Patrick Rambaud

BLEVE overpressure: Multiscale comparison of blast wave modeling

BLEVE overpressure modeling has been already widely studied but only few validations including the scale effect have been made. After a short overview of the main models available in literature, a comparison is done with different scales of measurements, taken from previous studies or coming from experiments performed in the frame of this research project. A discussion on the best model to use in different cases is finally proposed.

Prediction of Incoming Turbulent Noise Using a Combined Numerical / Semi-Empirical Method and Experimental Validation

The present paper investigates the case of a NACA0012 airfoil placed in a turbulent jet. The nozzle outlet diameter is equal to D = 0.041 m. The airfoil is placed at zero angle of incidence and with its leading edge located at 6D from the jet outlet. The chord of the airfoil is equal to D, and has a constant section over its span of 10D. The outlet velocity magnitude U0 is fixed to 13.2 m/s resulting in a Reynolds number based on the chord length of 36,000 and a Mach number of 0.04. The unsteady, three-dimensional incompressible flow around the airfoil is first computed with the LES module of the commercial solver FLUENT Rev. 6.2. The numerical flow results are compared with statistics on the velocity field (mean, RMS and spectra) obtained experimentally with hot wire anemometry on the same geometry and for the same operating conditions. This comparison reveals, as expected, that the mesh refinement influences the cut-off frequency resolution. Besides, an innovative procedure is proposed to compute from the same CFD computation, the noise radiated for the all frequency spectrum. The SYSNOISE Rev.5.6 solver, integrating Curle’s analogy, is used to predict the low frequency part of the noise spectrum, while Amiet’s theory is used to predict the high frequency range. The first one, limited to the computation of sound radiated by compact sources and then to the low frequency range, uses the unsteady pressure fluctuations on the airfoil stored during the CFD flow computation. The second one is a theory specially developed for airfoil sound radiation at high frequency and taking then into account, in an explicit way, non-compactness effects. The statistical flow data, needed by Amiet’s model, are fitted on the CFD data.

Numerical issues in the application of an amiet model for spanwise-varying incoming turbulence

The present paper investigates the possible application of Amiet’s theory in spanwise varying conditions. This study encompasses two different aspects. Firstly, all the parameters of the turbulent flow upstream of the airfoil have to be known to rescale the theoretical turbulence spectrum used in the turbulence interaction theory. In that framework, a methodology to compute the turbulence length scale upstream of the airfoil has been developed through the use of Fourier transforms of the upwash velocity of the incoming flow. Secondly, Amiet’s theory has been developed for airfoil noise radiation in uniform flow. In case of spanwise varying conditions, a Segmentation Method is proposed to predict the radiated noise. This method consists in cutting the airfoil in segments having each its own upstream flow conditions and to sum up the resulting individual emitted noise to obtain the total radiated noise in the far-field. This direct method reveals that spurious effect due to radiation angle and finite size of segments can appear. A rescaling based on listener position and a new Inverse Segmentation Method using a combination of large span airfoils are proposed to solve both problems. This method has been tested for several flow parameters and has shown its potential to reproduce correctly the radiated noise. The methods proposed in this paper have been combined and used on the jet-airfoil interaction case, for which acoustic prediction using Amiet’s theory has shown a satisfactory agreement compared to experimental noise measurements. Noise pollution is encountered in various industrial applications : noise emitted by landing gear in transport industry; noise from wings and high-lift devices in aeronautics; noise due to windshield wipers, rear-view mirrors, engine cooling systems and mufflers in car industry. It also exists in other domains of applications, as wind turbines or fans. In most of the cases, the noise produced is spread among all the frequency spectrum leading sometimes to a significant participation of high frequencies to the overall sound produced. The definition itself of the limit between low and high frequencies is not straightforward but a common rule is to consider the non-compactness limit of the problem. In such situations of non-compactness, the influence of the higher frequencies is an important factor to take into account in the noise predictions and specific methods for these frequencies have to be developed. Among the methods available and widely used to predict the low frequency components are the hybrid methods (indirect methods) in which the computation of the flow is decoupled from the computation of the sound. The computational cost to obtain the acoustic field is highly reduced compared to direct methods, computing the flow and its sound field together, when high Reynolds number and low Mach number are considered, as in most of industrial applications of interest. These hybrid methods consist in two steps : a) firstly, near the noise source, the flow field is obtained from an unsteady computation ; b) secondly, the acoustic source radiation is computed in the far-field by the use of an analogy. 1–3 This methodology is based on the substitution of the real flow by equivalent sources, computed as a post-processing of the flow data.

Experimental Characterization of Hypersonic Nozzle Boundary Layers and Free-Stream Noise Levels

Pitot pressure surveys of the Longshot hypersonic contoured nozzle in the von Karman Institute are presented. Measurements use a rake of pitot probes at four locations along the nozzle axis from half the nozzle length down to the exit. Laminar and turbulent numerical simulations are used for comparison. A good agreement is obtained with the turbulent computations conrming the existence of turbulent boundary layers along the nozzle walls. The uniformity of the free-stream parallel ow is characterized by a standard deviation of 3.5%. The free-stream noise levels determined from pitot pressure uctuations are about 7.5% and slightly lower inside the nozzle. These are comparable with the levels recorded in other similar hypersonic wind tunnels. Some ow divergences are shown to be possibly correlated to a discontinuity in the contour of the nozzle. The test time available at various locations is also determined and is shown to depend upon the location considered along and across the nozzle.

Assessment of Slag Accumulation in Solid Rocket Boosters: Part II, Two-Phase Flow Experiments

In solid propellant rocket motors incorporating a submerged nozzle, the entrapment of liquid residues of the combustion in the cavity formed in the surrounding of the nozzle integration part can lead to the accumulation of slag with a considerable mass. The long-term goal of the present study is to characterise experimentally the driving parameters of the slag accumulation in a stagnant area modelling the nozzle cavity. The experimental database will be used for validation of a numerical tool. In the present article measurements are shown in two-phase flow condition using a coldgas simplified model to determine effectively the main parameters that are influencing the droplet entrapment. Furthermore, the deformation of the accumulated liquid surface is analysed and presented as well.

Experimental and Numerical Multiphase-Front Fluid Hammer

The aim of this work is to study the fluid hammer and related multiphase phenomena taking place in a confined environment, where pressurized liquid faces vacuum conditions after opening an isolation valve. This paper describes the experimental facility built for this purpose, the tests performed, and shows the comparison with the numerical results obtained with a transient one-dimensional software based upon conservation equations. Because the multiphase phenomena taking place in the liquid front moving along the pipe are complex and difficult to interpret, a graphical tool by means of X-t diagrams was adapted by the authors to characterize the physical phenomena involved. Contrary to the classical water hammer analysis published in the literature so far, the X-t diagrams allow a simultaneous study in time and space for each of the magnitudes involved in the evolution of the fluid hammer.

Multiphase Investigation of Water Hammer Phenomenon Using the Full Cavitation Model

The aim of this work is to clarify some issues raised recently on the characterization of the water hammer, by presenting a numerical analysis on the multiphysics taking place. For this purpose, the commercial code CFD-ACE+ is used with the so-called full cavitation model, which takes into account the effect of absorption and desorption of a noncondensable gas. A comparison between two different compressibility models is conducted, making use of an inverse engineering technique, which allows the tracking of the medium speed of sound. Finally, a numerical reproduction of a valve opening is proposed, leading to a sudden acceleration of the fluid and consequent vaporization of the liquid. The numerical results confirm high sensitivity of the water hammer prediction regarding the amount of noncondensable gas present in the liquid.

Assessment of Slag Accumulation in Solid Rocket Boosters: Summary of the VKI research

In solid propellant rocket motors incorporating a submerged nozzle, the entrapment of liquid residues of the combustion in the cavity formed in the surrounding of the nozzle integration part can lead to the accumulation of slag with a considerable mass. The goal of the present study is to characterize experimentally the driving parameters of the slag accumulation in a stagnant area modelling the nozzle cavity. Furthermore, the experimental database is used for validating a numerical tool as well. In the present article a summary of the VKI (von Karman Institute) research carried out during the last four years is shown. Single-phase and two-phase measurements are performed using a cold-gas simplified model. Simultaneously, the experimental configuration is simulated numerically with the help of a commercial solver (CFD-ACE+). The final aim of the article is to point out the driving forces of the droplet entrapment process.

Hypersonic Boundary Layer Transition on a 7 Degree Half-Angle Cone at Mach 10

Hypersonic boundary layer transition experiments are performed in the low-enthalpy Longshot wind tunnel with a free-stream Mach number ranging between 12 ≥ M∞ ≥ 9.5 and Reynolds number between 12× 10 /m ≥ Reunit,∞ ≥ 3.3× 10 /m. The model is an 800 mm long 7 ◦ half-angle cone with nosetip radii between 0.2 and 10 mm. Instrumentation includes flushmounted fast-response thermocouples and pressure sensors. Boundary layer transition onset location is determined from the wall heat flux distribution. Nose bluntness has a strong stabilizing effect. No transition reversal could be observed at RB = 10RN for a Reynolds number based on the nosetip radius of ReRN,∞ = 123, 000. Increasing freestream unit Reynolds number results in larger RexB,e. Wavelet analysis of the boundary layer fluctuations shows that numerous wave packets are present during the transition process. Comparison with Linear Stability Theory results for second mode waves shows an excellent agreement for the most amplified frequencies. The N-factor of the wind-tunnel is 5 based on these computations and on the transition location measured experimentally. The convection velocity of the disturbances is closely approximated by the local boundary layer edge velocity for all conditions investigated. Schlieren flow visualization of the instabilities exhibits the typical rope shape of second mode disturbances for the sharpest nosetips. For nose bluntness larger than 4.75 mm, disturbances are mainly present at the edge of the boundary layer and within the inviscid shock layer. Their shape no longer presents the second mode typical structure although a frequency analysis of the disturbances is still compatible with second mode instabilities. Present results confirm the dominance of second mode waves in the transition process along a conical geometry for Mach numbers larger than 10.

Design of static pressure probes for improved free-stream characterization in hypersonic wind tunnels

Slender fast-response static pressure probes are tested in the Longshot hypersonic windtunnel with free-stream Mach number larger than 10 and Reynolds number 30000 . ReD . 60000 based on the probe diameter. They aim at improving the characterization of the freestream. Numerical simulations predict the wall pressure measured to be within 5% of the free-stream static pressure. The viscous effects along the probes are found experimentally to be limited for L/D > 16.5 in agreement with numerical results. Limited influence of the angle of attack of the probes is found for α < 2 ◦ thanks to the probing geometry used. The free-stream quantities at the nozzle exit (Mach, Reynolds...) are derived from the static pressure measurements and compared to those estimated assuming an isentropic nozzle flow expansion. The usefulness of static pressure probes is demonstrated through their ability to detect possible non-isentropic nozzle flow expansions which do not influence traditional measurements such as the test section stagnation pressure. This direct access to free-stream quantities allows to significantly improve the accuracy on the free-stream quantities and benefits to the characterization of hypersonic wind-tunnel flowfields.