The paper presents CFD results for the wake of a helicopter flying a low altitude at different advance ratios.

The wakes are assessed in terms of topology and velocity magnitudes. The structure of the wake near ground

changes rapidly with the advance ratio and its decay appears to be faster than what is suggested by theoretical

analyses. The results show clear the potential of modern CFD for use in helicopter safety and highlights the

need for detailed surveys of helicopter wakes using full-scale physical experiments

Barakos, G.N.

Cavallo, S.

Ducci, G.

Steil, R.

English

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Paper 109 
 
Page 1 of 7 
 
Presented at 44th European Rotorcraft Forum, Delft, The Netherlands, 19-20 September, 2018  
This work is licensed under the Creative Commons Attribution International License (CC BY). Copyright © 2018 by author(s). 
 
CFD ANALYSIS OF HELICOPTER WAKES IN GROUND EFFECT 
S. Cavallo , G. Ducci,  R. Steil, G. N. Barakos 
CFD Laboratory School of Engineering, University of Glasgow, G12 8QQ, Scotland, UK 
G. Gilbertini 
 Department of Aerospace Science, Politecnico of Milan, 20156 MI, IT 
 
Abstract 
The paper presents CFD results for the wake of a helicopter flying a low altitude at different advance ratios. 
The wakes are assessed in terms of topology and velocity magnitudes. The structure of the wake near ground 
changes rapidly with the advance ratio and its decay appears to be faster than what is suggested by theoretical 
analyses. The results show clear the potential of modern CFD for use in helicopter safety and highlights the 
need for detailed surveys of helicopter wakes using full-scale physical experiments.  
 
 
1. INTRODUCTION 
Helicopter wakes are of very complex structure, 
composed by large coherent vortical structures, 
inside a flow of smaller turbulent scales [1]. In 
classic helicopter research works, the wakes are 
represented by vortical filaments that can be of 
prescribed shape or computed in a dynamic 
fashion. The filaments are carriers of vorticity and 
are used for computing an induced flow field 
superimposed to a stream of air or, near ground, to 
an atmospheric boundary layer. This approach is 
adequate for initial evaluations of helicopter 
performance, but it lacks fidelity when it comes to 
delivering data necessary for safety operations 
where one helicopter encounters the wake of 
another. Predicting the details of the helicopter 
wake with high fidelity is a considerable task. Not 
only the employed method must be able to delivery 
accurate data, and do so efficient for routine use, 
but it must also be flexible enough to account for 
the effects of the atmosphere, ground, presence of 
multiple rotors etc. Wakes are a feature of all flying 
machines, and there are clear separation criteria 
between fixed wing aircraft from the International 
Civil Aviation Organization (ICAO), while the 
situation for helicopters is different, and specific, 
helicopter separation criteria have not been 
established yet. For hover-taxi, there is an existing 
guidance from Civil Aviation Authority (CAA) [2], 
which suggests respecting a minimum distance of 
three rotor diameters. However more detailed 
guidelines are needed, considering the size and 
weight of the helicopters and even the specific 
nature of some near-ground operations. 
2. CFD SOLVER 
The CFD solver HMB3 of University of Glasgow 
[3,4] has been employed in this work as a high-
fidelity method.  
3.  RESULTS AND DISCUSSION 
Firstly, simple rotor wake test cases are used to 
show the level of agreement between numerical 
predictions and test data. It should be made clear 
that the vast majority of rotor wake surveys cover 
the near-rotor flow region extending one or two 
rotor diameters downstream of the centre of 
rotation. Such studies are performed inside wind 
tunnels and although the thrust settings of the 
rotors are usually of the correct scale, when 
presented as a rotor thrust coefficients, the 
Reynolds number of these flows tends to be very 
low. This is not an ideal situation since the decay 
rates of turbulent wakes depend on Reynold’s 
number. There are of course additional 
complications with the use of the tunnels due to the 
constrains of test sections and their small size. This 
is not ideal since studies of wake encounters 
typically consider distances of the order of 50 to 70 
times the rotor diameter (provided ICAO guidelines 
are followed).  Figure 1 shows the size of the 
employed computational domain for some of the 
cases considered in this paper. The domain covers 
10 rotor diameters in the lateral direction and some 
35 diameters downstream. 
Table 1 provides a summary of experimental works 
related to detailed helicopter wake surveys. Again, 
most of the listed works correspond to wind tunnel 
measurements. The most comprehensive work 
cited in the literature is by Köpp [5] who measured 
full-scale wake using LiDAR. His study covered 
different helicopter weights and speeds and the 
measured wake strength is the main source of data 
in the open literature. More recent works include 
the studies of JAXA [6] again using a full-scale 
helicopter. The work of JAXA captures the 
helicopter wake but there are no explicit data 
published in the open literature, to guide the 
extraction of wake decay rates. 
Table 2 shows the most promising methods for the 
Page 2 of 7 
 
Presented at 44th European Rotorcraft Forum, Delft, The Netherlands, 19-20 September, 2018  
This work is licensed under the Creative Commons Attribution International License (CC BY). Copyright © 2018 by author(s). 
simulation of the far-wake region of the helicopter. 
The table divides the methods in low and higher 
fidelity groups and provides indications of the ability 
of the methods to predict the wake decay, account 
for the effect of the grounds, and even couple the 
method with other techniques or models such as 
flight simulation tools, or models of atmospheric 
turbulence.  
Table 3 describes the rotors used to model the 
wake of an approximate EC145 aircraft put 
together to simulate the experiments of JAXA. In 
this work the rotors were modelled as actuator 
disks [7] to reduce the overall computational costs. 
In this work an aircraft flying at various speeds at 
30 ft of altitude is simulated and for a main rotor 
thrust coefficient of 0.022. The results of Figure 2 
show clearly the ability of the method to capture the 
change of the wake topology. At hover and low-
speeds, a large ground vortex is visible. The flow is 
dominated by strong downwash and the wake of 
the rotor appears to be dominant for a radius 
covering 3 main rotor diameters before almost un-
disturbed flow is reached. As the speed increases, 
the main ground vortex appears to be partially 
under the front of the main rotor disk while some 4 
diameters around the aircraft appear to be 
influenced by its spatial development. The situation 
changes very rapidly as the speed reached 40kts 
with no ground vortex present ahead of the 
helicopter. Instead, a trailed wake is formed 
resembling that of a fixed wing aircraft. The wake 
is trailed behind the main rotor disk and reaches 
the ground at a distance of about 5 main rotor 
diameters behind the helicopter. It is interesting 
that at this speed the wake is mainly affecting the 
region immediately downstream of the helicopter 
and the extent of its influence in the lateral direction 
is significantly reduced reaching 1.5 rotor 
diameters only. Given the employed mesh density 
and the use of the Chimera [8] technique, the wake 
was well-resolved for up to 4 main rotor diameters 
downstream the rotor.  
To further look at the simulation data, the velocity 
magnitude (with the aircraft speed subtracted) was 
computed at several azimuth angles around the 
aircraft and at several distances from the main rotor 
hub. The results are shown in Figures 3-6 
corresponding to hover, and forward speeds of 
10,20 and 40 kts. As can be seen, for the hover and 
to some extent for the 10kts forward speed case, 
the flow appears to have a certain degree of 
symmetry around the aircraft. Especially for the 
hover case, there is very little variation in the 
azimuthal direction. At 10kts the symmetry is still 
there but there is a small degree of asymmetry 
looking at 0 degrees of azimuth. Since the aircraft 
speed is subtracted the ground velocity appears to 
be zero in these plots. Moving to the plots for 20kts 
on Figure 5, there is now a clear directionality in the 
flow. Although the ground vortex is now closer to 
the aircraft, the main feature in the plot is the 
distortion of the wake downstream of the aircraft. 
Especially near the rotor, it is now visible that at 
zero degrees of azimuth the velocity peak is not 
near the ground as the rotor wake changes from a 
toroidal shape of a trailed wake. At 40 kts the 
situation is now different with peak velocities 
appearing higher above the ground. It is this 
change of wake configuration between the toroidal 
and trailed forms that makes the use of simple rules 
for aircraft separation somehow more complicated 
for this case. 
The results so far show good predictions of 
velocities and of the overall flow topology and are 
perhaps suitable for a first estimate of the 
separation distances required between helicopters. 
Nevertheless the prediction of the wake decay is 
still difficult. To investigate the ability of the 
employed method in delivering estimates of decay 
the work of Kopp [5] was used. In this work the 
actuator disk method of HMB was employed to 
simulate the measurements of a Puma aircraft 
wake. The case corresponds to an advance ratio of 
0.16 and a thrust coefficient of 0.017. Due to the 
limitations of the employed CFD mesh and its loss 
of resolution after about 5 rotor diameters, it is not 
possible to compare the results with the 
experiments. Nevertheless, the swirl velocity of the 
CFD was extracted and is compared in Figure 7 
with measurements. The results show an overall 
fair agreement. Given that the employed mesh 
resolution is not very high, the comparison can be 
seen as encouraging.  
4. SUMMARY OF FINDINGS AND FUTURE 
WORK 
The obtained results suggest that using averaged 
Navier Stokes equations, an actuator disk model 
and moderate CFD grids can be a pragmatic way 
to simulate helicopter wakes for the purposes of 
investigating helicopter wake encounters. Using 
moderate speed computers to calculate a set of 
advance ratios and thrust coefficients at distances 
of up to 5 rotor diameters behind a rotor is not too 
expensive and can be carried out over a period of 
1-2 weeks. Using more advanced CFD techniques 
like adaptive meshing can bring this cost further 
down. The use of the data from studies like JAXA 
suggests that comparisons with full-scale 
helicopters is feasible and is perhaps a good way 
to avoid some of the complexities of wind tunnel 
data.  Where wind tunnel measurements, however, 
can be invaluable is in measuring at least the early 
decay rate of the wake. This is a difficult quantity to 
predict well since it requires adequate density of 
Page 3 of 7 
 
Presented at 44th European Rotorcraft Forum, Delft, The Netherlands, 19-20 September, 2018  
This work is licensed under the Creative Commons Attribution International License (CC BY). Copyright © 2018 by author(s). 
CFD grids to avoid effects of numerical dissipation 
and detailed modelling of the wake turbulence. It is 
expected that combining measurements with high 
fidelity CFD methods coupled with flight mechanics 
tools [9] can provide a robust framework to quantify 
the effect of wake encounters and could contribute 
to the development of more specific separation 
distances beyond what is currently employed.  
5. NOTATION 
CT Rotor thrust coefficient (CT = 0.022) 
𝑅 Rotor radius (R = 0.8 m) 
6. ACKNOWLEDGEMENTS 
This work was supported by the NITROS EU 
project on rotorcraft safety. The use of the wake 
measurements of JAXA is gratefully 
acknowledged.  
7. REFERENCES 
[1]  Johnson, W., Rotorcraft aeromechanics, Vol. 
36, Cambridge University Press, 2013. 
[2]  CAA, “CAP 490: Manual of Air Traffic Services 
Part 1,” 2015. 
[3]  Barakos, G., Steijl, R., Badcock, K., and 
Brocklehurst, A., “Development of CFD 
Capability for Full Helicopter Engineering 
Analysis ,” 31th European Rotorcraft Forum, 
Florence, Italy, Sept, 2005.  
[4]  Steijl, R., Barakos, G., and Badcock, K., “A 
Framework for CFD Analysis of Helicopter 
Rotors in Hover and Forward Flight,” 
International Journal for Numerical Methods in 
Fluids, Vol. 51, No. 8, 2006, pp. 819–847.  
[5]  Köpp, F., “Wake-vortex characteristics of 
military-type aircraft measured at Airport 
Oberp- faffenhofen using the DLR Laser 
Doppler Anemometer,” Aerospace Science 
and Technology, Vol. 3, No. 4, 1999, pp. 191–
199.  
[6]  Matayoshi, N., Asaka, K., and Okuno, Y., 
“Flight-test evaluation of a helicopter airborne 
LiDAR,” Journal of Aircraft, Vol. 44, No. 5, 
2007, pp. 1712–1720.  
[7]  Leishman, G. J., Principles of helicopter 
aerodynamics with CD extra, Cambridge 
university press, 2006.  
[8]  Steger, J. L., Dougherty, F. C., and Benek, J. 
A., “A chimera grid scheme.[multiple overset 
body-conforming mesh system for finite 
difference adaptation to complex aircraft 
configura- tions],” 1983.  
[9]  Wang, Y., White, M., Barakos, G., Tormey, P., 
and Pantazopoulou, P., “Simulation of a Light 
Aircraft Encountering a Helicopter Wake,” 
Journal of Aircraft, 2014.  
[10]  Heyson, H. H., “Analysis and Comparison 
With Theory of Flow-Field Measurements 
Near a Lifting Rotor in the Langley Full-Scale 
Tunnel,” Tech. rep., NATIONAL 
AERONAUTICS AND SPACE ADMIN 
LANGLEY RESEARCH CENTER HAMPTON 
VA, 1956.  
[11]  Caradonna, F. and Tung, C., “Experimental 
and analytical studies of a model helicopter 
rotor in hover,” 1981.  
[12]  Curtiss Jr, H., Erdman, W., and Sun, M., 
“Ground effect aerodynamics,” 1985.  
[13]  Teager, S. A., Biehl, K. J., Garodz, L. J., 
Tymczyszym, J. J., and Burnham, D. C., 
“Flight Test Investigation of Rotorcraft Wake 
Vortices in Forward Flight.” Tech. rep., 
FEDERAL AVIATION ADMINISTRATION 
TECHNICAL CENTER ATLANTIC CITY NJ, 
1996.  
[14]  Sjo ̈holm, M., Angelou, N., Hansen, P., 
Hansen, K. H., Mikkelsen, T., Haga, S., 
Silgjerd, J. A., and Starsmore, N., “Two-
dimensional rotorcraft downwash flow field 
measurements by lidar-based wind scanners 
with agile beam steering,” Journal of 
Atmospheric and Oceanic Technology, Vol. 
31, No. 4, 2014, pp. 930–937.  
[15]  Herges, T., Maniaci, D., Naughton, B., 
Mikkelsen, T., and Sjo ̈holm, M., “High 
resolution wind turbine wake measurements 
with a scanning lidar,” Journal of Physics: 
Conference Series, Vol. 854, IOP Publishing, 
2017, p. 012021.  
[16]  Sugiura, M., Tanabe, Y., Sugawara, H., 
Matayoshi, N., and Ishii, H., “Numerical 
Simulations and Measurements of the 
Helicopter Wake in Ground Effect,” Journal of 
Aircraft, 2016.  
[17]  Bauknecht, A., Ewers, B., Wolf, C., Leopold, 
F., Yin, J., and Raffel, M., “Three-dimensional 
reconstruction of helicopter blade–tip vortices 
using a multi-camera BOS system,” Experi- 
ments in Fluids, Vol. 56, No. 1, 2015, pp. 
1866. 
 
 
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This work is licensed under the Creative Commons Attribution International License (CC BY). Copyright © 2018 by author(s). 
 
Table 1: Summary of test cases with corresponding references. Measured data can be used for wake decay 
studies. 
 
 
 
 Low Computational Cost High Computational Cost 
 Dynamic 
Inflow 
Prescribed 
Models 
Free Wake Actuator 
Disk 
Grid Based Grid Free 
Vortex Decay Y N N Y Y Y 
Ground Effect Y N Y Y Y Y 
Coupling N N N Y Y Y 
 
Table 2: Comparison of candidate methods for simulation of helicopter wakes. 
 
 
 
 
 Main Rotor Tail Rotor 
Number of blades, b 4 2 
Rotor radius [m], R 5.5 0.978 
Non dimensional rotor radius, R* 13.415 2.385 
Chord length [m], c 0.41 0.22 
Non dimensional chord length, c* 1.0 0.537 
Cutout (radius) [m] 0.725 0.225 
Non dimensional cutout 1.7685 0.45 
Solidity 0.095 0.1432 
Number of revolution [rpm], N 200 2170 
Angular velocity, [1/s] 20.94 227.2 
Period of revolution, [s] 0.3 0.027 
Thrust coefficient 0.022 0.003 
 
 
Table 3: Details of the employed rotor model for the simulation of the JAXA wake survey. 
 
 
  
Author Year Theme Method 
Heyson [10] 1956 Full scale wind tunnel wake measurement Dynamic pressure rakes 
Caradonna and Tung [11] 1981 Pressure and wake strength in hover Pressure probes and hot-wire 
Curtiss [12] 1985 Wake measurements in ground effect Hot-wire 
Teager et al. [13] 1996 Wake measurement in forward flight LiDAR 
Köpp [5] 1999 Helicopter wake decay LiDAR 
Sjöholm et al. [14] 2014 Helicopter wake on large scale LiDAR 
Herges [15] 2017 Wind turbine wake geometry LiDAR 
Sugiura et al. [16] 2016 Helicopter ground effect LiDAR. Ultrasonic anemometer 
Bauknecht et al. [17] 2015 Helicopter tip vortex measurements Back Oriented Schilren 
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Figure 1: Computational domain relative to rotor disk radius, R. 
 
 
 (a)  
 
(b) 
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(c) 
Figure 2: Comparison of the wake geometry, for three different values of flight speed: 1(a) 15kt, 1(b) 20kt, 
1(c) 40kt. The ground effect is simulated for an altitude of 30ft. 
 
 
 
 
1.25R     2.0R     3.0R 
Figure 3: Velocity profiles (0kt). 
 
 
 
1.25R     2.0R     3.0R 
 
Figure 4: Velocity profiles (10kts) 
 
 
 
 
 
 
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1.25R     2.0R     3.0R 
Figure 5: Velocity profiles (20kt). 
 
 
 
1.25R     2.0R     3.0R 
Figure 6: Velocity profiles (40kt). 
 
 
 
 
 
Figure 7: CFD and measured velocities for Kopp’s case. The CFD was extracted at 5 rotor diameters behind 
the rotor and is compared with data gathered at 22 rotor diameters. 
 
 
Distance from vortex core [m] 
15 10 5 0 -5 -10 -15 
-10 
-5 
0 
5 
10 
15 
CFD at 5 Rotor Diameters 
Experimental data at 22 Rotor Diameters 


CFD Analysis of Helicopter Wakes in Ground Effect

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CFD Analysis of Helicopter Wakes in Ground Effect

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