Ground Vortex Research Articles

Lessons Learnt from the Simulations of Aero-Engine Ground Vortex

With the startup of the aero-engine, the ground vortex is formed between the ground and the engine intake. The ground vortex leads to total pressure and swirl distortion, which reduces the performance of the engine. The inhalation of the dust and debris through a ground vortex can erode the fan blade, block the seals and degrade turbine cooling performance. As the diameter of the modern fan blade becomes larger, the clearance between the intake lip and the ground surface is smaller, which enhances the strength of the ground vortex. Though considerable numerical studies have been conducted with the predictions of the ground vortex, it is noted that the accurate simulation of the ground vortex is still a tough task. This paper presents authors’ simulation work of the ground vortex into an intake model with different crosswind speeds. This paper tackles the challenge with a parametric study to provide useful guidelines on how to obtain a good match with the experimental data. The influence of the mesh density, performance of different turbulence models and how the boundary layer thickness affects the prediction results are conducted and analysed. The detailed structure of the flow field with ground vortex is presented, which can shed light on the experimental observations. A number of suggestions are presented that can pave the road to the accurate flow field simulations with strong vorticities.

High-fidelity simulation of the interaction between a blade cascade and an ingested vortex

The aim of this research is to analyze the flow generated by the interaction between a simplified linear blade cascade adapted from an industrial aircraft engine demonstrator and an ingested vortex with the aerodynamic characteristics of a ground vortex observed in experiments. Due to the lack of availability of experimental and high-fidelity numerical studies of this phenomenon in the literature, a representative academic test case with a high-fidelity hybrid RANS/LES resolution of turbulence, namely the Zonal Detached Eddy Simulation (ZDES) technique, is performed. After the description of the computational setup and the numerical parameters used, the instantaneous topology of this complex flow is discussed, especially the interaction of the compressor cascade with the dynamics of the vortex. The simulation shows the chopping of the vortex, its destabilization and its dislocation into turbulent structures downstream the blade cascade. Turbulence is finely resolved in the blade wake and in the chopped vortex by ZDES technique at a moderate computational cost thanks to the RANS treatment of attached boundary layers with an applicative value of Reynolds number (5⋅106). A URANS simulation is also performed and allows a good representation of the averaged vortical structures inside the vortex close to the phase-averaged reference simulation. Spectral analyses are performed on unsteady velocity signals from probes located inside and outside the vortex chopped by the blades, highlighting more intense axial velocity fluctuations from turbulent structures inside the transformed vortex than in the wake of the blades, these structures being convected downstream in the direction of the Outlet Guide Vane (OGV).

Characteristics and evolution of ground vortex under heaving ground conditions: Insights from large eddy simulation (LES) analysis

When an aircraft engine operates near the ground, the ambient vortices generated by the freestream velocity boundary layer and engine suction will gather and lift, forming ground vortices which may result in intake distortion, consequently affecting the engine operation stability. To develop a shipborne test platform, this study conducted large eddy simulation calculations using the ANSYS Fluent and based on the assumption of incompressible flow. The 3-D unsteady Navier-Stokes (N-S) equations were solved using the WALE sub-grid model. The analysis focused on the characteristics and evolution of ground vortices under heaving ground conditions within 1 s. The results indicate that the ground vortex exhibits strong unsteady characteristics. During the descending phase of the platform, the quantity of vortex formed beneath the intake duct would decrease while the trailing vortex becomes more pronounced. However, the structure of the inlet vortex remains relatively unchanged at each time step. The intensity of the vortex and pressure distortion coefficient exhibit significant fluctuations over time, and their temporal variations approximately follow a sinusoidal function, which is also the heaving motion of the platform. Generally, when the velocity of the platform reaches its maximum downward vertical value, the average vortex intensity of the ground vortex tends to be minimal while the pressure distortion coefficient approaches its maximum value. Conversely, when the velocity of the platform reaches its maximum upward vertical value, the opposite trend can be observed.

Numerical study on aerodynamic performance of an intake duct affected by ground effect

Abstract This paper numerically studied the aerodynamic performance of an intake duct affected by the ground effect on a mobile test bench. The simulations were conducted under no wind and headwind conditions. The time evolution of the ground effect indicates that the coherent structure of the vortex system is mainly composed of the ground vortex, the horse-shoe vortex, and the creeping vortex under no wind condition. And it is mainly composed of the ground vortex, the trailing vortex, and the creeping vortex under headwind condition. Compared to the results under no wind condition, the integral vorticity of the ground vortex is larger than that under the headwind condition. The difference of the total pressure recovery coefficient is small, and the total pressure distortion index is large. The results show that with the decrease of the velocity at the intake duct outlet, the intensity of the ground vortex decreases, and the total pressure recovery coefficient at the intake duct outlet increases.

An Optimal Numerical Strategy for Intake in Crosswind Conditions

Abstract Crosswind can reduce the operability of an aeroengine significantly at static or near static operating conditions. Computational fluid dynamics predictions of the flow at crosswind conditions will play an important part in future designs, however, accurate numerical predictions of the flow within the intake remain challenging even for simulations of cases with intake only. The main objective of this paper is to demonstrate the importance of numerical setup and to determine an optimal computational model for crosswind investigations that can be used by other researchers. By considering the flow to be inherently unsteady, the influence of inlet and exit boundary conditions, and grid sensitivity is studied by using unsteady Reynolds-average Navier–Stokes (URANS) simulations. Numerical predictions of the time-averaged intake pressure recovery (IPR) and the motion of the vortex on the ground are compared against the existing experimental data. The results show that for an intake under crosswind, the ground vortex that forms under the intake and the in-duct separation, when present, exhibit unsteady behavior that become stronger as the crosswind velocity is increased. The steady-state simulation is only representative at lower crosswinds. The intake flow separation and ground vortex predictions are influenced by the inlet boundary layer profiles. Moreover, acoustic reflection was observed at the intake exit boundary which propagates upstream, creating artificial unsteady frequencies in IPR. The reflection is generated from the use of uniform boundary conditions at the intake exit and is not dissipated in the grid due to the long wavelength; this can be mitigated by using a choked nozzle at the intake exit. Acoustic reflection was also observed at the far-field exit boundary. These reflections are caused from interaction of trailing vortex and far-field boundary.

Study on aerodynamic development in Formula One racing

Formula One (F1) represents the pinnacle of automotive engineering. Aerodynamics is a critical factor that determines the performance of these high-speed racing machines. This paper aims to provide a comprehensive overview of the evolution and advancements in aerodynamic design and technology in Formula One over the years. Starting from the rudimentary designs of the 1950s, this paper traces the progression to the highly sophisticated Computational Fluid Dynamics (CFD) simulations and wind tunnel tests that dominate the sport today. Innovations like ground effects, diffusers, DRS (Drag Reduction System), and vortex generators are examined to elucidate how they contribute to increasing downforce, reducing drag, and thereby enhancing performance. This paper touches upon the future prospects of aerodynamic research in Formula One, including sustainability considerations and potential areas for innovation. Through a synthesis of historical trends, technical breakthroughs, and empirical data, this paper may provide a holistic understanding of aerodynamics in the context of Formula One racing.

Ground test and numerical investigation on aerodynamic performance of a quad tilt-rotor aircraft in ground and water effects

Ground and water interferences significantly affect the safety of takeoff and landing for Quad Tilt-Rotor (QTR) aircraft. However, the mechanism is extremely complex. An unsteady numerical simulation method suitable for amphibious QTR aircraft is established and verified through ground tests. This study explores the distinct aerodynamic characteristics of the QTR aircraft in water and ground effects (IWE and IGE) and compares them to those out of ground effect (OGE). Results indicate that the wing, ground or water surface strongly affects the QTR's aerodynamic performance by influencing the downwash flow form the rotors. The lift and torque coefficients exhibit irregular fluctuations during IGE compared with OGE. However the distribution of lift and torque coefficients become more periodic and regular during IWE due to the flexible water surface, which serves as a flow rectifier accelerating the dissipation of ground vortices and rotor tip vortices. The soft water surface is impacted to form a “drainage area”, which increases the distance between the rotor and the water. Therefore, the water effect is not as strong as the ground effect. The higher the height of rotors above the surface, the weaker the interference effects. The water effect vanishes when the vehicle is at least 0.9 times the rotor diameter above the water surface while the ground effect vanishes when the vehicle is at least one rotor diameter above the ground.

Lift and Tip Vortices Generated by Tapered Backward-Swept and Forward-Swept Wings under Stationary Ground Proximity

An experimental investigation of the aerodynamics and near-field tip-vortex flow field behind tapered backward- and forward-swept wings with a stationary ground effect was conducted at Reynolds number (Re)=1.81×105. The results showed a large lift increase of 26.7% and 12.3% for the backward-swept wing (BSW) and forward-swept wing (FSW), respectively, with reduced ground clearance, along with a significant drag reduction of 45% and 30% for the BSW and FSW. For the BSW, a multiple-vortex system appeared in close ground proximity, consisting of a tip vortex, a corotating ground vortex, and a counterrotating secondary vortex. The ground vortex strengthens the tip vortex, whereas the secondary vortex negates its vorticity. For the FSW, the multiple-vortex system was not readily identifiable due to its unique geometry, which always keeps the inboard region of the wing at a close ground effect while leaving the tip region less affected by the ground effect. The root stall of the FSW also produced a continuously strengthening tip vortex with the increasing angle of attack. In contrast, the tip stall of the BSW led to a monotonically increasing vortex strength only up to the static-stall angle. Regardless of the wing model, the weak tip vortex also translates into a small lift-induced drag compared with the total drag. Finally, the lift force computed through the integration of the spanwise circulation distribution, inferred from the cross-flow measurements, at selected ground distances was also found to be in good agreement with the direct wind-tunnel force-balance data, with 101% and 98% consistency for BSW and FSW, respectively. The aerodynamics and tip-vortex measurements of both wing models outside the ground effect were also acquired to serve as a comparison.

Longitudinal Aerodynamic Characteristics of Ducted Fan Propelled Fixed-Wing VTOL Aircraft Hovering in Ground Effect

The aerodynamic performances and control characteristics of vertical takeoff and landing aircraft will be significantly affected by the ground effect during takeoff and landing. The longitudinal aerodynamic characteristics and main flow features of a fixed-wing vertical takeoff and landing aircraft hovering in ground effect are investigated in this paper, using Multiple Reference Frame based numerical simulations. The aircraft is propelled by three ducted fans. Results show that the overall morphology of the flow is characterized by a fountain, ground vortex, reingestion, and recirculation. When the thrust distribution between the front and aft ducted fans changes, the flow features change as well. As the aircraft approaches the ground, the ducted fan thrust decreases, the fuselage lift increases, and the total lift first decreases and then increases. The maximum lift increases by 9.6% and the minimum lift decreases by 3.0% compared with that out-of-ground effect. The magnitude of the drag is about 1% of the lift, which has little influence on the aircraft’s performance. The pitching moment gradually changes from the equilibrium state to a significant nose-up moment. The total power consumed increases at a specific rotational speed.

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.

Ground effects on the aerodynamics of a wing with slot type distributed propulsion system for VTOL applications

Abstract During take-off of a vertical take-off and landing (VTOL) aircraft, ground effects can cause a downward force on the aircraft body and wings. The downward force could substantially reduce the payload of the aircraft, which is undesirable. This paper investigates the ground effects related to VTOL applications with distributed propulsion. A slot jet is used to simulate the distributed propulsion system. A model of a wing with a slot jet placed near the trailing edge of the wing is investigated. The slot jet is almost perpendicular to the ground to provide a vertical thrust. Experimental and numerical methods are used to investigate the aerodynamic performance of this model. Theoretical analysis is carried out to understand the formation mechanism of the low pressure region on the lower surface of the wing, which causes the downward force. The flow physics of the jet inducing ground vortex is investigated. It is found that the convection term in the ground vortex area is the main source of the pressure reduction. Based on the flow mechanism, a redesigned configuration is proposed to reduce the negative effect of the ground vortex. The flow structures such as the tip vortex are also investigated.

Способ расчетно-экспериментального определения статистических характеристик коэффициента лобового сопротивления посторонних предметов

The paper analyzes the work on the problem of damage to gas turbine engine (GTE) elements by foreign objects thrown by a ground vortex. The main approaches to determining the initial conditions necessary for modeling the throwing of the foreignobjects at the entrance to the aircraft power plant, including their aerodynamic parameters,are considered. A computational and experimental method is proposed for determining the drag coefficient of foreign objects. The paper presents the results of testing of the proposed method, namely the statistical characteristics of the aerodynamic parameters of foreign objects

Lift Computation Through Crossflow Measurement Behind a Rectangular Semiwing in Ground Effect

Abstract The lift force of a rectangular semiwing in stationary ground effect was computed based on the spanwise circulation distribution inferred from wake crossflow measurement and was compared with the direct force balance data at Re = 2.48 × 105. The lift calculated was found to be in good agreement with the force-balance measurement for ground distances h larger than 10% of the airfoil chord c. In close ground proximity (h/c ≤ 10%), a large discrepancy however existed due to the appearance of a multiple vortex system consisting of a tip vortex, a corotating ground vortex, and a counter-rotating secondary vortex. The counter-rotating secondary vortex offset the vorticity of the tip vortex. By considering its contribution additive, the discrepancy observed between the lift forces computed and measured with the force balance in close ground proximity became marginal. The impact of ground effect on lift-induced drag was also discussed.

Unsteady Analysis of Aeroengine Intake Distortion Mechanisms: Vortex Dynamics in Crosswind Conditions

Abstract The formation of a ground vortex and its ingestion into an aero-engine intake under crosswind conditions play a significant role in the aerodynamic excitation of the fan. Using steady and unsteady numerical simulations, an analysis of the dynamics of several distortion features is presented. For a simplified intake at high crosswind velocities, there is a substantial movement of the ingested ground vortex at the aerodynamic interface plane (AIP). The ingested ground vortex follows a specific trajectory while varying both in size and strength. The transition from a periodic to an aperiodic regime of the intake distortion at the AIP occurs as the crosswind velocity is increased. Circumferential mode decomposition shows that the largest amplitude of the distortion occurs at the first circumferential mode, and the amplitudes of the higher modes decrease monotonically. Furthermore, the amplitude of the spatial harmonics is time-dependent, which may be an influential feature at the time of assessing fan forced response.

Aerodynamic Characteristics of a Ducted Fan Hovering and Transition in Ground Effect

Ducted fans installed on vertical takeoff and landing vehicles experience significant ground effect during takeoff and landing. The aerodynamic characteristics of a ducted fan hovering and transitioning in the ground effect are studied using numerical simulations in this paper. The flowfields are obtained by solving Reynolds Averaged Navier–Stokes equations with the Multiple Reference Frame approach. When a ducted fan hovers in the ground effect, the blade thrust increases due to the combined effect of the increase in the effective angle of attack of the blade and the increase in ambient pressure; the duct thrust decreases due to the combined effect of the decrease in the effective angle of attack of the duct and the increase in ambient pressure. Stall occurs at a certain advance ratio and angle of attack when transitioning in the ground effect. The ground effect delays the occurrence of stall at some advance ratios. The ground effect is hardly detectable at angles of attack less than 30° even if the height drops to 0.5 times the duct exit diameter. At this height and high angles of attack, the different positions and influence regions of the ground vortex at different advance ratios contribute to the different variation trends in the ducted fan performance.

A CFD/experimental comparative database to feed a predictive model for ground vortex characteristics

Most modern aircraft engines rely on the use of high Bypass Ratios (BPR) turbofans to achieve both high thrust and low specific fuel consumption. However, such configurations are prone to the formation of ground vortices during low-speed operations. This phenomenon arises under specific combinations of wind direction, velocity or inlet air speed, generating engine vibrations and leading to the suction of damaging abrasive particles. Its characterization in early design stages is crucial. In this work, a joint experimental and numerical exploration of operating conditions leading to ground vortex presence is carried out on a scaled wind tunnel configuration. Flow details are investigated for several working points obtained from a specific set of input parameters (intake speed, wind speed, ground clearance). A methodology suitable for both experimental and Computational Fluid Dynamics (CFD) works is developed to extract vortex characteristic quantities, based on a local pressure minimum and Q-criterion contours topology. A very good agreement is obtained when comparing vortex predictions stemming from CFD and experiments. This database shall be used to transpose experimental data probed outside of the nacelle to data within the nacelle using data analytics techniques, paving the way for future data driven predictive models.

Numerical Study of the Main Rotor Wake Structures and Induced Velocity Fields at the Tail Rotor Location When Flying Near the Ground

This paper considers the Mi-8 helicopter main rotor aerodynamics when flying near the infinite ground surface. The research is based on the free wake model developed by authors at Moscow Aviation Institute. The distance from the rotor's hub to the ground surface in the range of H = 6-16 m and the values of free stream (flight) velocity in the range of V = 0-15 m/s are considered. The results of the visualization for both rotor wake shapes and streamlines are obtained. The influence of the ground proximity on the rotor wake shape, including the formation of "supervortex" and "ground vortex" structures are analyzed. The induced velocity fields of the main rotor in the area of the tail rotor location for various azimuth positions relative to the main rotor axis are studied. The conclusion is made about the significant influence of the ground effect on the rotor wake structure and induced velocities field, including the area of the tail rotor location. Particularly, at slip flight with speed V = 10 m/s, when the distance to the ground surface H increases from 6 to 12 m, the value of the average induced velocity at the tail rotor plane is growing up to four times. The obtained data have allowed taking into account such effects of aerodynamic interference in the simplified mathematical model of the research flight simulator of JSC Helicopters Mil and Kamov.

Research on the ground vortex and inlet flow field under the ground crosswind condition

The ground vortex field and nacelle inlet flow field under ground crosswind condition are investigated experimentally and numerically. A test rig with ground plane and crosswind blowing devices have been built, the experiments have been carried out by DSA3217 Pressure Scanner and Hot Wire Anemometer to get inlet total pressure distribution and the unsteady characteristics of ground vortex, respectively. The characteristics of ground vortex are quantitatively analyzed respect to vortex intensity, frequency, and dynamics, then the study crosswind interact on ground vortex is conducted, by considering the direct influence of crosswind on inlet distortion and the indirect effect of crosswind on ground vortex. Finally, it is found that due to the ground vortex, the process that inlet distortion increases with crosswind is unsteady and unstable. According to the variation of vortex intensity and the influence level that ground vortex acted on the inlet flow field, the process of blowing away the ground vortex from the beginning to the end can be divided into four phases, the encouraging phase, the initial blowing phase, the rapid blowing phase and the end blowing phase.

Influence of gap-ratio on flow dynamics and heat transfer for a square cylinder approaching a moving wall in turbulent regime

Flow past a square cylinder near a moving wall is investigated for 0.1 ≤ G/D (gap-ratio) ≤ 1.0 at a constant Reynolds number of 22,000 using large eddy simulations. Influence of the gap-ratio on various flow features has been studied. It is observed that for G/D ≤ 0.3, a regular Kármán vortex shedding from the cylinder gets suppressed which causes a large change in the flow dynamics. Unlike the case of a stationary wall, no ground vortex is formed in the present case. For G/D = 1.0, turbulence is strong in the near wake region and for low G/D values (≤ 0.3) it gets weaker and shifts downstream. With a decrease in G/D, an early reattachment of the lower shear-layer with the bottom surface of the cylinder occurs while the upper shear-layer is deflected and K-H instability occurs away from the top surface. The so-called ‘extreme events’ which contain high frequency fluctuations are observed for G/D = 1.0 and these disappear for low G/D values. Large coherent structures are observed for high G/D (≥ 0.5) while for low G/D uniformly distributed small structures are formed. Evolution of enstrophy has been examined for G/D = 0.1 and the role of the strain-rate tensor in the enstrophy production has been investigated. The lift coefficients are negative for all G/D values while drag coefficients decrease with a decrement in G/D and are nearly constant for G/D ≤ 0.3. Due to the formation of large coherent structures, power spectra for high G/D follows -5/3 law over a large range of frequency. Unlike the case of a laminar flow, Nusselt number on the cylinder surface monotonically decreases with a decrease in G/D.

Effect of Ground Boundary Condition on Near-Field Wingtip Vortex Flow and Lift-Induced Drag

Abstract The ground proximity is known to induce an outboard movement and suppression of the wingtip vortices, leading to a reduced lift-induced drag. Depending on the ground boundary condition, a large scatter exists in the published lift-induced drag and vortex trajectory. In this experiment, the ground boundary condition-produced disparity in the vortex strength and induced drag were evaluated. No significant discrepancy appeared for a ground distance or clearance larger than 30% chord. As the stationary ground was further approached, there was the appearance of a corotating ground vortex (GV), originated from the downstream progression of a spanwise ground vortex filament, which added vorticity to the tip vortex, leading to a stronger tip vortex and a larger lift-induced drag compared to the moving ground. For the moving ground, the ground vortex was absent. In close ground proximity, the rollup of the high-pressure fluid flow escaped from the wing's tip always caused the formation of a counter-rotating secondary vortex, which dramatically weakened the tip vortex strength and produced a large induced-drag reduction. The moving ground effect, however, induced a stronger secondary vortex, leading to a smaller lift-induced drag and a larger outboard movement of the tip vortex as compared to the stationary ground effect.