[American Institute of Aeronautics and Astronautics 42nd AIAA Aerospace Sciences Meeting and Exhibit...

Numerical Modeling of Coanda Jet Controlled Nacelle Configurations

Jingshu Wu*, Lakshmi N. Sankar†, Shayne Kondor‡

[email protected], [email protected], [email protected] of Aerospace Engineering

Georgia Institute of Technology, Atlanta, GA 30332-0150

ABSTRACT

Modification of the aerodynamic

characteristics of axisymmetric nacelles with the use of tangential Coanda jets is studied. It is demonstrated that Coanda jets in the leading edge (lip) region of the nacelle can increase the entrainment of air flowing into the nacelle. It is also shown that Coanda jets in the trailing edge region of the nacelle can laterally spread the jet-fan exhaust, and may lead to a reduction in the exhaust velocity downstream. The leading edge jet slot position was found to have a significant effect on the flow field. It is concluded that Coanda jets may be useful in altering and improving the performance of a nacelle that has been designed for cruise conditions.

INTRODUCTION

During the past several years, there has been a growing interest in the use of jets (continuous, synthetic, pulsed, etc.) for flow control. It is widely documented that these active control concepts can dramatically alter the behavior of aerodynamic components such as airfoils, wings, and bodies. Tangential jets that take advantage of Coanda effect to closely follow the contours of the body are particularly effective in that they can entrain a large mass of surrounding air. This can lead to increased circulation in the case of airfoils [Ref. 1-3], or drag reduction (or drag increase if desired) in the case of bluff bodies such as an aircraft fuselage.

The purpose of this study is to determine if Coanda jets are useful in altering the aerodynamic characteristics of nacelles that have been designed for optimum performance only under cruise conditions. Such nacelles are not designed to capture an adequate amount of air during off-design conditions such as low speed take-off, where a bell-mouth nacelle will be more appropriate. It will be very beneficial if flow control techniques such as Coanda jets may be used to make the nacelle behave like a bell-mouth shape at low speed conditions.

A second example of off-design performance that is of interest is the alternation of the fan-engine exhaust flow. In VTOL configurations, it is often

desirable to reduce the exhaust velocity (and temperature of the jet) by broadening the jet and enlarging its footprint on the ground. It will be beneficial if Coanda jets can achieve this desirable effect.

Of course, these changes to the behavior of the nacelle may be achieved by other means such as variable configuration nacelles/nozzles. These devices require moving parts and require actuator mechanisms that increase the weight, operational cost, and acquisition cost of the nacelle. These devices may also be designed to operate only at a fixed number of settings. Coanda jets, on the other hand have no moving parts other than a throttle valve that controls the mass flow rate and the momentum of the jet, and may be continuously controlled. They may be turned off during off-design conditions with little or no impact on the performance of the configuration.

These advantages must, of course, be weighed against the need for a readily available compressed air supply, and the need for ducts, valves and nozzles that deliver the required amounts of air to the desired locations. Such a system level study is not attempted here, and the focus of the present work is primarily on the aerodynamic behavior of Coanda jet controlled nacelle configurations.

MATHEMATICAL AND NUMERICAL FORMULATION

Computational Grid

A configuration that is being experimentally studied at Georgia Tech Research Institute is being modeled in the calculations. The body-fitted grid was generated with the aid of GRIDGEN, a commercially available grid generator. In the case of axisymmetric flows, we generate a series of meridional planes which are uniformly spaced in the circumferential direction around the nacelle and center body. In view of the axi-symmetric nature of the flow, only a quarter of the nacelle is modeled and gridded. The grid generator and the flow solver are, however, capable of modeling the entire nacelle under non-axisymmetric flow conditions.

American Institute of Aeronautics and Astronautics

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42nd AIAA Aerospace Sciences Meeting and Exhibit5 - 8 January 2004, Reno, Nevada

AIAA 2004-228

Copyright © 2004 by Jingshu Wu, Lakshmi N. Sankar, Shayne Kondor. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

mailto:[email protected]




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Figure 1 shows a sample C-grid for the configuration being tested. This is referred to as Nacelle-A in this work. The grid points are clustered in the vicinity of the nacelle leading and trailing edges. Enlarged views of the grid in the vicinity of the leading and trailing edge jet slots are shown in figures 2 and 3a. In order to study the effects of the jet slot position on the flow field, a modified configuration referred to as Nacelle-B has also been studied, and an enlarged view of this configuration is shown in figure 3b.

UNavier-Stokes Solver

A three-dimensional unsteady viscous flow analysis developed at Georgia Tech by the present authors and their coworkers is being used. The compressible Navier-Stokes equations are solved in a generalized non-orthogonal curvilinear coordinate system. The present analysis solves the flow equations in a strong conservation form, so that shock waves may be properly modeled.

In this work, a customized version of this solver capable of modeling axisymmetric nacelles with center-bodies, as well as general 3-D nacelle configurations, was employed. The effects of the fan were modeling with the use of distributed body forces that add momentum and energy to the flow.

An implicit time marching scheme with good stability characteristics is being employed, making the analysis suitable for steady as well as unsteady flows involving pulsed jets. This scheme is fourth order accurate in space and first or second order in time. Versions of this solver has been extensively used to model flow over wings, rotors, and wind turbines and has been carefully validated [Ref. 4-6]. Three turbulence models (Baldwin-Lomax zero equation model, one equation Splart-Allmaras model, and a two equation k-ε model) are available. In this study, all the calculations were done using the Balwin-Lomax Turbulence Model. UBoundary Conditions

On the nacelle and center body, simple no-slip boundary conditions are applied except at the jet slot exit. Because the flow field in the engine should be symmetric, we only model one quarter of the nacelle configuration, and enforce periodic boundary conditions. Non-reflective boundary conditions are applied at the outer boundaries of C grid. At the jet exit, the total temperature, Jet Mach number, and the flow direction were specified. The jet was assumed to be subsonic, and the pressure values at the jet slot were equated to that over the nacelle, in the immediate vicinity of the jet slot.

In Coanda Jet-Controlled Nacelle (CCN) studies, an important parameter is the blowing momentum coefficient, CBµB, defined as follows:

SV

VmC jet

2

21

∞∞

=ρ

µ

& (2)

where S is the square of the reference length, typically the length of the nacelle from the leading edge (lip) to the trailing edge. In this simulation, the jet was assumed to be tangential to the nacelle surface at the jet slot. The effects of the fan were modeled as follows. In the experiments carried out by the third author at Georgia Tech, the fan was driven by air jets directed at the tip of the fan. The total enthalpy rise across the fan had been measured at several radial locations across the fan. In the present computational study, this total enthalpy rise and the local flow velocity just upstream of the fan face were used to compute the radial distribution of the pressure forces exerted by the fan on the flow. These forces were prescribed as body-force source term on the right hand side of the axial- momentum equation. The work done by the force field (total enthalpy rise) also appeared as a source term in the energy equation. If needed, the effects of the swirl may be modeled with the use of appropriate body-force terms in the azimuthal direction. This was not done in this work.

URESULTS AND DISCUSSION

As discussed earlier, the present solver has

been extensively validated for a variety of wing, rotor, wind-turbine, and airfoil configurations. For brevity, these validation studies are not presented here, and the reader is referred to the authors’ web site HTUwww.ae.gatech.edu/~lsankarUTH for related publications.

The reference values for these calculations were: ambient temperature T B∞ B = 294.168º K, ambient density ρB∞ B = 1.2 kg/mP

3P, and reference length L = 0.212

m. The effects of the fan were modeled as discussed using the measured total temperature change of 5º K across the fan.

These values closely match the nominal ambient conditions in the GTRI experiments. At this writing, experimental results are available for only a single operating condition. Comparisons with these experiments are discussed later.

Calculations have been done both for static operation of the nacelle-fan combination (with a freestream Mach number MB∞ Bset to 0.01), and for low speed forward flight conditions at a freestream Mach number MB∞ Bof 0.2 We first discuss the results for the MB∞ B=0.2 case.

UTake-off and Landing Scenarios (Free-stream Mach number MUBU∞ UBU=0..2);


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Baseline Configuration (No jets) Calculations have been done for a reference

condition where no jets are activated, and with the inclusion of body forces to model the fan effects. This provides a reference condition against which all other calculations may be compared. Figure 4a corresponds to Nacelle configuration A shown in figure 3a, while Figure 4-b corresponds to Nacelle-B shown in figure 3-b. No significant difference in the flow field was found for these two configurations, when the leading or trailing edge jets were not turned on. As expected, there is considerable separation behind the bluff center body and at the step region of the nacelle where air jets were used in the experiments to drive the fan. These air jets were not modeled in the present work. Effects of Leading Edge Jet on the Flow Entrainment

To evaluate the effects of the leading edge Coanda jets, a series of calculations were done where the jet velocity was gradually varied. A total of 6 cases (shown on Table 1 and Table 2 for Nacelle A and Nacelle B, respectively) were calculated. These tables also show the computed mass flow rate, and computed net thrust (fan thrust minus the pressure drag forces over the nacelle and the center-body).

Figure 5a and 5b show the effects of the leading jet at a jet Mach number equal to 0.4 for these two configurations. Figure 6a, 6b show the results of leading jet Mach number is equal to 0.8. Comparing the flow field in the vicinity of leading edge to the baseline results shown in figure 4, it is clear that the spillage in the leading edge has been eliminated as a result of the Coanda effects. Tables 1 and 2 show that an increased amount of air is being entrained into the nacelle as the leading edge jet Mach number is increased.

From the comparison of the flow fields for the Nacelle-A and the Nacelle-B configurations under identical conditions, we can see that the jet slot position also plays a significant role. For the Nacelle B configuration, the slot position is placed aft of the stagnation point (for the baseline configuration). From a comparison of Table 1 (Configuration A) and Table 2 (Configuration B) it is seen that the Nacelle B entrains more mass of air into the nacelle, while the net thrust force is also increased. Effects of Trailing Edge Jet on the Fan Exhaust

A set of calculations were done, again at free stream Mach number of 0.2, and a Coanda jet Mach number of 0.4 and 0.8. Figures 7 show the flow field of baseline (no jet) configuration near the nacelle trailing edge. It is seen that there is some flow separation at the bluff trailing edge of the nacelle, and that much of the flow downstream of the fan flows parallel to the nacelle axis, with very little spreading. Figure 8 and 9 shows the corresponding flow in the trailing edge region when the trailing edge jet Mach numbers are 0.4 and 0.8,

respectively. It is also clearly seen that there is considerable lateral spreading of the jet/fan exhaust. From the calculated mass flow rate shown on Tables 1 and 2 for Configurations A and B (Cases 4,5), it is seen that the trailing edge jet also increase the mass flux of engine. For the Mach 0.4 trailing edge jet, the mass flow rate through the nacelle increased by 6.1%, while for Mach 0.8 trailing edge jet case, it increases by 13.7% over the baseline. This lateral spreading of the jet may be beneficial in broadening the jet footprint. UEffects of Jet on the Static Operation of the Nacelle (Freestream mach number MUBU∞ UBU=0.01):

It is also of interest to determine what roles the jets play, if any, in altering the flow field around the nacelle under static conditions. To investigate this, we repeated the previous calculations (done at MB∞ B=0.2) for a lower free stream Mach number of 0.01. Comparisons of the predictions with GTRI Test

Experimental data for a case (Leading edge jet Mach number of 0.797, static condition MB∞ B=0.0, and ∆TB0 B = 5.5º K) was available at the time this paper was written. Calculations were done for this case to validate the present analysis. The measured mass flow rate of 1.9337 kg/s closely matches the computed value of 1.8497 kg/s. The measured thrust force of 216.257 N also closely matches the computed net thrust of 200.6 N. Baseline Configuration (No jets)

Calculations have been done for a reference condition where no jets are activated. Figures 10a and 10b show the flow field in the vicinity of the nacelle leading edge, for the Nacelle-A and Nacelle-B configurations. The corresponding mass flow rates and thrust values are shown in table 3, and Table 4, for these two configurations. It was found the backward facing slot step in the leading edge region of Nacelle-A triggered a separated flow. This separation also caused the entrained mass flow rate for the Nacelle-A to be less than the Nacelle-B as seen in Tables 3 and 4. Effects of Leading Edge Jet on the Flow Entrainment

Figure 11a, 11b show the flow field in the vicinity of the nacelle leading edge, for Nacelle-A and Nacelle-B, respectively. The leading jet Mach number was set to 0.4 in this case. Figure 12a, 12b show corresponding results when the leading jet Mach number was increased 0.8. Compared to the baseline configuration, it is clearly seen that the separation area in the leading edge of Nacelle-A has been eliminated. Because of this, for the static condition, Nacelle-A configuration showed an increased entrainment than Nacelle-B. As for the take-off and landing conditions (MB∞ B=0.2) discussed earlier, the use of leading edge jets


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caused more air to be entrained into the flow as seen in Tables 3 and 4 (cases 2 and 3). Effects of Trailing Edge Jet on the Fan Exhaust

Figure 13 shows the flow field of baseline configuration near the nacelle trailing edge under static condition (MB∞ B=0.01). Figure 14 and 15 show the modified flow in the trailing edge region, when the trailing edge jet Mach number is 0.4 and 0.8. The results are listed in tables 3 and 4 (cases 4 and 5). As in the take-off and landing conditions discussed earlier, the Coanda effect associated with the jet causes an expansion of the flow downstream of the nacelle. However, flow separation over the upper surface of the nacelle was observed.

UCONCLUDING REMARKS A computational study has been conducted to

assess the effects of leading and trailing edge Coanda jets on the aerodynamic characteristics of an axi-symmetric nacelle-centerbody-fan configuration. It was found that the leading edge and trailing edge Coanda jets can dramatically increase the mass flow rate through the nacelle and increase the net thrust if the leading edge slots are properly positioned. It was also found that trailing edge Coanda jets can play a beneficial role in spreading the exhaust stream, while increasing the net flow through the nacelle.

Additional computational and experimental studies are needed to further understand and quantify the aerodynamic phenomena that cause these effects.

UACKNOWLEDGEMENTS

This work was done under the support of

NASA Langley Research Center. Dr. Mark D. Moore was the technical monitor. We greatly appreciate the support and encouragement of our sponsor.

UREFERENCESU

1. Liu, Y., “Numerical Simulations of the

Aerodynamic Characteristics of Circulation Control Wing Sections,” Ph. D Dissertation, Georgia Institute of Technology, May 2003, Found at HTUwww.ae.gatech.edu/~lsankar/NASA_CCWUTH .

2. Englar, R. J., “Circulation Control Pneumatic Aerodynamics: Blown Force and Moment Augmentation and Modification; Past, Present, and the Future”, AIAA Paper 2000-2541, June 2000.

3. Liu, Y., Sankar, L. N., Englar, R., and Ahuja, K., " Numerical Simulations of the Steady and Unsteady

Aerodynamic Characteristics of a Circulation Control Wing," AIAA 2001-0704, 39th AIAA Aerospace Sciences Meeting, Reno, NV.

4. Wake, B. E. and Sankar, L. N., "Solutions of the Navier- Stokes Equations for the Flow About a Rotor Blade," UJournal of the American Helicopter SocietyU, April 1989.

5. Kwon, O. J. and Sankar, L. N., "Numerical Simulation of the Flow about a Swept Wing with Leading Edge Ice Accretions," UComputers and FluidsU, Vol. 26, No. 2, pp. 183-192, 1997.

6. Xu, Guanpeng, and Sankar, L. N., “Computational Study of Horizontal Axis Wind Turbines," UASME Journal of Solar Energy EngineeringU, Vol. 122, No. 1, February 2000, pp. 35-39.

5 American Institute of Aeronautics and Astronautics

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ZFrame 001 ⏐ 22 Oct 2003 ⏐ uvwFrame 001 ⏐ 22 Oct 2003 ⏐ uvw

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Figure 1: Axisymmetric body fit C-grid Figure 2: Nacelle-a, b at trailing edge jet slot

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Figure 3a: Nacelle-a at leading edge jet slot Figure 3b: Nacelle-b at leading edge jet slot

Figure 4a Figure 4b

Flow field for baseline configuration at leading edge, M B∞ B = 0.2

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Figure 5a Figure 5b

Flow field for leading edge jet, MBlead B = 0.4, MB∞ B = 0.2

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Figure 6a Figure 6b


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Figure 7

Flow field for Baseline configuration at trailing edge, MB∞ B = 0.2

XY


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XY



XY


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Figure 8 Figure 9

Trailing edge jet, MBtrailB = 0.4, M B∞ B = 0.2 Trailing edge jet, MBtrailB = 0.8, M B∞ B = 0.2

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ZFrame 001 ⏐ 08 Nov 2003 ⏐ uvwFrame 001 ⏐ 08 Nov 2003 ⏐ uvw

Figure 10a Figure 10b

Flow field for baseline configuration at leading edge, M B∞ B = 0.01

XY


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Figure 13

Flow field for Baseline configuration at trailing edge, MB∞ B = 0.01

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Figure 14 Figure 15

Trailing edge jet, MBtrailB = 0.4, M B∞ B = 0.01 Trailing edge jet, M BtrailB = 0.8, M B∞ B = 0.01


Table 1 Nacelle - A Case M B∞ B Ma (LE) Ma (TE) Mass (kg/s) ∆m (%) Thrust (N) ∆Th (%)

Base Line 0.2 0 0 1.73655851 0 144.567523 0 2 0.2 0.4 0 1.74717202 0.6 145.675817 0.767 3 0.2 0.8 0 1.81596227 4.6 141.58052 -2.07 4 0.2 0 0.4 1.84207773 6.1 143.389245 -0.816 5 0.2 0 0.8 1.97407341 13.7 149.098639 3.13 Table 2 Nacelle - B

Case M B∞ B Ma (LE) Ma (TE) Mass (kg/s) ∆m (%) Thrust (N) ∆Th (%)

Base Line 0.2 0 0 1.76047628 0 112.721374 0 2 0.2 0.4 0 1.77286898 0.7 126.642933 12.35 3 0.2 0.8 0 1.85388701 5.3 146.386068 29.87 4 0.2 0 0.4 1.87937324 6.8 114.575017 1.644 5 0.2 0 0.8 1.94098313 10.3 110.183216 -2.251 Table 3 Nacelle - A


Base Line 0.01 0 0 1.35189639 0 132.915063 0 2 0.01 0.4 0 1.43640359 6.25 149.123972 12.19 3 0.01 0.8 0 1.54896609 14.6 190.501206 43.32 4 0.01 0 0.4 1.41351129 4.6 133.677348 0.574 5 0.01 0 0.8 1.51137044 11.8 133.298965 0.289 Table 4 Nacelle - B


Base Line 0.01 0 0 1.60572984 0 158.671206 0 2 0.01 0.4 0 1.65261227 2.9 166.31233 4.816 3 0.01 0.8 0 1.67585425 4.4 183.899537 15.9 4 0.01 0 0.4 1.7456032 8.7 163.453844 3.014 5 0.01 0 0.8 1.85647704 15.6 163.596566 3.104

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