Spectral proper orthogonal decomposition (SPOD) is used to investigate the low-frequency behavior of a pressure-induced turbulent separation bubble (TSB) generated on a one-sided diffuser at a nominal velocity of 20 m/s. The input to the decomposition is the unsteady wall shear stress determined experimentally with an array of novel calorimetric shear-stress sensors. The SPOD spectrum indicates low-rank behavior for frequencies below 10 Hz, which correspond to Strouhal numbers [ ] when built with the average TSB length and the nominal incoming velocity. A low-order model constructed with only the first SPOD mode at each frequency up to 10 Hz reveals that the TSB is contracting and expanding in a manner reminiscent of the low-frequency breathing motion recently documented in other TSBs. A comparison between SPOD and space-only proper orthogonal decomposition (POD) indicates that both decompositions are able to capture the low-frequency breathing motion as long as space-only POD is applied on signals previously low-pass filtered at 10 Hz. However, the superiority of SPOD is revealed by its inherent capacity to express the frequency range amenable to low-rank behavior. In addition to offering new insights into the low-frequency dynamics of TSBs, the present results also demonstrate the usefulness of calorimetric shear-stress sensors for the study of turbulent separating and reattaching flows.

A significant improvement of thermal accelerometer bandwidth κ demonstrated. In a classic open-loop configuration, a micromachined thermal accelerometer having a width of 1500 µm has a sensitivity of 0.045°C/g and a bandwidth up to 66 Hz limited by gas thermal response. A new sensor architecture is proposed to improve bandwidth with no sensitivity decrease: a closed-loop configuration with the addition of two resistors placed close to detectors to rebalance their temperature. They directly act on the thermal response of the fluid. Thus, sensor bandwidth is increased up to 1 kHz with a similar sensitivity of 0.034°C/g.

A new single-axis gas thermal gyroscope without proof mass is presented in this paper. The device was designed, manufactured and experimentally characterized. The obtained results were compared to numerical simulation. The working principle of the gyroscope is based on the deflection of a laminar gas flow caused by the Coriolis effect. A bidirectional hot air flow is generated by alternating activation of two suspended resistive micro-heaters. The heated gas is encapsulated in a semi-open cavity and the gas expands primarily inside the cavity. The thermal expansion gyroscope has a simple structure. Indeed, the device is composed of a micromachined cavity on which three bridges are suspended. The central bridge is electrically separated into two segments enabling to set up two heaters which may be supplied independently from each other. The two other bridges, placed symmetrically on each side of the central bridge, are equipped with temperature detectors which measure variations in gas temperature. The differential temperature depends on the rotational velocity applied to the system. Various parameters such as the heating duty cycle, the type of the gas and the power injected into the heaters have been studied to define the optimal working conditions required to obtain the highest level of sensitivity over a measurement range of around 1000°/s. The robustness of the device has also been tested and validated for a shock resistance of 10,000 g for a duration of 400 µs.

This paper presents the design, manufacture, and testing of a gas composition sensor for measuring the composition of fission gases inside experimental rods coming from an experimental reactor such as the future Jules Horowitz Nuclear Reactor (JHR). The piezoelectric element is a cerium-modified bismuth niobate titanate (NBT) ceramic screen-printed onto an alumina substrate. These types of piezoelectric elements have been shown in the past to be capable of operating in environments subject to particular thermal and radiative stresses [1]. Thus, three NBT (Na _0.5 Bi _(4.5-x) Ce _(x) Ti ₄ 0 ₁₅ ) ceramics with different dopants of elemental cerium (Ce) were fabricated, characterized and compared to the state of the art, especially with respect to piezoelectric (d ₃₃ ) and dielectric (ε _r and tan (δ)) performance. A study of the electrical impedance at different temperatures concluded that cerium-modified NBT with a Ce dopant of x=0.04 and x=0.06 was suitable for temperatures up to 340°C. These two NBT variants were then integrated into an acoustic cavity to form the sensor. The performance of these sensors was tested with two gas mixtures: pure helium and a mixture of 80% helium and 20% xenon. These laboratory sensors have shown the possibility of characterizing the composition of the gas mixtures for limited lower pressures of 40 bar for a Helium - Xenon mixture. Finally, the sensor is tested in various ternary gas mixtures containing helium, xenon and krypton, with accuracy on the composition estimation of 5%.