The computational fluid dynamics (CFD) is currently considered as a vital device for the study of fluid dynamics and developing a new aircraft, Neal M. C 1. Advances had been made in understanding helicopter fluid aerodynamics using CFD, Gordon J.L 2. Deferent helicopter CFD analysis had been done comparing CFD analysis results and experimental data or blade element theory results.
The majority of helicopter CFD analysis reviewed in this literature showed that CFD analysis results were in reasonable agreement with experimental data and few showed a bit discrepancy between CFD analysis results and experimental data. The following paragraphs mentions different helicopter CFD analysis reviewed. 7 Perera GAPR et al 3 conducted a research on “helicopter main rotor aerodynamic simulation with CFD”. The main objective of their research was to analyze the selected bell 212 main rotor under two main helicopter aerodynamic theories named Blade Element Theorem and Momentum Theorem.
The CFD simulation for hover, Forward flight, HIE and HOE flying maneuvers were performed. The rotor blade of NACA 0012 aero-foil was used. To simulate these rotor blades they used rotating mesh, ANSYS Fluent, a surface and volume mesh continuum containing approximately seven million polyhedral cells and Finite Volume Method (FVM) as discretization technic. In hover, 800rpm and angle of attach of 2° was used.
Implicit unsteady flow solver with ideal as and SST (Mentar), K-Epsilon turbulent model, estimated drag, lift, and momentum coefficients were also taken. The simulation results and actual results were compared and further analyzed. Several deviations were observed between CFD results and real data calculation for bell 212 main rotor. The forecasted values of aerodynamic parameters for Bell 212 main rotors were little bit different than expected. In their conclusion, this particular fact was directly related to computational limitations associated with CFD. 8 However, in “CFD analysis of complete helicopter configuration-lessons learnt from the go-ahead project” performed by René S.
and George N. B. 4, the finding showed that pre-test computations for economic cruise condition had been in reasonable agreement with the experimental results. The comparison considered surface pressure at various places on the fuselage taking into account the relative coarseness of the used grids. CFD results of various patterns also agreed reasonable well. But, the discrepancies in the separated flow regions at the back of the helicopter were noticed.
By improving meshes, a better spatial resolution of the flow was found. The mesh quality was the key for accurate predictions as well as educated estimation of the flow regions where intensive interactions of the flow structures take place for a complex CFD computation. 9 Christian R. 5 Worked on “CFD Analysis of the Main-Rotor Blade of a Scale Helicopter Model using Overset Meshing”. The flow field was resolved utilizing star CCM+. Mesh continuum for volume and surface contained approximately seven million polyhedral cells and finite volume method discretization technic were used.
An implicit unsteady flow solver, ideal gas and SST K-Omega model were applied. Hover flight and forward flight had been evaluated. Forward flight was performed by varying angle of attach of rotor shaft and collective pitch angle and freestream Mach number of 0.128 (M=0.128) was used without including cyclic pitching motion. The flight case applying cyclic pitch motion had been evaluated at zero rotor shaft angle of attach and zero collective pitch angle. The experimental data for comparison had been taken from NASA report.
The results from the CFD for hover ?ight were in excellent agreement with the experimental data from wind tunnel. The CFD results for lift in cases of forward ?ight without applying cyclic motion coincided with the experimental data for lift. But there had been di?culties to produce a thrust for forward flight. It had been concluded that application of overset mesh to evaluate main rotor blades with application of computational fluid dynamic (CFD) does work. 10 Nik A.
et Al 6 analyzed “computational aerodynamics for hovering helicopter rotors”. Simulation of helicopter rotors in axial flight using the helicopter multi-block (HMB2) solver of Liverpool University for range of rotor tip speeds and collective pitch setting was conducted. The Parallel Helicopter Multi-block CFD solver was used and validated for the Caradonna and Tung model rotor in hover. Prediction of rotor hover performance, wake geometry and its strength using CFD methods were discussed. The blade loads, wake geometry and wake strength were analyzed and the impact of the number of mesh points on the blade loads and wake geometry were also investigated. Mesh of more than 3.6 and 9 Million points per blade were used.
Excellent agreement of the blade loads data and wake trajectories between CFD and experiment have been observed, and suggests that CFD can adequately resolve the loads and wake structure. 11 Nik A. et al 7 worked on “numerical analysis of an isolated main helicopter rotor in hovering and forward flight”. In this work aerodynamic characteristics of a 5-seater helicopter with various rotor geometry operating in forward flight mode were simulated with FLUENT software.
The main objective was to calculate the aerodynamic load generated by rotor during hovering and various forward flight velocity range. The method used to model rotating rotor was multiple references rotating frame and standard viscous k-? turbulent flow model. Rotor rotated in hover flight and forward flight. Calculation of coning angle and flapping angle was based on blade element theory. The comparison had been made between CFD results and blade element results.
The CFD simulation results and blade element theorem analysis was found to be in good agreement. 12 Nik A. R. N.
M. and Barakos G. 8 worked on “Performance and Wake Analysis of Rotors in Axial Flight Using Computational Fluid Dynamics Flow”. The aim of the work was to validate the HMB solver and to improve the existing basic knowledge about the studied subject in this research.
The analysis was carried out using HMB on rotors in hover and vertical ascend flight, the surface pressure over the blades, the performance of integrated rotor, and the trajectory of vortex wake. The results were evaluated with the experimental data of the UH-1H rotor. The investigation of detailed rotor velocity field of the tip vortex in hover flight was performed. HMB solver predicted well the rotor blade aerodynamic performance in comparison to experimental data and HELIX-I data.
Small discrepancies could be observed for low ascending rate. A strong similarity of the swirl velocity profile had been found. The reasonable agreement between predicted results and experimental data were found for hover flight and descent rate. Unsteady solution was suggested for rotor in vortex ring state.
This work validated HMB solver on rotor in axial flights utilizing several rotor test data. 13 Ulrich K. et al 9 worked on “CFD-simulation of the rotor head influence to the rotor-fuselage interaction” they investigated interaction phenomenon of fluid-structure of a rotorcraft in fast forward flight. Detailed model, involving the swashplate and the control rods was considered because of the great influence of the main rotor head on the wake structure. The rotor head configuration had been simulated in many variants to find the solution of the influence of the components. The Compact reconstruction fifth order Weighted Essentially Non-Oscillatory fluid state reconstruction scheme for an improved rotor wake conservation was applied.
An up wind HLLC Riemann was used to solve the flux computation. The fundamental variation of unsteady flow behaviors was noticed by analyzing the flow field and force. The substantial impact of incoming flow from the rotor wake was observed. The strong difference was found particularly in the region of low intensity of the wake after comparing different configuration. 14 Gupta R.
and Agnimitra 10 performed on” Computational fluid dynamics analysis of a twisted three-bladed H-Darrieus rotor” to evaluate the performance of a twisted three-bladed H-Darrieus rotor steady-state two-dimensional computational fluid dynamics analysis was studied utilizing FLUENT 6.2 software. Unstructured-mesh finite volume method coupled with moving mesh technique to solve mass and momentum conservation equations were applied for simulation of the flow over the rotor blade. The standard k-? turbulence model was chosen for pressure-velocity coupling Second-order upwind discretization scheme. drag coefficient , lift coefficient, and drag-to-lift coefficient were analyzed with respect to angle of attack (AoA) for two chord Reynolds numbers(Re).
The power coefficient (Cp) of the rotor and the effect of twist angle at the chord ends on effective performance of rotor were analyzed. Validation was made by using experimental data for twisted three-blade Darrius rotor. The comparison of the two approaches showed good agreement. 15 Khier w. et al 11 conducted the research on “Trimmed CFD Simulation of a Complete Helicopter Configuration”.
The aim of research was to evaluate the aerodynamic interference between the rotating elements and no rotating elements of the rotorcraft. It was performed utilizing the flight mechanics tool HOST weakly coupled to the RANS solver FLOWer. The flow over rotorcraft configuration under various flight conditions was simulated. The analysis showed a noticeable variation in the load distribution between isolated main rotor and full rotorcraft case. Slight disagreement in the computed pressure was found on rotor blades between the isolated rotor and the complete helicopter. The power consumption was found to be negligible.
Major variations in surface pressure and the fuselage loads were observed. 16 Fraunhofer IWES et al 12 worked on “Aerodynamic Simulation of the MEXICO Rotor “to validate open source CFD toolbox OpenFoam against the MEXICO data-set. The steady state and time-accurate simulations were conducted using the Spalart-Allmaras turbulence model for many operating cases. Axisymmetric inflow for three different wind speeds were used. The numerical data were evaluated with pressure distributions from many blade sections and PIV-flow data from the region located near the wake.
A good agreement between numerical results and experimental data was found. 17 Tung, C and Ramachandran K. 13 performed on” Hover performance analysis of advanced rotor blades” with aim of validating available hover flight prediction methods. This work used wake, an extensive set of loads and performance data as experimental basis. These data were captured from a pressure instrumented model UH-60 rotor. The model was had replaceable tips, comprising a tapered and a BERP-type tip which allowed evaluation of the effects of rotor blade configuration.
The central prediction method analyzed was a vortex embedded, free-wake and full-potential CFD method named HELIX-I. It was noticed that HELIX-I code provides great comparisons with the data comprising surface pressure, wake and performance. It was observed that the HELIX-I code provides a good compromise between comprehensive nature of Navier-Stokes methods and the speed of boundary integral methods. 2.2 ROTOR AERODYNAMICS 18 The helicopter main rotor generates vertical lifting force in against the aircraft weight and horizontal propulsive force (thrust) for forward flight. It provides a means of producing forces and moments to control the altitude and position of the helicopter 14. The knowledge of aerodynamics loads effects on the rotor blade dynamic response and environment in which the rotor operates is important.
Helicopter has ability to hover and to perform forward flight. 19 During hover, the helicopter main rotor blades move considerable volumes of air in a downward direction. This process accelerates the air to relatively high velocities and requires lots of horsepower. To perform hovering flight, the helicopter main rotor must generate lift (L) equal to the total weight (W) of the helicopter. with an increase of blade pitch and high rotor blades speed, the necessary lift for a hover is induced and reach a state of stable stationary hover. The rotor tip vortex affects negatively the effectiveness of the outer blade portions in hover flight.
The lift(L) of following blade is severely affected by vortex of preceding blade. For hovering the helicopter requires high power. This high-power requirement is effect of continuous creation of new vortexes and ingestion of existing vortexes. Unlike out of ground effect (OGE) operation, in ground effect (IGE) operation, the downward airflow patterns and outward airflow patterns tend to restrict vortex generation. Restriction of vortex generation results in increasing of efficiency of outboard part of the rotor blade and reduces overall turbulence of system produced by ingestion and recirculation of the vortex swirls 15. 20 In hovering flight, Collective pitch angle, tip Mach number and blade wake affect overall performance requirement for hover flight.
21 Collective pitch angle changes angle of attack of all rotor blades by an equal amount. The collective pitch is operated to control the average rotor blade pitch. Change in pitch angle changes the blade lift (L) and the average rotor trust (T) and increase the drag on blades. Increase in drag requires extra power 16. 22 A high rotor tip speed provides the rotor a high level rotational kinetic energy for a given radius.
The high rotor tip speed reduces the rotor design weight. However, compressibility effects and noise are two factors that oppose the use of high rotor tip speed. The compressibility effects increase rotor power requirements. With increase in Mack number the rotor noise increases rapidly. For maximum hover flight performance lower tip Mach number is required 16.
23 The wake due to the rotating blade comprises, in part, a vertical vortex sheet, with formation of concentrated vortex at the blade tip. The vortex sheet has a vorticity with vectors aligned mainly normal to and parallel to the trailing edge of the blade. Experiments have shown that blade tip vortices are almost fully rolled up within only a few degrees of blade 16. 24 For forward flight the rotor is tilted forward, and total lift and thrust forces are also tilted forward and generate resultant lift-thrust force. The generated resultant force that can be resolved into two components: lift (L) acting vertically upward and thrust (T) acting horizontally in the direction of flight.
In addition to lift and thrust forces, there is weight and drag. For steady forward flight, lift, thrust, drag, and weight must be in balance. When lift force exceeds weight, the helicopter accelerates vertically until both forces are balanced; if thrust is less than drag, the helicopter slows down until the forces are in balance. 15. 25 During forward flight, airflow moves opposite to the flightpath of rotorcraft.
The velocity of air-flow equals the velocity of rotorcraft in forward flight. The velocity of air flow across the blade is determined by: the point location of the rotor blade in plane of rotation at a given time, blade rotational velocity, and airspeed of the helicopter determine the velocity of the airflow across the blades. The airflow on rotating blade varies continuously with rotation of blade. The highest airflow velocity occurs over the one side of plane of rotation for advancing blade in a rotor system and decreases to rotational velocity over the nose. It continues to decrease until the lowest velocity of airflow occurs over other side for retreating blade 15. 26 When the helicopter begins to accelerate from a hover, the rotor system becomes more efficient.
Transitional lift results from improved rotor efficient due to acceleration of helicopter from hover flight to forward flight. As in-coming wind produced by helicopter movement enters the rotor system, vortices and turbulent stay behind and the airflow becomes more horizontal 15
