Increasing Yaw Angle Research Articles

Effect of yaw on wake and load characteristics of two tandem offshore wind turbines under neutral atmospheric boundary layer conditions

In this work, we numerically investigated the effects of yaw angle on the wake and power characteristics of two National Renewable Energy Laboratory (NREL) 5 MW wind turbines based on actuator line method (ALM) and large eddy simulation (LES) under a neutral atmospheric boundary layer (ABL) with specified offshore surface roughness. The turbines are placed in tandem, with a spacing of seven rotor diameters, and the yaw angles range from 0° to 30°. The results indicate that under coordinated yaw conditions, the wakes of the two turbines significantly shift with increasing yaw angles, encroaching on the trailing edge of the turbines. The expansion of the wakes also gradually weakens, leading to a reduction in width. The superposition of the wake generated by the downstream turbine diminishes, leading to both turbines exhibiting approximately comparable physical characteristics within their respective wakes. As the wake of the upstream turbine propagates downstream, a secondary low-speed region emerges between the primary low-speed zone of the wake of downstream turbine and the surrounding atmosphere. With the increase in yaw angle, this secondary low-speed region significantly enhances the rate of wake recovery while also inducing a more pronounced deflection of the wake, thereby demonstrating a stronger entrainment effect. Regarding load characteristics, the time history of power characteristics and the power spectral density (PSD) spectra indicate a good turbine response to the inflow. The power characteristics of the upstream turbine exhibit a scaling law is closely related to the yaw angle. The quantitative relationship is established between yaw angle and the power distribution of the turbines, alongside a proposed correlation between the yaw angle and the cos 2(γ) scaled power curve. The power of upstream turbine decreases and the power of downstream turbine gradually increases with the increase in yaw angle. It is further found that the downstream turbine demonstrates optimal performance at a yaw angle of 20°due to the influence of the yawed upstream turbine. These analyses provide insights into the characteristics of wind turbine arrays under yaw conditions from the perspective of unsteady wake features, interactions, and aerodynamic performance, which can aid in wind farm unit planning and control strategies.

Experimental study of vortex-induced vibration of stay cables installed with two types of perforated shroud light devices

To address both illumination issues and the common vortex-induced vibration (VIV) of bridge cables, a perforated shroud light device is proposed. The effect of the different porosities (20%, 35%, 42%, and 55%) on VIV of the rigid segmented shroud cable models is studied first by wind tunnel tests. It reveals that shrouds with porosities of 20% and 35% could reduce the VIV amplitude of the smooth cables by 26% and 46%, respectively. However, shrouds with porosities of 42% and 55% tend to increase the complexity and risk associated with VIV. Vibration measurements on flexible should cable with 20% porosity were conducted for both vertical and inclined models with an inclination of 35° and yaw angles of 0°, 30° and 60°. Shrouds with a porosity of 20% exhibit similar effects on vertical flexible cables as on horizontal rigid cables, in both cases mitigating VIV. The impact on the inclined flexible cables depends on the yaw angle. At the yaw angle of 0°, the shroud effectively mitigates VIV in flexible cables. However, at yaw angles of 30° and 60°, the same porosity level shroud exacerbates the risk of VIV in the inclined flexible cable. This may be due to the enhanced wind velocity along the cable axial direction and decreased penetrating flow rate through the shroud as the wind yaw angle increases. Overall, it was found that a shroud with a porosity of 20%–35% performed the best and was recommended for applications on cables with no inclination and inclined cable at yaw angle β = 0°.

A comparative study of hydrodynamic performance between ducted and non-ducted Archimedes spiral hydrokinetic turbines

The Archimedes spiral hydrokinetic turbine (ASHT), as a novel type of horizontal-axis hydrokinetic turbine, is well suited for deployment in the low-speed ocean current environment. Besides, integrating hydrokinetic turbines within a duct stands out as a prospective approach to enhance the efficiency of energy harvesting. In this study, a duct is implemented with the aim of enhancing the performance of ASHT. Using the commercial code program ANSYS-FLUENT, the Reynolds-averaged Navier-Stokes (RANS) equations are solved along with the SST k-ω turbulence model to elucidate the distinctive hydrodynamic characteristics of ASHT and its ducted configuration (DASHT). The findings revealed a consistent outperformance of the DASHT over the ASHT, achieving a notable maximum improvement of 122% in the power coefficient (CP). The ASHT exhibits turbulence near the blade tips, generating large spiral tip vortex, while the DASHT, with its duct, induces significant turbulence, resulting in a wider range of vortex structures. Furthermore, the DASHT shows better energy harvesting due to enhanced diffusion under yawed flow. The wake asymmetry and increased turbulence are observed as the yaw angle increases, affecting vortex structures differently in ASHT and DASHT. At higher yaw angle, the DASHT vortex splits into two parts, contributing to a more complex wake structure.

Effect of the yaw angle on the riblet performance

Direct numerical simulations are performed to investigate the effect of the yaw angle (α) on the drag-reduction performance by riblets in a turbulent channel flow. The yaw angles of α=0° and 90° correspond to the riblets parallel and perpendicular to the free-stream direction, respectively. It is well known that maximum drag reduction occurs at α=0°, and the drag-reduction performance by riblets is degraded with increasing α. We obtain the critical yaw angle below which drag reduction can be achieved and investigate the corresponding mechanism. From a parametric study, drag reductions are achieved for α<20°. As α increases, the form drag increases more rapidly than the skin-friction drag decreases, resulting in the increase in the total drag. At drag reducing and increasing yaw angles, the mean velocity profiles in wall units shift upward and downward, respectively, and the latter profile suggests that the drag-increasing riblet is similar to a rough surface. With increasing α, the wall-normal and spanwise velocity fluctuations and Reynolds shear stress increase, and stronger near-wall vortices are generated. By applying a scaling analysis to the momentum equation in the riblet-perpendicular direction and the FIK (Fukagata–Iwamoto–Kasagi) identity [Fukagata et al., “Contribution of Reynolds stress distribution to the skin friction in wall-bounded flows,” Phys. Fluids 14, L73–L76 (2002); Peet and Sagaut, “Theoretical prediction of turbulent skin friction on geometrically complex surfaces,” Phys. Fluids 21, 105105 (2009); Bannier et al., “Riblet flow model based on an extended FIK identity,” Flow Turbul. Combust. 95, 351–376 (2015)] to the riblet-parallel direction, the total drag is expressed as a linear function of sin2α.

Analysis of Diffusion Characteristics and Influencing Factors of Particulate Matter in Ship Exhaust Plume in Arctic Environment Based on CFD

The gradual opening of the Arctic shipping route has made navigation possible. However, the harm caused by ship exhaust emissions is increasingly severe. Therefore, it is necessary to study the diffusion characteristics of ship exhaust plumes during Arctic navigation. The study focuses on a merchant vessel as the subject of investigation, employing computational fluid dynamics (CFD) simulation techniques to analyze the diffusion characteristics of particulate matter (PM) within ship exhaust plumes under Arctic environmental conditions. The diffusion law of ship exhaust plume PM is clarified, and the influence of three factors, synthetic wind speed, yaw angle and chimney angle, on the PM diffusion is analyzed. It was found that after the PM was discharged from the chimney, the majority of the PM dispersed directly backward along with the external flow field, while a minor fraction lingered at the stern of the ship for an extended period before eventually diffusing backward. Among them, 1235 particles were captured within a range of 200 m from the stern, with a capture rate of 0.6%. When the synthetic wind shows a yaw angle, the capture rate of PM in the interval increases rapidly with the increase of yaw angle, while other factors have less influence on the capture rate of PM. This study provides foundational guidance for predicting PM diffusion from ship exhaust plumes in Arctic environments, thereby enabling more effective strategies for managing such emissions.

Numerical study on the influence of riblet yaw-angle on drag reduction rate with different immersion heights

This paper uses the Large-eddy simulation method to investigate the influence of riblet yaw angle on drag reduction rate with different immersion heights, revealing that the vortex structure is gradually transformed from riblet-tip vortex pairs to boundary vortex forms with the increase of yaw angle, and analyzing the influence mechanism of immersion height on the critical yaw angle. The result shows that the drag-reduction rate of riblets decreases as the yaw angle increases, and the critical angle of the riblet varies with the different immersion heights. When the immersion height is 0.7, 0.5, and 0.3, the critical yaw angle is close to 30, 40, and 50°, respectively. As the immersion height increases, the cross-flow velocity at the riblet tip is more accessible to reach the critical velocity at which a boundary vortex can be formed, resulting in a smaller critical yaw angle for larger immersion heights. Furthermore, with the yaw angle increases, the normal diffusivity of normal vorticity and spanwise vorticity increase and decrease, respectively, which indicates that the lateral flow fluctuations decrease beyond a critical yaw angle, while the Reynolds stress component due to energy dissipation induced by the streamwise velocity decreases. Meanwhile, with the formation of a boundary vortex, the ability of the riblet to reduce viscous shear stresses increases due to the reduction of near-wall velocity gradient, and the emergence of a high-pressure region on the windward side of the riblet and a low-pressure region on the leeward side, consequently augmenting the differential pressure drag. This work provides valuable insights for designing riblets with heightened effectiveness at larger yaw angles, thereby extending their applicability under diverse flow conditions.

Experimental and numerical investigations on the performance and wake characteristics of a tidal turbine under yaw

This paper presents an extensive examination of the wake characteristics and hydrodynamic performance of a 1:60 scale horizontal axis tidal stream turbine under yawed inflow conditions. Experimental investigations were conducted in a laboratory flume, while numerical simulations utilized the actuator line method and large-eddy simulation technique. The turbine was subjected to an inflow turbulence intensity of 6% and was tested at five different yaw angles (5°, 10°, 15°, 20°, and 30°). The findings demonstrate a substantial reduction in the power coefficient as the yaw angle increases, with a notable drop of 47.1% recorded at a yaw angle of 30°. Furthermore, numerical simulations of wake characteristics, including mean velocity and turbulence intensity, were examined and compared with measurements, yielding satisfactory agreement. The yawed wake flow exhibited asymmetrical distributions of mean velocity and deviations from the centerline. Additionally, distinct deformations of the yawed wake in the y-z cross sections were observed, indicating a discernible curled kidney shape that intensified with larger yaw angles. Moreover, interactions between the turbine wake and support structures affected the width and turbulence intensity of the near wake, introducing additional unsteadiness that accelerated wake recovery.

Analysis of the aerodynamic characteristics of an ejection seat system using computational fluid dynamics

In the present work, an investigation of the aerodynamic characteristics of an ejection seat occupant is carried out using the commercially available computational fluid dynamics software ANSYS Fluent. 3D Reynolds-averaged Navier–Stokes equations are solved to obtain the aerodynamic coefficients of the ejection seat system. For this analysis, an unstructured grid is generated for the ejection seat occupant using ANSYS meshing. Validation is carried out and the performance of three different turbulence models is analyzed at Mach 0.6. Based on the most suitable turbulence model, further analysis of the aerodynamic coefficients of the ejection seat occupant is calculated at Mach numbers of 0.35, 0.45, 0.55, 0.65, and 0.75. For all values of Mach, the angle of attack is varied from −15° to 15° in 5° increments and the yaw angle is varied from 0° to 60° in 10° increments. Based on the results, it is observed that the magnitude of the axial force decreases with increasing angle of attack and yaw angle. Similarly, the normal force coefficient and pitching moment coefficient decrease with increasing angle of attack. Finally, the side force coefficient, yawing moment, and rolling moment coefficients increase with increasing yaw angle.

Performance Analysis of Hydrokinetic Turbine Using Shroud Ratio Comparison under Yaw Misalignment Condition

This research aims to analyze the performance of hydrokinetic turbines under yaw misalignment conditions using descriptive statistical methods on coefficient of power (Cp) data. Tests were conducted at water velocities of 0.7, 0.9, and 1.1 m/s for three types of turbine shrouds consisting of turbines without shrouds, turbines with two different types of shrouds, at yaw angles from 0° to 25° with 5° intervals. The study concludes that the performance of each turbine type is significantly influenced by the combination of water flow velocity and yaw angle. The diffuser type has the highest Cp value at every yaw angle, but its performance decreases with increasing yaw angle. The Blade type has poorer performance compared to the diffuser at every yaw angle and has the best performance at a combination of 1.1 m/s velocity and 5° yaw angle. Meanwhile, the shroud type has more stable performance and is not greatly affected by variations in velocity and yaw angle. Based on the analysis of changes in average Cp values with changes in yaw angle at V 0.7 m/s, all three turbine types experienced an increase in Cp value at a yaw angle of 5, with the shroud experiencing the most significant increase. At V 0.9 m/s, the diffuser and shroud types were able to maintain their average Cp values at every yaw angle, while the blade type decreased with increasing yaw angle and experienced a significant decrease at a yaw angle of 25. At V 1.1 m/s, the diffuser and blade types experienced a decrease in performance with every increase in yaw angle, but the shroud type was able to maintain the same Cp value and even experienced a significant increase at a yaw angle of 5.

Flow Structures and Aerodynamic Behavior of a Small-Scale Joined-Wing Aerial Vehicle under Subsonic Conditions

Flow behavior and aerodynamic performance of a small-scale joined-wing unmanned aerial vehicle (UAV) was studied experimentally and numerically under various pitch and yaw angle combinations in subsonic flow conditions. Selected numerical results are compared against experimental results obtained using surface oil flow visualizations and force measurements, with additional simulations expanding the range of combined pitch and yaw configurations. Under zero-yaw conditions, increasing the pitch angle leads to the formation of symmetric ogive vortex roll-ups close to the fuselage and their significant interactions with the fore-wing. Additionally, contributions to lift and drag coefficients under zero-yaw conditions by the key UAV components have been documented in detail. In contrast, when the UAV is subjected to combined pitch and yaw, no clear evidence of such ogive vortex roll-ups can be observed. Instead, asymmetric flow separations occur over the fuselage’s port side and resemble bluff-body flow behavior. Additionally, these flow separations become more complex, and they interact more with the fuselage and fore- and aft-wings when the yaw angle increases. Lift and drag variations due to different pitch and yaw angle combinations are also documented. Finally, rolling and yawing moment results suggest that the present UAV possesses adequate flight stability unless the pitch and yaw angles are high.

A Numerical Study About the Flow Around Motorbike with Different Types of Fairing When Travel Through Crosswind

Riding a motorbike possess to a dangerous situation because of its stability and balance. Crosswind has a significant impact on riding since, with a strong gust, it can easily drive the motorbike off the road or into another lane or traffic. A numerical study has been conducted to investigate the aerodynamic loads of the drag, side, and lift coefficient and to analyze the flow pattern occurs of the motorbike with different types of fairing when travel under various angle of crosswind that was set ranging 0◦, 15◦, 30◦, 60◦ to 90◦. Two models of Yamaha YZF M1 with different types of fairing were designed using SolidWorks 2020 and then simulated using Ansys CFX with 25m/s as the velocity for main and crosswind inlet. The aerodynamic loads coefficient was calculated for each model and the streamlines flow were analyzed. The aerodynamic loads coefficient was found increases as the yaw angle increases. Half fairing model was found to have higher aerodynamic coefficient compared to fully fairing model since half fairing model was not aerodynamic shape.

Aerodynamic characterization of two tandem wind turbines under yaw misalignment control using actuator line model

Yaw control has proven to be promising in alleviating the wake effects that plague the efficiency of wind farms. In this work, the actuator line modeling (ALM) method is adopted to simulate the flows over two tandem turbines distanced by 3–7 rotor diameters, with the yaw angle of the upstream rotor varying from γ1=0° to 50°. The aim is to provide a comprehensive aerodynamic characterization of this simple wind farm under yaw misalignment control. With increasing yaw angle, the power generated by the downstream rotor increases, compensating the power loss in the upstream rotor, and resulting in significantly higher total power of the two turbines than that without yaw control. The maximum power output is achieved as the upstream wake of the yawed rotor is redirected away from the downstream rotor plane. Behind the downstream rotor, the secondary steering phenomenon is observed, where the wake is also redirected from the centerline. The use of the actuator line model also reveal unsteady aerodynamic characteristics that cannot be captured by lower-fidelity models. For the upstream rotor, the yaw misalignment results in time-varying change in the local angle of attack on the blade, giving rise to unsteady loading. The downstream rotor is partially submerged in the deflected wake incurred by the yawed upstream rotor. As the blade revolves into and out of the wake deficit, the blade experiences cyclic loading, leading to even stronger fluctuations in the aerodynamic loads than the upstream rotor. These analysis provides a comprehensive understanding of the yaw control effects on the two tandem rotors from the perspectives of aerodynamic performance, wake profiles, and unsteady characteristics. The insights gained from the present study can aid the design of collective yaw control strategies of wind farms, and lay the foundation for assessing the fatigue damage associated with yaw misalignment.

Study on Complex Wake Characteristics of Yawed Wind Turbine Using Actuator Line Method

In modern large-scale wind farms, power loss caused by the wake effect is more than 30%, and active yaw control can greatly reduce the influence of the wake effect by deflecting the wind turbine’s wake. The yawed wind turbine’s wake characteristics are complex, and a deep comprehension of a yawed turbine’s wake is necessary. The actuator line method combined with URANS (unsteady Reynold-averaged Navier–Stokes equations) is used to study the yawed wind turbine’s wake characteristics in this paper. Compared with an un-yawed wind turbine, a yawed one has two main characteristics, deflection and deformation. With an increasing yaw angle, turbine wake shows an increasing deflection. The results indicated that deflection at different height was different, the wake profile showed the biggest deflection at about the hub height, while the smallest deflection existed at the top and bottom of the yawed turbine’s wake. This can be visually demonstrated by the evolution of a kidney-shape velocity distribution at the vertical cross-section. Two-dimensional and three-dimensional presentations of velocity deficit distributions are presented in this paper. The evolution of an irregular kidney-shape distribution is discussed in this paper. It is formed by the momentum exchange caused by the counter-rotating vortex pair. The results indicated that the counter-rotating vortex pair was composed of the streamwise vortex flux brought by the tip vortex. Furthermore, when the wind turbine rotated clockwise and yawed clockwise, the negative vorticity of counter-rotating vortex first appeared in the upper left position.

Analysis of the Aerodynamic Characteristics of Small-Sized Car Vehicles under the Influence of Steady Crosswind

The crosswind is one of the factors that can lead to a car accident. Crosswind is defined as a side force that causes a vehicle to become unstable and deviate from its desired path. This article focuses on evaluating the flow behaviour and aerodynamic loads that affect vehicle stability when driving in a steady crosswind. In this study, a numerical approach is used with the ANSYS Fluent software as a platform to run the simulation. In this research, the crosswind flow angle (Ψ) is varied from 0° to 90°. The incompressible flow surrounding the vehicle is solved using the Reynolds-Averaged Navier-Stokes (RANS) equations in conjunctions with the k-ε turbulence model. The Reynolds number is utilized depending on the velocity of the vehicle which are 2.8×106 for high Reynolds number and 7.2×105 for low Reynolds number respectively. According to the findings, the crosswind has a significant quantitative and qualitative impact on aerodynamics. In terms of aerodynamic load, the side coefficient (Cs) increases as the crosswind yaw angle increases. When the crosswind yaw angle reaches 60°, there is a significant drop, but it remains almost constant when the crosswind yaw angle reaches 90°. In terms of flow structure, as the crosswind yaw angle increases, the vortex formation on the leeward region expands, increasing the vehicle stability imbalance. Finally, there is no significant difference in the quantitative aerodynamic characteristics of high and low Reynolds numbers.

Physical Modelling of Tidal Stream Turbine Wake Structures under Yaw Conditions

Tidal stream turbines may operate under yawed conditions due to variability in ocean current directions. Insight into the wake structure of yawed turbines can be essential to ensure efficient tidal stream energy extraction, especially for turbine arrays where wake interactions emerge. We studied experimentally the effects of turbines operating under varying yaw conditions. Two scenarios, including a single turbine and a set of two turbines in alignment, were configured and compared. The turbine thrust force results confirmed that an increasing yaw angle results in a decrease in the turbine streamwise force and an increase in the turbine spanwise force. The velocity distribution from the single turbine scenario showed that the wake deflection and velocity deficit recovery rate increased at a rate proportional to the yaw angle. The two-turbine scenario results indicated that the deployment of an upstream non-yawed turbine significantly limited the downstream wake steering (i.e., the wake area behind the downstream turbine). Interestingly, a yawed downstream turbine was seen to influence the steering of both the upstream and the downstream wakes. These systematically derived data could be regarded as useful references for the numerical modelling and optimisation of large arrays.

A Study of Offshore Wind Turbine Wake Effects in Yaw Conditions Using an Improved Actuator Line Method

AbstractThe study of wind turbine wakes is very important for the layout of offshore wind farms. The technique of regulating the yaw angles of the upstream wind turbine to lessen the influence on the downstream turbines has attracted continual attention in recent years. In this study, the wake interactions between a yaw wind turbine and a downstream wind turbine are investigated using a numerical technique based on the openfoam solver in conjunction with an improved actuator line method. The Gaussian anisotropic body force projection method and the integral velocity sampling method are the two fundamental components of the improvement of the actuator line method. The NREL 5-MW wind turbine benchmark model is used to test the numerical accuracy. The simulation of the wake effects from the upstream turbine in non-yawed conditions that follows has good agreement with the results that have been published in the literature. Finally, this work presents a number of predictions about the power coefficients and wake characteristics of two tandem-arranged wind turbines at various yaw angles based on these precise verification efforts. The results of the analysis in yaw conditions are used to derive the yaw wake characteristics and the optimal yaw angle range. As the yaw angle increases, the total power of the wind turbine increases and then decreases, and the upstream wake area decreases significantly. The total power reaches its maximum at 20–30 deg. The research content of this paper will provide an important reference for wind farm scheduling.

Tire dependence for the aerodynamics of yawed bicycle wheels

The aerodynamic characteristics of a modern road cycling wheel in cross wind are studied through force- and planar PIV measurements in the TU Delft Open Jet Facility. The performance of the 62 mm deep rim is evaluated for three tire profiles, and yaw angles up to 24°. All measurements are executed at 12.5 m/s (45 km/h) freestream- and wheel-rotational velocity. The wheel's rim-tire section in crosswind is found to behave similar to an airfoil at incidence, ultimately resulting in a reduction of the wheel's aerodynamic resistance with increasing yaw angle magnitude. This phenomenon, also referred to as the sail-effect, is limited by the stall angle of the tire-rim profile. The stall angle is found to depend critically on the tire's surface structure. Larger stall angles, resulting in lower resistance, are obtained if the tire profile triggers laminar-to-turbulent boundary layer transition.

Effects of yawed inflow and blade-tower interaction on the aerodynamic and wake characteristics of a horizontal-axis wind turbine

The aerodynamic characteristics of a wind turbine change dramatically under the influence of complex inflow and interaction between blade and tower. In this study, detailed full-scale CFD models of NREL 5 MW wind turbine are developed to investigate the specific aerodynamic characteristics with varying yaw angles and wind speeds based on OpenFOAM tool box. The total and local blade aerodynamic loads are analyzed by incorporating the effects of yawed inflow and tower. It is clearly noted that the blade load fluctuation amplitude and influence range due to the presence of tower gradually increase when the yaw angle becomes larger. Moreover, the three-point method is employed to extract the angle of attack (AOA) and induce factor of different blade sections within a rotation period. Generally, the fluctuation amplitudes of AOA and induce factor increase with the increase of yaw angle by ignoring the abrupt change induced by the tower shadow effect. Subsequently, the effects of blade rotation on the aerodynamic force of tower for varying wind speeds and yaw angles are considered, and a “negative pressure” state is observed when the blade passes in front of the tower. Finally, the wake effect under yawed inflow is revealed in details.

High-Order Large Eddy Simulations of a Wind Turbine in Ducted and Open-Rotor Configurations

Abstract High-order large eddy simulations are performed to study the performance and flow fields of a ducted wind turbine (DWT) operating at different tip speed ratios. To evaluate the effects of the duct, simulations with the same tip speed ratios are also performed on the corresponding open-rotor turbine. It is found that the ducted turbine consistently obtains higher power outputs than the open-rotor counterpart, and the duct itself enhances flow turbulence and blade trailing-edge vortices but weakens tip and hub vortices. Flow bifurcation is observed at the largest tip speed ratio and is identified to be caused by blade blockage effects. Comparative simulations are also performed on both turbines under different yaw angles. It is noticed that the ducted configuration is insensitive to small yaw angles and maintains higher power outputs than the open-rotor configuration at all yaw angles. Moreover, it is observed that the wakes of both configurations recover more quickly as the yaw angle increases.

Asymmetries and similarities of yawed rotor wakes

This paper describes an experimental study of the asymmetries and self-similarity/self-preservation of yawed rotor wakes and the similarity between different yaw angle scenarios. The wake asymmetry is found to increase with increasing yaw angle and also increases in the streamwise direction before the wake reaches a state of self-similarity. Furthermore, if we divide the profiles into left and right parts, the normalized profiles are the same for different streamwise locations even in the region where the profiles have not reached a state of self-similarity. The relation between the asymmetry of the velocity deficit profiles and the yaw angle is derived theoretically and validated by the present experimental measurements. A critical yaw angle is determined for the similarity in the streamwise evolution of the wake between different yaw angle scenarios. Specifically, there is a similarity between different yaw angle scenarios when the yaw angle is below the critical value. The results of this study will be helpful in understanding yawed rotor wakes and establishing a yawed rotor wake model.