Two-dimensional Laser Doppler Anemometer Research Articles

Effects of varying submergence and channel width on local scour by plane turbulent wall jets

An experimental program was carried out to understand the role of channel width and varying tailwater depth on the scour caused by a plane wall jet. To this end, four groups of tests, denoted as A, B, C and D, were conducted. In group A, during each test the tailwater depth was gradually increased from 2bo to 20bo in a systematic manner. Here, bo is the nozzle height at the exit. In group B, the tailwater depth was gradually decreased from 20bo to 2bo. Group C tests were conducted at fixed tailwater depths based on inferences made from groups A and B tests but correspond to low tailwater conditions. Group D tests were conducted at fixed tailwater conditions corresponding to a large submergence.Within each group, the jet exit velocity was varied from 0.75 to 1.16 m/s while the channel width was varied from 100 to 400 mm. A two-dimensional laser Doppler anemometer was used to characterize the velocity field. Present observations indicate that the velocity-time history can be used to characterize various regimes of flow. Distinct velocity signature patterns have been identified for the digging and refilling phases. These patterns also serve to distinguish between high and low submergence flow. The complete absence of the refilling phase indicates high submergence flow. Most previous studies made the distinction between high and low submergence based on the tailwater ratio. However, the present results show that the distinction also depends on the jet exit velocity. Though the channel width does not have a significant effect on distinguishing the flow as high or low submergence, the scour pattern is affected by the channel width. Narrower channels tend to provide for a two-dimensional (2D) scour pattern in a nominally 2D flow, whereas, wider channels provide for a three-dimensional mound region. These aspects should be borne in mind while conducting future scour experiments and in developing empirical equations that are commonly used to describe scour geometry.

Laboratory experiments of fine-scale mixing and mass transport within a coral canopy

Laboratory experiments obtained fine scale measurements of turbulent shear stresses and rates of mixing and mass transfer over a nonliving bed of the coral, Porites compressa, the dominant species found in Kaneohe Bay, Hawaii. A reef canopy was placed in a recirculating wave-current flume and flow was generated that simulated the flow characteristics of the reef flat of Kaneohe Bay. Turbulence and velocity structure under both unidirectional and wave-dominated currents were measured using a two-dimensional laser Doppler anemometer. Mass transport measurements were made using a planar laser-induced fluorescence technique in which the scalar transport of Rhodamine 6G dye, fluxed from the surfaces of the coral, was quantified. Results show that the action of surface waves, interacting with the structure of the reef, can increase instantaneous shear and mixing up to six times compared to that of unidirectional currents. Maximum shear and mass transport events coincided with flow separation within the wave-current boundary layer and the ejection of vortices into the flow. Wave action also acted to increase the vertical flux of water from within the coral structure. The combined effects of increased turbulent stress and fluid exchange from the interior of the canopy increased mass flux due to wave action 2.3±0.5 times that measured for comparable unidirectional currents.

Two-Dimensional Scour Hole Problem: Role of Fluid Structures

An experimental program was carried out to understand scour caused by a plane wall jet. A two-dimensional laser Doppler anemometer was used to characterize the velocity field at various locations in the scour hole region. Observations indicate that different types of flow structures influence scour at different time periods. Based on the present tests, the entire test duration is divided into five time zones. Following vigorous scour caused principally by jet shear forces and impingement at the start of the test and during early time periods, the flow was characterized by the presence of longitudinal axial vortices, turbulent bursts, and movement of the jet impingement point during the later stages. Attempts were made to distinguish the fluid structures at asymptotic conditions. The scour hole region was characterized by the presence of randomly forming and disappearing streaks, laterally located concave shaped depressions, rolling and ejection of the bed material. Through analysis of higher order moments and quadrant decomposition, sweep and ejection type events were observed, which can potentially contribute to scour.

Time-mean structure of secondary flows in open channel with longitudinal bedforms

Secondary motions are commonly present in open channel flows. This study aims to investigate, both experimentally and analytically, time-mean characteristics of cellular secondary flows generated by longitudinal bedforms. Experiments were conducted in a tilting, rectangular flume with six different longitudinal bedforms, including alternate bed strips with different roughness heights and bed ridges of wavy and rectangular shapes. Various flows were sampled using a two-dimensional laser Doppler anemometer (LDA) and a one-dimensional ultrasonic Doppler velocimeter (UDV). Experimental results demonstrate secondary flows appearing basically in cellular fashion over the modeled longitudinal bedforms. It is also shown that the cellular structures can be described analytically with kinematic considerations. The discrepancies between theoretical and measurement results are discussed. An empirical relationship between maximum vertical velocity and bed configuration is finally proposed based on the experimental data.

Secondary flows over artificial bed strips

Characteristics of open channel flows can be significantly modified in the presence of secondary flows. In this study, cellular secondary flows were artificially generated with alternate rough and smooth bed strips, which were aligned longitudinally in an open channel. Flow measurements were conducted with a two-dimensional Laser Doppler Anemometer system. Experimental results show that the distributions of the primary velocity and streamwise Reynolds shear stress that are altered in the presence of the secondary flows clearly deviate from those observed for two-dimensional open channel flows. The modified streamwise velocity can be represented by a function proposed in the log-wake form but varying periodically in the lateral direction. This study also demonstrates that the flow field can be linearized so that a flow quantity can be generally decomposed into two components, one being related to the average base flow and the other symbolizing the perturbation caused by the bed configuration. This perturbation-based approach finally provides an analytical description of the altered Reynolds shear stress distribution that differs from the linear profile used for two-dimensional open channel flows.

Analysis of Flow over the Perforated Plate in the Duct by Means of LDA and Numerical Simulation

Boundary layer flow over a multi holed plate is analyzed in this paper. Two perforated plates which have different diameter of holes and a flat plate are tested in the rectangular duct. The diameters of holes are 20 mm or 40 mm in staggered arrangement and the all plates are 10 mm in thickness. These studies are carried out by means of two-dimensional LDA (laser Doppler anemometer) and numerical simulation with Partial Cell method. The mean flow velocity and the relative turbulence intensity are 4.5 m/s and 2% respectively in both the experiment and the calculation. Comparisons of flows over the three plates are made for the mean velocity, turbulence intensity and Reynolds stress. In addition, the difference in turbulence flow structure depending on the diameter of holes has been discussed by experimental results.

Note on natural and forced breaking waves in deep water

Two laboratory investigations of natural and forced breaking waves in a two-dimensional wave tank are reported. They are considered to be simple representations of breaking waves that exist in a real ocean, in moderate and rough sea states. The first type (Crescent Waves) is usually observed on a sea surface in relatively calm weather. Initially, for a nonlinear initial wave train of wave steepness ak =0.34, a three-dimensional instability dominates the side band instability, and stable Crescent Waves are observed. The direction angle of radiated oblique waves as a most unstable mode, and associated growth rate are found to be in excellent agreement with Mclean's theory. The second type (strongly overturning breaking waves) was generated on the surface of essentially deep water using a wave energy focusing technique. Time evolution of the transient wave was observed using a high speed video image, and the inherent complicated unsteady flow fields was measured by means of a two-dimensional Laser Doppler Anemometer. We found that, 41% of the wave momentum was lost by the extreme wave plunging breaker. The maximum horizontal velocity at the overturning top of the crest was found to be equal to 1.4 times the phase velocity of the wave, in its most developed stage. The two-dimensional velocity field reveals that the flow is essentially irrotational up to the top of the crest. Surface wave height measurements before and after both an isolated plunging and spilling breaking wave, reveal a frequency downshift phenomenon in the corresponding spectral shapes which seems not to have been previously seen.

Measurement of Turbulent Flow in Standard Jar Test Apparatus

Although the jar test has long been used in coagulation and flocculation, there is little available information on vessel hydrodynamics. Vessel-average parameters such as the root-mean-square velocity gradient, \iG¯, have traditionally been used for hydraulic simultude between the jar test and the full scale. With the recognition that vessel-average parameters may not truly represent hydraulic conditions within a vessel, there is a need to characterize jar test hydrodynamics. Analysis of turbulent flow in a standard jar test apparatus was completed using a two-dimensional laser doppler anemometer. Three dimensional velocity measurements were used to determine the general flow field, dissipation rates, and turbulent length scales within the jar. Substantial variations in turbulent characteristics were found to occur within the vessel. Scaling relations were developed for various turbulent parameters. Values for scaling constants were found to be different than those reported for the Rushton turbine, which is by far the most studied mixing impeller. For example, “scaling constants” for local turbulent dissipation rates in the impeller zone were found to be in the order of five times less than the Rushton turbine. The result is significant because many researchers have accounted for inhomogeneties in mixing characteristics within the jar test apparatus by using findings from available data on the Rushton turbine.

Secondary Flow Velocity Patterns in a Pulmonary Artery Model With Varying Degrees of Valvular Pulmonic Stenosis: Pulsatile In Vitro Studies

The objective of this study was to characterize in detail the secondary flow velocity patterns in an in vitro model of a human (adult) pulmonary artery with varying degrees of valvular pulmonic stenosis. A two-dimensional laser Doppler anemometer (LDA) system was used to map the flow fields in the main (MPA), left (LPA), and right (RPA) branches of the pulmonary artery model. The study was conducted in the Georgia Tech right heart pulse duplicator system. A pair of counter-rotating secondary flows were observed in each daughter branch in which the fluid moved outwardly along the side walls and then circled back inwardly toward the center of the vessel. For the case of the "normal" valve, the two counter-rotating secondary flows were symmetric about the centerline. The strength of secondary flows in the RPA was much stronger than in the LPA. However, as the pulmonic valve became more stenotic, the two counter-rotating secondary flows in both the LPA and RPA were no longer symmetric. In addition, the strength of secondary flows in both daughter branches increased with increasing degree of valvular stenosis. The increment in the LPA was, however, greater than in the RPA. The study demonstrates the importance of analyzing complex biological flows from a three-dimensional viewpoint.

Axial flow velocity patterns in a pulmonary artery model with varying degrees of valvular pulmonic stenosis: Pulsatile in vitro studies

With the advent of noninvasive clinical techniques which can measure blood flow velocities (Doppler ultrasound), it is suggested that a fundamental knowledge of the axial flow velocity patterns in the pulmonary artery, and the changes caused by stenosis, may be used to support accurate diagnosis of valvular pulmonic stenosis. The present study was designed to characterize the axial flow velocity patterns in an in vitro model of a human adult pulmonary artery with varying degrees of valvular pulmonic stenosis. A two-dimensional laser Doppler anemometer (LDA) system was used to map the flow fields in the main (MPA), left (LPA), and right (RPA) branches of the pulmonary artery model. The study was conducted in the Georgia Tech. right heart pulse duplicator system. It was observed that the axial flow velocity patterns in the MPA and the LPA change dramatically with increasing degree of valvular stenosis. This indicates that the axial flow velocity patterns in these two branches are strongly influenced by the degree of valvular stenosis. The axial flow velocity patterns in the RPA, however, do not change much with varying degrees of valvular stenosis, indicating that the axial flow fields in the RPA are mainly influenced by the geometry of the bifurcation. It may be concluded therefore, that the changes in the axial flow velocity patterns in the MPA and LPA (rather than in the RPA) could be sensitive and reliable indicators of the severity of the defect.

In vitro hemodynamic characteristics of tissue bioprostheses in the aortic position

The in vitro hemodynamic characteristics of a variety of old and new generation porcine and bovine pericardial bioprostheses were investigated in the aortic position under pulsatile flow conditions. The following valves were studied: Carpentier-Edwards porcine (Models 2625 and 2650), Carpentier-Edwards pericardial, Hancock porcine (Models 242, 250, and 410), Hancock pericardial, and Ionescu-Shiley (standard and low-profile) bioprostheses. The pressure drop results indicated that the old design valves had performance indices in the range of 0.30 to 0.42, whereas the new low-pressure fixed designs have performance indices of 0.50 to 0.70. Flow visualization and velocity and turbulent shear stress measurements, conducted with a two-dimensional laser Doppler anemometer system, indicated that all tissue valve designs created jet-type flow fields. The intensity of the jets and turbulence levels were less severe with the new designs. The old designs created higher peak jet velocities and higher levels of turbulent shear stresses. On the whole, pericardial bioprostheses have better in vitro hemodynamic characteristics than porcine bioprostheses. These observations should have applications regarding the clinical choice of bioprosthetic valves and have implications regarding further improvements in the preparation and design of bioprosthetic valves.

Pulsatile flow velocity and shear stress measurements on the St. Jude bileaflet valve prosthesis.

The velocity and turbulent shear stress fields in the immediate vicinity of the St. Jude bileaflet valve were measured under pulsatile flow conditions with a two-dimensional laser Doppler anemometer system. The valve was studied in both aortic and mitral positions. In both positions, the valve created relatively centralized flow fields. However, a major portion of the flow occurred through the two side orifices. Regions of flow separation were observed adjacent to the valve sewing ring in the area of the valve pivot (hinge) mechanism. Elevated turbulent shear stresses were measured in both positions. Peak values of 760 dynes/cm2 and 2 000 dynes/cm2 were observed in the mitral and aortic flow chambers, respectively. Such turbulent shears could cause sublethal and/or lethal damage to blood elements. The regions of flow separation adjacent to the pivot mechanism could lead to tissue overgrowth and/or thrombus formation, which in turn could impede proper motion of the valve leaflets.

Results from a two-dimensional laser Doppler anemometer

Results obtained using a two-dimensional laser Doppler system are compared with those from the conventional one-dimensional system. The results show good agreement, but attempts to measure simultaneously from two channels with the three-beam system were abandoned due to the small, but significant variations in the two tracker systems. An alternative superposition fringe mode procedure is discussed.