Author

# Julien Weiss

Other affiliations: Bombardier Aerospace, University of Stuttgart, Université du Québec ...read more

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.

Topics: Wind tunnel, Turbulence, Boundary layer, Jet (fluid), Anemometer

##### Papers published on a yearly basis

##### Papers

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TL;DR: In this paper, the unsteady behavior 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.

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.

70 citations

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17 Jun 201948 citations

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TL;DR: In this paper, the authors measured wall static pressure and longitudinal velocity fluctuations in a pressure-induced turbulent separation bubble generated on a flat test surface by a combination of adverse and favorable pressure gradients.

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...

33 citations

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TL;DR: In this paper, the experimental and computational investigation of an isolated fuel droplet evaporation conducted in wind tunnel by suspending the droplet using supports of different sizes and materials is presented.

Abstract: Fuel droplet vaporization process involving heat and mass transfer holds key interest due to its application in wide range of energy systems. This manuscript presents the experimental and computational investigation of an isolated fuel droplet evaporation conducted in wind tunnel by suspending the droplet using supports of different sizes and materials. Different sizes of initial droplet diameter 1565–2775 μm, ambient temperature 303–403 K and varying ambient air velocity 0.4–2.7 m/s allowed the investigated Reynolds number to be varying between 30 and 275. K-type thermocouples ( d f = 76–812 μm) and glass fibers ( d f = 200–800 μm) are used for fuel droplet suspension. Use of thermocouples allowed acquiring the temporal variation of droplet temperature. Both experimental and computational investigations were carried out to quantify the heat conduction to fuel droplet through droplet support and its effects on the droplet evaporation rates. Gradients of droplet evaporation rates are found to be changing for very small support sizes while extrapolated to obtain values for pure convection cases. MATLAB code based on mathematical model is developed to see the outcome of varying support size, support material, ambient temperature, ambient velocity and droplet size on the droplet evaporation process. The average over estimation of mean droplet evaporation rates in the absence of heat conduction for linear extrapolation are found to be 30% and 8% for thermocouples and glass fibers respectively at U ∞ = 1.4 m/s and varying ambient temperatures while using 0.008 ≤ d f 2 / d o 2 ≤ 0.035 for linear extrapolation.

27 citations

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TL;DR: In this paper, 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, 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.

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.

23 citations

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TL;DR: There is, I think, something ethereal about i —the square root of minus one, which seems an odd beast at that time—an intruder hovering on the edge of reality.

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 …

33,785 citations

01 Jan 2016

TL;DR: random data analysis and measurement procedures is available in the authors' digital library an online access to it is set as public so you can get it instantly.

Abstract: random data analysis and measurement procedures is available in our digital library an online access to it is set as public so you can get it instantly. Our book servers spans in multiple countries, allowing you to get the most less latency time to download any of our books like this one. Merely said, the random data analysis and measurement procedures is universally compatible with any devices to read.

592 citations

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TL;DR: In this paper, a direct numerical simulation of the shock wave and turbulent boundary layer interaction for a 24 deg compression ramp configuration at Mach 2.9 and Re θ 2300 is performed.

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.

359 citations

01 Jan 2002

296 citations

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TL;DR: The most recent developments in the modelling of heating and evaporation of fuel droplets, the results of which were published in 2014-2017, are reviewed, and the most important unsolved problems are identified.

Abstract: The most recent developments in the modelling of heating and evaporation of fuel droplets, the results of which were published in 2014–2017, are reviewed, and the most important unsolved problems are identified. Basic principles of power law and polynomial approximations and the heat balance method for modelling the heating of non-evaporating droplets are discussed. Several approaches to modelling the heating of evaporating droplets, predicting different heating and evaporation characteristics, are compared. New results in modelling heating and evaporation of spheroidal droplets are identified. Basic principles of the Discrete Component Model and its application to biodiesel fuel droplets are summarised. Main ideas of the Multi-dimensional Quasi-discrete Model and its applications to Diesel and gasoline fuel droplets are discussed. New developments in gas phase evaporation models for multi-component fuel droplets are presented. A self-consistent kinetic model for droplet heating and evaporation is described. New approaches to the estimation of the evaporation coefficient, including those taking into account quantum-chemical effects, are summarised. Among unsolved problems, the effects of non-spherical droplets, limitations of the ETC/ED model, effects of the interaction between droplets, effects of the moving interface due to evaporation, modelling of complex multi-component droplets, modelling of droplet heating and evaporation in near- and super-critical conditions, development of advanced kinetic and molecular dynamics models and effective approximation of the kinetic effects are discussed.

267 citations