**3.10. Roughness material**

The control technique with the roughness material, which is one of the fundamental objectives of the chapter, can passively control the flow over wind turbine blade operating at low Reynolds number ranges. Regarding the development of vortex structures at the wake of VG applications [65], the flow is re-energized with the vortices produced by miniramps as denoted in **Figure 13**. Furthermore, the flow is inherently gaining momentum by means of that passive flow controller. Therefore, the separated flow because of adverse pressure gradients that occurred generally at leading edge of airfoils may be suppressed with re-energized flows, resulting in the occurrence of more stable flow characteristics without boundary layer separation.

As occurred in VG applications, the flow control method by means of roughness material is performed with similar ways by intervening the flow. Vortex sheds produced by roughness cause the flow in the boundary layer to gain more energy as seen in **Figure 14** [66]. Energized flow hinders the boundary layer flow separation and it ensures the flow to move along the airfoil surface by attaching. The vortex sheds can be used for a few different purposes over the surface of airfoils. For instance, the vortex sheds, which were used for recognition of flow phenomena at the study presented by Koca et al. [67, 68], gave the momentum to flow, resulting in lift recovery and even less vibration or noise for wind turbine blades.

Regarding identifying the role of roughness material on the flow characteristics over roughened NACA 4412 airfoil as indicated in **Figure 15**, investigations based on the force measurement, the smoke-wire, hot-film sensor (glue-on type), and hot-wire experiments have been performed by Genç et al. [8, 69]. The purpose of the experimental study was to determine the LSB and transition phenomena over uncontrolled NACA 4412 airfoil in detail and then was to observe how sandpaper as a roughness material affected the flow topology.

**Figure 13.** Vortex structures at the wake of VGs [65].

**Figure 14.** Comparison of ¼" roughness height and vortex shedding characteristics at different Re<sup>c</sup> numbers [66].

**Figure 15.** Representation sketch of the roughened airfoil.

The results, which were obtained from smoke-wire experiment and hot-film sensor, showing an integrated graph were denoted in **Figure 16** [8]. The streamlines obtained from smoke-wire experiment clearly revealed that LSB occurred between x/c = 0.3 and x/c = 0.7 for uncontrolled Traditional and New Types of Passive Flow Control Techniques to Pave the Way for High Maneuverability… http://dx.doi.org/10.5772/intechopen.90552 143

**Figure 16.** Comparison results of two different experiments at Reynolds number of 5 × 10<sup>4</sup> and α = 8°: (a) k/c = 0 (uncontrolled airfoil) and (b) k/c = 0.003 [8].

airfoil, while it was seen between x/c = 0.3 and x/c = 0.5 for the roughened airfoil. It means that using sandpaper causes LSB's size to shrink enormously. As physically speaking, the undulations acquired from voltage values, which were predefined how to obtain in Ref. [8], started to increase after x/c = 0.3, meaning the transition inception and separation point due to adverse pressure gradients in **Figure 16(a)**. However, the amount of undulations at x/c = 0.5 was less than that at x/c = 0.3, because small eddies having less energy in the aft portion of LSB caused the undulations amount to reduce. After x/c = 0.5 point, the obvious increment in undulations indicated that the flow in the boundary layer was fully turbulent because of energized vortices.

**Figure 17** [8] shows a combination graph consisting of numerical and experimental results for a roughened airfoil with k/c = 0.006 at Reynolds number of 5 × 10<sup>4</sup> and α = 8°. At first glance, APG exhibits a dominant role on flow and it causes the flow to separate from the airfoil surface of x/c = 0.3 as depicted in the flow visualization graph. Then, the flow reattaches to solid surface nearly at x/c = 0.6 by gaining momentum by means of roughness material. Same flow phenomena like boundary layer separation, reattachment and LSB are shown and proved with streamlines and Cp curves obtained from the numerical study. The peak point among separation (referred to as S) and reattachment (referred to as R) points reveal the LSB in C<sup>p</sup> curve. The trend of Cp curve is almost constant after separation point due to the presence of dead air region having as negligible as less flow phenomenon. The position of LSB is between x/c = 0.3 and x/c = 0.6 as shown in the smoke-wire experiment result. Besides, a mild peak at x/c = 0.5 indicates the transition point over the airfoil surface.

In addition to the results mentioned above, two more important results were obtained from aerodynamic force measurement results as seen in **Figure 18**. First, the stall phenomenon because of flow separation was pronouncedly postponed in **Figure 18(a)**. Second, lift coefficient (CL) in **Figure 18(b)** increased with the use of roughness material, resulting in enhancement of aerodynamic performance of airfoil. Moreover, it was clearly seen that roughness material gave good results, especially in the pre-stall region. Thus, the

**Figure 17.** The combined results obtained from the numerical and smoke-wire result for the roughened airfoil with k/c = 0.006 at Reynolds number of 5 × 10<sup>4</sup> and α = 8° [8].

Traditional and New Types of Passive Flow Control Techniques to Pave the Way for High Maneuverability… http://dx.doi.org/10.5772/intechopen.90552 145

**Figure 18.** Force measurement results at k/c = 0.006: (a) Re = 7.5 × 10<sup>4</sup> and (b) Re = 1 × 105 [8].

roughness material was firstly entitled as "the pre-stall flow control mechanism" in aerodynamic literature by authors in Ref. [8].

#### **3.11. Flexible membrane wings**

Another essential objective of the chapter in terms of the passive flow control device is doing a detailed survey on flexible membrane wings. The requirement for improving the flight capabilities of MAV and UAV leads to increasing concern in biologically inspired wings. It is well known that the wings of flying animals such as bats resemble a thin membrane-like material with a fixed leading edge and free, scalloped trailing edge that can be easily complied with the flow environment. Moreover, they can regulate the wing planform for a specific flight condition and their flight can be qualified by immensely unsteady and three-dimensional wing motions. A membrane wing is better able to tailor the atmospheric disturbances and makes the vehicle easier to fly [70, 71]. In other respects, the efficacy of the membrane comes from the ability of passive control through the flight as well as decreasing the weight of the wing [70]. Smith [72] paraphrased the emphasis on flexibility and wing stiffness in modeling the flapping motion and generation of the resultant force. A summary presentation study of the aerodynamics of micro air vehicles operating at low Reynolds numbers was carried out by Mueller and DeLaurier [73]. In order to come up with the negative effects of LSB for improving aerodynamic performance, researchers utilized flexible membrane wings for numerous practices such as hang glider, microlight, and UAVs and MAVs. An experimental investigation about time-dependent LSB formation on AR = 3 wing was conducted, and it was seen that in the membrane wings at low Re numbers the LSB was more prevalent. Leading-edge separations were influenced via both Reynolds number and leading-edge vortices occurring because of the separation bubbles led to time-dependent alterations on the vibration of the wing [74, 75]. At low AR, tip vortices delayed stall, exclusively at low Re numbers owing to affecting flow on the wing and separation bubble [76] and analysis of the

instant deformation found out spanwise and chordwise, which were due to the shedding of leading-edge vortices' farther tip vortices [77].

Rojratsirikul et al. [78, 79] searched flow and deformation characteristics of membrane wings with low aspect ratio via velocity and deformation measurement. They found membrane oscillations in second chordwise mode at higher incidences. The dynamic of membrane wing can be altered with excess length [80, 81] and support [82] of the wing. Genç [80] studied on a membrane wing with excess length. The results depicted that camber of membrane wing induced the separated flow; therefore, small separated regions were seen. Besides, Greenhalgh et al. [81] observed that increasing excess length caused to reduce separation incidence, and hysteresis interval concluded a restricted working area for the highest excess lengths. Arbós-Torrent et al. [82] considered the effects of the geometry of front and aft of the wing on the aeromechanics of membrane wings. It was stated that average camber-like membrane fluctuations altered with respect to the geometry and size of both front and aft supports. Besides, the front and aft support having rectangular cross-section everlastingly provided further lift and deformations of mean camber compared with circular cross-sectional support. Galvao et al. [83] studied experimentally on the compliant membrane wings modeled based on mammalian flight mechanics. They showed that three-dimensional (3D) flow and tip vortices were ascendant. Furthermore, the deformation of compliant wings increased with both incidence and deformation increasing. Bleischwitz et al. [84] surveyed membrane wings aeromechanics in ground effect. Digital image correlation (DIC) and proper orthogonal decomposition (POD) were used for obtaining membrane vibrations. It was seen that fluctuation modes were adequate to hold 90% of all deformation energy closes to stall. Moreover, structural modes of spanwise were ensured in virtue of POD in lift increment areas. On the other hand, Hu et al. [85] executed a study on the flapping flexible membrane wings. It was seen that oscillation provided significant aerodynamic benefits in unsteady state regime. In other respects, it was concluded that generally the rigid wing had better lift capacity for flapping wings. The flexibility of the wing affected its aerodynamics positively [86] and membrane wings had an increase in maximum CL during oscillating. Furthermore, an increase in reduced frequency led to an increase in maximum CL. Membrane wings have a higher slope of lift and postponed stall [87]. Additionally, the membrane kinematics was closely relevant to membrane tension and free-stream velocity [88].

As previously mentioned, numerous studies have been performed for a better understanding of flexible wings and examined their effects in terms of aerodynamic performance. Herein, it is important to give sight for the conducted researches about flexible membrane wing, which are tabulated in **Table 1**. A common result could be said from all these studies that membrane wings had favorable characteristics such as a higher lift-to-drag ratio and a higher maximum lift coefficient when compared to an equivalent rigid wing from the aerodynamics point of view.

Unlike these studies, Demir [98] investigated the deformation that occurred on the flexible membrane wing surface and how it affected the LSB. Moreover, he examined how LSB affected the vibrations that occurred on the membrane surface, the distribution of the flow characteristics, as well as the fluid-structure interactions between the membrane and flow both Traditional and New Types of Passive Flow Control Techniques to Pave the Way for High Maneuverability… http://dx.doi.org/10.5772/intechopen.90552 147


**Table 1.** Summary of pioneering studies on flexible membrane wing.

experimentally and numerically. An experimental study was conducted by Demir and Genç [74] in order to examine time-dependent circumstance of flow on flexible membrane wing and they noticed that the size of LSB altered with time because of the indecisive flow features of the wing. The indecisive behavior upon the flexible membrane wing brought about various deformation modes to constitute at various angles of attack. The results of time-dependent flow visualizations for angles of attack of α = 12° and α = 10° for different time intervals are given in **Figure 19** [75] and **Figure 20** [75]. Time-dependent attitudes of LSB was obviously seen as analogizing obtained results at miscellaneous times between t = 0.08 s and t = 0.20 s. The bubble size enlarged at t = 0.16 s and then was smaller at t = 0.20 s for Re = 2.5 × 104 at α = 12°, as seen in **Figure 19**. Additionally, as it is seen in **Figure 20**, the size of LSB enlarged until t = 0.12 s and then lessened at t = 0.16 s at α = 12° and Re = 5 × 10<sup>4</sup> . For this purpose, it can be deduced that bubble size varied with time because of the indecisive flow characteristics of flexible membrane wing.

**Figure 19.** Smoke wire flow visualization result of AR = 3 flexible membrane wing at y/s = 0.4 for α = 12° and Re = 2.5 × 104 [75].

**Figure 20.** Smoke wire flow visualization result of AR = 3 flexible membrane wing at y/s = 0.4 for α = 10° and Re = 5 × 104 [75].

As seen in **Figure 21**, vibrational modes in the middle section of the wing reduced and joined up at the tip region at α = 10° by the virtue of occurring separation bubble and these vibrational modes became a chordwise mode of two at α = 12° as seen in **Figure 22**. The holes Traditional and New Types of Passive Flow Control Techniques to Pave the Way for High Maneuverability… http://dx.doi.org/10.5772/intechopen.90552 149

**Figure 21.** Three-dimensional view of standard deviation of mean deformation of AR = 3 flexible membrane wing at α = 10° for Re = 5 × 10<sup>4</sup> .

**Figure 22.** Three-dimensional view of standard deviation of mean deformation of AR = 3 flexible membrane wing at α = 12° for Re = 5 × 10<sup>4</sup> .

formed by the separation bubble in the middle of the wing were illustrated with white dashed lines and the regions with red color showed the peaks.
