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SHOCK TUNNEL FREE FLIGHT FORCE MEASUREMENTS USING A COMPLEX MODEL CONFIGURATION Klaus Hannemann (1), Jan Martinez Schramm (2), Stuart Laurence (3), Sebastian Karl (4) (1) German Aerospace Center, Bunsenstraße 10, 37073 Göttingen, Germany, [email protected] German Aerospace Center, Bunsenstraße 10, 37073 Göttingen, Germany, [email protected] (3) German Aerospace Center, Bunsenstraße 10, 37073 Göttingen, Germany, present address: Department of Aerospace Engineering, University of Maryland, College Park, MD 20742, USA, [email protected] (4) German Aerospace Center, Bunsenstraße 10, 37073 Göttingen, Germany, [email protected] (2) ABSTRACT The free flight force measurement technique is a very attractive tool to determine forces and moments in particular in short duration ground based test facilities. With test times in the order of a few milliseconds, conventional force balances cannot be applied here. The technique has been applied in a number of shock tunnels utilizing models up to approximately 300 mm in length and looking at external aerodynamics. In the present study the technique is applied using a complex 1.5 m long hypersonic integrated supersonic combustion ramjet (scramjet) engine consisting of intake, combustor and thrust nozzle. For this type of engine the design objective is a combustor with efficient mixing and combustion within the shortest possible length, but still robust enough to operate in various operational conditions. In the framework of the EU co-funded project LAPCAT II, a M=7.4 scramjet powered small scale flight experiment (SSFE) configuration was designed. Since free jet testing of the complete combustion flow path is a mandatory step within the design roadmap of future engines, ground based testing of the SSFE engine was conducted in the High Enthalpy Shock Tunnel Göttingen (HEG) of the German Aerospace Center (DLR). This type of facility allows duplication of flight conditions in excess of M=8. Here tests were performed simulating Mach 7.4 flight conditions in approximately 28 km altitude. The numerically predicted thrust of the engine could be confirmed in HEG by free flight force measurements based on displacement measurements utilizing optical tracking. Combining these experimental results with computed aerodynamic data of the complete SSFE showed that for a selected flight condition a positive aero propulsive balance of the complete configuration could be achieved. balance). The design process of the scramjet engine is reported in [6]. The main focus of the experimental campaign in HEG was to measure the thrust generated by the scramjet engine. The free flight model technique in conjunction with optical tracking was applied. The thrust increment of the engine was determined by comparing the measured force in streamwise direction obtained by fuel on and fuel off experiments. In HEG the wings of the SSFE configuration were removed (see Figure 1). Combining nose-to-tail computations of the SSFE configuration and the HEG model configuration conducted by DLR and ESA-ESTEC using the DLR TAU code [7] with the DLR thrust measurements in HEG, the aero propulsive balance of both configurations could subsequently be analysed. While these results are already discussed in [2], the focus of the present article is on describing the complex model set-up and the optical tracking procedure. Details regarding the numerical modelling of the flow past the complete and the truncated SSFE configuration (see Figure 1) in HEG free stream conditions are given in [7]. 1. INTRODUCTION One goal of the EU co-funded research project LAPCAT II [13] was the design of a M=7.4 supersonic combustion ramjet (scramjet) powered small scale flight experiment (SSFE) configuration. Within the design phase of the SSFE the major goal was that the vehicle is able to generate net thrust (positive aero propulsive Figure 1. Schematic of the SSFE configuration (top) and the wind tunnel model configuration with removed wings used in the HEG test campaign (bottom) A summary of the force measurement technique development at HEG and an overview of the free flight technique utilizing light and stiff models for external aerodynamics investigations is given in [1]. 2. EXPERIMENTAL SETUP High Enthalpy Shock Tunnel Göttingen, HEG The HEG of DLR is one of the major European hypersonic test facilities. This free piston driven shock tunnel ([3],[4]) was commissioned for use in 1991 and has been utilized extensively since then in a large number of national and international space and hypersonic flight projects. Originally, HEG was designed for the investigation of the influence of high temperature effects such as chemical and thermal relaxation on the aerothermodynamics of entry or reentry space vehicles. Subsequent extensions of the HEG operating range included the establishment of a number of low total specific enthalpy conditions and of complete scramjet engine ground-based testing capabilities [3]. pulse of gas to a hypersonic convergent - divergent nozzle at stagnation pressures of up to 200 MPa, and stagnation enthalpies of up to 23 MJ/kg. Regarding the test gas, no basic limitations exist. The operating conditions presented in the present article are related to the test gas air. Additionally, operating conditions using nitrogen and carbon dioxide exist. Additional information about the working principle of the facility, its capabilities and former use can be found in, e.g., [3], [4]. The HEG operating condition used in the presented experimental investigation is HEG Condition XIII. In Table 1, the nominal reservoir and free stream conditions are given. p0 [MPa] 17.7 h0 [MJ/kg] 3.24 T0 [K] 2695 P T [Pa] [K] 2024 264 [g/m3] 27.2 u [m/s] 2398 M [-] 7.36 Table 1. Nominal reservoir and free stream for HEG operating condition XIII Figure 2. Photographic views of HEG Wind Tunnel Model Setup Compared to the utilization of wind tunnel models which are derived, e.g., from capsule type configurations (see, e.g., [1]), the configuration and the objective of the measurements considered here imply additional requirements. The SSFE engine is approximately 1.5 m long and in order increase the stiffness and to reduce the weight as much s possible, the main body of the wind tunnel model was manufactured from a solid block of aluminium (see Figure 4). The manufacturing of this part occupied a seven degrees of freedom CNC bed-type milling machine for a period of four month. Figure 3. Schematic of HEG The overall length of HEG is 62 m and it weighs 280 t. Approximately a third of its weight is contributed by an inert mass (see Figure 2, left picture) which is used to reduce the tunnel recoil motion. The compression tube is closed by a hydraulic oil system at the main diaphragm station. The shock tube is connected to the nozzle of the tunnel at the downstream closure, which is also driven by oil hydraulics to close and seal the tunnel. The compression tube has a length of 33 m and a diameter of 0.55 m. The shock tube is 17 m long with a diameter of 0.15 m. The HEG was designed to provide a Figure 4. SSFE wind tunnel model main body during manufacturing in the DLR workshop; in order to enhance the stiffness of the model it was machined from a solid block of aluminium An additional requirement is to implement suitable mechanisms to both release the model allowing freeflight during the test duration, and terminate its motion following the conclusion of this time period. In Figure 5 the hanging mechanism attached to the central beam of the HEG test-section roof is shown. The model is initially suspended in the test section by means of two kevlar/aramide threads. The longitudinal separation between the two threads is 590 mm, and a thread diameter of 0.8 mm was chosen to support the 85 kg heavy model. Each of the threads is arranged in a Vconfiguration passing through the model body and then attached on either side to the appropriate clamping mechanism. Employing V-configurations provides better alignment stability in the roll and yaw axes than single attachment points on the model. The use of a front-and-back two thread configuration, together with height adjustment at each of the clamping devices, means that the model height and angle of attack can be adjusted independently of one another and an appropriate initial alignment of the model relative to the support legs as well as to the flow direction can be easily obtained. In Figure 6 the attachment points of the front thread to the hanging mechanism as well as the two points of entrance of this thread into the model body are shown. that this will have a negligible effect on the angle of attack. For example, a 2 ms delay would produce a worst-case change of 5x10-4 deg. The model catching mechanism comprises two cylindrical buffers, each mounted between a pair of legs (see Figure 7). Each buffer consists of a steel rod with two O-rings of slightly larger diameter attached to provide a damped termination to the model motion without damaging the internal model components. The legs not only provide supports for the buffers, but also provide routes by which the fuel line and any required cables (depending on the particular configuration chosen – fully free flying or weakly constrained configuration) can enter into the model without being exposed to the flow. In order to allow the different initial angles of attack required by the experimental campaign, the height of both buffers can be adjusted via slots in the legs. Figure 7. Model catching mechanism; cut through the model showing the leg-mounted buffers used to terminate the model motion Figure 5. SSFE model hanging and height adjustment components The hydrogen injection system must be fully integrated into the model. Hydrogen is injected into the combustor by two semi struts at the front part of the combustor and one full strut further downstream [7]. The hydrogen supply system of the wind tunnel model consists of two individual injection units. Each unit supplies one strut assembly. The schematic given in Figure 8 shows the setup exemplary for one hydrogen flow path. Figure 6. Attachment points of the kevlar threads at the hanging mechanism (left) and the model (right) The height adjustment on both supports is realized by means of a central travelling screw with a guiding pin to either side to ensure alignment is maintained. To allow free flight during the test time, some method of detaching the threads is necessary. For this purpose razor blades are incorporated in the model close the entrance points of the threads (right part of Figure 6). The razor blades are mounted horizontally directly behind the threads; when the flow arrives, it will push the threads back against the razors, shearing them off without leaving any significant excrescences on the model surface. There may be a delay between the cutting of the front and rear threads, but it can be shown Figure 8. Schematic of the hydrogen injection system The hydrogen is supplied through a valve to a pipe crossing the wind tunnel wall. The pipe is hooked up to a thin and flexible capillary pipe. The capillary line is attached to the wind tunnel model and supplies the fuel to one litre steel fuel tanks (see Figure 9). Once the system is filled to the required pressure, the external valve is closed and the hydrogen is injected into the plenum via a fast acting solenoid valve a few milliseconds before the HEG run is initiated. The capillary pipe connection to the model was designed such that no impact on the model is generated during the free flight phase in case the weakly constrained configuration is applied. This was confirmed by dedicated test runs in HEG (see Figure 17). Figure 9. Hydrogen injection system integrated in the free flight wind tunnel (photo of the system (top) and schematic (bottom): (1) Pipe, (2) tanks, (3) pipe, (4) solenoid valve, (5) pressure tap, (6) plenum chamber below injector blocks. HEG run, i.e., to combine the force measurements with combustion flow path pressure measurements, the model is additionally equipped with 40 pressure transducers distributed on the intake-, combustor- and nozzle-section (Figure 10). In the combustor, all sensors are mounted on the side-walls; this choice was made based on CFD predictions showing that the effect of reflected shocks is much reduced in comparison to the lower wall, meaning that the measured pressure will be much less sensitive to the exact location of the shocks. This should, therefore, provide a more meaningful comparison with CFD results. In the nozzle section, at several downstream positions pressure gauges are located on both the nozzle centreline and sidewall; by this point downstream, the pressure gradients on the lower wall have become somewhat shallower in the downstream direction, making lower-wall measurements more worthwhile here. For realizing the completely free flying arrangement, on board data acquisition units are required. Due to constraints regarding the available space in the model the number of units and consequently the number of data channels are limited. In order to measure pressures at all selected locations simultaneously in one run [12], wire connections to an external data recorder are needed resulting in a weakly constrained arrangement. Figure 11. Schematic of the free flight wind tunnel model installed in the HEG test section Figure 10. Pressure sensor locations on the SSFE model intake (top), combustor (middle) and nozzle (bottom) In order to obtain additional information from each The integration of the free flight model in the HEG test section is shown schematically in Figure 11. The inlet is positioned in the core flow (test flow rhombus) of the hypersonic nozzle of HEG. However, the downstream part of the model is exposed to non-uniform flow outside the test rhombus, and, therefore, does not correspond to the situation experienced in flight conditions. For that reason differential force measurements using fuel on and fuel off conditions which allow eliminating the influence of the nonrepresentative flow past the model outside the test rhombus were conducted. Consequently, the tests in HEG provide the thrust increment which is generated solely by the internal propulsion flow path. 3. OPTICAL TRACKING The displacement of the model was recorded using a visualization based tracking technique. A symmetric trapezoidal shaped tracking object was installed on the model. This object is positioned on the top surface of the model just downstream of the cowl leading edge such that it lies inside the field of view of the HEG windows (Figure 12). The length of the top face of the object is 10 mm; the two sides have an angle of 45 degree and the overall height is 14.5 mm. The tracking object movement was visualized using the HEG Schlieren setup, consisting of a conventional Z-fold arrangement as shown in Figure 12. Additional information about the Schlieren setup and its utilization can be found in [4] and [11]. The Schlieren setup was adjusted such that the target was imaged on the camera chip covering the sensitive area. A Phantom v1210 digital camera was used. Two 1.5 m focal-length spherical mirrors (H1) collimate the beam from the light source to pass through the test section and then refocus it on the opposite side. The focal plane (FP) of the H1mirrors lies in the middle of the model on the tracking object. leading edge and ensured that any unstart of the intake would be detectable. The light source employed was a Cavilux Smart laser which provided repetitive 10 ns pulses at 690nm. The Phantom v1210 camera recorded the images, typically at 25,000 fps with a resolution of 896x512 pixels. A narrow band-pass filter was placed in the optical path slightly ahead of the knife-edge in order to remove any extraneous light, e.g., from tunnel selfluminosity. The visualization system also allowed observing the detachment of the front support threads. In Figure 13, three images resulting from one experiment are shown. The initial state of the model (before the arrival of the flow), the detachment of the front threads, and the clean configuration during the test time are visualized. In most of the HEG run the threads completely detached well before the onset of the steady test time. The basis of the methodology pursuit to evaluate the motion of the model is given in [1], [8], [9] and [10]. Figure 13. Image sequence showing the tracking object and cowl leading-edge region; (top) before flow arrival; (middle) during thread detachment; (bottom) during steady test time Figure 12. Position of tracking object on the SSFE model in HEG (top); setup of the Schlieren visualization system used for optical tracking of the model motion (bottom) The focal point is denoted by (R), and lens (L) is used to focus the image on the camera chip (FP). A vertical knife edge was placed at the focal point, which allowed the visualization of the flow structures at the cowl In order to locate the contour of the model, the recorded images are treated with a Sobel convolution mask [14] which determines the first order derivative of the pixel intensity in vertical and horizontal direction. This filter includes a discretised Gaussian smoothing. An example is given in Figure 14. Here an image recorded prior to the run is shown after treatment with the Sobel-Filter. The identified model contour is clearly visible. Here the pixel intensity represents the magnitude of the computed combined gradients in horizontal and vertical directions. acceleration within this time period are denoted by the bold black lines in Figure 16. 0.25 0.10 0.20 0.08 0.06 u [m/s] Figure 14. Tracking image after application of a SobelFilter (original image given in Figure 13 (top)) x [mm] 0.15 0.10 0.04 0.05 0.02 250 0 1 2 3 4 5 6 7 8 0.00 0 1 2 3 4 5 6 7 8 t [ms] t [ms] 30 25 20 15 2 10 5 0 350 300 0.00 a [m/s ] The next step in the processing procedure is to mark all pixels with intensity above a user defined threshold. The result of this process is shown in Figure 15. In the top plot, all marked pixels are shown in grey. The pixels marked in red and blue are subsets which the tracking algorithm identifies as pixels defining the model contour. This is done by comparing the analytical description of the tracking object with the coordinates of the marked pixels. pixels with a gradient value above user definded treshold pixels afer first optimization step ‐5 ‐10 1 2 3 4 5 t [ms] 6 7 8 y [Pixel] 200 Figure 16. Displacement (top left), velocity (top right) and acceleration (bottom) of the tracking object in x direction 150 100 50 Figure 15. Algorithm to identify pixels describing the tracking object; all pixels with a gradient value above a user defined threshold and set of contour pixels after first optimization step (upper); final set of pixels describing the contour (lower) The free flight technique applied to the SSFE model in HEG posed several challenges on the optical tracking technique. The model size of the SSFE is larger than the optical windows available at HEG, which means that only a part of the model (the tracking object) could be visualized. The assumption has to be made that the motion of the tracking object represents the motion of the complete model. Due to the mass of the model, the resulting motion is small. In Figure 16, it can be seen that the distance the model moves during the test time is only a fraction of a millimetre. The intake crotch region, the detachment process of the threads and the motion of the tracking object had to be visualized simultaneously. The observation of the intake crotch region is important to monitor the operation mode of the intake, i.e., to identify in case the intake would show unstart during the test time. The behaviour of the threads allows judging the release time of the model, and the target itself is used to determine the motion of the complete model, as already discussed. An example for the measured movement in x-direction of the tracking object for a fuel-off run is shown in Figure 16 in the upper plot. The displacement is smoothed by applying a Savietkzy-Golay-Filter. By subsequent differentiation with respect to time the velocity and the acceleration of the tracking object is obtained (Figure 16). For the run considered here, an approximately constant acceleration, i.e., force on the model is obtained between 4.5 and 7 ms. The interpolated displacement (using a quadratic fitting function) and the constant velocity increase, i.e., For the present measurements, a resolution of approximately 0.02 mm per pixel was obtained. Together with the error resulting from the camera timing, a precision of 10% can be determined when assuming a constant acceleration and using the displacements and times encountered in the experiments in an error propagation method (see e.g., [8]). This precision would hold if the determination of the motion would rely only on the information of one pixel. As discussed, the determination is based on a large number of pixels defining the contour of the target and the 0 ‐50 400 450 500 550 600 650 700 750 800 850 900 x [Pixel] 350 300 250 pixels afer first optimization step final set of pixels describing the contour y [Pixel] 200 150 100 50 0 ‐50 400 450 500 550 600 650 700 750 800 850 900 x [Pixel] precision improves by this statistics to roughly 1.5%. Nevertheless, the motion of the model deviates from the fitted motion assuming a constant acceleration by approximately 7%. Therefore, this value is taken here as a conservative error estimation. model does not represent this case. Based on TAU predictions the wind tunnel model configuration experiences a negative acceleration (net thrust), however, the non-representative external flow past the model in HEG generates a higher drag, and, therefore, a positive acceleration (net drag) even in the case of fuel injection. 30 Angle of attack ‐2° Fuel‐On Fuel‐Off 25 20 2 a [m/s ] 15 10 5 0 ‐5 Figure 17. Comparison of acceleration in streamwise direction using the weakly constrained and free flying model configuration As outlined above, several reasons were identified to consider a weekly constraint free flight model configuration (re-fuelling of the hydrogen tanks during run preparation, recording of pressure data from all gauges installed in the model). The line and cable connections were designed such that an impact on the force measurements is minimized. This approach was validated by comparing the force measurements obtained for the weekly constraint configuration with a completely free flying model configuration, i.e., without any connection between the model and the support system. The measured acceleration in x‐direction is compared in Figure 17. No measurable influence of the line and cable connections used on the weakly constrained configuration can be detected during the test time window. 4. RESULTS The results of the HEG SSFE force measurements and the comparison with computational predictions are reported in [2]. Here a summary of the key results is provided. The net thrust increment generated by the internal engine flow path is determined by subtracting the force measured with fuel injection from the force measured without fuel injection. The measured acceleration for the fuel off and fuel on (fuel equivalence ratio of ≈ 1.0) cases at an angle of attack of = -2o are given in Figure 18. In both cases the acceleration is positive, i.e., during the available test time, the model is moving downstream. However, the acceleration obtained from the run with fuel injection is significantly reduced compared to the run without fuel injection. It should be emphasized that the force acting on the model in HEG cannot be directly compared to the numerical prediction given in [2]. In the computations a uniform free stream is applied whereas in HEG the external flow past the ‐10 1 2 3 4 5 6 7 8 9 t [ms] Figure 18. Acceleration in streamwise direction derived from the measured displacement of the tracking object; angle of attack = -2o The difference between the absolute thrust and absolute drag are referred to as aero propulsive balance here. If the thrust exceeds the drag, the aero propulsive balance is positive, and vice versa (see [2]). The measured net force increment (thrust solely generated by the internal combustion flow path for a fuel equivalence ratio of ≈ 1.0) and = -2o is given in Table 2. Additionally, the corresponding numerical TAU code prediction is included. The numerical result under predict the measured thrust by 7.6%. = -2° Thrust [N], HEG Thrust [N], TAU Difference [%] 0.99 580 ± 42 536 7.58 Table 2. Measured and computed thrust increments of the internal combustion flow path; denotes the fuel equivalence ratio Figure 19. Aero propulsive balance for the SSFE configuration based on combining experimental (HEG) and numerical (TAU) data; fuel equivalence ratio ≈ 1.0 The aero propulsive balance determination for the complete SSFE configuration based on a combination of measured (HEG) and computed (DLR TAU code) data is summarized in Figure 19. Combining the measured thrust of the internal propulsion flow path and the computed drag of the complete vehicle for fuel-off condition, a positive aero propulsive balance of 73N is obtained for = -2o (see Figure 19). Even when taking into account the uncertainty of the thrust measurement, a positive balance of 73N - 42N = 31N is still obtained. 5. SUMMARY AND CONCLUSIONS The complete scramjet flow path of the LAPCAT II small scale flight experiment (SSFE) configuration was tested in the High Enthalpy Shock Tunnel Göttingen (HEG) of DLR. The thrust increment between fuelled and unfuelled operation was detected with the free flight technique (in fully free flight and/or weakly constrained configuration). This technique was used in HEG for the first time in combination with such a complex model configuration. For an angle of attack of =-2°, and an equivalence ratio of ≈ 1.0, the combination of measured and computed forces demonstrated that a positive aero propulsive balance could be achieved, i.e., the SSFE configuration would generate net thrust. It should be emphasized that the numerical predictions are based on the assumption that the boundary layer is fully turbulent. Since the flow on the external part of the vehicle and on the intake will be transitional, the above values represent a conservative estimate. ACKNOWLEDGMENTS A part of this work was funded by the ‘Long-Term Advanced Propulsion Concepts and Technologies II’ project investigating high-speed transport. LAPCAT II, coordinated by ESA-ESTEC, is supported by the EU within the 7th Framework Programme Theme7 Transport, Contract no.: ACP7-GA-2008-211485. Further info on LAPCAT II can be found on http://www.esa.int/techresources/lapcat_II. The support of the HEG team in preparing the wind tunnel model and during the performance of the test campaign is highly acknowledged. 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