A computational model is presented that can be used to simulate the development of the dust cloud that can be entrained into the air when a helicopter is operated close to the ground in desert or dusty conditions. The physics of this problem, and the associated pathological condition known as ‘brownout’ where the pilot loses situational awareness as a result of his vision being occluded by dust suspended in the flow around the helicopter, is acknowledged to be very complex. The approach advocated here involves an approximation to the full dynamics of the coupled particulate-air system. Away from the ground, the model assumes that the suspended particles remain in near equilibrium under the action of aerodynamic forces. Close to the ground, this model is replaced by an algebraic sublayer model for the saltation and entrainment process. The origin of the model in the statistical mechanics of a distribution of particles governed by aerodynamic forces allows the validity of the method to be evaluated in context by comparing the physical properties of the suspended particulates to the local properties of the flow field surrounding the helicopter. The model applies in the Eulerian frame of reference of most conventional Computational Fluid Dynamics codes and has been coupled with Brown’s Vorticity Transport Model. Verification of the predictions of the coupled model against experimental data for particulate entrainment and transport in the flow around a model rotor are encouraging. An application of the coupled model to analyzing the differences in the geometry and extent of the dust clouds that are produced by single main rotor and tandem-rotor configurations as they decelerate to land has shown that the location of the ground vortex and the size of any regions of recirculatory flow, should they exist, play a primary role in governing the extent of the dust cloud that is created by the helicopter.

/pdf/eulerian-simulation-of-the-fluid-dynamics-of-helicopter-295mo5z7mk.pdf

Eulerian Simulation of the Fluid Dynamics of Helicopter Brownout

: Wind or rotorwash generated by rotors, ducts, or jets effect the operational suitability and utility of future Vertical Takeoff and Landing (VTOL) aircraft. As the aircraft s physical size, weight, and disk loading increases beyond the range of current systems, the risk increases that rotorwash operational impact(s) may compromise an aircraft s ability to satisfy the Warfighter's needs. Early definition of anticipated rotorwash conditions will permit an assessment of operational suitability and determine whether specific design changes are warranted and/or if acceptable operational tactics, techniques, and procedures can be established. This report documents the assessment process, environmental limits, rotorwash modeling, and output display supporting the rotorwash operational footprint model. These elements graphically combine to display the rotorwash operational impact assessment on the ground environment as contour plots or footprints. The tools developed are for the single main rotor helicopter, tandem helicopter, and tiltrotor configurations, but they can be extended to encompass additional configurations. The rotorwash operational footprint displays the effect of winds generated by rotor thrust on the surrounding environment. These footprints can be used to evaluate compliance with aircraft performance specifications, verify safe separation distances, or inform design trade studies.

Rotorwash Operational Footprint Modeling

Quantitative and qualitative results of a series of experiments conducted on a rotor in ground effect at low
forward speeds are presented. The velocity over a wide area of the ground effect wake was measured using Particle Image Velocimetry (PIV), and the evolution of the flow is described as the forward speed increases. Helicopter brown-out was simulated through a series of flow visualisation experiments. The technique involved sprinkling a fine powder on the ground below and ahead of the rotor. This helps to validate the experimental simulation of the brown-out phenomenon. Larger dust clouds were observed at lower advance ratio, and the dust cloud penetrated into the areas of the flow including those where vorticity levels were of low or negligible magnitude.

/pdf/measurements-of-a-rotor-flow-in-ground-effect-and-1fu3f3adsk.pdf

Measurements of a rotor flow in ground effect and visualization of the brown-out phenomenon

The propulsive dynamics of a flexible undulating foil in a self-propelled swimming configuration near a wall is studied experimentally. Measurements of the swimming speed and the propulsive force are presented, together with image acquisition of the kinematics of the foil and particle image velocimetry (PIV) in its wake. The presence of the wall enhances the cruising velocity in some cases up to 25% and the thrust by a 45% , for swept angles of 160 and 240°. The physical mechanisms underlying this effect are discussed by studying the vorticity dynamics in the wake of the foil. Proper orthogonal decomposition is applied to the PIV measurements in order to analyse the kinetic energy modal distribution in the flow and to relate it to the propulsion generated by the foil.

https://iopscience.iop.org/article/10.1088/1748-3190/10/1/016003/pdf

Large-amplitude undulatory swimming near a wall.

The propulsive dynamics of a flexible undulating foil in a self-propelled swimming configuration near a wall is studied experimentally. Measurements of the swimming speed and the propulsive force are presented, together with image acquisition of the kinematics of the foil and particle image velocimetry (PIV) in its wake. The presence of the wall enhances the cruising velocity in some cases up to 25% and the thrust by a 45%, for swept angles of 160 and 240 degrees. The physical mechanisms underlying this effect are discussed by studying the vorticity dynamics in the wake of the foil. Proper Orthogonal Decomposition (POD) is applied to the PIV measurements in order to analyse the kinetic energy modal distribution in the flow and to relate it to the propulsion generated by the foil.

Large-amplitude undulatory swimming near a wall

The ‘ground effect’ is the aerodynamic phenomenon whereby the flow field around a vehicle, either an aircraft or a car, is constrained and altered by the presence of the ground. For an aircraft operating in ground effect, it is the creation of an effective air cushion between the lower surface of the wing and the ground that modifies the physics of the flow resulting in positive lift enhancement. For a car utilizing ground effect, the dominant feature is a low-pressure field between the vehicle and the ground caused by significant flow acceleration, similar to the ‘Venturi effect’, leading to downforce (negative lift) enhancement. Components such as wings and diffusers operate within this regime. The impact of ground effect is generally an increase in aerodynamic efficiency, that is, an increase in lift-to-drag ratio for aircraft or the equivalent downforce-to-drag ratio for cars. When a vehicle operates too close to the ground, the beneficial response diminishes. The ground effect can adversely affect the flight stability of an aircraft during take-off and landing as well as under cruise conditions in close proximity to the ground.



This chapter provides a brief summary concerning the history of ground effect vehicles. Various methods employed to study the phenomenon are introduced; these include empirical approximations, analytical methods, numerical simulations, and wind tunnel tests. Applications of ground effect to aircraft and vehicle design are also discussed. Advantages and risks are further examined with an emphasis on control and stability.


Keywords:

aerodynamics;
ground effect;
induced drag reduction;
wing-in-ground effect;
diffuser-in-ground effect;
wing-in-ground effect vehicles

Erjie Cui

Papers

Ground effect aerodynamics