The VKI Longshot gun tunnel is a reference short-duration hypersonic facility designed for aerodynamic and aerothermal studies at very high Mach numbers.

This wind tunnel operates with different test gases to simulate Earth and Martian atmospheric (re-)entries in a near perfect gas environment, making it ideal for aerothermodynamic investigations in the continuum regime. This includes the comprehensive aerodynamic and aerothermal characterization of orbital vehicles and space debris during atmospheric re-entry, as well as the investigations of fundamental phenomena like laminar-to-turbulent boundary layer transition, shock-boundary layer interactions, and viscous interaction effects. The facility is also a key resource for validating numerical codes due to its advanced flow characterization techniques.

Flow Environment

The Longshot's unique capabilities stem from its inertial piston, which compresses test gases to pressures up to 400 MPa while maintaining moderate temperatures around 2500 K, enabling extreme Reynolds numbers. The test gas can be expanded through one of the following nozzles:

• Mach 12 contoured nozzle (exit diameter: 426 mm)
• Mach 14 contoured nozzle (exit diameter: 541 mm)
• Mach 10 to 20 conical nozzle (exit diameter: 356 to 600 mm)

Reynolds numbers based on body length can reach up to 10^7, with useful test times ranging from 20 to 80 ms.

Instrumentation Techniques

Test articles are mounted in a 15 m^3 open-jet test section using a high-precision 6-axis positioning mechanism for various pitch, roll, and yaw configurations. Instrumentation techniques includes:

• 6-components balances for force/moment measurements (acceleration compensated),
• thin-films and coaxial thermocouples for fast-response temperature/heat transfer measurements,
• piezoresistive/piezoelectric pressure sensors for ultra-fast measurements (up to 1MHz),
• flow visualization techniques (Shadowgraph/Schlieren/LIF-based Schlieren) coupled with high-speed cameras,
• 6 degrees of freedom free-flight setups for the aerodynamic characterization of single/multiple bodies.

The data acquisition system features high-precision amplifiers and allows simultaneous sampling of over 80 channels at 500 kHz, plus 16 channels at 2500 kHz. Following a major renovation in 2017, the wind tunnel has been automated, significantly enhancing the repeatability and efficiency of the test campaigns.

Project Highlights

• Space debris: “Aerodynamic characterization of space debris and determination of the associated total heat load in order to validate numerical simulations”
• Intermediate eXperimental Vehicle (IXV): “Experimental study on the efficiency of control surfaces, generation of an aerothermodynamic database”
• Atmospheric Reentry Demonstrator (ARD): “Wind tunnel heat flux and pressure measurements on the heat shield under laminar and turbulent boundary layer conditions”
• Mars Sample Return Orbiter (MSRO): “Heat flux measurements to support the development of the thermal protection system”
• Hermes Spaceplane: “Generation of an aerothermodynamic database, investigation of control surfaces efficiency, and fundamental investigation of shock boundary layer interactions over compression corners”

Schlieren flow visualization on the IXV vehicle

LIF-based Schlieren flow visualization on the hypersonic laminar-to-turbulent boundary layer transition phenomenon.

Schlieren flow visualization on the space debris vehicles

Contact

Pr. Guillaume Grossir
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Phone: +32 2 359 96 36

 

Schematic of the VKI H-3 blowdown hypersonic tunnel equipped with interchangeable contoured nozzles enabling Mach 5 and Mach 6 operation

The VKI H-3 wind tunnel stands out as one of the few European hypersonic facilities enabling aerodynamic and aerothermal characterization of launchers, cruise vehicles, re-entry vehicles and space-debris.

Equipped with interchangeable axisymmetric contoured nozzles, the VKI H-3 blowdown wind tunnel produces exceptionally uniform (>99.7%) Mach 5 and Mach 6 free jets with core flow diameters exceeding 120 mm. The facility utilizes dry air supplied from a pebble-bed heater, capable of delivering stagnation pressures between 5 and 35 bar and achieving a maximum stagnation temperature of 550 K. This setup allows for a versatile range of free-stream unit Reynolds numbers, spanning from 3x10^6 to 35x10^6 per meter, within a finely characterized environment.

The test section is equipped with a high-accuracy five-degree-of freedom orientation mechanism and a rapid injection mechanism that enables model injection into the hypersonic stream in less than 0.1s. An efficient supersonic ejector further enhances the facility's capabilities, enabling extended test durations exceeding 30 seconds and supporting a high frequency of daily test runs.

Modern data acquisition is handled by a sophisticated National Instrument system, which supports a variety of instrumentation techniques. These include localized measurements using thermocouples and pressure sensors, as well as global measurement techniques such as oil flow, sublimation, Infra-Red thermography, and Temperature-Sensitive Paint (TSP). Aerodynamic data are typically gathered using three-component aerodynamic strain gauge balances. The tunnel also boasts high-quality shadowgraph and schlieren optical systems, along with advanced optical and non-intrusive measurement techniques like Laser-Induced Fluorescence (LIF), Planar Laser-Induced Fluorescence (PLIF), and Interferometric Laser Imaging for Droplet Sizing (ILIDS) for specialized requirements.

The main scientific contributions include the generation of comprehensive aerodynamic and aerothermal databases, validation of numerical simulation tools, and fundamental research into critical hypersonic phenomena such as natural and roughness-induced boundary layer laminar-to-turbulent transition, shock wave boundary layer interactions, liquid fragmentation in crossflow, and ablation processes...

Schlieren flow visualization in the VKI H-3 tunnel

Pressure rake used for the VKI H-3 nozzle flow uniformity characterization

The VKI S-4 tunnel is a blowdown wind tunnel operating at high stagnation pressure equipped with a planar Mach 3.5 contoured nozzle. It has a moderate-sized test section with a cross-sectional area of 80x100 mm^2. The tunnel's simplicity and cost-effectiveness make it ideal for academic purposes and both fundamental and applied research.

Equipped with a fine optical system, the VKI S-4 tunnel supports Shadowgraph, coloured-Schlieren and Background-Oriented Schlieren measurement techniques. Aerodynamic balances and a wide variety of other measurement techniques may be employed. Depending on the stagnation pressure conditions, the tunnel can achieve run times ranging from 8 to 25 minutes, during which the model incidence can be varied continuously.

The S-1/C wind tunnel is world-class turbine rig for testing large-scale, transonic, low-Reynolds number linear cascades.

This test bench is a continuous closed-circuit facility driven by a 615 kW axial flow compressor. A water/air cooler allows controlling the flow temperature at near atmospheric condition and dry air is maintained at all conditions. The mass flow is regulated by regulating the compressor rotational speed and via a by-pass valve. A vacuum pump allows lowering the tunnel absolute pressure to ~ 8,000 Pascal. This wind tunnel was originally used for external aerodynamics applications such as the study of laminar shock boundary layer interactions and various supersonic flow configurations installed in a converging/diverging test section. It was then converted into a high-speed, low-Reynolds number turbomachinery cascade rig by removing the converging/diverging test section and replacing the downstream elbow of the circuit by a large linear cascade test section.

The test section can fit a large-scale low-pressure turbine (LPT) cascade model; the upstream channel height can be adjusted between 375 and 650 mm whereas the maximum airfoil span is 225 mm; typical chord lengths can be as high as 80 mm. This guarantees a sufficiently high aspect ratio, even for transonic exit conditions and high turning. The linear cascade ensemble is made up of several blades (at least 10) and the two large circular sidewalls (1,120 mm diameter) allow a continuous adjustment of the inlet flow angle. Upstream passive grids are used to control the background turbulence intensity.

The upstream part of the test section is equipped with a high-speed rotating bar system composed of a disc of 625 mm diameter, driven by an electric motor of 30 kW spinning up to 3,500 rpm, and mounting up to 96 cylindrical bars made of molybdenum. The number, diameter and rotational speed of the bars can be adjusted to generate engine-like rotor wakes in terms of wake frequency and velocity triangles, as opposed to low-speed systems using a linear bar displacement. The integration of both turbulence generation mechanisms allows reaching the expected turbulence and wake patterns observed in low-pressure turbines.

The facility allows investigating the aerodynamics of high-speed low-pressure turbines for direct-driven or geared turbofan engines at correctly scaled engine conditions (Mach and Reynolds numbers). A wide range of engine operating regimes can be set, from sea-level take-off to high-altitude cruising: The exit Reynolds and Mach numbers can typically be varied between 20,000 and 300,000 (depending on the blade chord length) and 0.6 to 1.2 respectively.

Recently, the S-1/C rig has been upgraded with an injection/suction cavity system integrated underneath the cascade inner endwall and with upstream rotating bars to study the combined effects of purge/leakage flows and periodic wakes on the 3D unsteady flow of a transonic LP turbine cascade.

The wind tunnel is equipped to monitor the operating conditions and several measurement techniques allow a detailed time averaged and time resolved blade wall behavior and global performance definition.

The cascade inlet and outlet flows are surveyed at multiple planes using pneumatic wall static pressure taps, multi-hole pneumatic pressure probes, thermocouple sensors as well as miniature fast-response directional probes and hot-wires. The various probes can be displaced in pitch- and span-wise directions across multiple cascade pitches by means of accurate traversing mechanisms. The central blade of the cascade and the inner endwalls are alternatively instrumented with surface pneumatic pressure taps, fast-response pressure sensors and hot-film gauges in order to measure time-resolved blade velocity and pseudo shear stress distributions. To enable fast scans of the 3D blade aerodynamics, the central instrumented blade, equipped with one stream-wise series of pressure sensors or hot films, can be displaced along the span-wise direction by means of a bespoke traversing mechanism.

Scheme Continuous High Speed Cascade Wind Tunnel S-1