Measurements Of Velocity Fluctuations Research Articles

Observation of exceptionally strong near-bottom flows over the Atlantis II Seamounts in the northwest Atlantic

Knowledge of near-bottom ocean current velocities and especially their extreme values is necessary to understand geomorphology of the seafloor and composition of benthic biological communities and quantify mechanical energy dissipation by bottom drag. Direct measurements of near-bottom currents in deep ocean remain scarce because of logistical challenges. Here, we report the results of flow velocity and pressure fluctuation measurements at three sites with depths of 2573–4443 m in the area where the Gulf Stream interacts with the New England Seamounts. Repeated episodes of unexpectedly strong near-bottom currents were observed, with the current speed at 4443 m of more than 0.40 m/s. At 2573 m, current speeds exceeded 0.20 m/s approximately 5% of the time throughout the entire eight-week measurement period. The maximum flow speeds of over 1.10 m/s recorded at this site significantly surpass the fastest previously reported directly measured current speeds at comparable or larger depths. A strong correlation is found between the noise intensity in the infrasonic band and the measured current speed. The noise intensity and the characteristic frequency increase with the increasing current speed. Machine-learning tools are employed to infer current speeds from flow-noise measurements at the site not equipped with a current meter.

Near-field pressure and wake velocity coherence of a circular cylinder

Aerodynamic noise and unsteady loads resulting from the vortex shedding of a circular cylinder pose significant challenges in engineering applications. Understanding these challenges is closely related to pressure fluctuations on the cylinder surface. This experimental study conducted simultaneous measurements of surface pressure and velocity fluctuations within the subcritical Reynolds number range (14.7×103≤Re≤30×103) to investigate the influence of vortex shedding on near-field pressure. The experiments utilized a highly instrumented cylinder with mini-pressure transducers. The results revealed that surface pressure fluctuations exhibit maximum energy content near the cylinder's shoulders at the fundamental vortex shedding frequency (f0), aligning with pronounced lift fluctuations. The analysis of pressure–velocity coherence indicated that the most energetic flow structures resulting from vortex shedding significantly contribute to generating surface pressure fluctuations at the f0-peak frequency, extending over a considerable distance from the near- to far-wake regions. Additionally, the pressure fluctuations responsible for drag fluctuations are predominantly imposed at the base of the cylinder, primarily at the second harmonic (2f0), arising from flow structures shed at the end of the vortex formation region. Wavelet analysis provided insights into the temporal characteristics of surface pressure fluctuations, revealing amplitude modulation over time with multiple repetitive patches around the f0-peak frequency and close to the cylinder's shoulders, where the highest energy level predominates due to vortex shedding.

Logarithmic scaling of higher-order temperature moments in the atmospheric surface layer

A generalized logarithmic law for high-order moments of homogeneous passive scalars is proposed for turbulent boundary layers. This law is analogous to the generalized log law that has been proposed for high-order moments of the turbulent longitudinal velocity and is derived by combining the random sweeping decorrelation hypothesis with a spectral model informed by the attached eddy hypothesis. The proposed theory predicts that the high-order moments of passive scalar fluctuations within the inertial sublayer will vary logarithmically with wall-normal distance (z), and is evaluated using high frequency time-series measurements of temperature and streamwise velocity fluctuations obtained in the first meter of the atmospheric surface layer (ASL) under near-neutral thermal stratification. The logarithmic dependence with z within the inertial sublayer is observed in both the air temperature and velocity moments, with good agreement to the predictions from the proposed theory. Surprisingly, the proposed theory appears to be as, if not more, valid for transported passive scalars than for the longitudinal velocity.

Jet Installation Noise Modeling in Static and Flight Conditions Using Centerline Fluctuations

The present work is dedicated to the modeling of the low-frequency part of the jet installation noise (JIN) in flight conditions. It is known that the properties of JIN can be predicted based on the characteristics of the near field of an isolated jet. Unlike the static case, in the presence of a coflow, it is difficult to directly measure the structure of the pressure perturbations in the jet near field. To overcome this problem, we propose a technique based on hot-wire measurements on the jet axis, suitable both for static and flight conditions. The well-known agreement between a parabolized stability equations (PSEs) model and experimentally measured velocity fluctuations on the jet axis allows using the PSE approach for the reconstruction of the axisymmetric pressure fluctuations in the vicinity of the wing trailing-edge location. The first helical mode, which is also important for the jet installation noise prediction, is approximately reconstructed based on the fact that its properties are close to those of the axisymmetric mode. These pressure characteristics are then used as input in an analytical jet installation noise model. To confirm this approach, acoustic measurements of JIN in static and flight conditions are conducted for a laboratory subsonic jet installed near a flat plate simulating a wing. It is shown that the analytical model informed by the PSE-reconstructed pressure field is capable of capturing the main features of the low-frequency jet–plate interaction noise both in static conditions and in the presence of coflow.

Raindrop fall velocity in turbulent flow: an observational study

Abstract. Laboratory measurements of drop fall speeds by Gunn–Kinzer under still air conditions with pressure corrections of Beard are accepted as the “gold standard”. We present measured fall speeds of 2 and 3 mm raindrops falling in turbulent flow with 2D-video disdrometer (2DVD) and simultaneous measurements of wind velocity fluctuations using a 3D-sonic anemometer. The findings based on six rain events are, (i) the mean fall speed decreases (from the Gunn–Kinzer terminal velocity) with increasing turbulent intensity, and (ii) the standard deviation increases with increase in the rms of the air velocity fluctuations. These findings are compared with other observations reported in the literature.

Reduction of electromagnetic interferences in measurements of fast-changing air velocity fluctuations by means of hot-wire anemometer

Air flow velocity measurements are the key issues in examining the condition and progress of mine ventilation process. In addition to standard measurements carried out in the stock exploitation process, it is necessary to conduct cognitive scientific research in this area. One of the important issues here is the measurement of fast-changing fluctuations of air velocity, enabling the knowledge of the spectral structure of the tested flow. Hot-wire anemometers are used for such measurements both in laboratories and in industrial conditions. A frequent metrological problem is the presence in the measuring area of a high level electromagnetic interference (EMI) from electrical networks and devices. Due to the metrological properties of hot-wire anemometers, such as high gain and wide frequency bandwidth, measuring signal in the presence of electromagnetic interferences may be subject to strong disturbances. Therefore, it is necessary to develop methods for reducing or eliminating these disturbances. The article presents such a new method based on the use of an additional induction loop in the anemometric sensor for compensation or reduction of interferences.

Numerical Assessment of Density Ratio and Mainstream Turbulence Effects on Leading-Edge Film Cooling: Heat and Mass Transfer Methods

Abstract Within the context of leading-edge film cooling in a high-pressure turbine vane, the present study is a step forward toward modeling showerhead performance for a baseline geometry (namely four staggered rows of cylindrical holes) at engine-like conditions, starting from a previous investigation, at low-speed flow (exit isentropic Mach number of Ma2is = 0.2), low inlet turbulence intensity of Tu1 = 1.6%, and density ratio of DR ∼ 1. Those operating conditions, dictated by experimental constraints, were essential to validate results from delayed detached-eddy simulation (DDES) against off-the-wall measurements of velocity, vorticity, and turbulent fluctuations, for the coolant-to-mainstream blowing ratio of BR = 3 (momentum flux ratio of I = 9). Here, the potential of DDES is exploited to predict the aerothermal features of the flow in the leading-edge region in the presence of a larger density ratio (DR ∼ 1.5) and turbulent mainstream (Tu1 = 13%), while matching either BR or I. The experimental database contains surface measurements of film cooling adiabatic effectiveness (η), obtained by using the pressure-sensitive paint (PSP) technique. DDES predictions of η were computed using the species transport model (i.e., mass transfer), for comparison against the conventional thermal method, based on creating a temperature differential between the mainstream and the coolant (i.e., heat transfer). The simulated film cooling performance was found to depend on the method used, thus suggesting that other parameters than DR, BR, I, and Tu1 should be taken into account when the goal is matching engine-like conditions.

Control of Shock-Induced Separation of a Turbulent Boundary Layer Using Air-Jet Vortex Generators

An experimental analysis was carried out to study the control effectiveness and turbulence behavior of an array of steady air-jet vortex generators on a shock-induced boundary-layer separation. Control was applied to a 24° compression-ramp interaction at Mach 2.52 and . A parametric study varying the jet spacing revealed that an adequate interaction between the vortices is essential for effective control, and the most favorable control effect was achieved with a jet spacing of . This case was analyzed in more detail using particle image velocimetry. The results reveal a strong corrugation of the separation zone due to the injection of the jets with a substantial reduction in its area along the entire spanwise extent of the air-jet array, compared with the uncontrolled case. Measurements of velocity fluctuations along the interaction region also show that the jet injection results in a reduction in turbulent intensity after reattachment, contributing to an overall reduction in amplification factor across the interaction region. The expected increase in turbulent mixing, which is essentially responsible for the control effect, was confirmed in the region in-between jet injection and separation shock. Farther away from the surface, the flow is not affected, which reduces potential drag penalties.

Data-Driven Identification of Nonlinear Flame Models

Abstract This paper presents a data-driven identification framework with the objective to retrieve a flame model from the nonlinear limit cycle. The motivation is to identify a flame model for configurations, which do not allow the determination of the flame dynamics: that is commonly for industrial applications where (i) optical access for nonintrusive measurements of velocity and heat release fluctuations are not feasible and (ii) unstable combustion is monitored via multiple pressure recordings. To demonstrate the usefulness of the method, we chose three test cases: (i) a classical Rijke tube; (ii) an experiment of a laminar flame (EM2C case), (iii) and a high-pressure, turbulent premixed flame (German Aerospace Center (DLR) case). The procedure is as follows: First, acoustic network models for the three cases are generated for which the in-house software taX is employed. Next, the acoustic network models are embedded in an optimization framework with the objective to identify flame parameters that match the experimental limit cycle data: based on the instability frequency and pressure amplitudes, we formulate physical constraints and an objective function in order to identify the flame model parameters gain nopt and time delay τopt in the nonlinear regime. We demonstrate for the three cases that the identified flame parameters reproduce the unstable combustion processes and highlight the usefulness of the method for control purposes.

Measurements of pressure and velocity fluctuations in a family of turbulent separation bubbles

Abstract

On the multi-scale turbulent structure interactions within wind farms

The quantification of the scale-by-scale interaction of multi-wakes and background flow is required to estimate turbines behaviour within wind farms. The paper provides insight on the evolution of multi-scale turbulence of a model wind farm operating in turbulent boundary layer. The wind farm configured with aligned turbines set at streamwise spacing between units of 5 diameters by spanwise spacing of 2.5 diameters, corresponding to an array of 8x3 turbines. Howtwire anemometry was used to acquire high-resolution measurements of streamwise velocity fluctuations at various locations. Experimental results suggest that the contribution of multi-scale turbulence structures to the kinetic energy of the wake is highly dependent on the location inside the wind farm, the evolution is significantly modulated by turbine rotation, wake interactions and outer flow, large scale motions mainly dominate outer and far wakes, while small scales are popular in inner and near wakes.

Local Measurements of Radial Plasma Velocity Fluctuations in the FT-2 Tokamak Using Equatorial Enhanced Scattering

The possibility of local measurement of the fluctuation level of the radial plasma velocity using equatorial enhanced scattering of a narrow microwave beam in the upper hybrid resonance in internal regions of tokamak plasma is demonstrated. Restrictions of the proposed method at the plasma periphery where the density fluctuation amplitude increases and small-angle scattering of microwaves by the fluctuations along the path to the upper hybrid resonance and back becomes significant are clarified.

Flight Effects on Turbulent-Jet Wave Packets

An investigation is carried out to evaluate how an external uniform stream affects turbulent-jet coherent structures. The presence of this uniform stream outside the jet represents the effect of forward flight on the velocity field. Coherent structures are modelled as wave packets, and the flight effect is studied using the parabolized stability equations (PSEs). Measurements of mean base flows and axial velocity fluctuations on the jet centerline at assumed flight conditions are described. The velocity fields were supplied to a PSE model to retrieve the wave packet signatures of axial velocity fluctuations. Overall results are in good agreement with experimental measurements. There is an observed increase in wave packet wavelengths and phase velocities (with respect to the nozzle) as flight velocity increases. Also, axial amplification rates are determined and show stabilization in the near-nozzle region, which is confirmed by experimental power spectral densities on the jet centerline. This tendency holds as the freestream velocity increases.

Cohesive river bank erosion mechanism under wave-current interaction: A flume study

The effect of hydrodynamic forces due to combined action of surface waves and current on the riverbank is critical to understand sediment entrainment, transport and bank line retreatment process. In understanding the temporal effect of turbulent structures under induced wave-current flow, a series of laboratory experiments were carried out. Micro-Acoustic Doppler Velocimeter (ADV) and Ultrasonic Ranging System (URS) were used simultaneously for the measurement of velocity fluctuations and bank undercut depth increment. Modulation of the turbulent flow characteristics and the benefaction of turbulent bursting structures at the initiation of erosion process and before the failure of the cohesive bank due to undercut progression are discussed. The results show that velocity and Reynolds shear stress have direct dependence on the size and rate of the entrainment of cohesive aggregates from bank face. The effect of wave-current motion leads to an increase in shear stress at the lateral bank giving rise to erosion and transportation of sediment particles/aggregates. Quadrant analysis of the random velocity fluctuation under wave-current flow at the initiation of erosion process shows strong presence of ejection and sweep events. Findings from the present study may provide a better understanding on the design of cohesive bank erosion control measures.

LES of Turbulent Co-current Taylor Bubble Flow

Turbulent co-current Taylor bubble flow was modeled with LES alongside the VOF interface tracking solver. The method is first validated in the context of single phase turbulent pipe flow and three distinct laminar Taylor bubble flow cases. The turbulent co-current Taylor bubble flow is simulated using a moving frame of reference which is attached to the bubble, allowing for a straight-forward time averaging and Dirichlet-type flow recycling inlet boundary condition. In contrast to periodic inlet and outlet boundary conditions, the Dirichlet one assures that small bubbles which are lost by the Taylor bubble are removed from the computation once they leave the domain, therewith not interfering with the nose of the Taylor bubble. The transient loss of small bubbles which is observed in our simulations implies that the problem is inherently unsteady. We therefore present an averaging strategy which consists of combined time, space and ensemble averaging. This strategy allows systematic comparison of results across different computational meshes. The results show that the approach captures distinct features of turbulent co-current Taylor bubble flow well, in the sense that good agreement is observed with experimental data from literature in terms of the toroidal vortex flow field in the wake of the Taylor bubble. Agreement with measured velocity fluctuation profiles in the wake of the Taylor bubble is less pronounced, and is discussed in the paper. Nevertheless, the simulation approach of co-current Taylor bubble flow proposed in this paper may serve as a tool that will provide deeper insights in two-phase turbulent flow for future research.

A note on wind velocity and pressure spectra inside compact spherical porous microphone windscreens.

Simultaneous measurements of wind velocity and pressure fluctuations were conducted in a wind tunnel to investigate the wind noise source inside compact spherical open celled porous windscreens. The existing outdoor wind noise models are found to be inadequate to predict the wind noise inside a wind tunnel. This paper proposes a model to predict the interior stagnation pressure, which agrees with the wind noise measured inside the windscreen within a bandwidth, where the exterior turbulence-turbulence interaction pressure overestimates the wind noise level. The limitations of the proposed model and other potential sources for wind noise inside porous windscreens are discussed.

Turbulent velocity measurements in high Reynolds cryogenic helium facilities at Service des Basses Températures (SBT)

Due to its very low viscosity, cryogenic helium has been used for years to generate high Reynolds turbulent flows. The measurement of velocity fluctuations in such high Reynolds flows is however a challenging issue, as it is necessary to develop small size (typically micron-length), robust sensors, to measure the whole spectrum of fluctuations of the velocity, which may extend to hundreds of kHz and possibly higher. SBT has developed for years different facilities, in particular taking benefit of the refrigeration capacities available at CEA Grenoble. In this article we present the current status of developments of hot wire sensors at CEA. Different characterizations of Wollaston hot wires are shown, and measurements of velocity fluctuations in different conditions, in normal helium as well as in superfluid helium are shown.

Selection of optimal multi-hotwire probe in constant temperature anemometry (CTA) for transonic flows

To decouple dimensional flow perturbation quantities, a probe optimization method is proposed for multi-wire constant temperature anemometry in transonic flow conditions. Historically, hotwire measurements of density, velocity, and total temperature fluctuations in transonic flows are challenging due to the complexity of calibration and difficulties in obtaining a probe with favorably conditioned sensitivity matrix. Based on universal empirical correlation for heated cylinders in compressible flow, the current method relies on evaluation of wire-voltage sensitivities to density, velocity, and total temperature perturbations. To maximize the signal-to-noise ratio (SNR) of the decoupling, a two-step optimization-sifting procedure is developed. The first-step optimization maximizes a value function geared towards lower sensitivity-matrix condition numbers, better robustness against wire temperature-setting errors, and large sensitivities. From the resulting Pareto front, the second step sifts further through the candidate probes by decoupling simulated noisy voltage and ranking the probes by their decoupling quality. The performance of the optimal wire temperatures and diameters combination is contrasted against probes stemming from a naive selection from the parameter space. According to the artificial data, only the optimal probes enable decoupling with reasonable SNR. Finally, the research effort proposes guidelines to define a probe with desirable properties, applicable across a wide range of transonic flow conditions.

Direct measurements of steady heat transfer maintained by oscillating pipe flow in thermoacoustic system

This study reports direct measurements of steady heat transfer in a heat exchanger pipe of a thermoacoustic heat pump driven by an oscillating helium gas flow. From simultaneous measurements of gas temperature, pressure, and velocity fluctuations, the steady heat transfer rate is determined from the radial profile of the temporal mean temperature and from the axial distribution of the enthalpy flux density. The heat transfer coefficient h is determined to be given by h=2k/r0 consistently in the range of 1.6≤r0/δα≤6.6, where k is the gas thermal conductivity, r0 is the tube radius, and δα=2α/ω is the characteristic transverse length given by using the thermal diffusivity and angular frequency.

Ultrasonic measurement of sound velocity fluctuations in biological tissue due to ultrasonic heating and estimation of thermo-physical properties.

Tissue characterization in terms of the differences in thermo-physical properties of biological tissues was investigated in this study. The objective was to measure the ratio of variation in sound velocity due to ultrasonic heating and to derive the relational expression between the ratio and thermo-physical properties. The ratio of sound velocity variation before and after the temperature rise of tissue samples exposed to ultrasound was measured by ultrasonic pulse echo method. The thermo-physical properties were estimated for a tissue-mimicking material and porcine muscle and fat tissues due to theoretical expression. The transducer for heating had a resonance frequency of 3.2MHz, and the transducer for measurement of sound velocity variation had a resonance frequency of 5.2MHz. In the phantom study, the measured values of the temperature rise agreed with the values calculated by a finite element method (FEM). The estimated values of the temperature rise from the sound velocity variations of muscle and fat tissues were 0.36°C and 1.1°C, respectively. Also, the estimated values of thermo-physical properties agreed with the reference values within an error of 10%. The thermo-physical properties of the porcine tissues were measured by sound velocity variation due to ultrasonic heating within the safety regulations.