ASME Turbo Expo 2017: Turbomachinery Technical Conference and ExpositionJune 26 - 30, 2017, Charlotte, NC, USA

GT2017-63960

REDUCTION OF PRESSURE LOSSES IN A LINEAR CASCADE USING HERRINGBONE RIBLETS

Qiang Liu, Shan Zhong and Lin Li

School of Mechanical, Aerospace and Civil Engineering, Manchester University,Manchester, M13 9PL, UK

ABSTRACT

This paper presents the results from the first experimental assessment of herringbone riblets in reducing total pressure losses in a linear cascade of diffuser blades. The experiments were undertaken at Re=1×105, M=0.13 and a free stream turbulent intensity of 2%. Three cascade configurations were examined at a blade incidence angle of 0.8˚; Case A: the baseline case without surface modification; Case B: blades with smooth strips; Case C: blades with ribleted strips. In Case A, flow separation starts at 24.1%c from the blade leading edge followed by a complete stall, resulting in significant total pressure losses as measured by a five-hole probe on a cross-flow plane downstream. Seven smooth or ribleted strips were adhered on the blade suction surfaces along their span in Case B and Case C. In comparison to Case A, the average total pressure loss coefficient is decreased by 6.4% and 16.8% in Case B and Case C, respectively. The velocity vectors leaving the cross-flow measurement plane also appear to be more uniformly distributed with the average flow turning angle being increased by 4˚ and 10˚ in Case B and Case C respectively, indicating that the extent of flow separation in the cascade has been reduced substantially. Furthermore, a pseudo sound power analysis of hot-wire data in the blade wake reveals a reduction in the noise level of 1.1dB and 1.6 dB, respectively. These results hence provide strong evidence that a profound aerodynamic improvement can be achieved in a cascade with the use of herringbone riblets.

1 INTRODUCTION

Driven by the need to decrease blade counts so as to reduce the overall component weight, axial compressor blades are designed to bear high loading and hence are prone to flow separation, especially at off-design operating conditions. The tendency toward higher blade loading demands that methods, which are capable of reducing pressure losses in compressors over a wide range of operating conditions, are constantly being sought.

Both passive and active methods have been explored for their potential in suppressing flow separation in axial compressors. Active flow control methods, such as vortex generating jets [1], acoustic excitation [2] and, most recently, plasma actuators [3], have been actively studied. Nevertheless, passive control methods are often more preferred by industry due to simplicity in implementation and their low cost-to-benefit ratio. Numerous types of passive flow control devices of macro size have been investigated, including low profile vortex generators [4-6], cavities [7], leading edge fillets [8] and surface roughness [9]. These passive methods delay flow separation either by triggering boundary layer transition before flow separation starts or by introducing flow instabilities that promote transition in the separated boundary layer. However, despite their ability to reduce the size of separation region, an overall loss reduction may not be achieved because of the extra loss introduced by these macro scale structures.

Inspired by the micro surface patterns on shark skins, longitudinal riblets have been studied for their potential of reducing surface friction drag. The pioneering studies carried by Walsh et al. [10] reported a surface friction drag reduction of 8% on flat surfaces, when the spacing between the peaks of adjacent riblets takes the optimal value of 17 viscous wall units. Lietmeyer et al. [11] applied

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longitudinal riblets on compressor blades and obtained a reduction in profile losses of 7.2% when the riblets are applied after the transition point on the blade suction surfaces.

More recently, herringbone riblet patterns found on bird feathers began to receive research attentions. Chen et al. [12] carried experiments in a pipe flow at a range of Reynolds numbers and reported a friction drag reduction of 16% with the spacing between the peaks of adjacent riblets being 20 viscous wall unit; a level of drag reduction notably higher than that the traditional streamwise microgroove riblets can offer. A parametric investigation undertaken by Nugroho et al. [13] in a turbulent boundary layer revealed that the herringbone riblets generate counter-rotating streamwise vortices in the near-wall region, see Figure 1. It is believed that these vortices result in a pronounced spanwise moderation of the boundary layer, providing a mechanism for suppressing the near-wall turbulence production and hence reducing the surface friction drag.

Figure 1 COUNTER-ROTATING VORTEX MODE PROPOSED BY Nugroho et al. [13]

Considering the streamwise nature of the vortices produced by herringbone riblets, the authors of this paper hypothesize that these vortices may function in a similar way as those produced by traditional vane vortex generators hence offering an effective mechanism for suppressing flow separation. Therefore, in this study the effects of a novel bio-mimetic herringbone-riblet pattern on the pressure losses in a linear cascade were investigated for the first time. A state-of-the-art laser manufacturing technique was employed to produce well defined herringbone-riblet patterns. A five-hole probe was used to measure the total pressure loss and flow turning angle at a cross-flow plane downstream of the cascade. Furthermore, hot-wire measurements were also carried out to evaluate the effects of these riblets on the frequency spectra of the unsteady flow in the cascade which infer the local sound power levels. The objective of this work is to provide an experimental assessment of the effectiveness of herringbone riblets in flow separation control in a linear cascade at low Reynolds numbers whereby paving the way for their potential applications on compressors.

2 EXPERIMENTAL FACILITIES AND METHODS

2.1 The Linear Cascade

Our experiments were carried out in a cascade wind tunnel located at the University of Manchester. A schematic of the tunnel layout is shown in Error: Reference source not found. The maximum flow speed achievable in the test section is 120 m/s.

Figure 2 LAYOUT OF THE LINEAR CASCADE

z’

x’

y’c’c

p

Measurement plane

β1

l1

β2

Figure 3 SCHEMATIC OF THE BLADE CASCADE

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Error: Reference source not found shows a schematic of the cascade used in the experiments. The cascade is made up of 13 diffuser blades with a chord length of c=31.0 mm and a height of 51.2 mm. The maximum thickness of the blades is 2.5 mm at 34%c. The inlet and exit angles of the cascade are 64.2˚ and 3.9˚, respectively, resulting in a blade turning angle of 60.3˚. Adjustable tailboards are mounted at the end of the cascade to ensure a periodic outflow. In our setup, for a given blade cascade the blade incidence angle can be altered within the range of -10˚ < < +10˚ with the use of a set of interchangeable flow guide vanes with different exit angles, which is mounted between the contraction section and the cascade inlet. The main geometrical parameters of the cascade are summarized in Table 1.

Table 1 CASCADE DESIGN PARAMETERS

True Chord length (c) 31.0 mmAxial chord length (c’) 26.1 mmPitch (p) 15.9 mmPitch to chord ratio (ξ1) 0.513Span (sp) 51.2 mmAspect ratio (ξ2) 1.65Inlet blade angle (β1) 64.2˚Exit blade angle (β2) 3.9˚

2.2 Herringbone Riblets

The geometric parameters of riblets are defined in Error: Reference source not found. The ribleted strip was designed to have a length of lr=18 mm and a width of br=6.5 mm. The divergent angle of the grooves is =60˚, with 30˚ on each side of the centre line. The groove width and depth are 300 μm and 80 μm respectively. A summary of the geometry of riblet strips is given in Table 2. The herringbone riblets were made using an advanced laser manufacturing technique. The images of the actual riblets shown in Error:Reference source not found reveal that the microscopic structure of individual riblets is well defined.

Figure 4 GEOMETRIC PARAMETERS OF HERRINGBONE RIBLETS Figure 5 IMAGES OF HERRINGBONE RIBLETS

The riblets are produced on a foil with a thickness of 120 μm. The foil is then glued onto the suction surface of the blades as shown in Error: Reference source not found. A total of seven riblet strips are fixed across the span of each blade with spacing between the centreline of riblet strips of 6.5 mm. The ribleted foils stretch from 37%c to 90%c along the streamwise direction. The start location of the riblets is located between the location of flow transition and separation on the suction surface predicted by a CFD simulation of the same cascade. To ensure periodicity of the cascade flow, riblets are fixed on the suction surfaces of the six blades located in the middle region of the cascade.

Table 2 GEOMETRY OF THE RIBLETED STRIPS

Dimension

s 300 μmh 80 μmlr 18 mmbr 6 mm

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θ 60˚2.3 Experimental Methods

To evaluate the pressure losses in the cascade flow passage, measurements with a five-hole probe were performed. The probe has a diameter of a 2.7 mm. Before the experiments, the five-hole probe was calibrated at 20 m/s, 30 m/s, 40 m/s and 50 m/s for pitch and yaw angles up to 20˚ with 5˚ increments. With the aid of the calibration chart of the probe shown in Figure 7, the total pressure, the three velocity components and flow angles at the measurement point can be obtained by post-processing the data obtained with the probe in the subsequent experiments. Based on our analysis, the measurement uncertainty in total pressure and flow direction is less than 2% and 0.4˚, respectively.

6.5mm

37%c

6mm 45mm

10%c

LE TE

Figure 6 RIBLETED STRIPS ON SUCTION SIDE SURFACE

-10 -5 0 5 10-10

-5

0

5

10

Cppitch

Cpyaw

-10 -5 0 5 10-10

-5

0

5

10

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Cpyaw-10 -5 0 5 10

-10

-5

0

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10

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a) 20m/s b) 30m/s

c) 40m/s d) 50m/s

-10 -5 0 5 10-10

-5

0

5

10

Cppitch

Cpyaw

Cppitch

Cpyaw-10 -5 0 5 10

-10

-5

0

5

10

Figure 7 CALIBRATION CHART OF THE FIVE-HOLE PROBE

Figure 8 MEASUREMENT POSITIONS

The measurements with the five-hole probe were made on a cross-flow plane parallel to the cascade exit plane located at a chord length downstream. The probe was traversed along five spanwise lines 3.25 mm apart as shown in Figure 8. Each line had 43 equally spaced measurement points spanning across three flow passages. At each measurement point, 10000 data samples were collected at a sampling frequency of 1 kHz. With the total pressure data deduced from the measurement, the total pressure loss coefficient γ as defined below can be obtained,

(1)

where PT,in is inlet total pressure, measured at the centre of settling chamber with a Pitot tube, PT,out is the total pressure obtained at a given point on the measurement plane, PS,in is static pressure at the cascade inlet.

In this experiment, a hot-wire probe was used to capture the velocity fluctuations on the same measurement plane from which the effects of herringbone riblets on the unsteady characteristics of the flow can be deduced. The probe was traversed across the mid-span of a blade passage. The signals were collected at a sampling frequency of 50 kHz for 10 seconds and low-filtered at 20 kHz.

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3 RESULTS AND DISCUSSION

Our experiments were conducted at a free stream velocity of 45m/s and a turbulence intensity of 2%. The Reynolds number based on the chord length of the blades is 1×105. In order to examine the effects of herringbone riblets on the performance of the linear cascade, three test cases have been studied, Case A: the baseline case without surface modification; Case B: blades fitted with strips without herringbone riblets; Case C: blades fitted with strips of the same shapes and size as in Case B with herringbone riblets. A comparison between Case A and C enables the effects of herringbone riblet foil strips to be established whereas a comparison between Case A and B enables the effect of foil strips with a finite thickness to be assessed. The blades tested in three cases are shown in Error: Referencesource not found.

3.1 Oil Flow Visualization of the Baseline Flow

Figure 10 shows the oil flow visualization images obtained on the suction surface of four blades in the cascade in the baseline case without riblets (Case A). At a first sight, the most distinct feature in the flow over the suction surface appears as a laminar separation bubble located between s1 and s2. The start and end location of this bubble is very similar for all four blades (between 0.24c and 0.46c), indicating a good periodicity of the flow within the cascade. However, an observation of the movement of oil dots placed on the surface downstream of s2 reveals the presence of a reverse flow going all the way back from the trailing edge to s2. Therefore, it is speculated that the flow over the suction surface is dominated by a relatively stationary recirculation zone followed by a complete stall downstream as depicted in the schematic shown in Figure 11. Such a flow pattern is plausible for blades of a high turning angle. The above flow model is consistent with the presence of a region with high pressure losses revealed by the five-hole probe located directly downstream.

a) without strips b) with smooth strips c) With ribleted strips

Figure 9 THREE CONFIGURATIONS OF BLADES

5 6 7 8Blade number

s1s2

Figure 10 OIL FLOW PATTERNS ON FOUR BLADES WITHOUT REBLETS

Recirculation zone

stall

Separation position

s1

s2

Figure 11 SCHEMATIC OF FLOW PATTERN

3.2 Five-hole Probe Measurements

3.2.1 Total Pressure Loss Coefficient

Figure 12a shows the contour of total pressure loss coefficient, γ, in the baseline case. γ is periodically distributed across the cascade. In each blade passage, the flow can be divided in two zones; i.e. a high pressure loss coefficient zone (red) and a low pressure loss

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coefficient zone (blue and yellow). The former zone corresponds to the separated flow from the suction surface of the blade, whereas the latter zone corresponds to the potential flow in the blade passage. The slight contraction of the high total pressure loss zone away from the mid-span is consistent with the slight downstream shift of the start of flow separation towards the two endwalls as revealed in the oil flow visualisation images (Figure 10). In the baseline case, the average peak total pressure loss coefficient of the three passages, ap, is found to be 0.65 and the mean total pressure loss coefficient across the three passages, a, is 0.51.

Y

Z

X

Y

Z

X

Y

Z

X

a) Without stripes

b) With smooth stripes

c) With ribleted stripes

Pressure loss coefficient

z’

y’x’

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Y

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a) Without stripes

b) With smooth stripes

c) With ribleted stripes

Pressure loss coefficient

z’

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a) Without strips

b) Without smooth strips

c) Without ribleted strips

b) With smooth strips

c) With ribleted strips

Figure 12 COMPARISON OF TOTAL PRESSURE LOSS

With the smooth strips being added on the suction surfaces, the magnitude of the peak total pressure loss coefficient is reduced (see Error: Reference source not foundb). γap and γa are found to decrease to 0.61 and 0.48, respectively. Compared with the smooth strips, the ribleted strips apparently result in an even greater reduction in total pressure losses, giving arise to a γap of 0.49 and a γa of 0.43 (see Error: Reference source not foundc). Zones of high total pressure losses disappear, indicating that flow separation has been eliminated in this case. It is noted that the effect of the smooth and ribleted strips on the total pressure losses in different blade passages is not quite the same. This may be caused by slight variations in the process of adhering individual strips on the blade surfaces.

To aid comparison, the average peak total pressure loss coefficient and the mean total pressure loss coefficient for all the three cases are listed in Table 3. In comparison to the baseline case (Case A), the average total pressure loss coefficient is found to have been reduced by 6.4% and 16.8% in Case B and Case C, respectively.

Table 3 TOTAL PRESSURE LOSS COEFFICIENT

Parameters Case A Case B Case Cγap 0.65 0.61 0.49γa 0.51 0.48 0.43

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3.2.2 Flow Turning Angle

The blades tested in our experiment have a turning angle of 60.3˚. Due to the presence of flow separation in the blade passage, however, the actual flow turning angle will be reduced substantially. The flow turning angle hence indicates how well a blade passage is capable of guiding the flow in a predetermined flow path.

Figure 13 DEFINATION OF TURNING ANGLE AT MEASUREMENT POINTS

X Y

Z

Without foils

With ribleted foilsWith smooth foils

A B

X Y

Z

Without stripes

With ribleted stripesWith smooth stripes

A Bz’ X Y

Z

With out stripes

With r ibleted stripesWith smooth stripes

A Bz’

Without stripsWith smooth stripsWith ribleted strips

Figure 14 COMPARISON OF FLOW TURNING ANGLES BETWEEN THE THREE CASES

In our experiment, the variations in flow turning angles on the measurement plane are deduced from the pressure reading obtained with the five-hole probe. It is the angle between the incoming free stream flow velocity vector and the local velocity vector at a point of interest on the x’y’ plane, as is shown in Error: Reference source not found. For the present cascade setup, in an ideal situation the flow will leave the flow passage at a very small angle with respect to the ox’ axis. Due to flow separation, the velocity vectors will be deflected to the left and further away from the ox’ axis, resulting in a reduced flow turning angle.

Error: Reference source not found displays the velocity vectors leaving the measurement plane in the three cases. To aid illustration, the velocity vectors have been rotated by 90˚ clock-wisely around the oy’ axis so as to project them onto the y’z’ plane. To aid identification of the wake region, the contours of total pressure loss coefficient in the baseline case are superimposed onto the measurement plane. In the baseline case, the region of reduced flow turning angles appears to coincide with the region of high total pressure loss coefficient (region A) where the wake of the separated flow is located. In general, the addition of either the smooth strips or ribleted strips results in an increase in flow turning angle in region A and a slight decrease in flow turning angle in the potential flow (region B). Overall, the velocity vectors appear to be more uniformly distributed across the blade passages due to suppression of flow separation on the suction surfaces of the blades.

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The flow turning angles along the blade mid-span across the three blade flow passage are plotted in Error: Reference source not found to enable a direct comparison of the trend between the three test cases. Again compared to the baseline case, the magnitude of fluctuation in the flow turning angle is reduced with the smooth strips and it is reduced further with the ribleted strips. On average, the flow turning angle is increased by about 4˚ with the smooth strips and by about 10˚ with the ribleted strips.

3.3 Pseudo Sound Power Level

During the experiments, it was noticed that the addition of either smooth or ribleted strips on the cascade blades produces audible differences in the noise levels emitted by the cascade rig. A hot-wire probe was hence used to capture the difference in the unsteady flow characteristics downstream of the cascade blades in the three cases with an attempt to infer the impact of surface modification on the noise levels.

The hot-wire measurements were made along the mid span. Fast Fourier Transformation was then preformed on the probe voltage fluctuations using the Welch spectral estimation method. The sound pressure level (SPL) at a given frequency can be assumed to be proportional to the square of local velocity fluctuation, u2, at the corresponding frequency. In the present experiments, with the use of an uncalibrated hot-wire probe, the voltage fluctuations, e, instead of the velocity fluctuations is used in the analysis, i.e.

SPL≈10× log10 e2

(2)

Although the local mean velocity is increased at the measurement point with surface modification, the nonlinearity of the hot-wire calibration curve over the corresponding range of mean velocity can be regarded as negligible. As such, probe voltage fluctuations can be used to estimate the change in the sound power level.

Error: Reference source not found shows a comparison of the spectra of pseudo sound power level in the three cases at measurement point A (see Figure 8 for its location), which is close to the location of the free shear layer of the separated flow over the suction surface. Note that the absolute value of the power level has no specific meaning due to the analogy used. The frequency content above 10 KHz is not shown here since the anemometer used in this experiment does not respond to frequencies higher than 10 kHz. From Error: Reference source not found, one can see that at frequencies above 800 Hz or so, both ribleted strips and smooth strips are capable of reducing the sound power level to some extents. Nevertheless, the ribleted strips appear to be more effective in reducing the sound power level at frequencies lower than 800 Hz whereas the smooth strips produce almost no impact on this frequency range.

To quantify the difference in the overall sound pressure level relative to the baseline case, the root-mean-square values of hot-wire voltage fluctuations are used, i.e.

ΔSPL≈10×log10e2

eref2

(3)

where the reference value is that of the baseline value. It is found that in comparison to the baseline case the sound power level is reduced by 1.6 dB with the smooth strips and by 1.1dB with ribleted strips, respectively. The observed differences in sound power levels are a direct consequence of an alternation of the separated flow over the suction surfaces of the blades.

3.4 Discussions

Based on the results presented above, in comparison to the baseline case both the smooth strips and the ribleted strips appear to produce beneficial aerodynamic effects. More specifically, both of them act to reduce the extent of flow separation over the blade suction surfaces leading to a reduction in the total pressure loss, an increase in the flow turning angle and a reduction in the noise level produced by the flow. It is also evident that the ribleted strips produce a more profound effect in this regard.

In our experiment, the thickness of the foil strips is 120m and it is comparable in size to the local boundary layer thickness at the leading edges of the strips. Such a step may act as a surface roughness which causes the laminar boundary layer to breakdown into a turbulent one. We believe that the reduction of flow separation produced by the smooth strips is likely to be caused by the tripping of the laminar boundary layer by the leading edge of the strips.

On the other hand, we believe that it is the counter-rotating streamwise vortices generated by the highly directional surface patterns on the ribleted strips (similar to those proposed by Nugroho et al. [13]) which provide the mechanism behind the larger improvement in aerodynamic performance offered by the ribleted strips observed in our experiment. These vortices energise the low-momentum flow

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in the near-wall region by entraining the high-momentum fluid from the potential flow, which subsequently suppresses the massive flow separation on the suction surfaces of the blades.

Without stripes

With ribleted stripesWith smooth stripes

Turn

ing

angl

e(̊)

y’(inch)

A BWithou t stripes

With ribleted stripesWith smooth stripes

Turning angle(̊)

y’(inch)

A B

100 115 130 145 16010

20

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Without stripsWith smooth stripsWith ribleted strips

102

103

104-70

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-45

Pow

er(d

B)

f (Hz)

C

D

Blade with smooth stripesBlade with ribleted stripes

Blade without stripesWithout stripsWith smooth stripsWith ribleted strips

Figure 15 COMPARISON OF SPECTRA OF PSEUDO SOUND POWER LEVEL AMONG THE THREE CASES

Figure 16 FLOW TURNING ANGLES ALONG MID-SPAN

4 CONCLUSION

This paper presents the results from the first experimental assessment of herringbone riblets in reducing total pressure losses in a linear cascade of diffuser blades. The experiments were undertaken at Re=1×105, M=0.13 and a free stream turbulent intensity of 2%. Three cascade configurations were examined at a blade incidence angle of 0.8˚; Case A: the baseline case without surface modification; Case B: blades with smooth strips; Case C: blades with ribleted strips.

In Case A, flow separation starts at 24.1%c from the blade leading edge followed by a complete stall, resulting in significant total pressure losses as measured by a five-hole probe on a cross-flow plane downstream. Seven smooth or ribleted strips were added on the blade suction surfaces along the blade span in Case B and Case C. In comparison to Case A, the average total pressure loss coefficient is decreased by 6.4% and 16.8% in Case B and Case C, respectively. The velocity vectors leaving the cross-flow measurement plane also appear to be more uniformly distributed with the average flow turning angle being increased by 4˚ and 10˚ in Case B and Case C respectively, indicating that the extent of flow separation in the cascade has been reduced substantially. Furthermore, a pseudo sound power analysis of hot-wire data in the blade wake reveals a reduction in the noise level of 1.1dB and 1.6 dB, respectively. These results hence provide strong evidence that a profound aerodynamic improvement can be achieved in a cascade with the use of herringbone riblets.

In a follow-on experiment, we will investigate the effects of herringbone riblets at different angle of attacks and Reynolds numbers. A parametric study will also be performed to optimise the width and depth of the riblets for the blades.

5 NOMENCLATURE

Blade incidence anglebr Ribleted strip widthc Blade chord lengthc’ Blade axial chord length in local coordinate systemh Riblet groove depthlr Ribleted strip lengthp Blade pitch lengthPT,in Inlet total pressurePT,out Outlet total pressurePS,in Inlet static pressure

Re Reynolds numbers Riblet groove widthTu Turbulence intensityU Inlet freestream velocityξ1 Blade Pitch to chord ratioξ2 Blade aspect ratio γ Total pressure loss coefficientγa Average total pressure loss coefficientγap Average peak total pressure loss coefficientθ Riblet divergent angle

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6 ABBREVIATIONS

LE Leading edgeTE Trailing edge

7 ACKNOWLEDGEMENT

The authors would like to acknowledge the Engineering and Physical Sciences Research Council (EPSRC) in the UK and Dyson Technology Ltd for their financial support to this work. Our particular thanks also go to the aero acoustic team at Dyson Technology Ltd for many useful discussions. We also would like to thank, Mr Yihe Huang for his assistance during the manufacturing process of the riblets, and Mr Yufei Jin, for his assistance during the cascade experiments.

8 REFERENCES

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[2] Bernardini, C., Benton, S. I., and Bons, J. P., 2013, "The Effect of Acoustic Excitation on Boundary Layer Separation of a Highly Loaded LPT Blade," Journal of Turbomachinery, 135(5), p. 051001.

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[6] Hergt, A., Meyer, R., and Engel, K., 2006, "Experimental investigation of flow control in compressor cascades," ASME Paper No GT2006-90415, pp. 231-240.

[7] Bur, R., Corbel, B., Dé, J., and lery, 1998, "Study of passive control in a transonic shock wave/boundary-layer interaction," AIAA Journal, 36(3), pp. 394-400.

[8] Mahmood, G. I., and Acharya, S., 2007, "Experimental Investigation of Secondary Flow Structure in a Blade Passage With and Without Leading Edge Fillets," Journal of Fluids Engineering, 129(3), p. 253.

[9] Gbadebo, S. A., Hynes, T. P., and Cumpsty, N. A., 2004, "Influence of surface roughness on three-dimensional separation in axial compressors," ASME Paper No. GT2004-53619, , pp. 471-481.

[10] Walsh, M. J., 1983, "Riblets as a Viscous Drag Reduction Technique," AIAA Journal, 21(4), pp. 485-486.[11] Lietmeyer, C., Oehlert, K., and Seume, J. R., 2012, "Optimal Application of Riblets on Compressor Blades and Their Contamination Behavior,"

Journal of Turbomachinery, 135(1), p. 011036.[12] Chen, H., Rao, F., Shang, X., Zhang, D., and Hagiwara, I., 2014, "Flow over bio-inspired 3D herringbone wall riblets," Experiments in Fluids,

55(3), p. 1698.[13] Nugroho, B., Hutchins, N., and Monty, J. P., 2013, "Large-scale spanwise periodicity in a turbulent boundary layer induced by highly ordered

and directional surface roughness," International Journal of Heat and Fluid Flow, 41, pp. 90-102.

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