Helicopter blade tip vortex simulation by DLR

A blade vortex interaction (BVI) is an unsteady phenomenon of three-dimensional nature, which occurs when a

rotor blade A helicopter main rotor or rotor system is the combination of several rotary wings (rotor blades) with a control system, that generates the aerodynamic lift force that supports the weight of the helicopter, and the thrust that counteracts aerod ...

passes within a close proximity of the shed tip vortices from a previous blade. The

aerodynamic Aerodynamics, from grc, ἀήρ ''aero'' (air) + grc, δυναμική (dynamics), is the study of the motion of air, particularly when affected by a solid object, such as an airplane wing. It involves topics covered in the field of fluid dyn ...

interactions represent an important topic of investigation in

rotorcraft A rotorcraft or rotary-wing aircraft is a heavier-than-air aircraft with rotary wings or rotor blades, which generate lift by rotating around a vertical mast. Several rotor blades mounted on a single mast are referred to as a rotor. The Inter ...

research field due to the adverse influence produced on rotor noise, particularly in low speed descending flight condition or maneuver, which generates high amplitude impulsive noise.

Classes of blade vortex interactions

Literature distinguishes different classes of BVIs in

helicopter rotor A helicopter main rotor or rotor system is the combination of several rotary wings (rotor blades) with a control system, that generates the aerodynamic lift force that supports the weight of the helicopter, and the thrust that counteracts aero ...

s depending on the impacting

vortex In fluid dynamics, a vortex ( : vortices or vortexes) is a region in a fluid in which the flow revolves around an axis line, which may be straight or curved. Vortices form in stirred fluids, and may be observed in smoke rings, whirlpools in t ...

axis with respect to the blade span. Generally, It can be divided into four distinct types, which will be described as follows:

Parallel BVI

Parallel Parallel is a geometric term of location which may refer to: Computing * Parallel algorithm * Parallel computing * Parallel metaheuristic * Parallel (software), a UNIX utility for running programs in parallel * Parallel Sysplex, a cluster of I ...

BVI occurs when the vortex and the blade axes are nominally parallel. It is the BVI phenomenon that produces the largest-amplitude impulse (harmonic) noise, due to that the unsteady vortex moves towards to the downstream.

Perpendicular BVI

Perpendicular In elementary geometry, two geometric objects are perpendicular if they intersect at a right angle (90 degrees or π/2 radians). The condition of perpendicularity may be represented graphically using the ''perpendicular symbol'', ⟂. It can ...

BVI occurs when the axes are perpendicular and in parallel planes. Due to its low unsteadiness, the noise effect of perpendicular BVI are less significant with respect to parallel BVI. It produces a continuous broadband noise characterised by a much lower intensity compared to the impulse (harmonic) noise, which caused by parallel BVI.

Oblique BVI

Oblique Oblique may refer to: * an alternative name for the character usually called a slash (punctuation) ( / ) *Oblique angle, in geometry * Oblique triangle, in geometry * Oblique lattice, in geometry * Oblique leaf base, a characteristic shape of the ...

BVI occurs between the vortex and the blade when the axes are oblique. In helicopter research field, oblique BVI is a common phenomenon that looks like an intermediate action of parallel BVI and perpendicular BVI.

Orthogonal BVI

Orthogonal In mathematics, orthogonality is the generalization of the geometric notion of '' perpendicularity''. By extension, orthogonality is also used to refer to the separation of specific features of a system. The term also has specialized meanings in ...

BVI occurs when the axes of the vortex are in orthogonal planes. In the context of helicopter application, the orthogonal interaction usually exits between the tip vortices generated by the

main rotor A helicopter main rotor or rotor system is the combination of several rotary wings (rotor blades) with a control system, that generates the aerodynamic lift force that supports the weight of the helicopter, and the thrust that counteracts aerod ...

and the blade of the

tail rotor The tail rotor is a smaller rotor mounted vertically or near-vertically at the tail of a traditional single-rotor helicopter, where it rotates to generate a propeller-like horizontal thrust in the same direction as the main rotor's rotation. T ...

Means of BVIs prediction

As a predominant source of noise, BVI phenomenon can be detrimental to blade structure integrity as well because of the unsteady fluctuation of aerodynamics, such as vortex buffeting and dynamic stall in the retreating blade. Therefore, BVI becomes a prime concern in the helicopter research field. In order to understand the BVI flow characteristic more closely and suppress the

noise Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference aris ...

and

vibration Vibration is a mechanical phenomenon whereby oscillations occur about an equilibrium point. The word comes from Latin ''vibrationem'' ("shaking, brandishing"). The oscillations may be periodic, such as the motion of a pendulum—or random, suc ...

actively, it is important to predict the BVIs precisely. Recently, the tools for capturing BVIs can be divided into three parts, which will be described as follows:

Wind tunnel test

As for aerodynamic problems,

wind tunnel Wind tunnels are large tubes with air blowing through them which are used to replicate the interaction between air and an object flying through the air or moving along the ground. Researchers use wind tunnels to learn more about how an aircraft ...

test is a basic tool used in research. In 1994, researchers from German DLR, French ONERA,

NASA Langley The Langley Research Center (LaRC or NASA Langley), located in Hampton, Virginia, United States of America, is the oldest of NASA's field centers. It directly borders Langley Air Force Base and the Back River on the Chesapeake Bay. LaRC has foc ...

, and the US Army Aeroflightdynamics Directorate (AFDD) formed an international consortium to carry out a comprehensive experimental program which is denominated HART I (Higher Harmonic Control Aeroacoustic Rotor Test I) project at the large low speed facility of DNW (German-Dutch wind tunnel). In this test, a 40% scaled BO-105 rotor model along with a fuselage is used, a range of sophisticated measurement techniques are introduced to measure the noise level, blade surface pressure, tip vortices, blade motions, and structural moments with and without the application of HHC (Higher Harmonic Control) pitch control inputs. In 2001, an update program named HART II was conducted to improve the basic understanding and the analytical modeling capabilities of rotor BVI noise with and without higher harmonic pitch control (HHC) inputs, particularly the effect of rotor wakes on rotor

and

Analytical methods

The accurate simulation of the vortex structure in the wake is a crucial part on BVI research. Currently，the analytical methods for BVI phenomenon capturing are mainly based on the free wake model, which has highly efficiency but serious dependence on empirical parameters and cannot include air viscosity effect, furthermore, the

aerodynamics Aerodynamics, from grc, ἀήρ ''aero'' (air) + grc, δυναμική (dynamics), is the study of the motion of air, particularly when affected by a solid object, such as an airplane wing. It involves topics covered in the field of fluid dyn ...

calculated in free wake model are based on the

lifting-line theory The Prandtl lifting-line theory is a mathematical model in aerodynamics that predicts lift distribution over a three-dimensional wing based on its geometry. It is also known as the Lanchester–Prandtl wing theory. The theory was expressed inde ...

with the drawback in air load capturing and flow field describing, especially for the characteristics of

transonic Transonic (or transsonic) flow is air flowing around an object at a speed that generates regions of both subsonic and supersonic airflow around that object. The exact range of speeds depends on the object's critical Mach number, but transoni ...

flow.

Computational fluid dynamics methods

In the last fifty years, Computational Fluid Dynamics (CFD) methods experienced great development since the CFD method was first applied to the

helicopter A helicopter is a type of rotorcraft in which lift and thrust are supplied by horizontally spinning rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forward, backward and laterally. These attribut ...

research in 1970s. The development of rotor CFD has undergone three stages. * Full-Potential Equations *

Euler Equations 200px, Leonhard Euler (1707–1783) In mathematics and physics, many topics are named in honor of Swiss mathematician Leonhard Euler (1707–1783), who made many important discoveries and innovations. Many of these items named after Euler includ ...

Navier–Stokes Equations In physics, the Navier–Stokes equations ( ) are partial differential equations which describe the motion of viscous fluid substances, named after French engineer and physicist Claude-Louis Navier and Anglo-Irish physicist and mathematician G ...

( RANS, LES) The full-potential equations are based on the potential flow theory, but the result computed by this method is usually larger than the real one as it ignored the effect of wake. Currently, it can be applied for predicting BVIs as well due to the excellent advantages in computational efficiency. With the development of computer technology,

Euler Leonhard Euler ( , ; 15 April 170718 September 1783) was a Swiss mathematician, physicist, astronomer, geographer, logician and engineer who founded the studies of graph theory and topology and made pioneering and influential discoveries in ma ...

/ Navier-Stokes equations started to used for rotor aerodynamic research. Compared with the full-potential equation,

/ Navier-Stokes equations can not only accurately capture the nonlinear flow phenomenon of the rotor flow field, but can also capture the motion of the blade tip vortex in the computational domain. At present,

/ Navier-Stokes equations have become the dominant method in the field of rotor CFD of

. However, due to the complicate the rotor flow field, there are still many problems need to be solved, such as blade motion,

elastic deformation In engineering, deformation refers to the change in size or shape of an object. ''Displacements'' are the ''absolute'' change in position of a point on the object. Deflection is the relative change in external displacements on an object. Stra ...

, mesh density and rotor wake capture.

Hybrid methods

Currently, researchers have developed some kind of hybrid technologies to tackle the above problems. For example, high fidelity detached eddy simulation (DES) method was conducted to precisely predict air loads near the blade; the adapted Chimera grids method was used for accurately capturing the vortex shed by blades; the CFD/CSD (Computational Structure Dynamics) was carried out widely to more effectively account for the change of the flow field caused by the elastic deformation of blades. Meanwhile, some scholars have begun to introduce high-resolution discrete vortex model(DVM) into the CFD/CSD method. The CFD/CSD/DVM method can not only improve the accuracy of calculation of BVIs, but also effectively eliminate the shortcomings of CFD methods on numerical, furthermore, it can greatly decrease the computation sources. It is an important direction that is worthy of further development in BVI prediction.

References

External links

Rotor Analysis - Blade Element Momentum Theory

{{Helicopters and rotorcraft

Rotor Rotor may refer to: Science and technology Engineering *Rotor (electric), the non-stationary part of an alternator or electric motor, operating with a stationary element so called the stator *Helicopter rotor, the rotary wing(s) of a rotorcraft ...