Tze Pei Chong

A Parametric Study of Passive Flow Control for a Short, High Area Ratio 90deg Curved Diffuser

This paper represents the results of an experimental program with the aim of controlling the flow in a highly unstable 90 deg curved diffuser. The diffuser, which is an integral part of an open jet wind tunnel at the University of Southampton, has the unique configuration of extreme shortness and high area ratio. In this study, several passive flow control devices such as vortex generators, woven wire mesh screens, honeycomb, and guide vanes were employed to control the three-dimensional diffusing flow in a scaled-down model. Although less successful for vortex generators, the other devices were found to improve significantly the uniformity of the flow distribution inside the curved diffuser and hence the exit flow. This study suggests that a cumulative pressure drop coefficient of at least 4.5 at the diffuser exit with at least three guide vanes is required to achieve adequate flow uniformity at the diffuser exit. These flow conditioning treatments were applied to the full-scale diffuser with exit dimensions of 1.3×1.3 m2. Flow with comparable uniformity to the scale-model diffuser is obtained. This study provides valuable guidelines on the design of curved/straight diffusers with nonseparated flow and minimal pressure distortion at the exit.

Self-noise produced by an airfoil with nonflat plate trailing-edge serrations

This paper represents the results of an experimental study aimed at reducing the airfoil self-noise by the trailing-edge serration of four different sawtooth geometries (defined in the serration angle and length). These serrations have a common feature: all of the sawtooth patterns are cut directly into the trailing edge of a realistic airfoil. This configuration offers better structural strength and integrity. For the sawtooth trailing edges investigated here, the radiation of the extraneous vortex shedding noise in a narrowband frequency due to the partial bluntness at the serration roots is unavoidable. However, this narrowband component tends to be less significant provided that the serration angle is large and the serration length is moderate. Sound power was measured, and some of the sawtooth geometries have been shown to afford significant boundary-layer instability tonal noise and moderate turbulent broadband noise reductions across a fairly large velocity range. This paper demonstrates that a non...

On the Three-Dimensional Structure of Turbulent Spots

A detailed measurement of turbulent spots propagating in a laminar boundary layer over a flat plate was made at a zero pressure gradient and three favorable pressure gradients. Data were recorded across the span of turbulent spots at a number of streamwise locations along the plate using a hot-wire probe and surface-mounted hot films. In this work we aim at extending the existing studies that are largely based upon measurements made in the plane of symmetry of the spots and away from the wall and investigating the three-dimensional structure of turbulent spots and its dependence on streamwise pressure gradients. The results from the present experiment reveal some interesting aspects of the overall structure of the spots and the role that regions with negative velocity perturbations play in the spanwise growth of turbulent spots. The presence of a spanwise overhang is also confirmed that is believed to be responsible for the smaller spreading angle of spots measured at the wall. This finding is expected to have important implications to the development of improved transition models that are used to predict heat transfer and skin friction for turbine blades.

Aeroacoustic and aerodynamic performances of an aerofoil subjected to sinusoidal leading edges

This paper presents the preliminary results on the aeroacoustic and aerodynamic performances of a NACA65-(12)10 aerofoil subjected to 12 sinusoidal leading edges. The serration patterns of these leading edges are formed by cutting into the main body of the aerofoil, instead of extending the leading edges. Any of the leading edges, when attached to the main body of the aerofoil, will always result in the same overall chord length. The experiment was mainly performed in an aeroacoustic wind tunnel facility, although a separate aerodynamic type wind tunnel was also used for the force measurements. These sinusoidal leading edges were investigated for their effectiveness in suppressing the laminar instability tonal noise (trailing edge self-noise) and turbulence–leading edge interaction noise. The largest reduction in aerofoil noise tends to associate with the sinusoidal leading edge of the largest amplitude, and smallest wavelength. However, noticeable noise increase at high frequency is also observed for this combination of serration. In terms of the aerodynamic performance, increasing the serration wavelength tends to improve the stall angles, but the lift coefficient at the pre-stall regime is generally lower than that produced by the baseline leading edge. For a sinusoidal leading edge with large serration amplitude, the effect of the reduction in “lift-generating” surface is manifested in the significant reduction of the lift coefficients and lift curve slope. The sinusoidal leading edge that produces the best performance in the post-stall regime belongs to the largest wavelength and smallest amplitude, where the lift coefficients are shown to be better than the baseline leading edge. In conclusion, large amplitude and small wavelength is beneficial for noise reduction, whilst to maintain the aerodynamic lift a small amplitude and large wavelength is preferred.

Noise reduction using combined trailing edge and leading edge serrations in a tandem airfoil experiment

This work reports the noise reduction obtained using simultaneously serrated trailing edges and serrated leading edges in a tandem airfoil con guration. Two types of serrated trailing edge treatments are applied to the upstream airfoil, i.e., a sawtooth serration and a slitted sawtooth serration. The leading edge serrated airfoil was designed and manufactured by ONERA. Broadband noise reductions of between 5 dB and 8.5 dB are reported. It is shown over most of the frequency range that overall noise radiation is dominated by interaction noise, while trailing edge noise dominates the high frequency power spectrum at low mean ow velocity. Therefore, the overall noise reduction is dominated by the reductions in interaction noise, and particularly by the reduction of the downstream airfoil response R(f) due to leading edge serrations. However, additional reductions of the overall noise radiation of up 3.5 dB are provided by the use of the slitted sawtooth trailing edge rather than the sawtooth trailing edge on the upstream airfoil, at the high ow velocity U0 = 80m=s. This is shown to be most likely due to modi cations of the wake parameters, i.e., notably a faster decay of the turbulence intensity behind the slitted sawtooth trailing edge.

A Comparison of Spreading Angles of Turbulent Wedges in Velocity and Thermal Boundary Layers

Turbulent wedges induced by a three-dimensional surface roughness placed in a laminar boundary layer over a flat plate were visualized for the first time using both shear-sensitive and temperature-sensitive liquid crystals. The experiments were carried out at zero pressure gradient and two different levels of favorable pressure gradients. The purpose of this investigation was to examine the spreading angles of turbulent wedges indicated by their associated surface shear stresses and heat transfer characteristics and hence obtain further insight about the difference in the behavior of transitional momentum and thermal boundary layers when a streamwise pressure gradient exists. It was found that under a zero pressure gradient the spreading angles indicated by the two types of liquid crystals are the same, but the difference increases as the level of favorable pressure gradient increases with the angle indicated by temperature-sensitive liquid crystals being smaller. The results from the present study suggest that the spanwise growth of a turbulent region is smaller in a thermal boundary layer than in its momentum counterpart and this seems to be responsible for the inconsistency in transition zone length indicated by the distribution of heat transfer rate and boundary layer shape factor reported in the literature. This finding would have an important implication to the transition modeling of thermal boundary layers over gas turbine blades.

An Experimental Study of Tonal Noise Mechanism of Laminar Airfoils

This experimental works summarize the (1), aeroacoustic results of airfoil self-noise generated at Reynolds numbers between 1.0 x 10 and 6.0 x 10. The test facility is a newly built low noise and low flow turbulence open jet wind tunnel, where a NACA0012 airfoil was mounted to the exit nozzle horizontally. The noise measurements were performed at three effective angles of attack at 0.0, 1.4 and 4.2 degrees. In most cases, multiple discrete tones, embedded in a broad spectrum “hump”, were recorded. The boundary layers on the pressure side of the airfoil were mainly laminar in nature, although instability waves were predicted to grow at the adverse pressure gradient region of the airfoil, (2), aerodynamic data on the airfoil wake development in an attempt to study the noise source. The airfoil incidences were chosen as 0, 2 and 5 degrees. It was found that the radiated tones from the airfoil were always accompanied by localized large scale wake oscillation. The acoustic radiation of this wake tone is believed to be triggered by resonance/feedback between the wake oscillation and the point of first instability on the pressure side of the airfoil. Linear stability analysis on the boundary layer confirmed that the frequency at which maximum amplification of the Tollmien-Schlichting wave occurred always coincide with the main tone noise frequency. It is hoped that the results presented in this paper can shed some lights on the tone generation mechanism that characterise the airfoil noise with laminar boundary layers. Nom

Design of a low-noise aeroacoustic wind tunnel facility at Brunel University

The constructions of the aeroacoustic wind tunnel and the hemi-anechoic chamber are financially supported by the School of Engineering and Design at Brunel University.

On the Noise and Wake flow of an Airfoil with Broken and Serrated Trailing Edges

This paper represents the results of an experimental program aiming at exploring the effectiveness of several broken, saw-tooth shaped trailing edges to suppress the instability tone noise at acceptable aerodynamic loss at low Reynolds number flow. This paper also studies the noise sources through acoustic and wake velocity measurements. The test cases consist of several angles of attack from 0 o to 5 o at a fixed Reynolds number of 150,000. For an airfoil with a sharp trailing edge, instability tone noises are detected. The noise mechanism is identified as the aeroacoustic feedback loop which involves the diffraction of the Tollmien-Schlichting waves at the trailing edge. When the trailing edge is replaced with a broken and serrated type, most of these instability tone noises are suppressed, resulting in large noise reduction. The effectiveness of the tonal noise reduction also increases with the serration angle of the saw-tooth. The wake velocity spectra produced by a sharp trailing edge could be very different depending on the angle of attack. Laminar wake at the zero angle of attack ensures that no wake tones could be produced. At 3 o attack angle, the wake becomes very unstable with the presence of a spectra hump and multi-discrete components. It is proposed that these multi-discrete components could eventually produce wake tones in addition to the instability tones produced by the aeroacoustic feedback loop at the pressure side. Because of this, laminar airfoil at a moderate angle of attack always possesses serious noise issue. The wake velocity spectra for a broken, serrated trailing edge are characterized by vortex shedding at 0 o attack angle with no apparent bluntness-induced tone. At 3 o attack angle, the pressure imbalance between the suction and pressure side at each saw-tooth creates counter-rotating vortices. These vortices inhibit both of the spanwise coherence of the bluntness-induced vortex shedding and the multi-discrete components in the wake that would have been produced by a sharp trailing edge. The results in this paper demonstrate that a versatile and low noise laminar airfoil could be achieved when a broken, serrated type trailing edge is used.

Statistical-Empirical Modelling of Airfoil Noise Subjected to Leading Edge Serrations

With the objective of reducing the broadband noise from the interaction of highly turbulent flow and airfoil leading edge, sinusoidal leading-edge serrations were investigated as an effective passi...