4TH EUROPEAN CONFERENCE FOR AEROSPACE SCIENCES (EUCASS)
Fluid/Structure Coupling Under Confined
Low Acoustic Feedback
D.SAKAY, F.PLOURDE
Institut Pprime, UPR CNRS 3346, ENSMA, Université de Poitiers, BP 40109, 86961 Futuroscope
Chasseneuil Cedex, France.
S.PETITOT
Service Equipements Propulsifs et Mécanismes, Direction des Lanceurs, CNES, Rond Point de l’EspaceCourcouronnes, 91023 Evry Cedex, France.
Abstract
In the framework of the solid rocket motor (MPS) in Ariane 5 satellite launcher, a thermal
protection (PT) is required in order to prevent the combustion of the frontal face of the
propellant grain segments. The burning of the grain offers a higher rate than the ones
observed for the PT. During firing, it results in an annular obstacle protruding into the flow.
This annular protrusion exhibits high shear region development in which Vortex Shedding
Obstacle (VSO) occurs. A second source of aerodynamic instabilities can be attributed to the
burning of the propellant, along which vortices are created (VSP). The occurrence of
resonance is caused by the alignment of vortex shedding frequencies with one of the available
longitudinal acoustic modes of the chamber.
To better understand the impact on MPS stability of the behavior of the PTF during
combustion, CNES has decided to fund, in the frame of French research programs managed
by the launcher directorate, a precise characterization of the PTF behavior at low scale. The
present work falls then within the study of fluid/structure interaction and its effects on the
stability analyses. The approach followed represents a cold flow experimental analysis in
which the aerodynamic characteristics of the combusting grains are provided by injecting
cold air through porous walls. The use of a rubber based obstacle (PTF) is compared to the
use of a rigid aluminum one (PTR). When submitted to flow conditions, the PTF shows
vibration at low frequencies. Due to the rigid nature of the PTR, such low frequencies are not
observed. The movement of the PTF is able to interfere with the creation and interactions of
VSO and VSP structures, consequently modifying the impact of turbulent structures on to the
integrated nozzle. Such behavior weakens the communication of reflected acoustic waves
with vortex shedding phenomenon, and thus preventing the self-sustaining resonant behavior.
1. Introduction
The ARIANE 5 launcher presents two solid rocket motors (MPS P230), each engine has 230 tons of propellant
divided in three segments.1 Pressure oscillations at low frequencies have been observed in full scale firing tests of
the MPS P230.2,3,4 Oscillations in SRM can be linked to either combustion instabilities or aerodynamic
instabilities. The latter is usually driven by an interaction of the flow with changes in geometric parameters, and
will be the subject of this study.
Each propellant segment of the motor is separated by a thermal protection (PT) in order to prevent the surface in
each intersection from combustion. The burning of the propellant grain offers a higher rate than the ones observed
for the PT and, during firing, this result in an annular obstacle into the flow.
In order to fundamentally analyze the unstable mechanisms within SRM, cold flow experiments have been
conducted in a reduced model of the Ariane 5 rocket booster (MICAT). Experiments in this small scale set-up
have already been conducted throughout the last two decades,5,6,7,8 as well as numerical simulations
characterizing the role of wall injection in the occurrence of pressure instabilities.9,10,11,12
In the present work analyses are conducted for two obstacles (PT), constituting one reference case made in
aluminum (PTR) and one in a rubber based material (PTF). Measurements of movement of the PTF, instantaneous
velocity fields by PIV method and the chamber’s acoustic behavior (quantified by means of average and
fluctuating pressure intake) allows one to describe the flow field behavior in SRM.
4TH EUROPEAN CONFERENCE FOR AEROSPACE SCIENCES (EUCASS)
Vortex shedding in the wake of the obstacle generates turbulent structures (VSO). A second vortex shedding
mechanism arises along the injecting walls (VSP). The two mechanisms (VSO and VSP) can merge together and
may align with the longitudinal acoustic modes of the test chamber, emphasizing a resonant
phenomenon.13,14,15,16
Recent studies17 have shown that the physic properties and consequently the vibration mode of an emerging
thermal protection can influence the VSO generation and it's interaction with VSP by forcing a fluid-structure
coupling. Therefore, movement of the PTF has a significant influence on the flow organization and is the main
issue of this research.
By means of direct pressure fluctuation acquisition, the so-called “end of combustion” configuration is identified
as a resonant case i.e. strong acoustic and velocity correlation arises. Consequently, this experiment condition has
been considered as a reference case for the analyses of fluid/structure interaction.
In order to render possible PIV seeding concomitant with air injection, dedicated porous walls have been used.
However, such a change has a direct impact on the acoustic isolation, characterizing a low acoustic feedback
behavior.
2. Experimental Set-up
The experiment is conducted in a 1/40 scaled model of the Ariane 5 SRM P230. Figure 1 shows representations of
the experimental set-up, in which the propellant grain combustion is simulated by a cold gas injection through
porous bronze plates adapted to three injecting blocks. The air intake passes through a system of polyamide tubes
adapted with sonic nozzles. Under specific pressure conditions, these area reductions provide a uniform mass flow
rate distribution. Air is screened, filtrated and separated ensuring an equal flux distribution along the porous walls.
This composition induces a perpendicular injection that simulates the propellant burning.
The second and third injection blocks are separated by an aluminum obstacle (reference case). The latter is assumed
rigid and has a natural frequency higher than the one of the first acoustic mode of the i.e. no interaction may occur.
To introduce potential fluid/structure interactions, the rigid obstacle has been replaced by a rubber based material of
the same size and slightly different thickness with its natural frequency close to the first acoustic mode
The pressure data is acquired by use of two piezoelectric quartz sensors, these sensors are located near the inlet
section and work in a range of 0-10 bar with 0.015 mbar pressure accuracy.
The vibration of the obstacle was analyzed by laser vibrometry. The acquisition was carried out during a time
corresponding to 30720 time-steps. The data was divided in 200 points each composed by an average of 153 blocks
(defining a resolution of 2.5 Hz applied to a 0 to 500 Hz spectral range).
The instantaneous velocity measurements are obtained by means of a high speed PIV system. Due to the need of
particles seeding in the PIV visualization, a low acoustic feedback set-up has been studied. The injecting conditions
at porous walls are characterized by providing a 98% filtering of 8.5 µm sized particles. Previous experiments (high
acoustic feedback) only refer to grade 3 bronze porous plates providing a 98 % filtering of 0.7 µm particles. The
flow speed was kept constant at M=0.09. A 1000 Hz sample rate frequency is sustained along 6 seconds resulting in
6000 images. A multi-pass method is used with a reduction of the interrogation window and with a window overlap
of 50 % in both passes. A medium filter was set to strongly remove and iteratively replace by interpolation any
groups of spurious vectors, allowing calculations of averages and standard deviation.
3. Results
Experiments in a fully acoustically isolated (i.e. with grade 3 porous walls) set-up have been carried out. These
preliminary tests were conducted with pressure acquisition in order to correctly perceive the occurrence of a
resonant behavior. Figure 2-a and 2-b display respectively the pressure spectral response recorded at the head-end
for the high-resonant (rigid obstacle) and low-resonant (flexible obstacle) cases, being both fully acoustically
isolated configurations. It is clear in both cases that one simple frequency emerges in the spectral answer. The
latter is found close to a frequency that corresponds to the first longitudinal acoustic mode of the chamber. For
M  0.09, one can detect a linear increase of such peak despite low energy levels. For 0.09  M  0.105, a jump
in frequency first occurs and is followed by another linear increase.
In the fully resonant case, as shown in Figure 2-a, it is obvious that such frequency induces significant amplified
levels underlining the development of a resonant phenomena. Figure 2-b reveals the output obtained for the low
resonant case, in which one can easily notice a significant weakening in spectral energy even though the network
of pressure response is similar. To highlight our purpose, Figure 3 shows the spectral answer contained in the
pressure fluctuation at M=0.09 for both high and low-resonant cases.
While the first acoustic mode is
significantly marked for the high-resonant case, the maximum peak amplitude is reduced by a significant factor for
the low-resonant set-up. Such a behavior can be attributed to a diminution in dynamic/acoustic coupling
4TH EUROPEAN CONFERENCE FOR AEROSPACE SCIENCES (EUCASS)
mechanisms. In the light of a higher acoustic feedback, the role of a flexible obstacle in the energy reduction is
then confirmed.
In order to explain such a change in fluctuation activity it is first necessary to underline that the flexible obstacle is
bent under the flow load. One could then suppose that an inclination of the obstacle may be the main cause of the
reduced energy levels, i.e. by modifying vortices issued from the shear-layer as well as their interaction with the
injecting flow. However, Nunes18 conducted experiments with a rigid and bent obstacle and no such reductions
were observed. In order to confirm their previous assumptions, the authors performed measurements with the use
of a vibrating system adapted to a rigid obstacle. By forcing the obstacle vibration through a range of specific
frequencies it was concluded that a movement of the latter does change the acoustic response.
Having confirmed that the vibration of the obstacle is in fact responsible for the diminution of fluctuating
activities, a series of experiments measuring such movement were carried out. The vibration was confirmed
through data acquired through laser vibrometry. In order to perceive the non-homogeneous movement of the latter,
five different points were focused. All measured points were located on the same vertical level and with a constant
gap between each. Figure 4 shows the spatial location of each point and its respective displacement RMS. The
latter also contains a displacement spectrum for the exact middle position, in which the most excited frequency is
relatively close to the calculated frequency for its first natural vibration mode. Each examined point of the obstacle
exhibits low scattered excited frequencies and the different displacement levels confirm the obstacle 3D motion.
In order to better understand the relationship between a flexible obstacle set-up and instability level reductions, the
main issue is to measure the dynamic flow field close to the obstacle and to analyze the role of obstacle vibration
on vortex shedding phenomenon. From high speed PIV method, the flow pattern was registered and its vortices
interaction quantified. Instantaneous speed fields clarified the difference in flow behavior and allowed the creation
of average speed fields. Figure 5 shows such average fields, in which one can observe that the flow becomes more
homogeneous after the horizontal position 1 with a flexible obstacle. Lower averaged speeds are also achieved
immediately after the creation of the VSO structures, showing high discrepancies. A difference in interaction
amongst VSO and VSP can be responsible for the reduction in speed levels and earlier organization of the flow.
Consequently, a correct understanding of the vortical path was necessary. For that purpose, a study of the proper
values for the symmetric tensor (γ2) was carried. As granted by the works of Ismail19, the γ2 criteria is composed
by the sum of the rotational and the shear terms of the speed gradient tensor. The γ2 vorticity fields presented in
Figure 6 allows one to identify earlier interactions between the VSO and VSP structures associated in the flexible
obstacle case.
Consequently, a theory linking an early encounter among vortices and the eventual diminution in turbulent
fluctuation comes to justify attenuation of the resonant state. The turbulence intensity analyses were completed to
fully understand the role of such early vortical interaction in a reduced acoustic and velocity correlation. Figure 7
shows average turbulence fluctuation fields for both cases, from which one can detect a clear reduction in
turbulent intensity concerning the flexible obstacle arrangement. In given positions, the discrepancy rises to a
point in which measures are found to be half of its original values. Turbulent behavior is also attenuated near the
wake of the obstacle, stating an obvious difference in VSO creation. Similar to the previous flow analyses, the
stabilization of the flow happens around the horizontal position 1; confirming correlations amongst velocity,
vorticity and turbulence behavior.
4. Conclusion
The pressure data in a strong acoustic feedback set-up confirmed the occurrence of a strong resonant state for the
reference case. There was a significant reduction in energy levels for the flexible obstacle case, stating the
relevance of the subsequent flow and vibration analyses shown in this work.
The movement of the obstacle was confirmed through direct vibration measurements. The frequencies and
displacements of the latter were as foreseen in previous simulations.
High speed PIV data elucidated the different interaction amongst VSO and VSP structures between the PTF set-up
and the reference case. Such behavior is linked to a lowering of velocity levels and a visible diminution in
turbulent activity.
One must keep in mind that the velocimetry and vibrometry experiments were realized in a low acoustic feedback
case, and by so can only be used to demonstrate a difference in flow behavior. Deeper analyses of the vortices
interactions, as well as direct correlations between punctual velocity fluctuation, pressure instabilities and obstacle
vibration in a completely isolated and resonant case are in course and shall complete the study of such phenomena.
All those works were integrally funded by the Launcher Directorate of the CNES, the French Space Agency.
4TH EUROPEAN CONFERENCE FOR AEROSPACE SCIENCES (EUCASS)
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4TH EUROPEAN CONFERENCE FOR AEROSPACE SCIENCES (EUCASS)
Figure 1: Experimental set-up – schematic drawing of injection blocks, geometry and main components
Figure 2: Variations in spectral density for pressure fluctuation relative to flow speed.
a – Rigid obstacle
b – Flexible obstacle.
Figure 3: Frequency spectrum for pressure acquisitions with a flow speed of M = 0.09.
4TH EUROPEAN CONFERENCE FOR AEROSPACE SCIENCES (EUCASS)
Figure 4: Obstacle’s five vibration measurement points, respective displacement RMS and displacement
spectrum for middle position.
Figure 5: Average flow speed fields; Left – Flexible obstacle, Right – Rigid obstacle.
Figure 6: γ2 criteria for instantaneous vorticity field; Left – Flexible obstacle, Right – Rigid obstacle.
Figure 7: Fluctuation fields, turbulence intensity; Left – Flexible obstacle, Right – Rigid obstacle.