In order to survive, organisms in nature have undergone billions of years of evolution; their body structures have been adapted to the current environment and exhibit special functions on biological surfaces . For the purpose of drag reduction, valuable inspiration can be derived from rapidly moving animals, such as the “denticles” found on the surface of shark skin, which enable high-speed swimming , as well as the texture of bird feathers . The phenomenon of drag reduction can also be observed on the surface of plants. For example, there is a superhydrophobic structure on the surface of lotus leaves . A thin gas film captured by the superhydrophobic structure creates a slip interface between gas and liquid, which effectively improves the drag reduction and antifouling performance of lotus leaves . However, the structures on biological surfaces are rather complex and not directly applicable in practice. Therefore, researchers have explored the drag reduction mechanisms through replication or imitation of the microtexture found on biological surfaces; it was found that the contribution of microtextures to drag reduction primarily occurs within the boundary layer . Lang et al. constructed rectangular and sinusoidal grooves with 2 mm in width, 3 mm in depth, and 1 mm in spacing, thus mimicking the transverse grooves on the surface of dolphin skin. They observed the effect of the grooves on flow separation and boundary layer using digital particle image velocimetry. Xiao et al. analyzed the drag reduction mechanism of bionic microtextures and constructed simplified V-shaped, trapezoidal, and wavy ribs by grinding. Experimental and simulation studies on aeroengine blades with such microtextures showed that the drag reduction performance of wavy ribs is better than that of the other two structures. Tian et al. pointed out that, because of the complexity of microstructures on the shark skin surface, it is difficult to use a uniform method to characterize the skin surface. Triangular grooves or rectangular grooves can be used to simplify the microstructures on the shark skin surface to study the effects on hydrodynamics and aerodynamics.

Figure 1: Microtextures with different sectional shapes: (a) triangles; (b) trapezoids, and (c) ellipses. The variables s, h, L and U represent the spacing, height, length, and the inflow velocity, respectively.

The drag reduction effect of biomimetic microtextures can reduce friction and turbulence pulsation on blade surfaces, thus, improving the aerodynamic performance of blades . The research on drag reduction of microtextures on blade surfaces can be traced back to the 1980s. In 1982, Walsh et al. from NASA Langley Laboratory conducted a pioneering microtexture study on surfaces. Their experiments focused on longitudinal grooves with various shapes, revealing that symmetrical V-shaped grooves exhibited a remarkable drag reduction effect at low flow rates. The highest drag reduction rate (DRR) attained was 8%. Chamorro studied fans with a grooved surface and found that, under certain operating conditions, the drag reduction effect of local coverage on the textured blade surface surpassed that of a complete covering. Additionally, they designed microgrooves of various sizes on the suction surface to achieve the optimal drag reduction effect. Zhang et al. proposed a method to determine the placement position of microtextures by using finite element analysis. The suggested microtextures were arranged on blade surfaces and exhibited drag reduction compared to smooth blades. Mischo et al. improved the cooling capability of turbine blades by adding grooves to the blade tips of axial turbines. Experimental and numerical simulation results showed that the addition of grooves increased turbine efficiency by 0.2% and 0.38%, respectively. In order to reduce aerodynamic losses in turbines caused by tip leakage, Parkash et al. added grooves at the blade tips and verified their effectiveness through computational fluid dynamics (CFD) simulations. After the incorporation of grooves, the turbine efficiency improved by 0.1% to 0.2%.

It is evident that arranging microstructures on blade surfaces can optimize the aerodynamic performance of the blades, thereby achieving energy savings. However, the rational arrangement of microstructures on blade surfaces also requires investigation, as the shape, size, and placement position of microstructures can all affect drag reduction performance . Wu et al. investigated the effect of different sizes of triangular grooves on the drag of NACA 0012 airfoils, finding an optimal DRR of 9.65% when the microstructure dimensions were s = h = d = 0.1 mm. Liang et al. arranged various sizes of triangular microstructures on rotating disks and found through comparative analysis that microtextures with dimensions of s = 0.5 mm and h = 0.2 mm yielded a maximum DRR of 8.46%, with the DRR varying with changes in Reynolds number. Yang and Baeder designed wavy structures on the trailing edge (TE) of a wind turbine blade to reduce the recirculation flow and coherent vortex shedding; the influence of wave depth, TE thickness, and chord length on the drag force were investigated by numerical simulations. The results indicated that the maximum drag was observed at a ratio of wave depth/TE thickness = 0.25. Hossain et al. constructed inward and outward dimple-like structures on the upper surface of a NACA 4415 airfoil. The results from wind tunnel experiments showed that the dimples on the airfoil surface delayed flow separation; the inward dimples increased lift by 16.43% and reduced drag by 46.66%. However, while these researches have yielded encouraging achievements in the application of microtextures on blade surfaces, achieving good drag reduction performance relies heavily on the appropriate microtextures regarding size, type, and position. The determination of such microtextures is typically impractical for the following two reasons: (1) There is a lack of analysis on the flow field over smooth blades. The phenomenon of flow separation on blades occurs because of the complex curved surface in the air flow. The placement position of microstructures needs to be determined based on the locations of flow separation on blade surfaces. (2) The analysis of drag reduction performance is mostly based on numerical simulation results, with a lack of technology research regarding the processing of microtextures and of analysis of experimental results. On the one hand, the small size of microtextures may lead to significant errors in conventional machining methods, thereby increasing the difficulty of verifying microtexture drag reduction. On the other hand, the cost associated with experiments required for microtexture testing, such as wind tunnel tests, is high.

The present study employs numerical simulations, high precision milling, and wind tunnel experiments for solving the two problems discussed above. The main contributions of this article are as follows: (1) Based on the highly symmetrical characteristics of rotating machinery such as compressors, a new numerical simulation method for blade analysis was proposed to determine the flow field on the smooth blade surface and provide references for the placement of microtextures. (2) The influence of microstructure size on drag reduction of blade surfaces was analyzed to determine the optimal parameters for microstructures. The drag reduction mechanism of the microstructures was also analyzed based on simulation results. (3) A high-precision five-axis computerized numerical control (CNC) milling machine was used to process the microtextured blades. In order to obtain high-quality surfaces on the microtextured blades, three types of end milling tools were utilized to rough- and fine-mill blade and microtextures. The microtextured blades were tested in a wind tunnel to obtain drag reduction results.

This section introduces the modeling of microtextures on blade surfaces, as well as the equipment and processes in the actual processing of microtextures. Finally, the wind tunnel experiment platform used to measure the drag reduction of the microtextured blades is described.

Figure 3: Flow chart of the simplified numerical simulation method. Images used courtesy of ANSYS, Inc.

The compressor model has rotational symmetry, and each blade is uniformly installed on the compressor. Therefore, the compressor model was evenly divided according to the number of blades to obtain a calculation domain model including a single blade and a single flow channel. In the simulation setup, the walls on both sides of the channels were set as periodic boundaries, which can simulate the flow domain with symmetry and make up for the calculation error caused by the simplified model. Through the above simplification, the calculation cost can be greatly reduced while ensuring calculation accuracy.

The microtexture placement position is determined according to the flow field of the smooth blade. Flow separation occurs during high-speed air flow over the curved blade surface. Here, a reasonable arrangement of microtexture can effectively improve the drag reduction. Therefore, CFD was employed to simulate a single channel model of the blade and the flow separation region was obtained. In order to verify the accuracy of the simulation calculation, the theoretical calculated value of the angle of attack was compared with the simulation results.

First, the drag reduction performance of four microtextures was compared by numerical simulations to determine the geometric type with the optimal drag reduction. Then, different widths (w), spacings (s), and heights (h) of the microtextures were compared to determine the scale range with drag reduction. In the simulation setup, the initial conditions and the flow domain are consistent with the single flow domain of the blade. The coefficient of friction and the DRR from the simulation results were compared to determine the geometric types and size ranges of the microtexture with drag reduction performance.

According to the flow field on the smooth blade surface, groove and rib microtextures were arranged before and after the flow separation point on the suction surface. The difference between grooves and ribs will described in the passage referring to step 4 in the Results and Discussion section. Also, based on the microtexture types and size range determined in step 3, the drag reduction results of grooves and ribs with different size parameters were compared to determine the combination of microtexture parameters with the best drag reduction performance. This parameter combination was employed for machining microtextures on the blade surface, and the microtextured blade was placed in the wind tunnel for experiments.

Here, the details about the simulation setup method and the determination method of microtextures in step 1 and step 3 of the proposed method are described. To enhance readability, the relevant results from steps 2, 3, and 4 will be analyzed in the Results and Discussion section.

Figure 4: The size of flow domain and the single blade.

Figure 5: Meshed model of (a) a smooth blade surface and (b) a microtextured surface with 0.005 mm height of the first layer mesh according to y+ = 1.5. Images used courtesy of ANSYS, Inc.

Figure 6: Characteristic parameters of (a) grooves and (b) ribs.

The dimensionless size calculation formula of microtextures with drag reduction performance are as follows :

where μ is the dynamic viscosity, v is the kinematic viscosity, u is the average flow velocity, uτ is the wall stress shear rate, τw is the wall shear stress, and ρ is the density.

where δ is the thickness of the boundary layer. The flow condition around the flat plate wall can be determined by the dimensionless local Reynolds number.

where x is the distance from the inlet along the fluid flow direction. For Rex < 3 × 105, the flow in the boundary layer is laminar, and the following equation yields δL:

The flow is turbulent if Rex > 3 × 106, and the thickness of δT is calculated as:

Table 1: Microtexture parameters.

The overall process of experiments involves machining the microtextures on the blade surface and conducting experiments with the microtextured blades in a wind tunnel. The Results and Discussion section will give details about the determined microtexture types and sizes.

Table 2: Overview of the equipment used in the experiment.

a⌀(tool diameter in mm) × (cutting edge length in mm) × ⌀(shank diameter in mm) × (overall length in mm) × (number of flutes)F.

Figure 7: The manufacturing procedure of the microtextured blade.

Table 3: Tool parameters in different machining stages.

Figure 8: Experimental platform and schematic diagram. (a) Wind tunnel test platform. (b) Three-hole probe measuring device. (c) Schematic diagram of the wind tunnel test platform.

To ensure the accuracy of experiments, the velocity and angle of attack for blade heights of 0.25, 0.5, and 0.75 were selected to carry out multiple tests to verify the drag reduction effect of the microtextured blade. The specific steps of the experiment are as follows: (1) preparation of two blades in contrast, that is, one smooth blade and another blade with a microtexture on the surface; (2) test the smooth blade first; adjust the wind tunnel flow velocity and the angle of attack to 123.98 m/s and 52.8°, respectively; (3) measurement of the TP and P at the inlet and outlet, respectively, and calculation of ξ and observation of the wake loss distribution; (4) change of velocity and angle of attack to 130.67 m/s and 54.8°, respectively, and continuation according to step 3; (5) change of velocity and angle of attack to 137.54 m/s and 57.0°, respectively, and continuation according to step 3; (6) installation of the blades with microtextures and repetition of steps 3–5. The results obtained from the above steps will discussed in the “Results of the experiments” section.

Figure 10: Simulation values of attack angle at (a) span = 0.25, (b) span = 0.50, and (c) span = 0.75. x is the axial direction, y is the radial direction, and z is the rotation direction of the blade. Images used courtesy of ANSYS, Inc.

Table 4: The comparison of simulated and theoretical values of the angle of attack.

Table 5: Aerodynamic parameters of the smooth blade.