Laser Velocimetry Measurements Research Articles

Refractive index of shock-compressed liquid nitrogen up to 29 GPa

ABSTRACT We report the equation of state and optical properties of shock-compressed liquid nitrogen (LN2) by using a two-stage light-gas gun at pressure up to 29 GPa. Laser velocimetry measurements were used to investigate the transparency and refractive index of shocked LN2 as a function of density. As the density increased with increasing pressure and temperature (13–25 GPa), the refractive index increased up to 27% of pre-shot index of LN2. Evidently, such extreme conditions had no major influence on molecules, and no such dissociation was observed up to 25 GPa. The polarizability slightly decreased and thus supported the existence of intact diatomic molecular nitrogen. At 29 GPa, shocked LN2 dissociated, showing that it probably changed to a highly reflecting fluid. Altogether, these experiments showed how the density affects the refractive index without any change in chemical bonding and allocates the condition at which the temperature-driven dissociation takes place.

Influence of Airflow on Thermal Comfort in an Energy-Saving House

The current thermal properties of building external structures including openings are supposed to suppress effects of draft to thermal comfort. But is it true expectation? The subject of the paper is the analysis of airflow around the window in three types of heating systems with predominant radiant as heat transfer. The analysis is based on experiments in the climatic cabin representing the room of a low-energy family house. To obtain the relevant data, a series of particle image laser velocimetry measurements were performed to visualize and quantify the airflow in the investigated area. The paper present results of the thermal comfort of the inhabitants of the room, which prove that with the influence of draft it is necessary to count on modern energy-saving buildings. They prove that old well-known rules about the position of the heating surface apply to the most cooled constructions and bring interesting conclusions about underfloor heating.

Experimental evidence for superionic water ice using shock compression

In stark contrast to common ice, Ih, water ice at planetary interior conditions has been predicted to become superionic with fast-diffusing (that is, liquid-like) hydrogen ions moving within a solid lattice of oxygen. Likely to constitute a large fraction of icy giant planets, this extraordinary phase has not been observed in the laboratory. Here, we report laser-driven shock-compression experiments on water ice VII. Using time-resolved optical pyrometry and laser velocimetry measurements as well as supporting density functional theory–molecular dynamics (DFT-MD) simulations, we document the shock equation of state of H2O to unprecedented extreme conditions and unravel thermodynamic signatures showing that ice melts near 5,000 K at 190 GPa. Optical reflectivity and absorption measurements also demonstrate the low electronic conductivity of ice, which, combined with previous measurements of the total electrical conductivity under reverberating shock compression, provides experimental evidence for superionic conduction in water ice at planetary interior conditions, verifying a 30-year-old prediction.

Reverberation technique for yield strength experiments at ultrahigh pressure and strain rate

Ultrahigh pressure and strain-rate (pressure ⪢10 GPa and strain rate ⪢105 s−1) are now attainable using high-power laser systems. A laser pulse drives a shock through a reservoir material which then unloads onto the target specimen. Laser velocimetry measurements at the back surface of the specimen are used to infer the material response. Material strength experiments have been proposed in which the pressure wave reverberates within the specimen and the cumulative resistance to compression by material strength is measured from the recorded velocimetry signature. In this work, ultrahigh pressure and strain-rate reverberation experiments are performed in tantalum, aluminum, and copper to investigate the yield strength behavior in this extreme regime. The experimental results indicate that the measurement sensitivity to yield strength is dominated by the lateral wave effects or impeded by the occurrence of spall. However, computer hydrodynamic code simulations are used to develop an experimental design based on shaping of the laser drive pulse that circumvents these difficulties. Simulations using the Steinberg–Guinan yield strength model and applying the design indicate the capability of measuring a factor-of-2 strength deviation from that predicted by the model with accuracy equal to or greater than the velocimetry measurement error.

Nonparticle laser velocimetry and permanent velocity measurement by enhancedlight scattering

Enhanced light scattering (ELS) from a nonuniform transparent gas provides acomplex signal that can be recorded by a quadrature demodulation technique andanalysed to give instant and continuous monitoring of gas velocity. The methodby which the velocity of the gas flow is extracted from the ELS signal isdeveloped. It uses the Hilbert transform properties of analytical functions. Thismethod for obtaining the velocity as a function of time is checked bycomparing its histogram to ELS Doppler spectra. Using this continuousmonitoring, extremely fast velocity rates of change, up to 109m s−2,were observed.

Combined “Fluorescence” LDV (FLDV) and PDA Technique for Non-ambiguous Two Phase Measurements Inside the Spray of a SI-Engine

Laser velocimetry measurements in the vicinity of reflecting surfaces are still a major problem in many fluid mechanical applications such as measuring close to walls or wall film surfaces, respectively. Moreover, in any kind of two phase flow an unambiguous separation of the gas and the liquid phase is of particular interest. Commonly used techniques like Phase Doppler Analyzers (PDA) with size discrimination are limited to two phase flows where the smallest particle of the dispersed phase is significantly larger than the seeding particles. This condition can rarely be fulfilled in technically relevant spray/air systems for instance in automobile engines or gas turbines. One of the most promising approaches to overcome this problem is a correct phase discrimination using fluorescent tracer particles for the gas phase. In this paper different laser based velocimeters have been compared using the spray of a gasoline injection nozzle as a typical example. The working principle of the “fluorescence” LDV (FLDV) will be explained in detail. Moreover, the quality of the fluorescence signals and of the standard bursts received from Mie-scattering particles will be compared. Finally, the capabilities of combined FLDV and PDA measurements inside the spray of a SI-engine at unsteady conditions will be presented. The pros and cons of this technique will be discussed against the background of discriminatory two phase PIV measurements applied to the same spray.

Physical Analysis and Scaling of a Jet and Vortex Actuator

Our previous studies have shown that the Jet and Vortex Actuator generates free-jet, wall-jet, and near-wall vortex flow fields. That is, the actuator can be operated in different modes by simply varying the driving frequency and/or amplitude. For this study, variations are made in the actuator plate and wide-slot widths and sine/asymmetrical actuator plate input forcing (drivers) to further study the actuator induced flow fields. Laser sheet flow visualization, particle-image velocimetry, and laser velocimetry are used to measure and charactrerize the actuator induced flow fields. Laser velocimetry measurements indicate that the vortex strength increases with the driver reptition rate for a fixed actuator geometry (wide slot and plate width). For a given driver repetition rate, the vortex strength increases as the plate width decreases provided the wide-slot to plate-width ratio is fixed. Using an asymmetric plate driver, a stronger vortex is generated for the same actuator geometry and a given driver repetition rate. The nondimensional scaling provides the approximate ranges for operating the actuator in the free jet, wall jet, or vortex flow regimes. Finally, phase-locked velocity measurements from particle image velocimetry indicate that the vortex structure is stationary, confirming previous computations. Both the computations and the particle image velocimetry measurements (expectantly) show unsteadiness near the wide-slot opening, which is indicative of mass ejection from the actuator.

A Plexiglas Research Pump With Calibrated Magnetic Bearings/Load Cells for Radial and Axial Hydraulic Force Measurement

A research pump intended for both flow visualization studies and direct measurement of hydrodynamic radial and axial forces has been developed. The impeller and the volute casing are constructed from Plexiglas which facilitates optical access for laser velocimetry measurements of the flow field both inside the impeller and in the volute casing. The pump housing is designed for flexibility allowing for each interchange of impellers and volute configurations. The pump rotor is supported by three radial magnetic bearings and one double acting magnetic thrust bearing. The magnetic bearings have been calibrated to characterize the force versus coil current and air gap relationship for each bearing type. Linear calibration functions valid for rotor eccentricities of up to 2/3 of the nominal bearing clearances and force level of ±58 N (13 lbf) and ±267 N (60 lbf) for the radial and axial bearings, respectively, were found. A detailed uncertainty analysis of the force calibration functions was conducted such that meaningful uncertainty bounds can be applied to in situ force measurements. Hysteresis and eddy current effects were quantified for each bearing such that their effect on the in situ force measurements could be assessed. By directly measuring the bearing reaction forces it is possible to determine the radial and axial hydraulic loads acting on the pump impeller. To demonstrate the capability of the magnetic bearings as active load cells representative hydraulic force measurements for a centered 4 vane 16 degree log spiral radial flow impeller operating in a single tongue spiral volute casing were made. At shut-off a nondimensional radial thrust of 0.084 was measured. A minimum nondimensional radial thrust of about 0.007 was observed at the nominal design flow. The nondimensional radial thrust increased to about 0.019 at 120 percent of design flow. The nondimensional axial thrust had a maximum at shut-off of 0.265 and decreased steadily to approximately 0.185 at 120 percent of design flow. Two regions of increasing axial thrust, in the flow range 75 to 100 percent of design flow, were observed. The measurements are compared to radial and axial force predictions using classical force models. The direct radial force measurements are compared to a representative set of radial force measurements from the literature. In addition, the directly measured radial force at design flow is compared to a single representative radial force measurement (obtained from the literature) calculated from the combination of static pressure and net momentum flux distribution at the impeller exit.

Boundary-Layer Characterization on Moving Walls by an Embedded Laser Velocimetry Technique

An embedded laser velocimetry measurement (ELV) method, suited for unsteady boundary-layer measurements on oscillating airfoils of arbitrary shape, has been developed. This method, based on an ELV optical fiber option, is an extension of a previous one tested on one-dimensional longitudinal velocity field measured over an oscillating flat-plate model.

Investigations of angle bias errors in laser velocimetry measurements

Fig. 3 Ice shapes for 15,30, and 60 s on the leading edge of the NACA 0012 airfoil. Recent studies with the calculation method of Ref. 1 also indicate excellent agreement between calculations and measurements on clean airfoils where the Reynolds number is low. It will be interesting to extend this study to an iced airfoil operating at low Reynolds numbers. For example, typical Reynolds numbers for the blades of a rotorcraft near the hub may be around 2 x 10. For the NACA 0012 airfoil, the stall angle without ice accretion is 11 deg at this Reynolds number (Fig. 1). The question would be, will a small leading-edge ice reduce the stall angle as drastically as it does in the higher Reynolds number case, and what will that stall angle be?

Laser Velocimetry and Blade Pressure Measurements of a Blade‐Vortex Interaction

Laser Velocimetry and Blade Pressure Measurements of a Blade‐Vortex Interaction

Laser Velocimetry Measurements in a Double Volute Centrifugal Pump

Laser velocimetry measurements were taken in a double volute/single discharge centrifugal pump (0.60 specific speed, 1583 US units) with symmetrical volute halves. Blade‐to‐blade radial and tangential velocity profiles at the impeller exit are presented and compared to data for a similar single volute pump. Flow rates ranged from 40% of design flow to the design point. The blade‐to‐blade profiles were more uniform than for the single volute pump. Also, the average circumferential variations for the double volute pump were more symmetric than for the single volute pump. For the double volute geometry measurements indicate that radial inward flow (recirculation) was only present for flow rates below 60% of design flow, compared to 80% of design flow for the single volute pump. Velocity data was also used to determine volute losses, slip factor, and momentum contributions to the impeller radial forces. Volute losses were quantified and shown to increase for flow rates below 80% of design flow and were approximately 10% of the developed head at 40% flow. The efficiency in the double volute compared to the single volute shows decreased performance for flows above 55% of design flow, which is attributed to increased boundary layer friction; at low flow rates increased performance is ascribed to better control over the recirculation regions. Slip factors were symmetric around the volute but were lower than for a single volute pump. Finally, momentum contributions to the total impeller radial load were shown to be maximum at the design point, contributing 40% of the force developed by the pressure distribution; the significance diminished at lower flow rates and the contribution was negligible at 40% of the design flow.

Experimental investigation of fringe divergence errors in laser velocimetry measurements

The effects of fringe divergence on the accuracy of laser velocimetry measurements have been investigated experimentally to obtain data that is supportive of a previously published theory by Hanson. The theory has been generalized to show that a similarity solution in terms of the laser beam confocal parameter may be derived. Experimental results are in good agreement with the similarity profile, proving the validity of both the similarity parameter and the published theory. The theory is used to develop alignment criteria to avoid residual turbulence resulting from fringe divergence. 4 refs.

Assessing the Capability of Doppler Global Velocimetry to Measure Vortical Flow Fields

A new non-intrusive flow diagnostics instrumentation system, Doppler global velocimetry, is presented. The system is capable of making simultaneous three-component velocity measurements within a selected measurement plane at video camera rates. These velocity images can provide the researcher with spatial and temporal information about the flow field in a global sense. The investigation of a vortical flow above a 75° delta wing comparing standard three-component fringe-type laser velocimetry measurements with Doppler global velocimetry measurements is presented.

Exit flows from highly porous media

This paper presents laser velocimetry measurements of the streamwise component of mean velocities and turbulence intensities measured downstream from the exit plane of porous ceramic foams through which air is flowed. The recent development and commercial availability of porous ceramic foams has lead to their application in many fields. Their uses have extended to combustion, high-temperature fluid filtering, biotechnology, and as support matrix for catalysts. These applications have created an interest in their pore scale fluid mechanics, both within the porous matrix and along surfaces open to flow. One emerging application is porous ceramic burners which combust liquid or gaseous fuels within the pore matrix or along the surface of the ceramic. The ceramic foams have pore sizes ranging from 4 to 12 pores per cm (ppcm) and porosities of 85%. Mean velocities between 0.3 and 1.5 m/s were examined. Radial distributions of mean velocities show a jet-like structure through the pores, with local mean velocities reaching maximum values over two times the area mean velocity. Negative mean velocities were often observed between pores, suggesting that recirculation zones are present above the web-like struts surrounding the pores. Levels of turbulence intensities normalized by the area mean velocity ranged from 0.05 to 0.6 for the various flow rates and pore sizes. Turbulence intensities were found to increase with increasing pore size for a given flow rate.

Laser velocimetry seed particles within compressible, vortical flows

The ability of seed particles to penetrate and accurately track vortices is of critical importance to the analysis of laser velocimetry (LV) measurements within these flow structures. In applying a particle equation of motion to vortical flowfields which extend into the supersonic, compressible regime, two approaches are considered. First, an ideal, potential vortex is developed for a compressible flowfield. As an aid to the design and analysis of vortical flowfield surveys over a wide range of independent parameters, this model is used to plot the time and position at which any particle starting at rest within a vortex will begin to track the flow velocity within 3% error. As a specific application, the potential vortex is then used to estimate the dynamic bias of LV measurements taken within a vortex shed from a 75-deg delta wing at 20 deg angle of attack in a Mach 1.9 supersonic flow. The second approach uses a computationally derived Navier-Stokes flowfield solution in place of the potential vortex model. The computational flowfield method predicts that accurate LV measurements within the delta wing flowfield require seed particles no larger than 0.1-0.2 /tm in diameter, and defines the unseeded inner vortex core region. Both approaches show an increase in velocity bias which is nearly proportional to particle diameter, stressing the need for a monodisperse seed of known size to resolve particle bias in complex flows.

Characterization of MOCVD fluid dynamics by laser velocimetry

The process of metalorganic chemical vapor deposition (MOCVD) is critically dependent on the fluid dynamics of the reacting and carrier gases. Quantitative understanding of the basic fluid dynamics associated with a reactor design is central to the application of engineering techniques to achieve design improvements in efficiency, bulk production and growth uniformity. A laser velocimetry (LV) system has been adapted specifically to acquire such quantitative information for the low Reynolds number mixed convective flows commonly encountered in MOCVD reactors. The instrument uses three color-separated wavelengths of an argon-ion laser to make three simultaneous independent orthogonal measurements of the flow field at specified interior locations. The technique is noninvasive, provides high spatial resolution, and is compatible with reactors operating at growth temperatures. The fluid dynamics of a horizontal MOCVD reactor used for GaAs growth were characterized by laser velocimetry measurements. The full three-component LV system was employed to determine a three-dimensional map of the flow field inside the quartz reactor vessel. Data have been obtained from the vessel both at room temperature and heated to growth temperature by RF induction. The two cases were compared to evaluate the effects of gravity-induced convection and buoyancy. Significant convective effects were found in the heated reactor, while fluidic switching effects were observed during room temperature operation. Instrumentation details and flow field measurements are presented.

Measurement of the interaction between a rotor tip vortex and a cylinder

The transient interaction between a cylinder and the trailing vortex from a rotor in forward flight is studied. Phase-averaged laser velocimetry and surface pressure measurements made with flush-mounted microphones are used to study the velocity and pressure variations during such an interaction. Vorticity contours constructed from the velocity measurements exhibit the presence of a secondary structure with vorticity opposite in sense to that of the primary tip vortex. This structure moves rapidly around the tip vortex from upstream to downstream. The pressure variations caused by the tip vortex on the surface of the cylinder are smooth as the vortex core passes by, and no evidence is found of fine structure inside the vortex core region. After vortex interaction, the secondary structure causes large variations in the surface pressure before being dissipated. Calculations using measured vortex strength and speed data indicate that the distortions and deflections of the vortex immediately prior to impingement on the surface differ significantly from those computed using two-dimensional potential flow concepts. Nomenclature Cpu = unsteady pressure coefficient: difference between the instantaneous pressure and the local mean static pressure, normalized by the tunnel dynamic pressure R - rotor radius r = radial position along the rotor radius U = velocity component along the tunnel axis, positive going downstream Um = freestream velocity V = vertical velocity component, positive downward X = distance parallel to the tunnel axis, origin is at the rotor hub center Xb - distance parallel to the tunnel axis, measured from the nose of the cylinder Yb = distance along the lateral direction from the cylinder axis Zb = vertical distance from the cylinder axis, positive

Laser velocimetry measurements in a laboratory vaned diffuser

Laser velocimetry measurements were made within a laboratory radial vaned diffuser with three different blade configurations. Measurements were made through passages with four, six and eight blades installed at off design conditions. Also, in the eight blade diffuser measurements were made between the blade passage exit and diffuser exit so that the complete secondary flow could be defined. The flow was found to separate from the blades and form large separation zones. The separation zones consisted primarily of two vortices rotating in opposite directions. At the passage exit the separation region encompassed 23% of the circumferential area for the four blade diffuser, 45% for the six blade and 40% in the eight blade diffuser. Separation occurred at 23%, 27% and 50% from the leading edge of the blades for the 4, 6 and 8 bladed diffusers, indicating that more blades better controlled the separation. Turbulence intensities ranged from approximately 5% to 15% in the primary flow and reached a few hundred percent in the secondary flow within the separation regions.

Effect of measuring volume length on two-component laser velocimeter measurements in a turbulent boundary layer

The effects of finite measuring volume length on laser velocimetry measurements of turbulent boundary layers were studied. Four different effective measuring volume lengths, ranging in spanwise extent from 7 to 44 viscous units, were used in a low Reynolds number (Reθ=1440) turbulent boundary layer with high data density. Reynolds shear stress profiles in the near-wall region show that 〈u v〉 strongly depends on the measuring volume length; at a given y-position, 〈u v〉 decreases with increasing measuring volume length. This dependence was attributed to simultaneous validations on the U and V channels of Doppler bursts coming from different particles within the measuring volume. Moments of the streamwise velocity showed a slight dependence on measuring volume length, indicating that spatial averaging effects well known for hot-films and hot-wires can occur in laser velocimetry measurements when the data density is high.