Turbulent Shear Layer Research Articles

Measurements of Flow Around a Flap Side Edge with Porous Edge Treatment

Wind-tunnel experiments were performed to investigate a flap side-edge vortex, which is a contributor to airframe noise. The flowfield investigation showed that the peak turbulent stresses were contained in the shear layer that rolled up to form the flap side-edge vortex. The wake from the main element was also entrained by the side-edge vortex. The near-field pressure fluctuations where the turbulent shear layer impinged on the flap side edge were broadband in nature from a Strouhal number of 10 to 50. Hot-wire measurements on the downstream vortex identified a broadband instability centered around a Strouhal number of 13.2. A porous side-edge treatment was applied to the half-span flap to modify the flap side-edge flowfield. The effect of applying a porous side edge was to reduce the Reynolds stresses contained within the vortex and the shear layer that formed it. The porous material also had the effect of displacing the vortex further away from the flap surface. This led to a reduction in the broadband pressure perturbations measured at the flap side edge. Compared with the accuracy of the measurements of the aerodynamic forces, the aerodynamic impact of the porous flap side edge was almost negligible.

DISPERSAL OF PROTOPLANETARY DISKS BY CENTRAL WIND STRIPPING

We present a model for the dispersal of protoplanetary disks by winds from either the central star or the inner disk. These winds obliquely strike the flaring disk surface and strip away disk material by entraining it in an outward radial-moving flow at the wind-disk interface, which lies several disk scale heights above the midplane. The disk dispersal time depends on the entrainment velocity, vd = cs , at which disk material flows into this turbulent shear layer interface, where is a scale factor and cs is the local sound speed in the disk surface just below the entrainment layer. If ~ 0.1, a likely upper limit, the dispersal time at 1 AU is ~6 Myr for a disk with a surface density of 103 g cm–2, a solar mass central star, and a wind with an outflow rate and terminal velocity vw = 200kms–1. When compared with photoevaporation and viscous evolution, wind stripping can be a dominant mechanism only for the combination of low accretion rates (10–8 M ☉ yr–1) and wind outflow rates approaching these accretion rates. This case is unusual since generally outflow rates are 0.1 of accretion rates.

Free-surface fluctuations in hydraulic jumps: Experimental observations

A hydraulic jump is the rapid and sudden transition from a high-velocity supercritical open channel flow to a subcritical flow. It is characterised by the dynamic interactions of the large-scale eddies with the free-surface. New series of experimental measurements were conducted in hydraulic jumps with Froude numbers between 3.1 and 8.5 to investigate these interactions. The dynamic free surface measurements were performed with a non-intrusive technique while the two-phase flow properties were recorded with a phase-detection probe. The shape of the mean free surface profile was well defined and the turbulent fluctuation profiles highlighted a distinct peak of turbulent intensity in the first part of the jump roller, with free-surface fluctuation levels increasing with increasing Froude number. The dominant free-surface fluctuation frequencies were typically between 1 and 4 Hz. A comparison between the acoustic sensor signals and conductivity probe data suggested that the air–water “free-surface” detected by the acoustic sensor corresponded to about the boundary between the turbulent shear layer and the upper free-surface layer. Simultaneous measurements of free surface and bubbly flow fluctuations for Fr = 5.1 indicated that the frequency ranges of both sensors were similar ( F < 5 Hz) whatever the position downstream of the toe. The present results highlighted that the dynamic free-surface measurements can be conducted successfully using acoustic displacement meters, and the time-averaged depth measurements was a physical measure of the free-surface location in hydraulic jumps.

Effect of Mach number on transonic flow past a circular cylinder

The effect of Mach number on transonic flow past a circular cylinder is investigated numerically for the free-stream Mach number M ∞ from 0.85 to 0.98 and the Reynolds number 2×105 based on the diameter of the cylinder. The work provides an insight into several salient features, including unsteady and quasi-steady flow state, formation of local supersonic zone, and evolution of turbulent shear layer. Results show that there exist two flow states dependent of a critical Mach number M cr around 0.9. One is an unsteady flow state characterized by moving shock waves interacting with the turbulent flow in the near region of the cylinder for M ∞<M cr, and the other is a quasi-steady flow state with nearly stationary shock waves formed in the near wake for M ∞>M cr, suppressing vortex shedding from the cylinder. Some flow behaviors in the unsteady and quasi-steady flow states are revealed. From time evolution of flow structures, local supersonic zones are identified in the wake and generated by two typical processes, i.e. reverse flow behind the cylinder and shed vortices in the near wake. The convective Mach number M c of turbulent shear layers shed from the cylinder is identified nearly as M c<1 in the unsteady flow regime and M c>1 in the quasi-steady flow regime, resulting in different evolutions of the shear layers.

A Phase Diagram for Turbulent, Transitional, and Laminar Clay Suspension Flows

Abstract New phase diagrams for the dynamic structure of clay-laden open-channel flows are proposed. These diagrams can be used to distinguish between turbulent Newtonian, transitional, and laminar non-Newtonian flow behavior, on the basis of the balance between turbulent forces (approximated by the horizontal components of flow velocity and turbulence intensity) and cohesive forces (approximated by the suspended clay concentration and rheology). Stability regimes for five different flow types are defined using a comprehensive series of laboratory flume experiments at depth-averaged flow velocities ranging from 0.13 m s−1 to 1.47 m s−1, and at volumetric kaolinite clay concentrations ranging from 0.03% (= 0.8 g L−1) to 16.7% (= 434 g L−1). As clay concentration increases, five flow types can be distinguished: turbulent flow, turbulence-enhanced transitional flow, lower and upper transitional plug flow, and quasi-laminar plug flow. The turbulent properties of transitional flows are shown to be considerably more complex than the common notion of gradual turbulence damping. Turbulence-enhanced transitional flows display higher turbulence intensity than turbulent flows of similar velocity, with such enhancement originating from development of a highly turbulent basal internal shear layer within ~ 0.01 m of the bed. In lower transitional plug flows, the basal internal shear layer separates a lower region of high vertical gradient in horizontal velocity and strong turbulence from an upper region of plug flow with a much gentler velocity gradient and lower turbulence intensity. Kelvin-Helmholtz shear instabilities within the highly turbulent shear layer are expressed as distinct second-scale oscillations in the time series of downstream velocity. Turbulence damping dominates upper transitional plug flows, because strong cohesive forces, inferred to be caused by gelling of the high-concentration clay suspension, start to outbalance turbulent forces. In quasi-laminar plug flows, gelling is pervasive and turbulence is fully suppressed, apart from some minor residual turbulence near the base of these flows. With very few exceptions, all flows pass through the same development stages as clay concentration increases, regardless of their velocity, but the threshold concentrations for turbulence enhancement, gelling, and development of internal shear layers and plug flows are proportional to flow velocity. At flow velocities below ~ 0.5 m s−1, only low concentrations (< 0.75%) of kaolinite are required to induce transitional flow behavior, thus potentially affecting many slow-moving and decelerating clay flows in natural sedimentary environments. However, at flow velocities above 1 m s−1, clay concentrations of at least 6% are required in order for flows to enter the transitional flow phase, but even at these velocities the transitional flow phases make up a significant proportion of the flow phase space. By converting the experimental data to nondimensional Froude number (momentum term) and Reynolds number (cohesive term), it is shown that each boundary between the turbulent, transitional, and laminar flow phases can be described by a specific narrow range of Reynolds numbers. Within the duration of the experiments, settling of clay particles occurred only in plug flows of low flow velocity (and low Froude number), when the flows lacked the strength to support the entire clay suspension load.

Hairpin Vortex Solution in Planar Couette Flow: A Tapestry of Knotted Vortices

A numerical continuation method has been carried out seeking solutions for two distinct flow configurations, planar Couette flow (PCF) and laterally heated flow in a vertical slot (LHF). We found that the spanwise vortex solution in LHF identifies a new solution in PCF. The vortical structure of our new solution has the shape of a hairpin observed ubiquitously in high-Reynolds-number turbulent flow, and we believe this discovery may provide the paradigm for a hierarchical organization of coherent structures in turbulent shear layers.

NUMERICAL SIMULATION OF SHOCK WAVE AND TURBULENCE INTERACTION OVER A CIRCULAR CYLINDER

The interaction of shock wave and turbulence for transonic flow over a circular cylinder is investigated using detached-eddy simulation (DES). Several typical cases are calculated for free-stream Mach number M∞ from 0.85 to 0.95, and the physical mechanisms relevant to the shock wave and turbulence interaction are discussed. Results show that there exist two flow states. One is unsteady flow state with moving shock waves interacting with turbulent flow for M∞ &lt; 0.9 approximately, and the other is quasi-steady flow with stationary shocks standing over the wake of the cylinder for M∞ &gt; 0.9, suppressing the vortex shedding from the cylinder. Moreover, local supersonic zones are identified in the wake of the cylinder and generated by two processes, i.e., reverse flow and shock wave distortion induced the supersonic zone. Turbulent shear layer instabilities are revealed and associated with moving shock wave and traveling pressure wave.

Two-phase flow measurements in turbulent hydraulic jumps

Hydraulic jumps are commonly experienced in industrial applications and manufacturing processes, as well in rivers and canals. The hydraulic jump is the sudden transition from a high-velocity open channel flow to a subcritical flow motion. Despite nearly two centuries of studies, there is still a lack of knowledge on the two-phase flow properties of the turbulent shear layer and the roller. New series of experimental measurements were conducted in hydraulic jumps with Froude numbers between 5 and 8.5, and inflow Reynolds numbers between 38,000 and 62,000. The two-phase flow measurements included some vertical profiles of void fraction, bubble count rate, interfacial velocity and bubble chords. In the air–water shear region, the void fraction and bubble count rate distributions exhibited marked peaks, with the maximum of void fraction always above the location of the maximum bubble count rate. The dimensionless turbulent diffusivity coefficient was estimated. The dimensionless distributions of interfacial velocity compared favourably with some wall-jet flow equations. The data analysis showed that the mean bubble chord in the turbulent shear layer was between 1 mm and 6 mm. The probability distribution functions (PDF) of bubble chord time illustrated a broad spectrum with predominance to small bubbles compared to the mean.

Active control of mixing in turbulent separated shear layers and effects of forcing on the fractal geometry of scalar interfaces

This paper explores effects of active control on mixing and the fractal geometry of scalar interfaces. In particular, a pulsed plasma actuator is used to implement active mixing control of turbulent-separated shear layers, with the advantage that this actuator has no moving parts. The effects on mixing are determined based on the scalar power spectrum and the scalar probability density function from scalar images recorded with laser Mie scattering for fog-seeded freestream air and pure ambient air. In addition, the fractal geometry of the scalar interfaces is examined to determine the effects of the forcing. The Reynolds number of the shear layer based on the visual thickness is 3.0 × 104 at the imaging station. The range of Strouhal numbers of the forcing is 0.3 ≤ St ≤ 7.8. Effects of forcing on the scalar power spectrum are found in terms of significant modifications of the spectral scalar energy at the lowest wavenumbers which is observed to increase by as much as 50% and decrease by as much as 40% depending on the forcing frequency. These modifications of scalar fluctuations at the largest scales indicate that the forcing is able to inhibit and enhance, respectively, the formation of large-scale scalar structures, which can be denoted as Large Structure Suppression (LSS) and Large Structure Regularization (LSR). Shifts of the intermediate peak of the scalar probability density are found to occur away from and toward, respectively, the higher speed fluid concentration value for the most pronounced cases of LSS and LSR. Regarding the effects of forcing on the fractal geometry of scalar interfaces, evidence is presented that forcing has a relatively small effect on the fractal dimension value but a significant effect on the range of scales of geometrical self-similarity. The fractal dimension exhibits values in the range of D2∼ 1.56 +/− 0.02 to 1.65 +/− 0.02 depending on the forcing conditions, with the highest dimension value found at the highest forcing frequencies. The range of scales exhibiting geometrical self-similarity is found to be extended by up to 75% or reduced by up to 50% depending on the forcing frequency. Therefore, these results show that although the fractal dimension value is relatively robust, periodic forcing of the turbulent-separated shear layer can substantially alter the extent of the range of scales exhibiting geometrical self-similarity.

Investigation of the flow in a circular cavity using stereo and tomographic particle image velocimetry

The turbulent flow over a circular cavity with an aspect ratio of D/H = 2 is investigated by multi-planar stereoscopic particle image velocimetry and with tomographic particle image velocimetry (PIV). The main aim of the study is the flow topology and the turbulent structure of the asymmetrical flow pattern that forms inside the cavity at these specific conditions. The flow field is measured in the vertical symmetry plane to describe the overall recirculation pattern in the cavity and the turbulent shear layer developing from the separation point. In this specific regime the shear layer fluctuations are recognized as those caused by instabilities together with the effect of the incoming boundary layer turbulence. Additional observations performed at several wall-parallel planes at different height inside the cavity allow to further evaluate the secondary flow circulation generated by this asymmetric regime. The observed flow pattern consists of a steady vortex, occupying the entire cavity volume and placed diagonally inside the cavity such to entrain the external flow from one side, capture it into a circulatory motion and eject it from the opposite side of the cavity. The spatial distribution of the turbulent fluctuations also reveals the same structure. The tomographic PIV measurement returns a visual inspection to the instantaneous three-dimensional structure of the turbulent fluctuations, which at the investigated height exhibit a low level of coherence with slightly elongated vortices in the recirculating flow inside the cavity.

On attenuation of plane sound waves in turbulent mean flow

Plane sound waves in a smooth pipe turbulent boundary layer are known to be more strongly damped when the acoustic boundary layer becomes thicker than the viscous sublayer. The attenuation constants that govern this phenomenon are accurately predicted by the mathematical model proposed by M.S. Howe [The damping of sound by wall turbulent shear layers. Journal of the Acoustical Society of America 98(3) (1995) 1725–1730. Also in: Acoustics of Fluid– Structure Interactions, Cambridge University Press, Cambridge, 1998]. This model assumes uniform mean core flow. The present paper proposes a variant of this model which is based on the assumption of parallel sheared mean core flow. Predictions of the two approaches are compared.

Direct Reduction of Aero-Optical Aberrations by Large Structure Suppression Control in Turbulence

Direct reduction of aero-optical aberrations incurred by a laser propagated through a refractive turbulent flow has been achieved by introducing plasma into the flow at various pulsing frequencies. The plasma is generated by a dielectric barrier discharge device which operates at elevated pressure to force flow near the separation point of a single-stream shear layer that is generated in a variable pressure wind-tunnel testing facility with Re ∼ 6 x 10 6 and M ∞ ∼ 0.9. The levels of aero-optical aberrations present in the laser are determined by directly profiling the propagated laser using a high-resolution Shack-Hartmann wave frontsensor with ∼ 1000 microlenses. Reductions in the ensemble-averaged optical path difference root mean square of up to 27% were achieved. Whole-field shadowgraphs were recorded for various forcing frequencies and indicate that the observed aero-optical effects are the result of modifications in the level of organization of the large structures. Because the dominant contributions to the aberrations in unforced flows are caused by large-scale organized structures, our findings indicate that the mechanism by which the significant reduction is observed in the forced experiments is due to large-scale suppression of the turbulent structures directly effected by the pulsed plasma actuator. Conversely, increases in the aero-optical aberrations, which have also been observed at certain plasma pulsing frequencies, are the result of large-scale regularization of the structures within the turbulent shear layer. The presence of these large-scale suppression and regularization mechanisms is confirmed by the shadowgraphs at forced cases compared with the unforced case.

Sound propagation in narrow tubes including effects of viscothermal and turbulent damping with application to charge air coolers

Charge air coolers (CACs) are used on turbocharged internal combustion engines to enhance the overall gas-exchange performance. The cooling of the charged air results in higher density and thus volumetric efficiency. It is also important for petrol engines that the knock margin increases with reduced charge air temperature. A property that is still not very well investigated is the sound transmission through a CAC. The losses, due to viscous and thermal boundary layers as well as turbulence, in the narrow cooling tubes result in frequency dependent attenuation of the transmitted sound that is significant and dependent on the flow conditions. Normally, the cross-sections of the cooling tubes are neither circular nor rectangular, which is why no analytical solution accounting for a superimposed mean flow exists. The cross-dimensions of the connecting tanks, located on each side of the cooling tubes, are large compared to the diameters of the inlet and outlet ducts. Three-dimensional effects will therefore be important at frequencies significantly lower than the cut-on frequencies of the inlet/outlet ducts. In this study the two-dimensional finite element solution scheme for sound propagation in narrow tubes, including the effect of viscous and thermal boundary layers, originally derived by Astley and Cummings [Wave propagation in catalytic converters: Formulation of the problem and finite element scheme, Journal of Sound and Vibration 188 (5) (1995) 635–657] is used to extract two-ports to represent the cooling tubes. The approximate solutions for sound propagation, accounting for viscothermal and turbulent boundary layers derived by Dokumaci [Sound transmission in narrow pipes with superimposed uniform mean flow and acoustic modelling of automobile catalytic converters, Journal of Sound and Vibration 182 (5) (1995) 799–808] and Howe [The damping of sound by wall turbulent shear layers, Journal of the Acoustical Society of America 98 (3) (1995) 1723–1730], are additionally calculated for corresponding circular cross-sections for comparison and discussion. The two-ports are thereafter combined with numerically obtained multi-ports, representing the connecting tanks, in order to obtain the transmission properties for the charged air when passing the complete CAC. An attractive formalism for representation of the multi-ports based on the admittance relationship between the ports is presented. From this the first linear frequency domain model for CACs, which includes a complete treatment of losses in the cooling tubes and 3D effects in the connecting tanks is extracted in the form of a two-port. The frequency dependent transmission loss is calculated and compared to the corresponding experimental data with good agreement.

Large Eddy Simulation of Leading Edge Film Cooling—Part II: Heat Transfer and Effect of Blowing Ratio

Detailed investigation of film cooling for a cylindrical leading edge is carried out using large eddy simulation (LES). The paper focuses on the effects of coolant to mainstream blowing ratio on flow features and, consequently, on the adiabatic effectiveness and heat transfer coefficient. With the advantage of obtaining unique, accurate, and dynamic results from LES, the influential coherent structures in the flow are identified. Describing the mechanism of jet-mainstream interaction, it is shown that as the blowing ratio increases, a more turbulent shear layer and stronger mainstream entrainment occur. The combined effects lead to a lower adiabatic effectiveness and higher heat transfer coefficient. Surface distribution and span-averaged profiles are shown for both adiabatic effectiveness and heat transfer (presented by Frossling number). Results are in good agreement with the experimental data of Ekkad et al. [1998, “Detailed Film Cooling Measurement on a Cylindrical Leading Edge Model: Effect of Free-Steam Turbulence and Coolant Density,” ASME J. Turbomach., 120, pp. 799–807].

Fine-Scale Characterization of the Turbulent Shear Layer of an Instream Pebble Cluster

This study characterizes the shear layer and associated vortex shedding around an isolated submerged pebble cluster in a gravel-bed river. The approach combines flow visualization and high frequency three-dimensional velocity (acoustic Doppler velocimeter) measurements. Two vortex shedding modes in the wake of the cluster were identified: A small scale high frequency initial instability mode and a lower frequency mode that scales with cluster height. The lower frequency mode arose from the intermittent interaction and amalgamation of the small-scale instability vortices. Reynolds shear stresses, velocity spectra, and coherence functions indicated a dominance of longitudinal-vertical shedding vortices in the wake of the cluster. Simultaneous flow visualization was required to determine the nature and behavior of the shedding modes. Quadrant analysis revealed that Q2 and Q4 events contributed 80% of the local longitudinal-vertical component Reynolds shear stress, and demonstrated a dominance of ejection events in the wake of the cluster. Through flow visualization, the behavior of the shear layer was seen to vertically expand and contract with the passage of Q2 and Q4 events, respectively.

Control of diffuser jet flow: turbulent kinetic energy and jet spreading enhancements assisted by a non-thermal plasma discharge

An axisymmetric air jet exhausting from a 22-degree-angle diffuser is investigated experimentally by particle image velocimetry (PIV) and stereo-PIV measurements. Two opposite dielectric barrier discharge (DBD) actuators are placed along the lips of the diffuser in order to force the mixing by a co-flow actuation. The electrohydrodynamic forces generated by both actuators modify and excite the turbulent shear layer at the diffuser jet exit. Primary air jet velocities from 10 to 40 m/s are studied (Reynolds numbers ranging from 3.2 to 12.8 × 104), and baseline and forced flows are compared by analysing streamwise and cross-stream PIV fields. The mixing enhancement in the near field region is characterized by the potential core length, the centreline turbulent kinetic energy (TKE), the integrated value of the TKE over various slices along the jet, the turbulent Reynolds stresses and the vorticity fields. The time-averaged fields demonstrate that an effective increase in mixing is achieved by a forced flow reattachment along the wall of the diffuser at 10 m/s, whereas mixing enhancement is realized by excitation of the coherent structures for a primary velocity of 20 and 30 m/s. The actuation introduces two pairs of contra-rotating vortices above each actuator. These structures entrain the higher speed core fluid toward the ambient air. Unsteady actuations over Strouhal numbers ranging from 0.08 to 1 are also studied. The results suggest that the excitation at a Strouhal number around 0.3 is more effective to enhance the turbulence kinetic energy in the near-field region for primary jet velocity up to 30 m/s.

Three-dimensional effects of supersonic cavity flow due to the variation of cavity aspect and width ratios

Unlike the steady closed-type supersonic cavity flow, open-type cavity flow is divided into internal and external flows by turbulent shear layer. The cavity flow may cause resonance phenomena due to pressure oscillation, depending on the cavity geometry and the flow conditions. These phenomena may induce noise generation, structural damage, and aerodynamic instability. In this research, the flow characteristics of three-dimensional supersonic cavity flow of Mach number 1.5 were analyzed with the variations of aspect ratio and width ratio. Three-dimensional unsteady compressible Reynolds-averaged Navier-Stokes (RANS) equations were used with a turbulence model. For numerical calculations, the 4th-order Runge-Kutta method and the FVS method with van Leer’s flux limiter were applied. The numerical calculations were performed by using a parallel processing program with 16 CPUs. The sound pressure level (SPL) spectra of pressure variations were analyzed at the point of cavity leading edge. The correlation of pressure distribution (CPD) was also analyzed for the propagation of dominant oscillation pressure waves with respect to the reference point of the cavity leading edge. The dominant oscillation frequency was compared with the oscillation modes of Rossiter’s formula. Oscillation Mode 2 appeared as a dominant oscillation frequency regardless of the aspect ratio of cavity in the two-dimensional flow. Oscillation Modes1 and 2 appeared in three-dimensional cavities of small aspect ratios. However, as the aspect or the width ratio increases, only the mode 2 or 3 frequency appeared as a dominant oscillation frequency.

Detachment and attachment of an axisymmetric non-reactive jet with turbulent shear layer: Control by plasma actuator

The control of an axisymmetric air jet equipped with a small-angle diffuser exhaust using a single non-thermal plasma actuator is studied with a stereoscopic PIV system. A single Dielectric Barrier Discharge (DBD) actuator is used to separate a naturally attached flow (12° angle diffuser) or to reattach a naturally separated one (22° angle diffuser). The experimental set-up allows us to measure the flow modifications occurring at the bevel of the diffuser. The actuator can generate a local flow (electric wind) of 6 m s −1 used to modify an axisymmetric air jet with centreline velocities of 10, 20 and 30 m s −1. The flow modifications under actuation are investigated in terms of time-averaged, phased-averaged fields as well as eddy localization. The analysis is mainly focused on the lower primary jet velocity at which the actuator presents a high efficiency. The results highlight that a plasma actuator can force the flow separation or attachment in a few dozen of milliseconds and that this device can be effective to manipulate the coherent structures surrounding the potential core. The global conclusion of the present study is that a single DBD actuator could be an effective device to promote the vortex shedding at the diffuser lip of an axisymmetric air jet. This may results in an improvement of the mixing characteristics, at least in the near-field region.

Mixing Measurements in a Supersonic Expansion-Ramp Combustor

This paper reports results on molecular mixing for injection via an expansion-ramp into a supersonic freestream with M 1 = 1.5. This geometry produces a compressible turbulent shear layer between an upper, high-speed “air” stream and a lower, low-speed “fuel” stream, injected through an expansion-ramp at α = 30° to the high-speed freestream. Mass injection is chosen to force the shear layer to attach to the lower guide wall. This results in part of the flow being directed upstream, forming a recirculation zone. Employing the hypergolic hydrogen-fluorine chemical reaction and pairs of “flip” experiments, molecular mixing is quantified by measuring the resulting temperature rise. Initial experiments established the fast-chemistry limit for this flow in terms of a Damkohler number (Da). For Da ≥ 1.4, molecularly mixed fluid effectively reacts to completion. Parameters varied in these experiments were the measurement station location, the injection velocity of the (lower) “fuel” stream, the stoichiometry for the flip experiments, and the density ratio of the fuel and air streams. As expected, mixing increases with increasing distance from the injection surface. The mixed fluid fraction increases by 12% when changing the fuel-to-air stream density ratio from 1 to 0.2. Comparisons with measurements at subsonic (high-speed) “air” stream velocities show that the trend of decreasing mixing with increasing speed documented in free-shear layer flows is also encountered in these flows. The current geometry produces higher mixing levels than do free shear layers.

Particle tracking velocimetry (PTV) and its application to analyse free surface flows in laboratory scale models

The methodology and application of particle tracking velocimetry (PTV) for surface flow velocity measurements are presented in a conventional laboratory scale model characterised both by large vortices and turbulence, making use of the fact that PTV allows to take reliable measurements in flows exposed to strong deformations typical in turbulent shear layers. The experimental setup included a river reach with main channel and wide floodplain of different roughness. Surface PTV measurements were carried out by a CCD camera with 30 frames per second frame rate and vertical camera axis. In the evaluation of the recordings an algorithm suitable to be applied in flows with strong strains was needed, capable to find corresponding particles even by highly deformed neighbourhoods. For this aim a modification of the so-called velocity gradient tensor method was used. The reconstructed velocity fields were used first to visualize the mean flow field, then to identify and investigate the various flow structures by means of the swirling strength analysis.