Shock Tunnel Research Articles

Velocity and NO-Lifetime Measurements in an Unseeded Hypersonic Air Flow

A laser-induced fluorescence (LIF)-based nitric-oxide flow-tagging technique was applied to measure both velocity and NO lifetime in a hypersonic shock tunnel from two experimental test runs. The results were supported by an analytical profile proposed in this paper that provides a way to correct velocity measurements under unknown systematic error sources. This procedure provided velocities with discrepancies lower than 3% for a total of five measurements, and lower than 2% when compared with that obtained from a linear fit. Additionally, the comparison between the proposed and experimental profiles allowed us to obtain the fluorescence NO lifetime from only one image.

Performance of Copper Calorimeter for Heat Transfer Measurement in High Enthalpy Shock Tunnel

Copper calorimeter, based on a calorimetric principle, offers a solution for heat transfer measurement in high enthalpy situation, especially in the erosive flow of high enthalpy shock tunnels. In this study, we numerically investigated the measuring performance of copper calorimeters. Non-ideal effects, such as heat loss to the insulator around and replacement of the average temperature of the copper element by the junction temperature, were discussed in detail. The influences of copper element thickness, copper/constantan wires thickness and sensor diameter were also estimated, with the aim to provide theoretical guidance for the design of copper calorimeter. In addition, corresponding experiments in JF10 high enthalpy shock tunnel were carried out against the data of coaxial thermocouples for verification. Results showed that the non-ideal thermal environment of a copper calorimeter (heat exchange with its surroundings) would result in a smaller measuring heat flux comparing to the one actually loaded; proper thickness of copper element matching the effective test time of shock tunnel was suggested. Besides, preliminary experimental results with corrections showed reasonable agreement with the heat flux of thermocouples, with an average deviation of 8%. Over all, this gauge developed extends and supplements the high enthalpy shock tunnel heat transfer measurements made by other techniques.

Calculating shock arrival in expansion tubes and shock tunnels using Bayesian changepoint analysis

To understand the flow conditions generated in expansion tubes and shock tunnels, shock speeds are generally calculated based on shock arrival times at high-frequency wall-mounted pressure transducers. These calculations require that the shock arrival times are obtained accurately. This can be non-trivial for expansion tubes especially because pressure rises may be small and shock speeds high. Inaccurate shock arrival times can be a significant source of uncertainty. To help address this problem, this paper investigates two separate but complimentary techniques. Principally, it proposes using a Bayesian changepoint detection method to automatically calculate shock arrival, potentially reducing error and simplifying the shock arrival finding process. To compliment this, a technique for filtering the raw data without losing the shock arrival time is also presented and investigated. To test the validity of the proposed techniques, tests are performed using both a theoretical step change with different levels of noise and real experimental data. It was found that with conditions added to ensure that a real shock arrival time was found, the Bayesian changepoint analysis method was able to automatically find the shock arrival time, even for noisy signals.

Soft Computing Based Force Recovery Technique for Hypersonic Shock Tunnel Tests

A hemispherical model equipped with a three component accelerometer force balance has been tested in a shock tunnel at Mach 8.0 freestream conditions. A novel technique has been devised using the Artificial Neuro-Fuzzy Inference System (ANFIS) for recovering the forces experienced by the model during the experiments. Implementation of this methodology in calibration of the force balance showed encouraging agreement with the impulse forces recovered from the calibration tests. The same recovery procedure is then adopted to obtain the time history of the forces for 0[Formula: see text] and 15[Formula: see text] angle of attack experiments. The drag recovered in steady state is found to agree well with the conventional methods with minor discrimination for the lift and pitching moment. In light of the limitation of the accelerometer force balance theory due to the unaccountability of model dynamics, the force recovery technique proposed herein is found simple to implement and can be opted as a tool for prediction of the aerodynamic coefficients and force time histories.

Heat flux characteristics within and outside a forward facing cavity in a hypersonic flow

A forward facing cavity effect on heat flux variations inside the cavity and over the nose surface is studied over a typical projectile geometry using the shock tunnel experiments. The flow oscillation will tend to oscillate the shock, which in turn varies the heat flux over the nose surface as well as inside the cavity. The higher length to diameter (L/D) ratio of the cavity plays an important role in nose surface heat flux reduction and investigated for L/D = 1, 2 and 3 by a uniquely designed model. The cavity flow oscillation period is determined, using the base pressure and surface heat transfer signals measured inside the cavity with reference to the pitot probe steady test time and shock oscillation period. The frequency analysis for oscillations is performed using the fast Fourier transformation. Experimental results showed that, the mean nose surface heat flux reduced by ≈4.6% to ≈7%, ≈6% to ≈8% and 3% to ≈31.4 and the mean shock standoff distance (l) is increased by ≈13%, ≈25% and ≈35% for L/D = 1, 2 and 3 configurations respectively with reference to the baseline geometry (without cavity geometry). As L/D increases, the nose surface location on which the heat flux reduction is observed moves towards the cavity lip and more surface area is covered with lower heat flux. Cavity region heat flux variations are studied by incorporating the thin film sensors over the cavity surface and it is observed that, a significant ≈32.5% to ≈36%, and ≈28% to ≈58% lower mean heat flux for L/D = 2 and 3 respectively with reference to the baseline geometry stagnation point heat flux.

Combined free-stream disturbance measurements and receptivity studies in hypersonic wind tunnels by means of a slender wedge probe and direct numerical simulation

Combined free-stream disturbance measurements and receptivity studies in hypersonic wind tunnels were conducted by means of a slender wedge probe and direct numerical simulation. The study comprises comparative tunnel noise measurements at Mach 3, 6 and 7.4 in two Ludwieg tube facilities and a shock tunnel. Surface pressure fluctuations were measured over a wide range of frequencies and test conditions including harsh test environments not accessible to measurement techniques such as Pitot probes and hot-wire anemometry. A good agreement was found between normalized Pitot pressure fluctuations converted into normalized static pressure fluctuations and the wedge probe readings. Quantitative results of the tunnel noise are provided in frequency ranges relevant for hypersonic boundary-layer transition. Complementary numerical simulations of the leading-edge receptivity to fast and slow acoustic waves were performed for the applied wedge probe at conditions corresponding to the experimental free-stream conditions. The receptivity to fast acoustic waves was found to be characterized by an early amplification of the induced fast mode. For slow acoustic waves an initial decay was found close to the leading edge. At all Mach numbers, and for all considered frequencies, the leading-edge receptivity to fast acoustic waves was found to be higher than the receptivity to slow acoustic waves. Further, the effect of inclination angles of the acoustic wave with respect to the flow direction was investigated. An inclination angle was found to increase the response on the wave-facing surface of the probe and decrease the response on the opposite surface for fast acoustic waves. A frequency-dependent response was found for slow acoustic waves. The combined numerical and experimental approach in the present study confirmed the previous suggestion that the slow acoustic wave is the dominant acoustic mode in noisy hypersonic wind tunnels.

Performance of high mach number scramjets - Tunnel vs flight

While typically analysed through ground-based impulse facilities, scramjets experience significant heating loads in flight, raising engine wall temperatures and the fuel used to cool them beyond standard laboratory conditions. Hence, the present work numerically compares an access-to-space scramjet's performance at both these conditions. The Mach 12 Rectangular-to-Elliptical Shape-Transitioning scramjet flow path is examined via three-dimensional and chemically reacting Reynolds-averaged Navier-Stokes solutions. Flight operation is modelled through 800 K and 1800 K inlet and combustor walls respectively, while fuel is injected at both inlet- and combustor-based stations at 1000 K stagnation temperature. Room temperature walls and fuel plena model shock tunnel conditions. Mixing and combustion performance indicates that while flight conditions promote rapid mixing, high combustor temperatures inhibit the completion of reaction pathways, with reactant dissociation reducing chemical heat release by 16%. However, the heated walls in flight ensured 28% less energy was absorbed by the walls. While inlet fuel injection promotes robust burning of combustor-injected fuel, premature ignition upon the inlet in flight suggests these injectors should be moved further downstream. Coupled with counteracting differences in heat release and loss to the walls, the optimal engine design for flight may differ considerably from that which gives the best performance in the tunnel.

LIGS measurements in the nozzle reservoir of a free-piston shock tunnel

Free-piston shock tunnels are ground-based test facilities allowing the simulation of reentry flow conditions in a simple and cost-efficient way. For a better understanding of the processes occurring in a shock tunnel as well as for an optimal comparability of experimental data gained in shock tunnels to numerical simulations, it is highly desirable to have the best possible characterization of the generated test gas flows. This paper describes the final step of the development of a laser-induced grating spectroscopy (LIGS) system capable of measuring the temperature in the nozzle reservoir of a free-piston shock tunnel during tests: the successful adaptation of the measurement system to the shock tunnel. Preliminary measurements were taken with a high-speed camera and a LED lamp in order to investigate the optical transmissibility of the measurement volume during tests. The results helped to successfully measure LIGS signals in shock tube mode and shock tunnel mode in dry air seeded with NO. For the shock tube mode, six successful measurements for a shock Mach number of about 2.35 were taken in total, two of them behind the incoming shock (p $$\approx $$ 1 MPa, T $$\approx $$ 600 K) and four after the passing of the reflected shock (p $$\approx $$ 4 MPa, T $$\approx $$ 1000 K). For five of the six measurements, the derived temperatures were within a deviation range of $$6\%$$ to a reference value calculated from measured shock speed. The uncertainty estimated was less than or equal to $$3.5\%$$ for all six measurements. Two LIGS signals from measurements behind the reflected shock in shock tunnel mode were analyzed in detail. One of the signals allowed an unambiguous derivation of the temperature under the conditions of a shock with Mach 2.7 (p $$\approx $$ 5 MPa, T $$\approx $$ 1200 K, deviation $$0.5\%$$ , uncertainty $$4.9\%$$ ).

Effect of cowl shock on restart characteristics of simple ramp type hypersonic inlets with thin boundary layers

The effect of cowl angle on the restart characteristics of simple ramp type hypersonic inlets was experimentally investigated in shock tunnel equipped with schlieren imagery and static pressure measurement. The cowl shock strength is found to be a key factor that determines the inlet restart and makes the restart contraction ratios significantly deviate from the Kantrowitz criterion. Stronger cowl shock tends to degrade the inlet restart capability by causing larger separation bubble and higher pressure loss during the restarting process. In particular, a sensitive range of the cowl angles, within which the restart contraction ratio decreases rapidly, was identified. A design concept of multiple noncoalesced cowl shocks was thus proposed and proven to significantly improve the inlet restart capability.

SHOCK WAVE INTERACTION NEAR A CYLINDER ALIGNED NORMAL TO A BLUNTED PLATE—PART I: GAS FLOW AND HEAT TRANSFER ON A PLATE NEAR A CYLINDER

The flow around a cylinder mounted on a sharp or blunted plate is experimentally studied. The experiments are performed in a shock tunnel at Mach number M∞ = 5 for a wide range of Reynolds numbers ReL (based on the plate length): from 0.6 to 3.4 × 107. The varied parameters are the distance between the leading edge of the plate and the cylinder X0 and the bluntness radius of the leading edge of the plate. A panoramic method is used to investigate heat transfer. The work consists of two parts. Part I describes the results on the flow structure and heat transfer on the plate surface ahead of the cylinder and in its vicinity. It is shown that the heat transfer coefficient near the cylinder is significantly greater than that on the plate in the undisturbed region and is close in terms of the order of magnitude to the heat transfer coefficient on the frontal surface of the cylinder in the undisturbed flow. An increase in the bluntness radius of the plate to a certain level considerably decreases the maximum Stanton number ahead of the cylinder. At the transitional and turbulent states of the undisturbed boundary layer on the plate ahead of the cylinder, the change in the Reynolds number in the examined range has a minor effect on heat transfer enhancement near the cylinder on both the sharp and blunted plates. Investigations of the state of the boundary layer on the plate, which is not disturbed by the cylinder, confirm the existence of a reverse laminar–turbulent transition, which occurs when the bluntness radius increases. It is shown that the laminar–turbulent transition and its reverse transition lead to nonmonotonic changes in the peak Stanton number on the plate as a function of the bluntness radius of the leading edge and the distance between the leading edge and the cylinder.

Nose-Tip Transition Control by Surface Roughness on a Hypersonic Sphere

The surface heat flux on a 100 mm diameter hypersonic sphere was reduced through surface roughness on its forebody. The test model was subjected to a hypersonic freestream of Mach 8.8 and Reynolds number 1.98 million/m, in a shock tunnel. Forebody surface heat transfer rates measured on smooth and rough spheres, under the same free-stream conditions, were compared. The comparison of heat flux indicated an overall reduction in surface heating rates on the rough model, which could be attributed to the delayed nose tip transition. The surface roughness on the forebody of the model generated miniature cavities. Stability of the free shear layer over the miniature cavities and entrapment of the destabilizing vortices in the cavities, make the flow over the rough test model more stable than the attached boundary layer over the smooth model, under transitional conditions.

Prediction of Shock-Standoff Distance and Entropy Distribution for Forward-Facing Cavity

Experiments are performed on a cylinder with a forward-facing cavity at M ∞ = 10 in the FD-14A shock tunnel. The shock-standoff distance and oscillation characteristics are recorded by a high-speed movie, and the dynamic pressure transducer is used to capture the unsteady signal of cavity base. Based on experimental and numerical results, a prediction method for estimating the shock-standoff distance is proposed. Results of shock-standoff distance and oscillation frequency are obtained for experiments in the shock tunnel. The predicted oscillation frequency is in accordance with experimental results. Furthermore, the relation of shock shape and the entropy increase are combined to obtain the characteristics of entropy distribution. As the shock-shape of flat-nosed cylinders is more liable to be influenced than blunt-nosed cylinders with increasing Mach number, the location of the extreme value moves to the surface as the Mach number increases for flat-nosed cylinders, while it remains the identical location for blunt-nosed cylinders.

Effects of trips on the oscillatory flow of an axisymmetric hypersonic inlet with downstream throttle

Experimental investigations are conducted on an axisymmetric hypersonic inlet to evaluate the effects of trips on oscillatory flows. The model exit is throttled with a fixed block to generate oscillatory flows at a freestream Mach number of 6 in a conventional wind tunnel and a shock tunnel. Schlieren imaging and pressure measurements are adopted to record unsteady flow features. Results indicate that trips with a 1 mm thickness prominently suppress external separations, shorten oscillatory cycles, and modify pressure magnitudes. Trips can reduce the upstream movement ranges of separated shocks from nose regions to locations axially 142 mm downstream. The oscillatory cycles are shortened from 3.75 ms to 3.25 ms and from 4 ms to 3.13 ms in two facilities. Tripped cases generally exhibit higher pressure magnitudes than those of untripped cases, of which the increment is up to 21 times the freestream static pressure for the farthest downstream transducer in the shock tunnel. The effects of trips are related to the streamwise vortexes in wake flows, in which interactions between external separations modify the separated flow patterns and enhance the sustainment of the forebody boundary layers to backpressure. Flow processes causing increments of oscillatory frequencies and pressure magnitudes are analyzed, while the flow mechanisms dominating the processes still need to be clarified in the future.

Numerical investigation of the axial impulse load during the startup in the shock tunnel

A shock tunnel is a representative type of ground-based equipment in the hypersonic field, which has the advantage of free stream of excellent quality. Accurate measurements of aerodynamic forces in the shock tunnel are great challenges. The impulse loads during the start-up process induce the vibration of the model and its support. Aerodynamic signals are disturbed by vibrational signals. The identification of the starting time of the steady periodic vibration and excited natural frequencies of the whole structure would contribute to improve the accuracy of measurement. These two factors are closely related to the properties of the impulse forces. However, there is currently no published research on the impulse forces. In this paper, the type of impulse forces (drag history) acting on the sharp cone during the start-up process was investigated by numerical simulation. The distribution of the static pressure of the typical wave structures was found to have a significant influence on the types of the drag history. A formula, based on the physical analysis of start-up process, was put forward to estimate the starting time of the steady stage. Additionally, the subsequent analysis and design optimization such as vibration of structure and disturbing frequencies needed an analytical and simple formation of drag history. Thus, the drag history was approximated by numbers of sine functions. Different phases exhibited notable difference in composition. Also, a metric denoted as the energy coefficient was derived to identify the critical frequencies and simplify the analytical expression of the impulse force.

Hypersonic shock tunnel studies of Edney Type III and IV shock interactions

Of all the possible outcomes of the shock interaction problem, Edney Type-III and Type-IV are considered to be of great importance as they lead to high heat transfer rates on the surface in the vicinity of the interaction. The enhancement in heat transfer occurs because of shear layer attachment in Type-III interaction and impingement of supersonic jet in Type-IV interaction. In this study, unsteady nature of these interactions is studied in conventional shock tunnel at moderate enthalpy condition of 1.07 Mj/kg at flow Mach number of 5.62. A hemispherical model, 50 mm in diameter, mounted with thin film Platinum gauges is used along with a 25° wedge to serve as a shock generator. The schlieren images are captured along with the surface convective heat transfer rate measurements for the analysis of flowfield over the model during the test time. The pixel intensity scan is performed along several lines running horizontally to estimate the steadiness of the flowfield. These are correlated with the heat transfer rate measurements to understand the non-steady nature of the interaction during the small test times offered by the shock tunnel.

Force and Moment Measurements on a Free-Flying Capsule in a Shock Tunnel

Experiments to measure the forces and moments acting on a blunted-cone capsule model are conducted in the High Enthalpy Shock Tunnel Göttingen. A free-flying arrangement is employed, whereby the model is initially suspended by weak threads that are detached by the arrival of the flow, allowing unrestrained flight during the steady test time. High-speed shadowgraphy is used to capture the model motion. Induced forces and moments are measured using both internally mounted accelerometers and visualization-based tracking techniques. Improvements to the visualization-based techniques allow the elimination of several previously identified sources of error. Drag, lift, and pitching-moment coefficients are derived over a range of angles of attack up to 6 deg, and excellent agreement is obtained between the techniques. Results are also compared with those from numerical simulations, with agreement found to lie within the experimental error for the drag and pitching coefficients but slightly outside for the lift coefficients. The precision of the visualization-based results is such that uncertainty in the freestream conditions is the dominant source of experimental error in the drag coefficient, and it becomes comparable to or larger than the standard error in the lift and pitching accelerations at the maximum angle of attack tested.

Temperature characterization of a radiating gas layer using digital-single-lens-reflex-camera-based two-color ratio pyrometry.

The two-color ratio pyrometry technique using a digital single-lens reflex camera has been used to measure the time-averaged and path-integrated temperature distribution in the radiating shock layer in a high-enthalpy flow. A 70mm diameter cylindrical body with a 70mm long spike was placed in a hypersonic shock tunnel, and the region behind the shock layer was investigated. The systematic error due to contributions from line emissions was corrected by monitoring the emission spectrum from this region using a spectrometer. The relative contributions due to line emissions on R, G, and B channels of the camera were 7.4%, 2.2%, and 0.4%, respectively. The temperature contours obtained clearly distinguished regions of highest temperature. The maximum absolute temperature obtained in the experiment was ∼2920 K±55 K, which was 20% lower than the stagnation temperature. This lower value is expected due to line-of-sight integration, time averaging, and losses in the flow. Strategies to overcome these limitations are also suggested in the paper.

Overview of Flow Diagnosis in a Shock Tunnel

Overview of Flow Diagnosis in a Shock Tunnel

Aerodynamic measurement of a large aircraft model in hypersonic flow**Project supported by the National Natural Science Foundation of China (Grant Nos. 11472281 and 11532014).

Accurate aerodynamic measurements in the hypersonic flow of large aircraft models in tunnels have practical significance, but pose a significant challenge. Novel aerodynamic force measurement methods have been proposed,but lack theoretical support. The forms of the force signals techniques for signal processing and calculation of aerodynamics are especially problematic. A theoretical study is conducted to investigate the dynamic properties based on models of the draw-rod system and slender rods. The results indicate that the inertia item can be neglected in the rod governing equation; further, the solutions show that the signals of each rod are a combination of aerodynamic signals (with a constant value) and sine signals, which can be verified by experimental shock tunnel results. Signal processing and aerodynamics calculation techniques are also found to be achievable via the flat part of the signals.

Experimental investigation of Görtler vortices in hypersonic ramp flows

A sharp leading-edge ramp model with 15°, 20°, and 25° ramp angles is experimentally investigated at a freestream Mach number 7.7 and a unit Reynolds number 4.2 × 106 m−1 in the Aachen Shock Tunnel TH2. The objective of this paper is to analyze Gortler vortices in terms of shear layer length, flow curvature, and vortex related parameters. The spanwise heat flux variation caused by it and the streamwise heat flux enhancement during its tenure are also studied. Thermocouples, pressure sensors, schlieren imagery, and infrared imaging are used for the investigation. The boundary layer on the flat plate until separation is laminar. An important outcome is that the arc length at reattachment is constant irrespective of the ramp angle. Characteristic boundary layer thicknesses at different Mach number show that the momentum thickness is insensitive to compressibility and is used to determine the Gortler number. A model is presented to determine the Gortler number in terms of ramp angle and a constant based on separation angle, arc length, momentum thickness, and Reynolds number. The half of the vortex wavelength is equal to the boundary layer thickness just before reattachment. The length scale required for breakdown of Gortler vortices decreases with rising ramp angle and is analogous to peak heating length. The streamwise heat flux enhancement occurs during the tenure of Gortler vortices and the enhancement rises with ramp angle. Although the visibility of Gortler vortices through temperature variation is distinct, the spanwise heat flux variation is not too high. Moreover, the spanwise heat flux variation rises marginally with ramp angle.