In-ground Effect Research Articles

Numerical study of downwash flow on rice plant protection drone with computational fluid dynamics method

Unnamed Unmanned Aerial Vehicles (UAVs) are increasingly being utilized in various industries, including agriculture, to support the growing demand for food. UAVs streamline work processes and are particularly useful in the spraying method for plant protection. This study aims to analyze the characteristics of the downwash flow, which are influenced by factors such as flight altitude, airfoil profile, and the flying speed of the drone. Unlike previous studies that used 6-blade UAVs, this research focused on a 4-blade configuration. The study employed Computational Fluid Dynamics (CFD) to analyze drone geometry and input boundary conditions based on environmental factors. The drone's flying altitude significantly impacted downwash flow, particularly concerning In Ground Effect (IGE) and Out of Ground Effect (OGE) conditions. Unlike previous research, this study considered the airfoil profile of the propeller, which, along with the drag and lift coefficients from the airfoil geometry, affected the downwash flow. The drone's flying speed, related to the relative wind speed around its working area, also influenced pressure distribution and downwash flow speed. These factors significantly impacted downwash flow and determined the distribution of plant protection droplets on the rice field. The results indicated that increasing flight altitude reduced the ground effect, affecting the quadcopter's downwash. Similarly, flight speed had a similar effect on downwash as altitude. Based on these findings, the study recommended a flight altitude of 2 m and a speed of 2 m/s for optimal downwash and proper distribution of plant protection.

Stable hovering of proximity quadrotor UAV in-ground effect via model compensation controller

Stable hovering of proximity quadrotor UAV in-ground effect via model compensation controller

Ground Effect on the Thrust Performance of Staggered Rotor System

In this study, the thrust performance of a staggered rotor system in-ground effect (IGE) and out-of-ground effect (OGE) while considering the interaction on wake characteristics were investigated experimentally. A thorough comprehension of their performance holds significant importance for trajectory planning, aircraft design, landing safety, and energy-efficient landings. The complex interactions within staggered rotor systems and the impact of ground effects make rotor distance and ground interactions critical factors influencing near-ground flight performance. The study investigated the influence and enhancements of rotor thrust performance through an examination of rotor speed, lateral distance, and altitude. Experimental tests were conducted on two small-scale rotor models to assess the effects of these parameters. These experiments compared the performance of staggered rotor systems with isolated rotors, analyzing the competition mechanism between the thrust loss caused by interference and the thrust gain of rotors IGE. Furthermore, emphasis was placed on analyzing the thrust gain issues exhibited by staggered rotor systems under the condition of H = 2R. Additionally, the analysis was focused on identifying prominent relative positions for thrust performance and parameters for improving thrust performance in ground effects in staggered rotor systems.

Fountain Flow Visualization in Quadrotor Wake Decreasing Rotor Thrust In-Ground Effect

Wake visualizations and thrust measurements of a quadrotor model were conducted to clarify the relationship between thrust and wake flow structure in-ground effects. The adjacent rotor distance, rotor height above ground, and the presence of a plate representing a generic quadrotor centerbody were varied. Thrust decreased when lowering the quadrotor to the ground when the adjacent rotor distance was over 2.2 times the rotor radius. Laser light sheet flow visualization showed quadrotor wake flow structures. Furthermore, visualization using particle image velocimetry showed an averaged vector map of wakes and fountain flow at the center of the quadrotor wake, which caused circulation with rotor passing flow and decreased rotor thrust. The center plate increased the sum of the rotor thrust and plate lift; however, circulation flow formed under the plate and caused the rotor thrust to decrease the in-ground effect. The results clarified that fountain flow development decreased the quadrotor thrust in-ground effect. Reducing the adjacent rotor distance weakened the development of fountain flow in the quadrotor wake and eliminated the decrease in rotor thrust near the ground.

Long-wave deformation of in-ground-effect wake vortex under crosswind condition

The in-ground-effect (IGE) aircraft wake vortex poses great hazards to airplanes. Especially when the vortex column is deformed with a long wavelength, the vortex core position is harder to determine, and thus, the IGE wake vortex is essential to air traffic safety and requires more detailed research. In this study, to account for the effects of roll-up-induced structures, the wake vortex pair was initialized using the vortex sheet model proposed by Lin et al. [1]. The asymmetrical wing loading effect on IGE wake evolution was considered for the first time. The Large Eddy Simulation results showed that the crosswind strengthened the upwind vortex, while the long-term wake vortex evolution was not significantly influenced by the wind. Most importantly, in certain crosswind conditions, the wake vortex deformed sinusoidally and linked with its mirror counterpart. From a linear stability analysis, it was found that this long-wave deformation could only occur on the upwind side owing to the redistribution of the strain field caused by the near-ground wind shear. Due to secondary vortex generation, the deformation was suppressed and a low growth rate of α≈0.41 was recorded for the long-wave instability.

Prediction of aerodynamic characteristics of high-lift Common Research Model in ground effect

Abstract Reynolds Averaged Navier-Stokes (RANS) simulations are performed to investigate the aerodynamic characteristics of the NASA Common Research Model (CRM) in its high-lift (HL) configuration in close proximity to the ground. The RANS simulations are conducted at a moderate Reynolds number of $Re = 5.49 \times {10^6}$ and $M = 0.2$ with the use of the Spalart-Allmaras (SA) turbulence model. out of ground effect (OGE) simulation results are validated against available wind tunnel data before proceeding to in ground effect (IGE) simulations. The obtained computational results in the immediate vicinity of the ground with asymmetric aircraft attitudes demonstrate significant changes in the longitudinal and lateral-directional aerodynamic characteristics, which should be taken into account in flight dynamics analysis of aircraft during take-off and landing in crosswind conditions.

Propeller Ground Effect in Forward Flight

The study of the fixed-pitch small-scale propeller in-ground effect is extended to forward flight conditions at different incidence angles of the propeller. Force-based experiments, smoke flow visualization, surface flow visualization, and phase-locked particle image velocimetry (PIV) indicate the presence of three distinct performance zones as a function of the advance ratio and ground distance, where the ground effect changes drastically. The power required at constant thrust shows an increasing trend after a threshold advance ratio for each and then decreases steeply upon a further increase in the advance ratio. Phase-locked PIV revealed the presence of a fountain vortex and a ground vortex at distinct for each distance that correlates with an increase in power and a decrease in thrust, respectively. At advance ratios below the onset of the fountain vortex lies the optimal ground effect zone. An increase in the advance ratio beyond this threshold will result in power increments due to the formation of the fountain vortex. A further increase in the advance ratio initiates the onset of the ground vortex where the ground effect is significantly reduced. The onset of the fountain vortex and the ground vortex occurs at comparatively lower advance ratios for propellers at a positive incidence angle. A normalization method is developed for ground effect forward flight that collapses the in-ground-effect performance for various normalized heights, advance ratios, and incidence angles.

A Computational Examination of Side-by-Side Rotors in Ground Effect

This study investigates the interactional aerodynamics of hovering side-by-side rotors in ground effect. The 5.5-ft diameter, three-bladed fixed-pitched rotors are simulated using computational fluid dynamics at a targeted 5 lb/ft2 disk loading. Simulations are performed using the commercial Navier–Stokes solver, AcuSolve ®, with a delayed detached eddy simulation model. Side-by-side rotors are simulated at two heights above the ground (H/D = 0.5 and H/D = 1), and with two hub–hub separation distances (3R and 2.5R). The performance of side-by-side rotors in ground effect (IGE) is compared to isolated rotors out of ground effect. Between the side-by-side rotors IGE, a highly turbulent mixing region is identified where the wakes of each rotor collide. The flow fountains upwards, as well as exits outwards (along a direction normal to a plane connecting the two rotor hubs) with fountaining between the rotors reaching up to 0.75D above the ground. As blades at H/D = 0.5 traverse the highly turbulent flow, strong vibratory loading is induced and a thrust loss is observed over the outboard sections between the rotors that is large enough to negate any nominal ground effect benefits inboard. Side-by-side rotors at H/D = 0.5 with 2.5R hub–hub spacing produce peak-to-peak thrust oscillations up to 16% of the steady thrust. Rotors placed higher, at H/D = 1 are positioned above the turbulent mixing flow and produce significantly lower vibratory loads. The spacing between rotors at H/D = 0.5 and 3R hub–hub separation allows strong vortical structures to develop between the rotors which move from side to side over multiple revolutions. When the vorticity moves closer to one of the rotors, it produces a greater lift deficit over the outboard region and a stronger vibratory loading. For rotors closer together, at H/D = 0.5 and 2.5R separation, the vortical structures between rotors are constrained to a more concentrated area and show less side-to-side drift.

Experimental investigation of propeller noise in ground effect

The aerodynamic propeller performance and aeroacoustic characteristics of a single two-bladed propeller subjected to Ground Effect are investigated using experiments in an anechoic chamber. A strong increase in thrust, torque and power coefficients when a propeller operates in Ground Effect is demonstrated, consistent with previous studies. A low-fidelity Blade Element Momentum Theory model adapted for Ground Effect is implemented to cross-validate the aerodynamic results. The near-field and far-field spectra and their coherence are studied for various propeller distances from the ground plane and compared with the isolated case. The far-field noise measurements indicate that additional tonal peaks appear with the introduction of the ground plane when compared to isolated configuration. While in-Ground Effect, additional high-frequency broadband humps are observed at some polar angles not seen in isolated configuration. Two discrete regions, a shielded and reflection zone, with respective reduction and enhancement of the Overall Sound Pressure levels are observed. The primary reason for noise increase within the reflection zone is a result of pure acoustic reflection from the ground surface. The near-field measurements from an array of embedded high-response microphones in the ground plane indicate that the main source of the propeller noise is in the tip region. Near-field measurements reconfirm flow results from literature with near-field and far-field acoustic results. The present study may find important practical applications within Urban Air Mobility (UAM), especially in the design of vertiport landing pads and propellers for effective noise mitigation, aerodynamic performance, and prediction during the vertical take-off and landing phases of flight.

Review of experimental investigations of wings in ground effect at low Reynolds numbers

The ground effect-induced large lift increase and lift-induced drag reduction have long been recognized and utilized in the design and construction of wing-in-ground effect (WIG) craft. Various wing planforms have been employed in WIG craft. In this study, the experimental investigations of rectangular wings and delta wings of reverse and regular configurations at low Reynolds numbers are reviewed. For rectangular wings, both chord-dominated and span-dominated ground effects on the aerodynamics, tip vortex, and lift-induced drag are reviewed. For reverse delta wings, in addition to the experimental measurements of the aerodynamics and tip vortex flow at different ground distances, passive flow control utilizing Gurney flap, cropping, and anhedral are reviewed. The impact of ground effect on delta wings is also discussed. Suggestions for future investigations applicable to each wing planform in-ground effect are provided.

Aerodynamic Interactions of Counter-Rotating Coaxial Rotors Hovering in Ground Effect

With the renewed interest in counter-rotating coaxial rotor (CCR) vehicles, a comprehensive understanding of its performance under out-of-ground effect and in-ground effect (IGE) operating conditions is imperative. The current work reports on an experimental study of a modular three-bladed CCR system during IGE hovering conditions. Thrust and torque produced by the upper and lower rotors were individually measured for two different axial spacings between the rotors with varying rotor heights from the ground. Both fixed-collective (untrimmed) and constant thrust, torque-matched (trimmed) experiments were performed to understand the interdependent effects of rotor-to-rotor interactions and ground effect on the performance of the CCR and its individual rotors. The CCR performance increased monotonically with decreasing rotor height with performance qualitatively similar for the two rotor spacing cases. At the same time, individual rotor performance variation with rotor height showed nonmonotonic behavior due to complementary or opposing effects of the rotor-to-rotor interactions to the ground effect. In general, the lower rotor in CCR experienced the most relative improvement in performance during IGE operations and was also observed to degrade the upper rotor performance.

Numerical investigation of full helicopter with and without the ground effect

In the present work, the aerodynamic performance of the full helicopter PSP in hover flight is investigated using a simplified concept of multiple reference frame (MRF) technique in the context of high-order Monotone Upstream Centred Scheme for Conservation Laws (MUSCL) cell-centred finite volume method. The predictions were obtained for two ground distances and several collective pitch angle at tip Mach number of 0.585. The calculations were made for both out-of-ground-effect (OGE) and in-ground-effect (IGE) cases and compared with experimental data in terms of pressure distribution and integrated thrust and torque and vortex system.

Propulsive jet aerodynamics and aeroacoustics

AbstractComprehensive understanding of propulsive jet aerodynamics and aeroacoustics is key to engine design for reduced jet noise and infra-red signature in civil and military aerospace, respectively. Illustrated examples are provided of other aerodynamic/aeroacoustic problems involving jet development, including chevron nozzles, increased jet/wing/flap interference (as fan diameter increases), high acoustic environment (and potentially damaging screech) of supersonic jets on carrier decks and the strongly Three-Dimensional (3D) unsteady flow during the in-ground effect operation of Short Take-Off and Vertical Landing (STOVL) aircraft. To date, laboratory/rig test measurements have primarily been used to identify design solutions; increased use of Computational Fluid Dynamics (CFD) would help achieve cost/time reductions, but Reynolds-Average Navier–Stokes (RANS) CFD with statistical turbulence modelling has proven inadequate for such flows. The scenarios described are far removed from flows used to calibrate model constants, and predictive accuracy demands detailed insight into unsteady flow. Large-Eddy Simulation (LES), whilst computationally more demanding, offers a potential solution. Research undertaken to assess LES capability to address the challenges described is reviewed here. This demonstrates that tremendous progress has been made, indicating that LES can provide sufficiently accurate predictions, representing high value for engineering design. A series of validation studies of increasing realism to practical engineering systems is presented to underpin this conclusion. Finally, areas for further work are suggested to support the combined application of RANS and LES that is probably the optimum way forward.

Simple multiple reference frame for high-order solution of hovering rotors with and without ground effect

In the present work, the aerodynamic performance of the Caradonna and Tung and S-76 in hover were investigated using a simplified concept of multiple reference frame (MRF) technique in the context of high-order Monotone Upstream Centred Scheme for Conservation Laws (MUSCL) cell-centred finite volume method. In the present methodology, the frame of reference is defined at the solver level by a simple user input avoiding the use of mesh interface to handle the intersections between frames of reference. The calculations were made for both out-of-ground-effect (OGE) and in-ground-effect (IGE) cases and compared with experimental data in terms of pressure distribution, tip-vortex trajectory, vorticity contours and integrated thrust and torque. The predictions were obtained for several ground distances and collective pitch angle at tip Mach number of 0.6 and 0.892.

Effect of ground on the shape optimisation of a symmetric aerofoil at low angles of attack

Numerical investigation on the aerodynamic characteristics of an optimised NACA0012 aerofoil in-ground effect (IGE) has been performed. Gradient-based shape optimisation was carried out using the ANSYS® 19.0 Adjoint Solver to augment lift over drag ratio (L/D) by at least 10%, at various heights and angles of attack. SST k-ω turbulence model was chosen for the simulations, after its validation for out-of-ground effect (OGE) and performing wind tunnel tests for IGE. While the desired target of 10% increase in the performance parameter was easily achieved through optimisation at low angles of attack (α < 6°), the frozen turbulence assumption in Adjoint Solver limited large shape alterations at higher angles of attack. Upper surface of the aerofoil had larger changes from original camber when compared to the lower surface. Also, the optimised profiles had significant modifications towards x/c ≥ 0.8. This signifies the suitability of trailing edge morphing for such applications.

Quasi-Steady In-Ground-Effect Model for Single and Multirotor Aerial Vehicles

This Paper presents a new quasi-steady in-ground effect (IGE) model for small multirotor aerial vehicles such as quadcopter unmanned aerial vehicles. The model offers three major advances to the state of the art: 1) predicting the IGE thrust ratio without singularity, 2) relating the strength of the ground effect to the blade geometry (pitch angle, solidity, and radius), and 3) capturing the effect of multirotor configurations. Two IGE coefficients, based on blade element theory, are introduced and then analyzed using the method of images. Additionally, the thrust-loss phenomenon on multirotor IGE is observed and modeled empirically. Experiments are conducted to validate each component of the model, and the results suggest that the model can be used to predict the onset of the IGE for vehicle motion control and planning in the regime where IGE dominates.

Experimental and computational investigation on comparison of micro-scale open rotor and shrouded rotor hovering in ground effect

Various investigations on open rotor (OR) hovering in-ground effect (IGE) are carried out, but few papers report shrouded rotor (SR) hovering IGE. This paper compares aerodynamic performance and flowfield characteristics of OR and SR hovering IGE by both experimental measurements and computational fluid dynamic (CFD) simulations. Experimental results reveal that in IGE flight, the aerodynamic performance of SR is more sensitive than that of OR. And at constant power, SR offers more thrust than OR at the same ground distance. Ground has a great influence on thrust for OR below 2.2 rotor radius distance, while for SR it shows obvious effect below 1.5 rotor radius distance. It is also shown that normalized aerodynamic coefficients of OR and SR are independent on rotor speed. In addition, for OR the rotor thrust coefficient changes nearly linearly with the logarithmic distance from ground, while for SR it changes nonlinearly. Flowfield analysis by CFD shows that shroud changes the tip flow features and expands the slipstream area of SR. When ground distance gets small, back pressure below the rotor-disk plane increases, which is more obvious for SR than OR. Furthermore, shroud thrust of SR decreases because of tip leakage flow and flow separation.

Experimental study of a bio-inspired flapping wing MAV by means of force and PIV measurements

Abstract This study explores the aerodynamic performance of an X-Wing bio-inspired flapping wing micro air vehicle (MAV) underlying clap-and-fling motion by means of force and flow field measurements. Wind tunnel tests were first conducted to identify the operation region of current flapping wing MAV. The effects of flapping frequency and wing flexibility on force generation and power loading were evaluated using a miniature six-component force transducer. Flow visualization in the chordwise wake of the flapping wings is measured by means of phase-lock particle image velocimetry (PIV) measurement, results revealed the shedding behavior of the vertical structures by the wings during clap-and-fling motion showed a momentum surplus. Additionally, the in-ground-effect (IGE) is also examined aiming to understand the aerodynamic characteristics during take-off and landing of flapping wing MAV.

In-Ground-Effect Modeling and Nonlinear-Disturbance Observer for Multirotor Unmanned Aerial Vehicle Control

This paper focuses on modeling and control of in-ground-effect (IGE) on multirotor unmanned aerial vehicles (UAVs). As the vehicle flies and hovers over, around, or underneath obstacles, such as the ground, ceiling, and other features, the IGE induces a change in thrust that drastically affects flight behavior. This effect on each rotor can be vastly different as the vehicle's attitude varies, and this phenomenon limits the ability for precision flight control, navigation, and landing in tight and confined spaces. An exponential model describing this effect is proposed, analyzed, and validated through experiments. The model accurately predicts the quasi-steady IGE for an experimental quadcopter UAV. To compensate for the IGE, a model-based feed-forward controller and a nonlinear-disturbance observer (NDO) are designed for closed-loop control. Both controllers are validated through physical experiments, where results show approximately 23% reduction in the tracking error using the NDO compared to the case when IGE is not compensated for.

Assessment of S-76 rotor hover performance in ground effect using an unstructured mixed mesh method

In the present study, the aerodynamic performance of an S-76 rotor in hover was numerically investigated by using an unstructured mixed mesh flow solver. The study was made for the rotor for both OGE (out-of-ground-effect) and IGE (in-ground-effect) conditions, and the results are compared against each other. In the present mixed mesh methodology, body-fitted prismatic/tetrahedral mesh was adopted in the near-body flow domain to treat complex geometries easily and to capture the viscous layer on the solid surface more accurately, while in the off-body region away from the blades Cartesian mesh was used. To better resolve the flow characteristics and to prevent excessive numerical dissipation, a high-order accurate weighted essentially non-oscillatory (WENO) scheme was employed in the off-body flow region. An overset mesh topology was adopted to handle blade rotation and to exchange the flow variables between the two different mesh regions. The calculations were made for three different blade configurations, having swept-tapered, rectangular, and swept-tapered-anhedral tip shapes, and the results are compared with experimental rotor performance data in terms of thrust, torque and figure of merit. The predictions were obtained for a collective pitch angle sweep from 5 to 10 degrees at a tip Mach number of 0.60 for both with and without ground effects. The detailed flow characteristics, such as vorticity contours and tip-vortex trajectory, were also investigated.