PIV investigation of a space launcher model - · PDF filecharacteristic frequencies of the vortex shedding at Ma = 0.7, ... launch, because at moderate ... underexpanded nozzle plume

Sonderforschungsbereich/Transregio 40 – Annual Report 2011 109

PIV investigation of a space launcher model

By S. Scharnowski, C.J. Kähler, J. Windte † AND R. Radespiel †Institute of Fluid Mechanics and Aerodynamics, Universität der Bundeswehr München

Werner–Heisenberg–Weg 39, 85577 Neubiberg

In order to investigate the wake dynamics of a generic space launcher model at tran-sonic Mach numbers, high repetition rate PIV measurements were performed in thetrisonic wind tunnel facility at the Bundeswehr University Munich. The data is requiredto validate unsteady numerical flow simulations, but also to estimate the dynamics ofthe vortices present in the turbulent shear layer. In order to resolve the relatively highcharacteristic frequencies of the vortex shedding at Ma = 0.7, velocity fields were mea-sured with 4 kHz by using a high repetition rate PIV system. By using sophisticated postprocessing techniques the size, strength and orientation of the vortices could be esti-mated along with the dominant shedding frequency. The analysis shows that due to theturbulent nature of the wake, a range of characteristic frequencies exist rather then asingle dominant frequency, in contrast to the literature. The acquired data also allowsfor detecting single vortices and determine their size, energy, orientation, and rotationdirection. Furthermore, PIV measurements at the hypersonic Mach number 5.9 wereperformed at the Hypersonic Ludwiegtube Braunschweig on a similar space launchermodel. It was possible to measure the evaluation of the mean flow topology in the bound-ary layer as well as in the extension of the recirculation domain.

1. IntroductionThe transportation of satellites and other aero/astro space equipment requires effi-

cient systems that fulfill a number of demands, e.g., a high degree of reliability in ex-treme conditions comparable to those encountered in outer space, enough power tomobilize massive pieces of hardware and superior cost effectiveness. So far, numericalmethods have been employed for low cost aerodynamic designs of space launchers.Due to the large Reynolds numbers a RANS simulation is usually necessary to solvethe problem, but due to the significance of the large scale vortex dynamics, Large EddySimulations have to be used at least in the wake region. To cover such problems, thezonal LES approach was implemented recently [1]. To validate these quite novel sim-ulation concept, detailed measurements are required to estimate the dynamic featuresof the flow. A large variety of recent numerical investigations using Reynolds AveragedNaiver Stokes (RANS) methods [2], large eddy simulations (LES) [1] or detached eddysimulations (DES) [3] concentrate on the base flow of space vehicles. Furthermore, ex-perimental data for the validation of the numerical results were gathered in several windtunnel experiments [4–9]. The results of some of these investigations suggest the exis-tence of highly unsteady pressure fluctuations with large amplitudes in the base region,

† Institute of Fluid Mechanics, Technische Universität Braunschweig, Bienroder Weg 3, 38106Braunschweig

110 S. Scharnowski, C.J. Kähler, J. Windte & R. Radespiel

which lead to strong mechanical loads on the rocket nozzle, especially in the transonicregime, and could have damaging effects.

Additionally, several close collaborations to perform numerical investigations and ex-periments have taken place in the recent years. The FESTIP (Future European SpaceTransportation Investigations Program) and the RESPACE project (Key Technologies forReusable Space Systems) for example developed new technologies and new simulationmethods for reusable launchers [9,10]. The work presented here is a sub project of theSFB/TRR 40 program, founded by the German research foundation (DFG). The scien-tific focus within the SFB/TRR 40 is the analysis and modeling of coupled liquid rocketpropulsion systems and their integration into the space transportation system. Basedon reference experiments, numerical models are developed, which serve as a basis forefficient and reliable predictive simulation design tools. A combined optimization of themajor components under high thermal and mechanical loads, e.g. combustion chamber,nozzle, structure cooling and after body, is undertaken within the SFB/TRR 40 programto achieve an enduring increase in the efficiency of the entire system.

In this contribution, detailed analysis of the dynamics of the shear layer in the wake ofa blunt generic space launcher model will be presented. This is an continuative evalua-tion of the data set presented in Bitter et al. [11], where the implementation and perfor-mance of PIV on the space launcher configuration was presented.

The transonic range around Mach = 0.7 was of particular interest for the project part-ners as one of the objectives was to simulate the flow conditions shortly after the rocketlaunch, because at moderate altitudes, the ambient conditions (air pressure and den-sity) cause strong flow / structure interactions that can lead to high mechanical stressesin the involved components.

Furthermore, the hypersonic flow regime is of interest for the design of future spacelaunchers. Although, the mechanical loads are usually not crucial at this speed, sincethe density at higher altitude is rather small, high thermal loads may result from theunderexpanded nozzle plume downstream the vehicle base. While the results for thetransonic regime are presented in Sec. 2, the measurements at the hypersonic Machnumber 5.9 are discussed in Sec. 3

2. Transonic regimeThe transonic measurements were performed in the Trisonic Wind tunnel Munich

(TWM) at the Bundeswehr University Munich. The TWM is a blow down type wind tun-nel. The total pressure range of this wind tunnel is pt = [1.2 . . . 5]bar. This leads to aReynolds number range of Re= [7 . . . 80] · 106 m−1. The Mach number is adjusted in theLaval nozzle, which is continuously deformable and has a rectangular cross section. AMach number range between 0.3 and 3.0 can be obtained, with an accuracy of 0.005.The operating time of the TWM facility depends on the adjusted Reynolds and Machnumber, it reaches up to 300 seconds at Mach = 3. The maximum flow rate (240 kg/s)is achieved at Mach = 1 and pt = 5bar. In this case the run-time of the facility is ap-proximately 40 seconds. The facility is described in detail in [11]. The setup of the highrepetition rate PIV measurement system and the space launcher model are discussedin Sec. 2.1. Sections 2.2 and 2.3 show PIV results about vortex statistics for the wakeflow and shear layer dynamics, respectively. Most of the results of the transonic regimehave also been presented in [12].

PIV investigation of a space launcher model 111

FOVlight sheet

164.3

36°

54

Æ

21.5

Æ

FIGURE 1. Generic space launcher model with rear sting used at the TWM facility. The laser lightsheet and the field of view (FOV) for PIV are illustrated. Numerical values are given in mm.

2.1. PIV setup at the TWM facility

Due to the limited run time of the facility and the large number of recordings requiredfor reliable data, a high repetition PIV system using a Quantronix Darwin Duo Nd:YLFdouble pulse laser with a wavelength of 527nm, a pulse duration of tp ≈ 120ns, and alaser energy of 22 mJ per cavity at 1 kHz was used. The laser beam was transformedinto a light sheet using two spherical lenses with focal lengths of -40 mm and +50 mmfor adjusting the height of the light sheet, followed by two cylindrical lenses with focallengths of -25 mm and +50 mm for width adjusting.

The recordings were captured by using a Phantom V12 high repetition rate CMOScamera. By using a reduced resolution (1,280×500px) a recording rate of 8, 000 images/swas possible, which led to 4,000 PIV double frame images per second. The time betweenthe laser pulses was adjusted to 3µs, to restrict the particle displacement to 10pixelin order to keep out of plane loss of pairs on a small level. A Zeiss Sonnar T* AE2.8/180 mm objective lens with an f number of 4 was mounted in front of the camera.The working distance was 1.3m resulting in a particle image diameter of about 2 - 3 px.This allows for optimized spatial resolution and dynamic spatial range according to [13].

The tests were performed on a blunt axis symmetric aluminum model with a polishedsurface to avoid diffuse reflections at the wall, which would bias the near wall PIV velocitymeasurements, see [14]. The configuration consists of a 36 ◦ cone with a sphericalnose of R = 5mm and a cylindrical part of l = 164.3mm in length and a diameter of⊘D = 54mm. The total length, from nose to base, is l = 231.3mm. A rear sting inthe base of the cylinder was used for mounting the model in the test section. The fieldof view (FOV) was selected in order to determine the flow topology in the wake. Thelauncher model and the FOV are sketched in Fig. 1.

2.2. Vortex statistics

The acquired PIV data was evaluated by using a standard software (DaVis by LaVisionGmbH). A total number of 8,000 PIV double images was processed as followed:

(a) shift-correction of the raw images to compensate for model vibrations(b) local filter: subtract sliding minimum over 5×5px to reduce large scale background(c) masking model surface to enhance correlation procedure in the near wall region(d) vector calculation: multi-pass algorithm with decreasing window size down to 16×

16px with 75% overlap(e) post processing: rejecting vectors that differ from their neighbors by more than

two times the standard deviation of the neighbors (for each velocity component)The resulting 8,000 vector fields consist each of about 30,000 data points. Figure 2

illustrates the mean velocity distribution averaged over all computed vector fields.


X/D

Y/D

U /

U∞

−0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

−0.1

0

0.1

0.2

0

0.5

1

FIGURE 2. Mean velocity distribution and streamlines computed computed from 5, 000 PIVrecording for Ma = 0.7, ReD = 1.0 · 106.

X/D

Y/D

ωz /

U∞ in

m−

1

−0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

−0.1

0

0.1

0.2

−400

−200

0

200

400

FIGURE 3. Instantaneous vector field and estimated vorticity.

X/D

Y/D

λ ci /

U∞2 in

m−

2

−0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

−0.1

0

0.1

0.2

0

1

2

3x 10

4

FIGURE 4. Swirling strength and detected vortices for the vector field in Fig. 3.

The data set of 8,000 vector fields was used to detect vortices and identify theirstrength and size. Therefor, the swirling strength λci, the imaginary portion of the com-plex eigenvalue of the local velocity gradient tensor Eij , was computed by following thework of Adrian et al. [15]:

λci = max

(0,−Exy · Eyx −

Exx · Eyy2

+E2xx · E2

yy

4

)(2.1)

where the velocity gradient tensor is defined as follows:

Eij = ∂Vi/∂j. (2.2)

The swirling strength distribution, computed from the vector fields by using Eq. 2.1, wasbinarized using a threshold of 20% of the maximum of λci. Subsequently, the shape ofthe remaining connected areas (with λci > 20% · λci,max) was approximated by ellipsesusing the regionprops toolbox of MatLab (by MathWorks).

Figure 3 shows an exemplary instantaneous vector field and its rotation ωz. Theswirling strength λci for the same vector field is illustrated in Fig. 4 along with the de-tected vortices. The estimation of the vortex positions from the binarized image allowsonly for detecting vortices with a swirling strength stronger than the applied threshold.


X/D

Y/D

coun

ts

−0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

−0.1

0

0.1

0.2

0

100

200

300

400

FIGURE 5. Position of the 130, 000 detected vortices.

0

0.5

1

1.5

01

23

40

20

40

X/D

size in mm

coun

ts

(a) Vortex size distribution.

0

0.5

1

1.5

05001000150020002500

0

50

100

150

X/D

vortex enegy in a.u.

coun

ts

(b) Vortex energy distribution.

0

0.5

1

1.5

11.5

22.5

33.5

40

50

100

X/D

L1 / L

2

coun

ts

(c) Cross section ratio distribution.

0

0.5

1

1.5

−1.5−1−0.500.511.5

0

20

40

60

X/D

orientation

coun

ts

(d) Cross section orientation distribution.

FIGURE 6. Vortex statistics for different quantities along the X axes.

In order to detect also weaker vortices a cross correlation based wavelet analysis asdescribed in [16] would be required. However, the implemented method is capable ofdetecting the most energetic vortices and of identifying their size and position from thebinarized swirl distribution. Furthermore, fitting ellipses to the connected areas of thebinarized swirl distribution allows for estimating the vortex tubes orientation with respectto the measurement plane.

Approximately 130,000 vortices were detected in 8,000 vector fields. Their position,size (minor and major axis), and orientation was stored together with the swirling strengthat the center of the ellipses and the rotation direction (estimated from the sign of the ro-tation ωz). In Fig. 4 the rotation direction is indicated by the color of the ellipses, white


Strouhal number

time

in s

0 0.1 0.2 0.3 0.40

0.5

1

1.5

0.2

0.4

0.6

0.8

1

0 0.1 0.2 0.3 0.40

0.1

0.2

Strouhal number

ampl

itude

mean spectral amplitude

FIGURE 7. Temporal development of the shear layer dynamics: Spectral amplitude for 512 out of8,000 samples for varying times (top) and mean distribution averaged over the whole measure-ment time (bottom).

and red corresponds to counter clockwise and clockwise, respectively. Figure 5 illus-trates where the vortices were detected. The field of view was divided into 100 × 25boxes and the number of vortex centers per box is color coded in the figure. Most ofthe vortex centers are located within the shear layer as expected. At the beginning ofthe shear layer (X/D < 0.2) the vortex centers are located close to Y/D = 0. Furtherdownstream (X/D > 0.2) the vertical range is increasing.

The quantities size, swirling strength, axis ratio, and orientation are analyzed withrespect to the vortex center position in X direction. The results are plotted in Fig. 6.Figure 6(a) shows a histogram of the major axis length of the detected vortices for thedifferent X positions. Vortices smaller than 0.4mm could not be detected due to thefinite size of the interrogation windows and the resulting vector field grid spacing. Mostof the detected vortices have a size of around 1mm and the largest ones (larger than3mm) are located at X/D = 0.2. Figure 6(b) illustrates the energy distribution along theX axis. Whereas, it is assumed that the vortex energy is proportional to the product ofthe minor and major axis length and the swirling strength. The most energetic vorticesoccur at a position around X/D = 0.5. In part (c) of Fig. 6 the ratio of the major andminor axis of the detected ellipses is plotted as a histogram like before. Most vorticesappear nearly circular, however, some are stretched indicating that the vortex tubes cutthe measurement plane under a certain angle. The orientation of the detected ellipses isshown in Fig. 6(d). Whereas 0 and ±π/2 corresponds to stretching in X and Y direction,respectively.

2.3. Shear layer dynamics

Figure 7 illustrates the development of the power spectral density of the horizontal ve-locity component for an area in the shear layer far away from the base of the spacelauncher model (at X/D = 1.9...2.1 and Y/D = −0.2...0.1). The spectra were computed


from an ensemble of 8, 000 vector fields acquired at a frequency of 4 kHz. For each linein the upper part of Fig. 7 only 512 time steps of the data set were used to compute thespectral amplitude. The spectra were computed for each point within the selected areausing the method of Welch [17] as implemented in MatLab and, thereafter, one aver-aged spectrum was determined for each time step. A Hamming window function wasapplied to each 512 points of the data subset in order to cancel the largest side lobe ofthe Fourier spectrum [18]. The vertical axis shows the time difference between the firstvector field from the whole ensemble and the first one out of each 512 samples. Thehorizontal axis shows the Strouhal number which is defined as follows:

StD =f ·DU∞

(2.3)

where D is the model diameter (54mm), the free stream velocity is U∞ = 230m/s, andf is the frequency. Besides the temporal development of the power spectral density themean value over 1.9 seconds is shown in the lower part of Fig. 7.

Even though the mean spectrum shows a distinct maximum at StD ≈ 0.21 (corre-sponding to f = 890Hz), the temporal distribution has only intermittent wave packets.Thus, no regular periodic vortex shedding appears as assumed in the literature, com-pare [3, 4, 19, 20]. However, a Strouhal number around StD ≈ 0.21 seems to be morelikely than others.

3. Hypersonic regimeThe Hypersonic Ludwiegtube Braunschweig (HLB) is a cold Ludwieg type blow down

tunnel, utilizing a fast acting valve to separate the high pressure from the low pressuresection. The high pressure section consists of a 17 m long heated storage tube andcan be pressurized up to 30 bar. The low pressure section consists of a Laval nozzledesigned to generate Ma = 5.9, the adjacent 500 mm diameter test section, and a 6 m3

dump tank. The time with constant flow conditions in the test section, and thus the mea-suring time, is about 80 ms. The unit Reynolds numbers operative range is 3 ·106 m−1 to20 · 106 m−1. More detailed information on the design and operation of the HLB is givenin [21].

For all measurements the pressure in the storage tube and in the low pressure sectionwas set to (18.0± 0.2) bar and approximately 1 mbar, respectively. This corresponds toa Reynolds number of 16 · 106 m−1. The temperature in the middle of the storage tubejust before the valve was estimated to be (462± 10) K.

The model is made of Plexiglas and is painted in dull black. The geometry is similar tothat used in the TWM at Bundeswehr University Munich. However, the model is enlargedby the factor 2. A ring of glued corundum grains on the conical part of the space launchermodel was used to control the transition from laminar to turbulent flow (see. [22]).

The setup of the PIV measurement system and the space launcher model used at theHLB facility are discussed in Sec. 3.1. The PIV data evaluation is described in Sec. 3.2.Sections 3.3 and 3.4 show PIV results about boundary layer flow and the wake flow,respectively.

3.1. PIV setup at the HLB facility

Due to the high velocity of the flow at Ma = 5.9 (≈ 910 m/s) a PIV system with shortinter framing time is required. A PCO.4000 camera (by PCO AG) with a 4008 × 2672px CCD-sensor was used. The time ∆t between the two laser pulses could be set to


328.6

36

°

1

08

Ø

4

3Ø

FOV III FOV I

FOV II light sheet

FIGURE 8. Sketch of the space launcher model used at the HLB facility with laser light sheet andthe different fields of view (FOV). Numerical values are given in mm.

setup I setup II setup IIIarea of interest boundary layer wake overview wake close-upfield of view 66× 50 mm2 120× 90 mm2 75× 50 mm2

scaling factor 18.9 µm/px 34.1 µm/px 18.9 µm/pxPIV recordings 25 25 220working distance 350 mm 600 mm 400 mm∆t 0.5 µs 2.0 µs 0.35 µs

TABLE 1. PIV setups for hypersonic measurements at HLB.

350 ns. The objective lens was selected such that the maximum particle image motionbetween the two frames was less than 30 px: 180mmF/3, 5 macro by TAMRON EuropeGmbH with a f-number of 8. A working distance of 350 mm ensures a maximum numericaperture of the optical setup. A Q-Switched Nd :YAG double-pulse laser system (Brilliantby Quantel) with a pulse duration of 5 ns and a laser energy of 60mJ per cavity was usedto illuminate the tracer particles (oil droplets). The laser beam was transformed into alight sheet using two spherical lenses with focal lengths of −40 mm and +50 mm foradjusting the width of the light sheet, followed by a cylindrical lens with focal length −50mm for height adjusting. Finally, a mirror located in the rear sting mounting redirects thebeam to illuminate the investigated field of view. The light sheet thickness at the regionof interest was estimated to be 0.5 mm.

For the evaluation of the flow field three different fields of view were selected: one forthe boundary layer close to the model’s base and two to analyze the wake. The differentareas are illustrated in Fig. 8. Table 1 summarizes the size, the scaling and workingdistance as well as the number of recorded image pairs and the time ∆t between thelaser pulses.

3.2. Image processing technique

Both, the relatively low amount of acquired PIV recordings and strong variations inthe seeding density call for a sum-of-correlation data evaluation, which averages thenormalized correlation functions for the interrogation windows over the number of PIVrecordings [14]. A multi-pass cross-correlation algorithm with decreasing window size(from 128 × 128 px to 32 × 32 px, 50% overlap, and a Gaussian window weighting witha 4:1 stretching in x direction) was applied by using the evaluation software DaVis (byLaVision GmbH).

Prior the averaging of the correlation functions, the model movements in the raw im-ages were corrected for a reliable PIV evaluation down to near-wall regions. Further-more, a local subtract sliding minimum filter reduced CCD background noise and reflec-


(a) Raw image. (b) Filtered image.

FIGURE 9. Image pre processing: the raw images were shift corrected and a local subtractsliding minimum filter was applied.

tions in the PIV images in order to enhance the correlation peaks. The spatial filteringdiameter of 5×5 px, which is slightly larger than a particle image, was used as before inthe transonic case. Near-wall regions were masked due to a less reliable computation ofthe velocity vectors caused by laser light reflections which led to self-correlation of thereflections. In Fig. 9(a) and 9(b) an raw PIV recording and a filtered and shift-correctedone demonstrate the image preprocessing discussed above.

Finally, a post-processing algorithm, including median filtering, where outliers wereeliminated by a correlation peak ratio threshold of 1.3 (ratio of highest peak minus globalminimum to second highest peak minus global minimum), was applied.

3.3. Boundary layer results

In this section, the results of the boundary layer investigation on the end of the cylindricalpart of the space launcher model are presented. The number of image pairs is 25, asoutlined in Tab. 1. The field of view was 66×50 mm2, corresponding to 0.61×0.46 modeldiameters. The image preprocessing algorithm discussed above was applied. The nor-malized absolute velocity field derived from the measurements is plotted in Fig. 11 to-gether with the wake flow results. The figure visualizes the evaluation of the boundarylayer height within the rear end of the main body. A subset of the computed velocityvectors is shown.

The extracted velocity profile at x/D = −0.1 is illustrated in Fig. 10 using two differentmethods: window cross correlation as implemented in DaVis as well as a single-pixelapproach with averaged correlation peaks over 50 × 2 pixel as described in [23]. Bothmethods result in similar velocity profiles for the outer region. However, the single-pixelapproach yields more reliable results in the near wall region (see [13, 14]). The mea-surement points in the near wall region are quiet noisy. This is due to the rather smallnumber of PIV image pairs and due to the low density of tracer particle close to thesurface.

In the PIV results, the entire boundary layer height was resolved within approximately1000 pixel. The averaged correlation peaks over 50 × 2 px led to 500 boundary layersampling points in the direction normal to the wall at x/D = −0.1. Nevertheless, the


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++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

U/U∞

y/D

0 0.2 0.4 0.6 0.8 10.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

50 x 2 px

32 x 32 px

+

x

FIGURE 10. Velocity profile at y/D = −0.1 estimated for two different interrogation window sizes.

low thickness of the boundary layer (δ99 = 18.9mm) and the low optical magnification(necessary due to the high velocity) also reveal the challenging goal of resolving thenear-wall gradients in the flow, which is a necessary step for the validation of numericalflow simulations.

In order to resolve the logarithmic region and the decay of the streamwise velocitycomponent down to the wall, in this experiment, the magnification of the imaging systemmust be increased by a factor of 2 - 5 in order to resolve the viscous sublayer region with10 - 20 pixel. Therefore, a long-range micro-PIV technique with single-pixel resolutionevaluation in combination with a camera with much shorter inter framing time would fulfillthe demands if particles appear in this region.

3.4. Wake flow results

Finally, the results of the PIV measurements in the wake of the space launcher modelat setup position II and III are presented. For position II 25 double-frame images wereacquired in order to achieve an overview of the flow topology. The observed field of viewhad a size of 120×90 mm2, corresponding to 1.11×0.83 model diameters, as outlined inTab. 1. This leads to an axial wake resolution of roughly 1.2 model diameters behind thebase of the model. The image preprocessing discussed in Sec. 3.2 was again appliedto the raw data. For position III the field of view was reduced by decreasing the workingdistance (see Tab. 1) in order to obtain the highest possible numerical aperture.

As a general overview, it can be stated that it is feasible to characterize the spatialextension of the recirculation domain in the wake with the chosen PIV setup even at hy-personic Mach numbers. A distinct recirculation area with a clear reattachment locationat x/D = 0.6 develops for Mach = 5.9. However, the flow within the recirculation areaitself could not be resolved. This is most likely due to the very small size of the particlesthat can reach this area. The intensity of scattered light by these particles is to low forthe detection limit of the CCD sensor. To solve this inherent problem a locally seedingapproach is required.


FIGURE 11. Mean velocity distribution for all three measured setups: boundary layer at the endof the main body, wake overview and wake close-up.

4. ConclusionsThe results of the measurements in the transonic regime at the TWM facility show

the capabilities of high repetition rate PIV to resolve large-scale wake structures in tran-sonic flows. The developed approach allows for detecting single vortices in the wakeof the space launcher model. Statistical analysis of the vortex energy and vortex sizedistribution showed that the largest and most energetic vortices appear within the shearlayer. Consequently, it can be concluded that the nozzle should not extend to this region.Most of the detected vortices had a size of around 2% of the model diameter.

The temporal analysis of the wake’s frequency spectrum at Ma= 0.7 showed theexistence of small coherent vortex packages with a Strouhal number around StD ≈ 0.21,rather than periodic vortex shedding. It is confirmed that high dynamic loads caused byunsteady shedding vortices could interact with the nozzle (here: the sting) and couldlead to structural damaging.

The results of the measurements in the hypersonic regime at the HLB facility demon-strate the capability of the used PIV system to capture the development of the mean flowtopology in the boundary layer as well as in the extension of the recirculation domain atMa= 5.9. The boundary layer has a thickness of δ99 = 0.15 · D at the end of the mainbody (x/D = 0) and the reattachment line was found to be located at x/D = 0.6.

Next, the results of this investigation will be used to validate numerical simulationmethods and it will be interesting to see if the measurements about the vortex propertiesand dominant frequencies can be reproduced numerically for this complex flow problem.

AcknowledgmentsFinancial support from the German Research Foundation (Deutsche Forschungs-

gemeinschaft – DFG) in the framework of the Sonderforschungsbereich/Transregio 40(Technological foundations for the design of thermally and mechanically highly loadedcomponents of future space transportation systems) is gratefully acknowledged by theauthors.


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PIV investigation of a space launcher model - · PDF filecharacteristic frequencies of the vortex shedding at Ma = 0.7, ... launch, because at moderate ... underexpanded nozzle plume