Julien Weiss

Bio: Julien Weiss is an academic researcher from Technical University of Berlin. The author has contributed to research in topics: Wind tunnel & Turbulence. The author has an hindex of 11, co-authored 80 publications receiving 426 citations. Previous affiliations of Julien Weiss include Bombardier Aerospace & University of Stuttgart.

Unsteadiness in a large turbulent separation bubble

Abstract: The unsteady behaviour of a massively separated, pressure-induced turbulent separation bubble (TSB) is investigated experimentally using high-speed particle image velocimetry (PIV) and piezo-resistive pressure sensors. The TSB is generated on a flat test surface by a combination of adverse and favourable pressure gradients. The Reynolds number based on the momentum thickness of the incoming boundary layer is 5000 and the free stream velocity is . The proper orthogonal decomposition (POD) is used to separate the different unsteady modes in the flow. The first POD mode contains approximately 30 % of the total kinetic energy and is shown to describe a low-frequency contraction and expansion, called ‘breathing’, of the TSB. This breathing is responsible for a variation in TSB size of approximately 90 % of its average length. It also generates low-frequency wall-pressure fluctuations that are mainly felt upstream of the mean detachment and downstream of the mean reattachment. A medium-frequency unsteadiness, which is linked to the convection of large-scale vortices in the shear layer bounding the recirculation zone and their shedding downstream of the TSB, is also observed. When scaled with the vorticity thickness of the shear layer and the convection velocity of the structures, this medium frequency is very close to the characteristic frequency of vortices convected in turbulent mixing layers. The streamwise position of maximum vertical turbulence intensity generated by the convected structures is located downstream of the mean reattachment line and corresponds to the position of maximum wall-pressure fluctuations.

Unsteady Behavior of a Pressure-Induced Turbulent Separation Bubble

Abstract: Wall static-pressure and longitudinal-velocity fluctuations are measured in a pressure-induced turbulent separation bubble generated on a flat test surface by a combination of adverse and favorable pressure gradients. The Reynolds number, based on momentum thickness upstream of separation, is Reθ≃5000 at a free-stream velocity of Uref=25 m/s. The results indicate that the flow is characterized by two separate time-dependent phenomena: a low-frequency mode, with a Strouhal number St1≃0.01, which is related to a global “breathing” motion (i.e., contraction/expansion) of the separation bubble, and a higher-frequency mode, with a Strouhal number St2≃0.35, which is linked to the roll-up of vortical structures in the shear layer above the recirculating region and their shedding downstream of the bubble. These two phenomena are reminiscent of the “flapping” and “shedding” modes observed in fixed-separation experiments, though their normalized frequencies are different. The breathing mode is also shown to be str...

Method for the determination of frequency response and signal to noise ratio for constant-temperature hot-wire anemometers

Abstract: Determination of the frequency response of constant-temperature hot-wire anemometers is needed for measurements of high frequency turbulent fluctuations or when the bridge cannot be well adjusted because of too short a testing time. A method is proposed to determine the complete transfer function of a constant-temperature anemometer bridge in several milliseconds by means of an electrical test. The frequency response is used to perform postcorrection of the data, which enables the measurement of turbulent quantities at frequencies higher than the cut-off frequency of the system, when the bridge adjustment is not optimum. The technique, which is tested in the free stream of a supersonic wind tunnel at M=2.5, also enables a accurate estimation of the signal to noise ratio.

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

Random Data Analysis And Measurement Procedures

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Direct numerical simulation of supersonic turbulent boundary layer over a compression ramp

Abstract: A direct numerical simulation of shock wave and turbulent boundary layer interaction for a 24 deg compression ramp configuration at Mach 2.9 and Re θ 2300 is performed. A modified weighted, essentially nonoscillatory scheme is used. The direct numerical simulation results are compared with the experiments of Bookey et al. at the same flow conditions. The upstream boundary layer, the mean wall-pressure distribution, the size of the separation bubble, and the velocity profile downstream of the interaction are predicted within the experimental uncertainty. The change of the mean and fluctuating properties throughout the interaction region is studied. The low frequency motion of the shock is inferred from the wall-pressure signal and freestream mass-flux measurement.