances:

One of the most intensively studied principles of harnessing the energy from ocean waves is the oscillating water column device. The oscillating water column converts the motion of the water waves into a bi-directional airflow, which in turn drives an air turbine. The bi-directional axial Wells turbine as a candidate for oscillating water column power take-off systems was the object of considerable research conducted in the last decades. However, there is a lack of consistent data to support practical design considerations when pre-selecting a turbine for an oscillating water column power plant. Furthermore, to minimize the overall environmental impact of this technology requires the assessment of the aero-acoustic noise associated with a Wells turbine’s operation. The effect of cascade solidity and hub-to-tip ratio on the aero-acoustic performance of Wells turbine rotors is assessed systematically by numerical simulations and model scale testing. Based on the data from the study on these generic design parameters, new Wells turbine design charts were developed. For a given plant site, main machine dimensions are identified and a time domain model is used to predict the annual energy output and the annual equivalent sound power level. Maximum values of total-static peak efficiency and low sound emission can be achieved by selecting moderate values of cascade solidity and hub-to-tip ratio. On the other hand, high hub-to-tip ratio rotors in combination with high solidity are recommended for a maximum range of operation without stall, as the pressure head is varied. The designer of an oscillating water column system usually specifies selected operating conditions on dimensional turbine characteristics. Combinations of suitable rotor diameter and rotational speeds are selected using the design chart approach. When a fully operating, full-scale Wells turbine at fixed speed is assessed, higher cascade solidity maximizes the annual energy output at minimum acoustic emission. The drawback is a slightly larger rotor diameter when, e.g. compared with a lower cascade solidity.

Optimization of axial fans with highly swept blades with respect to losses and noise reduction

A low pressure axial fan with swept blades is optimized with respect to sound emission and efficiency. Noise is addressed by a modified sweep strategy. Regarding aerodynamics, geometrical parameters describing variations of the blade section and the hub contour are defined. The optimum in terms of maximal total-to-total fan efficiency at the design point is achieved by numerous CFD simulations embedded in the Simplex optimization method. Besides a moderate increase in efficiency at the design point, a remarkable extension of operating range is observed. The numerical results are successfully validated against experiment measurements. Acoustic measurements furthermore show a decrease in sound emission over the complete operating range.

Large Eddy Simulation of Acoustical Sources in a Low Pressure Axial-Flow Fan Encountering Highly Turbulent Inflow

A large eddy simulation (LES) was applied to predict the unsteady flow in a low-speed axial-flow fan assembly subjected to a highly “turbulent” inflow that is generated by a turbulence grid placed upstream of the impeller. The dynamic Smagorinsky model (DSM) was used as the subgrid scale (SGS) model. A streamwise-upwind finite element method (FEM) with second-order accuracy in both time and space was applied as the discretization method together with a multi-frame of reference dynamic overset grid in order to take into account the effects of the blade-wake interactions. Based on a simple algebraic acoustical model for axial flow fans, the radiated sound power was also predicted by using the computed fluctuations in the blade force. The predicted turbulence intensity and its length scale downstream of the turbulence grid quantitatively agree with the experimental data measured by a hot-wire anemometry. The response of the blade to the inflow turbulence is also well predicted by the present LES in terms of the surface pressure fluctuations near the leading edge of the blade and the resulting sound power level. However, as soon as the effects of the turbulent boundary layer on the blades become important, the prediction tends to become inaccurate.

Large Scale Inflow Distortions as a Source Mechanism for Discrete Frequency Sound from Isolated Axial Fans

The acoustic performance of isolated axial fans is strongly depending on the quality of the inflow. Current standards for the acoustic measurement of industrial fans require a certain undisturbed volume upstream of the fan. However, unexpected discrete frequency sound indicates that this may not be appropriate for reliable acoustic measurements. In this study, the flow field in the immediate vicinity of the fan rotor as well as in the entire surrounding room that is much larger than the undisturbed volume recommended by the standards is examined. Smoke visualizations and RANS simulations reveal that large scale flow structures develop in the room. This moderate flow in the room and not e.g. rotor selfinduced or tip clearance vortex-type structures give rise to the inflow distortion in the fan intake observed. This hypothesis is confirmed by measurements with a hemispherical flow conditioner placed relatively far upstream of the fan rotor which homogenizes the inflow and reduces the fans tonal noise considerably.