Low Reynolds Number Wind Tunnel Research Articles

Single-component force balance for the measurement of low-magnitude mean aerodynamic loads

A novel force balance is developed for the measurement of low-magnitude mean aerodynamic forces. The measurement of such small loads is often desired for low Reynolds number wind tunnel experiments, however commercially available options for force measurement do not have high enough sensitivity or accuracy. A force balance is designed to measure forces that range from mN to hundreds of milli-Newtons in magnitude. The sensitivity of the balance is derived theoretically, and the actual value is determined from calibration. The force balance demonstrates both a high degree of linearity, and low hysteresis over the desired load range. The measurement capability of the force balance is validated using measurements of the drag force on a circular cylinder, and the lift force on a square cylinder for Reynolds numbers between 1100 and 10 000. In each case, the force measured by the newly developed force balance shows good agreement with data from literature, and a level of accuracy currently not possible with commercially available sensors for such low-magnitude loads.

Coevolving advances in animal flight and aerial robotics

Our understanding of animal flight has inspired the design of new aerial robots with more effective flight capacities through the process of biomimetics and bioinspiration. The aerodynamic origin of the elevated performance of flying animals remains, however, poorly understood. In this themed issue, animal flight research and aerial robot development coalesce to offer a broader perspective on the current advances and future directions in these coevolving fields of research. Together, four reviews summarize and 14 reports contribute to our understanding of low Reynolds number flight. This area of applied aerodynamics research is challenging to dissect due to the complicated flow phenomena that include laminar–turbulent flow transition, laminar separation bubbles, delayed stall and nonlinear vortex dynamics. Our mechanistic understanding of low Reynolds number flight has perhaps been advanced most by the development of dynamically scaled robot models and new specialized wind tunnel facilities: in particular, the tiltable Lund flight tunnel for animal migration research and the recently developed AFAR hypobaric wind tunnel for high-altitude animal flight studies. These world-class facilities are now complemented with a specialized low Reynolds number wind tunnel for studying the effect of turbulence on animal and robot flight in much greater detail than previously possible. This is particular timely, because the study of flight in extremely laminar versus turbulent flow opens a new frontier in our understanding of animal flight. Advancing this new area will offer inspiration for developing more efficient high-altitude aerial robots and removes roadblocks for aerial robots operating in turbulent urban environments.

Components interaction effect evaluation of a small-capacity five-component internal balance system

With the development of small unmanned air vehicles, the urge for building the experimental facilities has emerged for the research on the low Reynolds number aerodynamics. In this paper, a five-component internal sting balance with the capacity of 10 N for forces and of 0.5 N·m for moments, which is to be installed in a close-circuit low Reynolds number wind tunnel for small unmanned air vehicle tests, was verified to investigate the interaction effect of its different components. Experimental setups and a sequence of evaluated procedures were designed to implement the application of different range of loads with pitch and roll angle coupled so as to investigate the interaction effect of different components by covering the full load range of sting balance. The sting balance was successfully verified with the designed experimental setups and procedures. Evaluation results showed that the interaction effect between balance components enhanced the measurement error and deteriorated the measurement precision. Moreover, it was also found that both precision and error depended on the loads. Relative error analysis revealed that the average relative error of all components was less than 2.1%. Hysteresis test showed that all balance component exhibited little hysteresis property except that side force component exhibited a mild hysteresis at lower load range.

Engineering Extrapolation to Flight Reynolds Number for Supercritical Airfoil Pressure Distribution Based on CFD Results

Pressure distribution of supercritical airfoil at flight Reynolds number could not be fully simulated except in cryogenic wind tunnel such as NTF (National Transonic Facility) and ETW (European Transonic Wind tunnel), which is costly and time resuming. This paper aimed to explore an engineering extrapolation to flight Reynolds number from low Reynolds number wind tunnel data for supercritical airfoil pressure distribution. However, the extrapolation method requiring plenty of data was investigated based on the CFD results for the reason of low cost and short period. Flows over a typical supercritical airfoil were numerically simulated by solving the two dimensional Navier-Stokes equations, with applications of ROE scheme spatial discretization and LU-SGS time march. Influence of computational grids convergence and turbulent models were investigated during the process of simulation. The supercritical airfoil pressure distribution were obtained with Reynolds numbers varied from 3.0×106 to 30×106 per airfoil chord, angles of attack from 0 degree to 6 degree and Mach numbers from 0.74 to 0.8. Simulated results indicated that weak shock existed on the upper surface of supercritical airfoil at cruise condition, that the shock location, shock strength and trailing edge pressure were dependent of Reynolds number, attack angles and Mach numbers. A similar parameter describing the Reynolds number effects factors was obtained by analyzing the relationship of shock wave location, shock front pressure and trailing edge pressure. Based on the similar parameter, airfoil pressure distribution at Reynolds number 30×106 was obtained by extrapolation. It was shown that extrapolated result compared well with simulated result at Reynolds number 30×106, implying that the engineering method was at least promising applying to the extrapolation of low Reynolds number wind tunnel data.

Engineering Extrapolation to Flight Reynolds Number for Supercritical Airfoil Pressure Distribution Based on CFD Results

Pressure distribution of supercritical airfoil at flight Reynolds number could not be fully simulated except in cryogenic wind tunnel such as NTF (National Transonic Facility) and ETW (European Transonic Wind tunnel), which is costly and time resuming. This paper aimed to explore an engineering extrapolation to flight Reynolds number from low Reynolds number wind tunnel data for supercritical airfoil pressure distribution. However, the extrapolation method requiring plenty of data was investigated based on the CFD results for the reason of low cost and short period. Flows over a typical supercritical airfoil were numerically simulated by solving the two dimensional Navier-Stokes equations, with applications of ROE scheme spatial discretization and LU-SGS time march. Influence of computational grids convergence and turbulent models were investigated during the process of simulation. The supercritical airfoil pressure distribution were obtained with Reynolds numbers varied from 3.0×106to 30×106per airfoil chord, angles of attack from 0 degree to 6 degree and Mach numbers from 0.74 to 0.8. Simulated results indicated that weak shock existed on the upper surface of supercritical airfoil at cruise condition, that the shock location, shock strength and trailing edge pressure were dependent of Reynolds number, attack angles and Mach numbers. A similar parameter describing the Reynolds number effects factors was obtained by analyzing the relationship of shock wave location, shock front pressure and trailing edge pressure. Based on the similar parameter, airfoil pressure distribution at Reynolds number 30×106was obtained by extrapolation. It was shown that extrapolated result compared well with simulated result at Reynolds number 30×106, implying that the engineering method was at least promising applying to the extrapolation of low Reynolds number wind tunnel data.

Control of a Transitioning Boundary Layer using Surface-Mounted Electro Active Polymers

Micro air vehicles (MAV) are a major focus of aerodynamics today with many military as well as civilian applications. MAV flight is dominated by the unsteady characteristics of low Reynolds number flows. Three-dimensional separations may occur at random on different parts of the wing, causing highly unsteady forces and moments, as well as full wing stall. In the present work an Electro-Active Polymer (EAP) actuator was examined as a feasible flow control actuator with application to low Reynolds number flows. The performance of the EAP was evaluated using a laser displacement device, where the displacements generated under selected driving frequencies and input voltages were recorded. It was shown that the deflections could reach up to 0.35 mm at frequencies below 40 Hz. The surface deflection was found to be parabolic across the span and width of the actuator. Hot-wire anemometry measurements were conducted on a flat plate equipped with a trailing edge flap in a low Reynolds number wind tunnel to study t...

Simulation and Experiment for Rigid and Flexible Wings of Flapping-Wings Microrobots

In this paper, an insect-based flapping-wing flying microrobot was built which can successfully fly in the sky. The unsteady aerodynamics associated with this microrobot was studied by using the method of computational fluid dynamics (CFD). On the basis of numerical simulation, the Fluid-Structure coupling mechanics for flexible flapping-wings were studied and discussed. According to the practically developed flapping-wing microrobot, a 2-D simulation model for flexible flapping-wings was established. Fluid-Structure coupling deformation and the effects of this model on the aero dynamic performance were analyzed, which have offered a theoretical basis for design of the aircraft with flexible flapping-wing. In order to verify the results of numerical simulation, aerodynamic performance tests have been conducted for the rigid and flexible flapping-wings in a low turbulence and low Reynolds number wind tunnel.

Two-Dimensional Low-Reynolds Number Wind Tunnel Results for Airfoil NACA 0018

The two-dimensional characteristics of airfoil NACA 0018 have been measured for Reynolds numbers between 0.15×106 and 1.0×106 to establish the lift, drag and moment curves that serve as input to performance calculations of vertical axis wind turbines. At the lower surface laminar separation occurs at low to medium angles of attack, which is of significant influence on the characteristics and the radiated noise. For the situation with a lower surface laminar separation bubble, span wise wake rake traverse measurements showed an irregular three-dimensional pattern. Noise reduction could be achieved with zigzag tape at the 70% to 80% lower surface chord station. Significant post-stall hysteresis loops occurred showing a high loss in lift.

Experimental and numerical study of the Mars Sample Return Orbiter in cold laminar hypersonic flow

This action has been undertaken in the framework of a collaborative program between NASA and CNES to support CNES in the development of the Mars Sample Return Orbiter (MSRO). The study, devoted to ONERA in this first phase of the program, consists in testing a 1/50 th size model in the hypersonic low Reynolds number wind tunnel R5Ch. Visualizations and heat-flux measurements have been carried out for different incidences in order to check the protection furnished to the satellite by the heat shield. The experimental measurements are discussed and compared with numerical results obtained with the Navier–Stokes finite volume solver FLU3M from ONERA. In addition, the influence on the measured heat flux of a partial cover, aiming at improving the thermal protection offered by the heat shield to the orbital vehicle is presented.