Fluid Mechanical and Acoustic Characterization of Low ...ltces.dem.ist.utl.pt/lxlaser/lxlaser2016/finalworks2016/papers/02... · This sound mechanism is governed by vortex shedding

18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016

Fluid Mechanical and Acoustic Characterization of Low-Pressure Axial Fans with different Blade Skew

Florian Zenger1,*, Stefan Becker1

1: Institute of Process Machinery and Systems Engineering, University of Erlangen-Nuremberg, Erlangen, Germany

* Correspondent author: [email protected]

Keywords: Laser Doppler Anemometer, LDA, Axial Fan, Blade Skew, Acoustics

ABSTRACT

Several parameters have been established that define the fan blade shape of low-pressure axial fans with the most important being the fan blade skew. Both the flow-field inside the fan and the sound emission of the fan is governed by this design parameter. In order to assign different parts in the acoustic spectrum to specific flow mechanisms, it is necessary to consecutively characterize the aerodynamics and the aeroacoustics of a fan. Hence, two fans with backward- and forward-skewed fan blades were investigated to identify differences in the flow-field and the acoustic spectrum. The flow-field at the suction and pressure side of the fans was measured with a laser Doppler anemometer, the acoustic field at the suction side was recorded with seven microphones. Results of the ensemble-averaged flow properties at the suction side showed that the axial velocity is clearly influenced by the fan angular position and that the distribution from hub to tip is dependent on the fan blade skew. There was hardly any influence of the angular position on the axial velocity at the pressure side. The distribution of the radial velocity at the pressure side showed that the flow is directed outwards by backward-skewed fan blades and directed inwards by forward-skewed fan blades. Values of the turbulent kinetic energy at the pressure side of the backward-skewed fan were significantly increased in the tip region. Broadband components in the acoustic spectra from 0.5 to 2 kHz were higher for the backward-skewed fan. Due to the outwards directed radial component, a longer flow path develops over the blade surface, resulting in a thicker boundary layer and consequently an intensified sound emission. For frequencies greater than 2 kHz, broadband components of the forward-skewed fan were more prominent. This sound mechanism is governed by vortex shedding on the fan blade trailing edges. It is illustrated that forward-skewed fan blades tend to induce a higher velocity perpendicular to the trailing edges of the fan blades which leads to an increased acoustic source strength on the trailing edges. Besides this, significant subharmonic components, i.e. non-corresponding to the blade passing frequency and harmonics, were found in the spectrum of the backward-skewed fan. The source mechanism for this phenomenon was linked to the complex flow field in the tip region. Backflow from pressure to suction side can interact with the fan blades which results in tonal noise radiation.


1. Introduction

Low-pressure axial fans are usually designed to achieve aerodynamic design specifications at a certain operating point. Besides these aerodynamic characteristics, the acoustic sound emission is an important part to be considered during new product developments. The acoustic sound emission is largely influenced by the blade geometry, i.e. blade skew (three dimensional stacking of the fan blade segments). Blade skew is a combination of blade sweep and blade dihedral (Vad (2011), Beiler (1996)), see also Section 2. Furthermore, the blade design has an effect on the aerodynamics of the axial fan. This can lead to different internal flow mechanisms in the specific fan, which can in turn have a retroactive effect on the aeroacoustic sound emission. It has been observed that forward-skewed fans have a lower sound emission than corresponding backward- or unskewed fans. Carolus and Beiler (1997) investigated the effect of blade skew on different noise mechanisms in axial fans. They concluded that several aeroacoustic sound sources are affected by the blade skew and that no single mechanism could explain the noise reduction. Furthermore, it was found that blade skew influences the three-dimensional flow field in axial fans: The flow in the hub region was observed to be increased by forward-skewed fan blades, resulting in a stabilization of the flow-field during fan passage. An experimental study on the flow-field in unskewed and forward-skewed axial fans using a laser Doppler anemometer was done by Meixner (1994). There was a substantial difference in the flow-field at the pressure side of the skewed and unskewed fans. The mean axial velocity for the forward-skewed fans was increased in the hub region and decreased in the blade tip region compared to values of the unskewed fan. In an accompanying study, Felsch and Meixner (1993) observed a 5 dB lower sound emission of the forward-skewed fan compared to the unskewed fan. Beiler (1996) made similar investigations of the flow-field in skewed and unskewed fans with a hot-wire anemometer. It was confirmed that sweep and dihedral (see Sec. 2) both have an effect on the flow-field. The flow in the hub region was increased by forward-skewed fan blades and the outlet velocity profile was more regular than in case of the unskewed and backward-skewed fan. The effect of blade skew on the radial velocity component was numerically examined by Agboola and Wright (1999). They found three major effects that are altered by the fan blade skew. First, forward-skew changes the direction of the radial velocity component which the authors directly linked to the noise reduction in forward-skewed axial fans. Second, backward-skew enlarges the region of momentum loss near the trailing edge. This can lead to flow stall and consequently to an increased sound emission. Last, the tangential velocity component was found to be lower in magnitude for the forward-skewed fan, which can lead to a higher efficiency.


Corsini and Rispoli (2004) made a numerical analysis the three-dimensional flow structures in an unskewed and a forward-skewed fan. The forward-skewed fan showed an inwards effect in the vicinity of the blade due to the reduced centrifugal fluid migration and also a reduction of the leading edge velocity peak near the blade tip. The flow-field at the outlet of skewed and unskewed fans was examined with a hot-wire anemometer and a five-hole probe by Li et al. (2007). As previously observed, the fluid moved in direction of the hub for the forward-skewed fan and correspondingly in direction of blade tip for the backward-skewed fan during the fan passage. Besides this, Li et al. (2007) concluded from lower velocity fluctuations at the pressure side in case of the forward-skewed fan that this can be the reason for a lower sound emission. Another experimental and numerical study on the blade skew effect on the downstream flow-field of axial fans was made by Hurault et al. (2010). It is mentioned that the radial equilibrium hypothesis is often wrongly assumed as there is a radial velocity component present. The radial component was decreased by an application of forward-skew and correspondingly increased by backward-skew. Investigations of the flow-field show that especially the radial velocity component is altered by the applied fan blade skew. While the mentioned examinations give a detailed overview of the three-dimensional flow mechanisms in axial fans, in most cases a specific link to the aeroacoustic sound emission is lacking. Therefore, the aim of this study is to give a detailed overview of the aerodynamic and acoustic characteristics of two low-pressure axial fans with forward- and backward-skewed fan blades and to illustrate flow phenomena which can account for certain parts in the acoustic spectrum.

2. Experimental Setup

Two generic fans with forward-skewed fan blades and backward-skewed fan blades were investigated, see Fig. 1. The fans had identical design parameters except for the blade sweep angle λ and dihedral angle νwhichdefinethebladeskew. Blade skew or three dimensional stacking is a combination of blade sweep and blade dihedral (Vad (2011), Beiler (1996)). Blade sweep describes a shifting of the blade airfoil sections in the flow direction (forward/positive sweep) or against the flow direction (backward/negative sweep). The sweep angle λ is defined as the angle between the radial direction and the projection of the blade stacking line of the swept airfoil onto the plane containing the radial direction and the chord line, see Vad (2011).


Fig. 1 Forward-skewed fan (left) and backward-skewed fan (right).

The dihedral angle ν is defined as the angle between the radial direction and the projection of the blade stacking line onto the plane containing the radial direction and the direction normal to the chord line, (Vad 2011). Both angles are shown in Fig. 2.

Fig. 2 Blade sweep angle λ and dihedral angle ν on a fan section.

The blade element theory for low-solidity fans (Carolus (2013), Pfleiderer C and Petermann H (2005)) was used for designing the fans. The detailed design procedure is described in Zenger et al. (2016). An important parameter during this procedure is the radial blade loading distribution


𝑟𝑐45(𝑟), with the radius 𝑟 and the circumferential velocity component at the outlet 𝑐45. This distribution can either be chosen to be constant (free vortex design) or depending on the radius (controlled vortex design). In this case a controlled vortex design was chosen according to

𝑟𝑐45 𝑟 = −𝑎 𝑏𝑟 − 𝑑 5 + 𝑐, (1)

with the coefficients 𝑎 = 5, 𝑏 = 5 [email protected]

, 𝑐 = 1.77FG

H and 𝑑 = 0.99 F

@A.C. The remaining design

parameters are listed in Tab. 1. The fan blade dihedral angle ν was chosen so that the axial installation space of the fans was minimal.

Tab. 1 Fan design parameters.

forward-skewed

fan

backward-skewed

fan

Total-to-static pressure coefficient 𝜓L@ 0.18

Flow rate coefficient 𝛷 0.18

No. of fan blades 9

Rotational speed 𝑛 in rpm 1486

Fan diameter 𝑑OPQ in mm 495

Hub diameter 𝑑RST in mm 248

Tip gap 𝑠 in mm 2.5

Sweep angle 𝜆 in ° (hub … tip) 0 … 55 0 … -55

The flow rate coefficient is defined as

𝛷 = WXYGZ[\]

^ _ (2)

and the total-to-static pressure coefficient as

𝜓L@ = 5`abc

d Z[\]Y_ G . (3)

In equation (2) 𝑉 is the volumetric flow, 𝑑OPQ the fan diameter and 𝑛 the rotational speed. In equation (3) 𝛥𝑝L@ is the total-to-static pressure difference (ambient to chamber pressure) and 𝜌 the air density. The fan total-to-static efficiency 𝜂L@ can be calculated with

𝜂L@ = X`abc5Y_j

, (4)

where 𝑀 is the measured torque on the fan shaft.


Fan Installation

The fans were installed in a short duct with an inlet bellmouth at the suction side and a diffusor at the pressure side, see Fig. 3.

Fig. 3 Fan installation.

The fans were driven by a motor at the pressure side outside of the duct. An optical fork sensor (one pulse per revolution) was mounted on the torque meter for determining the fan’s angular position. The test setup was integrated in a standardized inlet test chamber according to ISO 5801, ISO 5081 (2007) as shown in Fig. 4.

Fig. 4 Standardized inlet test chamber according to ISO 5801.


The room was completely equipped with absorbers to enable acoustic measurements with free-field conditions.

Laser Doppler Anemometer Setup

Fluid mechanical properties in 𝑥-, 𝑦- and 𝑧- direction were measured with a laser Doppler anemometer (LDA), Albrecht et al. (2003). A two-component LDA system in backscatter orientation with a 300 mW argon ion laser type Stellar-Pro-L Select 300 (Modu-Laser) with high energy output at 488 nm and 514.5 nm was used. Beam splitting, frequency shifting and signal processing was done with Dantec Dynamics equipment. For signal analysis including ensemble-averaging of the measurement values Dantec BSA Flow Software v5.20 was utilized. Transmitting fibers in the transmitter/receiver LDA probe type 2D FiberFlow (Dantec Dynamics) had a beam distance of 40 mm. Measurements were made at the suction side near the fan blade leading edges and at the pressure side near the fan blade trailing edges, see Fig. 5.

Fig. 5 LDA measurement positions.

For measuring axial velocity 𝑐o and circumferential velocity 𝑐p at the pressure and suction side, the LDA probe was placed sideways to the duct, fitted with a lens with 160 mm focal length. To obtain optical access to the flow field a small portion of the duct was replaced with a float glass pane. The probe was positioned parallel to the rotational axis for measuring the radial velocity 𝑐q on either the suction or pressure side, fitted with a lens with 800 mm focal length in order to avoid retroactive effects of the LDA probe on the flow-field. The different LDA probe positions including


the corresponding measured velocity components are shown in Fig. 6. The index 1 denotes values at the suction side and the index 2 denotes values at the pressure side.

The LDA probe was mounted on a three-axis traversing unit during all measurements. A total of 26 measurement points from 𝑟 𝑟rSsL ∈ 0.53, 0.98 along a line in radial direction (shown in Fig. 5 and Fig. 6) were investigated. Measurement stop criteria was set to 9 min measurement time or 1.5 ∙ 10y samples per position. The data were ensemble averaged in 2° bins depending on the fan’s rotational angle, resulting in a total of 180 bins per revolution. Each velocity component 𝑐z can be written as a combination of mean value 𝑐{ and fluctuation value 𝑐{|:

𝑐{ = 𝑐{ +𝑐{| (5)

with

𝑐{ = ?

}G~}� 𝑐{d𝑡}G}�

. (6)

Ensemble averaged values are marked with a tilde □.

The turbulent kinetic energy 𝑘was calculated according to

𝑘 = ?5 𝑐′F5 +𝑐′�5 +𝑐′�5 . (7)

Fig. 6 Schematic representation of LDA probe positions (left) and test setup for measuring axial and circumferential

velocity components at the pressure side (right). Rotational direction is clockwise as seen from the pressure/test

chamber side.


Sound-Field

The sound field was measured with seven 1/2 inch free-field microphones, type 4189-L-001 (Brüel & Kjær). The microphones were arranged in a semicircle with a radius of 1 m around the inlet bellmouth in a horizontal plane at the same height as the rotational axis, see Fig. 7.

Fig. 7 Schematic representation of microphone positions (left) and test setup for acoustic measurements with

backward-skewed fan (right).

3. Results and Discussion

All investigations were made at the design flow rate coefficient of 𝛷 = 0.18 which corresponds to a volumetric flow of 1.4 m³s-1. The volumetric flow was adjusted with the auxiliary fan during all measurements.

Flow-Field, Ensemble-Averaged Values

Ensemble-averaged values from the flow-field measurements at the pressure side are shown in Fig. 8. The mean velocity in axial direction 𝑐o? is represented by the contour plot. The vector-field illustrates the magnitude and direction of the velocity components in the 𝑦-𝑧-plane, i.e. radial velocity 𝑐q? and circumferential velocity 𝑐p?:


𝑐� ∙ 𝑒� + 𝑐� ∙ 𝑒� = 𝑐� ∙ 𝑒� + 𝑐� ∙ 𝑒� , (8)

with 𝑒□ being the unit vector in the corresponding coordinate direction. In general, the flow-field is governed by the fan blades. Plots of the whole fan showed a high symmetry corresponding to the nine fan blades. Hence, for a more detailed view of the flow-field only quarter fan segments are illustrated. Viewpoint in all plots is from the suction/test chamber side towards the fan.

Fig. 8 shows the ensemble-averaged distribution of mean axial velocity 𝑐o? (contour) and mean plane velocity components 𝑐q? and 𝑐p? (vector-field) on the fan suction side. The ensemble-averaged axial velocity at the suction side 𝑐o? is mainly influecend by the fan blade angular position. High velocity regions can be observed in vicinity of the trailing edges for both fans whereas low velocity regions are present near the blade tips of the blade leading edges. This effect is caused by the flow displacement of the fan blades. The axial velocity is alternating as each fan blades forms a flow obstruction where no direct axial through-flow is possible. The axial velocity is maximal near the trailing edges because the inflow is diverted by the fan blades. The axial velocity near the trailing edges in case of the forward-skewed fan is relatively constant from hub to tip with maxima near the hub and in the upper third of the fan blade whereas in case of the backward-skewed fan the axial velocity is maximal at the fan hub and then decreases towards the blade tips. This was already observed by Beiler (1996). Maxima near the fan hub are caused by the flow displacement of the fan hub whereas especially the flow-distribution from hub to tip is governed by the fan blade geometry. The vector-field of ensemble-averaged circumferential and radial velocity, 𝑐p? and 𝑐q?, shows that the flow at the suction side is directed against the rotational direction at the blade passage, i.e. in the area from leading edge to trailing edge and directed in rotational direction in between the fan blades. Radial and circumferential velocity are particularly high near the blade tip in case of the forward-skewed fan. This is in accordance with the higher axial velocity near the blade tip for the forward-skewed fan. Due to the decreased velocity near the blade tips in case of the backward-skewed fan, no alternating pattern can be observed in this region, i.e. the flow is in general directed against the rotational direction. Effects of an alternating combined inwards deflection from the blade tip and outwards deflection from the blade hub towards the region of high axial velocity is more prominent for the forward-skewed fan (peak axial velocity in the upper third of the fan blade) than for the backward-skewed fan (peak axial velocity near the fan hub). This can be attributed to the distribution of the axial velocity, as described above.


Fig. 8 Ensemble-averaged distribution of mean axial velocity 𝑐o? (contour) and mean plane velocity components 𝑐q?

and 𝑐p? (vector-field) at the fan suction side.

The ensemble-averaged distribution of mean axial velocity 𝑐o5 (contour) and mean plane velocity components 𝑐q5 and 𝑐p5 (vector-field) at the fan pressure side are depicted in Fig. 9. Compared to the results at the suction side there is only little influence of the fan blade angular position on the axial velocity. Influence of the duct boundary layer can be observed on the decreased axial velocity in the blade tip region for both fans. The same effect is caused by the hub boundary layer accordingly. The vector field of ensemble-averaged circumferential and radial velocity, 𝑐p5 and 𝑐q5, is now mainly governed by the circumferential velocity in rotational direction. The circumferential velocity is maximal at 𝑟 𝑟rSsL ∈ 0.6, 0.8 as expected from the blade loading distribution in Eq. (1). Concerning the radial velocity, it is obvious that the flow is directed inwards in case of the forward-skewed fan and outwards for the backward-skewed fan. Furthermore, at the blade tips of the backward-skewed fan the circumferential velocity is increased whereas this effect cannot be observed for the forward-skewed fan. As the circumferential velocity is 2-3 times higher than the radial velocity these effects cannot be recognized in detail on the basis of the vector-field in Fig. 9. Hence, contour plots of only the ensemble-averaged radial velocity 𝑐q5 at the pressure side are depicted in Fig. 10. From Fig. 10 it is obvious that there is a substantial difference in the distribution of the radial velocity for both fans which can be directly linked to the fan blade skew as described in Sec. 1. A relatively high radial velocity, i.e. an outwards directed velocity, can be observed in between two fan blades and at the hub of the backward-skewed fan. In the remaining regions mainly positive radial velocities can be found. In contrast, the velocity is only partially outwards directed in case of the forward-skewed fan.


Fig. 9 Ensemble-averaged distribution of mean axial velocity 𝑐o5 (contour) and mean plane velocity components 𝑐q5

and 𝑐p5 (vector-field) at the fan pressure side.

In addition, there are regions with a negative, i.e. inwards directed velocity, in the contour plot of the forward-skewed fan. This difference can be the cause of the increased circumferential velocity near the blade tips of the backward-skewed fan. As the fluid is transported outwards it reaches high velocities at the duct wall as the outwards motion is stopped and the movement merges to a circumferential motion.

Fig. 10 Ensemble-averaged distribution of mean radial velocity 𝑐q5 at the fan pressure side.


Another key parameter of the flow-field besides the velocity distribution is the turbulent kinetic energy. As plots of the ensemble-averaged turbulent kinetic energy at the fan suction side were almost identical for both fans only plots at the pressure side are shown, see Fig. 11. It is obvious that the discussed radial fluid migration for the backward-skewed fan has a major influence on the turbulent kinetic energy at the pressure side. While values of the turbulent kinetic energy in the blade tip region for the forward-skewed fan do not exceed 𝑘5 = 20 m2s-2, in case of the backward-skewed fan values of up to 𝑘5 = 50 m2s-2 can be found. This shows that there is a complex flow-field induced by the backward-skewed fan blades in the tip region. A reason for the high values of 𝑘5 for the backward-skewed fan can be regions with intensive backflow from the pressure side to the suction side caused by the pressure difference between the two sides. This is supported by the fact that there is a lower axial velocity at the suction side of the backward-skewed fan than of the forward-skewed fan. This can act as an additional flow-resistance in case of the forward-skewed fan, decreasing the potential for backflow from the pressure side. This can also have a major effect on the sound emission of both fans as will be discussed further throughout this section. Aside from the tip region, no major differences can be found in the two contour plots. The turbulent kinetic energy tends to be slightly increased at the trailing edges, possibly caused by the wake of the fan blades and in between two fan blades which can be induced by the concurrence of the flow through each blade channel.

Fig. 11 Ensemble-averaged distribution of mean turbulent kinetic energy 𝑘5 at the fan pressure side.


Sound-Field

As already mentioned, the complex flow-field particularly in the gap region can have a profound influence on the sound emission of both fans. Fig. 12 shows the sound pressure spectra of both fans. The pale lines show the sound pressure spectra of each microphone as depicted in Fig. 7, the bright lines show the averaged sound pressure spectrum over the seven microphone positions for each fan. The two spectra for each fan that are 15 dB below the average curve for 𝑓 > 1 kHz are from microphone positions 1 and 7, i.e. the microphones which are placed at an angle of 90° to the rotational axis. This suggest that the fans have no clear monopole radiation pattern as was previously observed by Bianchi et al. (2012) and Lallier-Daniels et al. (2016).

Fig. 12 Sound pressure spectra of forward- and backwars-skewed fan.

In general, the spectra consist of tonal and broadband components. Tonal components at the blade passing frequency of 𝑓�� = 225 Hz and harmonics result from steady blade forces (e.g. lift forces) and unsteady blade forces, i.e. blade forces induced by high inflow turbulence or a non-uniform inlet velocity profile, Wright (1976), Bommes et al. (2003), Carolus (2013). Levels of tonal components at the BPF and harmonics of both fans are in the same order of magnitude. For 𝑓 >1.2 kHz only tonal components of the forward-skewed fan are visible. Although the measurements were made in a test-chamber designed for generating an inflow with a very low turbulence there is still a substantial turbulence intensity present at the inlet, as also observed by similar experiments under laboratory conditions by Zanon et al. (2014), which can be the reason for these tonal components. A detailed study of the inflow conditions and its effects on the aeroacoustic sound emission of axial fans with the same test setup was studied by Zenger et al. (2016).


Broadband sound can be related to several mechanisms, e.g. turbulent ingestion noise and blade self noise, Wright (1976), Scharpf and Mueller (1995), Wojno et al. (2002), Carolus (2013), Roger (2013). Rotor self noise is a general term for boundary layer noise and trailing-edge noise. Turbulent boundary layer noise is caused by pressure fluctuations below the boundary layer, Sharland (1964), Carolus (2013) whereas trailing edge noise is produced by vortical structures that are convected past the trailing edge, Roger and Moreau (2004), Carolus (2013). Turbulent ingestion noise is generated by the interaction of the inflow with the fan blades, Scharpf and Mueller (1995), Carolus (2013), Roger (2013). The averaged broadband spectrum of the backward-skewed fan has greater levels than the averaged spectrum of the forward-skewed fan up to a frequency of 𝑓 = 2 kHz. It is expected that boundary layer noise is more dominant for backward-skewed fans in this frequency range. The induced outwards motion by the backward-skewed fan blades results in an increased boundary layer thickness as there is a longer available path before reaching the fan blade trailing edges compared to forward-skewed fan blades, Wright and Simmons (1990), see Fig. 13. This effect is visible in the frequency range from 0.5 < 𝑓 < 2 kHz where the curve of the averaged spectra of the backward-skewed fan is up to 10 dB above the curve of the forward-skewed fan. For 𝑓 > 2 kHz it is anticipated that trailing edge is the dominant sound mechanism, Wright (1976), Carolus (2013). Herold et al. (2016) showed from beamforming sound maps that trailing edge noise from forward-skewed fans is more prominent than from backward-skewed fans in this frequency range which can be reproduced in this investigation. According to Brooks and Burley (2001), the sound pressure level from blunt trailing edge vortex shedding can be predicted with

𝐿�,�� 𝑓 = 10log ��,��C.C��A ¡¢c£¤¥£¤

∙ 𝐻T�§∗, ©§

∗

��,��, ©��,��

𝜓 , (9)

see also Brooks et al. (1989). In Eq. (9) 𝑐ª,�� is the velocity perpendicular to the trailing edge, ℎ is the trailing edge thickness, 𝐿 is the blade-segment spanwise length, 𝐷R the high frequency directivity function, 𝑐 the speed of sound, 𝑥®T@¯�°¯� the observer distance and 𝜓 the solid angle between the blade surfaces immediately upstream of the trailing edge. Note that the trailing edges of both fans are expected blunt due to the manufacturing using vacuum casting. The velocity perpendicular to the trailing edge 𝑐ª,�� can be calculated with the velocity on the trailing edge 𝑐�� and the angle 𝛽 as illustrated in Fig. 13:

𝑐ª,�� = 𝑐�� ∙ cos 𝛽 . (10)

The angle 𝛽 is greater for the backward-skewed fan blade than for the forward-skewed fan blade, see Fig. 13. The same applies to the velocity 𝑐ª,��. In the prediction model from Eq. (9) the velocity 𝑐ª,�� is comprised with an exponent of 5.5. Hence variations in the velocity perpendicular to the


trailing edge can be expected to have a major impact on the sound pressure levels in the high frequency range, in this study for 𝑓 > 2 kHz. This effect can be further increased by the mentioned outwards migration of boundary layer fluid as this reduces the angle 𝛽 in Fig. 13.

Fig. 13 Schematic representation of flow conditions on the fan blade trailing edges on backward- and forward-

skewed fan blades, boundary layer fluid movement according to Wright and Simmons (1990)

Besides the discussed broadband components a very dominant subharmonic peak can be observed at 𝑓 = 325 Hz for the backward-skewed fan and at 𝑓 = 370 Hz for the forward-skewed fan, however with a lower magnitude, see Fig. 12. Similar as described by Magne et al. (2015) and Moreau and Sanjose (2016) this is caused by coherent vortices stemming from backflow in the tip region and their interaction with the fan blades. As these vortices rotate with a different angular velocity than the fan these peaks do not occur at the BPF. A major parameter for the development of such backflow from pressure to suction side is the flow-field in the gap region. There are several differences in the flow-field in the gap region, as previously discussed. This can lead to a more pronounced backflow mechanism for the fan with backward-skewed fan blades than for the fan with forward-skewed fan blades. This is supported by the fact that the ensemble-averaged turbulent kinetic energy in the gap region at the pressure side 𝑘5 is substantially higher in case of the backward-skewed fan, hinting at complex flow phenomena in this region. Hence the subharmonic component is considerably higher for the backward-skewed fan.


4. Conclusion

LDA measurements were performed at the suction and pressure side of two fans with backward- and forward-skewed fan blades. Results from the ensemble-averaged flow quantities showed that the distribution of axial velocity at the suction side is different for both fans. While the axial velocity from hub to tip was rather constant for the forward-skewed fan, the axial velocity was found to be increased near the blade hub and decreased in the tip region. However, the axial velocity at the pressure side was similar for both fans, indicating that there is an outwards directed flow induced by the backward-skewed fan blade. This radial migration could be confirmed with the vector plot of radial and circumferential velocity and with the contour plot of the radial velocity at the pressure side. In contrast to the forward-skewed fan where an inwards directed flow was observed, there was a substantial outwards directed radial component for the backward-skewed fan. Increased values of the turbulent kinetic energy indicate the existence of a complex flow phenomenon in the tip region of the backward-skewed fan. These observations were assigned to different sound mechanisms which can explain the stronger/weaker characteristic of different parts in the acoustic spectra for both fans. For the applied blade skew, broadband components of the forward-skewed fan were lower in the mid-frequency region and higher in the high-frequency region. This could be assigned to the boundary layer flow path length and to vortex shedding on the trailing edge. The most significant impact of the blade geometry could be observed on the subharmonic component of the backward-skewed fan which was linked to flow phenomena in the tip region. This study showed that the applied blade skew can have a profound impact on the noise radiation of axial fans. In order to avoid undesired components in the acoustic spectrum, e.g. subharmonic components, a suitable blade skew strategy can be chosen. In cases where a certain flow-field is desired at the pressure side, e.g. an outwards or inwards directed flow, the potential impact on the sound emission can be assessed.


References Agboola F, Wright T (1999) The effects of axial fan noise control by blade sweep on the radial component of velocity. AIAA-99-1862, doi: 10.2514/6.1999-1862.

Albrecht H-E, Borys H-E, Damaschke M, Damaschke N, Tropea C (2003) Laser Doppler and phase Doppler measurement techniques. Springer.

Beiler M (1996) Untersuchung der dreidimensionalen Strömung durch Axialventilatoren mit gekrümmten Schaufeln. Ph.D. thesis, University Siegen, Germany.

Bianchi S, Corsini A, Sheard A-G (2012) Experimental characterisation of the far-field noise in axial fans fitted with shaped tip end-plates. ISRN Mechanical Engineering, doi: 10.5402/2012/212358.

Blake W (1986) Mechanics of flow-induced sound and vibrations. Vol. II: Complex flow-structure interactions. Academic Press.

Bommes L, Fricke J, Grundmann R (2003) Ventilatoren. Vulkan.

Brooks T, Burley C (2001) Rotor broadband noise prediction with comparison to model data. AIAA Paper 2001-2210.

Brooks T, Pope S, Marcolini M (1989) Airfoil self-noise and prediction. NASA Reference Publication 1218.

Carolus T, Beiler M (1997) Skewed blades in low pressure fans: a survey of noise reduction mechanisms. AIAA-97-1591, doi: 10.2514/6.1997-1591.

Carolus T (2013) Ventilatoren - Aerodynamischer Entwurf, Schallvorhersage, Konstruktion. Vol. 3, Springer Vieweg.

Corsini A, Rispoli F (2004) Using sweep to extend the stall-free operational range in axial fan rotors. Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy 2004 218 pp. 129-139, doi: 10.1243/095765004323049869.

Felsch K-O, Meixner H-U (1993) Geräuschreduktion durch Sichelung der Schaufeln bei Axialventilatoren. Abschlussbericht zum Forschungsvorhaben AIF Nr. 8714.

Herold G, Zenger F, Sarradj E (2016) Analyzing noise components on skewed fans with a virtual rotating microphone array. Proceedings of 22nd AIAA/CEAS Aeroacoustics Conference, Lyon, France.


Hurault J, Kouidri S, Bakir F, Rey R (2010) Experimental and numerical study of sweep effect on three-dimensional flow downstream of axial flow fans. Flow Measurement and Instrumentation 2010 21 pp. 155-165, doi: 10.1016/j.flowmeasinst.2010.02.003.

ISO 5801 (2007) Industrial Fans - Performance Testing Using Standardized Airways. International Organization for Standardization, ISO 5801:2007.

Lallier-Daniels D, Piellard M, Coutty, B Moreau S (2016) Aeroacoustic study of an axial ring fan using Lattice-Boltzmann simulations and the Ffowcs-Williams and Hawkings analogy. Proceedings of the International Symposium on Transport Phenomena and Dynamics of Rotating Machinery ISROMAC 2016, Hawaii, Honolulu.

Li Y, Ouyang H, Du Z (2007) Experimental research on aerodynamic performance and exit flow field of low pressure axial flow fan with circumferential skewed blades. Journal of Hydrodynamics 2007 19 (5) pp. 579-586, doi: 10.1016/S1001-6058(07)60156-5.

Magne S, Moreau S, Berry A (2015) Subharmonic tonal noise from backflow vortices radiated by a low-speed ring fan in uniform inlet flow. Journal of the Acoustical Society of America 137 (1) pp. 228-237, doi: 10.1121/1.4904489.

Moreau S, Sanjose M (2016) Sub-harmonic broadband humps and tip noise in low-speed ring fans. Journal of the Acoustical Society of America 139 (1) pp. 118-127, doi: 10.1121/1.4939493.

Meixner H-U (1994) Vergleichende LDA-Messungen an ungesichelten und gesichelten Axialventilatoren. Ph.D. thesis, University Karlsruhe, Germany.

Pfleiderer C, Petermann H (2005) Strömungsmaschinen. Vol. 7, Springer.

Roger M (2013) Reduction of airfoil turbulence-impingement noise by means of leading-edge serrations and/or porous materials. AIAA Paper 2013-2108, doi: 10.2514/6.2013-2108.

Roger M, Moreau S (2004) Broadband self-noise from loaded fan blades. AIAA Journal 42 (3) pp. 536-544, doi: 10.2514/1.9108.

Scharpf D, Mueller T (1995) An experimental investigation of the sources of propeller noise due to the ingestion of turbulence at low speeds. Experiments in Fluids 18 pp. 227-287, doi: 10.1007/BF00195098.

Sharland I (1964) Sources of noise in axial flow fans. Journal of Sound and Vibration 1 (3) pp. 302-322.


Vad J (2011) Blade sweep applied to axial fan rotors of controlled vortex design. Ph.D. thesis, Hungarian Academy of Sciences, Budapest.

Wojno J, Mueller T, Blake W (2002) Turbulence ingestion noise, part 2: Rotor aeroacoustic response to grid-generated turbulence. AIAA Journal 40 (1) pp. 26-32, doi: 10.2514/2.1637.

Wright S-E (1976) The acoustic spectrum of axial flow machines. Journal of Sound and Vibration 45 (2) pp. 165-223, doi: 10.1016/0022-460X(76)90596-4.

Wright T, Simmons W-E (1990) Blade sweep for low speed axial fans. ASME Journal of Turbomachinery 112 pp. 151-158.

Zanon A, De Gennaro M, Kuehnelt H, Langmayr D, Caridi D (2014) Investigation on the influence of mesh topology and freestream turbulence intensity in broadband noise prediction af axial fans. Proceedings of ASME Turbo Expo 2014: Turbine Technical Conference and Exposition, Düsseldorf, Germany, doi: 10.1115/GT2014-26858.

Zenger F, Herold G, Becker S (2016) Acoustic characterization of forward- and backward-skewed axial fans under increased inflow turbulence. Proceedings of 22nd AIAA/CEAS Aeroacoustics Conference, Lyon, France.

Zenger F, Junger C, Kaltenbacher M, Becker S (2016) A benchmark case for aerodynamics and aeroacoustics of a low pressure axial fan. SAE Technical Paper 2016-01-1805.

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Fluid Mechanical and Acoustic Characterization of Low ...ltces.dem.ist.utl.pt/lxlaser/lxlaser2016/finalworks2016/papers/02... · This sound mechanism is governed by vortex shedding