Shear Probes Research Articles

Observations of a Kelvin-Helmholtz Billow in the Ocean

We identified a Kelvin-Helmholtz billow from vertical turbulence velocity and instantaneous heat flux signals obtained from airfoil shear probes and thermistors mounted on a research submarine. The vertical turbulence velocity indicates that the horizontal scale of the billow was about 3.5 m. The spectral slope of the vertical turbulence velocity component is close to −2, revealing the flow is two-dimensional. We show a remarkable agreement between the length scales of the observed billow and those computed from direct numerical simulations based on similar conditions.

An Intercomparison Study Using Electromagnetic Three-Component Turbulent Velocity Probes

Abstract An intercomparison study was performed with four Russian-made, electromagnetic probes capable of measuring three components of oceanic turbulent velocities and two single-axis velocity sensors familiar to western scientists, namely, a hot-film anemometer and an airfoil shear probe. The intercomparison measurements were conducted in a water flume tank in the Marine Scientific Production Corporation on Fishery Technology facility in Kaliningrad, Russia. Measurements were obtained in the turbulent region generated behind a grid at three different mean flow speeds (0.7, 1.1, and 1.7 m s−1). In all the intercomparison runs, data from the electromagnetic velocity probes behave in a manner expected from the sensor and filtering characteristics. On the average, turbulent velocity variances from the electromagnetic probes are about ±20% of those from both the hot-film and airfoil probes.

Dissipation Measurement with a Moored Instrument in a Swift Tidal Channel

Abstract A moored and autonomous instrument that measures velocity and temperature fluctuations in the inertial subrange using shear probes and FP07 thermistors has been deployed in a swift [O(1 m s−1)] tidal channel for eight days. The measured velocity signals are free from body vibrations for frequencies below 16 Hz in flows faster than 0.5 m s−1 and below 8 Hz for slower flows. At lower frequencies, fluctuations of torque on the instrument, due mainly to fluctuations of the ambient current, produce large pitching and rolling motions (≈4° peak) that can easily be reduced by minor mechanical changes. Vibrations at higher frequencies do not scale with flow speed and stem mainly from mechanical structures. The velocity spectrum is free from contamination by body motions between 0.2 and 20 cpm (the inertial subrange), and consistent estimates of the rate of dissipation of kinetic energy are obtained from the spectral levels of vertical and lateral velocity fluctuations within this range.

An Autonomous Ocean Turbulence Measurement Platform

Abstract The use of a small autonomous underwater vehicle (AUV) as a platform for making in situ flow measurements in the ocean environment is described. Two high-wavenumber shear probes and a dynamic Pitot tube, housed in a pressure vessel mounted on the nose of the AUV, allow measurements, in the dissipation range, of all three components of velocity. Microstructure temperature and possible body vibrations are monitored using auxiliary local probes. A conductivity–temperature–depth package, an acoustic Doppler current profiler, and a Marsh–McBirney current meter on board allow measurement of mean background conditions. Gathered data are stored on an onboard computer. The AUV can survey 7–11-km regions at a speed of 1.5–2 m s−1, its motion being uncoupled from that of any surface mother ship. The vehicle has relatively low manufacture and operational costs and can potentially operate in stormy conditions. The small-scale measurements allow the determination of estimates of in situ energy dissipation rate...

Turbulence Measurement from an Autonomous Underwater Vehicle

Abstract Horizontal profiles of the microstructure of velocity and temperature were obtained with a large autonomous underwater vehicle (AUV) using two piezoelectric shear probes, an FP07 thermistor, and three orthogonal accelerometers mounted on a sting at the forward end of the vehicle. A winter field trial in Narragansett Bay provided a run in the midwater pycnocline at 8-m depth that contained a thermal front, an ascending profile to 3-m depth, and a run in the weakly stratified surface layer at this depth. Although shear spectra were strongly contaminated by narrowband vibrations produced by the motor and actuators, this contamination was highly coherent with the measured acceleration and was removed with standard signal processing techniques. The corrected spectra agreed well with the Nasmyth universal spectrum for wavenumbers up to 40 cpm. The estimated rate of dissipation of kinetic energy varied from 0.8 to 250 × 10−8 W kg−1 and was consistent with the rate of production of turbulence by surface ...

A novel method for characterizing contact-mode shear wave transducers in immersion

The dominating shear motion of a contact-mode shear wave transducer is often accompanied by a weak parasitic longitudinal mode due to its finite size and constrained boundary conditions. The amplitude and phase of this out-of-plane vibration may be exploited for determining the shear polarization direction and for detecting any asymmetry or distortion of the shear transducer’s vibration pattern. In practice, the contact-mode shear probe is immersed in water and energized by an rf pulser to serve as the transmitter. A C-scan is then made of the face of the shear transducer using a focused longitudinal wave transducer as the receiver. The resulting image shows the out-of-plane longitudinal vibration of the shear transducer. For commercial contact-mode shear probes, this pattern usually consists of two crescent-shaped regions of higher vibration amplitude but of opposite phase. A line bisecting the two crescents and passing through their centers defines the shear polarization direction. Finite-element modeling of the shear displacement of a circular disk with constrained boundary confirmed the experimentally observed behavior. This approach provides a simple method for characterizing commercial shear wave transducers. [Work supported by Center for NDE, Iowa State University.]

Turbulence Measurement with a Moored Instrument

Abstract A new autonomous and moored microstructure measuring instrument has been tested in coastal waters. The instrument measures shear in the dissipation range of the wavenumber spectrum using four shear probes; temperature fluctuations using two FP-07 thermistors; temperature and salinity using three pairs of Seabird sensors; horizontal currents using two ducted rotors and a fluxgate compass; and body motions using three accelerometers and a pressure transducer. Data are sampled for 128 s every 5 min and reduced into ensemble and band-averaged spectra and statistical parameters. The processed data are transferred to a disk every 6 h along with one 128-s set of unprocessed data. In a 2-day experiment, the instrument experienced peak flows of 0.15 m s−1 and resolved dissipation rates in currents as slow as 0.03 m s−1. Spectra of velocity fluctuations agree closely with the Nasmyth universal spectrum. For dissipation rates, the lower limit of detection is determined by electronic noise, while the upper l...

Separation of turbulence velocity and temperature sound-speed variability with reciprocal transmission

Acoustical scintillation results from the combined contribution of scalar (i.e., temperature and salinity) and vector (velocity) fluctuations. Reciprocal transmission allows us to separate these components. In a turbulent flow, in which the measured scales lie within the vertical subrange, the velocity fluctuations can be used to derive the rate of turbulent kinetic energy dissipation. Reciprocal transmission measurements at 67 kHz were recently acquired as part of the U.S. Office of Naval Research ‘‘Moderate to High Frequency’’ research program. The results are interpreted in terms of the effective refractive index structure parameter and compared both with the independent shear probe measurements of Lueck (personal communication) and the recent theoretical analysis of Ostachev [Waves Random Media 4, 403–428 (1994)].

Measuring intramuscular fat in beef with ultrasonic frequency analysis.

Frequency analysis of Fourier spectra from ultrasonic signals was used for predicting intramuscular fat content of beef tissue. The most significant parameter in the frequency domain for predicting intramuscular fat concentration in beef was the number of local maxima. It represents the discontinuity of the Fourier spectrum caused by inhomogeneous fat concentrations in the longissimus muscle, which had the correlation coefficient .89 (P < .05) when a 2.25-MHz shear probe was used. The optimum frequency for predicting the amount of intramuscular fat content in the longissimus muscle was found to be 1.92 MHz. A multivariate regression model was developed using parameters in the frequency domain as follows: percentage of fat concentration = 1.790 - 2.373x (lower frequency) + .049x (bandwidth) + 1.178x (local maxima) (R2 = .82). Validation demonstrated that the multivariate model in the frequency domain was capable of predicting intramuscular fat concentration with an average of 1.17 percentage of fat error (P < .05). The multivariate model was most appropriate for predicting intramuscular fat below 4%. The mean accuracy of the model in the frequency domain was approximately 79%.

Profiler Measurements of Vertical Velocity Fluctuations in the Ocean

Abstract A method is described for measuring the vertical component of velocity fluctuations due to three-dimensional turbulence in the ocean from a freely falling microstructure profiler. The dynamic pressure measurement relies on a commercially available and very sensitive piezoresistive differential pressure transducer. At nominal profiler fall speeds of 0.9 m s−1, the noise limit in vertical velocity fluctuations is ∼3 mm s−1. Scaled turbulence spectra from shear probes and from the new sensor demonstrate the validity of the measurement. Although hydrodynamic noise (eddy-shedding) at frequencies near the peak in the dissipation spectrum precludes full resolution of the high wavenumber portion of the spectrum, the high end of the inertial subrange of the turbulence is resolved. At low wavenumbers, the measurement is limited by the finite size of the profiler. It is anticipated that the method will be useful in examining turbulent fluxes using an eddy-correlation technique.

Heat/mass transfer from surface films to shear flows at arbitrary Peclet numbers

An efficient numerical method is described for studying the combined conductive and convective transport from a surface film on a planar boundary to a fluid in simple shear flow. Such problems arise most commonly in the use of hot film anemometers, electrochemical shear probes, or in simple models of chemical reactions. The method is illustrated by calculating the total flux (Nusselt number) and variation of the flux along the surface for isothermal circular disks at arbitrary values of the Peclet number (dimensionless shear rate) and is compared with asymptotic results valid at high and low Peclet numbers. The theoretical low Peclet number results of Phillips [Q. J. Mech. Appl. Math. (in press)] lie within 2% of the numerical results for Peclet numbers as high as P=2. At high Peclet numbers, a theoretical estimate for the neglected flux from the edge regions is used, together with the numerical simulations, to propose a correction to the standard one-term asymptotic expression. This approximate relationship remains within 7% of the numerical calculations for Peclet numbers as small as P=5. In addition, results for the total heat transfer as a function of Peclet number are presented for isolated elliptical disks at two orthogonal orientations with respect to the flow and an asymptotic expression for high Peclet numbers is presented for arbitrary disk orientations.

A dual film anemometer probe for measuring arterial shear reversal

Wall shear rate was measured in the canine thoracic aortal in vivo by means of a hot-film anemometer and flush-mounted shear probe. Simultaneous measurement of blood temperature at the probe tip was accomplished by means of a thermistor film located close to the hot-film shear sensor on the probe tip. Blood temperature measurements were utilised to monitor the reversal of the thermal boundary layer produced by the heated shear sensor during blood flow reversal. Temperature data were utilised in an attempt to correct wall shear rates by adjusting the overheat ratio in response to the varying near wall temperature. Wall shear rate data and blood viscosity data were utilised to compute wall shear stress values. The average change in peak shear stress resulting from temperature corrections was 16%.

Hot-film wall shear probe measurements inside a ventricular assist device.

Wall shear rates at eleven sites within the Penn State Electric Ventricular Assist Device (EVAD) were determined with the pump operating under conditions of 30 and 50 percent systolic duration and a mean flow rate of 5.8 L/min using a flush-mounted hot-film probe. Probe calibrations were performed with the hot-film in two orientations relative to the flow direction: a standard orientation and an orientation in which the hot-film was rotated by 90 deg from the standard orientation. The magnitude and direction of the wall shear stress at each site within the EVAD were estimated from ensemble averaged voltage data recorded for similar standard and rotated film orientations. The results indicate that, during diastole the wall shear stress direction around the pump's periphery for both operating conditions is predominantly perpendicular to the inflow-outflow plane (in the direction of the pusher plate motion) and reaches a peak value of approximately 350 dynes/cm2. The highest wall shear stresses were found near the prosthetic aortic valve (inside the EVAD) under the 30 percent systolic duration condition and are estimated to be as high as 2700 dynes/cm2. Peak shear stress values of 1400 dynes/cm2 were observed in the vicinity of the prosthetic mitral valve under both operating conditions. The results suggested that the valve regions are substantially more hemolytic than other wall regions of the EVAD; the magnitudes of the wall shear stresses are sensitive to operating conditions; and that wall shear in the direction of pusher plate motion can be significant.

Bottom Stress Estimates from Vertical Dissipation Rate Profiles on the Continental Shelf

Abstract Measurements of the near-bottom distribution of the turbulent dissipation rate on the continental shelf west of Vancouver Island are used to calculate bottom stress. A free-failing vertical profiler with microstructure shear probes was used to measure the dissipation rate, from near the surface to within 0.15 m of the bottom. The shear probes measure velocity gradients at scales within the viscous subrange of the turbulence and therefore directly measure the rate at which kinetic energy is dissipated by viscosity. Friction velocities are computed from the formula uast; = (ϵκz/ρ)⅓, where the dissipation rate ϵ is measured in the constant stress layer. The technique is more reliable than estimates of the dissipation rate obtained by fitting spectral slopes to velocity spectra at scales in the inertial subrange. Near-bottom current measurements indicate that the bottom stress values obtained from the turbulent measurements are well correlated with the current magnitude. An estimate of the drag coeff...

The frequency response of electrochemical wall shear probes in pulsatile flow.

The frequency response of surface-mounted electrochemical mass transfer probes used to deduce wall shear rates has been investigated experimentally for the case of fully developed laminar pulsatile flow in a straight tube. Generally good agreement is found with the asymptotic results obtained by Lighthill's methods. The significance of the results with regard to the investigation of models of pulsatile flows of physiological interest is discussed. It is concluded that the frequency-dependent phase and amplitude corrections required to obtain accurate wall shear measurements are of such magnitudes as to render impractical the use of electrochemical probes to determine wall shear rates in these flows.

In-vitro wall shear measurements at aortic valve prostheses

Wall shear distributions during the cardiac cycle at the valve rings of Starr-Edwards. Björk-Shiley and Lillehei-Kaster aortic valves are measured and compared with thresholds reported for shear-induced trauma of blood components. Further, for the disk valves, the influence of pulse rate on wall shear stresses is evaluated. Hot film anemometry with flush-mounted wall shear probes is used as measurement technique in a pulsatile flow mock circuit. The experimental systolic data support the better hemodynamic characteristics of the disk valves over the ball valve also with respect to the threshold shear stresses of flow induced blood trauma. These results are confirmed by postoperative clinical studies, where lower LDH-values are found with the disk than with the ball valves. During diastole, however, high shear stresses are measured and calculated at the valve ring of the Björk-Shiley prosthesis, which can be referred to the non-overlapping closing mechanism. This result is discussed with respect to the often observed thrombus formation at the disk downstream of the smaller orifice of the Björk-Shiley valve.

Pacific Equatorial Turbulence

In January and February 1979 a free-fall vehicle was deployed at 150°W near the equator to sample velocity and temperature microstructure. Airfoil shear probes on the vehicle are able to resolve the peak of the velocity gradient spectrum, and permit computation of the rate of dissipation of turbulent energy (ε¯). These measurements indicate maximum values of ε¯ of 10−7 W kg−1 (10−3 Cm2 s−3) to be within 0.5° of the equator, above the velocity core of the undercurrent, similar in location to the observed distribution in the Atlantic (Crawford and Osborn, 1979a). However, a secondary maximum, below the core of the undercurrent found in the Atlantic, was not observed in the Pacific. A comparison of profile-averaged values of &ε¯ with the zonal current and density profiles suggests that temporal variations in &ε¯ are related to the strength of the zonal shear and the magnitude of the stable stratification above the 150 m deep core of the undercurrent. The vertical eddy viscosity due to turbulence (KM) computed from these measurements is of order 0.001 m2 s−1 for the region between 20 and 140 m depth, with larger values neat 20 m, smaller values near 140 m. Upper limits to the vertical turbulent heat flux computed from the formula of Osborn (1980) yield a broad maximum of downward diffusion of heat of as much as 20 W m−2 between 40 and 120 m depth, and an average value for the turbulent eddy diffusivity for heat (KH) of <0.001 m2 s−1. The distribution of both KH and &ε¯ are close to log-normal for data near the equator above the core, and show log-normal standard deviations of 1.1 and 1.0, respectively.

Hot-film wall shear probe for measurements at flexible walls

A new wall shear probe for measurement at flexible walls, such as membranes or arteries is presented. The design is based on commercially available hot-film wall shear probes. Rapid fixation at the membrane and sealing of the probe hole is guaranteed by a cap screw. The membrane is pressed between the screw and a flange. The probe is calibrated in stationary Couette flow by means of a calibration device which was described in an earlier paper (Tillmann and Schlieper, 1979). Application of the probe in an artificial blood pump chamber is demonstrated and first results for the wall shear stress distribution at different sites of the chamber are presented.

Position sensitivity of hot-film shear probes

The position sensitivity of hot-film shear probes was studied with the use of pulse-echo ultrasound. The ultrasonic probe was placed opposite the hot-film probe across a rigid tube. The accuracy of measuring hot-film probe position by ultrasonic methods was studied with the aid of a mechanical micrometer. A similar study was conducted in vivo on the thoracic aorta of dogs to maintain flush conditions between the hot-film probe and arterial wall. The sensitivity of hot-film probes to position with respect to a rigid wall was studied. Probe position was determined by ultrasonic methods. The results showed a 20% deviation in hot-film anemometer bridge voltage with a change in probe position of 0.3 mm from the wall. This could result in a 50% error in computed wall shear stress. Resolution of the method of ultrasonic positioning of the hot-film probe was determined to be 0.1 mm.

A device for the calibration of hot-film wall shear probes in liquids

The device described consists of two concentric cylinders with the outer one rotating, with Laminar Couette flow established within the gap. As the inner stationary cylinder holds the probes, the analytical determination of the shear stress at the wall of the inner cylinder allows the calibration of the probes.